ASSESSING NITROGEN PROCESSING NEAR A SWINE WASTE LAGOON IN A MANTLED KARST TERRAIN, NORTHWESTERN ARKANSAS

The Illinois River basin of northwestern Arkansas is a karst terrain dominated by thin, rocky soils that generally are unsuitable for row crop agriculture. Animal production is, therefore, the predominant agricultural practice in the basin, and waste storage lagoons at confined animal-feeding operations are a potential point source of water with elevated nutrient concentrations. The U.S. Department of Agriculture's Natural Resource Conservation Service (NRCS) has developed Waste-Storage Practice (WSP) 313 in an attempt to avoid seepage from such lagoons. The objective of this investigation is to determine the effectiveness of WSP 313 in preventing seepage from a swine facility waste lagoon at the University of Arkansas Savoy Experimental Watershed, located in the Illinois River basin. This objective was met by sampling water from the lagoon and springs, wells, and shallow ground-water “interceptor” trenches downstream from the lagoon during base-flow and high-flow conditions. Samples were analyzed for nitrogen species, total phosphorus, chloride, metals, and stable isotopes of nitrate, dissolved inorganic carbon, and dissolved organic carbon to assess nitrogen processing in shallow ground water. Concentrations of nitrogen species and chloride in wells and springs are low but above background concentrations. Ammonia concentrations are relatively high in lagoon water but relatively low in shallow ground-water “interceptor” trenches near the lagoon, suggesting rapid nitrification of lagoon seepage. Nitrate isotope data are being collected to fingerprint nitrate sources in water from springs and wells downstream from the lagoon, and to determine if denitrification is occurring during base-flow conditions. Varied nitrate concentrations and mixing models indicate dilution of nitrate during high-flow conditions. A water budget is being prepared that, when coupled with nitrate concentration data, will indicate whether or not lagoon seepage is adversely affecting water quality downstream of the lagoon.