I
n the United States and parts of Europe, groundwater contamination is recognized as a major environmental problem. Similar attention has yet to be paid to this issue in many countries worldwide, despite the fact that their groundwater is or soon will be a crucial resource. In Mexico, for example, very little information has been published on the quality of groundwater or the significance of existing or Dotential threats to it. The goals of this paper are, first, to summarize public knowledge regarding the groundwater resource and potential sources of contamination in the Mexico City area, a n d , s e c o n d , to make recommendations for actions to improve the understanding and protection of the resource based on the f i n d i n g s of p u b lished studies from other parts of the world.
-~
(2). The current water use in the MCMA is approximately 60 m3/s (3, 4 ) , and demand is increasing (Figure 2). Approximately 45 m3/s, or 75% of the total water used, are drawn from an aquifer system that underlies the lacustrine plain (4-8). An additional 15 m3/s are pumped from two distant basins (4).Because water from such outside sources is limited and expensive, proper management a n d protection of t h e groundwater resource beneath the
MCMA is critical to the future of its inhabitants.
The groundwater resource As illustrated in Figures 3(a)and 4, three important zones can be distinguished in the Basin of Mexico: the lacustrine, the transition, and the mountainous (9-23). The mountainous area, the product of volcanic activity, directs water from precipitation toward the central part of the Basin, either in surface runoff or in subsurface flows. 1 The lacustrine clay deposits are presedt in upper and lower formations, 30 to 70 m thick, and are divided by a hard layer (Capa Dura) composed predominantly of silt and sand (20). The clay layer is considered an aquitard because it is considerably less permeable than the Capa Dura or underlying sediments. The area between the lacustrine clays and the mountains is k n o w n as t h e transition zone. The boundary between the lacustrine and the transition zones is generally defined as the edge of the t h i c k u p p e r clay formation. In much of t h e t r a n s i t i o n zone, clays, if present at all, are interbedded with silts and sands. In areas close to the volcanoes, the transition zone is comprised of fiactured basalts. In general, the surficial media in thetransition zone have a relatively high permeability as compared to that of the lacustrine clays. Thus, the majority of the recharge of the aquifers occurs through - the transition zone
POTENTIAL FOR
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CONTAMINAT10N
The metropolitan area Mexico City is located i n an originally closed hydrologic basin, which was opened artificially i n the late 1700s. Figure 1 depicts the Basin of Mexico, which encompasses the Distrito Federal and Darts of several states iMexico, Hidalgo, Tlaxcala, and F'uebla). Before the rise of the Aztec Empire (100 A.D.), a series of lakes covered approximately 1500 km2 in the Basin. Now a highly urbanized area, hereafter called the Mexico City Metropolitan Area (MCMA), covers much of the lacustrine (lake-related) sediments and parts of the surrounding mountains. As shown in Figure 2,the population of the MCMA approached 20 million in 1990 (I) and is predicted to reach 25 million by the year 2000 794 Environ. Sci. Technol., Vol. 27, No. 5, 1993
MARISA MAZARI Universidad Nacional Aut6noma de M6xico Mexico City, Mexico
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D 0U G L A S M M A C K A Y University of Waterloo Waterloo, Ontario, Canada
f 9.~~, 101. ~~.
The main aquifers in the Basin of Mexico are composed of alluvial and volcanic material ranging in thickness from 100 to 500 m (20, 23). The aquifers were subject to artesian pressure in the past, with
0013-936W93/0927-79404.00/0 0 1993 American Chemical Society
Historically, the baslII Mexico has been one of 1 it densely populated - 1 the world
overall hydraulic gradients and water flow in an upward direction through the overlying clay aquitards. However, the hydrologic regime was impacted significantly by the utilization of groundwater resources. Now the gradients and flow in the upper part of the Basin deposits are generally downward, toward heavily pumped zones (IO, 13. 1 4 ) . As shown in Figure 3(b), the aquifers in the Basin are currently exploited through several well fields: Xochimilco-TlBhuacChalco, Zona Metropolitono, Lago de Texcoco, and Teoloyucan-Tizayuca-Los Reyes-Chiconautla (25). The names in italic are used for brevity in Figure 3(b) and subsequent discussion. Pumping of wells i n the Zona Metropolitana began i n 1847. By 1925, groundwater extraction had led to subsidence in Mexico City of approximately 1.25 m (16).In 1940, 150 deeper production wells were installed, which accelerated subsidence in the city. In 1954, use of most of the Zona Metropolitana wells was stopped and new wells were installed to exploit other portions of the aquifer system. The current extraction rates within the aquifer subsystems are as follows: Xochimilco,
27 m3/s; Zona Metropolitana, 8.3 m3/s; Texcoco, 5.2 m3/s; and Chiconautla, 4.5 m3/s (4). Although
these changes reduced subsidence in the central MCMA, they have increased subsidence in the ChalccXochimilco area (4. 17, 18). At present there are approximately 1200 registered wells, of which about 40% are used only occasionally during the dry season. The wells draw from depths of 70 to 300 m, as illustrated schematically in Figure 4. Currently, the groundwater extraction rate, 45 m3/s, is much higher than the rate at which the aquifer is replenished by natural recharge (the estimated yearly total, most of which is received during the rainy season, is equivalent to a constant rate of 25 m3/s) ( 4 , 7, 291. This high rate of overexploitation is the major groundwater management problem in the MCMA. It is important to recognize, however. that the problem is compounded by the threat of contamination. Aquifer vulnerability Potentiallv the most important route for traLsport of contaminants into the aquifers is through the surficial materials in the transition
zones. It is, after all, through these zones that a majority of the recharge of the aquifers occurs. Contaminants released to the subsurface in the transition zones could migrate themselves or be carried down by infiltrating water toward the aquifers. Later, we will discuss what is known about contaminant sources in the transition areas. The importance of these sources and this potential route of aquifer contamination are considerable because many of the supply wells now draw from zones within or near the transition zone (Figure 3(a)]. Although the vulnerability of the aquifer in the transition zone is obvious, there are other potential areas of vulnerability. These are areas within the lacustrine clays, which may have higher permeability than is aenerally assumed. The ciays that oGerlie much of the aquifer in the Basin of Mexico have been considered an effective barrier to downward migration of water and surface contaminants. Thus the main aquifer generally has been considered hydrogeologically closed to contamination that orieinates in the lacustrine area (5,6 , I z ) . The reliance on the lacustrine clays to act as an efficient barrier to contamiEnviron. Sci. Technol., VoI. 27, No. 5. 1993 795
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nants is based in large part on the assumption that the clays act hydrogeologically as relatively homogeneous, impervious units. However, there are at least two ways by which the integrity of the clays may have been breached: human activities and natural fracturing. Engineered breaches of the clays. Over the years a wide variety of ac796 Environ. Si. Technoi.. Voi. 27. No. 5, 1993
de Hidalgo
7,
tivities in the Basin, such as drilling, excavating, and pile driving, have penetrated all or a portion of the lacustrine clays. The most ohvious complete penetration of the clays occurs when a well is installed into the underlying aquifer. An issue of potential importance is the downward migration of water and contaminants along the outside
of wells that were installed without the now common procedures (grouting and sealing) to prevent this problem. If significant at all i n the MCMA, this problem is likely to be limited to the older wells because drilling controls have been strict in the past 40 years. A more important aspect of some of the older wells may be that they were retired from use but their interiors not sealed. Thus, if an old water supply well has been cut off or covered during construction of new surface facilities and its top is left open near the surface, the well may act as a drain, drawing water and contaminants directly into the aquifer. Abandoned wells have been identified as potentially significant short circuits for contaminant migration through aquitards in some areas of the United States, particularly where urbanization has occurred in previously agricultural areas, such as the Santa Clara (“Silicon”) Valley in California (20,21). Other engineering activities are likely to have affected the hydraulic integrity of the lacustrine clays in the MCMA. To our knowledge, no overview of this topic is available, but the potential importance is illustrated by two examples. A deep drainage system and a subway system were installed in the MCMA, both of which required extensive excavations into the clays. The vulnerability of the aquifer to contamination could be increased for the following reasons: (1) the excavations completely penetrate the clays at any point, which is known to be true for portions of the deep drainage system, (2) water and contaminants are able to move downward along the exterior of the buried structures, or (3) water and contaminants in the interior of the structures are able to leak out in significant quantities. Although these issues were likely to have been taken into account during the design of both the subway and the deep drainage systems, they warrant reexamination considering the aging of the installations and the damage that may have occurred because of earthquake activity in the Basin. Natural fracturing of the clays. Recent studies suggest that cracks and fractures exist in the clays in various parts of the Basin, for example, in the Lago de Texcoco, along Rio Chnrnbusco, and in the Valley of Chalco (22,23). Surface cracking is known to result from the subsidence process (24, 25). In addition,
fractures in the upper clay formation may, under some circumstances, develop as a result of ponding from the first heavy rains following the dry season (22). It is important to know how deep the fractures go and whether they
persist. Theoretical calculations (personal communication, J. AIberro, Instituto de Ingenieria, Universidad Nacional Aut6noma d e Mexico, 1991; 26) predict that fractures in the clay would be closed at some depth because of compression
and the plastic characteristics of the clay, provided they are not filled with other materials. In the Texcoco area of the MCMA, Rudolph et al. (23)observed fractures from the surface, in shallow test pits, and in continuous core samples taken to
Environ. Sci. Technol.. Vol. 27, NO. 5. 1993 797
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Generalized section of the Basin of Mexico
I Volcanic rocks Permeable sands, gravels, etc.
depths of approximately 10 m. Many of the fractures were filled with silt and sand of apparent aeolian origin. Based on matching of measured geochemical profiles with computer simulations, Rudolph et al. ( 2 3 ) concluded that transport of chemicals through the upper clay formation in the Lago de Texcoco area occurs more rapidly than would be expected for the unfractured clay. Thus, they concluded that chemical migration in the clay aquitard is dominated by water movement through the fractures. These findings are consistent with an increasing body of evidence on the importance of fractures to solute transport through clay deposits in the United States and Canada (27-31). It is now clear that in many areas clayey deposits that were viewed as long-term protection of underlying aquifers may not be pro788 Enviran. %I. Technol., Val. 27, NO.5,1993
II
viding complete protection ( 3 2 ) . Whether contaminant transport through such media may lead to significant alteration of the quality of the underlying groundwater resonrce depends on various factors including the extent and interconnection of fracturing: the hydraulic gradients through the clays: the size and properties of the aquifer: the locations of extraction wells: and the locations, types, and amounts of contaminants released by human activity. Considering the irreplaceable nature of the MCMA's groundwater resource, it would seem crucial for investigations to be conducted to increase understanding of these factors. Contaminant sources in the MCMA Based on our review, the following are some contaminant sources that warrant continued or increased
Production wells
attention in the MCMA. It is certain that there are others of importance, but this summary may illustrate the range of issues to be addressed. Landfills. Mexico City generates approximately 12,000 tons of solid waste per day, 48% of industrial origin and 52% domestic (33). The locations of the main landfills in the MCMA are indicated in Figure 3(c). The landfills were originally outside of the urban zone, but the city has expanded to surround them. Some industries store portions of their solid wastes on their own property, though information on this is not readily available. Other clandestine dump sites also exist in the urban area, but most are believed to receive mainly domestic waste. Some c a n y o n s a n d old mines i n the mountainous areas contain noncompacted fill or garbage (9).
The main landfills were designed for municipal waste, and thus are not lined or outfitted specifically for containment of hazardous waste. Nevertheless, all of them have received mixed domestic and industrial wastes. Two of the main landfills, which were closed in 1983 and 1986, have been under investigation by the Departamento del Distrito Federal (DDF) for several years. As found at similar facilities worldwide (e.g., 34, the landfills are releasing to the subsurface leachate containing a complex mixture of contaminants (DDF, unpublished data). At one site the contamination is moving through permeable fractured basalts; at the other the migration is apparently occurring through fractured clay. At least one of the landfills remaining in operation is located i n the transition zone, where there are minimal natural barriers to subsurface migration. Although the importance of the existing or potential contamination from landfills in the MCMA has yet to be addressed in documents that are available to the public, this issue clearly warrants continued attention. Petroleum refining, transport, and storage. In the MCMA, a likely source of significant subsurface contamination is the petroleum refinery (Refinerfa 18 de Marzo), that was operated by PEMEX (Petrbleos Mexicanos) from 1948 until 1991. As illustrated in Figure 3(c), the refinery is located in the transition zone. The potential for release of significant quantities of petroleum products from such facilities is best exemplified in Mexico by the explosion in Guadalajara in early 1992, which apparently resulted from gasoline released into sewers by a PEMEX facility. Such releases may contaminate soils and groundwater (35). This is a common problem at U.S. refineries, particularly the older ones, where releases of petroleum products have occurred because of leaking tanks, pipes and other equipment, as well as spills. To assess whether a significant problem exists at the PEMEX facility in the MCMA, it is necessary to conduct soil and groundwater surveys such as have been conducted at many U.S. sites. One of the first goals of such studies should be to determine whether the aquifer is vulnerable to contamination in the area of the refinery. The location of the refinery in the transition zone raises the possibility that the natural subsurface barriers to contami-
nant infiltration may be discontinuous and relatively ineffective. In addition to considering the refinery itself, such surveys should include the pipelines that were used to transport petroleum products in liquid and gaseous form to and from the refinery, especially where they cross the transition zone or areas in which t h e lacustrine clays are deeply fractured. Gasoline stations. After the 1992 explosion in Guadalajara, a program was initiated to identify and correct leaks at gas stations in the MCMA. In investigations to date, leaks have been identified in approximately 80% of the 253 PEMEX stations in the Distrito Federal; a major program for repair of the gas stations a n d replacement of the underground tanks is now under way (personal communication, J. M. Lesser, Lesser y Asociados, S. A,, QuerBtero, MBxico). Further work will be necessary to determine whether the leaks have led or will eventually lead to significant impacts on the underlying groundwater, but it is quite reasonable to expect that, in at least some of the cases (notably gas stations in the transition zones), petroleum hydrocarbons have been able to penetrate into the subsurface in significant quantities and to significant depths. Electronics industries. Electronics industries typically use large quantities of synthetic halogenated organic solvents. In the United States, many users of halogenated solvents (dense nonaqueous phase liquids, or DNAPLs) have discovered that significant amounts of the solvents have been released through spills and leaks. The solvents may, under some circumstances, infiltrate into subsurface media to significant depths (35, 36-38), subsequently dissolving slowly into the passing groundwater. At many sites, this process has led to the contamination of billions of gallons of groundwater (36, 38). In the MCMA there are on the order of 100 manufacturers of electronic products, most of them concentrated in the northern section (39).Some of these electronics manufacturers appear to be located in the transition area, with little or no surficial clays to protect the underlying aquifer. High priority should be given to the investigation of these industries, because elsewhere very significant contamination of groundwater has often been discovered to result from seemingly small releases of solvents (36).
Other commercial and industrial sources. Experience in the United States and other countries suggests that a variety of other commercial and industrial activities may have released significant quantities of contaminants to the subsurface. Releases of halogenated solvents in DNAPL form are likely from drycleaning establishments (which in Mexico use tetrachloroethylene) and facilities that clean and degrease metals (such as metal fabricators, train yards, subway facilities, and airports). Releases of significant quantities of petroleum hydrocarbons (light nonaqueous phase liquids, or LNAPLs) can be expected from facilities that store or use fuels (e.g., airports and train yards). A variety of sources of inorganic contaminants may also pose threats. For example, investigations by Mexican scientists have shown that a chromate plant that disposed of a large amount of residues on its property has caused serious local groundwater contamination problems in the northern part of the MCMA(40). Efforts should be made to identify other such potential sources of organic and inorganic contaminants in the MCMA and to determine whether they are located in areas of high aquifer vulnerability (e.g., the transition zone or in areas of significant fracturing of the lacustrine clays). For example, based on the information readily available, the Mexico City International Airport is located in the lacustrine zone (Figure 3). Thus, it might be expected that the aquifer underlying the airport would be reasonably well protected from the organic and other contaminants that are likely to have been released from the airport-unless the surficia1 clays are significantly fractured. Only site-specific field studies can allow further assessment of the potential for groundwater contamination from this and other similarly situated sources. Wastewater disposal. In the MCMA, 26% of the population has no sewer service (42); in these areas wastes are disposed of locally, often in cesspools, septic tanks, or openpit latrines. As is evident by comparing Figure 1 and Figure 3(a), considerable areas of the transition zone are populated. Thus, there is the potential for microbial pathogens a n d other contaminants present in the domestic waste to migrate from the surface downward through relatively permeable media Environ. Sci. Technol., Vol. 27, No. 5,1993 799
toward the aquifer. Whether this is a significant problem is unknown. The wastewater system of the MCMA is illustrated in Figures 3(d) and 4. The system includes three main unlined sewer canals (Gran Canal del Desagiie, Rio de 10s Remedios, and Canal Nacional), sewers, reservoirs, lagoons, pumping stations, and the Deep Drainage System (42-44). The wastewaters flow toward the north eventually into the Requena and Endhd reservoirs. Some of the wastewater is used for irrigation in the state of Hidalgo. " Thirest flows toward the Gulf of Mexico t h r o u g h t h e TulaMoctezuma-PBnuco River system. The domestic waste-
Because the canals are unlined in large part, water and contaminants could migrate from the canals down toward the aquifers. This is of special concern in the northern part of the Basin where the canals traverse the transition zone [Figures 3(a) and 3(d)]. To date, no monitoring has b e e n c o n d u c t e d to d e t e r m i n e whether significant migration of contaminants has occurred from the canals in the transition zone, an activity that would appear to be of high priority considering the proximity of supply wells in that area.
30 to 50 m and operating by gravity (42, 44). In the central area of the MCMA, this system is constructed primarily in the lacustrine clays, although parts required excavation into the aquifer. As the system leaves the Basin to the north, the tunnels cross the transition zone. The Deep Drainage System is thought to be leak proof, but the occasional necessity of repairs during the dry season suggests that shortterm leaks are possible. To our knowledge there is no monitoring conducted to determine whether the Deep Drainage System has released significant amounts o f contaminants to the aquifer or transition zone.
THERE is VERY LITTLE INFORMATiON ON
storm wat er runoff. More than 90% of the liquid industrial wastes, which total approximately 1.5 million tons annually, are discharged untreated to the sewer system ( 4 5 ) . The characteristics of these wastes are poorly understood, but given the variety of industries in the MCMA it is likely that some of t h e liquid wastes discharged contain hazardous chemicals. The sewer pipes are likely to leak, either because of improper installation or, more likely, deterioration or disruption by building activities, subsidence, and earthquakes. Contaminants in wastewater leaking from sewers in the transition and mountainous areas might be transported toward the aquifers by the wastewater itself or by infiltrating precipitation. Some of the sewer pipes empty or are pumped into a system of open canals. The Gran Canal del Desagiie was constructed at the beginning of the century. Originally, wastewater flowed by gravity along the Gran Canal, but subsidence of the city has led to the need for pumping stations along the canal. The Rio de 10s Remedios, originally a natural river that crossed the northern part of the MCMA, has for several decades been used to carry domestic a n d industrial wastewater. There is no man-made lining beneath either of these canals, except at junctions and other sites subject to erosion. 800 Environ. Sci. Technol., Vol. 27,No. 5,1993
THE BASIN OF MEXICO The potential for subsurface contamination by the canals also exists in the lacustrine area because there are significant downward hydraulic gradients, which are likely to increase over time (18, 46). In fact, recent research at the confluence of the Gran Canal del Desague and the Rio de 10s Remedios appears to confirm that infiltration of contaminated water has begun (46, 47). Tritium and several organic contaminants (halogenated solvents and surfactants) have been detected at low concentrations at depths which, although relatively shallow at this time, are consistent with migration dominated by fracture flow ( 4 6 , 4 7 ) . Indeed, fractures have been observed in cores at this site to a depth of 15 m ( 4 6 ) . Because the downward hydraulic gradients are expected to increase with time, the progress of this infiltration would be expected to accelerate. These findings underscore the need to replace these two open sewer canals with pipes, which has been planned to occur between 1992 and 1995. During the 1970s the Deep Drainage System was built to allow disposal of flood waters during the rainy season [Figure 3(d)]. Today the system handles both runoff and wastewater during the rainy season. The system is composed of three large (3 to 5 m diameter) tunnels constructed at depths ranging from
contamination in the MCMA, certainly too many to investigate simultaneously given the economic constraints in Mexico. However, these sources can be prioritized with some confidence by considering the hydrogeology in the MCMA as well as conclusions from years of groundwater investigations elsewhere. For example, it is clear that a first step in setting priorities should be to prepare more precise maps of aquifer vulnerability and contaminant sources throughout the Basin. Where the two overlap significantly, monitoring activities should be undertaken as soon as possible to determine whether significant contamination has already occurred. To date, extraction wells in the MCMA have generally been used to monitor the groundwater quality. Studies in the United States have illustrated that monitoring of supply wells is a poor way to detect the onset of groundwater contamination, in part because supply wells tend to draw from deeper portions of aquifers and often from very wide vertical intervals (37). Thus, when contamination is noted in supply wells, it is likely that it has already affected relatively large volumes of the subsurface. In the United States and elsewhere, smaller diameter and shorter screened monitoring wells are used to pinpoint contaminant sources and provide information on contaminant distribution and water flow. With this more detailed un-
derstanding, it has often been possible to halt the expansion of the contaminant plume before supply wells are affected. For these reasons, there should be increased emphasis in the MCMA on the installation of monitoring wells, particularly near probable sources of significant contamination in areas of high aquifer vulnerability, such as the transition zone or deeply fractured portions of the lacustrine clays. Of apparently high priority is the investigation of electronics manufacturing industries or other users of halogenated organic solvents located in or near the transition zone in the northern part of the MCMA. Other potentially important contaminant sources, which have been under investigation by DDF, are the landfills located on permeable soils. A third and major potential source is the sewage system. Because the sewage system conveys considerable amounts of domestic and industrial wastes, the possibility that the sewers and unlined canals release significant amounts of contaminants into t h e subsurface should be evaluated by monitoring activities where the potential for downward migration is high. Although the potential for the release of various contaminants to the subsurface in the MCMA is very high, it does not necessarily follow that for each class of contaminants or for each site the threat to the groundwater resource is significant. Recent research suggests that many of the more mobile petroleum compounds may degrade during migration in the subsurface, especially if sufficient oxygen is available (e.g., 471. This finding appears to explain why, although petroleum contamination of US. soils and groundwater is a serious and continuing problem near contaminant sources, the petroleum hydrocarbons are relatively rarely found in significant concentrations in public supply wells (37, 48). There are, however, cases in the United States in which the petroleum hydrocarbons persist to form long plumes of contamination that do affect supply wells. Field and laboratory studies should be conducted in the MCMA to determine whether petroleum hydrocarbons pose a significant current or future risk to the groundwater resources-that is, to determine whether the aromatic compounds have appeared in wells or formed persistent groundwater plumes. If so, then the sources should remain high priorities for additional inves-
tigation. If not, these sources may be deemed of lower priority for the time being. Within the past decade, there has been a tightening of environmental legislation in Mexico and a promulgation of more restrictive regulations for drinking water: for drinking water supply sources: for disposal of industrial wastewater and liquid hazardous waste: and for the design, construction, and operation of landfills. These are encouraging steps, which will undoubtedly prove beneficial over the long term. However, the regulatory infrastructure in Mexico is unlikely to be able to implement these regulations effectively or completely for some time. Furthermore, the new regulations cannot reverse the contamination that has already occurred. Thus, as noted above, concerted efforts are needed to locate and address the hot spots of existing contamination, perhaps preventing further migration of contaminants and degradation of the groundwater supply.
Summary Groundwater in the MCMA is a critical resource that is known to be overexploited and likely to be vulnerable to, or already affected by, contamination from a wide variety of sources. Given the known exploitation, past studies have focused on the quantity of water available. There is very little information on present or potential contamination of the soils and groundwater in the Basin of Mexico. Apparently there have been some efforts over the past few years to assess the quality of the groundwater resources, but neither the results of these efforts nor their implications have been made public. There exists no comprehensive examination and comparison of the potential routes by which contaminants may reach the groundwater resource in the MCMA. This paper is a first step toward that goal, but without a much more extensive overview, it will be difficult to set reasonable priorities for actions within the MCMA to protect the aquifers. Nevertheless, because the MCMA has much in common with metropolitan areas throughout the world, it is very likely that soil and groundwater contamination similar to that observed elsewhere has occurred and continues in the MCMA. Thus the lessons drawn from experience elsewhere are very likely to apply to the MCMA. Historically, the Basin of Mexico has been one of the most densely
populated areas in the world. The area has survived several population declines over the past 2000 years, each brought on in part by health or environmental crises ranging from exhaustion of cropland to epidemics of new diseases brought by Europeans. In recent years, the MCMA has been assaulted by new problems, such as contamination of air and water, which have accompanied industrialization. In its response to groundwater contamination, Mexico may save valuable time and resources by drawing from the best approaches that have emerged through costly and time-consuming trials in other countries throughout North America, Europe, and Asia. Acknowledgments Many people i n Mexico, the United States, and Canada contributed generously of their time, information, and ad-
M u r i s a M a z a r i i s an associate researcher in the Centro de Ecologia at the Universidad Nocional Autdnoma d e M6xico (UNAM). She has degrees in biology from UNAM, in applied hydrobiology from the University of Wales Institute of Science and Technology, and in environmental science and engineering from UCLA. For the past 10 years she has researched water contamination problems in Mexico.
Douglas M. Mackay is adjunct professor in the Centre for Groundwater Research at the University of Waterloo, Ontario, Canada, ond a visiting scientist at the UCLA Deportment of Civil Engineering. His research focuses on transport and fate of organic chemicals in groundwater and groundwater decontamination technologies. He received a B.S. degree in engineering and an M.S. degree and Ph.D. in civil engineering from Stanford University. Environ. Sci. Technol., Vol. 27, No. 5. 1993 801
vice. In Mexico, this includes J. M. Lesser (Lesser y Asociados, S.A.) and, from the Universidad Nacional A u t 6 noma de MBxico (UNAM), J. Sarukhtin, C. Cortinas, R. Iturbe, S. GonzBlez, A. Noyola, C. Cruickshank, I. Herrera, A. CortBs, J. Durazo, D. Piiiero, and E. Ezcurra. In the United States, this includes W. Glaze (University of North Carolina) and D. Perry (University of CaliforniaLos Angeles). In Canada, this includes C. Pitre, A. Ortega, R. Farvolden, J. Cherry, and D. Rudolph at the University of Waterloo. This work was funded i n part by t h e Hewlett F o u n d a t i o n through its support of the UCLA Environmental Science and Engineering Program; the UC MEXUS Consortium Grant for Dissertation Research; the UCLA Latin American Center Program on Mexico; the Centro de Ecologia, UNAM; and the University of Waterloo. Part of this work was funded by scholarships to M. Mazari from Consejo Nacional de Cienc i a y Tecnologfa, UNAM, a n d UC MEXUS.
References Resultados Preliminares, X I Censo General de Poblacidn y Vivienda; Instituto Nacional de Estadfstica, Geograffa e Informitica: Mexico City, 1990; p. 75. Cabrera, G. In Problemas d e l a Cuenca de MBxico; Kumate, J.; Mazari, M., Eds.; El Colegio Nacional: Mexico City, 1990; pp. 31-60. Departamento del Distrito Federal. Estrategia Metropolitana para el Sistema Hidrdulico del Valle de M6xico; Departamento del Distrito Federal y Gobierno del Estado de MBxico: Mexico City, 1989. Mazari, M.; Mazari, M.; Ramirez, C.; Alberro, J. In Volumen Rad1 J, Marsal; Ovando, E. et al., Eds.; Sociedad Mexicana de Mecdnica de Suelos, A. C.: Mexico City, 1992; pp. 37-48. Secretaria de Agricultura y Recursos Hidriulicos. Estudio para evitar la contaminacidn del acuifero del Valle de MBxico; Durazo, J.; Farvolden, R. N., Eds.; Comisidn de Aguas del Valle de MBxico: Mexico City, 1988. Herrera, I.; CortBs, A. Ingenieria Hidrdulica en MBxico; Comisidn Nacional del Agua: Mexico City, 1989; pp. 60-66. Murillo, R. In El Subsuelo de la Cuenca del Valle de MBxico y s u Relacidn con la Ingenieria de Cimentaciones a Cinco Aiios del Sismo; Ovando, E.; Gonzilez, F., Eds.; Sociedad Mexicana de Mecinica de Suelos, A. C.: Mexico City, 1990; pp. 109-18. Distribucidn de Pozos en la Zona Metropolitana de la Ciudad de MBxico; Unpublished map; Departamento del Distrito Federal: Mexico City, 1991. Secretaria General de Obras; Manual de Exploracidn GeotBcnica; Departamento del Distrito Federal: Mexico Citv, 1988. (10) Makal, R. J.; Mazari, M. El Subsuelo de la Ciudad de M6xico,2nd ed.; Congreso Panamericano de Mecdnica de 802 Environ. Sci. Technol.,Vol. 27, No. 5, 1993
Suelos y Cimentaciones; Facultad de Ingenieria, Universidad Nacional Autdnoma de MBxico: Mexico City, 1969. (11) Marsal, R. J.; Mazari, M. Series Instituto de Ingenieria No. 505; Univer-
sidad Nacional Autdnoma de MBxico: Mexico City, 1987. (12) Marsal, R. J.; Mazari, M. In El Subsuelo de la Cuenca del Valle de M6xico y su Relacidn con la Ingenieria de Cimentaciones a Cinco Aiios del Sismo; Ovando, E.; Gonzilez, F., Eds.; Sociedad Mexicana de Mecinica de Suelos, A. C.: Mexico City, 1990; pp. 3-25. (13) Ortega, A.; Farvolden, R. N. J. Hydrol. 1989, 220,271-94. (14) Durazo, J.; Farvolden, R. N. J. Hydrol. 1989, 112, 171-90. (15) Geohidrologia del Valle de Mdxico;
Lesser, J. M., Ed.; Departamento del Distrito Federal: Mexico City, 1987; contract 7-31-1-0211. (16) Gayol, R. Revista Mexicana de Ingenieria y Arquitectura, 1933, XI. (17) Mazari, M.; Alberro, J. In Problemas de la Cuenca de MBxico; Kumate, J,; Mazari, M., Eds.; El Colegio Nacional: Mexico City, 1990; pp. 83-114. (18) Ortega, A,; Cherry, J. A.; Rudolph, D. L. Ground Water, in press. (19) Ramirez, C. In Problemas de l a Cuenca de M6xico; Kumate, J.; Mazari, M., Eds.; El Colegio Nacional: Mexico City, 1990; pp. 61-82. (20) Mackay, D. M.; Roberts, P. V.; Cherry, J. A. Environ. Sei. Technol. 1985, 29(5), 384-92. (21) Santa Clara Valley Integrated Envi-
ronmental Management Project. Stage Two Report; Hinman, K. et al., Eds.; Office of Policy, Planning, and Evaluation. U.S. Environmental Protection Agency: Washington, DC, 1987. (22) Alberro, J.; Hernindez, R. In El Subsuelo de la Cuenca del Valle de M6xico y s u Relacidn con la Ingenieria de Cimentaciones a Cinco Arios del Sismo; Ovando, E.; Gonzilez, F., Eds.; Sociedad Mexicana de Mecdnica de Suelos, A. C.: Mexico City, 1990; pp. 95-108. (23) Rudolph, D. L.; Cherry, J. A,; Farvolden, R. N. Water Resour. Res.
1986,1,65-76. (31) D’Astous, A. Y. et al. Canadian Geotechnical Journal 1989, 26, 43-56. (32) Cherry, J. A. In Proceedings of the In-
ternational Symposium on Contaminant Transport in Groundwater; Stuttgart, Germany, 1989; 11. (33) Departamento del Distrito Federal. Direccidn General de Servicios Urbanos: Direccidn de Desechos S61idos; Unpublished report. Mexico City, 1990. (34) Bramlett, J. et al. “Composition of Leachates from Actual Hazardous Waste Sites’’; U.S. Environmental Protection Agency. Office of Emergency and Remedial Response: Washington, DC, 1987; EPA/600/2-87/043, (35) American Petroleum Institute. The Migration of Petroleum Products in Soil and Ground Water. Engineering and Technical Research Committee: Washington, DC, 1972. (36) Mackay, D. M.; Cherry, J. A. Environ. Sei. Technol. 1989, 23(6), 630-36. (37) Mackay, D. M.; Smith, L. A. In Regional Ground Water Quality; Alley, W., Ed.; Van Nostrand Reinhold: New York, 1992; chapter 13. (38) “Dense Nonaqueous Phase Liquids-A Workshop Summary”; Office of Research and Development. U.S. Environmental Protection Agency: Washington, DC, 1992; EPA/GOO/R92-030. (39) Islas, P. La
Industria EldctricaElectrdnica en la Zona Metropolitana de la Ciudad de M6xico: Un Andlisis Ambiental; Tesis de Licenciatura (Biologia); Universidad Nacional Autdnoma de MBxico; Mexico City, 1992; p. 59. (40) GutiBrrez-Ruiz, M. E.; VillalobosPefialosa, M.; Miranda, J. A. Rev. Int. Contam. Ambient. 1990, 6, 5-18. (41) Atlas de la Ciudad de MBxico; Departamento del Distrito Federal y El Colegio de MBxico: Mexico City, 1987. (42) Memoria de las Obras del Sistema de
23-43. (29) Keller, C. K.; van der Kamp, G.;
Drenaje Profundo del Distrito Federal, Vol. I-IV; Talleres Grificos de la Nacidn; Departamento del Distrito Federal: Mexico City, 1975. (43) El Sistema Hidrdulico del Distrito Federal; Direccidn General de Construccidn y Operacidn Hidriulica; Guerrero, G.; Moreno, A,; Gardufio, H., Eds.; Departamento del Distrito Federal: Mexico City, 1982. (44) El Sistema de Drenaje Profundo de la Ciudad de MBxico; Direccidn General de Construccidn y Operacidn Hidriuh a ; Departamento del Distrito Federal: Mexico City, 1988. (45) Inventario y Politicas de Gestidn de Desechos Industriales en la Ciudad de Mdxico; Departamento del Distrito Federal; Mexico City, 1992. Unpublished report. (46) Pitre, C. V. M.S. Thesis, Department of Earth Sciences, University of Waterloo, Ontario, Canada, 1993. (47) Mazari, M. D.Env. Dissertation. University of California-Los Angeles, Los Angeles, CA, 1992. (48) Rifai, H. S. et al. J. Environ. Eng. 1988,
Cherry, J. A. Canadian Geotechnical Journal 1986,23, 229-40. (30) Pankow, J. F. et al. J. Contam. Hydrol.
114(5), 1007-29. (49) Hadley, P. W.; Armstrong, R. Ground Water 1991, 29(1), 35-40.
1991, 27(9), 2187-201. (24) Hiriart, F.; Marsal, R. J. In El Hun-
dimiento de la Ciudad de M6xico: Proyecto Texcoco; Vollimen Nabor Carrillo; Secretaria de Hacienda y CrBdito Pdblico: Mexico City, 1969; pp. 109-47. (25) ResBndiz, D.; Zonana, J. In EI Hundimiento de la Ciudad de MBxico: Proyecto Texcoco. Voldmen Nabor Carrillo. Secretaria de Hacienda y Credit0 Pdblico. Mexico City, 1969; pp. 203-27. (26) Terzaghi, K. Theoretical Soil Mechanics; Wiley: New York, 1948; pp. 144-55. (27) Williams, R. E.; Farvolden, R. N. J. Hydrol. 1967, 5,163-70. (28) Grisak, G. E.; Cherry, J. A. Canadian Geotechnical Journal 1975, 22(23),
