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Abstract: Geohydrologic Controls on the Distribution and Origin of Brackish Water in the Mississippi River Alluvial Aquifer of Eastern Morehouse Parish, Louisiana, and Southern Chicot County, Arkansas

G. F. Huff

Analyses of groundwater samples from eastern Morehouse Parish, Louisiana, and southern Chicot County, Arkansas, indicate the presence of brackish water in the Mississippi River alluvial aquifer. Most of the brackish water in the Mississippi River alluvial aquifer of southern Chicot County is derived from saline waters in underlying aquifers of Tertiary age. This brackish water is introduced into the Mississippi River alluvial aquifer by the regional groundwater flow pattern. Within an area of anomalously-elevated salinity (dissolved solids concentrations >1000 mg/L) in southern Chicot County, Br/Cl ratios of brackish water indicate that a small part (approximately 5%) of the solutes may be derived from brines within the Smackover Formation of Jurassic age. The Smackover Formation i present at depths of approximately 5500 to 6500 ft below sea level in southern Chicot County. The presence of the area of anomalous salinity in conjunction with the reported location of the intersection of two regional wrench faults, one of which may penetrate into the Smackover Formation, supports the hypothesis that upward flow is channeled through the area of intersection of these faults.

Once introduced into the Mississippi River alluvial aquifer of southeastern Arkansas, brackish water flows to the southwest into Morehouse Parish along a buried fluvial channel eroded into the underlying Cockfield aquifer. Estimated flow velocities indicate that southwestward movement of brackish water in the fluvial channel could be measured on a human time scale.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994