Groundwater Quality at the Independence Basin in Central Mexico: Implications for Regional Development
By Adrian Ortega Guerrero
Centro de Geociencias, Campus Juriquilla,
Universidad Nacional Autónoma de México
Regional and local studies were integrated for sustainable management of groundwater resources in six municipalities of the State of Guanajuato in central Mexico (San José Iturbide, Dr. Mora, San Luis de la Paz, San Diego de la Unión, Dolores Hidalgo, San Felipe and San Miguel Allende). Different hydrogeologic methods were used for understanding of the process controlling groundwater flow conditions and water quality at the watershed scale. A Geographic Information System (GIS) was used to integrate all physical and chemical information of the basin. The activities included geology, geophysics, hydrogeochemistry, environmental isotopes, groundwater flow and chemical modeling and an inventory of potential sources of contamination within the watershed.
Results show that the size of the watershed comprises 6,840 square kilometers. The thickness of the granular aquifer reflects a variation of 50 to 400 m in thickness overlying a volcanic fractured aquifer. Groundwater quality showed important implications for human health and agricultural practices, where arsenic and fluoride are present far above drinking water limits, impacting more than 4,000 inhabitants in an area of 300 square kilometers of the watershed, and apparently increasing with time.
The presence of major ions in groundwater and their impact on water quality are consistent with reactions of volcanic rock mineral dissolution, which exist at the recharge areas. Sodium concentration restricts about 18,000 hectares of the total irrigated through aspersion and 12,000 hectares by gravity. Radioactive isotopes show that recharge does not reach the water table, indicating that groundwater is not renewed on an annual basis. High arsenic, fluoride and sodium content are associated to residence time in the order of thousands of years.
An educational program in groundwater was designed for the above-mentioned municipalities to understand the groundwater situation and implement adequate solutions in the short and long terms, in coordination with the water authorities.