EFFECTS OF MIXING METEORIC WATER AND SALINE GROUNDWATER

The geochemistry of spring water discharged from carbonate aquifers in the midcontinent region of the U.S.A. is controlled by the mixing of meteoric and saline waters and by subsequent water-rock interactions. At the field site, located outside of Bromide, OK, six springs discharge brackish water from the carbonate bedrock. As a result these springs provide an ideal location to study the role of mixing and water-rock interactions on the geochemical signature of spring waters in a carbonate aquifer. Our conceptual model is that mixing between a saline water from the Arkoma basin and a meteorite water, where the amount of meteorite water that mixes with the saline water is controlled by the size of the recharge area, followed by water-rock interactions along the independent subsurface flow paths accounts for the variations in water chemistry.

Results from four sampling events show that the concentrations of major ions discharged at the six springs varied over an order of magnitude. Differences in ion concentrations between the measured concentrations and the concentrations that were calculated based on a conservative mixing model were similar. This result indicates that most water rock interactions evident at the springs can be explained by the proportions of meteoric and saline waters being mixed together in the subsurface. The concentrations of calcium and strontium were elevated relative to the conservative mixing model. As anhydrite deposits are not common in the Arbuckle and Simpson Groups the elevated calcium and strontium concentrations are likely the result of the dissolution of carbonate minerals in the host bedrock. The saturation states of the majority of spring waters with respect to calcite and to dolomite were near equilibrium or undersaturated. Very few were oversaturated. The pCO₂ of the spring waters ranged from -1.8 and -1.2. These values were dissimilar from the conservative mixing model and appear to be related to the dissolution of calcite (and/or dolomite) and the outgassing of CO₂ along the flow path. The results from the conceptual model indicate that the mixing of different proportions of meteoric and saline waters incorporated with reactions involving carbonate dissolution explain the majority of the water-rock interactions at the study location.