Toxicity of oiled wetland sediments influenced by natural and enhanced bioremediation

Abstract

A multi-phase bioremediation project was initiated raphics. following the rupture of several oil and gas pipelines during extensive flooding of the San Jacinto River near Houston, Texas, in October of 1994. A small cove and its associated wetlands were set aside for petroleum bioremediation studies. Phase I began in December of 1994 and monitored sediment toxicity associated with intrinsic petroleum degradation. Acute toxicity was evaluated using the Microtox 100% Test on sediment elutriates from 21 plots around the study site over a period of 7 months. Sediment toxicity was elevated near the mouth of the cove and decreased to non-detectable levels within 4 months. Acute toxicity was correlated with sediment total petroleum hydrocarbon (TPH), and gas chromatography-mass spectrometry (GC-MS) target saturate concentrations. Phase 11 began in March 1996 with the controlled application of weathered oil and selected nutrient amendments to evaluate their effects on bioremediation. The amendments evaluated were inorganic nutrients and inorganic nutrients plus an alternate electron acceptor. The effects of treatments on sediment toxicity and mutagenicity were assessed using Microtox 100% and Solid-phase Tests and amphipod, cytochrome P450 Reporter Gene System (RGS), Toxi-ChromoPad and Salmonella/microsome assays. Significant toxicity was detected (Microtox*) in all plots that were oiled followed by a rapid decrease that was correlated to petroleum losses. Amphipod mortality was initially high for all oiled sediments and decreased significantly over time. However, a significant increase in mortality was observed in plots receiving nutrient amendments after 140 days, possibly due to elevated ammonia levels. The Toxi-chl.omol3adTM and Salmonella assays were not sensitive to petroleum addition. Significant correlations were identified between Microtox and GC-MS target analyse levels in total sediment extractions. Subsequent GC-MS analyses of sediment elutriates revealed that Microtox toxicity was strongly correlated with fluorenes, naphthalenes, phenanthrenes and dibenzothiophenes. In conclusion, significant toxicity was detected in oiled sediments that decreased to near background levels within months. The addition of treatments did not consistently alter sediment-associated toxicity with the exception of the amphipod assay. The Microtox, amphipod and P450 RGS assays were sensitive to weathered oil. However, the Toxi-ChromoPad and Salmonella assays were not sensitive under the experimental conditions.