Impact of sulfuric and nitric acids on carbonate dissolution, and the associated deficit of CO2 uptake in the upper-middle reaches of the Wujiang River, China.

ABSTRACT

Carbonate weathering and the CO2 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 CO2 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 CO2 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 Ca2+ and Mg2+ were the dominant cations (55.87-98.52%), and HCO3- was the dominant anion (63.63-92.87%). The combined concentrations of Ca2+ and Mg2+ commonly exceeded the equivalent concentration of HCO3-, with calculated [Ca2++Mg2+]/[HCO3-] equivalent ratios of 1.09-2.12. The mean measured groundwater δ13CDIC 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 [SO42-+NO3-]/HCO3- showed that additional SO42- and NO3- 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 CO2 to the total aquatic HCO3- 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 CO2 uptake by carbonate weathering across the study area was 14.92%, which suggests that previous workers may have overestimated the absorption of CO2 by carbonate weathering in other karstic areas worldwide.