Insecticide and metal contamination of a mixed cover agricultural watershed

Abstract

Although pesticides used to control detrimental insects have changed since the wide spread use of highly persistent organochlorine insecticides in the 1940s through the 1970s, pesticides still present environmental problems. Random sampling of major watershed features of Otoucalofa Creek was conducted to study the distribution and concentration of residual and current use insecticides and metals in a mixed cover agricultural watershed. Samples of water, soil, sediment and fishes were taken from Otoucalofa Creek watershed and tested for 23 contaminants, including persistent organochlorine and current-use insecticides, PCBs and metals. Otoucalofa Creek watershed was chosen because of its location in the loess hills of northern Mississippi, USA, and the fact that its drainage includes 10,100 hectares of forest, 6,100 hectares of rowcrops, 4,900 hectares of pasture, and the small town of Water Valley (pop. 4,147). The greatest concentrations of insecticides were found in fish, but the highest concentrations of metals were found in soil and sediments. Six of 7 metals and 11 of 15 insecticides tested were present in fish samples. Arsenic and mercury averaged 0.67 μg/g and 0.31 μg/g, respectively, in soil samples and 0.25 μg/g and 0.03 μg/g in sediment samples, but were in lower concentrations in fish tissue. Relatively high concentrations of zinc were found in soil and sediment. DDT and its metabolites averaged 118 ng/g in fish, averaged only 1.90 ng/g in sediments and were undetectable in soil. Contamination of water was dependent upon storm related discharge and is therefore a transient phenomenon. Water samples taken during normal flow had the lowest concentrations of both metals and insecticides. Storm flow (High Stage) samples had significantly higher concentrations of insecticides and metals than did normal flow. Since arsenic is the only persistent contaminant still being applied to agricultural land, concentrations of residual insecticides should steadily decline. Current use insecticides were only detected seasonally and in extremely low concentrations, however, this transient, short term exposure, which may not have been as detrimental with less toxic organochlorine pesticides, is more significant with acutely toxic current use compounds.