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

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 CO2(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 (δ13CDIC) 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 CO2(g) and the δ13C of CO2(g) in the vadose zone and from the soil zone at a background location. Near neutral pH and high concentrations of SO42−, Ca2+, and Mg2+ and the positive correlation between Ca2+ + Mg2+ and SO42− + HCO3− 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 δ13CDIC of soil water and perched groundwater (leachate) is consistent with the dissolution of carbonates with heavy δ13C and the loss of CO2(g) from solution to the vadose zone by acid dehydration of HCO3− and diffusion. The concentration of CO2(g) in the vadose zone was high when compared to atmospheric and the δ13CCO2 was enriched relative to background soil CO2(g), consistent with CO2(g) production from acid dehydration of HCO3− and from DIC loss as CO2(g) due to high pCO2 in the leachate samples. Geochemical and isotopic modeling suggest that the DIC concentrations and δ13CDIC 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 CO2(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 CO2(g).

Research Highlights
► We studied acidification and neutralization during groundwater carbonate evolution.
► H+ neutralization by CaCO3 produces high concentration of dissolved inorganic carbon.
► Neutralization of H+ by HCO3− creates high concentration of CO2 in the vadoze zone.
► Stable carbon isotopes show evidence of CO2 from CaCO3 in groundwater.
► Groundwater carbonate evolution is not always controlled by soil CO2.