The present doctoral thesis comprises three different research chapters which are assigned to different stages of the standard procedure of contaminated site remediation projects. The first research chapter is attributed to the investigational phase dealing with a mineralogical and geochemical characterization of a chromium (Cr) contamination in alluvial sediments. Analyses revealed that Cr formed a black layer in the underground and that it is present in its trivalent state (Cr(III)) as discrete grains or as coatings around mineral grains. Phase investigations showed that it precipitated as Cr(III) hydroxide (Cr(OH)3) and Cr(III)-Ca-containing hydrocalcite (idealized stoichiometry: CaCr23+(CO3)2(OH)4·H2O). Hydrogeochemical models established an evolution scenario for the black layer and predicted a longitudinal spread of the black layer of approximately 37 meters downgradient from the source and a maximum transversal spread of approximately 10 meters. In the second research chapter, the impact of injecting sodium dithionite (Na2S2O4) on the in-situ reduction of hexavalent Cr (Cr(VI)) at two hot spots, both located at the same contaminated site, was investigated, hence it is assigned to the remediation phase. Results revealed that part of the Cr(VI) is mobilized due to interaction of injected ions with the soil. With ongoing injection, the reducing strength of the reducing agent was sufficient to reduce available Cr(VI) to Cr(III). During the treatment, different remediation phases, characterized by excess chromate and excess (bi)sulfite were identified and it was shown that the presence of iron is the rate-limiting step in the reduction process. Two additional groundwater samples were taken half a year and one year after terminating the injection and analyzed, both indicating a rebound of Cr(VI). For the third research chapter (also assigned to the remediation phase), a new approach was tested to combine a boron doped diamond electrode (BDD), zero-valent iron (Fe(0)) in a fluidized bed reactor and ultraviolet (UV) radiation to decompose dissolved organic contaminants. Additionally, the influence of adding H2O2 or H2O2 + H2SO4 to the system was investigated and tetrachloroethene, methyl-tert-butyl-ether (MTBE) and clopyralid were chosen as test substances for the experiments. The treatment methods were tested alone, in any combination of two (+ H2O2) as well as in combination of all three of them to identify emerging synergy effects. Results showed that the treatment methods alone were able to decrease the organic contaminant concentrations, yet an efficiency increase was obtained by combining the methods. The highest decomposition rates were achieved by combining all treatment methods and the addition of H2O2 + H2SO4.