[en] Full text: The multi-isotope approach using stable isotopes of various light elements offers possibilities to recognize chemical processes within the aquifers, interactions between ground and surface water, quantification of the balance of water or dissolved compounds in anthropogenic stressed landscapes. Especially in mining areas, the quality of ground- and surface water is one of the main problems during and after remediation measures. Caused by lowering of groundwater level, sulfide oxidation and consequently acidification of surface and groundwater systems are common processes. Successful remediation strategies require knowledge of the chemical and physical processes proceeding in dump sites, and of the groundwater flow dynamic. Experienced in using sulfur, hydrogen and oxygen isotopes in post-mining landscapes, we will present three key examples for assessing the sulfate reduction in dump sediments, the water balance of mining lakes, and sulfate input from different sulfur sources. (i) Sulfate reduction: During the long time saturation process the groundwater system of dumps can turn to reducing conditions as indicated by S- and O-isotope signatures of dissolved sulfate. Mainly in the overburden dump sediments of Cospuden mining area (south of Leipzig, Germany) sulfate reduction follows a continuous trend in time from younger to older parts. The input of oxidizing solutions can prevent the reduction process. Evidently, sulfate reduction is still underdeveloped in the conveyor bridge dump caused by penetration of sulfate and iron rich solutions from the weathering zone. A spatial and temporal development was evaluated in different age structured dump sediments using the δ³⁴S and δ¹⁸O values of sulfate. (ii) Lake water and sulfate balance: The acidic mining lake 111 (Lusatia mining area, Germany, pH-value 2.6) exists for more than 40 years and reached stable hydrological and hydrochemical conditions about 30 years ago. Isotope data (H, O) were used to determine the annual groundwater in- and outflow of the lake and to calculate the amount of sulfate, iron, and acidity that is carried into the lake by groundwater. For the hydrological balance water samples for ¹⁸O-analyses were taken from sampling wells around the lake representing the in- and outflow area, from springs at the lake shore, and from the lake itself. The calculation of the hydrological balance by δ¹⁸O-values was carried out by a hydrological model. Besides the acquired field data long-term average values for precipitation, evaporation, temperature, humidity, and isotopic composition of the precipitation were taken into account. The calculated balance proposed an average residence time of the lake water of about 20 years. Considering the lake water sulfate as a mixture of the dump- and aquifer-input, the δ³⁴S-values of lake- and groundwater-sulfate combined with the established annual inflow was used to calculate the annual sulfate input, and based on the hydrochemical data of the inflowing groundwater likewise for the annual iron-input. (iii) Sulfate balance during flooding: Different sources of sulfur have to be considered for the sulfur budget of mining lakes (in process of filling up): dissolved sulfate from aquifers in the surrounding, dissolved sulfate from water used for artificial flooding (river water or a drainage water from a neighboured mine), and sulfate from the interaction of lake water with aerated sediments bearing oxidized sulfides. Balance investigations can be supported by δ³⁴S if the contributing sulfur sources can be characterized by known and sufficient different isotope signatures. The accompanying flooding of an extended system of abandoned open pits north of Leipzig (Goitsche) with water from the river Mulde was monitored by sulfur isotopic composition. The starting point was the existence of uncovered sediments with high primary sulfide content in parts of the future lake bottom and a very low pH in the drainage water. Thus, acidification of the lake water was apprehended. Actually, the influence of highly depleted sulfides (-25 per mille CDT) characterized the pre- and initial phase of flooding. In the later phase, δ³⁴S was controlled more by groundwater than by river water (about +4.4 per mille) due to the much higher mean sulfate concentration in groundwater. Because of a large variation of δ³⁴S values and sulfate concentrations measured in groundwater samples, only the δ³⁴S of the mean groundwater input can be estimated. Based on this result, the contributions of the three mentioned main sources in the sulfur balance have been estimated. (author)