A county-level assessment of ground water contamination by pesticides

A pesticide screening model was integrated with a geographic information sy stem (GIS) for evaluating the ground water vulnerability to pesticide conta mination in Albemarle County, Virginia. The attenuation factor (AF), an ind ex of pesticide mass emission from the vadose zone, was used to evaluate th e relative contamination potential of 70 pesticides used in the county. Res ults for only three pesticides - atrazine, dicamba, and lindane - are discu ssed in this paper. Spatial (land use, soils, and hydrogeology) and relatio nal (soil and pesticide properties) data lavers were combined with the AF m odel within the GIS environment for spatial computation of AF: for actual a nd 2 m ground water depths. For each pesticide, a ground water vulnerabilit y map with five contamination potential categories (high, medium, low, very low, and unlikely) was generated, based on the spatial distribution of AF for each cell size of 0.27 acre (0.11 ha). To consider the variability in p esticide transport, model simulations were performed for "maximum," "averag e," and "minimum" scenarios of pesticide leaching. Under the average leachi ng scenario (2 m depth), the three pesticides were found to have very low t o low levels of contamination potential in some areas. For maximum leaching scenario (2 m depth), contamination potential of these three pesticides in creased to low to medium levels. When actual ground water depth was used, n o significant contamination potential was indicated by any of the three pes ticides. The modeling approach mas evaluated using the data from a limited monitoring study in Albemarle County. Although agreement between the model prediction and actual pesticide detection was observed, extent of the data was not sufficient enough to draw firm conclusions about the model's succes s in predicting contamination potential. A sensitivity analysis of the meth odology revealed that significant uncertainty can be involved in predicting the county-scale contamination potential; To make better use of this study , it is recommended that a comprehensive pesticide monitoring be undertaken in Albemarle County.