Bioavailability of Reversibly Sorbed and Desorption-Resistant 1,3-Dichlorobenzene from a Louisiana Superfund Site Soil

Microcosm batch studies were conducted to study the biodegradation of 1,3-dichlorobenzene (1,3-DCB) from the aqueous (soil free) and soil phases. For soil phase experiments, a freshly contaminated soil and a soil containing only the desorption-resistant or irreversibly bound or non-labile fraction of the contaminant were used. These experiments were designed to simulate biodegradation at Superfund site assuming sorption/desorption equilibrium was reached. The presence of the soil reduced the rates of biodegradation significantly. Nearly 100% of the total 1,3-DCB in the aqueous phase was biodegraded by enriched bacterial cultures within 7 days compared to about 55% over a 6-week incubation period from the freshly contaminated soil. The biodegradation in the soils containing only the desorption-resistant fraction of the contaminant was considerably lower (about 30%). It is believed that for freshly contaminated soil, 1,3-DCB readily desorbed into the aqueous phase and was available for microbial consumption whereas for soils containing mostly the desorption-resistant fraction of 1,3-DCB, the contaminant availability was limited by the mass transfer into the aqueous phase. Our earlier studies concluded that about 20–30% of the sorbed contaminant is tightly bound (even larger for weathered or aged soils) and is not easily extractable. This fraction is typically present in micropores or chemically bound to soil humic matter and thus is not readily accessible for microbial utilization. The findings presented here for 1,3-DCB are in agreement with those reported for other chemicals in the literature and could have implications to the current remedy, the monitored natural attenuation at the Petro Processors Inc. Superfund site in Louisiana.