Identification and quantification of contributions to karst groundwater using a triple stable isotope labeling and...

Although karst groundwater systems provide critical ecosystem services in many regions worldwide, anthropogenic contamination has seriously degraded groundwater quality. Properly elucidating geochemical processes, quantifying contributions of natural and anthropogenic end members, and then protecting karst aquifer systems remain challenging from scientific and engineering aspects. To identify the hydrochemical processes and quantifying contributions of end members (especially, contamination end members), 49 samples were collected from cave waters (CW), artesian springs (AS), and gravity springs (GS) in a karst watershed in Guiyang, China. With increased anthropogenic contamination, the CW, AS, and GS characterized by a Ca-Mg-SO₄^2--HCO₃^- composition often had pH and SO₄^2- concentrations exceeding USEPA secondary drinking water standards. That is attributed to the influence of water-rock interaction, rainfall, and anthropogenic sources (mainly, sewage and fertilizers), in agreement with the compositions of δ³⁴S_SO4, δ¹⁸O_SO4, and ⁸⁷Sr/⁸⁶Sr as well as the results of principal component analysis and correlation coefficients. Based on an end-member mixing model, contributions of rainfall and anthropogenic sources were 47% and 33% of GS, 52% and 41% of CW, and 58% and 35% of AS, respectively. It suggests that the karst groundwater quality is predominantly controlled by rainfall and anthropogenic sources (especially, land use). Results may be applied to properly evaluate the impacts of natural and anthropogenic sources on karst aquifers, coupled with actions to efficiently control potential contamination end members.