The Baton Rouge fault, South Louisiana: a barrier and/or conduit for vertical and/or lateral ground water flow?

There is considerable interest in the role that faults play as barriers and conduits for fluid flow. The Baton Rouge (BR) fault is a listric fault that cuts a thick siliciclastic sequence of complexly interbedded fluvial-deltaic sands and mudstones. Aquifer sands to a depth of 1000 m north of the fault are the principal supply of fresh water to the metropolitan and industrial Baton Rouge area. Sands near the fault are becoming increasingly contaminated by brackish water. Authors of a recent study of the geochemistry of ground waters in southern Louisiana concluded that saline contamination has been produced by the dissolution of deep salt and that the saline waters have migrated vertically up faults into shallow aquifers.

While it is probable that the elevated salinities near the BR fault reflect salt dissolution, a more 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 brine appear to be faults associated with the domes rather than regional listric faults. A detailed study has been done of the spatial variations in salinity calculated from logs for wells on either side of the BR fault. Most of the logs were run in the 1960s, so the log information provides a snapshot in time of the salinity structure prior to significant groundwater contamination. The spatial variations in salinity across the fault are consistent with natural lateral interfingering of freshwaters derived from the north and brackish waters from the south. A 2004-2005 study of chloride concentrations in the ground water showed that the highest chloride concentrations occur at mid-depth in the aquifer system rather than the base, as might be expected if salt transport were up the fault.