GROUND-WATER DISCHARGE OF AGRICULTURAL PESTICIDES AND NUTRIENTS TO ESTUARINE SURFACE-WATER

This research investigated the transport of land-applied nutrients and pesticides from unconfined aquifers to tidal surface waters of Virgin ia's coastal plain. Ground water, estuarine surface water, ground wate r discharge, upland soil, and offshore sediment samples were collected from May 1992 until February 1993 from four agricultural sites. Sampl es were analyzed for inorganic nitrogen and phosphorus and five pestic ides: atrazine, cyanazine, alachlor, metolachlor, and carbofuran. Pest icides from aqueous samples were determined by liquid-solid phase extr action followed by gas chromatography-electron capture detection (GC-E CD) an/or by pesticide-specific immunoassay. Soil and sediment samples were analyzed by extraction and gas chromatography/mass spectrometry (GC/MS). Nutrient measurements indicated that fertilizer nitrogen was moving from the ground water to the surface water. and nitrogen fluxes across the sediment-water interface were correlated with fresh water discharge rates. Mean nitrate-N flux was 2.48 mg/m(2) . hr with a maxi mum value of 30.98 mg/m(2) . hr. Pesticides were detected in more than half of the upland soil samples. in approximately 40 percent of the g round water samples, and in just under 20 percent of the seepage meter samples. Pesticides were nor detected in any of the offshore sediment samples or surface water samples. Alachlor and metolachlor were detec ted in upland soil samples at concentrations ranging from 10 to almost 500 mu g/kg. All five pesticides were found in ground water samples a t concentrations generally below 1 mu g/L, with alachlor, atrazine, an d metolachlor most frequently Found. Alachlor, atrazine, cyanazine, an d metolachlor were detected in water discharging across the sediment-w ater interface and entering estuarine waters at concentrations ranging from 0.05 to 0.5 mu g/L. These levels were generally consistent with the amount of dilution due to the mixing of fresh ground water and sal ine pore waters prior to discharge across the sediment-water interface . Based on all positive detections of pesticides in ground water disch arge, which represented approximately 18 percent of all samples, avera ge flux rates of cyanazine, metolachlor, alachlor, and atrazine were 0 .32. 0.37, 0.80, and 1.12 mu g/m(2) . hr, respectively These findings indicate that submarine ground water transport of both nutrients and p esticides does occur. and this transport route should be considered wh en implementing agricultural management practices. The levels of nitro gen transport to surface water appears significant. The overall levels of pesticide movement through ground water, although generally quire low, represent a transport route that is commonly neglected in watersh ed management.