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VI ABSTRACT In this work different methods are used to detect and correlate exothermic reactions, which is caused by iron disulphide oxidation in dump material. Hydrogeochemical as well as, classical analysis methods, such as RDA (XRay diffractometry), RFA (X-Ray fluorescens) and DTA (differential thermal analysis) are used. Additionally an interpretation of the thermal bands of a scene, recorded by a DAIS 7915 Sensor, together with measurements taken with a radiation thermometer is done. The investigation area, which was selected as an example, covers the former lignite open cast mining Witznitz II, which belongs geologically to the Weißelster basin and the natural-space of Leipzig low-land bay, and lies about 5 km south of the city of Borna. It was developed 1945 from the former open cast mining „Dora and Helene“, which were started in the year 1922 and in the meantime established. The final desactivation of the Witznitz open cast mining took place during the years 1991 to 1993. Altogether approx. 257 millions t brown coal were produced. The remaining hollow cavity of the open mining, if it were not filled with dump material, would be filled with the ascending groundwater and additional water up to the year 2008. The remaining hole lake Kahnsdorf was already partly filled during the investigation. The final level of the lake is reached at 126 m NN. The hydrogeochemical analyses show clear features of the iron disulphide oxidation with exception of the samples from the apron wells and the effluent streams Pleiße und Whyra. This applies to the groundwater samples of the slope with drawnwater and the water from the remaining hole lake Kahnsdorf. Common to all influenced samples is a low pH, high electrical conductivity, very high concentration of sulphate, iron, magnesium and aluminium ions, as well as the heavy metals typical for this pH and were deposited in the once reducing environment. The results could be well assigned to the materials. The differential thermal analysis, which were carried out for all samples at different levels up to a meter of depth from the western and eastern embankment areas of the remaining hole lake Kahnsdorf, give rise to five samples (all originate from a depth > 50 cm) in the result for exothermic reactions. In three samples exothermic reactions can be attributed to iron disulphide oxidation. The RDA und RFA analyses confirm the presence of pyrite in two samples, for six samples Jarosit, acknowledged the new formation of minerals, which develops during the iron disulphide oxidation. One sample contained gypsium, a mineral new formation, which forms the termination of the iron disulphide decomposition. This makes clear that in wide area of the dumps the conversion processes are to a large extent finished in the surface. The measurement of the surface temperature of the sediments with the radiation thermometer show no anomalies on the western embankment of the remaining hole lake Kahnsdorf, which suggested exothermic processses that could be due to the iron disulphide oxidation. However the eastern embankment shows a positive temperature changes from the embankment shoulder to the bank area, which also persist in 15 cm depth. The highest temperatures were measured close to groundwater surface, what indicates the possibility of exothermic process. A very good correlation exists between the sea-water surface temperatures, those calculated with the DAIS 7915 sensor, and the results of the radiation thermometer measurement. The correlation, which exists between the measured soil-surface temperatures of the two measuring systems, can be called good. The Thermal classification of the DAIS 7915 data does not show any correlation with the temperatures, which are to be assigned to the oxidation in the places, where exothermic reaction took place. The method, as it was applied here, does not provide an appropriate proof of exothermic reactions. The data for the necessary correlations are practically not receivable for the locations which can be measured. The applied methods led only partly to sucess. The hydrogeochemical analysis in combination with the RDA and the dump substrate map proved to be a practical and also an economically feasible assessment. An initial state can be described by pH and conductivity measurements and then completed by the mineralogical deposit. This enabled to determine how much material already weathered over the dump flows. This method combination can sucessfully be used for the rehabilitation of dumps in the brown coal open cast mining.