The contribution of human activities to dissolved inorganic carbon fluxes in a karst underground river system: Evidence from major elements and δ13CDIC in Nandong, Southwest China

Generally, the DIC in karst groundwater is dominantly derived from carbonate dissolution by carbonic acid. However, recently increases in the inorganic carbon flux have been linked to human activities, which nitric and sulfuric acids may contribute to carbonate dissolution. In order to quantify the sources and fluxes of DIC, and evaluate the carbon isotopic evolution of groundwater in Southwest China, the carbonate dissolution by carbonic, sulfuric and nitric acids was evaluated by hydrochemistry and δ13CDIC of groundwater. The results show that: (1) groundwater collected from residential and agricultural areas, showed higher DIC concentrations and δ13CDIC than those in groundwater collected from forested and grass land areas; (2) the contributions of carbonate dissolution by carbonic acid to total DIC concentrations in groundwater collected from forested and grass land areas averaged 99%; (3) the contributions of carbonate dissolution by carbonic acid to total DIC concentrations in groundwater, collected from residential and agricultural areas, varied from 40% to 77% with a mean percentage of 62%; (4) while the contributions of carbonate dissolution by sulfuric and nitric acids to total DIC concentrations in groundwater, collected from residential and agricultural areas, varied from 23% to 60% with a mean percentage of 38%; and (5) the δ13CDIC approaching a value of around −14‰, with a molar ratio between (Ca2++Mg2+) and HCO3− of around 0.5 in groundwater, indicated that the carbonate was dissolved by soil CO2 from C3 vegetation under open system conditions. While the δ13CDIC varying from −5‰ to −11‰, with a variational molar ratio between (Ca2++Mg2+) and HCO3− of 0.5 to 0.8 in groundwater, indicated that carbonate dissolution was controlled by soil CO2 (from C3 vegetation), HNO3 and H2SO4. Also, this study indicated that the amount of soil or atmospheric CO2 consumed during carbonate weathering should be critically evaluated when sulfuric or nitric acids are involved. Thus, not only the exports of inorganic carbon have been enhanced, but also the concentrations of nitrate and sulfate in karst groundwater have been elevated due to carbonate dissolution by sulfuric or nitric acid.