Ground Water Discharge of Agricultural Pesticides and Nutrients to Estuarine Surface Water

Abstract

AbstractThis research investigated the transport of land‐applied nutrients and pesticides from unconfined aquifers to tidal surface waters of Virginia's coastal plain. Ground water, estuarine surface water, ground water discharge, upland soil, and offshore sediment samples were collected from May 1992 until February 1993 from four agricultural sites. Samples were analyzed for inorganic nitrogen and phosphorus and five pesticides: atrazine, cyanazine, alachlor, metolachlor, and carbofuran. Pesticides from aqueous samples were determined by liquid‐solid phase extraction followed by gas chromatography‐electron capture detection (GC‐ECD) and/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/m2hr, with a maximum value of 30.98 mg/m2hr. Pesticides were detected in more than half of the upland soil samples, in approximately 40 percent of the ground water samples, and in just under 20 percent of the seepage meter samples. Pesticides were not detected in any of the offshore sediment samples or surface water samples. Alachlor and metolachlor were detected in upland soil samples at concentrations ranging from 10 to almost 500 μg/kg. All five pesticides were found in ground water samples at concentrations generally below 1 μg/L, with alachlor, atrazine, and metolachlor most frequently found. Alachlor, atrazine, cyanazine, and metolachlor were detected in water discharging across the sediment‐water interface and entering estuarine waters at concentrations ranging from 0.05 to 0.5 μg/L. These levels were generally consistent with the amount of dilution due to the mixing of fresh ground water and saline pore waters prior to discharge across the sediment‐water interface. Based on all positive detections of pesticides in ground water discharge, which represented approximately 18 percent of all samples, average flux rates of cyanazine, metolachlor, alachlor, and atrazine were 0.32, 0.37, 0.80, and 1.12 μg/m2hr, respectively. These findings indicate that submarine ground water transport of both nutrients and pesticides does occur, and this transport route should be considered when implementing agricultural management practices. The levels of nitrogen transport to surface water appears significant. The overall levels of pesticide movement through ground water, although generally quite low, represent a transport route that is commonly neglected in watershed management.