ESTIMATED RATES OF DISSOLUTION OF A SALT DOME UNDERLYING THE MISSISSIPPI RIVER ALLUVIAL AQUIFER, IBERVILLE PARISH, LOUISIANA

The Mississippi River Alluvial Aquifer (MRAA) is important source of groundwater for several municipalities and industries in south-central Louisiana. In western Iberville Parish, the aquifer is recharged by the Mississippi River to the east, and there is diffuse discharge in the Atchafalaya Basin to the west. Recharge waters from the Mississippi are fresh, but salinity levels in the western portions of the aquifer are as high as 10,000 mg/L. It has previously been determined that the dissolution of the shallow Bayou Choctaw salt dome is one of the sources of elevated salinity in the aquifer. It is of interest in terms of understanding rates of groundwater salinization, the tectonic history of the dome, and the fact that the dome is used as a crude oil storage facility to estimate the rates of dissolution of salt.

The MRAA was deposited following the Late Wisconsin sea level low stand ca. 20,000 ybp. The top of salt appears to be nearly coincident with the base of the MRAA at a depth below ground surface of 200 m, and the salt is capped with an approximately 150 m thick mass of carbonate caprock which extends upward into the MRAA. It seems more likely that the caprock has formed as a result of MRAA groundwater flow which dissolved salt and converted anhydrite to calcite than there having been a pillar of caprock jutting 150 m above the ground surface prior to the deposition of the MRAA. Published analyses indicate the salt contains 3.7 wt. % anhydrite. Converting all the anhydrite to a 150 m thickness of calcite caprock having an assumed porosity of 0.2 from literature values would require the dissolution of an approximately 2800 m thickness of salt over a 20,000 y period of time with an average dissolution rate of 240 t/y of halite and an uplift rate of salt of 0.14 m/y. Present lateral groundwater flow velocities in the MRAA are approximately 100 m/y. Given the differences in chloride concentrations in waters up the flowpath and down the flowpath from salt the estimated dissolution rate could be produced by groundwater flow through the lower 0.4 m or so of the sediments immediately overlying salt. However, there have probably been significant variations in fluid flow and dissolution rates over time.