Pesticide Contamination of Groundwaters in the Mahantango Creek Watershed

Abstract

AbstractWaters from 20 wells in a primarily agricultural Pennsylvania watershed were analyzed for the most heavily and extensively applied pesticides as determined by farm survey. Those analyzed were atrazine, metolachlor, cyanazine, alachlor, terbufos, chlorpyrifos, fonofos, and carbofuran. No metolachlor, alachlor, terbufos, chlorpyrifos, fonofos, or carbofuran were detected. However, atrazine concentrations ranging from 13 to 1110 ng/L were found in 14 of the 20 wells. These findings suggest widespread atrazine contamination of the groundwaters, but at extremely low concentrations. Cyanazine was found above trace concentrations in one well. Although there was only one record of application, simazine was found in seven wells within the range of 10 to 170 ng/L. The degree and spatial distribution of atrazine contamination depended mostly on the frequency and distribution of corn production. The highest atrazine concentrations were associated mostly with continuous corn production. Aquifer rock type, depth to water table, and the degree of recharge short‐circuiting estimated from PO4 contamination of groundwater, were concluded not to be important factors. Generally, Cl provided a better index of atrazine contamination than did NO3 because extensive denitrification occurred in several wells. The probability of atrazine being found above the minimum detection limit was high when Cl exceeded 3 mg/L or NO3‐N exceeded 4 mg/L.