Storage and transport in cave seepage- and groundwater in a South German karst system

Papers published in Hydrology and Earth System Sciences Discussions are under open-access review for the journal Hydrology and Earth System Sciences Abstract We investigated one of the best-known and second largest karst areas in Germany (Blautopf Catchment) that offers a unique access waters of the unsaturated zone through a large cave system. As tracers for water flow and storage we measured stable isotopes (18 O/ 16 O and D/H ratios) in precipitation, seepage-and groundwater. 5 The precipitation showed a distinct seasonal cycle with δ 18 O values between −2.6 and −22.6‰ during summer and winter, respectively. However, the isotope signals in seepage water in the caves as well as the discharge were completely buffered and ranged around an average δ 18 O value of −10‰. This value also matched the long-term average of the precipitation. In addition, the homogeneous isotopic composition 10 of the Blautopf Spring was against expectation for its highly variable discharge (0.3 to 32 m 3 s −1) that is typical for a fast responsive karst system. We explain the isotopic similarity between cave seepage and the Blautopf Spring (as an integral signal for groundwater) by nearly complete mixing of the water already in the vadose zone. The latter can be divided into the compartments soil, epikarst and rock matrix that all 15 have good storage capacities and also allow diffusive exchange of solutes between mobile and less mobile matrix water. The above approach revealed new aspects about turnover and flow paths of the infiltrated water and thus helps to constrain the risk by pollution to the groundwater.