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GEOSPHERE Architecture of the aquifers of the Calama Basin

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Research Paper
1438Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
Architecture of the aquifers of the Calama Basin, Loa catchment basin, northern Chile Teresa 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, USA 2Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile 3Department of Earth & Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853-1504, USA 4Earth 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.
GEOSPHERE
doi:10.1130/GES01176.1
CORRESPONDENCE: [email protected]
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 2015 Revision received 13 May 2015 Accepted 17 June 2015 Published online 5 August 2015
OPEN ACCESS
GOLD
This paper is published under the terms of the CC-BY license.
© 2015 Geological Society of America
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.
N
uncertain
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
http://catalogo.corfo.cl /cgi-bin/koha/opac-detail.pl ?biblionumber=5041
Study of the Water Resources of the Loa River Basin. Volume 2, Appendices.
CORFO 1977 Corporación de Fomento de la Producción, Santiago, Chile
Hidrogeología de la Segunda Región con referencia especial a las zonas investigadas, in Recursos hidráulicos del Norte Grande,
CHI-69/535 Hydrogeology of the Second region with reference to investigated zones, in Hydraulic Resources of the Grand North
DGA 2001 Dirección General de Aguas, Ministerio de Obras Públicas, Santiago, Chile
Actualización delimitación de acuíferos que alimentan vegas y bofedales, Región de Antofagasta
Informe Técnico, S.I.T. 76 Updated delimitation of the aquifers which supply springs and wetlands, Antofagasta region
Mayco 2013 Mayco Consultores, report prepared for Dirección General de Aguas, Ministerio de Obras Públicas, Santiago, Chile
Informe Final: Levantamiento información hidrogeológica Región de Antofagasta
http://documentos.dga.cl /SUB5493.pdf
Final Report: Hydrogeological information for Antofagasta region
Montgomery 2009 Montgomery & associates for Minera El Tesoro, report to Dirección General de Aguas
Informe Annual 2009 Monitoreo Hidrogeológico, Sector Campo de Pozos Minera El Tesora
Montgomery 2010 Montgomery & associates for Minera El Tesoro, report to Dirección General de Aguas
Anexo C: Diagramas esquemáticos de la construcción de los pozos, Minera El Tesoro, Calama, Chile
Provided by Dirección General de Aguas
Appendix C: Schematic diagrams of construction of wells, Calama, Chile, El Tesoro Mining Company
Minera Leonor 2007 Aquaconsult for Minera Leonor Informe Annual Monitoreo Comprometido con DGA Período: Año 2007
Provided by Dirección General de Aguas
Annual Monitoring Report Promised to DGA for 2007
EIA 2005 Knight Piésold S.A. report to Servicio de Evaluación Ambiental (Environmental Evaluation Agency), Santiago, Chile
CODELCO Chile División CODELCO Norte Proyecto Mansa Mina Estudio de Impacto Ambiental
http://seia.sea.gob.cl/archivos /EIA/2013102201/EIA_6313 _DOC_2128727783_-1.pdf
Environmental impact study for Mansa Mina project of CODELCO Norte Division of CODELCO Chile
EIA 2011 Aquaconsult report to Servicio de Evaluación Ambiental (Environmental Evaluation Agency), Santiago, Chile
Informe Final para Estudio de Impacto Ambiental: Estudio Modelamiento Hidrogeológico Subcuenca Cluster Toki para Proyecto Quetena
http://seia.sea.gob.cl/archivos /Anexo_2-1__1_de_6_.pdf
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
1441Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5

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.
1442Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
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†
September– October 1916
(L/s)
1 Below future position of the Conchi Dam 2200 1900 2100 880 2 After junction of Salado and Loa Rivers 3200 3100 240 3 At Angostura 3500 3200 4 Northeast of Calama Hill 4200 3600 1200 5 Near La Cascada§ 1300 3700 2400 640
Loa River before junction with San Salvador River 1200 Loa River after junction San Salvador River 2200 690 A.D.1993–2000 San Salvador River before junction with Loa River 600 Loa 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.
1443Jordan et al. | Architecture of aquifers, Loa basinGEOSPHERE | Volume 11 | Number 5
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…

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