Pesticide Leaching Potential by Watershed for 13 Crops
Source: Natural Resources Conservation Service - National Resources Inventory
Map Description:
Even when the label instructions are carefully adhered to, a small portion of pesticides applied on farm fields sometimes reaches surface and ground water, as evidenced by the detection of pesticides in water quality monitoring studies. Pesticide loss from farm fields depends on the natural characteristics of an area (soil properties, climate, and terrain), properties of the chemicals used, and farm management practices. The relationships among these factors are complex. Pesticides that leach or runoff on one soil type may not significantly leach or runoff with another soil type. To devise and implement policies for reducing pesticide loss from farm fields, decision-makers need to know where in the country the potential for these losses is the greatest.

Using national-level databases, a simulation was conducted of potential pesticide loss from farm fields on the basis of the factors that are known to be important determinants of pesticide loss, including:1. intrinsic potential of soils to leach or runoff pesticides, 2. chemical properties of the pesticides, 3. annual rainfall and its relationship to leaching and runoff, 4. cropping patterns, and 5. chemical use. Annual pesticide losses were estimated by Don Goss (Texas Agricultural Experiment Station, Temple, Texas) for a variety of soils and climates using the field-level process model GLEAMS (Groundwater Loading Effects of Agricultural Management). Leaching and runoff estimates were generated for 240 pesticides applied to 120 soils for 20 years of daily weather from each of 55 climate stations distributed throughout the United States. Pesticide runoff was movement beyond the edge of the field, including both pesticides in solution and pesticides adsorbed to soil material and organic matter. Pesticide leaching was movement beyond the bottom of the root-zone. Separate estimates were made for irrigated and nonirrigated conditions. These pesticide loss results were then integrated with a national chemical use database and the 1992 National Resources Inventory (NRI) to simulate potential pesticide loss for cropland throughout the coterminous United States.

NRI sample points were treated as representative fields. Land use data for 1992 was used. Thirteen crops were included in the simulation: barley, corn, cotton, oats, peanuts, potatoes, rice, sorghum, soybeans, sugar beets, sunflowers, tobacco, and wheat. Fruits, nuts, and vegetables were not included in the simulation because the NRI does not include data on specific crops for these categories. Estimates of percent acres treated with pesticides and application rate by crop and by state were obtained from Gianessi and Anderson for over 200 pesticides. These estimates represent average chemical use for the time period 1990-93. Pesticide use was imputed onto NRI sample points on the basis of the crop grown and the state in which the NRI sample point was located. The potential for pesticide loss from each representative field was estimated using the state average application rate, percent acres treated, and the percent annual pesticide loss estimates. The maximum percent leaching and runoff loss over the 20-year period was imputed to NRI sample points using match-ups by soil and proximity to the 55 climate stations. The total loss of pesticides from each representative field was estimated by summing over the loss estimates for all the chemicals that could have been used on the crop grown on the representative field, after adjusting for the percentage of the acres treated. Average watershed loadings were obtained by aggregating the pesticide loss over all the representative fields in the watershed where one of the 13 crops were grown using the NRI expansion factors as weights, and then dividing by the acres of nonfederal rural land in the watershed. Red areas of the maps include 25 percent of the watersheds with the highest scores. The maps show how the potential for pesticide loss varies around the country, assuming general chemical use practices and cropping patterns. Actual pesticide loss will differ from these simulated results because of the wide variety of application rates that farmers use, changes in the crops grown since 1992, and the management practices in use.

Research has shown that, with proper management, most of the potential for pesticide loss can be eliminated. Crop residue, for example, affects the water-holding capacity of the soil and thus the movement of water that carries the pesticides from the field. Organic matter content of the soil can be increased with appropriate management practices, thus increasing the adsorption of some pesticides onto soil particles and retaining them in the soil long enough for degradation by chemical and biological processes. Conservation tillage can reduce the soil loss from the field, and thus reduce loss of pesticides adsorbed to the soil particles. The potential pesticide loss shown in the maps does not adjust for reduction in losses resulting from these management practices. The maps do show, however, where in the country the need for careful farm management is the greatest, and where the likelihood of water quality impacts from pesticide loss from farm fields is the greatest.

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