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Delaware Oil Spill Bioremediation Field Study

From Tech Trends August 1995

Delaware Oil Spill Bioremediation Field Study

By Albert D. Venosa, EPA National Risk Management Research Laboratory

Last summer, an important field study was undertaken on the shoreline at Fowler Beach in the Delaware Bay. As part of a carefully designed study, 540 gallons of light crude oil were intentionally released onto fifteen replicate plots to evaluate bioremediation. The research design overcame the flaws of previous studies reported in the literature that based conclusions on comparisons between one large nutrient-treated plot and one large control plot. The problem with those experiments is that no replicate plots were established to provide a basis for estimating experimental error.

The objectives of the Delaware study were to obtain highly credible statistical evidence (1) to determine if bio-remediation with inorganic mineral nutrients and/or microbial inoculation enhanced the removal of crude oil contaminating mixed sand and gravel beaches; and (2) to compute the rate at which such enhancement takes place. Biodegradative loss of oil from the plots was tracked by GC/MS analysis of oil analytes normalized to hopane, a non-biodegrable compound present in all crude oils. A randomized complete block design was used to assess treatment effects. Three treatments were evaluated: a no nutrient control; water soluble nutrients (biostimulation); and water soluble nutrients supplemented with a natural microbial inoculum from the site (bioaugmentation). An unoiled and untreated plot served as a background control for microbiological characterization and baseline bioassays. Five replicates (blocks) of each of four plots (20 plots in all) were set up in random fashion so that whatever inferences could be ascertained from the data would be applicable to the entire beach, not just the test plots.

The experimental design provided the best chance to detect if significant treatment effects existed. Statistically significant differences between treated and untreated plots were observed at weeks 2, 4, 8 and 14 for the alkanes and weeks 8 and 14 for the polycyclic aromatic hydrocarbons (PAHs). First order rate constants for treated plots were significantly different from those of the untreated plots. At no time were any significant differences observed between plots treated with nutrients alone and plots treated with nutrients and the indigenous inoculum. Notable from the data is the fact that the hydrocarbon biodegradation occurred to a significant extent in the untreated plots. This was attributed to the high levels of background nitrogen measured on the beach from Delaware Bay (upwards of 1 milligram (mg) nitrate-nitrogen per liter interstitial pore water continuously over the course of the 14-week study). Had the background nutrient levels been closer to those typical of Prince William Sound beaches (i.e., a thousand-fold less), the slopes of the control plots would likely have been much flatter, thus giving rise to highly significant differences between treated and control plots for both alkanes and PAHs, perhaps as early as two weeks after the release. The latter conclusion is speculative, since such low background nutrient levels were not encountered in Delaware. However, the goals of the project were met: bioremediation via nutrient enrichment was demonstrated unequivocally with statistically credible data; computed biodegradation rates were high, with an alkane half-life of approximately two weeks and a PAH half-life of approximately four weeks; and bioaugmentation even with indigenous organisms does not stimulate further declines in hydrocarbons beyond simple nutrient addition.

Based on the fact that nutrient levels in the area were high enough to support significant biostimulative hydrocarbon decay, the recommendation would be not to encourage active bioremediation activities if an oil spill were to occur along the Delaware Bay shoreline. However, for coastlines having low natural input levels of nutrients, bioremediation should definitely be considered a major cleanup option.

The study was conducted under a Clean Water Act, section 311, permit. Section 311 allows the Administrator to approve the intentional discharge of less than 1,000 gallons of oil for research programs. In October 1994, the Administrator received an application from the State of Delaware to intentionally discharge 540 gallons of crude oil onto a private beach for a bioremediation study. The proposal was strongly supported by EPA's Office of Research and Development and Region 3, the U.S. Coast Guard, the U.S. National Oceanographic and Atmospheric Agency, the U.S. Department of Interior and several Delaware environmental groups. After an intra- and interagency review, EPA granted the permit in June 1994.

For more information on the study, call Albert D. Venosa at EPA's National Risk Management Research Laboratory at 513-569-7668. A report will be available in the Fall; to get on the mailing, send a FAX request to Albert Venosa at 513-569-7105.


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