Hydrogeochemical processes and vulnerability of a typical karst underground river system, southwest China

This study presents an investigation of the hydrogeochemical processes and potential vulnerability of a typical underground river system in southwest China. Physical and chemical data were collected from both traditional manual sampling and continuous automatic monitoring. Analysis of high-frequency monitoring data from the outlet spring, Jiangjiaquan spring (HQ), of the underground river system suggests the existence of several physico-chemical processes with distinctive characteristic response to storm events. Due to the impacts of sources and migration influencing factors, seasonal variations of major chemical components of the outlet spring are much different. For the entire karst watershed, spatial variability of surface and subsurface water geochemistry was significantly influenced by land use and lithologic characteristics. Principal component analysis (PCA) reveals that agricultural activities, water-rock interactions and soil erosion are potential influencing factors and the main sources for the chemical composition of JJQ spring. Nutrients from agricultural activities can be brought into the underground river by ways of diffuse recharge from leaching of soils, as well as focused recharge from surface flow via sinkholes. Water-rock interactions in the karst aquifer provide the evidence of diffuse recharge that contributed to groundwater quality. Soil erosion mainly causes focused recharge of contaminated water with particulate matters including iron, manganese and aluminum that carried into the underground river from sinkholes via surface flow.