The chemical processes and their thermal effects taking place inside a dump of an open-cast mine during a reclamation process are a matter of interest among geoenvironmental engineers and scientists. Fiber-optic-distributed-temperature sensing (DTS) was applied together with a soil ground water monitoring system (SGM-System) at the dump in the open-cast mine (Cospuden) for investigating those processes. An optical fiber was mounted at the entire length of a double-pipe SGM-System. The temperature was measured monthly, along the depth of 50 m, with a spatial resolution of 0.5 m. The accuracy of the temperature gauge was 0.05 K. The system was utilized for the localization of heat production zones in the subsurface, for the determination of the temperature variation's penetration depth from the surface into the soil of the dump and for the determination of the influence of the SGM-System on the subsurface temperature balance. The results were compared with a heat transport model and then interpreted.