IMPACT
ASSESSMENT OF WASTEWATER TREATMENT PLANTS ON
CHAPALA LAKE
The Lerma river flows along 750 km on the central Mexican highlands before flowing into Chapala Lake. Excess of water overflows into the Grande de Santiago river which flows into the Pacific Ocean, but the last time it reached a sufficient level to overflow was in 1977.
The mean annual runoff in the Lerma River basin is about
4,740 Hm3 but only 1,900 Hm3 reach the lake, and this only happens during the four months of rainy season. Evaporation is high, 76 % of the water is lost every year. The remainder is used for supplying potable water to Guadalajara Metropolitan area and for the agriculture that develops.
In such a context, the quality of the lake water reflects the events occurring upstream over an area of 48,215 km2. As most of the water evaporates, pollutants accumulate day after day and their concentration increases.
In the Lerma River basin, 44 urban type wastewater treatment plants have been built, thanks to the action of the River Basin Committee which started functioning in 1990. This sanitation programme, prepared by the Committee, covers the 100 main urban centres of the basin. In compliance with the new propositions that working groups will present during the next Committee meeting, the sanitation programme could be extended to 50 additional urban centres in order to have all people wanting a water supply and sanitation system covered by 2001.
The Mexican Institute for Water Technology (IMTA) carried out a survey with the objective of assessing the impact on the lake waters of the sanitation works built in the basin.
The study concluded that BOD was reduced, this being directly related to the sanitation programme. However, all the treatment plants built for urban wastewater are traditional and, therefore, nutrient removal is practically null. Agricultural
discharges loaded with fertilizers should also be considered as they increase the nutrient concentration and cause overfeeding favourable to eutrophication.
The effort made for river basin sanitation should continue but it should be completed by a regulation on fertilizer utilization and perhaps a change in fertilizer components.
WORKING
GROUP FOR THE INTEGRATED
SANITATION OF THE TURBIO RIVER SUB-BASIN,
TRIBUTARY OF
THE LERMA RIVER
Following the death of aquatic birds in the Silva dam-reservoir - located in the North-East of the State of Guanajuato - at the end of 1994, a dialogue started on February 9, 1995 between the three hierarchical governmental levels, operating organizations of Leon, San Francisco and Purisima, industrialists and NGOs concerned with environment.
A commission was created for the integrated sanitation of the Turbio river in order to study several projects:
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Environmental industrial park in Leon regrouping 109 tanneries to treat their wastewater.
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San Francisco and Purisima municipalities in Guanajuato State are going to jointly build and manage a treatment plant
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Potable water supply and sanitation service of Leon started building a plant to treat the town wastewater.
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48 industries and 6 pig breeding farms, that directly discharge their wastewater into the Turbio, will establish a treatment programme which should be operating in January 1998.
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Finally, the Institute for Scientific Studies of Guanajuato, the Iberoamerican University, the Foundation for Environment, the National Water Commission and the Directorate for the Environment of Guanajuato State form a working group in charge of finding the causes of bird death and classifying the sediments of Silva dam, of establishing fauna and flora inventories, environmental education, physico-chemical analyses of heavy metals found in water and silt of 11 streams in the Turbio River sub-basin.
Victor Oseguera Green
Information Centre of Chapala-Lerma Basin
E-mail : lermaham@sparc.ciatcq.conacyt.mx
Creation of
Nazas River Basin is considered, within the framework of the National Project for Modernizing Water Management (PROMMA), as one of the main basins for which the creation of a River Basin Committee is planned.
The basin covers an area of 71,906 km2 and is straddled on the Coahuila and Durango States. Population is estimated at 1.615 million inhabitants, 34 % of which are in rural areas and 66 % in urban areas. This region only comprises two towns of more than 100,000 inhabitants.
95 % of the resources are used for irrigation.
The mean water shortage significantly increases in dry periods. The Nazas river basin encounters problems of aquifer overexploitation, irregular water allocations for users, and deterioration of groundwater quality caused by the progressive lowering of the water table. A regulation concerning overexploited aquifers is needed to have a potable water reserve. The modernization of irrigation methods and the rehabilitation of hydroagricultural infrastructures are also indispensable.
The creation of a River Basin Committee will enable to prepare and implement programmes and activities to improve water management.
José Luis Montalvo Espinoza
National Water Commission for the Lagoon Region
Fax : (52-91) 17 42 81
