Sulfate variations as a natural tracer for conduit-matrix interaction in a complex karst aquifer

Large areas of Europe, especially in the Alps, are covered by carbonate rocks, and karst springs are an important source for drinking water supply. Because of their high variability and heterogeneity, understanding the hydrogeological functioning is of particular importance for protection of karst aquifers. In this study, hydrogeochemical investigations characterized the water of a spring draining a complex carbonategypsum karst system in the Alps. The reaction of the spring to a rainfall event was examined to identify the relevant hydrological processes controlling the hydrochemistry of the spring, and to understand water-rock interactions and conduit-matrix exchange. A fast and marked reaction of discharge and electrical conductivity was observed. The main cations are Ca2+ and Mg2+, which showed a distinct decrease after the rainfall. Bicarbonate and sulfate were identified as major anions. Although HCO3- showed only minor fluctuations, SO42- decreased by 72% after the rain event. Comparisons of ion ratios show that both carbonate and gypsum rocks influence the water chemistry of the spring. The rainfall event caused a dilution effect, but dilution alone cannot explain the observed water chemistry. A conceptual model of the spring behaviour during low-flow and high-flow conditions, including conduitmatrix interaction, was developed, which can explain the observations. This study aims to give new insights into the highly dynamic exchange processes between karst conduits and the surrounding matrix, and the results demonstrated that (a) during low-flow conditions, the spring is characterized by high sulfate content, but after rainfall events, the water chemistry is dominated by bicarbonate. These findings show the dependency of water chemistry from the lithology; (b) a change in water chemistry is associated with a significant shift from low-flow to high-flow conditions; (c) conduitmatrix exchange is an important factor as shown by the discharge-sulfate relationship and clearly influences the behaviour of the spring.