Groundwater quality implications of wastewater irrigation in Leon, Mexico

Wastewater from the city of Leon, Mexico, has been used for irrigation of agricultural land close to the city for the last 40 years. The wastewater contains a significant percentage of industrial effluent from the extensive tanning industry, including high concentrations of salt and hexavalent chromium compounds, and is used untreated. The irrigated area overlies an aquifer which provides an important part of the municipal water supply. Infiltration of the irrigated water has led to the formation of a localised layer of shallow, poor-quality water above the regional aquifer. Investigation has shown changes in tie quality of the infiltrating wastewater as it passes through the soil and shallow aquifer layers. These significantly reduce the concentrations of chromium, sodium, sulphate and nitrogen. However, high chloride concentrations remain to pose an immediate threat to groundwater quality in the lower part of the aquifer and chloride concentrations in groundwater from some deep municipal supply boreholes are now rising rapidly.