Abstract

There is considerable interest in the role that faults play 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 to a depth of 2800 ft 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. It has been proposed that the saline contamination has been produced by the dissolution of deep salt and that saline waters have migrated vertically up the fault into shallow aquifers. An alternative hypothesis is that the source of saline contamination lies 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 calculated from wire line logs for boreholes on either side of the Baton Rouge fault and of chloride concentrations of groundwater samples from wells near the 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 later 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 high chloride zone extends west-east at shallow depth north of the fault, and an area of low chloride concentration exists at depth south of the fault. These chloride anomalies presumably reflect the existence of leak points across the fault. 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. While it is probable that the elevated salinities in waters near the Baton Rouge fault ultimately 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.