The effect of sulfuric acid neutralization on carbonate and stable carbon isotope evolution of shallow groundwater

Abstract

Abstract Carbonate neutralization of sulfuric acid has been observed in groundwater impacted by anthropogenic activities such as mine waste disposal. Our aim in this study is to provide greater insights as to how dissolved inorganic carbon (DIC) generation and CO 2(g) production from acidification and neutralization reactions affect the carbonate and stable carbon isotopic evolution of groundwater. We measured the concentrations of DIC and major ions and the stable carbon isotope ratio of DIC ( δ 13 C DIC ) in water samples from a metal sulfide- and carbonate-rich mine tailings pile considered an analogue to natural environments where acid generation and neutralization occur. In addition, we measured the concentrations of CO 2(g) and the δ 13 C of CO 2(g) in the vadose zone and from the soil zone at a background location. Near neutral pH and high concentrations of SO 4 2− , Ca 2+ , and Mg 2+ and the positive correlation between Ca 2+ + Mg 2+ and SO 4 2− + HCO 3 − is evidence that acidity produced by metal sulfide oxidation is neutralized by carbonates. Soil water and perched groundwater (saturated zone above the water table) had significantly higher DIC concentrations compared to groundwater, which suggest that DIC production from acid neutralization occurs primarily in the vadose zone where metal sulfide oxidation generates acidity. The range in δ 13 C DIC of soil water and perched groundwater (leachate) is consistent with the dissolution of carbonates with heavy δ 13 C and the loss of CO 2(g) from solution to the vadose zone by acid dehydration of HCO 3 − and diffusion. The concentration of CO 2(g) in the vadose zone was high when compared to atmospheric and the δ 13 C CO2 was enriched relative to background soil CO 2(g), consistent with CO 2(g) production from acid dehydration of HCO 3 − and from DIC loss as CO 2(g) due to high pCO 2 in the leachate samples. Geochemical and isotopic modeling suggest that the DIC concentrations and δ 13 C DIC of shallow groundwater is due to: (1) mixing of DIC in leachate in the vadose zone with infiltration from precipitation and/or lake recharge and (2) “open system” carbonate evolution controlled by CO 2(g) in the vadose zone produced from sulfuric acid neutralization by carbonates. The results of this study suggest that in natural and anthropogenic settings where sulfuric acid produced by metal sulfide oxidation is neutralized by carbonates, the carbonate evolution of shallow groundwater is not described by the classical model ascribed to soil zone CO 2(g) .