THE FATE AND TRANSPORT OF NITRATE THROUGH THE UNSATURATED ZONE AT A SITE IN NORTHWESTERN MISSISSIPPI

Annually in the United States, about 12 million tons of nitrogen are applied each year as commercial fertilizer causing contamination of surface and groundwater resources. In the Mississippi Delta, large amounts of agricultural chemicals are applied to crops on an annual basis, but are rarely detected in groundwater. Previous studies have indicated that the shallow alluvial aquifer in the Delta is unaffected by anthropogenic activities at the surface because of an overlying impervious clay layer. However, model simulations have indicated that the alluvial aquifer is recharged by a small percentage (5%) of rainfall. In 2005, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program began a study in the Bogue Phalia basin to assess how environmental factors and agricultural practices affect the source and transport of agricultural chemicals. Two wells located in a cotton field (surveyed as very fine sandy loam and silty clay) in Bolivar County, Mississippi were sampled for inorganic compounds, nutrients, and field parameters from June 2006 to November 2008. Nitrate was detected at concentrations ranging from 7.2 to 13 mg/L in a shallow well screened near the water table from 27 to 32 feet, but was not detected in a deeper well screened from 70 to 120 feet located approximately one-quarter mile from the shallow well. In June 2008, depth interval sampling was conducted in test holes drilled adjacent to the shallow well to better define the occurrence of nitrate at five depths ranging from 32.5 to 60 feet - between the depths of the shallow and deep wells. Nitrate concentrations decreased with depth in the water column, and there were no detections below a depth of 36 feet. Data indicate that some nitrate is being transported through the unsaturated zone into the alluvial aquifer, but it is being converted fairly quickly into ammonia and nitrogen gas under strong, reducing conditions in the aquifer. The data imply that the aquifer may not be as invulnerable to anthropogenic activities as previously thought.