HEAT USED TO QUANTIFY AND EVALUATE THE ROLE OF GROUNDWATER AND SURFACE-WATER INTERACTION ON NUTRIENT TRANSPORT TO THE SHALLOW ALLUVIAL AQUIFER UNDERLYING NORTHWESTERN MISSISSIPPI

Between April 2007 and November 2008, the U.S. Geological Survey (USGS) has collected various hydrogeologic and water-quality data to evaluate the role of groundwater and surface-water interaction on the transport of agriculturally applied nutrients to the shallow (less than 25 feet deep) sand and gravel aquifer underlying the Mississippi Alluvial Plain in northwestern Mississippi. Despite stringent best management practices, agricultural activities seemingly contribute to nutrient and pesticide loads in the region's groundwater and surface waters. A pervasive, near-surface, semi-impermeable clay layer appears to substantially inhibit movement of nutrients into the shallow alluvial aquifer. However, many streams and ditches are incised below the clay layer into more permeable material, thus suggesting another, more direct route for nutrient transport into the underlying alluvial aquifer. Such transport would be further enhanced by declining water levels in the aquifer. Previous investigations by the USGS showed that during periods of high surface-water flow, groundwater flow reverses direction, and the stream changes from a gaining stream (groundwater flow into the stream) to a losing stream (surface-water flow into the streambed sediments and potentially into the shallow alluvial aquifer). A one-dimensional model developed for the investigations considered only the movement of water in the vertical direction (into and out of the streambed). The present investigation expands on the previous model by evaluating both the vertical and horizontal flow components and couples this information with water-chemistry data.