The source of high concentration pesticide hotspots within groundwater is of considerable concern. This paper reports on a simulation intensive study designed to examine if and how nonpoint source (NPS) pesticide applications can result in regional-scale groundwater contamination with local high-concentration hotspots. For this study, a hotspot is defined to have a concentration of at least one order of magnitude greater than the maximum contaminate level for the pesticide of interest. To address the hotspot question synthetic data were prepared for a generalized regional-scale saturated NPS system. The synthetic data were constructed by generating 1, 2-dibromo-3-chloropropane (DBCP) water-table loading scenarios that vary in space and time, creating alternative porous media realizations for different large-scale features, and designing well networks to simulate pumping. Fluid flow and solute transport simulations were conducted in a straightforward concept-development mode, for the individual boundary-value problems that result from the different synthetic data combinations. The simulations were conducted hierarchically, with only the data combinations that result in high concentrations being used at the next simulation level. The results from the more than 11 000 NPS simulations scenarios considered in this study yielded only 15 DBCP hotspots. Generating hotspots for label recommended NPS applications, within contaminated regional-scale groundwater systems, required the combination of elevated water-table loading concentrations and high pumping rates. The effort reported here provides a foundation for future work focused on the sources of pesticide hotspots in regional-scale groundwater systems.

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