Available at: https://digitalcommons.calpoly.edu/theses/1319
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Dr. Yarrow Nelson
Remediation of weathered petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), dioxins, and polychlorinated biphenyls (PCBs) through monitored natural attenuation, in-situ biostimulation, and/or bioaugmentation was assessed using laboratory-scale microcosms. These contaminants of interest (COIs) have persisted in Santa Susana Field Laboratory (SSFL) soils for over 40 years in some cases. The objective of this United States Department of Energy (DOE)-funded study was to determine the potential of the aforementioned remediation methods to reduce COI concentrations in soil and estimate potential biodegradation rates of COIs in SSFL soils.
Several types of soil microcosms were established: one set of microcosms was run without amendments to estimate natural attenuation rates at the site; biostimulation was tested by addition of nitrogen and phosphorus, rice hulls, and biosurfactant (soya lecithin), another set was augmented with the white-rot fungus Phanerochaete chrysosporium, and gamma-irradiated microcosms served as sterilized controls. Soil samples were collected and analyzed for dioxins, PCBs, PAHs, and extractable fuel hydrocarbons (EFH) after 0, 4, and 8 months of incubation. Soil contamination in the microcosms initially consisted of primarily heavily chlorinated dioxins and PCBs, longer petroleum hydrocarbons (21-40 equivalent carbon chain length), and PAHs with 4-6 aromatic rings.
Small decreases in PAH, PCB, and dioxin soil concentrations were observed, but these decreases were not statistically significant. EFH concentrations were inflated at the final sampling event, but they appeared to reduce for two of three soils (Soils A and C) tested at the second sampling event. No COI concentration reductions were statistically significantly during 8 months of incubation. Because petroleum hydrocarbons were primarily longer-chain hydrocarbons in the C21 to C40 EFH range, it is likely that lighter hydrocarbons had been preferentially degraded, leaving the more recalcitrant longer-chain hydrocarbons in the soil. Dioxin concentrations appeared to decrease in some cases, but these reductions were not statistically significant at the 95% confidence level. Larger PAHs (4-6 rings) comprise the majority of residual PAH soil contamination. Given that concentrations of these PAHs have not decreased significantly during this 8-month long study, it is likely that these larger PAH contaminants are somewhat recalcitrant and will take a long time to biodegrade. Similarly, little or no PCB biodegradation was observed which is not surprising because the PCBs are heavily chlorinated, and bacterial biodegradation of these highly chlorinated compounds is reported to occur only under anaerobic conditions. The primary dioxin congener present in soils was octachlorodibenzodioxin (OCDD), which is the heaviest-chlorinated dioxin congener. Like PCBs, this compound requires anaerobic conditions for reductive dechlorination, and these are not present at the site. Total dioxin concentrations decreased in the microcosms amended with Phanerochaete chrysosporium, although this decrease was not statistically significant due to variability of dioxin concentrations measured in the soil. No decrease in tetrachlorodibenzodioxin toxicity equivalence was observed with P. chrysosporium bioaugmentation, and this parameter is important in terms of dioxin toxicity.
Soil vapor analyses performed at the site indicate highly aerobic soil conditions. To mimic site conditions as closely as possible, experimental microcosms were maintained incubated in aerobic conditions. Although fungi have been reported to degrade PCBs and dioxins under aerobic conditions, the microcosms augmented with Phanerochaete chrysosporium did not show statistically significant biodegradation of PCBs.
Contaminant sequestration in the soil may also have contributed to the lack of observed biodegradation because the COIs at this site are highly weathered. However, even microcosms augmented with a surfactant (soya lecithin), which would be expected to solubilize sequestered COIs, did not show significant biodegradation.