Use of Spatial Variations in Groundwater Salinity to Assess the Conduit-Barrier Properties of the Baton Rouge Listric Fault, Southeastern Louisiana
F. Colleen Wendeborn and Jeffrey Hanor
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 siliciclastic sequence of interbedded sands and mudstones in southeastern 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. These sands are becoming increasingly contaminated by the migration of brackish water northward. 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. A detailed study has been done of spatial variations in salinity, calculated using borehole wire line logs from either side of the Baton Rouge fault, and of chloride concentrations in groundwater samples from wells near the fault. 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 rather than vertical migration upward. 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. While vertical migration of saline water up the BR fault may be possible, the 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 water table. Conduits for upward transport of brine appear to be faults associated with the domes rather than regional listric faults.
AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013