Fluoride, nitrate and water hardness in groundwater supplied to the rural communities of Ensenada County, Baja California, Mexico

The hydrogeochemistry of 26 wells belonging to ten different aquifers in the county of Ensenada, Baja California, is studied. These wells are all used to supply the rural communities in the region, which comprise ~37,000 inhabitants, excluding the city of Ensenada. High total dissolved solids (TDS) concentrations (maximum 7.35 g l−1) indicate that salt is a ubiquitous contaminant in the aquifers due to seawater intrusion. The aquifers that support extensive agriculture activities (Maneadero, San Quintin, San Simon and El Rosario) are characterized by higher N–NO3 concentrations (maximum 20 mg l−1) derived from fertilizers. Fluoride concentrations exceed the 1.5 mg l−1 Mexican official limit in only four wells. The enrichments of F− in the southern aquifers are thought to be associated to water–rock interactions controlled mainly by Na–Ca equilibrium reactions with fluorite, as suggested from high dissolved Na concentrations in these waters. In the northern aquifer of Maneadero, no enrichment of Na is found and a geothermal source for F− is likely. Water is hard to moderately hard, with Ca/Mg ratios >1. Although drinking water directly from the tap is not a common practice in these localities, most sources have concentrations of major ions and TDS that exceed the Mexican official limits.