Pergamon Geothermics, Vol. 26, No. 5/6, pp. 613426, 1997 ID 1997 CNR Ekevier Science Ltd Printed in Great Britain. All rights reserved 0375-6505/97 $17.00+0.00 PII: s0375-6505(97)ooo13-8 ISOTOPIC ZONING AND ORIGIN OF THE AQUIFERS IN THE DISCHARGE AREA QF THE GEOTHERMAL FIELDS OF AHWACHAPAN AND CHIPILAPA, EL SALVADOR VICENTE TORRES RODRiGUEZ, * PETER BIRKLE, * EDUARDO GONZALEZ PARTIDA, * DAVID NIEVA, * MAHENDRA PAL VERMA,* ENRIQUE PORTUGAL MARiN* and FEDERICO CASTELLANOST *Institute de Investigaciones Elkctricas, Departamento de Geotermia, A.P. l-475, Cuernavaca, Morelos, C.P. 62001, Mixico t Comisidn Ejecutiva Hidroelkctrica de1 Rio Lempa, GEOCEL, Santa Tecla, La Libertad, El Salvador (Received January 1995;accepted February 1997) Abstract-The northern discharge areas of the Ahuachapin and Chipilapa geothermal fields can be subdivided into four different zones based on their structural position, and the isotopic and chemical composition of their waters. In general, the contribution of geothermal waters from these two fields was estimated to be less than 10%. Elevation effects are of little importance, whereas a slight trend towards higher isotopic values with increasing water temperatures may exist. The NNW-SSE-trending Escalante and Agua Caliente faults represent lateral groundwater barriers, and provide vertical conduits for the ascending geothermal waters. The western discharge areas seem to be more influenced by the Ahuachapan geothermal field, whereas those to the east are more influenced by the Chipilapa field. Groundwaters in the Northern Plain are mainly from shallow northward- flowing aquifers. These waters show temperature effects, mixing with geothermal waters and are affected by the geology of the area. However, none of these factors alone can explain the isotopic variations observed in the waters of the northern discharge areas. 0 1997 CNR. Published by Elsevier Science Ltd. Key words: El Salvador, Ahuachapin, Chipilapa, discharge area, isotopic zoning, geothermal fluids. INTRODUCTION The geothermal fields of Ahuachapin and Chipilapa are located in the western part of El Salvador. The strong influence of the local fault systems over the flow of thermal fluids in the 613
Transcript
Pergamon Geothermics, Vol. 26, No. 5/6, pp. 613426, 1997
ID 1997 CNR Ekevier Science Ltd
Printed in Great Britain. All rights reserved 0375-6505/97 $17.00+0.00
PII: s0375-6505(97)ooo13-8
ISOTOPIC ZONING AND ORIGIN OF THE AQUIFERS IN THE DISCHARGE AREA QF THE GEOTHERMAL FIELDS OF AHWACHAPAN AND
CHIPILAPA, EL SALVADOR
VICENTE TORRES RODRiGUEZ, * PETER BIRKLE, * EDUARDO GONZALEZ PARTIDA, * DAVID NIEVA, * MAHENDRA PAL VERMA,* ENRIQUE PORTUGAL
MARiN* and FEDERICO CASTELLANOST
*Institute de Investigaciones Elkctricas, Departamento de Geotermia, A.P. l-475, Cuernavaca, Morelos, C.P. 62001, Mixico
t Comisidn Ejecutiva Hidroelkctrica de1 Rio Lempa, GEOCEL, Santa Tecla, La Libertad, El Salvador
(Received January 1995; accepted February 1997)
Abstract-The northern discharge areas of the Ahuachapin and Chipilapa geothermal fields can be subdivided into four different zones based on their structural position, and the isotopic and chemical composition of their waters. In general, the contribution of geothermal waters from these two fields was estimated to be less than 10%. Elevation effects are of little importance, whereas a slight trend towards higher isotopic values with increasing water temperatures may exist.
The NNW-SSE-trending Escalante and Agua Caliente faults represent lateral groundwater barriers, and provide vertical conduits for the ascending geothermal waters. The western discharge areas seem to be more influenced by the Ahuachapan geothermal field, whereas those to the east are more influenced by the Chipilapa field.
Groundwaters in the Northern Plain are mainly from shallow northward- flowing aquifers. These waters show temperature effects, mixing with geothermal waters and are affected by the geology of the area. However, none of these factors alone can explain the isotopic variations observed in the waters of the northern discharge areas. 0 1997 CNR. Published by Elsevier Science Ltd.
The geothermal fields of Ahuachapin and Chipilapa are located in the western part of El Salvador. The strong influence of the local fault systems over the flow of thermal fluids in the
613
614 V. Torres Rodriguez et al.
Fig. 1. Geological and structural map, including main fault systems, for the AhuachapBn-Cerro Blanc0 and Cuyanausul-El Tortuguero geothermal fields.
area (Gonzilez Partida et al., 1997) suggests that these fields be referred to by the names of the controlling graben structures, so that the Ahuachapan and Chipilapa geothermal fields should be named Ahuachapan-Cerro Blanc0 (ACB) and Cuyanausul-El Tortuguero (CT), respectively. The ACB field is structurally related to the Concepcibn de Ataco caldera system, and the CT field to the Cuyanausul stratovolcano (Figs 1 and 2).
The two geothermal fields are separated by the Chipilapa Uplift, a tectonic horst (Figs 1 and 2; Torres Rodriguez et al., 1992). Geological (Gonzalez Partida et al., 1992, 1997) and hydrogeological (Torres Rodriguez et al., 1992) investigations showed the structural independence of both fields, The existence of a low conductivity zone in the area of the Chipilapa Uplift also suggests the structural separation of the two fields (Flores Luna et al., 1991; Romo et al., 1997). On the other hand, the homogeneous talc-alkaline composition of the volcanic rocks in both areas (Gonzalez Partida et al., 1997) indicates that they originated from a chemically similar magma.
Isotopic Zoning and Origin of Aquifers, Ahuachapbn and Chipilapa, El Salvador 615
323 I I I I I I I I I
SXk: 322 -
NORTHERN PLAIN 10 km
N 321 -
Fig. 2. Hydrogeological map of the recharge and discharge zones in the Ahuachapan~erro Blanc0 and Cuyanausul-El Tortuguero region. Arrows indicate inferred groundwater flow directions. MS = moderate-salinity groundwaters; HS = high-salinity water; SULF = high-sulfate water; TURIN = groundwater of the Turin area; ACB = Ahuachapln-Cerro Blanc0 geothermal system;
CT = Cuyanausul-El Tortuguero geothermal system.
The Ahuachaphn-Cerro Blanc0 and Cuyanausul-El Tortuguero geothermal fields provide the main recharge to the shallow aquifers of the Northern Plain (Fig. 2). The northern discharge areas are structurally controlled by the Chipilapa, La Capilla, Escalante and Agua Caliente faults (Figs 1 and 2).
616 V. Torres Rodriguez et al.
The purpose of the present paper is to define the various hydrochemical regions within the discharge area of the ACB and CT geothermal fields based on isotopic and geochemical data. The origin, processes and causes for isotopic fractionation, such as elevation and temperature effects on the discharge waters, are discussed.
METHODOLOGY
The data analyzed correspond to three sampling periods between April 1990 and January 1991. The isotopic “0 and *H compositions of 51 samples were determined at the Instituto de Investigaciones ElCctricas (IIE), Cuernavaca, Mtxico, and the major element chemistry at the geochemistry laboratory of the Comisi6n Ejecutiva HidroelCctrica de1 Rio Lempa (CEL), El Salvador (Appendix A). The results presented are part of an extensive multidisciplinary project whose results are presented in this special issue.
PREVIOUS WORK
Nieva et al. (1990) and Nieva et al. (1997) used chemical compositions to classify water types from the Northern Plain and recharge areas. In the discharge areas they distinguished waters of low salinity (TURIN), moderate salinity (MS), high salinity (HS), high sulfate concentration (SULF) and of a mixed type. A regional correlation, however, was not possible.
Little hydrochemical information is available for the Cuyanausul-El Tortuguero geothermal system due to the lack of deep wells; the deep drilling program was restricted to the AhuachapLn-Cerro Blanc0 field. Average isotopic values of -4.3% for 6’*0 and -46.4%0 for 6D were measured in waters from Ahuachapdn production wells (i.e. AH-6, AH-19, AH-22, AH-24, and AH-31; Torres Rodriguez et al., 1992). A theoretical concentration of 2200-2400 ppm of chloride and a temperature of 225-230°C were estimated for the geothermal water giving rise to the HS waters (Nieva et al., 1997). Isotopic values of -47%0 for 6D and - 3.5% for aLgO were registered for the total discharge of well CH-7B. This well is located on the western edge of the Cuyanausul Graben (Fig. 2) but its lower section is structurally related to the Chipilapa Uplift, which is part of the ACB field (Fig. 2; Nieva Gbmez et al., 1993; Nieva et al., 1997).
HYDROGEOLOGY OF THE AREA
The recharge zone for the Ahuachapin-Cerro Blanc0 and Cuyanausul-El Tortuguero fields comprises the areas of the Caldera de Concepcibn de Ataco and the Cuyanausul volcano (Fig. 2). Groundwaters descend from the volcanic chain in a more or less northward direction. In the areas of the Caldera de Concepci6n de Ataco and of the present geothermal production (ACB field), a semi-confined aquifer consisting of interlayers of porous, permeable pumice and dense tuff layers forms the main recharge system (Torres Rodriguez et al., 1992). In addition, the production area also presents a perched aquifer at a shallow depth. Deeper groundwaters within the northern part of the recharge region are heated by an active magma chamber (Gonzhlez Partida et al., 1992).
The geothermal activity in the area is evident by the fumaroles on the volcanic chain, and the hot springs in the Northern Plain. Production wells in AhuachapLn-Cerro Blanc0 tap hot geothermal steam and brine (up to 265°C; Truesdell et al., 1989).
Isotopic Zoning and Origin of Aquifers, Ahuachapbn and Chipilapa, El Salvador 617
Geothermal and meteoric waters flow towards the discharge area in the Northern Plain through a shallow, unconfined aquifer in pyroclastic deposits. The southernmost discharge areas form rivers and springs close to the small town of Turin. In general, the geothermal outflows in the discharge areas are associated with fault systems.
RESULTS
Isotopic variations within the Ahuachaprin-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge area
The discharge area in the Northern Plain can be divided into four zones (Fig. 2) based on their structural control and water chemistry: areas with waters of medium salinity (MS), high salinity (HS), and high sulfate content (SULF) can be distinguished, as well as a zone near Turin (TURIN). Nieva et al. (1997) use a more hydrochemically oriented classification of water types (Types 1, 2 and 3) and their mixtures. Terms such as “high salinity” should not be considered to be quantitative, but are used in comparison to the other zones.
MS zone:
- west of the major NNW-SSE-trending fault system (Escalante and Agua Caliente faults; Fig. 2)
- medium-salinity groundwaters
HS zone:
- east of the major NNW-SSE-trending fault system (Escalante and Agua Caliente faults)
- high-salinity groundwaters
SULF zone:
- large area north of Turin - low-salinity and low-to-high-sulfate groundwaters
TURIN zone:
- area close to Turin (N and E) - low-salinity and low-sulfate groundwaters.
The characteristic composition of these zones, with the range of measured values, is given in Table 1.
Table 1. Chemical and isotopic characteristics of waters from the discharge areas. The values in parenthesis represent single sites at the edge or outside these areas
MS HS SULF TURIN
Cl [wml 37-100 9&190(345) 2-19(83) 22-28 6~) [%I -50 to -53 -50 to -58 -50 to -57 -52 to -59 aI80 [%o] -7.0 to -7.4 -6.9 to -7.7 -6.7 to -7.2 (7.8) -7.4 to -7.8 Temperature [“Cl 37-61 28-59 2630 28-30 SO4 hml 22-36 1 l-22 2-85 9-19 SO4 (most common value) 30 21 45 13 bpml
618 V. Torres Rodriguez et al. -8.50 -8.30 -8.10 -7.90 -7.70 -7.50 -7.30 -7.10 -6.90 -6.70 -6.50
-59.00
-61 .OO
Fig. 3. 6”O vs 6D for waters from the AhuachapanCerro Blanc0 and Cuyanausul-El Tortuguero discharge areas. HS = high-salinity groundwaters; LML = Local Meteoric Line; MS = moderate-
salinity water; SULF = high-sulfate water; TURIN = groundwater of the Turin area.
Although some samples from the MS, TURIN and SULF zones show overlapping values, their isotopic compositions have different trends (Fig. 3). The isotopic composition of the HS area with its large distribution overlaps all the other areas. The MS area, west of the Escalante and Agua Caliente fault systems, has a narrow range of 6’*0 and 6D. TURIN is characterized by low 6180 values (- 7.4 to - 7.8%), whereas the SULF zone has (with the exception of sample 724; Appendix A) significantly higher 6”O values (- 6.7 to - 7.2%0).
The various areas can be easily differentiated using plots of chloride concentration vs 6D (Fig. 4) or vs 6180. The range of 6D values is very similar for each zone, whereas the chloride
Fig. 4. 6D vs chloride for waters from the Ahuachapan-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge areas. HS = high-salinity groundwaters; MS = moderate-salinity water;
SULF = high-sulfate water; TURIN = groundwater of the Turin area.
Isotopic Zoning and Origin of Aquifers, Ahuachaprin and Chipilapa, EI Salvador 619
concentrations are characteristic for each zone. SULF and TURIN have the lowest chloride values (2-19 and 22-28 ppm, respectively; Table l), with the exception of sample 523 from the eastern edge of the SULF area (74.6 ppm; Appendix A). Also east of the fault systems, the HS area shows the highest chloride concentrations (96-190 ppm) of the entire discharge region, and a maximum value (345 ppm) occurs 3 km north of this area (sample 740). The MS zone shows intermediate chloride concentrations, ranging between those of the HS and the TURIN and SULF zones.
Origin of isotopic variations In the previous section various hydrochemical zones within the discharge area of the
Ahuachapin-Cerro Blanc0 and Cuyanausul-El Tortuguero geothermal fields were described. Here we will discuss the processes that can cause isotopic fractionation and thus the formation of compositionally different types of groundwaters.
Elevation effects. In general, stable isotopes, such as oxygen-18 and deuterium, show lower 6 values with increasing elevation due to fractionation processes during precipitation. Figure 5 shows values for 6i*O vs 6D from springs and wells, distributed over the recharge and discharge areas, but classified according to their elevation. Three elevation areas were distinguished: less than 700 m a.s.l., between 700 and 1000 m a.s.l., and more than 1000 m a.s.1.
The Local Meteoric Line (LML) is based on the average values for precipitation in El Salvador (i.e. -47O& for 6D and - 7%0 for 6180; Stewart,-1990; Nieva et al., 1997). During October and November 1990, an average precipitation value of -38.6%0 for 6D and - 5.9% for 6180 were measured at five climatological stations in the Ahuachapan-Cerro
OXYGEN- 18 - DEUTERIUM
700 - 1000 m
Fig. 5. Effect of elevation on 6180 and 6D for waters from the Ahuachapin-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge areas. LML = Local Meteoric Line.
620 V. Torres Rodriguez et al.
OXYGEN- 18 - DEUTERIUM
-11.0 -0.0 -7.0 -5.0 -3.0
WO%,
Fig. 6. hi80 vs 6D for waters from the Ahuachapan-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge areas (open squares), for geothermal waters from the Ahuachapan~erro Blanc0 field (solid
squares), and for rain collected in the field (solid circles). LML = Local Meteoric Line.
Blanc0 geothermal field (data point “RAIN 1990” in Fig. 6). This value agrees very well with the LML.
A slight increase in isotopic values with elevation can be observed when comparing waters from less than 700 m a.s.1. with those from 700-1000 m a.s.l., which contradicts normal trends. The values for groundwater from more than 1000 m a.s.1. coincide with the LML.
Temperature eficts. Increasing temperatures are normally related to higher evaporation rates, producing isotopic fractionation with enrichment in heavier isotopes in the residual liquid. In Fig. 7 isotopic values from wells and springs are grouped according to temperature. Waters with temperatures below 22°C between 22 and 35°C and between 35 and 60°C were distinguished without considering their regional distribution. It can be demonstrated that cold water ( ~22°C) has a typical meteoric composition, whereas the other two types show a slight trend towards higher isotopic values due to evaporation and/ or geothermal effects.
Geothermal water influence. The geothermal fields of Ahuachapan-Cerro Blanc0 and Cuyanausul-El Tortuguero are located approximately 5-10 km south of the discharge area under study (Fig. 2). Piezometric levels indicate northward-flowing groundwaters (Torres Rodriguez et al., 1992), which makes the influence of geothermal water in the Northern Plain very likely. The ACB field is represented by the series of AH-wells from Ahuachapan, and by well CH-7B at the eastern edge of the Chipilapa Uplift (Fig. 2). No deep wells were drilled in the CT field.
Isotopic Zoning and Origin of Aquifers, Ahuachapcin and Chipilapa, El Salvador
Fig. 7. Effect of temperature on 6’*0 and 6D for waters from the Ahuachapln-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge areas. LML = Local Meteoric Line.
The indications of low geothermal influence in the northern discharge areas are as follows.
(1) The isotopic compositions of waters from deep ACB wells are distinct from those of springs and shallow wells in the northern discharge area (Fig. 6). The ACB field has typically heavy 6’*0 values, whereas deuterium is less affected by geothermal fluid-rock interactions. Waters from the four discharge areas are located to the right of the LML, with the exception of sample 457 (TURIN). Their proximity to the LML indicates considerable dilution with shallow groundwaters as the hot fluids flow towards the discharge areas. None of the discharge areas is characterized by a single fractionation process, such as interaction of geothermal fluids with the surrounding rocks, which would result in an oxygen shift with little change in 6D values. Evaporation effects would produce trend lines with slopes smaller than that of the LML.
(2) The mixing proportions in water from two different sources are given by
Cm - Ca n -- m - cs - c, (1)
where c,,, indicates the isotopic concentration of a mixed water-type, in this case the composition of the discharge area. c, and cs correspond to the two end members, the “standard aquifer water-type” (ca; see below) at one extreme and geothermal water from Cuyanausul-El Tortuguero or Ahuachapan-Cerro Blanc0 (cs) at the other. n, gives the proportion of geothermal water in the mixture.
Hydrothermal alteration strongly affects the concentration of oxygen-18 in groundwaters, while hydrogen isotopes are not affected because minerals contain little
622 V. Torres Rodriguez et al.
Table 2. Average isotopic compositions of different waters from the Ahuachapin-Cerro Blanc0 and Cuyanausul-El Tortuguero areas
hydrogen compared to water. Thus, to detect the effect of geothermal water on the discharge area of Cuyanausul-El Tortuguero and Ahuachapan-Cerro Blanco, it is appropriate to use 6180 as an indicator.
The mixing ratio and the origin of groundwater in the discharge area were calculated, using a “standard aquifer water-type”, which is not affected by geothermal dilution processes (Table 2). Its value has been derived from wells and springs with the lowest 6’*0 and 6D values. Isotopic data from fluids of various Ahuachapan wells were used to obtain a representative average value for the Ahuachapin-Cerro Blanco-produced geothermal waters.
Given the prevailing groundwater flow directions (Fig. 2) we considered contributions from the Ahuachapan-Cerro Blanc0 field to the MS discharge area, and to the HS, SULF and TURIN zones to be influenced by the waters from the Cuyanausul and El Tortuguero field, and probably from another geothermal system possibly associated with the Las Ranas volcano and/or a NW-striking graben system (Fig. 2). The average composition of Ahuachapan geothermal waters (AH-series; Table 2) was used to determine mixing proportions in the MS area. Due to the lack of data for the Cuyanausul-El Tortuguero field, waters from the easternmost geothermal well (i.e. CH-7B) were used as a “standard geothermal water-type” in the HS, SULF and TURIN zone calculations.
Using mean 6180 values from Table 2 and eqn. (l), average percentages for the influence of geothermal water (n,) in springs and shallow wells were calculated (see Table 3). Table 3 also includes the maximum values observed (n,max). The calculated average geothermal contribution in the intermediate HS zone is 5%; major recharge probably comes from the Chipilapa Uplift and the Cuyanausul-Tortuguero area. The small geothermal influence in the TURIN area suggests large dilution and/or an independent, shallow aquifer system. The
Table 3. Proportions of geothermal waters (n,) in the mixed waters of the northern discharge areas of the Ahuachapin-Cerro Blanc0 and Cuyanausul-El Tortuguero
geothermal fields
Discharge area nm [Xl
MS 9 HS 5 SULF 10 TURIN 2
n, max[%]
12 15 22
5
Isotopic Zoning and Origin of Aquifers, Ahuachapcin and Chipilapa, El Salvador 623
springs in the MS area contain about 9% of waters with a composition similar to that of the Ahuachapan-Cerro Blanc0 geothermal production wells.
The SULF area represents a special case. If eqn. (1) is used, a mixture with up to 22% of geothermal waters would be indicated. However, the composition of these waters is altered by mixture with geothermal steam condensate (Nieva et al., 1997).
(3) The typical oxygen isotope shift towards higher a’*0 values resulting from the interaction of geothermal fluids with the surrounding rocks was not found in any of the waters of the discharge areas (Figs 3 and 6).
Geologic control. The dominant geologic structure in the discharge area is the NNW- SSE-trending Escalante and Agua Caliente fault system (Fig. 2). The isotopic compositions (Fig. 3) and salinities (Fig. 4) are different on both sides of the faults, indicating that they constitute a groundwater barrier. Hydrogeological studies show that the groundwater flow is northwards (Torres Rodriguez et al., 1992). Therefore, it can be postulated that the MS area, located west of the Agua CalienteEscalante fault system, derives its recharge from the Ahuachapan-Cerro Blanc0 field, and the eastern zones (HS, TURIN, SULF) from the Cuyanausul-El Tortuguero field. On the other hand, differences between the TURIN and SULF zones cannot be explained by the generally N-S-trending geologic structures. Fault systems tend to form barriers, but may also facilitate the ascent of geothermal fluids. Waters from a spring located 3 km north of the HS area on a NW-SE-trending fault (sample 740), have the highest chloride content of the entire discharge area (about 345 ppm), indicating the influence of ascending geothermal fluids.
DISCUSSION
Based on eqn. (l), stable isotope data indicate that SULF waters represent the highest geothermal fluid contribution in the Cuyanausul-El Tortuguero and Ahuachapin-Cerro Blanc0 discharge areas. However, low salinity values and low temperatures suggest sources other than geothermal waters. This contradiction can be explained by the model of Nieva Gomez et al. (1992), which proposed mixing of secondary geothermal steam with shallow groundwaters in the SULF area. Another explanation could be heating by conduction from surrounding rocks.
The TURIN waters are characterized by low values in salinity, sulfates, temperatures, and 6’*0, and a wide 6D range. All these factors suggest a negligible geothermal water contribution, estimated at 2% using eqn. (1). In the TURIN area, waters from springs and shallow wells are derived from an independent, shallow aquifer system (AS system).
The MS zone presents groundwaters with the most homogeneous isotopic and geochemical compositions of all the discharge areas, suggesting very little interaction of the shallow aquifer system with other groundwaters. Mixing with geothermal fluids (calculated to be about 10%) is indicated by their temperatures (3958°C) and high 6D values (average - 5 l%o), which are the closest to the composition of geothermal waters.
The groundwaters of the HS zone, with their high salinities and wide range of isotope values, seem to result from a mixing of waters from various sources. Geologic structures, such as the Escalante fault, could facilitate the ascent of hot (up to 5O’C) waters. Isotope values for the HS waters are similar to those of the cold water springs, indicating that their composition could be a mixture of shallow groundwaters from the SULF, TURIN or MS areas. This would suggest the hypothetical groundwater flow directions shown in Fig. 2.
624 V. Torres Rodriguez et al.
CONCLUSIONS
The groundwaters of the Ahuachapin-Cerro Blanc0 and Cuyanusul-Tortuguero discharge areas are characterized by very homogeneous hydrochemical and isotopic compositions. A combination of various processes, such as conductive heating, evaporation, mixing, elevation and temperature effects, as well as the presence of geological barriers, affect the distribution of the various water types. The influence of waters from the Ahuachapin-Cerro Blanc0 and Cuyanusul-Tortuguero geothermal fields on the northern discharge areas is small as a result of major dilution with meteoric and shallow groundwaters. The waters from the SULF area are characterized by adiabatic cooling and related steam loss effects, and represent a mixture of steam and geothermal and meteoric waters. This mixing also explains the low chloride concentration in the SULF zone. The groundwaters near the town of Turin (TURIN) seem to show small geothermal influences. Probably their origin is related to shallow groundwaters from the Cuyanausul- El Tortuguero area.
The Escalante-Agua Caliente fault system forms a natural groundwater barrier within the discharge zone: the area west of the fault system (MS zone) shows the influence of shallow groundwaters and geothermal waters, both from the Ahuachapan-Cerro Blanc0 area. The isotopic and chemical composition of groundwaters from the region east of the Escalante fault (HS zone) overlaps those of all surrounding discharge zones.
Waters from the Ahuachapin-Cerro Blanc0 field seem to feed the western part of the discharge region, whereas the HS, SULF and TURIN areas are recharged by the Cuyanuasul-Tortuguero field, or by an as yet unidentified catchment area further to the east.
Acknowledgements-The authors would like to thank Marcel0 Lippmann and Alfred Truesdell for their valuable reviews of the manuscript.
APPENDIX A
Isotopic and chemical compositions of waters from the Ahuachaphn-Cerro Blanc0 and Cuyanausul-El Tortuguero discharge areas (see location map in Nieva et al., 1997)
Sample Domain Date Coord. X Coord. Y m a.s.1. 6”0[%,] 6D[%] T [“Cl (3 [PPd so4 bpd
Flores Luna, C., Romo Jones, J. M., Vega Aguilar, R., Esparza Hernandez, F., Gomez Treviiio, E., Garcia Hemandez, V. H. and Ramirez Hemlndez, J. (1991) Exploration magnetotelurica de1 camp0 geotirmico de Ahuachapin Chipilapa, El Salvador, C.A.: informe final. Internal report, Comisibn Ejecutiva Hidroelectrica de1 Rio Lempa, La Libertad, 167 pp.
Gonzalez Partida, E., Gardufio Monroy, V. and Renteria Lopez, D. (1992) Estudios geocientificos y de ingenieria de reservorios para el proyecto de factibilidad de1 camp0 geotermico de Chipilapa: informe final geovolcanologico. Internal report, Instituto de Investigaciones Electricas, Cuemavaca Morelos, 113 pp.
Gonzalez Partida, E., Torres Rodriguez, V. and Birkle, P. (1997) Plio-Pleistocene volcanic history of the Ahuachapan geothermal system, El Salvador: the Conception de Ataco Caldera. Geothermics 26, 555-575.
626 V. Torres Rodriguez et al.
Nieva, D., Verma, M. P., Santoyo, E., Portugal, E. and Campos, A. (1997) Geochemical exploration of the Chipilapa geothermal field, El Salvador. Geothermics 26, 589-612.
Nieva Gbmez, D., Gonzilez Partida, E., Romo Jones, J. M., Torres Rodriguez, V. and Verma Pal, M., (1993) Informe final de la sintesis geocientifica. Internal report, Instituto de Investigaciones Elictricas, Cuernavaca Morelos, 66 pp.
Nieva G6mez, D., Verma Pal, M., Portugal Marin, E. and Santoyo Gutitrrez, E. (1990) Estudios geocientificos y de ingenieria de reservorios de1 campo geotCrmico de Chipilapa. Estudio geoquimico: informe final (Rev. 1). Internal report, Instituto de Investigaciones Eltctricas, Cuernavaca Morelos, 69 pp.
Nieva G6mez, D., Verma Pal, M., Portugal Marin, E. and Santoyo GutiCrrez, E. (1992) Estudios geocientificos y de ingenieria de reservorios para el proyecto de factibilidad de1 campo geot&mico de Chipilapa. Estudio geoquimico: informe final (Rev. 2). Internal report, Instituto de Investigaciones ElCctricas, Cuernavaca Morelos, 79 pp.
Romo, J. M., Flores, C., Vega, R., VBzquez, R., Pirez Flores, M. A., G6mez Treviiio, E., Esparza, F. J., Quijano, J. E. and Garcia, V. H. (1997) A closely-spaced magnetotelluric study of the AhuachapPn-Cerro Blanc0 geothermal field, El Salvador. Geothermics 26.
Stewart, M. K. (1990) Environmental isotope study of Ahuachapin Geothermal Area: a reassessment. CEL Internal report, La Libertad.
Torres Rodriguez, V., Berlanga Gutibrrez, J. M., Borja Diaz, M. A., Contreras Lbpez, E., Faz Ptrez, P., GonzPlez Partida, E., Lesser Illades, J. M., Martinez Muiioz, R., Montiel Serafin, F., Nieva G6mez, D., Pal Verma, M., Parkman Cuellar, P., Portugal Marin, E., Posadas Gonzilez, D., Renteria Torres, D., Santoyo GutiCrrez, E. and Suirez Meneses, J. (1992) Estudios geocientificos y de ingenieria de reservorios para el proyecto de factibilidad de1 campo geot&mico de Chipilapa. Informe final hidrogeolbgico. Internal report HDG-IF-006X1-1, Instituto de Investigaciones ElCctricas, Cuemavaca Morelos, 465 pp.
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