The groundwater in the Baton Rouge area is dominated by sodium bicarbonate, making it very soft. It has a pH of 8 to 9 and is thus slightly alkaline rather than acidic. It also has very low levels of iron and manganese and is free from hydrogen sulfide. What are the geochemical processes that have given rise to this high quality groundwater?
The origin of sodium-bicarbonate groundwaters in siliciclastic aquifers in general has often been attributed to cation exchange. According to this model, microbial oxidation of organic carbon produces carbonic acid which dissolves calcite, producing dissolved calcium and bicarbonate. The calcium is exchanged for sodium adsorbed on clays, and the net result is sodium-bicarbonate groundwater. However, our research, which is based on spatial variations in water chemistry and thermodynamic considerations, supports the hypothesis that the process in the Southern Hills aquifer system is driven by dissolution-precipitation reactions primarily involving silicate minerals, not calcite dissolution and cation exchange. There is a progression of geochemical reactions down the groundwater flow path from north to south which results in the evolution of meteoric recharge waters to high pH, sodium-bicarbonate groundwaters. Throughout the system, oxidation of organic carbon produces carbonic acid which reacts with plagioclase feldspars having an average composition of approximately Ab0.70An0.30. 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, and pH exceeds 8. Silica concentrations now actually decrease down gradient. Some Ca is preferentially removed from solution over Na by adsorption on newly-formed smectite. However, 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. High alkalinity and high pH may also be responsible for the low concentrations of Fe and Mn, which should precipitate out as siderite. The waters never become sufficiently reducing to produce H2S. Our silicate dissolution hypothesis can be tested by careful petrologic examination of the aquifer sediments.
Key words: sodium-bicarbonate groundwater, siliciclastic aquifer, groundwater quality, Southern Hills aquifer system