Geochemical evolution of groundwater beneath Mexico City

Journal of Hydrology 258 (2002) 1±24

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Geochemical evolution of groundwater beneath Mexico City W.M. Edmunds a,*, J.J. Carrillo-Rivera b, A. Cardona c a

British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK b Instituto de GeografõÂa, UNAM, 04510, Mexico DF, Mexico c Posgrado en Ciencias de la Tierra, UNAM, 04510, Mexico DF, Mexico Received 22 February 2000; revised 28 March 2001; accepted 31 May 2001

Abstract The geochemical evolution of groundwaters along a 24 km ¯ow beneath Central Mexico City path from the Sierra de las Cruces towards Lake Texcoco has been investigated using stable isotopes, radiocarbon and major and trace elements to determine the natural baseline conditions, the extent of any contamination and the effectiveness of the overlying aquitard seal. Modern groundwaters of low salinity (,200 mg l 21) are found up to 11 km from the outcrop area and groundwater ages of up to 6000 yr bp occur in the middle part of the section. Groundwater stable isotope ratios d 18O and d 2H lie close to the global meteoric water line, indicating that the groundwater originates from local rainfall. The groundwater chemistry may be interpreted as the result of inputs from the source area with progressive water±rock interaction down the horizontal ¯ow gradient. A redox boundary is found at 9 km along the line of section, coincident with the start of the con®ned section. Relatively low nitrate concentrations (below 9 mg l 21 NO3 ±N) are found in the aerobic waters; low concentrations of NO3 in the aerobic waters and low Cl re¯ect inputs prior to the modern development. Some elements (Cr, U, As, Se, Sb) increase their concentration with distance (time) as far as the redox boundary, but low concentrations occur in the reducing aquifer section. The chemistry of several major ions (Mg, Na/Cl, K) as well as trace elements such as Li, Rb, Ba re¯ect the weathering of the basaltic mineral assemblage (feldspars and ma®c minerals) and their increases are generally proportional to residence time; phosphate, F and I concentrations indicate a probable source from apatite in the basaltic or rhyolitic rocks. A borehole in the east of the city (some 17 km downgradient) intercepted thermal water (Si geothermometry indicates 1638C at depth). This water gives a distinctive composition indicating possible addition of metamorphic CO2 which has then reacted with the igneous rocks. Increases in B and Cl are derived from volatiles trapped in the glass or vesicular basalt. A thermal anomaly found in the middle section of the heavily pumped aquifer is interpreted as the up-coming of warmer water from medium to greater depth mainly from basalts, rhyolites and possibly limestones. The geochemistry indicates that groundwater beneath Mexico City is of good quality and there is no obvious evidence of leakage of inorganic compounds from surface sources of contamination through the aquitard. The younger groundwater drawn from the western outcrop area is generally of good inorganic quality. Increased drawdowns in the con®ned aquifer have induced ¯ow of warmer water with higher Cl from depth. The resources in the aquifer represent an important reserve of good quality water which need to be properly managed as a high quality resource as part of integrated plans for the City's future supplies. q 2002 W.M. Edmunds. Published by Elsevier Science B.V. All rights reserved. Keywords: Groundwater; Hydrogeochemistry; Mexico City; Isotopes; Trace elements; Pollution

* Corresponding author. Fax: 144-1491-642345. E-mail address: [email protected] (W.M. Edmunds). 0022-1694/02/$ - see front matter q 2002 W.M. Edmunds. Published by Elsevier Science B.V. All rights reserved. PII: S 0022-169 4(01)00461-9

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W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

1. Introduction Mexico City is one of the largest conurbations in the world with some 20 M inhabitants and growing at around 2% per annum. About 70% of its total water supplies, amounting to an estimated 60.3 m 3 s 21 is obtained from groundwater. Original conditions were those of a fully saturated artesian basin even some 100 yr ago. Groundwater beneath Mexico City has been progressively exploited since the mid-nineteenth century, gradually becoming sub-artesian and with water levels having declined by around 80 m. Piezometric levels continue to fall by over 1 m yr 21 (Birkle et al., 1998), as a result of uncontrolled exploitation. Groundwater withdrawal has led to progressive land subsidence and over the past century the land surface has fallen by up to 9 m in the centre of the city causing structural damage (AIC (Academia De La InvestigacioÂn Cientõ®ca), 1995). The actual city area was formerly occupied by several lakes (Texcoco Ð the largest, and Tenochtitlan), but they were progressively drained as the city expanded. Many natural springs abounded at the edge of the artesian basin, but these were all exhausted by the early 20th century (Durazo and Farvolden, 1989). The city obtains its groundwater supply from more than 3500 boreholes tapping granular and fractured volcanic units. These boreholes are located within the city boundaries; for the purposes of this paper the aquifer is termed the Mexico City Aquifer System (MCAS). The MCAS lies towards the south of the Basin of Mexico (Fig. 1). The geological units included in the MCAS have a continuous and regional distribution: they are found elsewhere within the Basin of Mexico as well as in neighbouring basins. These units belong to the Mexican Trans-volcanic Belt with dimensions of 950 by 110 km in length and width, respectively (Mooser et al., 1997). Numerous studies dating from 1966 describe the physical hydrogeology of the MCAS. Investigations of groundwater in the near-surface lacustrine sediments (CAVM (Comision De Aguas Del Valle De Mexico), 1966) demonstrated the link between

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groundwater, abstraction and subsidence. Further detailed investigations on subsidence due to groundwater extraction were carried out by Marsal and Graue (1969) and Herrera et al. (1974), among others. These investigations were carried out using pure hydraulic calculations where the chemical response was neglected. More recently, basin-scale investigations have been carried out using isotope hydrology (Cortez and Farvolden, 1989) and, using a steady state numerical analysis of the ¯ow regime (Ortega and Farvolden, 1989). Rivera and Ledoux (1991) further investigated the links between water abstraction and land subsidence whilst Rudolph and Frind (1991) developed a theoretical approach to the aquitard response to groundwater extraction, ®nding that signi®cant interpretative errors may arise if the stress dependence of the hydraulic parameters is ignored in this type of system. Carrillo-Rivera (1998) has examined the investigations of subsidence and the need for 3D modelling which takes into account the actual abstraction rate, the response due to inter-basin hydraulic connection. Hydrogeochemical investigations have been carried out at regional and local scales on the nearsurface aquitard to investigate contaminant transport to the aquifer beneath. Ryan (1989) regionally investigated the extent of local pollution by nitrogen compounds whilst Rudolph et al. (1989) studied the local movement of solutes in the overlying aquitard. Both concluded that fractures in the aquitard might allow the migration of organic and inorganic contaminants into the aquifer beneath. Mazari and Mackay (1993) refer to the relevance of aquitard fractures in the vertical transport of some organic compounds beneath a sewage water canal at a site to the north of Texcoco lake. At the same site, Pitre (1994) de®ned the extent to which inorganic species had seeped into the underlying aquitard. Most hydrogeological studies of the MCAS therefore have centred on groundwater hydraulics and the vertical movement of water from the overlying aquitard. Water quality investigations have been restricted to the determination of major ions and primarily at a

Fig. 1. (a) The setting of the Basin of Mexico showing the location of Mexico City in relation to the main ¯ow directions of groundwater and surface water towards Lake Texcoco. (b) Plan of Mexico City area showing the aquitard area as well as the 1990 extent of con®ned groundwater. The geological cross-section of Fig. 2 is indicated. Groundwater sites sampled in the present study are shown together with the line of cross section, approximately normal to the original ¯ow direction, used in the geochemical interpretation.

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W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

local scale to check against drinking water standards; no integrated hydrogeochemical study of the MCAS aquifer exists. Groundwater management of the MCAS during the last half of the 20th century is likely to have caused various changes in the groundwater ¯ow regime which might also have induced water quality variations. Groundwater abstraction from the MCAS is also considered to have induced some migration of natural water with higher salinity as development has proceeded (Cardona and HernaÂndez, 1995). Furthermore there has been concern at the possible deterioration of groundwater quality beneath the city, due to the poor management and disposal of potential contaminants in both liquid and solid forms (Mazari and Mackay, 1993). Thus several possibilities exist for the modi®cation of the initial baseline water chemistry by either lateral or vertical in¯ows. In this paper, the main objective is to identify chemical changes taking place along a line approximately along the original direction of groundwater ¯ow towards the lake, considering also the potential of vertical in¯ow components. The speci®c targets are to (i) use trace elements, stable isotopes and radiocarbon to determine the evolution and age of the groundwater under the natural conditions during the recent geological past and to de®ne to what extent the prevailing groundwater quality represents initial (pre-development) natural baseline conditions in the MCAS. (ii) detect the extent of any contaminant inputs from Mexico City to the con®ned parts of the aquifer beneath. (iii) de®ne the extent to which the drawdown produced in the MCAS in the past 40 yr has caused reversed ¯ow, i.e. mass transport as well as chemical gradient reversal, (iv) determine which are the predominant geochemical processes taking place along the inferred horizontal groundwater ¯ow lines, (v) establish if such evolution matches that described from other sedimentary basins, and (vi) estimate the recharge areas for the groundwater. Against a better understanding of the groundwater chemistry of the basin, the prospects for improved management of the resource are highlighted.

2. The study area The Basin of Mexico (Fig. 1a) is one of the largest of a series of closed catchments located in the Mexico Trans-volcanic Belt (Mooser and Molina, 1993; VaÂzquez-SaÂnchez and Jaimes-Palomera, 1989). The basin has a drainage area of about 9600 km 2; however the actual area covered by this research (Fig. 1a and b) considers just the city development which has an extent of about 2400 km 2. The abrupt relief of the surrounding mountains, with altitudes in excess of 5000 m amsl, slopes towards the ¯at-lying centre of the basin to an altitude of approximately 2230 m amsl. The climate is subtropical but with a signi®cant variation in the mean annual precipitation across the basin Ð between 1800 mm in the southern Sierra Chichinautzin, 1100 mm in the west (Sierra de las Cruces) and 1200 mm in Sierra Nevada, to 600 mm towards the remnant Lake Texcoco in the centre of the basin. The rainy season is mainly in the summer months. The mean annual temperature is 12±148C in the southern Sierra Chichinautzin and in the Sierra de las Cruces and 158C in Lake Texcoco. The potential annual evapotranspiration is around 1400 mm. Rainfall produces negligible surface runoff in the permeable rocks of the Sierra Chichinautzin to the south. In contrast, substantial runoff is generated in the Sierra La Cruces to the west of the city, a condition that has caused severe historical ¯ooding. 2.1. Geology The Basin of Mexico is a closed basin located on a graben structure developed during the Oligocene where a thick sequence of volcanic and lacustrine materials was deposited. In those times the basin drained to the south. The runoff outlet was closed during the Pleistocene as the result of a series of volcanic activities (De Cserna et al., 1987). This magmatic activity resulted in extensive lava ¯ows that formed the Sierra de Chichinautzin to the south of the basin. The extrusive events lapsed for suf®cient time around 700,000 yr bp, allowing a substantial layer of ash, inter-bedded with extensive alluvial and lacustrine deposits, to accumulate in the various lakes formed after the closure of the basin. The hydro-stratigraphic pro®le of the area is shown in Fig. 2. The Cretaceous limestone, sequence

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

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Fig. 2. Geological cross-section of the western part of the Mexico Basin along the line shown in Fig. 1b.

consists of limestones, sandstones and shales, outcropping beyond the southern catchment limits and is the deepest identi®ed geological unit. These rocks are considered to have a total identi®ed thickness, in the centre of the basin, in excess of 1500 m. The Mixhuca borehole, Fig. 1b encountered this unit at 1581 m below ground surface and karstic and fractured features are indicated, based on the total loss of drilling ¯uid and the drop of about 8 m of the drilling tools (VaÂzquez-SaÂnchez and Jaimes-Palomera, 1989). The Middle Tertiary Volcanic Unit includes clastic material of Eocene age, basalts and rhyolites of Oligocene age and Miocene andesites with a collective thickness of about 3000 m. The Lower Pliocene deposits are characterised by the occurrence of lacustrine and pyroclastic material with a total thickness of 600 m. The Plio-Quaternary sequence consists mainly of basaltic-andesites and andesites, Pliocene andesites and Plio-Quaternary pyroclastics dominate the highlands and are interstrati®ed with contemporaneous alluvial and lacustrine deposits, their total thickness being in excess of 1000 m. The Quaternary-Recent deposits cover the entire ¯oor of the basin and comprise 600 m of ¯uvial and alluvial deposits. A lens of ®ne material is included within these deposits; its top part outcrops in Lake Texcoco

but underlies the rest of the plain except at its margins. This unit is represented mainly by microfossils, volcanic ash, and to a minor extent, lacustrine clays interbedded with sands, silts and occasionally gravels. Its total thickness increases gradually from the edge of the plain to Lake Texcoco where it attains approximately 300 m, forming the main aquitard con®ning the aquifer. Petrographic analyses of the Middle Tertiary basalts show phenocrysts of andesine, oligoclase, sanidine, quartz, augite and hypersthene; the matrix is composed of feldspar and quartz with minor apatite, magnetite, ilmenite and zircon. The rhyolites and andesites have similar mineral composition with quartz, an essential component of the former but being rare in the latter. Both show porphyritic texture with phenocrysts of andesine, hornblende, feldspar and augite. The Pliocene andesites are lava ¯ows, ®ne to medium grained and porphyritic in texture with plagioclase, quartz, olivine and some ma®c minerals. The Quaternary basaltic-andesites and andesites have porpyhritic texture with pyroxene and olivine set in a glassy matrix. The late Quaternary to Recent deposits are represented by volcanic ash and lacustrine clay interbedded with sand, silt and occasionally gravel. The deposits are composed of 5±10%

4 4 5,4 5,4 5,4 5,4 5 5,4 5 5 5,3 5,3 5,3 5 4 5 5 5,4 4 5,4 5 5 5,4 5,4 5,4 5,4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

1 4.8 5.2 6 5.3 5.9 6.9 6.4 8.6 9.9 8.9 10.6 11 13.8 16.7 16 15.7 15.8 17.3 17.9 18 17.9 22.5 23 23.3 24

Lithology Distance (km)

Map No. 406 279 276 260 280 200 200 245 150 120 300 230 300 220 nd 208 180 150 150 150 120 70 200 193 175 202

19.4 17.9 18.3 18.3 19.1 18.7 20.3 18.8 19.1 18.8 24 20.4 24.8 27.2 44 26.9 23.7 21.2 24 24 23.7 23.1 23.4 23 23.5 23.4

8.27 7.15 7.26 8.03 7.13 7.43 7.31 7.12 7.01 7.31 7.75 7.56 7.63 7 nd 7.1 7.17 7.3 7.6 7.67 7.81 7.77 8.3 8.24 7.97 7.85

Depth (m) Temp. (8C) pH p 652 167 184 700 268 285 243 253 253 267 223 114 160 2 67 nd 2 70 2 85 2 31 2 92 2 57 2 46 2 120 2 108 2 141 2 190 2 163

Eh (mV) 255 303 276 323 275 431 411 395 600 322 202 326 221 511 nd nd 1230 1590 1037 925 770 885 604 483 831 706

SEC (mS cm 21)

Table 1 Field information and major ion data for samples taken along the line of section shown in Fig. 1b

23.7 19.9 19.8 18.3 17.8 23.1 21 26.2 31.4 20.4 21.8 26.2 34.9 72.6 362.0 198.3 146.4 171.4 108.4 96.4 108.0 113.2 90.5 67.1 113.6 102.7

Na 4.39 4.97 4.68 4.18 3.76 5.32 4.22 5.23 6.83 4.81 3.79 5.88 7.04 10.47 71 12.56 18.62 14.64 9.68 8.18 9.07 9.81 13.49 12.39 15.93 13.65

K 9.0 15.0 12.4 17.4 16.0 26.8 24.3 21.2 41.3 18.8 8.6 14.7 2.7 12.6 25.7 36.5 38.7 46.0 26.8 21.0 18.6 20.4 8.3 7.5 13.9 10.0

Ca 8.4 12.7 10.8 12.9 12.5 21.7 18.5 17.5 31.8 15.8 4.0 12.4 1.7 11.8 190.0 24.9 31.4 62.2 38.4 28.6 25.9 28.3 14.8 12.9 24.6 17.0

9.9 5.3 5.0 5.3 9.4 11.8 9.5 11.5 21.9 5.3 2.0 6.5 1.0 27.2 650 162 129 170 62 57 72 82 36 28 70 68

Mg (mg 1 21) Cl 107 122 117 122 117 183 132 190 209 178 104 156 122 224 nd 497 453 566 271 262 293 317 234 195 337 293

2.8 20.6 10.7 20.8 10.6 27.2 35.8 18.5 64.5 6.2 1.7 4.2 2.3 2.8 35.8 4.2 0.0 45.5 157.1 90.7 31.6 19.2 27.2 12.4 49.7 8.9

HCO3 SO4

0.5 2.6 1.2 2.7 2.4 5.1 4.4 4.1 9.1 0.5 , 0.2 , 0.2 , 0.2 , 0.2 , 0.2 0.4 1.1 , 0.2 , 0.2 , 0.2 , 0.2 , 0.2 0.4 , 0.2 , 0.2 , 0.2

NO3 ±N

6 W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

sand-sized particles, most of which are calcareous ooliths; 55±65% of silt-sized siliceous microfossils, mostly diatoms, with biochemically precipitated calcite. 20±30% is composed of clay sized particles (montmorillonitic) of which as estimated 10% is smectite and the rest biogenic and volcanic silica, bentonite and minor phases (DõÂaz-RodrõÂguez et al., 1998); the remaining 5±10% is organic material. In general these sediments contain up to 10 wt% organic carbon (Mesri et al., 1975), as well as high Na (up to 9200 ppm) and Mn (up to 1920 ppm). There are four major outcropping areas in the highlands with contrasting geological unit distribution which may act as recharge areas Fig. 1a: Sierra Las Cruces to the south-west with rhyolites, andesites and dacitic rocks; the southern boundary of the basin is represented by the Sierra Chichinautzin, composed of basaltic units; to the east the Sierra FrõÂa includes andesites and dacites, and to the north the Pachuca Sierras have an heterogeneous distribution of andesites, rhyolites and basaltic rocks (De Cserna et al., 1987). The city area also contains volcanic vents and outcrops of basaltic lavas that date from historic times. 2.2. Hydrogeology The MCAS consists of a highly compressible aquitard (Quaternary-Recent deposits) partially overlying a heterogeneous and anisotropic aquifer system formed in both fractured and granular deposits (Fig. 2). Borehole depths are around 200 m depth (Table 1) and a reasonable amount of data are available only on the hydraulic properties of the Plio-Quaternary and Quaternary-Recent deposits. The properties of the deeper aquifer units are not known within the area of the MCAS. However, evidence from outside MCAS (and the Mixhuca borehole) indicates that the Cretaceous limestones are highly permeable due to existing karstic features (VaÂzquez-SaÂnchez and Jaimes-Palomera, 1989). Several hundreds of litres per second are continuously extracted from the Middle Tertiary Volcanics for the dewatering of a mine district in the northern part of the Basin of Mexico, in Pachuca, Hgo. (Carrillo-Rivera et al., 1999). No evidence of the hydraulic conductivity of the Lower Pleistocene unit however is available. The aquitard has been hydraulically modelled as a main source of induced water, representing some 17.5%

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(AIC (Academia De La InvestigacioÂn Cientõ®ca), 1995) and up to 75% (Herrera et al., 1982) of the total extraction into the MCAS by the exploitation of units 4 and 5; however no chemical evidence was used to support that model. Groundwater movement is controlled, in addition to the head differences, by lithological variation and geological structure. Under natural conditions recharge is inferred to have occurred in favourable areas in the permeable uplands surrounding the basin, whereas discharge occurred as springs at the edge of the plain. Discharge is also found through the lacustrine sediments towards the centre of the plain and by evaporative discharge through Lake Texcoco (Durazo and Farvolden, 1989). Groundwater discharge conditions in the valley as shown by boreholes constructed in the latter half of the 18th century was expressed by an elevated artesian pressure producing heads of several metres above ground level (AIC (Academia De La InvestigacioÂn Cientõ®ca), 1995). From a ¯ow system perspective, the area of the former Lake Texcoco, and that of the, plain are considered the main former discharge of groundwater ¯ow in the basin of Mexico as a whole (Ortega and Farvolden, 1989). The accumulation of salts in Lake Texcoco is largely the result of the evaporative discharge of groundwater at points of structural weakness (Rudolph et al., 1989). According to the permeable nature of the geological units in the highlands limiting the basin, various ¯ow systems in the horizontal plane would have developed. Discharge areas of regional and intermediate ¯ow systems, as described by Toth (1998), were found in the Lake Texcoco and piedmont areas, respectively. Each regional ¯ow systems might also have developed distinctive hydrogeochemical signatures, according to lithology and residence time. After about half a century of intensive groundwater abstraction, artesian pressure in the groundwater system in the basin has been lowered and indications of regional discharge areas have been reduced. A change from con®ned to uncon®ned conditions: a potentiometric surface about 30 m below the level of the bottom of the aquitard has been identi®ed. Further, the bottom of ancient Lake Texcoco is now about four metres above the level of the soil surface of Mexico City. As a consequence, the ancient discharge

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W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 3. Temperature and hydrochemical variations (pH, alkalinity, Cl and SO4) along the line of cross-section shown in Fig. 1b. The position of the redox boundary is also shown.

area is likely to have been shifted from its original position. Present groundwater ¯ow in the horizontal plane from west to east has a similar pattern to the original pre-pumping conditions from the surrounding mountains to the Lake Texcoco area Fig. 1b. However, horizontal hydraulic gradients have been increased and lateral ¯ow has been enhanced, and the main out¯ow area has been displaced towards major drawdown areas that are the result of continuous abstraction from more than 3500 boreholes across the city (Fig. 1b). The reduction of the hydraulic head is estimated, at current rates of groundwater extraction in the Mexico City area (about 51 m 3 s 21), to be about one metre per annum (Birkle et al., 1998). Natural

regional groundwater discharge is likely to be negligible except by evapotranspiration. Thermal waters have been discharging during historic times within the area of Mexico City considered here. The PenÄon de los BanÄos borehole (Site 15, Fig. 1b) is considered to intercept the discharge from a regional ¯ow system (Cortez and Farvolden, 1989), having a well-head temperature at the present day of 448C. This borehole is situated at the site of a former thermal spring used by the Aztecs before the 1500s, however the ¯ow started to decline with the increase in the groundwater abstraction after the 1950s. At other locations along the middle of the line of section (Fig. 3) there is thermal water ¯ow induced from depth and the evidence for this is discussed below. The horizontal hydraulic conductivity (Kh) of the exploited part of the aquifer (Units 4 and 5) has values that vary from 1:5 £ 1023 to 8:1 £ 1028 m s 21. Lower Kh values are found towards the centre of the plain in alluvial lacustrine deposits (VaÂzquez-SaÂnchez, 1995). Values of the storage coef®cient vary from 4 £ 1024 to 7 £ 1025 and from 7:3 £ 1022 to 1:7 £ 1021 for the con®ned and water table conditions, respectively. The computed vertical (regional) hydraulic conductivities for the main aquitard have been estimated to be in the order of 6 £ 1029 m s 21 (Ortega et al., 1993; VaÂzquez-SaÂnchez, 1995). Ortega and Farvolden (1989) numerically modelled vertical recharge at 1:1 £ 1028 to 1:9 £ 1028 m s 21 per square metre of recharge zone (350±600 mm yr 21). A hydrological balance for the area related to Mexico City was derived by Herrera et al. (1994) who obtained a value of 15.3 m 3 s 21 for the natural recharge, being 19% of the total precipitation (80.8 m 3 s 21). This value is about half that of the calculated range of recharge rates in the southern part of the basin of 21.2±30.0 m 3 s 21 obtained by adding together the recharge rates in sierras Nevada, Las Cruces and Chichinautzin as calculated by Birkle et al. (1998). Several constraints are expected from water samples collected from municipal abstraction boreholes, since they may be inadequate to detect contaminated in¯ows. Regarding direct man made activities (sewage ef¯uents, oil spills, leachates) some authors estimate that even as much as 0.1% contribution of contaminated in¯ows above the drinking water standards for any species would be diluted below

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

detection limits. At present, the most plausible pathway for surface contaminants to follow, besides those of the fractured rock and piedmont areas, is the region of the valley ¯oor covered by the aquitard where this unit has been broken by human activities (abandoned boreholes, excavations fully penetrating the aquitard, buried structures) as well as through natural fracturing. Studies have been based on matching measured geochemical pro®les with computer simulations representing the ¯ow through the upper part of the aquitard where fractures need to be the dominant feature; however, the related storage coef®cient used in such calculations yield values grater than one (Rudolph et al., 1989). The most likely mechanism for aquifer contamination is from point source rather than diffuse sources. On the other hand, some investigations indicate that fracture openings would be closed at depth because of compression and the plastic characteristics of aquitard material (Mazari and Mackay, 1993). On the other hand, in the middle and south-east part of the studied section there are responses that need to be given further consideration. In the middle section the observed increment in temperature (and salinity) implies that obtained water has a regional ¯ow input from beneath. This in¯ow has usually been neglected as most hydraulic modelling invokes horizontal ¯ow and tends to ignore chemical data. So far, an aquitard water input fails to explain the observed temperature increment. Leakage from the aquitard might also be expected to be saline. The aquitard has a wide variety of TDS concentrations; in nearby Texcoco area values from about 37,000±53,000 mg l 21 have been reported (Marsal and Graue, 1969), in Sosa Texcoco site TDS of 195,000 mg l 21 have been identi®ed (Rudolph et al., 1989); a salinity study of the water in the aquitard to the east of the study area reported contrasting vertical variation in TDS content with values from 110 to . 11; 400 mg l 21 (DGCOH, 1996). However, there is a lack of relation between salinity with depth, as well as in the horizontal dimension. Further ®eld investigation carried out by Pitre (1994) in the aquitard, under a canal that transports sewage water, found that transport of solutes would take, in the location of his study area (some 10 km to the north-west) from 65 to 135 yr to reach the aquifer unit beneath.

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3. Methods A representative set of samples was collected from 26 active water supply boreholes penetrating different lithological units in the MCAS (Table 1) and located on a west±east transect crossing Mexico City (Fig. 1b). Boreholes prior to sampling had been pumping continuously for more than six months, most of them having been in operation for several years. Under natural conditions groundwater ¯ow took place from west to east over some 30 km from the recharge area towards the main discharge area in Lake Texcoco. Intensive pumping in the last 50 yr has changed the local hydraulic gradients resulting in some instances in a groundwater velocity increase along the same ¯ow direction. In other places the ¯ow has been reversed and in places some lateral displacement has occurred towards the piezometric lows. However hydraulic parameters suggest that maximum advective displacement (up-gradient) has been of the order of 2 km. Consequently, considering the length of the section and distance between boreholes, samples are expected to basically represent the original horizontal ¯ow conditions. Temperature, electrical conductivity, Eh and pH were measured in the ®eld using an in-line ¯ow cell to ensure the exclusion of atmospheric contamination and to improve measurement stability. Since dissolved oxygen could not be measured, Eh is used here as the main tool for indicating aerobic and anaerobic waters (Edmunds et al., 1984). Two ®ltered (0.45 mm) samples were taken at each site in acidwashed, well rinsed low density polyethylene bottles. One sample for major cation and trace element determination was acidi®ed to make 1% in HNO2 3 ; producing a pH around 1.5 suf®cient to stabilise trace metals; ®ltered, unacidi®ed samples were collected for anion analysis. All inorganic and stable isotope analyses were carried out at the British Geological Survey (Wallingford). Alkalinity was measured on untreated samples by volumetric titration using bromocresol greenmethyl red indicator. Major cations together with SO4 (measured as total S), Si, Al, P, V, Fetotal, Mn, Sr and Ba were analysed by ICP-AES using an ARL 34000C optical emission spectrometer. Chloride, NO3, NH4, Br, I and F were analysed by automated colorimetry, other trace elements by ICP-MS (Fison

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W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

(per-cent modern carbon) where the errors are well within 1%. All reported values had ionic balance within 5%, except samples 4, 7 and 9 which showed less than 10% error. The accuracy of ICP-AES and ICP-MS analyses was controlled using appropriate laboratory standards and checked during each batch using international reference standards, including USGS standard 1643d. Inter-comparisons were also made for certain elements (Ba, Mn, Li, Zn) by comparing results produced both by ICP-AES and ICP-MS. The ICP-MS data are used here in preference, only because of the lower quantitation limits as compared with ICP-AES.

4. Groundwater chemistry 4.1. Groundwater evolution along the ¯ow line Ð major element trends

Fig. 4. Major ion relationships (Na/Cl, Mg/Ca,K and Br/Cl) along the line of cross-section shown in Fig. 1b. The plot also shows the position of the redox boundary.

PQ1 instrument) and As by hydride-generation ICPAES (using an ARL 341 hydride generator). Total As was analysed by pre-reduction of all As(V) to As(III) in samples 24 h before analysis using 5% KI (2.5 ml 20% KI in 7.5 ml sample solution), followed by online hydride generation using 1% w/v NaBH4 in 0.1% w/v NAOH (Trafford, 1987). Total organic carbon (TOC) was determined using a TOCSIN total carbon analyser. Stable isotope analyses (O,H,C) were carried out by mass spectrometry (VG Micromass 602C) and radiocarbon analyses by AMS (accelerator mass spectrometry) at Tucson, Arizona following sample preparation by the NERC Radiocarbon Laboratories at East Kilbride, Scotland. Results are reported as pmc

The principal characteristics of the groundwaters are shown in Table 1 where results are arranged in the order of distance along the 24 km west±east line of section from uncon®ned to con®ned conditions (Fig. 1b). Sample locations are projected onto this line of section which runs approximately along the original (pre-development) ¯ow line from outcrop eastwards. It includes the thermal site of PenÄon de los BanÄos (448C), adjacent to the international airport. A window of uncon®ned groundwater (samples 18± 22) is also included, coinciding with the volcanic outcrops of Iztalpalapa. The groundwater temperature near the western end of the section is 19 ^ 18C and increases to a fairly uniform 23 ^ 18C near the centre of the basin (Fig. 3). All boreholes have similar drilled depths and production intervals (Table 1). A distinct temperature anomaly however is found in the central part of the cross section (around 278C) and these boreholes are spatially related to the thermal source of borehole 15 (PenÄon de los BanÄos). This borehole, although not particularly deep probably indicates a natural up¯ow with further up coning of deeper water induced by development and this is discussed below. The salinity of the groundwater is low near the western end of the section and there is a slight increase along the line of section partly related to

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

the thermal anomaly. Chloride may be used as a conservative element, which mainly re¯ects atmospheric inputs. However internal sources of salinity (e.g. of geothermal or volcanic origin) may be found in the basin as well as salinity induced from the discharge area of lake Texcoco. Chloride concentrations are typically between 5±10 mg l 21 at the western end of the section (Fig. 3) and in the centre of the basin are variable between 25 and 100 mg l 21. There are, however, some anomalies with very low Cl ( , 2 mg l 21) at two sites around 14 km along the line of section and a group with over 100 mg l 21 between 25 and 30 km from the western end of the cross section. There is an overall increase in HCO3 across the aquifer with an inverse relationship with pH. The origin of chloride may be clari®ed using bromide, which can also be regarded as a conservative element. The (molar) ratio mBr/Cl of local rainfall is not available but is expected to lie just above the marine ratio …1:57 £ 1023 † most probably as a result of Br enrichment moving inland (Berg et al., 1980; Edmunds, 1996). The Br/Cl ratios in the MCAS groundwaters near to the recharge area lie well above the expected rainfall signature (Fig. 4). Most of these waters also have the lowest Cl (see Fig. 3) suggesting a signi®cant enrichment in Br is taking place early in the recharge process. The origins of this are uncertain but it may re¯ect the addition of aerosols from biomass and fossil fuel burning. With increasing Cl the Br/Cl decreases eastwards towards the value of PenÄon de los BanÄos (site 15). This is consistent with an internal source of Cl derived from the leaching of basaltic rocks with a reported ratio of 1:5 £ 1023 (Fuge, 1978). The trends in the major elements in the MCAS groundwaters as well as in the redox characteristics follow a progressive sequence of geochemical evolution. There is little or no apparent evidence from major elements that groundwaters have been affected by pollution from the city above but this possibility is discussed below. A series of chemical constituents (major and trace elements as well as isotopes) are used in the aquifer cross sections to follow the evolution of the groundwater chemistry; chloride is used as an inert reference element against which to study the water±rock interaction. Despite the intensive pumping in the MCAS, a progressive sequence of geochemical evolution is observed, which mirrors those

11

observed along ¯ow lines in carbonate (Edmunds et al., 1987) and non-carbonate (Hendry and Schwarz, 1990) sedimentary basins. A principal feature of the groundwaters is the enrichment in Na relative to Cl (Fig. 4) giving molar ratios up to 6 in the initial 11 km of the ¯ow line, although beyond this point the higher salinities from internal sources of Cl mask the effect to give slightly lower values. The high mNa/Cl ratios are indicative of strong water±rock interaction, shown also by increases in mNa/Ca and alkalinity along the line of section. Most groundwaters remain considerably below saturation with respect to calcite. This enrichment in Na at the expense of Ca indicates the release of Na mainly from the glassy matrix in the alkaline volcanics and their sedimentary derivatives. Andesine is one of the more commonly reported minerals in the volcanic rocks in this region (VaÂzquez-SaÂnchez and Jaimes-Palomera, 1989). Its incongruent dissolution to produce kaolinite would yield Na, Ca, HCO3 and SiO2 in 1:1:3:2 relationship. While this ratio is not observed, the dissolution of volcanic glass is likely to dissolve some 10 times faster than the crystalline basaltic minerals (Gislason and Eugster, 1987), Na being released preferentially to Ca. Another feature is the signi®cant enrichment in mMg/Ca representing the weathering of the ma®c minerals. An extreme composition is found in the PenÄon de los BanÄos sample (site 15) which has Ca: 25.7 and Mg: 190.0 mg l 21, respectively, which represent the extensive water±rock interaction in the deeper ¯ow system. Similarly, there is a progressive increase in K across the aquifer from 4 to 15 mg l 21 indicating progressive reaction of minerals in the volcanic sequence (probably biotite); in sample 15 an extreme enrichment again is found with the potassium concentrations reaching 71 mg l 21. The concentrations of sulphate are variable but some very low values are to be found in the intermediate section of the aquifer (11±26 km) which are considered to be input related (possibly rainfall prior to urban development) rather than the result of sulphate reduction, since this group spans the redox boundary (see below). The major element data thus de®ne two main groups of compositions with a more hydrogeochemically evolved set of waters being found further downgradient than 13 km from the recharge area.

12

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 5. Plot of d 18O and d 2H for groundwaters from the Mexico City aquifer system with plots of rain water trends from data in the literature.

4.1.1. Isotope geochemistry The d 18O and d 2H values from the MCAS aquifer are shown in relation to the global meteoric water line (GMWL) and to the local meteoric water line estab-

Fig. 6. Plot of carbon isotopes ( 14C as percent modern carbon) and d 13C, d 18O and d 2H along the line of cross-section shown in Fig. 1b The plot also shows the position of the redox boundary.

lished by Cortez and Farvolden (1989) in Fig. 5. The groundwaters occupy a narrow ®eld close to the GMWL as well as the local line. A closer examination of the evolution along the ¯ow line (Fig. 6) reveals that in the middle section (east of 11 km from the start of the section) the mean values are very slightly heavier than those nearer to the recharge area (by 0.35½ for d 18O and 2½ for d 2H). The values overall compare with those reported from the eastern margins of the MCAS (Issar et al., 1984) as well as lying close to the mean value for local rainfall (Cortez and Farvolden, 1989), suggesting rapid recharge with no signi®cant evapotranspiration losses. In contrast with this rather homogeneous set of data for the MCAS, water extracted from the aquitard has been reported as having d 18O of 2.1 and 230½ for d 2H (Pitre, 1994). The shallow groundwaters in the vicinity of Lake Texcoco also show a distinct isotopic enrichment, unlike the thermal water emerging at borehole 15, (d 18O and d 2H values of 29.1 and 259½, respectively). Radiocarbon activities were measured on seven selected samples to establish the groundwater residence times. In addition d 13C values were obtained to correct the radiocarbon data and derive ages. Both stable and radioisotopes show a clear evolution along the line of section (Table 2; Fig. 6). Radiocarbon, expressed as percent modern carbon, (pmc) at 6 km along the line of section has a value of 81.1 pmc, indicating modern groundwater. At 10±11 km along the ¯ow line values between 50.9 and 53.8 pmc

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

13

Table 2 Isotopic data for samples collected along the line of section shown in Fig. 1b Map no.

d 18O

d 2H (½)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

2 10.3 2 10.2 2 10.2 2 9.9 2 10.4 2 10.2 2 10.5 2 10.4 2 10.3 2 10.2 2 10.6 2 10.4 2 10.2 2 9.6 2 9.1 2 9.5 2 10 2 9.9 2 10 2 10.3 2 10.1 2 10 2 10.3 2 10.1 2 10 2 10

2 72 2 71 2 75 2 71 2 71 2 71 2 72 2 71 2 70 2 72 2 71 2 71 2 71 2 65 2 59 2 62 2 70 2 67 2 69 2 76 2 70 2 69 2 70 2 72 2 68 2 69

d 13C (½)

14

C (pmc)

Age (corrected)

2 13.7

81.1

Modern

2 13.8

50.9

Modern

2 13 2 6.3 0.8

53.8 13.1 1.9

Modern 5880

2 4.6 2 6.3

13.4 12.7

3370 6080

indicate slightly older water. Between 14 and 18 km from the recharge area relatively uniform older water (12.7±13.4 pmc) is found. The thermal water from PenÄon de los BanÄos has a barely detectable radiocarbon content (1.9 pmc). This trend in evolution is clearly supported by d 13C where values of 13.0±13.8½ in the youngest groundwaters imply a stoichiometric reaction between soil CO2 and a carbonate source with d 13C near zero. It is unlikely that volcanic sources of carbon are involved here. Values for carbon isotopes from CO2 in volcanic terrain including mantle-derived CO2 lie in the range 23 to 28½ (Hoefs, 1997). The source of inorganic carbon in the aquifer is likely to be secondary carbonates derived from the mobilisation of metamorphic CO2 originating from marine carbonates. The enrichment in 13C found in the deeper groundwaters can best be explained by reaction with a carbonate source with a carbon isotope ratio near 0½. Deep drilling has proved Cretaceous limestone beneath the volcanics and PenÄon de los BanÄos water has a strongly enriched

13

C composition (0.8½) which clearly indicates the isotopic exchange and an approach to water±rock equilibration with a heavy carbon source. Groundwater ages have been derived (Table 2) using the Pearson model (Fontes and Garnier, 1979) in which the measured d 13C values have enabled corrections to be made, using a value of 100% modern carbon together with a soil CO2 value of 226½ and carbonate of 0.8½. Using this model groundwaters from outcrop to 10 km along the ¯ow line are modern with radiocarbon contents from 51 to 81 pmc, but the samples from 14 km onwards, with values around 13 pmc yield corrected ages between 3300± 6000 yr bp. This age would imply that a component of the water had been moving from outcrop towards the discharge areas, towards Lake Texcoco, prior to human intervention. 4.1.2. Redox-related processes A redox boundary is found in the aquifer some 12 km along the line of section, marked by a change

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W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 7. Variations in redox related parameters (Eh, NO3, Fe and Mn), along the line of cross-section showwn in Fig. 1b, de®ning the position of the redox boundary.

in redox potential (Eh) of over 200 mV (Fig. 7) which must coincide with the complete reaction of oxygen (Edmunds et al., 1984). This is emphasised by the fact that to the east of this point NO3 ±N concentrations are consistently low, but to the west nitrate concentrations are above detection limits, although remaining below 10 mg l 21 NO3 ±N. The limit of high nitrate waters at about 10 km along the line of section may mark the maximum penetration of groundwater from the modern era and beyond this, waters with low nitrate (#0.5 mg l 21 NO3 ±N) but with Eh as well as low FeT indicating aerobic conditions, may re¯ect naturally low total nitrogen inputs from pre-industrial times. This hypothesis is supported by the low concentrations of both SO4 and Cl in this section of the aquifer (from 14 to over 20 km). The concentrations of TOC, are variable and moderately high (mean value 4.5 mg l 21). There is no clear trend in TOC along the line of section with the higher values being found in the reducing as well as in the aerobic section (Fig. 7). The highest value (14.6 mg l 21) compares with values for natural levels of TOC up to

Fig. 8. Plot of redox-sensitive trace elements (Mn, U, Cr, As, Se, Sb) in the Mexico City aquifer, relative to the redox boundary, along the line of section shown in Fig. 1b.

30 mg l 21 in pore waters from the aquitard (Pitre, 1994). The control on the consumption of oxygen in the aquifer may either be microbiological or possibly chemical (Ottley et al., 1997) with Fe 21 being released from the incongruent solution of silicates. 4.1.3. Minor and trace elements A wide range of minor and trace elements have been investigated to shed further light on the origins of water,

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

15

Table 3 Selected trace element data for samples collected along the line of section shown in Fig. 1b Map No.

Al mg l 21

Cr mg l 21

Co mg l 21

Ni mg l 21

Cu mg l 21

Ge mg l 21

Rb mg l 21

Mo mg l 21

Pb mg l 21

U mg l 21

As mg l 21

Se mg l 21

Sb mg l 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

1.41 10.33 2.8 2.9 1.39 6.77 1.4 1.06 8.4 1.91 1.82 3.43 13.38 3.47 3 1.96 24 1.31 0.93 2.2 2.37 2.06 2.86 2.25 2.19 1.67

1.83 3.86 4.94 2.38 2.98 2.36 2.32 4.23 1.82 2.31 1.72 1.33 0.2 , 0.02 0.21 0.08 0.39 0.03 0.04 0.08 0.05 0.05 0.04 0.06 0.08 0.14

, 0.01 0.04 0.06 0.04 0.03 0.11 0.03 0.04 0.03 0.01 , 0.01 0.06 0.02 0.05 0.06 0.04 0.03 1.37 0.06 0.03 0.02 0.02 , 0.01 , 0.01 0.04 0.04

0.08 0.62 0.59 0.38 0.21 0.90 0.34 0.56 0.68 0.16 0.14 0.42 0.10 0.28 1.67 0.45 0.25 7.95 0.85 0.39 0.23 0.33 0.36 0.23 0.32 0.32

0.96 1.38 1.1 1.17 0.89 1.02 0.54 1.04 0.84 0.94 0.75 1.36 0.32 0.57 0.49 9.88 0.36 1.19 1.21 0.69 0.24 0.39 0.78 0.34 0.28 12.8

0.09 0.06 0.03 0.11 0.10 0.15 0.16 0.13 0.08 0.14 0.14 0.08 0.25 0.30 1.24 0.48 0.68 0.08 0.28 0.67 0.71 0.85 0.74 0.46 0.85 0.70

8.69 7.83 7.66 8.32 7.80 11.20 8.52 10.06 10.44 6.01 8.17 6.14 9.91 17.03 144.60 12.54 14.13 16.73 9.30 8.47 9.79 9.82 15.72 16.32 21.94 18.54

0.84 0.41 0.78 0.32 0.46 0.44 0.60 0.59 0.22 0.34 2.83 1.29 5.88 7.13 72.30 5.90 1.39 7.86 3.81 3.84 4.38 4.49 6.00 3.80 5.04 3.98

0.24 0.72 0.30 0.30 0.21 0.39 0.32 0.23 0.67 0.15 0.34 0.90 0.47 0.40 0.24 2.55 0.58 0.67 0.53 0.41 0.52 0.48 0.27 0.17 0.50 1.74

0.56 1.06 1.00 1.72 1.51 4.18 2.72 2.26 6.90 2.00 0.99 2.21 0.29 0.05 10.90 0.02 0.01 10.03 1.26 0.42 0.24 0.78 0.00 0.00 0.00 0.05

0.49 0.82 0.93 0.89 0.34 0.37 0.43 0.23 0.84 0.47 1.29 1.75 3.56 1.90 nd 0.38 0.94 7.62 5.95 1.88 0.78 0.53 0.24 0.08 0.26 0.60

0.89 0.81 0.92 0.96 0.38 , 0.01 1.06 0.23 0.35 , 0.35 0.48 0.99 0.46 0.42 nd 0.59 1.28 1.24 1.25 1.43 1.14 0.37 0.54 0.40 0.62 1.58

0.05 0.03 0.06 , 0.02 0.02 0.04 0.04 0.06 0.06 , 0.01 0.13 0.09 0.16 0.04 , 0.26 0.08 , 0.03 0.06 , 0.02 , 0.02 , 0.02 , 0.02 , 0.02 , 0.02 , 0.02 0.02

geochemical processes and residence times, augmenting evidence provided by the major ions and isotope ratios. Element speciation described below has been examined using the phreeqc code (Parkhurst, 1995). The change from oxidising to reducing conditions is accompanied by distinct changes in other element concentrations, for example the increase in Fe 21 from below 20 to generally above 300 mg l 21. Several redox sensitive trace elements are investigated (Fig. 8) including Mn, U, Cr, As, Se, Sb the solubility and speciation of which may vary with pH and Eh of the groundwaters. Chromium concentrations in the aerobic section are between 0.2 and 4.9 g l 21, with concentrations near to or below the 3s detection limit of 0.024 g l 21 in all the anaeobic groundwaters. Chromium is an abundant trace element in basaltic rocks. It is present here as the oxyanion CrO 32 and the pattern of its occurrence suggest that concentrations represent the natural baseline under the prevailing redox conditions in a basaltic aquifer unit.

Uranium shows a similar geochemical behaviour to Cr with concentrations in the aerobic section being stabilised by the formation of oxyanions, the most abundant being …UO2 †…CO3 †22 2 : In the reducing section, concentrations of U are below 0.5 g l 21 although two sites (18 and 15) contain high values near 10 mg l 21 suggesting a different control. Manganese concentrations are low in the aerobic section but show an increase before the redox boundary and thereafter increase or are higher in the anaerobic groundwaters. This re¯ects the stability range of Mn 21 which is considerably larger than for Fe 21; manganese in other natural ¯ow sequences (Edmunds and Smedley, 2001) in the absence of redox controls tends to increase with residence time and distance. The concentrations of Ni and Co which are geochemically related to Mn and which are both common trace elements in basaltic rocks remain below 1 mg l 21 although with one main anomaly (borehole 18), which has a relative low temperature, 21.28C.

16

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 9. Plot of Group I and Group II metals plus B, P and F in the Mexico City aquifer along the line of cross-section shown in Fig. 1b.

Several elements in addition to Cr and U form oxyanions which are stable under aerobic conditions (As, Sb, Se, Mo). Arsenic (as As V) and antimony both show the same behaviour, along with Cr and U, of an increase in the aerobic section of the aquifer with

lower concentrations beyond the redox barrier, similar to the classic pattern leading to roll-front mineral deposits (Wanty et al., 1987). Some outliers are found in the reducing waters, notably boreholes 18 and 19, but both As and Sb follow the trend for lower concentrations in the deeper anaerobic groundwater. Molybdenum, like the other oxyanion-forming elements also shows an increase to around 6 g l 21 in the aerobic groundwater but then maintains high concentrations throughout the anaerobic section. This is in line with trends found in sedimentary aquifers where Mo accumulates in shallow groundwaters with increasing residence time, irrespective of redox conditions (Edmunds and Smedley, 2001). The high concentration of Mo (72 mg l 21) in borehole 15 also suggests that this element (probably derived from reaction of feldspar) also increases with increasing temperature under the reducing conditions. Germanium concentrations are at or below the 1 mg l 21 level but show systematic increase along the ¯ow direction (Table 3). Germanium, geochemically closely related to Si, is thought to accumulate as the result of continuing water±rock reaction and like Mo, this increase may be proportional to residence time but possibly also to temperature increase acting as a geothermometer. However the concentration (1.24 mg l 21 Ge) in borehole 15 is only slightly higher than the average value (0.4 mg l 21) for the reducing waters. Copper, Pb and Zn (Table 3) show no systematic trends across the aquifer and their occurrence seems to be unrelated either to residence time or geothermal in¯uence. For each of these three elements some outliers exist but no obvious explanations are apparent. The Group I and II metals have also been used as tracers of evolution along ¯ow lines in carbonate (Edmunds et al., 1987) and in sandstone aquifers (Edmunds and Smedley, 2001) and are shown for the MCAS in Fig. 9 (see also Tables 3 and 4). Strontium and Ba are released from carbonate minerals and also from K-feldspars during incongruent dissolution processes. Lithium, Rb, Cs are released mainly from feldspars and some clay minerals. Of these elements Ba is the only one likely to be limited by solubility controls in dilute groundwaters and in the MCAS the highest Ba concentrations are found in those waters lowest in SO4. Barium is however well below barite solubility limits (as calculated by phreeqc) and in theory could have reached much higher than the

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

17

Table 4 Minor element data for samples collected along the line of section shown in Fig. 1a Map No.

Si mg l 21

Sr mg l 21

Ba mg l 21

Li mg l 21

Ba mg l 21

FeT mg l 21

Mn mg l 21

Zn mg l 21

F mg l 21

Br mg l 21

I mg l 21

PT mg l 21

TOC mg l 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

29.9 29.7 27.5 24.0 27.7 26.7 27.7 28.4 26.6 29.9 32.9 30.6 34.6 40.0 58.9 36.3 31.7 27.8 28.3 26.5 29.5 29.5 25.7 24.2 26.1 24.4

90 151 126 178 151 255 223 214 399 166 73 128 27 100 770 263 334 576 277 219 196 219 97 85 155 118

11.4 19.7 17.1 17.9 17.5 22.0 24.3 24.7 63.1 15.5 19.3 23.4 13.7 36.1 340.0 145.6 155.0 7.6 51.5 40.8 48.0 52.1 39.2 33.1 73.2 64.5

15.6 22.0 15.9 16.1 11.1 24.8 11.5 10.5 10.8 18.4 29.7 22.9 29.3 196 1550 318 130 229 116 85 107 120 89 55 116 74

50.5 70.2 76.6 51.0 52.4 58.2 45.5 72.2 46.2 50.9 78.4 111 165 2128 14500 4927 2381 2511 810 695 949 1087 482 303 877 682

7.6 17.0 8.0 9.1 0.6 10.3 2.2 1.0 3.0 8.2 6.8 19.6 35.9 387.0 , 20 493.4 357.9 37.7 112.6 120.4 73.1 101.0 8.7 8.4 20.9 19.4

1.7 2.5 1.5 1.8 0.0 1.8 0.1 0.0 2 0.3 1.6 1.0 18.6 30.9 108.0 ,3 266.3 430.7 500.9 146.4 134.8 87.2 106.0 25.5 26.0 59.9 39.2

30.7 45.0 19.1 99.4 11.9 158.0 11.5 15.5 201.0 16.0 21.2 11.9 16.4 17.1 , 20 34.6 19.0 13.6 15.5 11.1 13.9 12.5 16.6 15.0 10.6 15.1

110 130 140 110 110 100 100 140 100 120 150 170 250 640 60 350 300 270 360 360 320 330 260 240 230 240

3 63 34 5 35 64 28 62 85 26 14 37 16 69 1410 243 328 474 192 194 234 255 124 114 238 244

2.4 3.3 3.7 5.7 2.6 4.4 2.8 4.7 7.1 4.1 2.3 3.3 2.4 8 72 3.7 12.2 22.8 7.6 8.2 10.2 11.3 6.8 7.1 9.4 12.7

0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.2 0.1 0.2 0.3 0.6 , 0.5 0.2 0.3 0.3 0.2 0.2 0.3 0.3 0.2 0.2 0.2 0.2

1.53 nd 0.51 nd 1.93 2.67 0.91 nd 7.09 1.7 nd nd 6.24 0.85 nd nd 5.56 4.82 nd nd nd nd nd nd 9.65 nd

observed concentrations. However, Ba is probably source-limited being a rare element in basaltic rocks. Water from borehole 15 is near to saturation with barite. In this aquifer unit Sr concentrations are low and no diagnostic trends are seen down-gradient for this element. Lithium together with Rb and Cs do, however, show distinct down-gradient increases (Fig. 9). Lithium has been shown to be a reliable indicator of groundwater residence times (Edmunds and Smedley, 2001) and has been used elsewhere in the Sierra Madre Occidental (Carrillo-Rivera et al., 1996) and the Mexican Trans-Volcanic Belt (Carrillo-Rivera, 1998) to discriminate between groundwaters of thermal and shallow origins. Lithium in the MCAS shows a 10-fold increase from 10±20 mg l 21 to over 200 mg l 21 down-gradient; the abrupt change in Li and some other trace elements is noted, coincident with the onset of reducing conditions. Rubidium increase is about threefold (from 7 to 21 mg l 21) and

Cs about 8-fold from 0.05 to 0.40 mg l 21. Much higher concentrations are seen in borehole 15 (Rb, 145 and Cs, 10.2 mg l 21). These trends indicate that the elements, despite being of low geochemical abundance in basaltic rocks are being leached both with time and with increasing temperature. The halogens F and I also show an increase in concentration along ¯ow lines in the MCAS aquifer (Fig. 9, Table 4). The initial concentrations (about 100 and 2 mg l 21, respectively) are close to atmospheric source values, after allowing for some evapotranspiration and the downgradient increases must represent mixing or reaction, or both. Fluorine concentrations (with the exception of borehole 15) lie below ¯uorite saturation and no solubility control has therefore been reached. The F/Cl and I/Cl ratios show a consistent decrease as Cl increases (Fig. 10), which suggests that the source of Cl is different to that for I and F. The F and I are likely to be released from reaction with apatite. This contrasts with Br which shows no

18

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 10. Plot of molar ratios for Br/Cl, I/Cl, F/Cl, B/Cl against Cl in the Mexico City aquifer.

absolute increase; the Br/Cl slope is an expression of the dilution of the initial ratio by a salinity source lower in bromide. Boron increases signi®cantly along the ¯ow path as shown in Fig. 9. The relationship between B and Cl as shown by the mB/Cl in Fig. 10 remains almost constant with increasing Cl, suggesting that both elements are derived from the same source, but different from the F and I. Borehole 15 also shows the same trend in B/Cl as the shallow groundwater. It is most likely that the internal source of B and Cl are the same and are derived from volatiles most probably trapped and then released during weathering of glass or vesicles in the volcanic rocks.

5. Discussion 5.1. Groundwater origins and overall quality evolution The similar stable isotope signatures, lying close to the local meteoric water line, provide conclusive evidence that the groundwater in the MCAS is derived

from local rainfall. The absence of fractionation suggests that rapid in®ltration with only limited evapotranspiration has occurred. Neither is there evidence of an offset in the isotopic data due to altitude effects, which suggest a well-mixed system. The very low chloride concentrations also support the stable isotope evidence that rainfall recharge represents a major input to the system. Modern rainfall in Mexico City has Cl in the range 0.2±4.0 mg l 21 (CEA, 1995) with a weighted mean value of 0.5 mg l 21. Using a mean Cl value of 5 mg l 21 in the modern groundwaters and assuming this to be all rainfall-derived would give a minimum recharge rate of 10% of rainfall (amounting to 80±100 mm yr 21); other modern, diffuse Cl sources are, however, likely in this suburban location which would allow this estimate to be exceeded. The values of 1±2 mg l 21 in the pre-industrial groundwaters are more likely to represent true atmospheric input values (modi®ed for evapotranspiration) and the difference between the two values might indicate the extent that dust and industrial aerosol ratios may have increased in the modern era. The increased Br/Cl ratios in the most dilute groundwaters are indicative of the source and denote enrichment above marine values from vegetation decay or burning of fuels. The groundwater chemistry of the uncon®ned section of the basin has evolved naturally by recharge from the surrounding highlands. Recharge in the higher fractured volcanics hasprovided an opportunity for development of deep regional ¯ows that contrast in travel time, depth and water±rock interaction with those evolved in the piedmont areas. After travelling at shallow depths water in the piedmont areas ¯ows through the volcanic sequence of the MCAS. Until the modern era discharge would have occurred naturally at springs occurring near the con®ned/uncon®ned boundary through the aquitard or via faults to Lake Texcoco. The evidence of the natural ¯ow system is still well preserved. Despite some disturbances to the groundwater (described below), the residence time at around 20 km along the sampled ¯ow line is 3± 6000 yr. Assuming a natural gradient from west to east towards lake Texcoco a radiocarbon ¯ow velocity of circa 4 m yr 21 is indicated. This value re¯ects the natural ¯ow rates in the con®ned aquifer controlled by upward leakage. Thus the lines of ¯ow suggested by the water chemistry and isotope patterns can be

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

explained by recharge from the western Sierras towards Lake Texcoco. The con®ned part of the aquifer is affected locally by a higher geothermal gradient expressed geochemically by higher Si and some trace elements (such as Li). The thermal water at PenÄon de los BanÄos is also well characterised, for example by high K and Mg/Ca ratios, as well as high B, Li and several other trace elements. From the geochemical trends across the MCAS there is evidence that the PenÄon de los BanÄos type thermal water is likely to be an important source of induced recharge to abstraction boreholes in its vicinity. The groundwater temperatures imply either a variable geothermal gradient across the basin or, more likely, that the water is being drawn up by abstraction, from greater depths beneath the city, as documented from other basins in the Mexican Transvolcanic Belt (Cardona and Carrillo-Rivera, 1998). Hydrogeochemical indicators (the redox boundary, radiocarbon and trace element distributions) express a hiatus between the uncon®ned and con®ned sections of the aquifer. The older, slower moving waters in the con®ned aquifer contrast with a more rapid ¯ow system which would have discharged as springs around the edges of the con®ned basin before abstraction began. Silica geothermometry for borehole 15 can be interpreted with quartz being the main control of dissolved silica. An equilibrium temperature of 1638C at depth is calculated (Fournier, 1981). The mean geothermal gradient across the sub-basin of Mexico City is unknown.Direct temperature logging in nearby borehole PP-1 reports a temperature of 878C at a depth of 1800 m (SHCP, 1969). The computed 3.168C/100 m temperature gradient yields an equilibrium depth of at least 2500 m which is similar to the recorded thickness of the Tertiary±Quaternary sequence (VaÂzquezSaÂnchez and Jaimes-Palomera, 1989). 5.2. The natural baseline and human impacts on water quality The combined evidence from major and trace element, and radiocarbon data shows that the overall geochemistry can be explained satisfactorily through a series of natural processes involving the volcanic sediments and extrusive rocks, which form the

19

MCAS, with groundwater ¯owing towards the centre of the basin and leakage losses (up to the modern era) of freshwater ¯ushing the overlying aquiclude. This has produced the observed baseline chemistry, strongly dominated by contributions from the geological units forming the aquifer. This natural baseline provides insight into the natural processes but also provides a foundation for recognising modern inputs as a basis for future management. In Fig. 11 the chemical variation is summarised in a series of box plots which highlight the median values for the concentrations of each element. These have been ordered according to the element abundance in sea water which is also shown for reference. Several other elements which have been analysed but not included in Table 3 (Ag, Tl for example) are also shown. This plot is a statistical summary and does not discriminate the details of the variations due to geochemical processes which are shown in the downgradient plots. It does, however, provide a good insight into the relative abundance of major and trace elements which are the result of the natural water±rock interaction. No element concentrations in the aquifer are found to exceed limits of potability, with the exception of Mn which has been found above drinking water limits of 0.05 mg l 21. The geochemical results provide little or no evidence of contamination or disturbance of the MCAS resulting directly from urbanisation or drainage of the aquitard as a result of excessive drawdown. This general lack of contamination is inferred from the observed concentrations of relatively mobile and inert elements Cl, NO3 ±N, total P, HCO3, Zn, B, and TOC where the concentrations may most probably be explained by natural occurrence. Of these, the TOC is present at relatively high concentrations in both modern and palaeowaters. One sample (borehole 9) has higher than average TOC and Cl plus high NO3 and Zn; this is possibly the only sample that might be in¯uenced by contamination. The concentration of B which is a good indicator of waste waters are relatively low and conform to the overall pattern that would result naturally from geochemical evolution downgradient. The present study indicates that natural processes control the overall chemistry of the aquifer and that no contamination is discernible in the abstraction boreholes. The possibility of incipient contamination cannot, however, be ruled out on

20

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

Fig. 11. Box plot of all elements determined in the Mexico City Aquifer, arranged in order of abundance in sea water. Box values indicate median concentrations, 75 and 95% concentrations as well as maxima and minima.

present evidence, since very low concentrations may be diluted and unrecognisable. Further sampling with sensitive indicators such as CFCs is needed to help resolve any uncertainty. 5.3. Induced changes in the ¯ow regime In comparison with a predominantly west to east ¯ow gradient which existed until early in the twentieth century, the intensive extraction in the MCAS has displaced the area of lowest hydraulic head towards Mexico City area. Consequently, a new hydraulic state is being achieved broadening the gradients according to the amounts of abstraction. Under natural conditions, groundwater ¯ow beneath the city is expected to move towards its lowest part, i.e. Lake Texcoco. The geochemical trends suggest that although the piezometric gradients have been signi®cantly disturbed, this is not (yet) translated into a signi®cant reversal of quality (mass transfer). Some geochemical indicators of modern recharge (e.g. pollution indicators) were expected in boreholes near the recharge window of the Iztapalapa fractured basalt outcrops within the city limits. From the available geochemical evidence it is implied that any

municipal waste waters that could circulate through these have not yet reached the sampled boreholes. Water balance modelling (Herrera et al., 1982, 1989) suggests that a signi®cant fraction (up to 75%) of the abstracted water from the MCAS is induced into the abstraction area from the aquitard. However, the observed stable isotope and chemical data strongly contradict this conclusion. Observed chemistry of water samples and d 18O and d 2H in the pore water in the aquitard suggest that the abstracted groundwater bear little relation with the former. Water temperature in the aquitard is from 19.9 to 20.48C (Pitre, 1994) and the observed difference of up to 78C with the exploited aquifer water is inconsistent with the conceptual model used in the water balance modelling. Vertical ¯ow upwards is an important component that should be considered in the water balance, mainly where thick aquifer units are involved, and could be satisfactorily traced using temperature variations and salinity (trace elements) response with abstraction time in standard pumping test procedures (Carrillo-Rivera et al., 1996). The con®ned groundwater is therefore considered to be little altered ancient water, disturbed only by borehole abstractions. Reversed horizontal groundwater ¯ow from east to west could be demonstrated

W.M. Edmunds et al. / Journal of Hydrology 258 (2002) 1±24

if water with higher salinity were encountered. Although saline waters overlie the aquifer, created by evaporation of groundwater discharge to form Lake Texcoco, no corresponding salinity increase is noted. Present day groundwater velocity estimates are of the order 40 m yr 21 (hydraulic gradient ˆ 0.00159, hydraulic conductivity ˆ 0:00008 m s 21, effective porosity ˆ 0:1; VaÂzquez-SaÂnchez, 1995). Considering advection as the main transport mechanism and neglecting hydrodynamic dispersion, it is considered that horizontal ¯ow could have been reversed in the area of borehole 25 with ancient water travelling westwards some 1600 m in the last four decades of intensive abstraction in central Mexico City. The role of upward ¯ow from lower horizons has so far received little consideration, although in recent papers this phenomenon has been proposed to explain water chemistry changes in relation with space and time (Panno et al., 1994; Carrillo-Rivera et al., 1996). Observed groundwater temperature and chemical evidence indicate that in the MCAS, regional groundwater ¯ow has been induced vertically into the abstraction area of the city. Isotopic data (Fig. 5) provide further support, since d 18O, d 2H and 14C, as well as Na/Ca, Li and B compositions of water from boreholes with the higher temperatures are similar to those of the deeper water (borehole 15) from PenÄon de los BanÄos. 6. Conclusions Under natural ¯ow conditions groundwater ¯ows from the highlands through fractured volcanic rocks through the continental sedimentary material, beneath the lacustrine aquitard sediments from the highlands, to the discharge area of Lake Texcoco. This ¯ow may be recognised by the well-preserved, progressive hydrogeochemical changes in element ratios, salinity and redox changes resulting from water±rock interaction along the ¯owpath. Groundwaters presently found beneath the city have ages of up to 6000 yr bp and indicates natural radiocarbon ¯ow velocities of around 4 m yr 21; thermal water properties indicate lengthier and deeper ¯ows that have a greater age. This preservation of near-natural baseline conditions is interesting since these groundwaters are found beneath the intensively developed area of one

21

of the largest cities in the world. Little or no evidence of urban waste water is found to date, con¯icting with the pure hydraulic modelling evidence that signi®cant quantities of water being drawn down through the aquitard and are contributing to the ¯ow. However, water drawn from abstraction boreholes in the main body of the aquifer may not detect traces of incipient pollution for which more precise sampling near the top of the aquifer is needed. Higher temperatures in the central part of the ¯ow line suggest either that there are strong variations of heat ¯ow across the basin and/or that up-coning of thermal water is occurring. The latter is the most probable explanation for the additional water required for the water balance. The drawdown produced in the water levels by the intensive abstraction in the area over the past 40 yr has caused a slight reversal of horizontal groundwater ¯ow although this is not recognisable in the water quality. Local groundwater salinisation from the saline waters of the lake is a possible scenario with current aquifer management. However, the geological control by the thick overlying aquitard (in excess of 300 m), the relative low horizontal hydraulic conductivity and the fractured nature of the strata beneath the exploited part of the aquifer indicates that the aquifer is more susceptible to inputs from upward vertical ¯ow; this is water of acceptable quality. The hydraulic resistance for upward water movement via fractures is lower than that required for pure horizontal ¯ow. Available geological evidence is not conclusive regarding vertical ¯ow through fractures. However, in terms of contaminant control and water balance the upward ¯ow appears to be highly signi®cant as compared with the currently held view of leakage through the aquitard as described by Rudolph et al. (1989) and Ortega et al. (1993). In conclusion, although piezometric decline continues, the quality of water in the MCAS remains high, representing mainly the residual water from preindustrial times. So far, modern groundwater ¯owing from the area of Sierra de las Cruces does not present inorganic problems of potability, although under the prevailing aerobic conditions there is little scope for natural de-nitri®cation or remediation. It is proposed that the aquitard presents a more effective barrier to pollution than has previously been noted. It is important to con®rm and monitor this for example by investigating in some detail the water quality in the pore

22

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water pro®les in the lacustrine ®ne-grained sediments beneath the city. The ®rst indicators of any drainage from the metropolitan area might be enriched values of d 18O and d 2H as well as increases in mobile ions such as Cl and B from the aquitard as inferred by Pitre (1994). Speci®c indicators such as CFCs might be used to test these conclusions. The aquitard therefore appears to be an effective barrier which has so far released its pore waters through compaction rather than transmitting water by drainage. The large clay thickness therefore appears to offer the capacity to greatly retard any urban contamination; in fact, high quality groundwater still remains and can be protected by careful management. Such management includes proper borehole design, construction and abstraction rates as to avoid point source in¯ow of surface contaminated water. It is likely that if abstraction could be signi®cantly reduced then a return of piezometric levels might be possible whilst maintaining water of high quality. This situation has occurred in other artesian basins around the world such as London, UK. Here the relaxation of abstraction allowed the water level recovery to take place, although with the added problem of ¯ooding of deep man-made structures such as basements and tunnels (Wilkinson and Brassington, 1991). One of the solutions to the water supply problems of Mexico City once the renewable quantities are more reliably established, might be the enhancement of vertical induced regional ¯ows through the knowledge of the functioning of the prevailing ¯ow system, thus minimising undesirable environmental impacts. The re-injection of treated waste waters (Birkle et al., 1998), might appear to be clearly an alternative solution which needs, however, to be considered under actual environmental, economic, ®nancial, health, social and technical constrains. Acknowledgements We would like to thank the Municipal Authorities for allowing access to the boreholes investigated in the study, and especially to Federico Mooser. We also thank the British Council and CONACyT (Mexico) for the study grant provided through link MXC/991/ 55. In particular we thank Janice Trafford for supervising the analyses of the groundwaters and George

Darling for carrying out the analyses of stable isotopes. The radiocarbon analyses were carried out with the help of Charlotte Bryant in the NERC Radiocarbon Laboratory at East Kilbride (Reference Numbers AA24154-24161). This paper is published with the permission of the Director, British Geological Survey. Natural Environment Research Council. References AIC (Academia De La InvestigacioÂn Cientõ®ca), 1995. El agua y la ciudad de MeÂxico, Comite Binacional: Academia de la InvestigacioÂn Cientõ®ca, Academia Nacional de IngenierõÂa, Academia Nacional de Medicina and National Academy of Sciences. p. 353. Berg, W.W., Crutzen, P.J., Grahek, F.E., Gitlin, S.N., Sedlacek, W.A., 1980. First measurements of total bromine and chlorine in the lower stratosphere. Geophys. Res. Lett. 7, 937±940. Birkle, P., Torres Rodriguez, T., Gonzalez Partida, E., 1998. The water balance for the Basin of the Valley of Mexico and implications for future water consumption. Hydrogeol. J. 6, 500±517. Cardona, B.A., Carrillo-Rivera, J.J., 1998. SituacioÂn hidrogeoloÂgica de las cuencas de San Luis PotosõÂ. Aguascalientes e Hidalgo como referencia conceptual del funcionamiento del agua subterraÂnea en la regioÂn centro del paõÂs. Simposio Internacional de Aguas SubterraÂneas. LeoÂn, Guanajuato (Diciembre 7±9), pp. 69±83. Cardona, A., HernaÂndez, N., 1995. Modelo geoquõÂmico conceptual de la evolucioÂn del agua subterraÂnea en el valle de MeÂxico. IngenierõÂa HidraÂulica en MeÂxico 10, 71±90. Carrillo-Rivera, J.J., 1998. Monitoring of exploited aquifers resulting in subsidence, example: Mexico City. In: Van Lanen, H.A.J. (Ed.). Monitoring for Groundwater Management in (Semi-)Arid Regions. Studies and Reports in Hydrology No. 57UNESCO, pp. 151±165. Carrillo-Rivera, J.J., Cardona, A., Moss, D., 1996. Importance of the vertical component of groundwater ¯ow: a hydrogeochemical approach in the valley of San Luis Potosi, Mexico. J. Hydrol. 185, 23±44. Carrillo-Rivera, J.J., Cardona, A., Hergt, T., Huizar, A.R., Kobr, M., 1999. Marco geoloÂgico, hidrologõÂa subterraÂnea, hidrogeoquõÂmica, anaÂlisis geomorfoloÂgico y registros de temperatura en la subcuenca del rõÂo de las Avenidas. Final Report, CAASIM, Hidalgo, Mexico, 268 p. CAVM (Comision De Aguas Del Valle De Mexico), 1966. Datos del Valle de MeÂxico, PerõÂodo 1959±1963. BoletõÂn de MecaÂnica de Suelos 4 MeÂxico. CEA, 1995. Analisis de agua de lluvia. Centro de estudios de la atmosfera, UNAM. Unpublished data. Cortez, A., Farvolden, R.N., 1989. Isotope studies of precipitation and groundwater in the Sierra de las Cruces, Mexico. J. Hydrol. 107, 147±153. De Cserna, Z., De la Fuente-Duch, M., Palacios-Nieto, M., Triay, L., Mitre-Salazar, L.M., Mota-Palomino, R., 1987. Estructura geoloÂgica-gravimetria sismicidad y relaciones neotectoÂnicas

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