The behavior of chemicals over long periods of time must be investigat
ed to assess the effectiveness of bioremediation. Three long-term, fie
ld-scale studies are presented and demonstrate the effectiveness of bi
oremediation at reducing chemical concentrations, mobility, and toxici
ty in soils and sludges over long time-periods. Two case studies descr
ibed the land-treatment of wood-treatment site soils and sludges while
the third case study described the land-treatment of oily wastes from
petroleum industry operations. These long-term studies confirmed that
chemicals in bioremediation residues are contained in treated soils,
that chemical concentrations continue to decline after active remediat
ion stops, and that toxicity is reduced. Even though significant conce
ntrations of chemicals were present in the residuals, the toxicity and
mobility of the residual chemicals and, therefore, the risk associate
d with them, had been substantially reduced,