Hydrochemical indications of human impact on karst groundwater in a subtropical karst area, Chongqing, China

The application of combined isotopic and hydrochemical compositions may be useful for evaluating water quality problems in karst aquifers in which it is difficult to distinguish the sources of solutes from the natural background of those due to human activities. Multiple isotopes (δ13C–DIC, δ34S–SO42−) and chemical parameters were measured in rainwater, groundwater and sewage in order to elucidate the solute sources and impacts from human activities and natural background in the Laolongdong karst catchment in Chongqing Municipality, SW China. Overall, the dissolution of carbonate rock controls Ca2+, Mg2+ and HCO3− content in rainwater and karst groundwater. SO42− originated mainly from gypsum dissolution in karst groundwater. Carbonate rocks in the studied site could be dissolved jointly by H2CO3 from the natural CO2–H2O reaction and other acids (organic acids and HNO3) from sewage and soils. Sewage discharge from urban areas and agriculture activities lead to the increase of NO3−, PO43− and Cl− in karst groundwater. To protect and sustainably utilize the karst aquifer, sewage originating from urban areas must be controlled and treated and the use of fertilizer should be limited.