SOURCE AND FATE OF ELEVATED LEVELS OF CHLORIDE IN THE MISSISSIPPI RIVER ALLUVIAL AQUIFER, IBERVILLE PARISH, LOUISIANA

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 chloride concentrations. 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 chloride levels in the western portions of the aquifer 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 chloride levels or whether the chloride can be remediated had not been determined. Possible sources of elevated chloride levels 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 elevated chloride levels 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, 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. These chemical signals are consistent with several other studies of saltwater contamination of freshwater aquifers, including a broad regional study of shallow South Louisiana aquifers whose salinization can be traced back to dissolution of salt domes in deep Miocene sands. It is possible that because of the higher Mississippi river stages caused by leveeing, the brackish aquifer waters now are being flushed out of the system. The ultimate fate of these brackish waters is diffuse discharge into the Atchafalaya basin.