Multiple isotope approach to the determination of the natural attenuation potential of a high-alpine karst system

(3)H, delta(18)O and delta(2)H measurements on groundwater and chemical and isotope analyses on groundwater sulfate were used to determine the origin and mean transit times of groundwater and the sources affecting groundwater sulfate in a high-alpine karstic catchment area in southern Germany. Modelling results using a lumped parameter approach yielded mean transit times of water between 4 and 12 years for different karst springs. However, results obtained from delta(18)O and delta(2)H measurements on groundwater showed that a calculated mean transit time of 12 years for groundwater flow in one karst system can only be explained by mixing of young and old tritium-free ice and snowmelt water. Groundwater sulfate characterized by delta(34)S values of around 20 parts per thousand and delta(18)O values of ca. 12% in concert with sulfate concentrations of approximately 11 mg/L are probably affected by evaporites. In contrast, delta(34)S values of around 6 parts per thousand, and delta(18)O values up to 9.4 parts per thousand, in concert with sulfate concentrations between 1 and 7 mg/L mainly derive from atmospheric deposition. However, a delta(34)S value of -7.3 parts per thousand identified for one of the karst springs demonstrates that the oxidation of sulfide-containing minerals must be considered as an additional source, affecting sulfate concentrations in groundwater. The estimated transit time distribution of groundwater shows relatively high mean transit times between 2 and 5 years with a low contribution of very short transit times of less than 1 year. The high contribution of long mean transit times of groundwater in the alpine karst system, which represents an important drinking water resource, is an important function for drinking water supplies. (C) 2008 Elsevier B.V. All rights reserved.