Source(s) of salinity in the Mississippi River Alluvial Aquifer, Iberville Parish, Louisiana

Abstract

The Mississippi River Alluvial Aquifer extends from Southern Illinois to the mouth of the Mississippi and is comprised of fluvial sands and gravels of Late Pleistocene age. Several areas of the aquifer in Arkansas, Mississippi, and Louisiana are affected by elevated levels of salinity. One such area occurs in Iberville Parish, LA, where the aquifer is 150-240 m in thickness and is capped by 23-38 m of clay. Recharge of the aquifer from the Mississippi River on the east is fresh, but salinity is high in the western portions of the aquifer and chloride levels are as high as 1,000 mg/L. The aquifer is an important source of water for several municipalities and industries, but prior to this study the source(s) of the elevated salinity levels or whether the high salinity can be remediated had not been determined. Possible sources of elevated salinity included remnant marine water from the last major transgression, recent encroachment of marine water, dissolution of one or more of the five salt domes in the area, and anthropogenic contamination. The source of salinity has been determined through mapping of spatial variations in salinity from well logs and from chemical analysis of well waters. The westward salinization of aquifer water represents a broad regional process of mixing with deeper saline waters and dissolution of salt domes, not contamination by anthropogenic point sources. The hydrogen and oxygen isotopic systematics of the aquifer waters indicate meteoric sources, not marine. The low Br/Cl and high Na/Cl ratios are consistent with a saline endmember produced by subsurface dissolution of salt domes, not a marine source. The dissolution of the shallow Bayou Choctaw salt dome is the principal source of elevated salinities in the aquifer, although there could be contributions from deeper domes in underlying Miocene sediments.