GEOCHEMICAL EVIDENCE FOR VERTICAL MIGRATION UP THE BATON ROUGE-DENHAM SPRINGS FAULT SYSTEM

The Baton Rouge aquifer system, located in south-central Louisiana, is a vital source of drinking and industrial water. The Quaternary-Neogene aquifer system is cut by an east-west trending fault, the Baton Rouge-Denham Springs (BR-DS) Fault that separates the system into an up-thrown north side (predominantly freshwater) from a down-thrown south side (predominantly brackish water). The southward dipping fault is thought to act as a partial barrier to updip movement of groundwater across the fault from the south to the north. The discharge of fresh water for industrial and drinking uses on the up-thrown side has produced a northward moving fresh water-brackish water boundary. Previous researchers have assumed that the source of the salt water is updip lateral movement within the aquifer system across the BR-DS Fault. Geochemical evidence in this study shows that the salt source is from deep brines moving up the fault system after dissolving Jurassic-age halite in Louan Salt diapirs. This vertical movement poses two problems: (1) calls into question the isolation of waste waters in deep-well injections below the aquifer system and (2) is resulting in a decrease in water quality within the aquifer system.

The geochemical evidence for halite dissolution includes the ratios of Na/Cl, Br/Cl and 87Sr/86Sr in the aquifer fluids. Ratios of Na/Cl and Br/Cl form linear mixing lines in which the saline end-member source has a molar Na/Cl ratio near unity and a molar Br/Cl ratio of 0.0005. These ratios would result from a marine fluid dissolving more than 70 g of recrystallized halite per liter of fluid. Diapirs of Louann salt at greater depths are the only known source of recrystallized halite. The measured 87Sr/86Sr ratios in the brackish groundwaters in the aquifer ranged from 0.70788, near the seawater value at the Eocene-Oligocene boundary up to 0.70837, characteristic of Lower Miocene seawater. These ratios increase within fluids as they move through clastic units, making them minimum ratios, corresponding to minimum ages of the source brine. The minimum ages are older than the ages of the Pleistocene, Pliocene, and Upper Miocene age of the formations (aquifers) containing the brackish waters.