1438Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
Architecture of the aquifers of the Calama Basin, Loa catchment basin, northern ChileTeresa Jordan1*, Christian Herrera Lameli2, Naomi Kirk-Lawlor3, and Linda Godfrey4
1Department of Earth & Atmospheric Sciences and Atkinson Center for a Sustainable Future, Snee Hall, Cornell University, Ithaca, New York 14853-1504, USA2Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile3Department of Earth & Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853-1504, USA4Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA
ABSTRACT
In the Loa water system of the Atacama Desert in northern Chile, careful management of groundwater is vital and data are sparse. Several key man-agement questions focus on aquifers that occur in the Calama sedimentary basin, through which groundwater and Loa surface water flow to the west. The complexity of the two major aquifers and their discharge to wetlands and rivers are governed by primary facies variations of the sedimentary rocks as well as by faults and folds that create discontinuities in the strata. This study integrates geological studies with groundwater hydrology data to docu-ment how the aquifers overlay the formations and facies. Neither the phreatic aquifer nor the confined or semiconfined aquifer, each of which is identified in most basin sectors, corresponds to a laterally persistent geological unit. The variable properties of low-permeability units sandwiched between units of moderate to high permeability cause a patchwork pattern of areas in which water is exchanged between the two aquifers and areas where the lower aqui-fer is confined. The westward termination of most of the sedimentary rocks against a north-trending basement uplift at an old fault zone terminates the principal aquitard and the lower aquifer. That termination causes lower aquifer water to flow into the upper aquifer or discharge to the rivers. The regionally important West fault juxtaposes formations with differing lithological and hy-draulic properties, resulting in some exchange of water between the upper and lower aquifers across the fault.
INTRODUCTION
The Loa River water system of northern Chile’s Atacama Desert (Fig. 1), in the Antofagasta region, exemplifies the high stakes involved in sustainable management of scarce water resources. The Loa surface and groundwater system supplies the great majority of water used in Antofagasta Region, and meets much of the municipal and agricultural demands. It is vital to Anto-fagasta Region copper mining, which constitutes ~50% of Chile’s copper pro-duction (Servicio Nacional de Geología y Minería, 2011), which in turn supplies one-third of the world’s copper needs. However, a key property of the Loa sys-tem is the scarcity of surface water.
The Loa groundwater and surface water are inseparable resources; the Loa River is the discharge channel of the groundwater of a >34,000 km2 area basin. The coincidence of modern climate, topography, and geology leads to a geography in which recharge occurs far from where humans use the water resources, and groundwater aquifers supply the vast majority of stream water. The aridity of the region sharply restricts the number of human inhabi-tants and extent of native plants or animals. However, under different climate states during the past few millennia the water flux was greater than now (Rech et al., 2002; Latorre et al., 2006); this leads to great uncertainty in estimations of how much of the current water flow is renewable versus fossil (Houston and Hart, 2004).
Under the Chilean water code, water is treated as a property independent of land, to which a permanent right is granted for a given production rate from a given extraction site. Although the water code treats groundwater rights and surface-water rights separately, for the Loa system the regulatory body (Direc-ción General de Aguas, hereafter DGA) treats them as strictly coupled. In cur-rent practice, all requests for additional rights or for changes in the points of extraction are subject to an environmental impact review by an independent agency, the Ministerio del Medio Ambiente.
Initial management efforts distributed rights to exploit surface and ground-water when only a limited understanding of the natural system was avail-able. Now management practices strive to minimize both economical and eco logi cal problems, yet to do so requires improved information about the natural system. The desire to better inform the management of this coupled natural-human resource system is a central motivator for this study. Further-more, because the Loa system is at the junction of a natural extreme (e.g., an arid to hyperarid climate) and an atypical water governance approach (e.g., pri-vate water rights), lessons from the Loa system may be broadly useful to those who are considering regional-scale groundwater management strategies.
Because of extreme aridity at low elevations, precipitation that is likely to lead to direct recharge of aquifers occurs only in the eastern mountainous fringe of the hydrologic basin (Fig. 1B) (CORFO 1977, see Table 1; Houston, 2009). At lower elevations there is extensive exchange of water between aqui-fers and rivers (DGA 2001, see Table 1; Houston, 2006), but the locations and net outcomes of those exchanges are not well documented and constitute major themes for ongoing research.
CITATION: Jordan, T., Herrera L., C., Kirk-Lawlor, N., and Godfrey, L., 2015, Architecture of the aquifers of the Calama Basin, Loa catchment basin, north-ern Chile: Geosphere, v. 11, no. 5, p. 1438–1474, doi:10.1130/GES01176.1.
Received 11 February 2015Revision received 13 May 2015Accepted 17 June 2015Published online 5 August 2015
OPEN ACCESS
GOLD
This paper is published under the terms of the CC-BY license.
1439Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
This paper focuses on the rocks through which the groundwater flows in the central sector of the Loa system, within and adjacent to the Calama Valley (Fig. 1B), in which an upper phreatic aquifer and a lower confined aquifer are routinely described (Figs. 2 and 3) (CORFO 1977, see Table 1; Houston, 2004). There are three primary purposes of this paper: to clarify the spatial distribu-tion of the rocks with hydraulic conductivity favorable to function as aquifers; to identify where the lower aquifers discharge to the surface water system; and to identify the most likely sectors in which water is exchanged between upper and lower aquifers. Although the data available for hydraulic properties are
sparse, the combined use of knowledge of the sedimentary architecture of the Calama sedimentary basin and of piezometric head enables informed extrap-olation of the hydraulic data laterally and vertically. For the first time for the Loa system, we analyze the controls on the spatial variability of major aquifers imposed by the complex stratigraphic architecture, and the uncertainties that remain. Examination of the state of knowledge reveals sectors of the ground-water basin for which it is most critical to obtain hydrochemical, geophysical, and hydrological data with which to monitor the impacts of water extraction or to constrain parameters in a numerical model.
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Central D
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Figure 1. (A) Inset map shows location of the Loa hydrologic system in west-central South America. (B) Map of the three types of basin pertinent to the hydrology of the Loa system. Base is a digital elevation model in which tan colors show elevations below ~2500 m, gray indicates higher ele-vation, and blue is the Pacific Ocean. The surface catchment basin is outlined in red (solid where persistent surface drainage to the Loa is clear; dashed where ground-water flow is a major intermediate step). The maximum plausible extent of the groundwater basin is outlined in black (solid line for borders defined where there is little possibility for recharge; dash-dot line where there is possible recharge but there are no direct constraints on the validity of this selection of the ground-water basin border). The Calama sedimen-tary rock basin is outlined by the evenly spaced dashed black line. Blue lines mark the Loa River and its main tributaries. Rivers (R., river name) and mountains (S., mountain name) mentioned in the text are labeled. Only areas above 4000 m (patterned regions) have a combination of sufficient precipitation and soil prop-erties suitable to significant infiltration of rainfall and snowmelt and thus direct recharge (Houston, 2009). The formally de-fined boundaries of the Loa surface water basin are located at U (dividing upper and middle Loa) and M (dividing middle and lower Loa). C—Calama city (at the western margin of the ~50 km × 50 km Calama Val-ley); D—Conchi Dam. Boxed area shown in Figure 2.
1440Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
A broader objective is to advance appreciation that knowledge of sedimen-tary basin architecture is valuable for regional hydrogeology research. Barthel (2014) drew attention to the need for improved fundamental regional hydro-geological approaches. The hydrocarbon resource industry routinely utilizes the architecture of entire sedimentary basins as a tool to predict reservoir flow properties. Likewise, for groundwater systems within sedimentary basins, the
large-scale architecture of the strata likely plays a major role in determining the continuity of hydraulic properties. This paper demonstrates an application of knowledge of sedimentary basin architecture to a major sector of a coupled groundwater–surface-water basin.
After describing the existing management premises and the physical context of the study area, we describe the available piezometric framework
TABLE 1. UNPUBLISHED REPORTS CONTAINING DATA CITED IN TEXT
Report identification, year Organization Title Report identification or URL English translation title
CORFO 1973 Corporación de Fomento de la Producción, Santiago, Chile
Estudio de los Recursos Hídricos de la Cuenca del Río Loa. V. 2 Anexos
Final report on study of environmental impacts of the Quetena Project: Hydrogeological model study for the Toki cluster subbasin
Matraz 2012 Matraz Consultores Asociados, Universidad Politécnica de Cataluña, report prepared for Dirección General de Aguas, Santiago, Chile
Estudio Acuífero de Calama, Sector Medio del Río Loa, Región de Antofagasta
http://documentos.dga.cl/SUB5431v1.pdf
Study of the Calama aquifer, middle sector of the Loa River, Antofagasta region
GAC 2012 Gestión Ambiental Consultores Anexo 8. Componente Hidrogeología DIA construccion paseo Rio Loa, Calama: Región de Antofagasta, Chile, prepared for City of Calama
Codigo BIP: 20191503-0 Appendix 8. Hydrogeology component: DIA construction of Loa River tourist route, Calama: Antofagasta region, Chile
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Figure 2. Piezometric map of the phreatic aquifer in the middle Calama groundwater system, superimposed on a digital elevation model. Filled circles mark locations of wells used in creat-ing map; red circles distinguish wells with time-series reports of water levels (Table 8). Black numbers and thin lines are piezometric contours; gray numbers and lines are land surface con-tours. Heavy black lines are faults. Elevations are in meters above sea level. A single italicized gray number notes the elevation of the bed of the Loa River north of the region with piezometric data. Solid piezometric contours indicate nearby well control, whereas dashed lines indicate long-distance extrapolation of sparse data. Data largely correspond to before 2005. Dashed lines in the southern sector (Llalqui area, L, and farther to the east) are from Houston (2006). The piezometric contours of previous reports (Fuentes Carrasco, 2009; EIA 2011, Matraz 2012, and Mayco 2013, see Table 1) were modified using wells and exploration boreholes consulted for this project (Table 7), which are identified with black dots. Hills in the midst of the Calama Basin sedi mentary fill that expose deformed Eocene and older rocks are marked by cross-hachured zones. Blue diamonds are locations with stream flow data noted in Table 2. Locations: Ch—Chintoraste hills; O—Ojos de Opache region; C—Calama (star); H—Calama Hill; T—Talabre area; LC—La Cascada waterfall. The rock units in contact with a thin alluvial fill below the Loa and Salado Rivers are indicated by colors. The rectangle marks the extent of Figure 3.
∆∼
A B
Figure 3. (A) Piezometric map of the lower aquifers for region in box in Figure 2, superimposed on a digital elevation model. Data largely correspond to before 2005. Labels and symbols for elevation and topographic contours, piezometric contours, landforms, and locations are as in Figure 2 (tr—extensive tufa carbonate deposits). Wells and exploration boreholes consulted for this project are identified by dots; red dots distinguish wells with time-series reports of water levels (Table 8). Hills in the midst of the Calama Basin sedimentary fill that expose deformed Eocene and older rocks are marked by cross-hachured zones. Heavy black lines mark faults. The piezometric contours of previous reports (EIA 2005, EIA 2011, Matraz 2012, and Mayco 2013, see Table 1) are modified based on available well data (Table 7). (B) Differential head of lower and upper aquifers. Green shows regions where head of lower aquifer exceeds that of upper aquifer (Lo>Up). Red shows regions where head of upper aquifer is higher than that of lower aquifer (Up>Lo). Blue shows areas with near equality of the two (Δ~0). Sectors with question marks are regions in which the contours in either A (or Fig. 2) are poorly constrained by data. Red circles indicate wells for which time series of head data are available that show a minimum human impact (Table 9); large diameter circles are for upper aquifer and small diameter circles are for lower aquifer. Stars indicate locations at which the impact on head levels of the variability of nearby well pairs is analyzed (Table 9). In the green areas there is a tendency for the lower aquifer to recharge the upper aquifer. In the red regions there is a tendency for the upper aquifer to recharge the lower aquifer.
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of the aquifers, then illuminate the spatial distribution of the geological units in which the aquifers and aquitards occur across the middle part of the Loa ground water basin. The paper concludes with identification of hydrogeologi-cal trends and unresolved problems.
Water Management Hydrological Premises and Uncertainties
The Loa hydrologic system is located on the western flank of the Andes Mountains in northern Chile and extends westward to the Pacific Ocean coast (Fig. 1). The generally accepted premise is that the natural water basin is at steady state such that recharge equals combined flows out of the basin plus extraction plus evapotranspiration. An alternative conceptual model holds that some flow results from head decay established during times of wetter climate (Houston and Hart, 2004).
Based on empirical relationships between precipitation and elevation as well as temperature and elevation, combined with the topography of the basin, the DGA (2003) estimated that the total annual available recharge of the Loa surface and groundwater hydrologic basin is 6.4 m3/s. However, the Loa River discharges only 0.6 m3/s to the Pacific Ocean (Salazar, 2003) (Table 2). The dif-ference, 5.8 m3/s, is attributed to evapotranspiration and consumptive water use. Large uncertainties exist with this steady-state model. A trend is found for water to flow predominantly in the subsurface in the upper parts of the basin, in a combination of surface channels and subsurface flow in the middle Loa basin, and in surface channels in the lower Loa basin. It is thought that the final significant transfer from subsurface to surface flow occurs just west of
Calama city (Salazar, 2003). Consequently, the official measure of the water in the system available for ecosystem use, and potentially for additional human use, is given by the discharge in the Loa and San Salvador Rivers west of this assumed final location of transfer from aquifers to surface streams.
The water balance model that underpins water management decisions is informed by a set of long-term stream gauging stations that exist within the central Loa basin as well as by a small set of monitoring wells (Fig. 2; Table 2). However, there are long reaches of the Loa River where flow is not measured, or where only single-year gauging campaigns have been reported (Matraz 2012, see Table 1). There has not been a previous analysis of the poten-tial for hydraulic interconnections among the rocks that contain the aquifers, although extensive monitoring plans have been developed to try to demon-strate the presence or absence of hydraulic connections. Although recently published geological and stratigraphic studies illuminate the stratigraphic and spatial positions of units that may function as aquitards or aquifers, the result-ing insight into the likely complexity of the groundwater system has not been integrated into basin-scale water management assessments that are important to the integrated management of the groundwater and surface water system.
The lack of understanding of the architecture of the sedimentary-hosted aquifers within the Calama Basin contributes to a lack of understanding of where groundwater exits the middle sector of the Loa groundwater basin. This gap in knowledge is particularly relevant to deriving, let alone monitoring, a water budget. Absent data regarding the western distribution or terminations of the aquifers, an arbitrary location of where discharge to the Loa is mea-sured could produce misleading information, especially for monitoring of the impacts of operating well fields. An outcome from this paper, a data-based
TABLE 2. REPRESENTATIVE LOW-FLOW MEASUREMENTS FOR THE MIDDLE AND LOWER SECTORS OF THE LOA RIVER
Pre-1979: mean of 1–7 single-date measurements (from Corporación
de Fomento de la Producción, 1973*)
Post-1979 inauguration of Conchi Dam; means of
DGA monthly means†
Stationnumber inFigure 2 Descriptive location
September–October 1916
(L/s)
August 1918(L/s)
June–July 1961(L/s)
June-July 1969(L/s)
Mean of July values 1990–2000 unless noted
(L/s)
1 Below future position of the Conchi Dam 2200 1900 2100 8802 After junction of Salado and Loa Rivers 3200 3100 2403 At Angostura 3500 32004 Northeast of Calama Hill 4200 3600 12005 Near La Cascada§ 1300 3700 2400 640
Loa River before junction with San Salvador River 1200Loa River after junction San Salvador River 2200 690 A.D.1993–2000San Salvador River before junction with Loa River 600Loa River at shore of Pacific Ocean 3600 600 Salazar (2003)
*Corporación de Fomento de la Producción (CORFO 1973; see Table 1) Estudio de Los Recursos Hídricos de la Cuenca del Río Loa, Anexos (Studies of the Water Resources of the Loa River Basin, Appendices). Universidad de Chile, Departamento de Recursos Hidraúlicos.
†Data reported by Dirección General de Aguas (DGA), http://snia.dga.cl/BNAConsultas/reports. §Pre-1973 reports cite station “Loa en Chintoraste”; post-1990 data reported by DGA at location “Loa en La Finca”; see Table 1.
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hypothesis for the locations of aquifer discharge west of Calama city, should be considered when planning monitoring stations.
The extraction of water has had significant impact on stream flow. Stream gauge measurements from the early twentieth century provide data least af-fected by extraction (Table 2; 1916 data). Some water would have been di-verted then for agricultural use, and the first sluice, built in 1915, supplied the early copper industry. By the 1960s there was important extraction of water for mining purposes, and in 1979 the high Conchi Dam was built to reduce the im-pacts of the rare floods. The result (Table 2) was a decrease between the early 1960s to the 1990s by >50% in stream flow. By the 2000s, the regulatory agency DGA began to tighten evaluations of petitions for additional extractions, in response to the assessment that the water system was in deficit.
Basic Components: Surface Water, Groundwater, and Sedimentary Rocks
Three distinct types of basin are important to the Loa system hydrology. From a hydrological perspective, the Loa basin is tightly coupled to conti-nental-scale landforms and is extensive, whether one considers its surface catchment area (first type of basin, the surface-water basin) or its groundwater recharge-discharge footprint (second type of basin, the groundwater basin) (Fig. 1). The third basin type is geological, and pertains directly to the physi-cal properties of the aquifer rocks. The Calama sedimentary basin forms the subsurface rocks of a central sector of the Loa hydrological basin (~2400 km2; sedimentary basin) and constitutes one of two major regions for groundwater
exploitation (Fig. 2). All three classes of basin are important, and it is necessary to clarify which basin (surface catchment, groundwater, or sedimentary) is the focus of various parts of the analysis.
Surface Catchment Basin
The surface catchment basin (33,570 km2; Tables 2 and 3) is limited at the topographic crestline in the Andes Mountains on the east, the Pacific coast on the west, and ~lat 21°S and 23°S on the north and south, respectively. The Loa River main stem measures 440 km in length, with a series of four orthogonal reaches, each 50–150 km long (Fig. 1). The first broad valley through which the surface drainage system passes is the 50 km by 50 km Calama Valley, the focus here, whose low-relief floor is ~2200–2800 m above sea level. The two main tributaries, the upper Loa River and the Salado River, join within the Calama Valley.
The general attributes of the topography, characterized by mountains with elevations >4000 m in the east and lowlands toward the west, set the bound-ary conditions for groundwater recharge and water flow. In the Atacama Des-ert precipitation increases with elevation (Table 3) and evaporation is intense. Precipitation currently capable of recharging the aquifers only occurs above 3500 m above sea level (asl) but, after consideration of evapotranspiration, recharge is most likely above 4000 m elevation (Fig. 1) (DGA, 2003; Houston, 2009). The area in the Loa catchment with widespread potential for modern recharge is the eastern mountains (Western Cordillera and Altiplano; Fig. 1), although there may have been recharge at elevations below 4000 m during
TABLE 3. SURFACE-WATER DRAINAGE BASIN OF THE LOA RIVER
Divisions Boundaries and dimensions Properties References
East: crest line, Andean peaks (> 6000 m asl)West: Pacific Ocean coastlineNorth: lat ~21°SSouth: lat 23°SArea: 33,570 km2
DGA (2005)Berenguer et al. (2005)
Upper Headwaters in Western Cordillera to junction of Loa River with Salado River
Arid to hyperarid, precipitation 5–200 mm/yr; sparse vegetation on hillslopes
Houston and Hartley (2003)
Middle Juncture Salado River to junction San Salvador River (location M in Fig. 1)
Hyperarid, precipitation 2–5 mm/yr; steep topographic gradient (from >2900 m elevation below Conchi Dam (location D) to ~2300 m at Calama city (location C) to 1200 m at juncture with San Salvador River (location M in Fig. 1); except ~20 km reach, passes through narrow canyon incised 20–200 m into either consolidated sedimentary rocks, volcanic rocks, or crystalline basement
Houston and Hartley (2003)
Lower Junction San Salvador River to Pacific Ocean Hyperarid, precipitation <1 mm/yr; flows in canyon entrenched tens to hundreds of meters depth below the average surface of the central depression; gains water only from aquifer discharge focused at single tributary (Amargo River)
Houston and Hartley (2003)
Note: asl—above sea level; DGA—Dirección General de Aguas.
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wetter times of the Pleistocene or Holocene (Latorre et al., 2002; e.g., Rech et al., 2002). Outside of the eastern highlands, the Loa system surface water bodies are fed by aquifers (DGA, 2003).
The San Salvador River rises at springs just north and west of Calama city (Fig. 2). The Loa and San Salvador Rivers are ~5 km apart and are parallel as the south and north canyon boundaries of a 65-km-long east-trending valley that has a table-like planar surface. With the exception of ~20 km distance of the Loa main stem, the Loa and San Salvador Rivers pass through narrow canyons incised 20–200 m into either consolidated sedimentary rocks, volcanic rocks, or crystalline basement.
River flow data from the early austral spring season in a year prior to ex-tensive consumptive use of the Loa River system (September 1916) indicate that natural flow of the middle Loa River where it entered the Calama Val-ley (~2200 L/s) was increased substantially by influx from the Salado River (~1000 L/s) (Table 2). The San Salvador River carried ~600 L/s. Between the junctions of the Salado and the San Salvador, the Loa River flows through the hyper arid Calama Valley and the Calama sedimentary basin. Exchanges be-tween surface water and groundwater are suggested by both downstream in-creases and decreases in Loa River flow in reaches where there are no tributary streams (Table 2; e.g., gains between stations 2–4; losses between stations 4–5).
Groundwater Basin
The boundaries of the groundwater basin are not well known (Table 4), both in the western region (Coastal Cordillera, Fig. 1) and in the eastern high-lands, where the border may coincide with the surface-water basin or may extend east of the surface catchment, beneath the Altiplano Plateau. The areal extent, 34,000–65,000 km2, is very uncertain (Fig. 1).
The Loa groundwater basin can be considered to include three geograph-ical sectors, upper, middle, and lower; the lower groundwater basin is not discussed herein. The eastern limit of the upper, or eastern, groundwater basin likely occurs among the volcanic centers that form the Western Cor-dillera and that cover broadly the southwestern Altiplano Plateau (Fig. 1). Those volcanic peaks overlie laterally extensive Miocene and Pliocene pyro-clastic volcanic deposits and interbedded epiclastic sands and gravels (de Silva, 1989; Montgomery et al., 2003; Houston, 2007) that compose aquifers in some of the upland basins (Mardones Perez, 1998; Montgomery et al., 2003; Houston, 2007; DGA, 2003). A reasonable but unproven extrapolation is that some water recharged east of the surface catchment divide flows as groundwater westward into the surface water Loa catchment (Pourrut and Covarrubias, 1995; Houston, 2007).
TABLE 4. GROUNDWATER BASIN OF THE LOA RIVER
Divisions Boundaries and dimensions Geological setting of aquifers Climate and recharge potential References
Overall East: within Andes MountainsWest: Pacific Ocean coastlineNorth: lat ~21°SSouth: lat 23°SArea: ~34,000 km2 (similar to
surface drainage) to 65,000 km2
(speculative maximum)
Precipitation increases with elevation: areas from the Calama Valley westward receive <5 mm/yr on average; mountain regions to the north and east receive 60–200 mm/yr
DGA (2003); Houston and Hartley (2003)
Upper At latitudes of the Calama Valley, western margin is the border of the Calama sedimentary basin
Hydraulically conductive volcanic-related deposits, probably inclusive of porous volcanic ash deposits, detrital sedimentary deposits, and fractured dense volcanic deposits
Widespread potential for modern recharge at elevations >4000 m (western Cordillera and Altiplano); potential recharge during Pleistocene and/or Holocene wet intervals at elevations <4000 m
DGA (2003); Houston (2009); Rech et al. (2002a); Latorre et al. (2002)
Middle Calama Valley and eastern ~10 km of Salvador-Loa valley; western limit defined by the emergence at surface of the rocks that, to east, compose lower aquifer and a major confining unit; south margin poorly constrained
Sedimentary rocks of Calama Basin Evapotranspiration exceeds precipitation DGA (2003); Montgomery 2009, 2010; El Tesoro 2010; EIA 2011; Matraz 2012 (see Table 1)
Lower Western ~50 km of valley between San Salvador and Loa Rivers, and entire region farther west and farther downstream
Combination of thin sedimentary units near Batea-María Elena, thick sedimentary units near Quillagua, and impermeable basement rock everywhere else
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The middle sector of the groundwater basin (Table 4) is strongly linked to the Calama sedimentary basin. Across the Calama topographic valley the water table drops nearly 500 m, from ~2700 m asl to 2240 m asl at Calama city, with a further drop of >100 m to the western limit of the aquifers (Fig. 2). Groundwater exploration, monitoring, and production in some parts of the Calama Valley have demonstrated that aquifers extend to several hundred meters depth. A phreatic aquifer extends locally to a depth as great as 100 m (Houston, 2006), and lower aquifers occur in a depth range of 100–300 m (Fig. 3A) (inclusive of DGA, 2003; EIA 2011, Matraz 2012, and Mayco 2013 in Table 1). These depths greatly exceed the thickness of unconsolidated sediment (Blanco and Tomlinson, 2009; Tomlinson et al., 2010), and are within the compacted Ceno zoic sedimentary rock. To date, no aquifers are known within the Paleo-zoic or Mesozoic highly indurated strata, and therefore these units plus plu-tonic rocks, metamorphic rocks, and lava flows are treated as the hydraulic basement. Fracture flow may be possible within the rock units treated as hy-draulic basement, but is not discussed here.
Calama Sedimentary Basin
The Cenozoic sedimentary rocks of the Calama Basin compose the third cat-egory of basin (Table 5) and host the major aquifers of the middle Loa ground-water basin. Both the Calama Valley and the mesa-like surface between the San Salvador and Loa Rivers (Fig. 4) are the modern expressions of this long-lived sedimentary basin. The Calama Basin is composed of moderately consolidated sedimentary rocks of Eocene–Pliocene age (May, 1997; May et al., 1999, 2005; Blanco et al., 2003; Jordan et al., 2006; Blanco, 2008; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) (Fig. 5). The depocenter of the Cenozoic sedimen-tary basin has shifted in location at least four times (Fig. 6). First, Eocene-age conglomerates and volcanic-associated strata reach ~1000 m thickness and occur mostly in the southern third of the valley (Fig. 6D). This first stratigraphic
unit encompasses the confined aquifers in some sectors of the groundwater basin. Second, Oligocene to lowermost Miocene strata reach ~2000 m in thick-ness beneath the north-central part of the valley (Fig. 6C). Third, the overlying lower and middle Miocene strata spread widely across what is now the Calama Valley (Fig. 6B). In the northern part of the basin some facies of these units host both the lower and upper aquifers, but in the southern and western part of the basin the fine-grained facies that corresponds to this time slice forms an important confining layer. Fourth, the upper Miocene and Pliocene Opache and Chiquinaputo Formations extend across most of the Calama Valley as well as northward along the lower reach of the surface water upper Loa basin and west-ward across the mesa-like surface between the San Salvador and Loa Rivers (Figs. 4 and 6A). Parts of the middle Miocene strata and much of the upper Miocene–lower Pliocene strata constitute the phreatic aquifers.
Geohydrological Framework
Water in the Loa catchment north and east of the focus region enters per-meable units of the subsurface (Houston, 2007), but those units are not later-ally continuous with possible host rocks of the Calama Valley focus area. The possible aquifer rock units within the Calama sedimentary basin are not tabu-lar or continuous sheets. Thus water must pass from one set of stratigraphic units to another set, in order to exit the middle section of the Loa system. A comparison of the large amount of mapping data added by recent surface and subsurface geological studies (e.g., Jordan et al., 2006; Blanco, 2008; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) to the hydrogeological reports, both published (Houston, 2004) and unpublished (EIA 2005 in Table 1), reveals that the stated stratigraphic position of the gravels that serve as confined aqui-fers is commonly inconsistent with the recent geological mapping.
Although the Calama sedimentary basin strata are little deformed com-pared to other Cenozoic basins of northern Chile, deformation by faults and
TABLE 5. SEDIMENTARY BASINS WITHIN THE LOA SYSTEM
Divisions Boundaries and dimensions Relationships to surface and groundwater basins
Properties References
Calama ~2400 km2
21.8°–22.6°S68.3°–69.2°W
Salado and Loa Rivers join in basin; San Salvador River rises from springs in basin; overlaps with middle Loa groundwater basin
Eocene–Quaternary strata; maximum thickness >2000 m
Naranjo and Paskoff (1981); May (1997); May et al. (1999, 2005); Jordan et al. (2006); Blanco (2008); Blanco and Tomlinson (2009); Tomlinson et al. (2010)
Batea area Within Central Depression valley22.2°–22.7°(?)S69.2°–69.6°(?)W
Junction of San Salvador and Loa Rivers within basin
Miocene and younger strata; maximum known thickness ~100 m
Naranjo and Paskoff (1982); May (1997)
Pampa del Tamarugal (Quillagua)
Within Central Depression valley;21.8° to farther north than limit of
surface water basin (~21°S)69°–69.7°S
Loa River passes through basin before final west-directed reach
Miocene and younger strata; near path of Loa, maximum thickness ~150 m
Sáez et al. (1999, 2012); Jordan et al. (2010); Nester and Jordan (2012)
1446Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
folds are important to the groundwater and surface-water systems. A set of faults and associated folds (Mesozoic and Eocene faults of Cerros de Guacate and Sierra de San Lorenzo; the Cere fault; Table 6) controlled the topographic margins of the Calama Basin while sedimentary rocks accumulated. Four major and one minor tectonically controlled fault sets traverse the sedimentary basin (Table 6; Fig. 7) and bound hydraulically conducting stratigraphic units. An additional set of nontectonic folds and fractures creates a large-scale and laterally continuous zone of likely hydraulic discontinuity within the Calama Valley (Fig. 7; Table 6). Some of the faults (e.g., West fault, Milagro fault; Table 6) displace by kilometers the continuity of rocks that are the hosts for the lower
aquifer. Only the Chiu Chiu monocline (Table 6; Fig. 7) offsets significantly the strata that host the upper aquifer. The result of the primary stratigraphic pat-terns and secondary deformation is a complex architecture of the strata that are plausible aquifers and aquitards beneath the surface of the Calama Valley and San Salvador–Loa Valley.
The geological units herein are the lithological units mapped by Marinovic and Lahsen (1984), Marinovic et al. (1995), Blanco and Tomlinson (2009), and Tomlinson et al. (2010). These mappable lithological units are not strictly the same as the aquifers and aquitards defined by hydraulic conductivity. Rather, this paper documents the positions in space of the geological units whose facies
A′
B′
C′D′
E′
Figure 4. Simplified geological map of Calama Valley and eastern sector of the San Salvador–Loa Valley, showing only the surface distribution of units discussed in this paper because of their role as hosts of aquifers or as major aquitards. Shades of green identify units associated with a lower aquifer; yellow indicates a unit that is associated with both a lower and upper aquifer. Shades of orange identify units that are considered in some areas to be aquitards. Distribution of units compiled from Blanco and Tomlinson (2009) and Tomlinson et al. (2010). Abbreviations as in Figure 2. Faults and major folds are shown as thick blue lines (names in Fig. 7), but not all of them deform surface units. Locations of geological and piezometric cross sections are marked as solid black lines, and area of Figure 9 is defined by box. Dashed thin black lines show seismic reflection profiles.
1447Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
are both anticipated, based on textures observed in outcrop, as well as demon-strated in boreholes, to have appropriate porosity and permeability to serve as aquifers or as aquitards. Thus the maps and geological cross sections place bounds on the distributions of the host rocks of the aquifers and aquitards.
MATERIALS AND METHODS
Data for groundwater are from reports that companies file with the Chilean agencies Ministerio del Medio Ambiente (Ministry of the Environment) and/or DGA. Those reports systematically record the static water level and, in some cases, useful aquifer parameters (transmissivity and storativity). The maps of piezometric surfaces (Figs. 2 and 3A) were generated using both a compila-tion of piezometric contours from previous reports (EIA 2005, EIA 2011, Matraz 2012, Mayco 2013, and Minera Leonor 2007, see Table 1) and data from 118 wells (Tables 1 and 7). Time series of water levels are available for more than 50% of these wells (Table 8; Figs. 2 and 3A). Wells reported to be production wells were not used unless a time series was available from which to iden-tify the impacts of pumping, and therefore to select data prior to that impact. Likewise, monthly time series enabled recognition of the impacts of pumping at nearby wells (e.g., Fig. 8), and exclusion of those data. Ideally the maps would represent a single month in a single year, for a time prior to human intervention in the hydrological system. In reality, the data on which the maps are based represent either the oldest reported water levels for each well or data for 2003–2005, which were the earliest years of widespread well monitoring ( Table 8). Overall, the oldest measurements used were recorded in 1993 and for a few sectors the earliest monitoring wells reported are as recent as 2011. For the phreatic aquifer, the surface of the water in the Loa and San Salva-dor Rivers was included in the data set. Although the data sources routinely indi cate whether each well measures an upper or lower aquifer, the depths of screened intervals are reported for only 39 of the wells, and the year of construction is known for fewer than half of the wells. Well integrity problems may affect the segregation of water in these wells, allowing upper aquifer water to affect the recordings of lower aquifer head. Nevertheless, the clear distinctions between upper and lower aquifer heights of most near-neighbor wells (Table 9) indicates that many of these wells successfully restrict water entry to desired intervals in a single aquifer. In the primary data sets (Table 1) there are a few examples of wells whose data suggest the mixing of the two aquifers, and we avoided use of those wells. Further evidence that these moni-
tor ing wells successfully isolate the waters of the two aquifers is provided by hydro chemistry studies. For example, Matraz 2012 (see Table 1) examined water chemistry for 134 wells that overlap with the set listed in Table 7 (19 wells in common for upper aquifer; 24 well in common in lower aquifer) and interpreted from the sulfate concentrations that the upper aquifer and lower
Figure 5. The major sedimentary units found in three zones of the Calama sedimentary basin. The west of Calama city column applies to the area near Ojos de Opache (O in Fig. 4), based on May (1997) and May et al. (1999, 2005), subsurface data, and new mapping. The east of Calama Hill column applies to the southern margin of the Calama Basin (east of H in Fig. 4), based on Blanco (2008) and Tomlinson et al. (2010). The Eastern Flank basin column applies to the eastern central sector of the basin (general region of the Llalqui lowlands, L in Fig. 4), based on May (1997), May et al. (2005), Jordan et al. (2006), Blanco (2008), and Blanco and Tomlinson (2009). Quat.—Quaternary; Fm.—formation.
1448Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
aquifer waters are distinctive. For those hydrochemistry monitoring wells, the well owners are required to maintain official certification of the well integrity.
Subsurface data for rock properties come from a small fraction of the nu-merous boreholes that have been drilled in the study area to explore for miner-als within the rocks underlying the sediments of the Calama Basin. In addition to a very small number of published analyses of exploration boreholes (May, 1997; Blanco, 2008), this study used reports of lithologies from 44 exploration boreholes (Table 7). For 15 of those well reports, the driller or mudlog records include mention of depths at which water or wet rock was encountered. This study utilizes geological information regarding aquifer and aquitard lithologies from 131 groundwater wells and logged mineral exploration borehole records that appear in reports prepared for the DGA (e.g., Matraz 2012, see Table 1) or to comply with environmental impact and mitigation regulations (e.g., EIA 2005, see Table 1). These were put in the public domain through the website of Chile’s Environmental Evaluation Service (Ministerio del Medio Ambiente, http:// sea .gob .cl). In those reports the lithological data appear either in detailed borehole-specific illustrations (e.g., EIA 2011, see Table 1) or embedded within geological cross sections (e.g., EIA 2005, see Table 1).
Geophysical profiles collected for minerals exploration, groundwater stud-ies, and petroleum exploration exist in the study area, although only a small fraction of the results is published (e.g., interpretations of seismic reflection profiles collected for petroleum exploration: Jordan et al., 2006; Blanco, 2008; gravity survey: Matraz 2012 and Mayco 2013, see Table 1). Additional exam-ples consulted for this study that were embedded as supporting documents within environmental impact analyses include NanoTEM™ (http:// zonge .com .au /capability /method /nano -tem) profiles (GAC 2012, see Table 1) and TEM (Transient Electromagnetic) profiles (EIA 2011 and Mayco 2013, see Table 1).
The lithological information from the boreholes and geophysical profiles was combined with data from geological maps (Marinovic and Lahsen, 1984; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) and stratigraphic studies
A
B
C
D
Figure 6. Paleogeographic maps of the principal Cenozoic stages of fill of the Calama Basin, stacked as they occur in the subsurface. Colored and patterned zones show regions in which strata of each time slice occur. The patterns and colors emphasize areas with properties suitable to function as aquifers (shades of yellow) or dominated by facies that are prone to low hydraulic conductivity (shades of orange). Each layer portrays a time slice rather than a geological depth slice. Note that the location of potential aquifer host rocks shifts laterally in successively deeper time slices. (A) Late Miocene–Pliocene basin. Both the limestone-dominated facies (pale yellow, brick pattern) and marginal conglomeratic facies (yellow, gravel pattern) are potential aquifers. R.—river; Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T—Talabre area. (B) Early and middle Miocene basin. Where the pattern is orange mudstone, the Jalquinche Formation dominates and there is little or no potential for facies suited to serve as aquifers. Where the pattern is yellow gravel, conglomerate and well-sorted sandstone of the Lasana Formation and alluvial gravels along the basin margins occur and are plausible aquifers. The Lasana Formation contains thick intervals of mudstone locally that would serve as aqui-tards. (C) Oligocene–earliest Miocene basin. The Yalqui Formation is shown by conglomerate symbols, but a mud-rich matrix displayed in some of its sparse outcrops indicates that part of this unit is unlikely to serve as an aquifer, and so it is represented by a pale orange color. (D) Eocene basin. Conglomerates of the Calama Formation are represented by yellow gravel symbols. Conglomerates west of the West fault are interbedded with volcanic deposits and mudstones of the Chintoraste complex (in orange).
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(May, 1997; May et al., 1999, 2005; Jordan et al., 2006; Blanco, 2008; Blanco and Tomlinson, 2009) to map the distributions of sedimentary units that are inde-pendently reported to hold the aquifers and form the aquitards. Our geological field work and satellite image analysis using Google Earth led to creation of a new geological map for the study area west of Calama city (Fig. 9). Combin-ing surface information, geophysical data, and borehole data, geological cross sections (Figs. 10–14) were created to illuminate major attributes of the three- dimensional distribution of the mapped units of major importance to aquifer and aquitard architecture.
A subset of both the mineral exploration boreholes and the groundwater well reports document the depths to water-bearing rocks or indicate the depth and lithology at which groundwater wells are screened for water entry (e.g., EIA 2005, see Table 1). Those reports are of special value in this analysis and were used to determine the direct connections between a rock unit and an aquifer. The few TEM profiles provided data for the depth to aquifers at lo-cations between wells. The positions of the piezometric surfaces (from Figs. 2 and 3A) and of the corresponding borehole-specific aquifers were super-imposed on the geological cross sections (Figs. 10, 11, 13, and 14).
TABLE 6. FAULT SETS WITHIN THE LOA SYSTEM
Major fault and/or fold
General location and orientation
Geological time of activity and kinematics in the region Geohydrological impact References
Mesozoic and Eocene faults west and south of basin
North-trending, at western margin (Cerros de Guacate and Sierra de San Lorenzo) and southern margins (Sierra Limón Verde) of Calama basin
Cretaceous–Paleogene faults and folds created highlands against which Calama Basin Paleogene and Neogene strata abut
In the plain between the San Salvador and Loa Rivers, the positions of paleoridgelines can be readily identified on satellite images, through the thin Opache Formation cover
Created limits to the Calama Basin and to the most suitable aquifers
Mpodozis et al. (1993); Tomlinson et al. (2010)
West fault system
North-trending, on west side of Calama Hill (divides Calama Valley from Loa–San Salvador Valley)
Eocene and early Oligocene: kilometer-scale right-lateral slip
Mid-Oligocene–early Miocene: ~37 km left-lateral displacement; locally, to 600 m of down-to-the-west displacement
Middle Miocene–Pliocene strata disturbed (hundreds of meters) by right-lateral displacement
Net left-lateral displacement 35 ± 1 km
Sedimentary units hosting lower aquifer are discontinuous across fault
Zone near fault highly fractured; fault cutting bedrock is a flow barrier
Impacts on permeability in sedimentary rocks bounding the fault are not documented
Tomlinson and Blanco (1997); Tomlinson et al. (2010); Araya Torres (2010)
Milagro fault East-trending reverse fault located between Talabre and Calama Hill
North side displaced up by ~1000 m; Eocene Incaic deformation
Creates major discontinuity in rocks that host the lower aquifer
Blanco et al., (2003); Blanco (2008); Blanco and Tomlinson (2009); Tomlinson et al. (2010); Jordan et al. (2006)
Loa fault Northeast-trending; in north-central Calama Valley to north of Talabre
Oligocene(?) offset; ~2000 m down-to-east normal slip, possible accompanying strike slip
Late Miocene small-magnitude folds over buried trace of fault
Western boundary of Yalqui Formation depocenter; little impact on middle Miocene and overlying strata so probably little impact on groundwater
Jordan et al. (2006); Blanco (2008); Blanco and Tomlinson (2009)
Cere fault East-northeast-trending; NW boundary Calama Valley
Miocene–Pliocene normal reactivation of Paleozoic fault
None identified; plausible impact on infiltration in bedrock below Loa River
Tomlinson et al. (2012)
San Salvador–Loa Valley fault set
Set of east-trending, near vertical, small offset faults
North of Chintoraste hills, adjacent canyon of Loa River and possibly adjacent Canyon of Ojos de Opache
Eocene–OligoceneFew data, but one strand displays horizontal
striations indicative of strike slip and tens of meters of displacement
Create physical discontinuities in aquifer host rocks and aquitards by juxtaposing across faults metasedimentary rocks, subvolcanic intrusives, lava flows, ignimbrites, and volcaniclastic conglomerates; brecciation may reduce hydraulic conductivity
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RESULTS
Aquifers and Hydraulic Parameters
In the Llalqui area (Figs. 4 and 13) in the eastern part of the Calama Val-ley, the Opache and Chiquinaputo Formations contain the upper aquifer (Fig. 15A–15C) (Houston, 2004, 2006, 2007). A few wells document a lower aquifer, which is locally artesian. The aquifer corresponds to rocks at >200 m depth that are capped by the Jalquinche Formation mudstone and/or an ignimbrite, with lateral variability in the thicknesses of those low-hydraulic-conductivity units. Houston (2004) presented evidence that the Sifón Ignimbrite is an effective confining layer between the two aquifers.
In the central sector of the Calama Valley, most wells encounter an upper phreatic aquifer. In south-central areas the phreatic aquifer occurs in a lime-stone-dominated rock (EIA 2005 and EIA 2011, see Table 1), the Opache For-mation. In the north-central area, the upper aquifer is variably in limestone mapped as the Opache Formation or in sandstone of the upper part of the Lasana Formation. A lower set of aquifers also occurs widely; at some lo-cations the wells are artesian (e.g., northeast of location T in Fig. 3B). In the north-central region the lower aquifer occurs in conglomerate below 80–130 m depth (Figs. 10, 13, and 14); current geological mapping places this conglomer-ate in the Lasana Formation. A mudstone is considered to be the confining unit and attributed commonly to the Jalquinche Formation. However, Blanco and Tomlinson (2009) reported that the Jalquinche mudstone facies is only a few
Figure 7. Zones of faults and associated folds in the study region. Table 6 describes the physical properties, sense of offset, and age of offset of each fault system. The geology of the rock units underlying the river bed alluvium is shown for compari-son to Figures 2 and 4. R.—river; S.—sierra; Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T—Talabre area.
Note: Italics indicate wells for which the elevation is approximate. For these, the original report provided location data, but not elevation data. The elevation was estimated from Google Earth elevations.
*Locations given in Universal Transverse Mercator (UTM Zone 19S). Most locations are reported by primary sources relative to datum PSAD56, but in some sources the datum is not stated and might be WGS84. The location uncertainty caused by inconsistent use of these datum systems is 400–450 m.
†T. Jordan 2012 (own interpretation), extracted information from original CODELCO exploration well logs and driller reports. §Dirección General de Aguas (DGA) monitoring reports available monthly, http://snia.dga.cl/BNAConsultas/. **http://www.codelco.com/prontus_codelco/site/artic/20120926/asocfile/20120926174425/monitoreo_ambiental_0912.pdf.††See text for reference.
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meters thick in the north-central area, and therefore is probably not an effec-tive confining layer. Instead, mudstone intervals within the Lasana Formation (Blanco and Tomlinson, 2009) are more likely candidates for the low-transmis-sivity layers in that area.
South of Talabre (T in Fig. 2) but north of the Loa River, there is a region where there is no phreatic aquifer even though there appears to be lateral con-tinuity of the limestone-rich unit. In that area, several wells prove a confined lower aquifer in a conglomerate unit (EIA 2005, see Table 1).
Both upper and lower aquifers are found near and west of Calama Hill (sec-tor between H and O in Figs. 2 and 3) (Mayco 2013, see Table 1). An upper phreatic aquifer exists in unconsolidated alluvium, karstic limestone, calcar-eous sandstone, sandstone, and conglomerate. The calcareous upper part is referred to as the Opache Formation (Fig. 15D) and the lower detrital strata has informal unit names (e.g., black sandstone). A lower aquifer occurs in conglomerate referred to in most reports as the Calama Formation (Fig. 16C), although evidence presented in the following indicates that the aquifer is in multiple sedimentary and volcanic units. Above the lower aquifer is a thick clay-rich siltstone, commonly attributed to the Jalquinche Formation.
West of the central Calama Valley and north of the San Salvador River and Calama city, where the surface elevations rise toward the northwestern moun-tain range, the confining layer is an ignimbrite (EIA 2011, see Table 1). However, that ignimbrite pinches out northward so that the two aquifers become one phreatic aquifer (EIA 2011, see Table 1).
The sparse data for the hydraulic properties of the rocks are summarized in Table 10. Typical permeability for the upper aquifer in the central Calama Valley (L and T in Fig. 4) is 0.7–1.2 m/day. In the San Salvador–Loa Valley area west of Calama Hill (sector between H and O, Fig. 4), productive well fields in the upper aquifer report higher permeability (3.9 m/day average) (Fuentes Carrasco, 2009). Houston (2004) reported that some upper aquifer horizons in the eastern part of the study area have much higher permeability (120 m/day). Heterogeneity was also emphasized by Fuentes Carrasco (2009), who consid-ered the upper aquifer west of Calama Hill to contain elongate channels of exceptionally high permeability.
Figure 8. (A) Variability during 2003–2005 in discharge of water in the Calama Valley. Loa River discharge monitored by summing reported releases by two paths from the Conchi Reservoir. Letter abbreviations at top are months of the year. (B–F) Variability during 2003–2005 in head of water in the Calama Valley. Monitored heads of lower and upper aquifer wells, organized by general location in the Calama Valley and San Salvador–Loa Valley. Black lines indicate water levels in the Loa River and in the upper aquifer. Blue lines indicate water levels in the lower aquifer. Well locations are in Table 7, and data are described in Tables 8 and 9. Locations of well pairs selected for direct comparisons of upper and lower aquifer variability are shown in Figure 3B. B is a shallow well in eastern Calama Valley near Salado River that is monitored by the Direccíon General de Aquas. It is reported as depth in meters below the surface (m.b.s.) because contradictory elevation data for the well are published. C and D are in the northeastern part of the Calama Valley (m.a.s.l.—meters above sea level). E–G are monitoring wells in the central part of the basin. H is located near the southern margin of the basin. I and J are located west of Calama city, in the San Salvador–Loa Valley.
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The permeability of the conglomeratic lower aquifer in the eastern region ranges from 1 to 4 m/day (Table 10), with localized horizons of much higher fissure permeability (40–100 m/day) (Houston, 2004). In the central Calama Val-ley as well as in the area west of Calama Hill, well tests for the lower aquifer display generally higher permeability, 2–21 m/day (Table 10).
Data for the basement rocks upon which the Eocene–Quaternary sedimen-tary rocks accumulated and for units reported to serve as confining layers con-firm that they are significantly less permeable. The few well tests conducted for the mudstone or ignimbrite that overlies the lower aquifer reveal perme-ability of ~10–3 m/day (Table 10). Araya Torres (2010) documented that most permeability values for basement rocks in the adjacent mountain range on the west side of the basin, where hundreds of measurements exist because of mining-related geotechnical studies, are 10–3 to 10–6 m/day.
Groundwater Flow
Across the eastern, central and northern sectors of the Calama Valley, the piezometric surface of a phreatic aquifer (Fig. 2) declines from 2660 m in the northeast to ~2300 m at the West fault, driving flow toward the southwest.
West of Calama city, the flow direction is west-southwest (Fig. 2) where the water table declines 90 m to the springs of Ojos de Opache.
The aquifers discharge to surface water bodies in several areas. In the cen-tral Calama Valley, short-term stream gauging campaigns document signifi-cant transfer of water (e.g., several hundred liters/second at multiple locations) from the aquifers into the south-trending reach of the Loa River (between blue diamonds 2 and 3 in Fig. 2) (EIA 2005 and Matraz 2012, see Table 1). West of Calama city (Fig. 2), examples of discharge into the Loa River occur near the location of the La Cascada waterfalls (LC in Fig. 2) and through diffuse zones of springs that produce extensive wetlands in two tributary canyons on the Loa’s north bank (blue diamond 5 in Fig. 2). Although no gauging stations document the flow of the Loa above and below those springs, at times of low flow there is a visually pronounced downstream increase in the flow of the Loa River. Similarly, the San Salvador River is entirely spring fed, primarily from a set of springs at Ojos de Opache (O in Fig. 2; CORFO 1973, see Table 1). Klohn (1972) reported that the water chemistry of surface streams, springs, and wetlands in the area between Calama city and Ojos de Opache reveals a connection between the upper aquifer and the surface waters.
For a set of wells with roughly monthly measurements during the inter-val of time corresponding to most of the hydrological data, 2004–2005, the
TABLE 9. VARIABILITY THROUGH TIME OF RIVER DISCHARGE AND OF HEADS IN UPPER AND LOWER AQUIFERS, CALAMA VALLEY REGION (continued)
Location description
Location (numbered star,
Fig. 3B)Wells
compared Years data
Months overlap upper and lower
aquifer data
Height on map at star center
(m)
Difference in piezometric
height from map(m)
Maximum variability in nearby wells
(m)Maximum
heightMinimum
height
Would relative vertical positions
change seasonally?
8
lower aquifer LE-1** September 1994–January 2008
September 2004–January 2005
2175 30 2.1 2176.050 2173.950
not with sampled variabilitylower aquifer LE-2†† July 1995–
January 2008 4.6 2177.300 2172.700
upper aquifer SI-8C 8 July 2004–6 January 2005 2205 0.14 2205.070 2204.930
upper aquifer SI-23C 7 September 2004–14 January 2005 0.689 2205.345 2204.656
9
lower aquifer OBS-7C§§ July 2007–June 2011 July 2007–
June 2011
2110 44 2.3 2111.150 2108.850not with sampled variability
upper aquifer OBS-7L*** July 2007–June 2011 2154 0.35 2154.175 2153.825
*Aquifer tested 18 December 2004.†Range reported here for only 2003–January 2004 because of aquifer tests on nearby wells later in 2004.§Aquifer test 14 September 1993.**Variation reported 1994–2005; range during 2003–2005 is ~1.5 m.††Variation reported 1994–2006; range during 2003–2005 is ~1 m.§§Water level for 2008; variation 2007–2008.***Water level for 2008; variation 2007–2008.
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time-variable groundwater levels (Fig. 8) reveal a mixture of natural variabil-ity and human management. An annual cycle of increased river flow during austral summer (December–March) is perceptible in the Loa River flow (Fig. 8A) after it exits the Conchi Reservoir to pass into the Calama Valley, although the seasonal variations are managed and spikes in discharge are likely related to managed flow through dam drains. The Chiu Chiu well (Fig. 8B) offers a history of an upper aquifer well that is tightly coupled to the other major
river that is a conveyance from the high-elevation parts of the catchment, as it is located <500 m from the Salado River near its juncture with the Loa. This well displayed a small-magnitude increase in the phreatic water table late in 2003, and a subsequent slow decline. That September increase predated by three months the anticipated annual precipitation cycle in the catchment high-lands (Chaffaut, 1998). For wells located farther downflow and at considerable distance from the rivers (Fig. 3B), neither the upper nor lower aquifer heads
? ?
??
Uncertain where confining layer terminates
0 m0 m
2000
2000 2100
to B
San Salvador
Loa
0 1 2 3 4 km
Ojo de Opache
E
Ch
LC
ON
F2
F1
F3
F4?
F5?
N
Opache Fm. and underlying sand and gravel
Chintoraste pyroclastic andvolcaniclastic complex
undifferentiated basement
Jalquinche Fm. Intrusive igneous rockPleistocene(?) terrace or modern channel sediments
conglomerate overlyingChintoraste complex
Mesozoic stratatufa carbonate platform
altitude (m) above sea level2000
inferred basement ridge
100 m
C′C
Figure 9. Geological map of the Calama Basin west of Calama city, in the eastern sector of the San Salvador–Loa Valley (lo-cation is shown by box in Fig. 4). Mapped relations are based on field work and our satellite image analysis. Dashed-line faults (thick black lines) are inferred based on satellite image interpretation and sparse data. Solid-line faults were mapped in the field. Gray lines identify zones in which the structural grain of basement rocks can be discerned through the small-scale relief even though the basement unit is draped by the surface unit, the Opache Formation (Fm.). Shades of green iden-tify units associated with a lower aquifer. Blue patches are areas covered by tufa carbonate deposits that are many meters thick. The brown line labeled 0 m (solid where mapped; dashed where inferred) traces the position where the Jalquinche Formation pinches out below the base of a thin sandstone unit that underlies the Opache Formation (Fig. 5). The green line labeled 0 m (solid where mapped; dashed where inferred) traces where the Chintoraste unit pinches out. Between the brown and green lines, there is no re-gionally extensive aquitard, and therefore water-bearing horizons in the Chintoraste unit will have direct contact with the base of the rocks that regionally host the upper aquifer. Ch—Chintoraste hills; LC—La Cas-cada waterfalls; ON—Ojos de Opache at Nacimiento spring.
1460Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
A
B
A′
D′
B′
Figure 10. (A) Geological cross section A-A’ through the main part of Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. Mudstone of the Jalquinche Formation and conglomerate and coarse sandstone of the Lasana Formation interfinger extensively. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.
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display a seasonal variability that is tightly related to the upper Loa and Salado inflow to the Calama Valley, even with a phase shift. For those wells, rapid fluc-tuations in water levels are more likely the result of pumping of nearby wells (e.g., Figs. 8C, 8I) than natural causes. For the wells that are distant from the rivers, the range of variability of the head during 2003–2005 is 1–13 m, inclu-sive of large values interpreted to reflect human management (Table 9). With the available data, identification of natural variability is subjective, but during
2003–2005 the natural range is interpreted to be <2 m for both the upper and lower aquifers (Table 9).
The elevations of the aquifer surface in the north-central Calama Valley and of the Loa River bed constrain the plausible geology of the aquifer in the area north of the well data. The elevation of the phreatic aquifer in the two north-ernmost control wells (Fig. 2) is ~2610 m asl in the valley center and ~2660 m asl near the western margin of the valley. Following the piezometric gradient
AB′
B
A′E′
Figure 11. (A) Geological cross section B-B’ across the eastern part of the San Salvador–Loa Valley and southwestern part of the Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exagger-ation. The rocks associated with the two aquifers are also iden-tified where construction (screened intervals) and associated geology are reported for wells near the line of section.
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to the northeast, the landscape surface elevation rises to 2720–2800 m asl near the Loa River, before dropping 40–60 m to the river bed. Although the Opache Formation is exposed widely in the northern part of the Calama Valley, it was removed by erosion at the Loa canyon. Instead, the bedrock exposed at the depth of the river bed is within the Lasana Formation (Figs. 2 and 4). If a phre-atic aquifer is recharged by the Loa River south of the Conchi Dam (Fig. 2), the most likely aquifer host is the Lasana Formation. Groundwater must then flow southwestward into what become both the lower and the upper aquifers.
Through the central and northern sectors of the Calama Valley, the head of the lower aquifer declines ~400 m and the form of its piezometric surface is similar to that of the upper aquifer. The flow direction is from northeast to southwest (Fig. 3A). For the lower aquifer in the western part of the Calama Val-ley and San Salvador–Loa Valley, the pattern of the piezometric surface is more irregular. Immediately west of Calama Hill, the piezometric gradient is low (~10 m/km) over a 5-km-wide zone before transitioning westward to a steep slope (~20 m/km) in the region of the Ojos de Opache springs (Fig. 3A). Although sparse data south of Calama Hill (H in Fig. 3) suggest that lower aquifer water may flow toward the Loa canyon south of Calama city, there are no control wells near the Loa River southwest of Calama city or river gauges west of station 5 (Fig. 3) with which to verify possible lower aquifer conditions. Station 5 (Fig. 3A) marks the western point at which the Opache Formation, the typical upper aquifer host rock throughout the western part of the basin, crops out at the base of the Loa canyon. West of station 5, extensive volumes of carbonate
deposits occur along some sectors of the river bed north of Chintoraste hills (tr in Fig. 3A), and spring-like tufa carbonate deposits occur along the traces of some east-trending faults on the walls of the Loa canyon. These deposits suggest spring drainage that is not tied to the upper aquifer.
The relative heights of the piezometric surfaces of the upper and lower aquifers vary across broad regions (Fig. 3B). In most of the northern Calama Valley west of the Loa River and in a zone along the west-central margin of the valley, the lower aquifer head is above that of the upper aquifer, produc-ing localized artesian or near-artesian conditions (Fig. 3B). For well pairs 1 and 6 (Fig. 3B; Table 9), the variability of head over the reported months is sufficiently large to plausibly reverse these relative piezometric heights in some months. Nevertheless, persistently flowing artesian wells near well pair 1 demonstrate the robustness of the relative pressures within some parts of the northern region. In other broad areas, especially one in the south-central region and another near and west of Calama city, the head in the upper aqui-fer is higher than that in the lower aquifer (Fig. 3B). There are two smaller regions in which the lower aquifer head is similar to that of the upper aquifer (Fig. 3B). In most of the subareas these relative heights are robust over the 2003–2005 data years (Table 9).
Geology
Geohydrological Consequences of Faults and Folds
The long-lived, north-trending, oblique-slip West fault system occurs near the boundary between the Calama Valley and the San Salvador–Loa Val-ley ( Reutter et al., 1996). Due to lateral offset during the mid-Oligocene–early Miocene, the Eocene and lower Oligocene strata in the southwestern Calama Valley would not have formed in continuity with similar-aged deposits in the San Salvador–Loa Valley (Figs. 7, 10, 11, and 14). Within the Calama Basin, ver-tical offset across the West fault apparently displaces the contact of the crystal-line basement with strata by <200 m (Figs. 11 and 14).
Where the West fault zone cuts crystalline basement rock in the >800-m-deep Chuquicamata open pit mine, the zone of fault gouge and breccia is at least 3 m wide (Tomlinson and Blanco, 1997). Araya Torres (2010) documented that the fault within the mine region is a barrier to groundwater flow; however, that data set examined the hydraulic conductivity of fracture zones related to basement rock and did not evaluate hydraulic properties where the faults cut the moderately lithified sedimentary units that are the aquifers.
The east-trending Milagro fault system (between locations H and T in Fig. 7) formed contemporaneously with accumulation of the Eocene Calama Forma-tion, and is buried by post-Eocene strata along most of its trace (Blanco, 2008; Tomlinson et al., 2010). Available outcrop and subsurface data imply that it is a north-dipping reverse fault in its central sector (Fig. 10) and that it declines in offset in the western and eastern sectors, where folds dominate the structure (Fig. 14). In general, lower aquifer rocks north of the Milagro deformation zone
C′
Figure 12. Geological cross section C-C’ in the San Salvador–Loa Valley south of Loa River. There are no known boreholes in close proximity to this cross section.
1463Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
0
kilometers
vertical exaggeration 5:1
2000
1000
3000
Loa Fault
0 10 20 30 km
vertical exaggeration
15:1
2000 m
cross-section A-A′
east end seismic line
NW SE
Chiu Chiu monocline
D D′cross-section E-E′A
B
upper aquifer
lower aquifer
piezo- metric
surface
water- bearing rocks
Lasana Formation conglomerate or sandstone
Ignimbrite
Yalqui Formation (conglomerate)
El Yeso Formation (gypsum)
Yalqui Formation (muddy conglomerate)
Upper Miocene-Pliocene Chiquinaputo Formation
Opache Formation
Jalquinche Formation (mudstone)
alluvial or wetland deposits, unconsolidated
basement rocks (compactedstrata; volcanic and intrusive)
folded strata (schematic)
fault (arrow indicates sense of displacement)
Figure 13. (A) Geological cross section D-D’ through the central Calama Valley. The line of profile matches seismic line 99–06 interpreted by Jordan et al. (2006) and the cross section of Blanco and Tomlinson (2009). Thick black line is a fault. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.
Figure 14. (A) Geological cross section E-E’ through the western sector of the Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpre-tations. Dashed unit contacts and faults indicate a high degree of uncertainty about the position. Mudstone of the Jalquinche Formation and conglomerate and coarse sandstone of the Lasana Formation interfinger extensively. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.
1465Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
occur in the Lasana Formation, whereas lower aquifer rocks near and south of this fault occur in the Calama Formation (Figs. 10 and 14).
The fault within the basin with the largest vertical displacement, the north-east-trending Loa fault (Fig. 7; Table 6), displaces the pre–middle Miocene units and is a major discontinuity in the deeply buried Yalqui Formation (Figs. 10 and 13). Well data in the northern part of the Calama Valley are insufficient to test whether the fault impacts the aquifers (Figs. 10 and 13).
In the western sector, in the San Salvador–Loa Valley, near-vertical, small-displacement (tens to hundreds of meters), east-trending faults that are exposed in the canyon of the Loa River (Figs. 7 and 9) have an important local impact on the groundwater system. Because these faults juxtapose rocks of markedly different physical properties, such as coarse Eocene conglomerate against Jurassic metasediments, hydraulic conductivity changes abruptly. Pre-liminary data suggest that groundwater is forced to the surface along one of
these faults, to form extensive calcium carbonate mineral deposits along the Loa River bed (tr in Fig. 3) and at paleosprings located on the canyon walls. There are no stream gauge data for the Loa River at suitable locations to test this discharge hypothesis. With many fewer constraints, it is inferred that parallel faults of similar magnitude occur 3–4 km to the north. These inferred faults would control the east-trending walls of the Quebrada de Opache can-yon, and might control vertical displacement by tens of meters of some of the lithologic units described in the water monitoring wells (Fig. 14, southern extreme of cross section E-E’). To date, the potential affects by these faults on the hydrology of the Ojos de Opache springs area have not been considered, and no wells monitor the region down-gradient (southwest) of this set of in-ferred faults.
The final pair of structures known to cause major displacement of the strata that serve as aquifers are the Chiu Chiu monocline and accompanying
B
CD
A
Figure 15. Photographs of rocks that host the upper aquifer illus trate the lithological diversity as well as some of the pri-mary porosity and fracture porosity. (A) Opache Formation lime-stone in a quarry wall in the southeastern part of the Calama Valley (near 22.45745°S, 68.731°W). The limestone has little visi-ble porosity except in the upper 1 m, but fractures that are both parallel to bedding and approximately perpendicular to bedding (arrows) display centimeter-scale open space. (B) Chiquinaputo Formation conglomerate bed (upper half) and cross-bedded sandstone bed (lower half) in the northeastern part of the Calama Valley, south of the Salado River (near 22.3°S, 68.5°W). (C) Opache Formation limestone near northern limit of study area (near 22.031°S, 68.620°W) where the Opache is only ~3 m thick. Note the abundant centimeter-scale vugs. (D) Opache For-mation limestone in the southwestern part of the study area, in the San Salvador–Loa Valley (near 22.4924°S, 69.0081°W). Note that the upper left half of outcrop is well bedded, whereas the rock of the right half is entirely broken into meter-scale breccia. The brown coloration and white vertical streaks in the lower left area are indicative of alteration and mineralization during water seepage. An active spring exists at the same horizon 25 m to the left. Note person for scale in white oval.
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Salado syncline (Figs. 7 and 17). At gross scale, the monocline is a gentle fold with down-to-the-east sense of stratigraphic offset of 100–200 m (Blanco and Tomlinson, 2009). The synclinal axis is a broad, shallow, and complexly folded zone, within which the Loa and Salado Rivers flow. The Chiu Chiu monocline is younger than the Opache Formation and older than the Quaternary Chiu Chiu Formation. The serpentine form of the monocline and evident subsidence of the area encircled by the monocline (Fig. 17) led Blanco and Tomlinson (2009) to interpret it to be the result of the subsurface dissolution and removal of an evaporite-rich unit located at many hundred meters depth (Fig. 13A). The 2–3-km-wide syncline would thus represent the topographic low formed above the area of maximum subsurface material loss and subsidence. The vertical position of the strata that contain the upper aquifer rises >100 m from east to west across the monocline (Fig. 13). South of the Loa River the monocline diminishes progressively in relief. Fractures related to the original dissolution and subsidence along the axis of the syncline, as well as fractures related to strain within overlying units (Blanco and Tomlinson, 2009), may have en-hanced permeability through some rock units.
Distribution of Aquifer Host Rocks East and North of Calama Hill
Three cross sections (Figs. 10, 13, and 14) illustrate the geometry of the strata that contain the aquifers in the Calama Valley. Cross sections A-A’ (Fig. 10) and E-E’ (Fig. 14) are approximately parallel to the groundwater flow di-rection; cross section D-D’ (Fig. 13) is essentially perpendicular to ground-water flow.
The regionally extensive limestone of the upper Miocene and Pliocene Opache Formation, the host for the upper aquifer in many areas, displays a va-riety of facies (Figs. 15A, 15C), some with centimeter-scale vugs (Fig. 15C) and microkarst (May et al., 1999; Houston, 2004). Both bedding-parallel and nearly vertical fractures are common (Fig. 15A) and likely contribute to the hy draulic conductivity of the Opache. Laterally toward all the basin boundaries, the Opache grades to conglomerates (Figs. 6A and 17). In the northeastern Calama Valley, the Chiquinaputo Formation, which is also a host to the phreatic aquifer (Houston, 2004), interfingers with the Opache limestone (Figs. 5 and 13) as well as locally underlying the limestone (Blanco, 2008). The Chiquinaputo consists of well-sorted fluvial gravels (Fig. 15B). Zones of siltstone within the Chiquina-
A
B
C
Figure 16. Photographs of rocks that are reported to host the lower aquifer. (A) The Yalqui For-mation of the eastern extreme of the Calama sedimentary basin (near 22.4°S, 68.3°W) consists of pebble conglomerate interbedded with coarse sandstone. There is a strong vertical hetero-geneity at the decimeter scale, yet individual layers are well sorted and locally cross-bedded (upper 30 cm). (B) The Yalqui Formation at a position ~10 km more centrally located within the Calama sedimentary basin compared to A (near 22.401°S, 68.364°W). The angular clasts, poor sorting, and fine-grained matrix together produce a texture that is unlikely to support high val-ues of hydraulic conductivity. (C) Calama Formation near the western margin of Calama Valley, exposed on Calama Hill (near 22.45°S, 68.88°W). The gravel is clast supported, with moderately well sorted, rounded to subrounded cobbles and pebbles.
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puto Formation, reported by Blanco (2008), likely have poor hydraulic conduc-tivity and diminish the continuity of flow within the upper aquifer. Elsewhere, the basin-margin conglomerates are alluvial fan gravels (May, 1997; Blanco, 2008) that are not well sorted and likely have low hydraulic conductivity.
The Eocene Calama conglomerate (Figs. 6D and 16C) is the host to the lower aquifer in most of the southern Calama Valley. Blanco et al. (2003) and Blanco (2008) documented that, in outcrop, the lower 100 m of the Calama Formation contains several interbeds of andesite lava overlain by ~450 m of alluvial and fluvial conglomerate cemented by either gypsum or clays. The Calama Formation pinches out northward near the Eocene-age Milagro fault
and fold system (Figs. 7, 10, and 14) (Blanco, 2008; Tomlinson et al., 2010). In the eastern sector of the Calama Valley, depths to a local lower aquifer coincide with the interpreted depth to the Yalqui Formation (Figs. 6C and 13). Although facies of some exposures of the Oligocene–lowest Miocene Yalqui Formation are suitable to serve as a lower aquifer (Fig. 16A), much of the Yalqui Forma-tion is a matrix-supported conglomerate (Fig. 16B) that is not likely to have adequate permeability. The kilometer-scale lateral variations from clast-sup-ported to matrix-supported conglomerate texture (Figs. 16A, 16B) seen in out-crop suggest that aquifers in the Yalqui Formation in the eastern sector of the basin must be laterally complex. There is inadequate information in the zone
TABLE 10. VALUES OF PERMEABILITY REPORTED IN PUBLISHED AND UNPUBLISHED REPORTS, ORGANIZED BY GEOGRAPHICAL SECTOR AND HYDROGEOLOGICAL UNIT
Sector in central Loa groundwater basin
Geological unit(Formation name used by source)
Local hydrogeological
role
Matrix permeability range
(m/day)
Matrix permeability average
(m/day)Source of data Comments
Eastern (Llalqui) (Fig. 4, broad area surrounding location L)
Opache and Chiquinaputo Upper aquifer 1–2 Houston
(2004)Aquifer test; matrix
permeability
Opache and Chiquinaputo Upper aquifer ≤120 Houston
(2004)Aquifer test; fissure
permeability
Lasana? and Yalqui (Calama) Lower aquifer 1–4 Houston
(2004)Aquifer test; matrix
permeability
Lasana? and Yalqui (Calama) Lower aquifer 40–100 Houston
(2004)Aquifer test; fissure
permeability
Sifón Confining 0.00003 Houston (2004) Outcrop data and theory
Central (Fig. 4, broad area surrounding location T)
Opache Upper aquifer 0.04–2.25 1.2 EIA 2005Average of 8 tests
in 3 wells
Lasana conglomerate
memberUpper aquifer 0.53–0.79 0.7 EIA 2005 Average of 3 tests
in 1 well
Jalquinche Aquitard or confining 0.000321–0.0062 0.0017 EIA 2005 Average of 11 tests
in 4 wells
mostly Lasana conglomerate;
(Calama)Lower aquifer 0.71–23.1 5.5 EIA 2005 Average of 19 tests
in 9 wells
Northwestern margin(Fig. 4, near West fault
northwest of location H)Calama Lower aquifer 2.0 0.02–9.96 EIA 2005 Average of 11 tests
in 6 wells
Calama city–West fault region (Fig. 4, between locations H and O)
Opache Upper aquifer 3.9Fuentes Carrasco
(2009)Pump tests
Calama Lower aquifer 20.6 0.24–106.22 EIA 2005 Average of 6 tests in 3 wells
Note: Matraz 2012 (see Table 1) summarized the span of permeability values from 205 wells for an upper aquifer (1 x 10–3–3 x 102 m/day), lower aquifer (1 x 10–5–1 x 102
m/day), aquitard (1 x 10–6 to <5 x 10–3 m/day), and basement (1 x 10–9–1 x 10–5m/day), ranges similar to those compiled here. The report does not permit examination of spatial variations of those values.
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between the Talabre, Llalqui, and Calama Hill areas (T, L, and H in Fig. 3A) to deduce how the gravel-dominated lower aquifer host rocks change across the Chiu Chiu monocline.
In the northern and eastern Calama Valley the middle Miocene Lasana For-mation (Fig. 5) is the most likely candidate to be the gravel reported to host both the lower and upper aquifers (Figs. 10, 13, and 14). The Lasana Formation is at least 100 m thick where its lower member crops out along the Loa canyon north of Chiu Chiu (Fig. 4) (Blanco, 2008). Blanco (2008) and Blanco and Tom-linson (2009) described fluvial conglomerate, sandstone, and siltstone in the lower member, in repeated complex fining-upward series several meters thick, that become progressively dominated by siltstone to the south and west. The upper member has a higher percentage of mudstone (Blanco, 2008). Aquifers composed of the Lasana Formation are likely internally complex and laterally limited by facies changes. Blanco (2008) described preferred orientations of the sedimentological features of the lower member that may cause a favored south-southwest alignment of conductive aquifer properties. The Lasana Formation is age equivalent to and interfingers laterally with the Jalquinche Formation clay-rich siltstone unit (Figs. 5, 13, and 14), which functions as an aquitard (Table 10).
Widespread ignimbrites in the eastern and northern extremes of the Calama Valley (Ramírez and Gardeweg, 1982; Marinovic and Lahsen, 1984; de Silva, 1989) may be aquitards. The distribution of the Sifón Ignimbrite is well established in the east-central and southern parts of the Loa catchment basin. In the northern and easternmost sector, published maps and reports do not clarify the locations of the boundaries between the Sifón Ignimbrite and other very thick ignimbrites (e.g., Cupo, Divísico, Rio Salado, and lower San Pedro Ignimbrites; e.g., de Silva, 1989) in similar stratigraphic positions. Herein, these five ignimbrites are treated as a single map unit (Figs. 4, 10, and 13). Although locally at least part of the 1–100-m-thick ignimbrites is welded tuff, more widely the ignimbrites are not welded. The effectiveness of these ig-nimbrites to impede water flow is unresolved: common vertical fractures may serve as flow paths (Montgomery et al., 2003), yet water-pressure data in the Llalqui area demonstrate that the Sifón Ignimbrite locally confines an artesian aquifer (Houston, 2004, 2007).
In the southern part of the Calama Valley a major aquitard or confining unit is created by the fine sandstone and mudstone of the middle Miocene Jalquinche Formation, which is laterally extensive (Figs. 5 and 9–14) and as much as 200 m thick (May, 1997; May et al., 2005; Blanco, 2008; Blanco and Tomlinson, 2009; Tomlinson et al., 2010). Red mudstone is an important com-ponent of the Jalquinche Formation (Blanco, 2008), and the reddish color is suggestive of a diagenetic Fe-rich clay. Fine-grained reddish sandstone is both common and rich in gypsum (May, 1997; Blanco, 2008). Its color and gyp-sum content together suggest that even in the sandstone facies the primary porosity may have been occluded. North of the Talabre area (T in Fig. 4) the Jalquinche Formation thins to only 10 m, and is replaced by coarse-grained facies of the Lasana Formation (Fig. 6B) (Blanco and Tomlinson, 2009).
Not only are the geological units that serve as aquitards laterally variable from mudstone to ignimbrite, in addition their effectiveness as aquitards is heterogeneous within a single geological unit. Both the Jalquinche Formation and the ignimbrites change markedly in thickness as well as pinch out entirely. In some parts of the central Calama Valley the coarse facies of the lower mem-ber of the Lasana Formation either underlies the Sifón Ignimbrite or underlies a thick mudstone interpreted as the Jalquinche Formation or as a facies vari-ation within the Lasana Formation (Figs. 10, 13, and 14); however, elsewhere available well data are not conclusive that any of these units is the aquitard (see especially Fig. 10, where boundaries are dashed lines). As a consequence of the heterogeneity of the overlying set of aquitards, down-gradient flow in deep aquifer horizons is likely to pass laterally from phreatic zones to confined zones (Fig. 3B).
Distribution of Units West of Calama Hill
Three cross sections, B-B’ (highly oblique to groundwater flow), C-C’, and the southern 15 km of E-E’ (the latter two subparallel to groundwater flow; Figs. 12, 13, and 14), illustrate major changes in the distribution of Calama
Figure 17. Map of the upper Miocene–Pliocene Opache For-mation (brick pattern), the Chiquinaputo Formation, and lat-erally equivalent alluvial conglomerates (both the Chiquina-puto and marginal conglomerates are represented by gravel pattern), showing the positions of interpreted subsurface dissolution pathways. The positions are constrained by seis-mic reflection data where the line is solid. Where dashed, the positions are constrained only by subtle variations in landforms and are speculative. The path of the syncline con-trols the position of the lower Salado River (R.) and a reach of the Loa River. Hypothetically, the southern dissolution path may also influence groundwater flow south of Calama Hill (H) in an area where there are no monitoring wells. Ch—Chintoraste hills; O—Ojos de Opache region; S.—San.
1469Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
Basin strata near the West fault and in the region where the Calama Valley meets the San Salvador–Loa Valley. Cross section C-C’, drawn south of the Loa River, is isolated by the deep Loa canyon from the regional groundwater flow. Nevertheless, the exposures in this southern zone permit detailed under-standing of the architecture of the sedimentary and volcanic units (Figs. 2 and 9), unlike the area between the Loa and San Salvador Rivers where only the Opache Formation is exposed. It is assumed that cross section C-C’ (Fig. 12) approximates the geometries and depths of formations that are appropriate for the aquifer-hosting rocks between the two rivers.
Rocks related to a local Eocene volcanic center (Mpodozis et al., 1993; Trumbull et al., 2006) at the Chintoraste hills are an important part of the geo-hydrological setting west of the West fault (Figs. 4 and 9). Pyroclastic deposits are abundant, as well as epiclastic conglomerates, minor lava flows, and sub-volcanic intrusives. The Chintoraste hills are a circular feature ~4 km in diam-eter with an outer ring of outward-tilting (~40°–50°) ignimbrites and a center that includes contact-metamorphosed Mesozoic strata. Reports from mineral exploration boreholes in the 10-km-wide valley between the West fault and Chintoraste hills indicate widespread pyroclastic deposits, primarily tuffs, and interbedded coarse fluvial siliciclastics. The lithologic distribution sug-gests that pyroclastic deposits near the Chintoraste center interfinger north-ward and eastward with conglomerates. In some of the mineral boreholes, water was reported within these deposits.
Capping the localized Chintoraste volcanic center is an Oligocene(?) or Miocene conglomerate that constitutes a more widespread sheet, ~10 m thick (Fig. 6). From a depositional history perspective, the Eocene Chintoraste strata and the overlying conglomerate sheet are very different. However, few subsur-face data exist that enable differentiation of this capping conglomerate from Chintoraste lithologies.
Borehole reports and paleospring locations suggest that the lower aquifer west of the West fault is dominated by pyroclastic deposits and interbedded conglomerates. This study attributes most of these water-bearing horizons to the Chintoraste complex (Figs. 11 and 14) and the overlying thin conglomerate sheet. A consequence of this interpretation of the lower aquifer host rocks is that, at the West fault, down-gradient flow in the lower aquifer (Fig. 3) transfers from an eastern host rock dominated by conglomerate (Calama Formation) to a western host rock dominated by pyroclastic facies and volcaniclastic facies (Chintoraste unit).
In the area west of Calama Hill (H in Fig. 4), the Opache Formation lime-stone and immediately underlying sands and gravels serve as the regionally extensive phreatic aquifer (Figs. 11, 14 and 15D; Table 10). The sub-Opache medium-grained sandstone to well-sorted cobble conglomerate ranges in thickness between 0 and 4 m (May et al., 1999) to 20–30 m (our mapping), too thin to distinguish in Figure 9 as a separate map unit. The outcrop belt of the sandstone and conglomerate at the base of the Opache Formation is associ-ated with important springs.
The uppermost major unit with poor capacity to transmit water is a part of the Jalquinche Formation. It is composed mostly of gypsum-rich and clay-rich
mudstones and very fine to fine sandstones. Heterogeneities in the Jalquinche Formation encompass local horizons of coarser sandstone and conglomerate. Overall, the thickness of the Jalquinche Formation varies markedly, from 0 to 200 m (May, 1997), while it thins westward, toward a basement paleoridgeline near which it pinches out (brown line, Fig. 9).
Rocks with properties suitable to act as aquifers are heterogeneous in this western area. A persistence throughout the Neogene of topography that fun-neled rivers through the narrow San Salvador–Loa Valley (Figs. 6A–6C) would likely have created preferred elongations of sedimentary facies in an east-west direction. Near what is today the canyon of the Loa River exists evidence of paleo–Loa River positions, expressed both in cross sections of channel forms and in landforms. One stratigraphically deep example is a paleochannel with an apparent width of 250 m that cuts the Chintoraste unit and has well-bedded siliciclastic fill that underlies the Jalquinche Formation. Shallow examples may either control zones of karst in limestone of the Opache limestone or control the distribution of Quaternary gravels.
To the west of Calama Hill, bedrock of the Precordillera constricts the sedi-mentary units that form the aquifers. This is especially true of the upper aqui-fer, where the horizontal distribution and vertical relationships of rocks with relatively high permeability are reduced from an eastern wide area (~25 km north-south distance) to a narrow western area (~5 km wide, measured north-south) (Fig. 18). Although the width of the lower aquifer also narrows west-ward near Calama Hill, the Chintoraste volcanic and volcaniclastic unit may continue at a similar elevation for more than 10 km southward, underlying a broad valley for which there are few data (Fig. 4). Given that the Jalquinche Formation coarsens southward from the Loa River, and given that the scant borehole reports in the southern valley reveal no evidence of a Jalquinche-like mudstone, an aquifer within that southern valley is likely to be phreatic.
In addition to the narrow passage between the north and south bedrock boundaries of the San Salvador–Loa Valley, water that passes north of Chinto-raste hills in the lower aquifer encounters a second major bedrock constric-tion. A north-trending bedrock ridge separates the eastern sedimentary basin domain from a western domain of high-standing folded Mesozoic metasedi-mentary rocks and Cretaceous–Eocene intrusive bodies (Figs. 9 and 12), with a north-trending fault at the boundary. South of the Loa River the western bedrock domain and faulted boundary have a thin cover of Miocene–Pliocene gravel; between the San Salvador and Loa Rivers there is a thicker cover of Opache Formation, which obscures the details of the bedrock ridge (Fig. 9). On the east side of that bedrock ridge (west end of section C-C’, Fig. 12), both the Jalquinche mudstones and the Eocene Chintoraste unit with its permeable in-terbeds thin westward. In the intercanyon plain the host rocks for the regional lower aquifer shallow westward until they are in direct contact with the base of the thin sandstone and gravel that underlies the Opache Formation. In Fig-ure 9, the brown contour marked 0 m traces where the Jalquinche Formation pinches out, and the green contour labeled 0 m traces where the Chintoraste unit pinches out. Between the brown and green lines and for some distance to the east of the brown line, where the Jalquinche is thin, there is no effective
1470Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
A
B
x′
z′
z′
Figure 18. (A) River locations, major faults, and the posi-tions of two schematic geohydrology cross sections (black lines). The reach of the Loa River that probably recharges in part the Calama Valley aquifers is highlighted blue, and river elevations of 2660 m, 2760 m, and 2865 m are noted in that reach. The brown line marks the border of the Calama sedimentary basin and its potential aquifer host rocks. The pale green area encompasses both the Jalquinche Forma-tion and the ignimbrites that effectively confine a lower arte sian aquifer in some area. The tan zones within the green region mark areas where the two aquifers are in pres-sure balance or the upper aquifer head exceeds that of the lower aquifer, from Figure 3B. The dotted pattern indicates regions where no data test the effectiveness of potential confining units. Contours for the head in the lower aquifer are simplified from Figure 3A. There are no well data in the northern sector of the basin and no known suitable confin-ing layer there, so hypothetical head contours are shown (blue) to tie to the elevations of the Loa River. The blue area in the northeast map corner is a zone above 4000 m elevation where primary recharge likely occurs. The star marks the location of Calama city. Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T— Talabre area. (B) Schematic cross sections of the geo-hydrology of the Calama Valley. The vertical scale is exag-gerated relative to the horizontal scale, and neither is exact. The moderate- to high-permeability zones (white units) are a set of formations in which a mosaic of sedimentary facies exist and, by inference, variable permeability exists that is not illustrated. The black arrows indicate parts of those units that contain groundwater. The arrows indicate the part of the vector of groundwater flow that resolves to the plane of the cross section; additional flow may occur into or out of the plane of section. The blue arrows indi-cate flow across the low-permeability geological units that separate the upper and lower aquifers, inferred based on relative heads of the lower and upper aquifers (Fig. 3B; Table 9). Open arrows indi cate exchanges of the rivers with the aquifers.
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aquitard and therefore there is a single aquifer that is phreatic. Thus within a span of 10–15 km west of the West fault, it is inferred that the lower aquifer connects to the upper aquifer (intercanyon region) and can discharge directly to both rivers.
At a smaller scale near the Loa River, two intersecting sets of faults control the position of the western boundary of the rock unit that hosts the lower aqui-fer. The first fault set is north trending (F1 and F3, Fig. 9) and forms the western boundary of the Eocene Chintoraste unit. The second set, east trending and of small displacement, juxtaposes the Chintoraste unit against the Mesozoic impermeable basement (F2 in Fig. 9). That set also juxtaposes multiple lith-ologies within the Eocene volcano-sedimentary package against one another, some likely permeable and some of low permeability. Locally, the result is that the lower aquifer host rocks terminate at a corner.
DISCUSSION
Overview
For the Loa system, the existing subsurface and surface flow data are in-adequate to quantify numerous parts of the hydrological system. It would be convenient to assume that aquifer properties are laterally homogeneous, so that sparse data can be widely applied. However, the geological properties of the sedimentary basin that hosts the major aquifers of the Loa system point to-ward considerable heterogeneity (Fig. 18). This paper contributes an improved understanding of the architecture of the rocks of suitable hydraulic conduc-tivities to serve as aquifers or as aquitards within and adjacent to the Calama Valley, and their influences on the preferential flow pathways and possible discharge regions.
Within the Calama Valley the groundwater supplied from the eastern mountains and from the northern mountains join at multiple levels. At the sur-face, the baseflow-fed Salado River and baseflow-fed Loa River merge. In the subsurface, groundwater enters from the highlands and encounters sedimen-tary rocks of the Calama Basin. Figure 18 illustrates in a simple geohydrolog-ical sketch the vertical changes in hydraulic conductivity but displays only the most rudimentary aspects of the horizontal variability. The steep regional topo-graphic gradient imposes a strong piezometric gradient that directs ground-water to the southwest and then west across the study area.
The aquifers of the north sector of the Calama Valley are filled at least in part by infiltration of water from the Loa River into the conglomerate and sandstone of the Lasana Formation along the sector of the river south of the Conchi Dam (Figs. 2 and 18). Additional groundwater may enter the Calama Valley from the peaks above 4000 m elevation beyond the northeast end of Figure 18 section X-X’, passing beneath the bed of the Loa River in the lower parts of the Lasana Formation. Once in the Lasana Formation, the groundwater migrates southwest and encounters lenses and formations of variable permeability, with the out-come that the groundwater is progressively split into multiple aquifers (Fig. 18B).
Groundwater in the east-central Calama Valley, east of the Loa River, is sourced by water that precipitated in the highlands to the east and northeast, by recharge that may be both direct (Houston, 2007) as well as indirect, e.g., by infiltration of Salado River water into its alluvial bed. The extent of or locations where the groundwater from the eastern aquifers (L, Llalqui area, Fig. 18A) mixes with the water in the central Calama Basin aquifers are not yet docu-mented (Fig. 18B; note question marks where the eastern and northern cross sections should connect).
From the perspective of the broad Loa water system, within the Calama Valley there is no net increase in water because there is essentially no direct precipitation. However, within the study area the rivers exchange with the aquifers at various locations. A key river-groundwater exchange may occur near the northern limit of the Calama sedimentary basin, where the Loa River likely loses water into the Lasana Formation, which down-gradient hosts both a lower confined aquifer and the upper phreatic aquifer (Figs. 10 and 14). Else-where in the basin, east of the West fault, the available data suggest that only the upper aquifer exchanges with the rivers. However, at the West fault and within 20 km distance to its west, most of the water in the lower aquifer must discharge to the upper aquifer or to the rivers directly.
Calama Valley and West Fault System Hydrogeology
Across the western and central Calama Valley the piezometric surfaces of the two aquifers are of similar elevations in some sectors, but elsewhere the lower aquifer piezometric surface is higher than that of the upper aquifer (Fig. 3B). Within those broad areas (Fig. 3B) the aquitard likely permits a small de-gree of slow flux upward and the lower aquifer may recharge the upper aqui-fer. The sedimentary rocks that serve as the aquifer are highly hetero geneous across this region. Near the north-trending reach of the upper Loa River and the Chiu Chiu monocline, southward and westward thinning of the Sifón (Blanco, 2008; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) and Cupo Ignimbrites (Figs. 10 and 13) lead to their loss of effectiveness as confining units. Similarly, lateral variations in the thickness of the Jalquinche Formation and mudstones in the Lasana Formation lead to transitions in their capacity to be effective confining layers (Figs. 10, 14, and 18B).
In areas in the western and central Calama Valley where the head in the upper aquifer exceeds the piezometric surface of the lower aquifer (red in Fig. 3B), we expect that slow downward flow from the upper aquifer may recharge the lower aquifer. The narrow strip north and east of the Talabre region (Fig. 3B) where the pressure in the two aquifers is similar corresponds to a transition zone from excess head in the lower aquifer to excess head in the upper aquifer.
The piezometric gradient of the lower aquifer in the 5 km east of the West fault is markedly steeper than in the central sector of the Calama Valley (broad region around T in Figs. 3A and 14). A less pronounced increase in gradient occurs also within the upper aquifer east of the West fault (Fig. 2). Changes in the piezometric gradient of the lower aquifer may reflect the Milagro deforma-
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tion zone and related changes in the thickness of suitable aquifer host rocks (Fig. 14), but lithological data for the lower aquifer are not sufficient to test this hypothesis.
The heads of the two aquifers come into equilibrium near the West fault zone, which comprises the West fault and a subsidiary parallel fault, located 1 km to the east (Fig. 3B) (Tomlinson et al., 2010). The Loa River turns sharply to the south near the trace of the eastern branch of the fault, continues ~6 km in the zone between the two faults, and then resumes its westward direction (Fig. 7). In the subsurface, rocks that host the lower aquifer, the Eocene Calama Formation conglomerate to the east and Eocene Chintoraste pyroclastic and sedimentary rocks to the west, meet at the West fault (Figs. 10, 11, and 14). Lower aquifer water flow from east of the West fault to west of the West fault system navigates through permeability pathways that are not stratigraphically continuous. Although those deeper units are discontinuous, the Jalquinche Formation and the upper aquifer-bearing Opache Formation accumulated after most of the displacement across the West fault zone, and likely underwent much less disruption (Tomlinson et al., 2010). Nevertheless, the nearly equal heads (blue zone north of H in Fig. 3B) likely indicate an active flow between the lower and upper aquifers through a less effective aquitard. The faults may increase the heterogeneity of rock units that would otherwise act as aqui-tards, and they may place low-conductivity rocks adjacent to high-conductivity rocks. Recent research shows that large faults can effectively produce a greater hydraulic connection between shallow and deep aquifers (Bense et al., 2013).
San Salvador–Loa Valley Hydrogeology
The interpretation that the lower aquifer in the western region is hosted by the Chintoraste complex and a thin overlying conglomerate is a departure from prior interpretations, which ascribed the aquifer to the Calama Forma-tion. The borehole geology and the outcrops at Chintoraste hills reveal that the host rocks west of the West fault are much more pyroclastic and volcaniclastic than is the Calama Formation (Blanco et al., 2003; Blanco, 2008). Furthermore, considering that tens of kilometers of left-lateral displacement along the West fault are interpreted to have postdated accumulation of the Eocene Calama Formation (Tomlinson and Blanco, 1997), it is unlikely that the Calama Forma-tion continues to the west of that major fault.
Although the Loa River turns abruptly southward where it intersects the east branch of the West fault and parallels the fault set for ~2 km (Fig. 7), potential exchanges between the river and aquifers cannot be quantified with the sparse stream gauge data (Table 2), especially because several irrigation channels tap the river in this reach. Only a few kilometers farther west, widespread springs discharge from the upper aquifer to the Loa River channel and to the main tribu-tary to the San Salvador River. Much of the discharge occurs because the upper aquifer is intersected by the canyon walls (e.g., springs near LC, Fig. 9).
Whereas at the West fault the piezometric height of both aquifers is ~2240 m (Figs. 2 and 3A), throughout the area with data west of the fault the piezometric
height of the upper aquifer is 20–40 m higher than that of the lower aquifer. This relative loss of head in the lower aquifer might occur for either of two reasons. First, the rocks with high permeability below the aquitard layer might thicken westward, allowing more vertical dispersion in the lower aquifer of water that infiltrated across the West fault. Second, part of the water from the lower aquifer east of the fault might have transferred upward within the fault zone into the upper aquifer. Data are not available to test which explanation is more viable. Whatever the cause, the result is a zone with relative pressures that create a hydraulic gradient favoring downward seepage of upper aquifer water into the lower aquifer, across the intermediate Jalquinche Formation (Fig. 18).
Unlike prior studies, this study concludes that within 20 km west of the West fault the lower aquifer discharges significantly, if not completely, to the surface water system, because the host rocks for the lower aquifer unit thin between impermeable basement and a thinning confining unit. This lower aquifer dis-charge is in part direct, because the host rocks crop out in the San Salvador, Ojos de Opache, and Loa canyon walls (Fig. 9). Although hydro logi cal evidence of groundwater discharges into the Loa River canyon where the Chinto raste unit crops out is lacking, extensive carbonate deposits in the bed of the Loa River north of Chintoraste hills (Fig. 9) and paleospring carbonates located above the modern water level on the canyon walls are hypothesized to be by-products of lower aquifer springs. The lower aquifer discharge is also indirect in part, through the upper aquifer between the San Salvador and Loa canyons. This indirect discharge is inferred from the westward termination against a basement ridge of a major aquitard, the Jalquinche Formation (brown dashed line in Fig. 9), and of the subjacent units with lithologies that are suitable for moderately high permeability (green line in Fig. 9). Borehole data imply that little mudstone separates the Chintoraste unit from the sub-Opache sandstone of the upper aquifer (westernmost 4 km E-E’, Fig. 14), and hence water could migrate easily from the lower aquifer to the upper aquifer.
Important improvements in knowledge of the hydrogeology of the criti-cal region west of Calama city will require geological mapping at high spatial reso lu tion. In addition to providing greater precision on positions of formation boundaries, mapping is needed to specify facies variations in the water-bear-ing rocks, to establish the positions of channelized strata of high hydraulic conductivity, and to relate paleospring deposits to the modern groundwater hydrology. Given the widespread cover by the Opache Formation of underly-ing complex lateral changes in aquifer-host units and aquitard units, and the vertical canyon walls along the Loa River, novel observation techniques and high-resolution geophysical surveys may be needed.
Intersection of the Aquifers of the Eastern and Central Sectors
A lack of publicly available piezometric data for the eastern sector of the basin (from Llalqui to the Salado River; Figs. 2 and 3A) results in very little documentation of what happens to either aquifer near the Chiu Chiu mono-
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cline (Figs. 13B and 18). In that sector, where the Loa River flows parallel to and immediately east of the monocline, a short-term stream gauge campaign conducted late in a summer season (March) indicated that the upper aquifer discharges ~700 L/s to the river (Matraz 2012, see Table 1), accounting for about a quarter of the surface water flow. Another discharge from the upper aquifer to a spring on the north bank of the Loa River, ~100 L/s, occurs at the southern crossing of the Loa River canyon over the monocline (Fig. 4) (Matraz 2012, see Table 1). This spring location suggests a likely structural control on upper aqui-fer groundwater flow, which is combined with insight into aquifer host rocks developed elsewhere in the Calama Valley to put forth a hypothesis for ground-water flow near the Chiu Chiu monocline. The lower aquifer occurs at a depth exceeding 200 m (hosted in the Yalqui Formation and a conglomerate that may be age equivalent with the Lasana Formation; Fig. 13; Blanco and Tomlin-son, 2009), with a considerable thicknesses of two overlying low-permeability units (Jalquinche Formation and Sifón Ignimbrite). Given the westward dip of the aquifer host strata and aquitards east of the Loa River and their higher elevations across the monocline west of the river (Fig. 13B), it seems likely that the Salado-Llalqui region groundwater does not cross the monocline to mix with the northern source region groundwater of the Talabre area. Blocked by the rise in elevation of the aquifer hosts at the monocline, lower aquifer water may flow south into the region where the thick (hundreds of meters) Eocene Calama Formation is expected to occur (Figs. 4 and 6D). Within the Calama conglomerates, water may continue southward until the monocline tip is reached, south of which it then flows westward between, and paralleling, the Loa River and the southern basin boundary. Some lower aquifer water would thus flow south of Calama Hill (H in Fig. 4). An absence of data near and south of the west-flowing reach of the Loa River (Fig. 3A) precludes any further evaluation of this hypothesis.
CONCLUSIONS
The integration of data describing spatial variations in sedimentary facies and their thicknesses has the potential to improve understanding of any com-plex groundwater system located within the fill of a sedimentary basin. This Calama Basin study integrated information about lateral variations in the po-tential to store and transmit water with an assessment of the primary aquifers, and thereby clarified the spatial distribution of the units with which the con-fined or semiconfined aquifer system is associated. The results are data-based hypotheses for recharge of the aquifers of the northern Calama Basin and dis-charge west of Calama city.
The results suggest that neither the upper nor lower aquifer corresponds to a laterally persistent geological unit. A comparison of piezometric maps for the two major aquifers implies that there is a patchwork pattern of areas in which water exchanges between the two aquifers, areas where the lower aquifer is confined, and areas where a phreatic aquifer is absent (Figs. 3B and 18). Across the central Calama Basin this pattern results from the lateral variability in thick-
ness and in hydraulic properties of the aquitards, which are both ignimbrites and mudstones. Much of the lateral variability in aquifer properties results from facies changes in the middle Miocene Lasana and Jalquinche Formations, as well as in the upper Miocene to Pliocene Opache and Chiquinaputo Formations.
Folds and faults add to the architectural complexity of the aquifers. A princi-pal example occurs west of Calama city, where a fault-controlled, north-trend-ing basement ridge against which most of the Calama Basin sedimentary rocks terminate controls discharge from a lower, semiconfined aquifer to springs and rivers. A second important example is the West fault, across which pie-zometric gradients change and groundwater flow navigates major changes in the host rocks.
Two general conclusions of this analysis should be useful in the design of studies that will improve understanding of the coupled surface water and groundwater system. First, the spatial variations of the aquitards exert a key control on the exchanges of water between the upper and lower aquifers. Addi-tional research focused on the sedimentary architecture of the middle and late Miocene sedimentary basin, with a focus on the aquitard facies, would likely lead to better understanding both of preferential flow pathways and the loca-tions where there are exchanges between an upper and lower aquifer. Second, faults and folds near which specific sedimentary units terminate, change in thickness, or change in elevation likely have groundwater flow consequences. The hypothetical consequences can be tested by monitoring the groundwater and surface water at locations near those faults and folds, in both upflow and downflow directions, or with new geophysical and geochemical studies.
ACKNOWLEDGMENTS
We thank Nicolás Blanco Pavez and Andrew J. Tomlinson of the Servicio Nacional de Geología y Minería (Chile), Luís Baeza Assis, Jorge Jemio Figueroa, and Manuel Bucci Ramirez of CODELCO (Corporación Nacional del Cobre de Chile), and Luís Rojas B. and Arturo Beltrán Schwartz of the Dirección General de Aguas (Chile) for initial access to locations, reports, and data, as well as for in-depth discussions of the Loa system. We appreciate the early encouragement by and discus-sions with John Houston. Alex Covarrubias Aranda and Rodrigo Riquelme Salazar of Universidad Católica del Norte, Oscar Cristi of the Universidad del Desarrollo, Gary Libecap and Eric Edwards of University of California Santa Barbara, and Lovell Jarvis of University of California Davis con-tributed greatly to our understanding of the management of the Loa water. U.S. National Science Foundation grant OISE-1037929 enabled Jordan, Godfrey, and Kirk-Lawlor to gain an understand-ing of the multifaceted challenges for water management in the study area. A Fulbright Fellowship for Jordan in 2012 provided partial support of this project. We also thank the makers of Google Earth and the agencies and companies who acquire satellite images displayed on Google Earth for the free availability of this information and tool, which were fundamental for this study. A critique of an earlier manuscript by Maria-Theresia Schafmeister was very helpful. We are grateful for reviews by Andrew Tomlinson and John Houston that led to improvements in the manuscript.
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