Inadequate disposal of tailings resulting from processing and other mining and metallurgical activities causes often severe contamination of surface streams and groundwater reservoirs, due to oxidation of the inherent sulfide phases and the subsequent generation of leachates characterized by high acidity and elevated concentration of hazardous elements. A relatively low cost alternative to pump and treat approach aiming at prevention of contamination or clean up of contaminated groundwater involves the application of in-situ permeable reactive barriers (PRBs). A number of low cost compounds may be used as reactive media in order to provide alkalinity, activate precipitation and sorption mechanisms and finally lead to clean up of the contaminated plume. Laboratory column tests were conducted to study and assess (a) the potential of limestone and red mud barriers to remove several inorganic contaminants from synthetic solutions simulating acidic leachates under dynamic flow conditions and (b) the volume of reactant required for efficient operation under the conditions examined over long periods of time. The speciation/mass transfer computer code PHREEQC and the MINTEQ database were used for geochemical modeling of the process. Both reactive media tested caused within the PRB the development of conditions that favored removal of heavy metal ions mainly by precipitation, co-precipitation and adsorption. Fe is mainly removed as goethite and ferrihydrite, Al as boehmite and gibbsite, Cu, Mn, Zn, Co and Ni as metal hydroxides and Cd by sorption/co-precipitation on/with iron and aluminium compounds. Sulfates are removed as gypsum. The toxicity of both reactive media at the end of the treatment period regarding all heavy metals studied is limited. The experimental results underline the potential of limestone and red mud permeable reactive barriers to remove, over a long period, high loads of inorganic contaminants from acidic leachates generated at mining sites and endangering the quality of aquatic systems. They also provide useful information on the efficiency of PRBs operating under field conditions. (C) 2003 Elsevier Ltd. All rights reserved.