ORIGIN OF SODIUM-BICARBONATE GROUNDWATERS IN THE SOUTHERN HILLS AQUIFER SYSTEM, SOUTHEASTERN U.S.: SILICATE HYDROLYSIS

The origin of sodium-bicarbonate groundwaters in siliciclastic aquifers has been variously attributed to calcite dissolution and Ca-Na cation exchange on clays and/or to irreversible hydrolysis reactions involving Na-silicates. Our research, which is based on spatial variations in water chemistry, mass balance relations, and thermodynamic considerations, supports the hypothesis that the process in the Southern Hills aquifer system, southwest Mississippi and southeast Louisiana, is driven by dissolution-precipitation reactions primarily involving silicate minerals, not by cation exchange. Our evidence supports the following scenario: oxidation of organic carbon produces carbonic acid which reacts with detrital plagioclase feldspars having an average composition of approximately Ab₀.₇₀An_0.30. In the up-gradient part of the system, the incongruent dissolution of plagioclase produces kaolinite and releases dissolved silica, Na, Ca, and HCO₃ into solution. As the silica concentration and the sodium/hydrogen ion activity ratio in the waters increase down gradient, the waters become saturated with respect to (Na,Ca)-smectite. The plagioclase dissolution reaction now switches to: plagioclase + kaolinite + dissolved silica + carbonic acid → (Na,Ca)-smectite + dissolved Na, Ca, and HCO₃. As the groundwaters evolve in composition, the Na/H activity ratio increases significantly, but silica concentrations now actually decrease down gradient. Some Ca is preferentially removed from solution over Na by adsorption on newly-formed smectite. In the most distal parts of the aquifer system the high bicarbonate alkalinity and high pH result in the waters becoming saturated with respect to calcite, and precipitation of calcite is another probable sink for Ca. Silica concentrations level off to values consistent with chalcedony saturation.