ORIGIN OF SODIUM-BICARBONATE GROUNDWATERS BY SILICATE HYDROLYSIS

Groundwaters dominated by dissolved sodium-bicarbonate represent an important end-member continental water type. Host rocks for such waters include siliciclastic aquifers, granodiorites, basalts, and anorthosites. The sources of CO₂ incorporated in bicarbonate are largely thought to be microbial, although mantle CO₂ has been invoked in one study. Two principal hypotheses address the sources of dissolved Na: 1) the base exchange hypothesis, where dissolution of CaCO₃ by carbonic acid releases dissolved Ca which displaces Na adsorbed on clay minerals; 2) the silicate hydrolysis hypothesis, where incongruent dissolution of Na-silicate minerals produces dissolved Na and HCO₃ and solid reaction products.

The Southern Hills aquifer system of Mississippi and Louisiana, USA is an excellent natural laboratory for studying the role of silicate hydrolysis in the origin and evolution of Na-HCO₃ groundwaters in siliciclastic aquifers. There is a large body of water analyses, and the physical hydrogeologic setting over flow paths of 120 km is well established. The waters vary systematically in composition down dip from barely altered meteoric waters having compositions of 0.1 mM HCO₃, pH = 5.5; to highly evolved waters with 3.8 mM HCO₃, pH = 8.4. The down-dip waters then mix with saline waters produced by the subsurface dissolution of salt domes.

The concentration of dissolved HCO₃ serves as a reaction progress variable. From HCO₃ = 0.1 to 1.5 mM, in the up-gradient part of the system, there are approximately linear increases in dissolved silica, Na, and Ca with increasing HCO₃, consistent with dissolution of detrital plagioclase feldspars having an average bulk composition of approximately Ab₀.₇₀An_0.30 and the precipitation of kaolinite. With increases in HCO₃ > 1.5 mM, the Na/H activity ratio increases significantly, but silica and Ca concentrations now decrease. The plagioclase dissolution reaction can now be described as: plagioclase + kaolinite + dissolved silica + carbonic acid yields (Ca>Na)smectite + dissolved (HCO3 + Na>>Ca). Downdip precipitation of calcite is another possible sink for Ca. Theoretical reaction path modeling calculations of incongruent silicate weathering have been presented in the literature for over 50 years. Other than this study, however, there have been few documented field examples over such a wide range of HCO₃ concentrations.