CRUDE-OIL HYDROCARBON BIOREMEDIATION AND SOIL ECOTOXICITY ASSESSMENT

In this study, we determined the limits and extent of hydrocarbon biod egradation, earthworm and plant toxicity, and waste leachability of cr ude oil-containing soils. Three oils (heavy, medium, and light of API gravity 14, 30, and 55, respectively) were mixed into silty loamy soil s containing low (0.3%) or high (4.7%) organic carbon at 4000-27 000 m g/kg TPH. Hydrocarbon bioremediation in these artificially weathered o ily soils usually followed first-order removal rates in which 50-75% a nd 10-90% of the total petroleum hydrocarbons (TPH) were degraded in 3 -4 months for the low and high organic soils, respectively. Gas chroma tographic profiles (simulated boiling point distillation of saturates and aromatic compounds) showed that, after bioremediation, hydrocarbon s 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 C-11-C-22, C-23 -C-32, and C-35-C-44, respectively. Most oily soils were initially tox ic to earthworms in which few animals survived 14-day bioassays. In a solid phase Microtox test, most oily soils had EC50 values that were l ess than or equal to 50%. Seed germination and plant growth (21-day te st, 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 germ ination 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 co lumn or batch extraction experiments. However, after bioremediation, m ost of the aromatic compounds were no longer leachable from the soils. These data demonstrate that treated oily soils lose their toxicity an d potential to leach significant amounts of BTEX. These nontoxic soils contain 1000-8600 mg/kg residual hydrocarbons as TPH. Furthermore, th ese data suggest that the remaining petroleum compounds may be bound o r 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 provi de a basis for a framework in which petroleum hydrocarbon-containing s oils can be evaluated by ecological assessment methods such as biodegr adability, ecotoxicity, and leaching potential of regulated substances .