VARIATIONS IN GROUND WATER CHEMISTRY IN HINDS AND MADISON COUNTIES, MISSISSIPPI

Municipal water wells have been routinely analyzed for inorganic and organic compositions by the Mississippi Department of Health and the results are archived at the U.S. Geological Survey in Jackson. Analysis of these data demonstrates significant changes in inorganic geochemical compositions of ground waters in Madison and Hinds counties in central Mississippi, although each county shows its own unique characteristics.

Two distinct groups of ground waters have been found in Madison County. Group one waters that have high (350 mg/L) total dissolved solids (TDS) occur at shallow depths (<250 m), and are restricted to the southeastern part of the county. Group one waters also display high concentrations of Cl, SO4, and Na. Group two waters have low TDS (100 mg/L), come from depths of 250 – 500 m, and are distributed throughout the county. Group two waters have low concentrations of Cl, SO4 and Na. In Hinds County, however, no distinct depth-related grouping is found relative to TDS, Cl, SO4, or Na. But, ground water compositions show major east-to-west variations. Groundwater having low TDS (>250 mg/L), Cl, SO4, and Na are found on the eastern part of county. But concentrations of all elements systematically increase westward in such a way that ground waters on the western edge of the county have TDS as high as 900 mg/L.

Surface and subsurface geological analysis of the two counties suggest that regional variations in groundwater compositions can be related to following: (1) the location of the recharge area, (2) geological processes around the Jackson dome, (3) rock – water interaction with clastic-dominated aquifers, and (4) brine intrusion along faults.