ARSENIC IN GROUNDWATER AT THE SOUTHERNMOST END OF THE CORDILLERAN

The Independence Aquifer is located in the Lerma-Chapala Basin, in the southernmost end of the Mexican Cordilleran. The mining districts of Guanajuato and Mineral de Pozos are located near the recharge zones of this aquifer. Groundwater is the main source of water for agricultural, urban and industrial uses in the Independence Basin with an area of about 7,000 square kilometers. Groundwater is extracted from a granular aquifer of lacustrine origin that covers another aquifer of a fractured nature in the volcanic rocks that dominates Mexico’s high plateau. A total of 206 samples of groundwater were taken, in 1998, from the 2,500 wells that extract water from both aquifers, in order to determine the distribution of major ions and trace elements associated with the groundwater flow system in this unitary basin, in addition to environmental and radioactive isotopes. This work reports the presence and distribution of arsenic in groundwater in the Independence Basin. Concentration of arsenic in groundwater varies between 0.001 and 0.1 mg/l. An area of about 300 square kilometers presented concentrations above 0.04 mg/l that represents the Maximum Contaminant Level for arsenic suggested by the Mexican Department of Health. High arsenic content is associated to the fracture aquifer and groundwater with residence time in the order of thousands of years. Arsenic quantities in groundwater may potentially increase in the future if adequate criteria are not established for managing the aquifer.