Stable isotopes (S, Cl) and hydrochemical variations in a karstic ground water system, Guiyang, SW China

A groundwater system in a karstic terrain is easily subject to pollution and its remediation is difficult once contaminated by human activities. The groundwater samples collected in both winter and summer seasons at Gulyang, SW China, show a wide range of variations in chemical composition and dominance of Ca 2., Mg 2+, SO42-, and HCO3 ions. The anthropogenic inputs include CI, NO3, SO42-, Na +, and K +, as judged from the relationships between different elemental ratios and sewage samples. C1- concentrations of all water samples in the study area vary from 0 to 3.9 mmol/L in winter and are higher than in summer. In most of the samples, SO4 zconcentrations are high, ranging from 0.1 to 11 mmol/L. The studies show that the cycling of sulfur plays an important role in controlling water chemistry and fate of contaminants in groundwater. In order to understand the source of SO42- and the geochemical cycling of some elements in the karstic groundwater environment, we have carried out a study on the variations of S and C1 isotope compositions, in addition to other isotopic tracers such as 87Sr/86Sr and ~13C studied previously. The ~37C1 values of the groundwater samples present a seasonal variation, ranging from 0.00%0 to +2.03%0 in winter and from -1.46%o to +0.29%0 in summer. The surface water samples show a similar range of variations as observed for groundwater. Two rainwater samples collected at Gulyang have a distinct isotopic signature (mean value is --3%0) compared to the ~37C1 values reported in literature. These results indicate that ground water is mainly derived from precipitation and suggest a fast recharge of the karst system by surface water. The G34S values of SO42 in the groundwater range from -20%0 to +22%0, with most of the values between -15 9/00 and +10 9/00 and no seasonality is observed for the G 34S values. Correlations between G34S and C1/HCO3, Ca/SO4, G13C and 87Sr]86Sr are in favor of mixing between at least 3 sulfur sources into the karst system: dissolution of sulfate evaporites, oxydation of sulfide minerals present in coal-containing strata and anthropogenic inputs including atmospheric inputs and domestic discharge. The covariations between G34S, G13C, and 87Sr]86Sr indicate that sulfuric acid produced via oxidation of sulfide minerals and organic sulfur probably takes a significant part in the dissolution of carbonate rocks in this karstic region.