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Trends in the Potential for Environmental Risk from Pesticide Loss from Farm FieldsRobert L. Kellogg, Natural
Resources Conservation Service, USDA PURPOSE OF STUDYEnvironmental indicators of the potential for pesticide leaching and runoff from farm fields were constructed to provide policy makers and national program managers with information to help assess whether existing programs and policies have been effective in reducing externalities from agricultural activities. Twelve pesticide-related time series were estimated for 1972 through 1997 for watersheds at the 6-digit Hydrologic Unit level:
ANALYTICAL METHODSA model was constructed to estimate pesticide loss from farm fields using the following data and information:
The National Pesticide Loss Database was used as the basis for all but the time series for pounds applied. The National Pesticide Loss Database provides estimates of the 95th percentile of annual mass loss and concentrations at the bottom of the root zone and the edge of the field for 120 soils, 55 climate stations, and 243 pesticides. Separate estimates are available for irrigated and nonirrigated conditions. Pesticide leaching and runoff losses were estimated using the pesticide fate and transport model GLEAMS. GLEAMS is a process model that uses as inputs soil parameters, field characteristics, management practices, pesticide properties, and climate to estimate pesticide leaching and runoff losses. (For additional details on the National Pesticide Loss Database, see "The National Pesticide Loss Database: A Tool for Management of Large Watersheds.") The analytical framework consists of about 5,000 resource polygons representing the intersection of 48 states, 280 watersheds at the 6-digit Hydrologic Unit level, and 1,400 combinations of climate and soil groups. Seven crops are currently included in the analysis: corn, soybeans, wheat, cotton, sorghum, barley, and rice. A total of 94 pesticides were included in estimates for 1972-86, and up to 192 pesticides were included in estimates for 1987-97. The framework was constructed using the NRI after imputing the pesticide loss data onto NRI sample points by crop, soil group, and climate group. Concentration and mass loss estimates in the National Pesticide Loss Database are relative to application rates so that, when multiplied by application rates specific to crops and states, the final estimate of the annual concentration or mass loss is obtained. Estimates of pounds of pesticides applied, mass loss, and annual concentrations were obtained for pesticides used on the 7 crops in each of the 5,000 resource polygons for each of the 26 years. On average, 75 percent of the cultivated cropland in the 48 states was incorporated into the simulation. Map 1 and map 2 show the spatial distribution of the approximate coverage for 1982 and 1992. (Note on the map legends: In the process of converting maps to the World Wide Web the legends on certain maps became unreadable due to their small size. As a temporary solution the legends are presented in a separate file until we can update the maps. Simply look up the map number to find the appropriate legend.) Environmental risk was assessed using Threshold Exceedence Units (TEUs), which measure the extent that the annual concentration exceeds a water quality threshold. For each pesticide used in each of the 5,000 resource polygons, the threshold-concentration ratio was calculated. Where the threshold concentration was exceeded, the ratio was multiplied by the acres treated to obtain estimates of Threshold Exceedence Units (TEUs). Acres treated were estimated for each resource polygon using acres-planted data and state-level estimates of the percentage of acres treated derived from the Doane and NASS survey databases. Thus, TEUs account for the amount the threshold was exceeded and the amount of acres treated in an area. TEUs were derived in this manner specifically for the purpose of comparing risk from multiple pesticides over space and time. They are a relative measure of risk (the higher the TEU score, the higher the risk), but do not measure absolute levels of risk. (TEUs are similar in concept to the acre-feet volumetric measure, since they are a multiple of acres times a measure of magnitude at a point.) TEUs per watershed were obtained by summing TEUs over all pesticides used in each resource polygon, then summing over all resource polygons in each watershed. (For information about another application using TEUs, see "An Information Aid for Assessing Possible NRCS Involvement in the State Management Plan Process for Regulation of Pesticides.") The estimated indexes of potential risk change over space and time because of: 1) changes in acres planted, 2) changes in percent acres treated, and 3) changes in the suite of pesticides used, and 4) changes in application rates of the more mobile and/or persistent pesticides on vulnerable acres. The estimated indexes are based on national-level databases, and so caution should be exercised in making local interpretations. Management factors are not included in the indicators. The algorithm for the potential risk index is:
where:
SUMMARY OF FINDINGSThe 8 pesticide risk indexes indicate that the Nation's pesticide policies during the last twenty six years have succeeded in reducing overall environmental risk, in spite of slight increases in acres planted and pounds of pesticides applied. Nevertheless, there are still areas of the country where there is no evidence of progress, and areas where risk levels for protection of drinking water, fish, algae and crustaceans remain high. Observations supporting this conclusion are:
The authors plan to extend the indexes to include additional crops in the future, but including additional crops is unlikely to alter the broad trends shown here because of the small acreage involved. Appendix: List of Figures and Maps |
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