Crude Oil Hydrocarbon Bioremediation and Soil Ecotoxicity Assessment

In this study, we determined the limits and extent of hydrocarbon biodegradation, earthworm and plant toxicity, and waste leachability of crude oil-containing soils. Three oils (heavy, medium, and light of API gravity 14, 30, and 55, respectively) were mixed into silty loamy soils containing low (0.3%) or high (4.7%) organic carbon at 4000−27 000 mg/kg TPH. Hydrocarbon bioremediation in these artificially weathered oily soils usually followed first-order removal rates in which 50−75% and 10−90% of the total petroleum hydrocarbons (TPH) were degraded in 3−4 months for the low and high organic soils, respectively. Gas chromatographic profiles (simulated boiling point distillation of saturates and aromatic compounds) showed that, after bioremediation, hydrocarbons in oily soils decreased from 70 to 90%, from 40 to 60%, and from 35 to 60% for those carbon number species in the range of C11−C22, C23−C32, and C35−C44, respectively. Most oily soils were initially toxic to earthworms in which few animals survived 14-day bioassays. In a solid phase Microtox test, most oily soils had EC50 values that were ≤50%. Seed germination and plant growth (21-day test, wheat and oat but not corn) were also significantly reduced (0−25% of controls) in untreated soils containing the medium and light crude oils but not the heavy oil. Bioremediated soils were neither toxic to earthworms, inhibitory in the Microtox assay, nor inhibited seed germination after 5 (high organic soil) or 10−12 (low organic soil) months of treatment. Water-soluble hydrocarbons (e.g., O&G and BTEX) could leach from pretreated soils (medium and light crude oily soils) in column or batch extraction experiments. However, after bioremediation, most of the aromatic compounds were no longer leachable from the soils. These data demonstrate that treated oily soils lose their toxicity and potential to leach significant amounts of BTEX. These nontoxic soils contain 1000−8600 mg/kg residual hydrocarbons as TPH. Furthermore, these data suggest that the remaining petroleum compounds may be bound or unavailable in that they are not (a) biodegraded further, (b) toxic to soil-dwelling species (earthworms and plants), and (c) susceptible to leaching and subsequent impact to groundwater. These findings provide a basis for a framework in which petroleum hydrocarbon-containing soils can be evaluated by ecological assessment methods such as biodegradability, ecotoxicity, and leaching potential of regulated substances.