Impact of sulfuric and nitric acids on carbonate dissolution, and the associated deficit of CO uptake in the upper-middle reaches of the Wujiang River, China.

Carbonate weathering and the CO consumption in karstic area are extensive affected by anthropogenic activities, especially sulfuric and nitric acids usage in the upper-middle reaches of Wujiang River, China. The carbonic acid would be substituted by protons from sulfuric and nitric acids which can be reduce CO absorption. Therefore, The goal of this study was to highlight the impacts of sulfuric and nitric acids on carbonate dissolution and the associated deficit of CO uptaking during carbonate weathering. The hydrochemistries and carbon isotopic signatures of dissolved inorganic carbon from groundwater were measured during the rainy season (July; 41 samples) and post-rainy season (October; 26 samples). Our results show that Ca and Mg were the dominant cations (55.87-98.52%), and HCO was the dominant anion (63.63-92.87%). The combined concentrations of Ca and Mg commonly exceeded the equivalent concentration of HCO, with calculated [Ca+Mg]/[HCO] equivalent ratios of 1.09-2.12. The mean measured groundwater δC value (-11.38‰) was higher than that expected for carbonate dissolution mediated solely by carbonic acid (-11.5‰), and the strong positive correlation of these values with [SO+NO]/HCO showed that additional SO and NO were required to compensate for this cation excess. Nitric and sulfuric acids are, therefore, suggested to have acted as the additional proton-promoted weathering agents of carbonate in the region, alongside carbonic acid. The mean contribution of atmospheric/pedospheric CO to the total aquatic HCO decreased by 15.67% (rainy season) and 14.17% (post-rainy season) due to the contributions made by these acids. The annual mean deficit of soil CO uptake by carbonate weathering across the study area was 14.92%, which suggests that previous workers may have overestimated the absorption of CO by carbonate weathering in other karstic areas worldwide.