PESTICIDES AND NUTRIENTS IN SOUTHERN UNITED-STATES SHALLOW GROUND-WATER AND SURFACE RUNOFF

Nutrient and pesticide concentrations in shallow groundwater (3m) and surface runoff were determined for conventional-till (CT) and no-till (NT) soybean watersheds. Groundwater NO3-N concentrations were similar for both tillage systems, averaging about 7-8 ppm, but exceeded 10 pp m for some storms. Annual mean groundwater NO3-N concentrations were o nly 0.34 ppm in a riparian zone downslope from the CT watershed. Highe r nutrient concentrations in NT surface runoff reflected surface resid ue leaching. Runoff from both watersheds, 2 days after a broadcast app lication of 0-20-20, had high nutrient concentrations beneath the decr eased during subsequent storms. Pesticide concentrations in groundwate r of the NT watershed were as high as 250 ppb at a depth of 1.5 m with in 1 week after application (1st rainfall). Concentrations beneath the CT watershed were <10 ppb maximum. One month after application, pesti cide concentrations in groundwater beneath both watersheds had decreas ed to about 10% of their respective 1-week values. Similar total pesti cide losses in runoff occurred for both tillage systems. NT reduced se diment loss but increased pesticide movement into the soil profile. Re sults of companion studies with corn at another site indicated similar trends for nutrients and pesticides in shallow groundwater. Within 11 months after application, herbicides were still detectable at very lo w concentrations (<1 ppb). Sod insecticides, applied at planting, were not found in groundwater. Herbicides and insecticides were detectable in both the water and sediment phases of runoff for 5 months after ap plication. At the 1.5-m depth, the mean NO3-N concentration in groundw ater for conventional-, reduced-and no-till corn was 4.78 ppm compared with 6.96 ppm at the 3-m depth.