ORIGIN OF WATERS CAUSING SALINIZATION OF THE BATON ROUGE AQUIFER SYSTEM, LOUISIANA

A major environmental and economic problem confronting the Baton Rouge, Louisiana metropolitan area is the ongoing salinization of fresh groundwater in the area. It has been hypothesized that the saline waters contributing to groundwater contamination were formed at depth by the dissolution of Middle Jurassic salt and have migrated up the Baton Rouge listric growth fault into the aquifer system. An alternative hypothesis proposes that the saline waters have formed instead by dissolution of the St. Gabriel salt dome 6 km to the south of Baton Rouge, have migrated vertically up faults related to salt tectonics, and are migrating to the north at relatively shallow subsurface depths.

To test the second hypothesis we have begun mapping out spatial variations in salinity and temperature above the St. Gabriel salt structure as a means of establishing potential pathways and mechanisms of upward transport of saline waters. Salt has been encountered in two wells at depths of 3400 m and approximately 4300 m. Salinity has been determined from the spontaneous potential response on 46 logs over salt. Most of the logs cover a depth range of 1200 to 2400m. Temperature gradients have been determined from corrected bottom hole temperatures derived from information on the log headers. Computed temperature gradients range from 18 to 36 ^oC/km. A series of slice maps at selected subsurface depths has been made to document spatial variations in salinity. Salinities range from 33 to 192 g/L at a depth of 1200 m. There is the indication that several areas of high salinities also correspond to localized areas of elevated temperature gradient. Such a correspondence would be consistent with the advective transport of brine and heat upward from salt. A series of logs from shallow groundwater wells in the area will be used to establish salinity variations at more shallow depths.

The structural geology above the salt structure is complex, with the presence of a large number of major normal faults possibly related to salt dissolution. We are in the process of attempting to correlate spatial variations in temperature and salinity with areas of fault compartmentalization.