Characteristics and controls of the hydrochemistry and carbon isotope of thespring water in the Hongjiadu Basin of Guizhou

Spring water is an important source of drinking water for the residents in the Hongjiadu Basin of Guizhou. However, with the rapid increase in human activities, the quality of the spring water has continuously degraded over recent years. To investigate the sources and geochemical processes of contamination in the springs, 15 representative water samples were collected for analyses of the characteristics of hydrochemistry and isotope of dissolved inorganic carbon (DIC). The spring water samples are dominated by Ca2+, Mg2+, HCO-3 and SO2- 4. Sulfuric and nitric acid, together with carbonic acid, are involved in the weathering of carbonate rocks. Spring S01 and S09 are considered unsuitable as drinking water sources due to exceeding concentration limit of NO-3. Na+, Cl- , K+ and NO- 3 in the springs mainly originates from sewage, fertilizer (e.g. chemical fertilizer and manure); SO2-4 originates from sulfide oxidation in coal seams, acid rain, and sewage. The characteristics of δ13CDIC in the Hongjiadu Basin are thereby considered to be mainly impacted from soil CO2 and weathering of carbonate rocks. Meanwhile, the participation of sulfuric and nitric acid in carbonate rock weathering leads to positive δ13CDIC values and increasing concentrations of SO2-4 and NO-3 in the spring water, whereas the degradation of anthropogenic organic matter, bacteria sulfate reduction and denitrification result in negative δ13CDIC values. These factors caused the δ13CDIC values of the spring water from the study area to fall within the range of -17.72‰ to -8.74 ‰ with a average of -11.58‰. This study confirms that the isotope of δ13CDIC, combined with hydrochemical analyses, is useful to examine the carbon biogeochemical processes and trace the pollutants sources in groundwater in karst areas.