Ecotoxicological assessment of soils of former manufactured gas plant sites: Bioremediation potential and pollutant mobility

Analytically well-characterized soils from four different former manufactur ed gas plants (MGP) sites contaminated by coal tars were used in tests of e xtensive biodegradation of polycyclic aromatic hydrocarbons (PAHs) in stirr ed reactors. In all cases, the extent of biodegradation was limited to 80-1 00% for 2- and 3-ring PAHs, 40-70% for 4-ring PAHs, and below 20% for 5- an d 6-ring PAHs. The capacities to transfer pollutants to water Were compared for leachates from soils that had or had not undergone biological treatmen t Leachate analysis involved determination of PAHs and bacterial tests of a cute toxicity (Microtox) and genotoxicity (SOS Chromotest). For some untrea ted soils, PAH leaching was observed, and positive responses to the Microto x test were well correlated to-the concentrations of naphthalene and phenan threne. Biologically treated soils had lost all capacities for leaching as concluded from PAH determinations and responses to the Microtox test. All s oil leachates were devoid of genotoxic effect, in accordance with the low c oncentrations observed of mutagenic PAHs. The results of this risk-based ap proach for assessment of MGP soils showed that pollutants remaining after b iological treatment were unavailable for further biodegradation and that th e extent of leaching had been reduced to the level that it did not represen t a significant threat to groundwater.