Buffering of sodium concentration by cation exchange in the groundwater system of a sandy aquifer

The water chemistry data obtained from a sandy aquifer in the Trout Lake basin of northern Wisconsin, USA were reinvestigated in order to explain the chemical behavior of sodium concentration in groundwater. Monovalent cations such as sodium and potassium dissolved in groundwater generally remain within narrow concentration ranges. In contrast, calcium and magnesium concentrations show continuously increasing trends due to continuous weathering of aquifer materials, of which reactive minerals are dominated by sodic plagioclase. The similar trend observed in groundwaters contaminated with NaCl-deicer suggests that this unusual chemical behavior of monovalent cation is caused by cation exchange reactions. In other words, when Na ions are introduced into the groundwater system, only a minor fraction of the Na ion (∼5%) partitions into the aqueous phase, whereas the majority replaces divalent cations adsorbed on the exchangeable sites of the sediments. However, the cation exchange capacity (CEC) measured for the sandy sediment (∼4 meq/kg) is insufficient to cause the observed consistent Na concentration under a batch condition (stagnant groundwater with no molecular diffusion) of cation exchange. Results of transportive exchange calculations suggest that Na concentration can be effectively "buffered" as the NaCl-contaminated groundwater flows even when the CEC was very low (1 meq/kg) because flowing groundwater can interact with more cation exchange sites than that under stagnant conditions. It is also suggested that the buffering of Na concentration due to cation exchange can partly explain the "Ca-excess problem" that has been noted previously in the study area.