ABSTRACT

The potential for contamination of shallow ground water accompanies exploration for and production of oil and gas. Brackish water occurs locally in the surficial, otherwise freshwater, Mississippi River alluvial aquifer in east central Louisiana. This brackish water is thought to be the result of contamination by oil-field brines from the Wilcox Group, the principal hydrocarbon-bearing unit in east central Louisiana. To aid in increasing the precision with which sources of shallow ground-water salinity can be determined, we studied the geochemistry of brines from the Wilcox Group and brackish water from the Mississippi River alluvial aquifer.

Regional ground water flow patterns and spatial variations in brine chemistry are consistent with the hypothesis that Wilcox Group brines acquired salinity by dissolution of diapiric Louann salt in the Mississippi and Gulf Coast Salt Dome Basins. The ⁸⁷Sr/⁸⁶Sr ratio of Wilcox Group brines increases with depth in east central Louisiana. This trend in ⁸⁷Sr/⁸⁶Sr ratios with depth may reflect mixing of a brine having ⁸⁷Sr/⁸⁶Sr ratios within the range of mid-Jurassic seawater, originating in the Mississippi Salt Dome Basin, with a more radiogenic brine originating in the Gulf Coast Salt Dome Basin.

A field test using samples of brackish water from contaminated wells in the Mississippi River alluvial aquifer of east central Louisiana demonstrates that where oil-field brine contamination is suspected in the shallow subsurface, ⁸⁷Sr/⁸⁶Sr ratios of brackish water can be used to infer the ⁸⁷Sr/⁸⁶Sr ratio of a potential contaminant brine and thereby constrain the depth from which brine contamination originates. Depth of origin of a contaminant brine could, in turn, constrain potential pathways of brine movement into the shallow subsurface, such as plugged and abandoned oil wells and test holes.