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WATER QUALITY AND HEALTH CONCERNS 39 5 Water Quality and Health Concerns VULNERABILITY OF THE AQUIFER Given the tremendous size and population of the Mexico City Metropolitan Area (MCMA) and its dependence on the aquifer for almost three-quarters of its drinking water supply, the protection of ground water quality is of utmost concern. Wastes from domestic, industrial, and commercial activities contain a variety of human pathogens and toxic contaminants that may pose a hazard if not properly managed. The potential for these contaminants to leach into the ground water depends on many factors, such as the composition of soils and geologic materials, the depth of the water table, the recharge rate, and environmental factors that can influence the mobility or degradation of contaminants (National Research Council, 1993). In the MCMA, these characteristics differ among the three major hydrologic zonesâthe lacustrine zone of the valley floor, the transition zone in the piedmont region, and mountain zone (the hydrogeological character of these zones is discussed in Chapter 3). The transition zone is of particular concern because of the combination of natural permeability, the rapid urban growth, and the increase in the number of water supply wells pumping water from this area. Problems with land use include a large proportion of settlements without sewage collection, unlined drainage canals carrying untreated domestic sewage and industrial wastewater, unlined solid waste landfills, and poorly controlled storage and disposal of hazardous waste (Mazari and Mackay, 1993). The mountain zone is not urbanized to the same extent as zones of the lower elevations. However, human settlements on the mountain slopes increase the potential for contamination.
WATER QUALITY AND HEALTH CONCERNS 40 Until recently, Mexican authorities had believed that the lacustrine clays, which underlie much of the urban area, formed an impermeable, protective layer that prevented downward migration of contaminants. However, depressuring and consolidation of the clay layers have led to the development of fractures that can accelerate downward migration of contaminants (Alberro, 1993; Rudolph et al., 1991; Pitre, 1994). Since at least the 1940s, the flow gradients have been downwards towards the heavily pumped areas in contrast to the historic, artesian conditions (Carrillo, 1948). Modeling results and field studies at selected sites show that compounds have migrated a longer distance in the clays than would have been predicted based on a homogenous clay formation, supporting the hypothesis that contaminants are moving along fractures (Rudolph et al., 1991). While these studies have shown downward migration of contaminants and the potential for ground water contamination, no studies have yet determined if these contaminants have actually reached the main aquifer in exploitation. A recent study in the heavily pumped Chalco plain (Ortega et al., 1993) found significant consolidation of clays, and the authors concluded that subsidence will continue with the release of salts and other soluble chemical constituents from the clay aquitard into the main aquifer below. Ortega and co-workers concluded that a better understanding of the aquifer's response to different pumping scenarios would be required for protection and long-term management of the aquifer. The lack of wastewater treatment and the practice of using open ditches to carry untreated sewage is widespread in Mexico and the rest of Latin America (Pan American Health Organization, 1990b; Cech and Essman, 1992) and is a general concern for public health. The MCMA generates an estimated 44 cms of wastewater (ComisiÃ³n Estatal de Agua y Saneamiento, 1993) and according to the Federal District, more than 90 percent of industrial liquid wastes are discharged into the sewer system (Lesser y Asociados, S.A., 1993). The combined sewer system transports wastewater and rainfall runoff through a primary network 1,212 km long and a secondary network 12,326 km long. The new deep drainage system penetrates below the clay aquitard in some places and intercepts the main aquifer. During periods of heavy rain, wastewater exfiltrates from the deep tunnels into the surrounding subsoil and can create problems in many locations within the lacustrine zone where protection from the clay layer is no longer effective. Unlined canals pose an additional risk of underground water contamination, particularly in the transition zones where the soil is highly permeable (Mazari and Mackay, 1993). There are many abandoned wells in the area, some of which are open near the surface or otherwise poorly sealed. Many are located close to unlined drainage canals that contain untreated domestic and industrial wastewater. These abandoned wells may provide an alternate and more direct route for contamination of the water supply aquifer from the surface.
WATER QUALITY AND HEALTH CONCERNS 41 The MCMA is the most important industrial zone in Mexico, and contains about 45 percent of the nation's industrial production. According to the National Institute of Nuclear Investigations, the amount of hazardous waste generated in the Federal District is about 3 million metric tons per year, of which more than 95 percent are process effluents or treated effluents discharged to the municipal sewage system. The remainder, or some 150,000 tons, are solids, most of which are sent to municipal waste dumps or to illegal dumps. In addition to the wastes currently generated, there are about 40 million tons of old hazardous wastes produced since the 1940s, when the area's industrialization sharply increased. While the Federal District contains records on the number and types of industries, the other states in the Basin of MexicoâMexico, Hidalgo, Tlaxcala, and Pueblaâdo not (see AIC-ANIAC, 1995 for more details). While there have been investigations on the type of contaminants produced at different facilities, and on the migration of contaminants in the subsoil of the MCMA, the committee has not been made aware of any studies examining the geologic formations of the main water supply aquifer which could confirm contamination of the water supply from industrial sources. The production and management of hazardous wastes are regulated by the 1988 General Law of Ecological Balance and Environmental Protection. This law imposes restrictions and controls on producers of hazardous wastes, requires registration and permits of affected companies, documentation of industrial processes, and management requirements. Notwithstanding the provisions of the law and regulations, the actual practice of hazardous waste management in the MCMA is seriously compromised by the lack of facilities for recycling, treatment, or disposal of such wastes. Two companies in the metropolitan area are authorized for recycling of specific types of wastes. No disposal sites in the Basin of Mexico are authorized to receive hazardous materials. Another water quality concern is the risk from pesticide application on agricultural lands. While there is no reliable information on the extent or severity of water contamination caused by pesticides in Mexico, the Pan American Health Organization (1990a) has identified several river basins where pesticides may be a problem, including the Lerma River Basin that supplies part of the drinking water to Mexico City. Pesticide residues have been detected in human adipose tissue in samples from the Mexico City population (Albert et al., 1980). Human exposure to pesticides occurs more readily through direct consumption of agricultural products or by runoff from agricultural fields that may contaminate surface waters used as drinking water sources (National Research Council, 1993). However, the leaching of pesticides through the subsurface into ground water is another potential pathway for contamination.
WATER QUALITY AND HEALTH CONCERNS 42 MONITORING AND SANITARY CERTIFICATION The Ministry of Health is responsible for certification of the quality of water intended for human consumption, and has promulgated a set of standards covering requirements for water supply systems, transportation of drinking water, and sampling procedures. As a result, the quality of drinking water in the MCMA is monitored for inorganic chemicals, organic chemicals, and bacteriological and physical parameters. The water department of the Federal District maintains a Central Control Laboratory at Xotepingo that performs all water quality analysis for the Federal District. The laboratory has established a system of monitoring and water quality analysis to evaluate water supply installations such as wells, treatment plants, pumping stations, and storage tanks. Water is also sampled at household taps within neighborhoods of the 16 counties of the Federal District. Analysis of drinking water quality is performed at one of four monitoring levels, depending on the types of water quality problems anticipated. The most basic sampling is called Level A which is used to detect bacteriological contamination and to yield information regarding free chlorine residuals, acidity, electronic conductivity, temperature, and turbidity. Level 2A additionally samples minimum physical and chemical characteristics prevalent in the Federal District and include total alkalinity, chlorides, color, oxygen demand, total hardness, and ammonia nitrogen. Level 3A monitoring fulfills the requirements for drinking water standards established by the Ministry of Health (Table 5.1). These standards include the additional parameters of chlorine, calcium hardness, magnesium hardness, fluorides, total dissolved solids, nitrates, nitrites, organic nitrogen, sulfates, methylene blue active substances, potassium, sodium, aluminum, arsenic, barium, cadmium, copper, cyanide, iron, lead, magnesium, manganese, mercury, nickel, silver, selenium, and zinc. Level 4A is an intensive sampling procedure where site-specific problems are suspected, and where sampling may include synthetic organic compounds, biological and chemical oxygen demand, radon, and other human pathogens. The application percentages of these levels of analysis in 1992 were 70 percent A samples, 15 percent 2A samples, 10 percent 3A samples, and 5 percent 4A samples. Comparable water quality data is monitored by the State of Mexico's water and sanitation commission (ComisiÃ³n Estatal de Agua y Saneamiento). The State of Mexico, however, does not have as high a level of infrastructure or personnel as does the Federal District for water quality monitoring (see AIC-ANIAC, 1995). QUALITY OF THE WATER SOURCES Ground water quality in the MCMA varies, and some of this variation is due to characteristics of the region's geologic formations. Water from the superficial
WATER QUALITY AND HEALTH CONCERNS 43 TABLE 5.1 1991 Standards Developed by the Ministry of Health Used for Certifying the Quality of Drinking Water for Human Use. Parameters Standarda Aluminum 0.20 Arsenic 0.05 Barium 1.00 Cadmium 0.005 Calcium Hardness, as CaCO3 300.00 Chemical Oxygen Demand 3.00 Chromium (VI) 0.05 Copper 1.50 Cyanide 0.05 Fluoride 1.50 Iron 0.30 Lead 0.05 Magnesium Hardness, as CaCO3 125.00 Manganese 0.15 Mercury 0.001 Nitrates, as N 5.00 Nitrites, as N 0.05 Selenium 0.05 Sulfate 250.00 Total Alkalinity, as CaCO3 400.00 Zinc 5.0 Carbon-Chloroform Extractables 0.30 Carbon-Alcohol Extractables 1.5 Organic Nitrogen, as N 0.10 Phenols 0.001 Color, Pt-Co Units 20 Free Chlorine (overdosed water) 0.20 1.00 Methylene Blue Active Substances 0.50 pH 6.9â8.5 Taste and Odor No Obj. Turbidity, NTU (silica scale) 10 Fecal Coliforms, MPN, no./100 ml 0 aMilligrams per liter unless otherwise indicated. Published in Articles 211â213 of the General Health Law, Ley General de Salud. Diario Oficial de la FederaciÃ³n el 14 de junio de 1991.
WATER QUALITY AND HEALTH CONCERNS 44 lacustrine clays is of very poor quality due to its high concentration of dissolved salts (from 1,000 up to 130,000 milligrams per liter, Rodriquez, 1987). For this reason, all production wells that provide water for potable purposes currently draw from deeper than 400 meters in order to tap the higher quality water from the main aquifer. Nevertheless, some intrusion of saline waters into the main aquifer has been documented (Lesser-Illades et al., 1990). Salts and total dissolved solids in production wells generally increase from the foothills toward the center of the plain, correlating to where salinity increased in the old lakes. Elevated concentrations of sulfur, iron, and manganese, derived from volcanic geology of the region, have been detected in specific areas (Bellia, et al., 1992). Although not a serious problem, wells have been closed in the few locations where inorganic chemical concentrations are higher than the water quality standards (SARH, 1988). The Federal District's Central Control Laboratory at Xotepingo has identified and mapped additional ground water parameters that indicate the potential for organic and/or biological contamination. As of 1993, testing at the well head revealed areas of non-compliance with color, total solids, ammonia, organic nitrogen, nitrates, chemical oxygen demand, and hardness. These problems tend to be localized in the eastern section of the Federal District and in portions of the surrounding well fields as shown in Figure 5â1. During the same period, water quality standards were not met for physical/chemical constituents in 31 percent of the wells, or for bacteriological properties in 21 percent of the wells. In some of these locations where these water quality problems have been detected, additional treatmentâincluding oxidation, filtration, and activated carbon adsorptionâis performed at the well head as part of a pilot program; otherwise, the wells are closed if a public health threat is indicated. The State of Mexico reports that 23 percent of its 242 water supply wells for its service area do not meet the standards for coliform bacteria, and 11 percent do not meet the standard for inorganic constituents. An increase in concentration of hydrogen sulfide has been reported from 21 wells, although there is no standard for hydrogen sulfide. Information on surface water quality, as provided by the Federal District and the National Water Commission, indicates that the major surface water sources for the MCMAâCutzamala River, Magdalena River, and Madin Damâare of generally acceptable quality except for high levels of fecal coliform in the Cutzamala River (AIC-ANIAC, 1995). As described in Chapter 4, these surface water sources undergo chemical coagulation, filtration, and chlorination. Ground water is normally treated only by chlorination. Thus, all water is at least disinfected. Surface water from many small springs contributes 0.7 cms to the MCMA water supply. Testing reported by the Federal District in 1993 indicated that a high proportion of springs did not meet water quality standards for physical/chemical constituents (38 percent) or for bacteriological
WATER QUALITY AND HEALTH CONCERNS 45 FIGURE 5â1 Well field areas supplying the Federal District where water quality testing indicates particular parameters are not in compliance with water quality standards. Source: Federal District's Central Control Laboratory at Xotepingo.
WATER QUALITY AND HEALTH CONCERNS 46 properties (76 percent). Information was not available on whether these sources from surface springs receive treatment other than disinfection with chlorine. WATER QUALITY IN THE DISTRIBUTION SYSTEM The Federal District's Central Control Laboratory at Xotepingo analyzes the water quality within the distribution system by sampling at certain hydraulic installations (entrance points for water delivered to the distribution system, pumping stations, and system storage tanks). Sampling is also performed at household taps at designated street crossings in 1,270 neighborhoods within the 16 counties. Table 5.2 shows the percent of household tap samples in compliance with the residual chlorine standard (0.2 milligrams/liter) among the 16 counties of the Federal District. Compliance ranged from 87 to 100 percent and was notably low among southeastern counties (Iztapalapa, TlÃ¡huac, and Xochimilco). Testing of water where it first enters the distribution system indicates potential microbiological contamination at four entrance points in the East and South service districts (refer back to Figure 4â2). Average sampling in 1993 reported to the committee by the Central Control Laboratory showed elevated color at Cerro de Estrella and Metro Cubico; high organic nitrogen at La Caldera and relatively high levels of turbidity at Metro Cubico and Xotepingo. There are 326 re-chlorination stations within the distribution system. This additional disinfection step apparently accounts for the percent compliance being as high as it is at the neighborhood level (Table 5.2). The State of Mexico also monitors its drinking water quality within the distribution system and at household taps; although sampling is performed less intensively than in the Federal District. Household tap water samples in the 17 metropolitan counties were analyzed in 1993 for the presence of residual chlorine. The presence of chlorine residual was less than 90 percent in 12 of the counties, with some counties reporting as low as 16.6 and 47.3 percent of samples with detectable chlorine (Table 5.3). According to the State of Mexico water authorities, water quality deterioration in general at consumer taps is attributed to infiltration of poor quality water from the surrounding media into a leaky system, and from precipitation of salts (mainly from calcium, magnesium, iron, and manganese) in the distribution lines. Leaks in the distribution system are a major cause of concern for both water quality and water supply. When the soil is permeated by sewage from leaking sewers or from other sources, such as unlined canals carrying sewage, then leaky pipelines will be infiltrated with contaminated water when pressure is low. According to the Federal District's water quality laboratory, neighborhoods that experience more frequent interruptions in service have poorer quality water compared to neighborhoods with a constant supply.
WATER QUALITY AND HEALTH CONCERNS 47 TABLE 5.2 The Percent of Household Tap Water Samples That Meet the Standard for Chlorine Residual (0.2 milligrams per liter) Among the 16 Counties in the Federal District. Results are averages for 1993 as reported by the Federal District Central Control Laboratory at Xotepingo. Federal District Counties Number of samples Percent compliance for chlorine residual Alvaro ObregÃ³n 7,060 95 AtzcapotzÃ¡lco 5,520 99 Benito JuÃ¡rez 3,107 96 Coyoacan 6,979 97 CuajimÃ¡lpa 1,337 97 Cuauhtemoc 2,555 96 GustÃ¡vo Madero 12,419 94 IztacÃ¡lco 3,572 96 Iztapalapa 19,210 87 Magdalena Contreras 1,709 93 Miguel HidÃ¡lgo 2,952 95 Milpa Alta 1,110 95 TlÃ¡huac 4,023 87 Tlalpan 4,148 95 VenustiÃ¡no Carranza 3,414 95 XochimÃlco 4,215 89 Source: AIC-ANIAC, 1995. Rivera et al. (1979) conducted the first independent study of the presence of human pathogens in tap water in the Federal District and found 10 out of 25 samples containing one or more of the active forms of pathogenic organisms. A more recent study of the bacteriological quality of water entering a Mexico City hospital (Juarez, et al., 1992) found that up to 90 percent of the samples were unacceptable based on either chlorine concentration or total coliforms. The irregularity in water supply makes household water storage tanks a necessity. These water tanks, or tinÃ¡cos, are common on almost all household rooftops and are used to store water when water pressure in the system is inadequate. In many instances, the tanks are open and not cleaned regularly, permitting the residual chlorine to dissipate and encouraging the growth of microorganisms. Microbiological contamination of the household water tanks may be due to contamination at the well head, infiltration of contaminants in the leaky water distribution system, or contamination by airborne microorganisms in those storage tanks that are left uncovered and exposed (Rivera, et al, 1994). The standard levels of chlorine (0.2 milligrams/liters) maintained in the distribution system as it reaches the customer's tap are not sufficient to inactivate
WATER QUALITY AND HEALTH CONCERNS 48 microorganisms that may have entered the pipelines. The value of maintaining a chlorine residual is to prevent the growth of slime in the system and, more importantly, to be a marker as to whether recontamination may have occurred. Recontamination can use up the chlorine residual. Therefore its absence is a cause for concern. TABLE 5.3 The Percent of Household Tap Water Samples with Detectable Chlorine Residuals Within Metropolitan Counties of the State of Mexico. Results are averages for 1993. State of Mexico Metropolitan Counties Percent of samples with a detectable chlorine residual Atizapan de Zaragoza 81.4 Huixquilucan 89.4 Naucalpan 88.9 NicolÃ¡s Romero 80.9 Tlalnepantla 89.8 CuautitlÃ¡n Izcalli 76.1 CuautitlÃ¡n 16.6 Coacalco 100.0 TultitlÃ¡n 55.8 Ecatepec 96.7 NezahualcÃ³yotl 97.4 TecÃ¡mac 93.7 Chicoloapan 100.0 ChimalhuacÃ¡n 83.3 La Paz 47.3 Chalco 56.0 Ixtapaluca 72.2 Source: AIC-ANIAC, 1995 Wastewater delivered by the Grand Canal is used to irrigate 5,500 hectares in the Chiconautla area. Outside the watershed, part of the raw wastewater is used to irrigate about 80,000 hectares of farmland in the State of Hidalgo, a practice that has evolved since 1934. Protection of public health is managed through crop restrictions rather than wastewater treatment. In 1991, the Urban Development and Ecology Ministry (SEDUE), in coordination with the Health Ministry (SSA) and the Agriculture and Hydraulic Resources Ministry (SARH), established a standard that prohibits the use of untreated wastewater on crops which may be eaten raw or ones that are grown on the surface of the ground. However, irrigation with untreated wastewater may still pose health problems (Shuval, 1986), and irrigation with treated wastewater must be carefully controlled. As an example, a study in the Xochimilco region, south of Mexico City,
WATER QUALITY AND HEALTH CONCERNS 49 identified significant levels of fecal coliform bacteria in the soil and on leafy vegetables. Irrigation comes from the Xochimilco canals that receive treated wastewater, but the 10 kilometer open canal is subject to contamination from adjacent farms and human settlements (Sepulveda et al., 1987). In addition to direct exposure to raw sewage in open canals, humans can be exposed to airborne dried fecal dust in areas where raw sewage can dry out. This dust may become suspended as air-borne particles and may contain free-living protozoa. Although most protozoa isolated from air-borne particles are non- pathogenic, there have been air-borne pathogenic protozoa identified in specific locations in the metropolitan region (Rivera et al., 1994). WATER-RELATED HEALTH CONCERNS Infectious gastro-intestinal diseases are the major water-related health concern in the MCMA, as well the rest of the country. Children are especially vulnerable to these diseases, which often result in acute diarrhea and sometimes death from dehydration. In 1991, the acute diarrhea rate in all of Mexico was 3,233 illnesses per 100,000 inhabitants; 46 percent of these cases occurred in children less than 5 years old. In 1991, census data reported that infectious intestinal diseases are the second leading cause of infant mortality nationwide with a mortality rate of 278.4 per 100,000. It is the third leading cause of infant mortality in the State of Mexico and fourth in the Federal District (mortality rates are 450 and 156.7 per 100,000 respectively; INEGI, 1991a). Acute diarrhea is prevalent in the MCMA, and some areas show higher incidence and mortality than others. Figure 5â2 (a and b) shows incidence of diarrhea disease (morbidity) and mortality rates in the general population by the 16 counties in the Federal District and in 12 of the 17 counties in the State of Mexico. Average sickness and mortality rates are most elevated in the more rural, southeastern jurisdictions of the Federal District (Milpa Alta and Tlahuac) and in many of the more rural counties in the State of Mexico. As explained in Chapter 6, many of these same areas have generally lower access to in-house piped water (see Figure 6â1). Due to the administration of oral rehydration therapy, mortality from diarrheal diseases has dropped over the past decade. However, such treatment does not address the cause of diarrheal diseases. Protozoa parasites, including Giardia and Entamoeba histolitica, are prominent causal agents of diarrhea. Amoebic dysentery, which is endemic in Mexico (Pan American Health Organization, 1990b), is transmitted by the cyst form of Entamoeba histolitica, often by fecal-contaminated drinking water (McFadzean and Pugh, 1976). The normal level of chlorine has little or no effect on encysted amoeba (Rose et al., 1991). Giardia infections in young chil
WATER QUALITY AND HEALTH CONCERNS FIGURE 5â2 Morbidity (a) and mortality rates (b) due to acute diarrhea in the general population by county in the Mexico City Metropolitan Area. Data was collected by sanitary jurisdictions, which do not correspond exactly to counties in the State of Mexico. For this reason, âno dataâ is indicated for five counties. Source: INEGI, 1991a. 50
WATER QUALITY AND HEALTH CONCERNS 51 Mexico City include Balantidium coli, Naegleria fowleri, and species of Acanthamoeba (Rivera et al, 1978; 1983; 1984; 1986). Exposure to pathogenic species of Naegleria and Acanthamoeba can cause disorders of the central nervous system and even death, especially in young children (Naeglaria). The pathogens are believed to be acquired through the nose while swimming. Children playing in the water are particularly at risk. For this reason, authorities should ensure that waters destined for recreational use receive proper treatment. A great variety of enteric viruses may be ingested with non-potable water, including subgroups of polioviruses, coxsackie viruses, echoviruses, and infectious hepatitis viruses. These viruses can cause respiratory, gastro-intestinal, and central nervous-system disorders. Infectious hepatitis produces liver diseases, and Hepatitis A is probably endemic in Mexico (Cech and Essman, 1992). In 1986, two large outbreaks of acute hepatitis occurred in the State of Morelos, south of Mexico City, in areas without public water or sewer service. This was the first documented epidemic of water-borne, non-A, non-B hepatitis in Latin America (Okun, 1991). Of all the diseases known to cause diarrhea, the bacteria that causes cholera, Vibrio cholera, is known for its most severe symptoms. In 1991, cholera outbreaks occurred in Peru and spread to most other Latin American countries. In Mexico, there were 2,690 cases, of which a small percentage (2â3 percent) occurred in the Mexico City Metropolitan Area (Craun et al., 1991). Other bacteria of health significance transmitted by polluted water (and food contaminated with polluted water) include Salmonella, Shigella, Camplylobacter foetus, Yersinia enterocolotica, and E. coli (Sarti-Guttierez et al., 1989; Castro, 1991) Cryptosporidiosis is one of the most serious of all microbial diseases because it can cause infection at a very low dose, survives well in water environments, and may be resistant to levels of disinfection generally applied to drinking water (Rose, 1993). As evidenced by the outbreak of more than 400,000 cases and more than 100 deaths in Milwaukee in April, 1993, contamination can occur even though drinking water regulations are not exceeded (Fox, 1993; Rowan, 1993). No known drug therapy is currently effective for this infection (Soave, 1990), and people who have immune system deficiencies, such as AIDS patients, infants, and cancer patients, are particularly at risk. Because of its similarity to other diarrheal diseases, Cryptosporidiosis will not show up separately unless an effort is made to find it and thus data on its occurence is scarce. Morbidity and mortality from waterborne infectious disease may be lower among industrialized countries than in other developing countries, but these risks persists and may be more important than previously realized even in highly industrialized countries (Craun et al., 1994a). While much waterborne disease is associated with contaminated water sources and inadequate water treatment, protection of water quality during its distribution to the customer is of equal importance.
WATER QUALITY AND HEALTH CONCERNS 52 Health Concerns from Toxic Chemicals In addition to the problems typical of developing countries, such as high prevalence of infectious diseases caused by fecal pollution, Mexico is experiencing problems facing industrial societies (Pan American Health Organization, 1990a, 1990b). Toxic chemical contamination of water, as well as air, soil, and food, are on the rise in Mexico. Chemicals of concern are nitrates, toxic metals and other inorganic pollutants, assorted volatile and semi-volatile organic solvents, agricultural pesticides, herbicides, and radio-chemicals. Toxic leachate from improperly disposed chemical wastes, leaky underground storage of industrial and energy-generating products, rainwater contaminated by air pollution, agricultural runoff, and mining wastes, are all potential contributors. Some chemicals could cause acute or chronic toxicity. Others can be genotoxic, producing carcinogenic, mutagenic, or teratogenic effects. According to the Pan American Health Organization (1990a), cancers are beginning to emerge as increasing risks in Mexico and other Latin American countries; although, they remaind overshadowed by mortality from communicable diseases. The by-products of chlorine disinfection (e.g., trihalomethanes) in finished water have been a subject of health concern in industrialized countries. Trihalomethanes are one of many by-products formed when water containing organic compounds is chlorinated for disinfection purposes. The issue of the balance between the risk of disinfection by-products (which are low-level, long- term risks) and the risk from infectious microorganisms continues to be examined (see for example, International Life Sciences Institute, 1992, Craun et. al, 1994b). In the face of continued high infant mortality rates from water-borne disease in Mexico, the risks of chronic disease from disinfection by-products are a relatively low priority.