POSSIBLE INFLUENCE OF GEOTHERMAL POWER ON GROUNDWATER QUALITY: THE CERRO PRIETO GEOTHERMAL FIELD

Geothermal energy has been considered as a low environmental impact energy source. Although it is not widely used worldwide, México is the fourth ranked country generating geothermal power. Besides, Cerro Prieto, northwest of México, close to the USA border is the second largest geothermal plant of the world, with a capacity of 720 MWe.

The influence of this facility on groundwater quality of the surrounding agricultural area was evaluated by means of chemical and isotopic determinations. Water was sampled from deep and shallow agricultural wells, canals, geothermal production wells, an evaporation pond, and piezometers. Since water is mainly used for irrigation, results were compared with national and international irrigation standards. Concentrations of As, Pb, Cd, Crtot, Cr(VI), Cu, Cd, Hg, B in agricultural wells showed that water is suitable for that use. Iron was below the standards in all but one well. However, chloride levels were above irrigation limits in all but 4 sampled sites. Sodium is not included in Mexican standards; however concentrations were above FAO limits in all sites. Lithium is neither included in the Mexican law; concentrations in the agricultural zone were below FAO standard but surpassed it in all geothermal samples.

Clear concentration differences were determined between samples from the geothermal and the agricultural zone. Infiltration of irrigation water to wells was shown by the isotopic determinations. Hydrogeochemistry, spatial concentration distribution of elements related with geothermal brines and of trace elements shown by isolines, jointly with isotopic values (d²H, d¹⁸O, d³⁴S, d¹³C) did not show degradation of the groundwater quality of the agricultural zone by the influence of the geothermal plant.