MISSISSIPPI EMBAYMENT AQUIFER POSSIBLE SOURCE OF PHOSPHORUS IN SURFACE WATERS

A geochemical survey of the Mississippi Embayment aquifer system in 1998 revealed higher than expected phosphorus concentrations in groundwater from the Mississippi River Valley alluvial aquifer. The median total dissolved phosphorus (P) concentration (0.30 mg/L P) of groundwater from the alluvial aquifer exceeded the US Environmental Protection Agency’s (USEPA) stream nutrient criteria (of 0.1 mg/L P) and also the national background for P in groundwater (0.02 mg/L P). The alluvial aquifer, which underlies the northwest part of Mississippi and adjacent states, is the most heavily-used aquifer (for agriculture) in the State, and produces 1.3 billion gallons per day in Mississippi alone during the summer irrigation season. Irrigation return flow from the alluvial aquifer can account for a high percentage of the water in streams during summer low flow periods. This could potentially contribute to the Gulf of Mexico hypoxic zone. A larger-scale geochemical survey was conducted during the summer of 2010 by the US Geological Survey to characterize P concentrations of the aquifer. This equally distributed, 46-well survey yielded P concentration median, maximum, minimum, and means of 0.62, 1.17, 0.12, and 0.59 mg/L P, respectively. Every sample exceeded both the USEPA’s stream nutrient criteria and the national background for P in groundwater, yet the data do not appear to follow any spatial pattern. Another geochemical survey of the alluvial aquifer conducted in 2010 by the Mississippi Department of Environmental Quality was considered in the analysis of the aquifer P concentrations. The datasets were shown to have statistically similar distributions through a one-way analysis of variance, and therefore were combined for further analysis, increasing the number of samples to 93 across the area. The overall median of all datasets was 0.62 mg/L P, but increasing the number of samples did not diminish the random spatial dispersion of P. A small number of wells which were sampled more than once indicate that there may be a temporal pattern to the data. These data show a decrease in P through time with respect to the irrigation season. For example, one particular well in Bolivar County, MS showed a range of P values between 1.33 mg/L (sampled on June 26, 2008) and 0.54 mg/L (sampled on November 4, 2008).