EVOLUTION OF GROUNDWATER COMPOSITIONS IN THE MISSISSIPPI RIVER ALLUVIAL AQUIFER, SOUTHEASTERN LOUISIANA

The Mississippi River Alluvial Aquifer (MRAA) has been an important source of water for municipalities and industries in West Baton Rouge and Iberville parishes, southeast Louisiana. The aquifer is recharged by the Mississippi River on the east, and groundwater discharges into the Atchafalaya River basin in the west. The aquifer consists of an approximately 150 m thickness of Pleistocene sands and gravels. A clay unit 5 to 10 m in thickness divides the aquifer into upper and lower units. There have been a number of significant water quality issues which have limited the utilization of the aquifer. These include local areas of elevated groundwater salinity, hardness, and dissolved iron. Previous work by us has shown that the areas of elevated salinity are the result of dissolution of salt domes and are not remnant marine waters, as previously believed. The present study has focused in on controls on hardness and dissolved iron in the freshwater upper and lower portions of the aquifer. A large database on the composition of Mississippi River water as a function of discharge and river stage makes it possible to put realistic constraints on the composition of the waters recharging the aquifer. These recharge waters have an initial hardness of ca. 100 mg/L as CaCO₃ as a result of high river water concentrations of Ca and Mg. Hardness and bicarbonate concentrations in the upper aquifer groundwaters increase over three-fold as the result of production of carbonic acid and the dissolution of carbonates. In contrast, silicate dissolution is more important in the lower aquifer, and there are marked increases in sodium and bicarbonate. There is a strong redox gradient away from the recharge zone, and river water sulfate is quickly reduced and dissolved iron introduced along the groundwater flow paths.