The study assessed the application of separate or combined biological and physical/chemical treatment techniques to high-strength hazardous dumpsite or landfill leachates, extractable wastes and spills, or sludges. Four types of real-world toxic, concentrated, complex wastes served as prototype waste streams: industrial landfill leachate, leachate from an industrial sludge impoundment, forced extract from impounded industrial sludge, and ground water contaminated with chlorinated solvent. Field pilot plants were constructed and operated to demonstrate sequential aerobic/anaerobic soil-based microbial treatment for leachates from two sites and for ground water contaminated with 1,1,1-trichloroethane at a third site. Even in the presence of high concentrations of inorganic salts, organic carbon reductions up to 99% were readily achieved. The levels of chlorinated solvents extracted in subsurface water were reduced from 5 mg/L to less than 20 ng/mL (greater than 99.6%).