EVALUATION OF PESTICIDES OCCURRENCE IN GROUNDWATER OF RIO VERDE FLUVIAL AQUIFER, SAN LUIS POTOSI, MEXICO

The Rio Verde fluvial aquifer is characterized by highly permeable sediments. The main economic activity in this area is the highly profitable irrigated agriculture, supported by groundwater. High fertilizer and pesticide application loads indicate a contamination potential to local drinking water supplies, hence the interest in understanding this aquifer. The highly variable precipitation in the area is characteristic of the semi-arid conditions. The study aimed to define the presence of diffuse contamination indicators in groundwater. Three field surveys along the year allowed the identification of diffuse contamination indicators, such as nitrate and organochlorine (OC) and organophosphate (OP) pesticides. The main agricultural area obtains groundwater from an unconfined aquifer in unconsolidated sediments (100-300 m thickness), with hydraulic conductivity from 1.7-2.3 m/day and porosity 0.18-0.22. Groundwater flow determined from hydraulic head distribution is from the southwest area to the natural discharge zone in the northeast; hydraulic and chemical data helped the identification of direct natural recharge along the ephemeral Morales creek (SW-NE direction). Nitrate values up to 259.1 mg/l (M=35 mg/l) were a robust indicator for diffuse contamination derived from irrigation return impacting baseline chemistry; change in nitrate concentration indicates seasonal effects. Several dissolved pesticides in groundwater were above the detection and quantification limits. Total OC pesticides (M=0.2644 ug/l, σ=0.2114 ug/l) and total OP pesticides (M=0.048 ug/l, σ=0.0612 ug/l) for the dry season showed the highest concentrations. Despite analyzing 30 pesticides, the main identified dissolved pesticides in groundwater were 5 OC (Dieldrin, heptachlor, gamma-chlordane, endosulfan-I, endosulfan) and 5 OP (O, O, O-Triethyl phosphorothioate, methyl parathion, diazinon, parathion). Dieldrin and heptachlor concentrations for some sites were above drinking water standards posing a public health risk. Considering the relatively shallow depth to the water table (5-30 m) and the high pesticide load in the region, soil type (phaozem) is probably the main factor preventing pesticide leaching to the saturated zone.