Saline Contamination Pathway Assessment Using Spatial Variations in Groundwater Salinity, Baton Rouge Fault, Southeast Louisiana

There has been considerable interest in the role of faults as barriers and conduits for fluid flow. The Baton Rouge fault is a listric fault that cuts a thick siliciclastic sequence of complexly interbedded fluvial sands and mudstones in southeast Louisiana. Aquifer sands north of the fault, to a depth of 1000 m, are the principal supply of fresh water to the metropolitan and industrial Baton Rouge area. Sands near the fault have been increasingly contaminated by brackish water. The vertical migration up the fault of saline waters produced by the dissolution of deep salt has been proposed as the source of contamination of shallow aquifer sands. An alternative hypothesis is that the source of saline contamination is south of Baton Rouge and that saline waters have migrated laterally across the fault.

A detailed study has been done of the spatial variations in salinity as calculated from wire line logs for wells on either side of the Baton Rouge fault. Most of the logs were run in the 1960s, and the information provides a snapshot of the salinity structure prior to significant groundwater contamination. The spatial variations in salinity across the fault are consistent with natural lateral interfingering of fresh waters derived from the north, and brackish waters from the south. A 2004-2005 study of chloride concentrations in the groundwater showed that the highest chloride concentrations occurred at mid-depth in the aquifer system rather than the base, as might be expected if salt transport were up the fault. The most likely source of the saline contamination lies to the south, where dissolution of salt domes has produced saline plumes which extend upward all the way to the ground surface. Conduits for upward transport of salty water appear to be faults associated with the domes rather than regional listric faults.