the wastewater through four artificial exits located at the northern end of the basin. The main components of the drainage system, labeled in Spanish, are shown in Figure 4–4. The system network is over 10,000 kilometers long with 68 pumping stations, numerous dams (presa), lagoons, and regulatory tanks for flow control, 111 kilometers of open canals, 42 kilometers of rivers (rio) used primarily for drainage, and 118 kilometers of underground collectors (interceptor and emisor) and tunnels.

Based on the 1990 census (INEGI, 1991a), 82 percent of the 15 million residents in the MCMA are connected to the sewer system. About 6 percent use septic tanks, and over 9 percent are not serviced by any kind of drainage system. However, differences within the service areas are notable, with some counties supporting less than half the residents on a sewer system. Additional information on sewer service is provided in Chapter 6.

Domestic and industrial wastewater discharges and stormwater are collected in the secondary network (consisting of small pipe service at the neighborhood level), and then carried by the primary network into the General Drainage System flowing out of the basin to the north. The State of Mexico reports that the total dry weather flow for the MCMA, which consists mainly of untreated municipal wastewater, is estimated at 44.4 cms (Comisión Estatal de Agua y Saneamiento, 1993). During the rainy season, the region experiences many storms of high intensity and short duration. A single storm can produce up to 70 millimeters (about 3 inches) of rainfall, representing 10 percent of the total annual precipitation. Because of this rainfall pattern, and the irregular geography, the general drainage system was designed to carry 200 cms over a 45 hour period (Departamento del Distrito Federal, 1969; See AIC-ANIAC, 1994 for a detailed description of the drainage system).

Currently, 90 percent of the municipal wastewater from the MCMA remains untreated and is diverted out of the Basin of Mexico through the general drainage system. The untreated wastewater is then used to irrigate 80,000 hectares of farmland in the Valley of Mezquital in the State of Hidalgo to the north. Irrigation return flow drains into tributaries of the Panuco River, which empties into the Gulf of Mexico.

The approximately 10 percent of wastewater treated in the MCMA is for local reuse projects such as ground water recharge and agricultural and urban-landscape irrigation. There are 13 wastewater treatment plants in the Federal District and 14 in the State of Mexico service area treating a total flow of 2.62 and 1.69 cms respectively (Departamento del Distrito Federal, 1992b; Comisión Estatal de Agua y Saneamiento, 1993).

Table 4.3 gives the combined flow during both the dry and rainy seasons and the characteristics of the wastewater as it exits the basin through the Grand

Front Matter (R1-R16)

1 Overview (1-3)

2 The Mexico City Metropolitan Area (4-7)

3 Description of the Mexico City Aquifer and its Exploitation (8-18)

4 Water Supply, Distribution, and Disposal (19-38)

5 Water Quality and Health Concerns (39-52)

6 Water Demand Management (53-70)

7 Institutional Issues (71-77)

8 Conclusions and Recommendations (78-88)

References (89-98)

Appendix A: Committee Member Biographies (99-104)

Appendix B: Study Affiliates (105-107)

Spanish Version: El Suministro de Agua de la Ciudad de Mexico: Mejorando la Sustentabilidad (108-123)

1 Panorama (124-126)

2 La Zona Metropolitana del Valle de Mexico (127-130)

3 Descripcion del Acuifero de la Ciudad de Mexico y su Explotacion (131-142)

4 Abastecimiento, Distribucion y Aguas de Desecho (143-163)

5 Calidad del Agua y Problemas de Salud (164-178)

6 Administracion de la Demanda de Agua (179-198)

7 Aspectos Institucionales (199-205)

8 Conclusiones y Recomendaciones (206-217)

Referencias (218-227)

Apendice A: Biografias de los Miembros del Comite (228-234)

Apendice B: Asociados al Proyecto (235-237)

Errata (238-238)