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Editor's Notes

• #11: Biomarker results from soil sample location TS-A-10 showed a strong decrease in C17:pristane and C18:phytane over the monitoring period (Figure 4). The untransformed ratio for C18:phytane decreased from 1.8 to 0.93. A strong correlation (R2=0.985) was indicated. For this same population the C17:pristane ratio also trended downward with a good correlation (R2 = 0.8424).
• #12: Natural log transformed data from sample location TS-A-10 provided further evidence of this trend in both C17:pristane (R2=0.8669) and C18:phytane ratios (R2=0.961). Results of natural log transformed data from TS-A-10 indicated that C17:pristane ratio trended at a slope of -0.028, and a standard error of 0.1575 giving an adjusted R2 value of 0.8193. The C18:phytane ratio for that sample location trended at a slope of -0.096, and a standard error of 0.0643 giving an adjusted R2 value of 0.9311.
• #13: Linear regression statistics for natural log transformed data from sample location ES-4 indicate a C17:pristane ratio trend with a slope of -0.015, and a standard error of 0.1793 giving an adjusted R2 value of 0.4666. C18:phytane ratio trend has a slope of -0.014, and a standard error of 0.1286 giving an adjusted R2 value of 0.6427.
• #14: A critical review of the response and monitoring approaches described herein could provide value to future oil spill responses where bioremediation is considered. While response measures clearly achieved interim remediation goals in this case, questions remain about the specific bioremediation approach and how it could be improved upon. The experience of the responders in this circumstance should provide knowledge to improve treatment efficiency and cost-effectiveness of bioremediation technologies for use at oil spill responses. Furthermore, this critical review should provide assurances for agencies and stakeholders concerned with responding to an oil spill. The bioremediation literature review and site-specific feasibility studies presented above should raise awareness of the technology in order to expedite inter-agency consensus on bioremediation as an oil spill response strategy. Experience shows that quick response to an oil spill of this type may improve the efficiency of bioremediation approaches. While significant TPH concentration reductions were observed over the monitoring period in all treatment areas, other lines of evidence should be considered prior crediting the bioremediation approach as clearly successful. As discussed above, Brown et al. explain that TPH concentrations are not sufficient measures of treatment progress because of the extensive error associated with the sampling protocols and analysis, as well as the heterogeneity of the site being studied. EPA’s literature review suggests that TPH data should be normalized to reduce these errors. In the previous sections, EPA has presented their own normalized data as well as data provided by consultants to the RP. Several of these individual sampling point plots show significant reductions in peak-height ratios during the monitoring period. However, the same results for many other sampling points do not show peak-height ratio reductions. Plots of compiled, normalized field data for all the soil samples collected by EPA show only a modest downward trend in both Divisions A & B. Because the Suisun Slough Oil Spill was a “spill of opportunity,” rigorous experimental controls were not built into the design of the monitoring plan. Field conditions in this case have introduced complexities that preclude more clear evidence of bioremediation. For example, tilling up fresh oil from below, which may not have degraded due to oxygen limitations would re-contaminate surface soils. In Division A, cleanup contractors cast fresh oil-contaminated soils into the treatment area during cleanup activities. Furthermore, EPA’s literature review identified pristane and phytane as inadequate biomarkers over longer-term treatments. The use of pristane and phytane therefore may underestimate biodegradative losses under certain conditions. Loss of these isoprenoids creates falsely high biomarker ratios, particularly after significant time has passed since the occurrence of the spill. Since treatment was not begun until greater than 30 days after the spill occurred, it is possible that natural attenuation occurred perhaps even to the extent that isoprenoids were degraded. In addition, comparative analysis of fingerprints from the “fresh” and “weathered” product qualitatively indicated that physical losses had occurred, as expected. In addition to these complexities, the depth of biological activity relative to the sampling depth may also be an important reason why these normalized mean concentration downward trends are not steeper. ERT has concluded that oxygen bioavailability or supply rapidly becomes limiting in the treatment area, except at the very surface (Dr. H. Allen, personal communication ). In this case, tilling improved soil aeration but perhaps not to the extent required to meet microbial oxygen demand. The presence of nutrients, especially ammonia, permits biological activity as long as there is bioavailable oxygen. In addition, the presence of nitrate allows fermentation to occur in the absence of adequate oxygen. Even under tillage conditions, biodegradation will be limited to the contact surfaces at the air interface; oxygen limitation will occur within 3 millimeters or so of the wet surface. Therefore, soil samples collected from the top 3 inches of surface soil could mask the effect of biodegradation (Dr. H. Allen, personal communication ). Drawing on the various lines of evidence EPA has concluded that both weathering and biodegradation of the spilled fuel oil (TPH-D or DRO) occurred in the affected areas. Biostimulation and tilling enhanced these processes to the extent feasible considering that they were not employed early enough in the spill event to properly evaluate their efficacy. It is not totally clear that some biological activity was occurring before biostimulation was begun, but on the surface it does not appear to have been a factor prior to the addition of nutrients. Weathering certainly occurred, with the loss of the lighter hydrocarbons, which ironically, as well as being somewhat toxic to the microorganisms, assisted in making the heavier hydrocarbons more readily available to microbial activity. In fact, DRO is perhaps an ideal hydrocarbon range for bioremediation, being neither too heavy nor too light (Dr. H. Allen, personal communication ).