Use of the Gleams Model to Estimate Pesticide Overland and Subsurface Transport in USDA Forest Service Nursery Applications

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The USDA Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) model was used to estimate pesticide concentrations entering surface water systems draining USDA Forest Service nurseries and subsequent risks from human oral exposure to the stream water. The GLEAMS model was employed to determine the primary pathways of pesticide loss, pesticide build-up in soil after successive applications, and pesticide concentrations in field runoff, eroded soil, and leachate. Pesticide residues in field runoff and adsorbed to eroded soil, both estimated by GLEAMS, were diluted into runoff volumes from treated and untreated areas to determine pesticide concentrations in the stream systems draining the nurseries. Where appropriate, pesticide residues from leachate were included. Pesticide concentrations in leachate and leachate volumes, both estimated by GLEAMS, and site-specific characteristics were used to make judgements about the susceptibility of drinking water aquifers to contamination by pesticides. The results of the GLEAMS analysis indicated that the pathways for nursery pesticide loss can be determined based on pesticide-specific parameters obtained from the open literature; however, the relative magnitude of the loss through each pathway must be estimated using nursery site-specific parameters.