Characterization of TPH biodegradation patterns in weathered contaminated soil

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Virginia Tech

Two weathered, petroleum contaminated soils were studied to determine if weathered products are amenable to bioremediation and to determine which TPH (total petroleum hydrocarbon) fractions were degrading during particular time frames under different remediation alternatives. Delineation of fractional degradation patterns results in inferences regarding the efficacy of different treatment methods on various petroleum products. A sandy loam and a clay soil were both studied to determine if soil matrix affects the degradation patterns. The experimental matrix included sacrificial static microcosms, soil columns and aerated slurry reactors. Both soils were evaluated under all bioreactor configurations using both a nutrient amended water and a water lacking nutrients. Controls were also used to evaluate abiotic losses.

Biodegradation rates generally followed a biphasic pattern, initially rapid then followed by a slow or stagnant period. Degradation rates increased from static microcosms to soil columns to slurry reactors. The slow phase was controlled by the presence of recalcitrant compounds which decreased in number and concentrations from static microcosms to columns to slurry reactors, and generally with nutrient addition. Nutrient addition enhanced degradation for all sandy soil treatments, but only slurry reactor treatment for the clay soil. The entire TPH spectrum was broken down into five minute parcels based on GC elution time. The compounds that eluted quickest generally were the easiest to degrade. The fraction that effectively covered the TPH components in gasoline was well removed under all treatment modes. All nutrient amended studies resulted in rapid essentially complete removal of the light fractions within two weeks. The fraction encompassing the middle distillates such as diesel fuel and jet fuel was degraded under all treatment methods, however only the slurry reactors resulted in final TPH levels that would have met regulatory limits. Fractions that eluted after 15 minutes were not effectively degraded by the static microcosms or the soil columns for either soil, eliminating in situ bioremediation as a viable treatment alternative for crude oil, fuel oil and gas oil contamination, at concentrations present in this study. Persistence of recalcitrant compounds was the major factor leading to the poor biodegradation observed in the static microcosms and soil columns. Fractional degradation was highly dependent on the initial concentration of the fraction. Generally, fractions present in the largest concentrations degraded fastest.