Salt-water intrusion and nitrate contamination in the Valley of Hermosillo and El Sahuaral coastal aquifers, Sonora, Mexico

 The Valley of Hermosillo coastal aquifer, state of Sonora, northwestern Mexico, has been over-exploited for the last four decades, in order to maintain agricultural activity in one of the most important irrigation districts of the Mexican Republic. The over-exploitation has resulted in the development of several drawdown cones and in the lowering of the water table to as much as 50 m below mean sea level. Contamination of the aquifer in the form of salt-water intrusion from the Gulf of California and high nitrate concentrations is the consequence of human activities. A hydrogeochemical zonation of the aquifer, based on the presence of different water families, led to the identification of a coastal band approximately 30 km wide that is affected by salt-water intrusion. Conductivity of the sampled water and the interpretation of the ratio Na/Cl×1000 was used to identify the location of three major intrusion plumes in this coastal band. The background nitrate contamination of the aquifer is about 4 ppm, but contents as great as about 17 ppm occur in some wells. Irrigation with raw sewage and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for localized peaks of the nitrate concentration.