Movement of Pesticides to Groundwater in an Irrigated Soil

Abstract

Abstract The movement of pesticide residues to ground water was studied on a commercial farm southwest of Portage la Prairie, Manitoba. The site had sandy soil with low organic matter content, a high water table, a tile drain system and an irrigation system using river water. Records were available from the beginning of commercial operation in 1979 describing pesticide usage on a field by field basis. A total of 21 different pesticide formulations were used in the 5 years of operation. An initial (1981) random sampling of the tile drain water did not detect any pesticide residues in the outflow at the 0.02 ug/L level. A subsequent extensive sampling (1982) detected residues of chlorothalonil on eight occasions ranging from 0.06 to 3.66 ug/L in the tile drain outflow. Ground water from one of two wells in the northwest quarter was found to contain chlorothalonil at a level of 10.1 to 272.2 ug/L in 1982 and 0.4 to 9.0 ug/L in 1983, carbofuran at a level of 11.5 to 158.4 ug/L in 1982 and < 0.5 to 1.0 ug/L in 1983, and carbofuran phenol (not quantified) in 1982 and 1983. RP-HPLC Kow data indicated that a number of the pesticides used on the farm could be as mobile or more mobile than chlorothalonil which had been detected in the ground water in two consecutive years; however, of the other pesticides only carbofuran was detected in the ground water. With a Kow lower than that of chlorothalonil, carbofuran was expected to be more mobile than chlorothalonil, and to appear in the water sooner, but this was not observed in the field samples.