Metales pesados en el agua superficial del río San Pedro durante 1997 y 1999

A surface water quality study was conducted for the San Pedro River during the years 1997 and 1999. The study was done from the river’s headwaters near to the City of Cananea, Sonora, up to the U.S./Mexican border. Surface water samples were analyzed for pH, electric conductivity, sulfates and some totals heavy metals: Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn, using the recommended EPA and Mexican National Water Commission sampling and laboratory methodologies. The phase I was done during February-November of 1997 with four quarterly sampling events (February, May, August, and November). The project phase II was done in April and August of 1999. In the proyect, samples were collected at 8 sampling stations. Surface water parameters were compared with the Water Quality Ecology Criteria of the Secretaria de Desarrollo Urbano y Ecologia (SEDUE 1989). The experimental data showed high total heavy metals values for Cd, Cu, Fe, Mn, Ni, Pb, Zn, electric conductivity and sulfates, and low values of pH for the sampling stations near the pollution sources; i.e., next to the Deposito Concentradora Vieja. The values of pH, electric conductivity, total heavy metals and sulfates were higher in both phases, than those dictated by the Water Quality Ecologic Criteria. Most of the evaluated parameters during the phase I presented their highest average values during May, August, and November 1997. The highest average values in the phase II were detected in August 1999. Mining activies were found to be the main source of pollution of the surface water and sediments in the San Pedro River. Wastewater discharges of untreated raw sewage coming from the city of Cananea were found to be the second most important source of pollution for the river.