Delineating vulnerability of karst aquifers using hydrochemical tracers in Southwestern China

Karst groundwater is particularly vulnerable to contamination. Long-term hydrochemistry auto-monitoring combined with laboratory analyses has shown that the chemograph demonstrates changes in the system. In order to determine the primary groundwater vulnerability, a relationship between hydrochemical changes and the origin of vulnerability in six karst spring catchments in Southwestern China is discussed. The reason and physical meaning of the hydrochemical changes in the spring are explained based on tracer tests and physical–chemical process in the S31 spring. The results prove that the hydrochemical changes show the origin of vulnerability and also can be its expression. Generally, autogenic recharge in bare karst is concentrated in epikarst zone, forming bypass flow and shaft flow, which causes dilution of Ca²⁺. The chemograph reflects the percentage of concentrated and diffuse recharge. The higher the ratio of concentrated recharge, the higher the grade of groundwater vulnerability. Due to Ca²⁺, HCO₃ ⁻, SEC, pH, and temperature in water can change for kinds of reasons during their transport, more karst systems with variable natural and human factors are discussed. Protective layer, allogenic water recharge, and transform of surface stream and subterranean river can change the origin shape of hydrochemical curve. Under this circumstance, more indicators need to be monitored, especially contaminant-related ions such as nitrate, in order to clearly understand the contribution of concentrated and diffuse recharge to water chemistry. After comparing the traditional groundwater vulnerability assessment and the methods based on hydrochemical change, rainfall, protective layer, epikarst, transient storage and karst network should be more concerned when the work is done. Auto-monitoring of karst is more and more popular in Southwestern China; therefore, it is suggested that the hydrochemical methods should be used as a primary expression of vulnerability, giving a direct reference for karst water management.