In A Review of Literature Related to Oil Spill Dispersants (1997 – 2008, total of 430 papers) this comprehensive report considers the impact, effectiveness and related factors for using dispersants in oil spills. I’ve called out some important excerpts here. If you don’t have time to read it all, at least review the sections that are in bold.

First, it is very important to note that many of the most troubling concerns mentioned in this literature review are followed by a statement like this:

Data are currently limited and further studies are recommended.

What this means is that in the last 40 or so years that dispersants have been in use, we still haven’t conducted sufficient testing on how the dispersants impact marine ecosystems and human health. We simply don’t KNOW how deadly or how toxic the dispersants are, and worse, we don’t even know for certain if they are EFFECTIVE!! What we know for sure is that we don’t know enough, and that there’s a reasonable chance that the dispersants do more harm than good, both in the short and long term.

Of the recent toxicity studies, most researchers (about 75 %) found that chemically-dispersed oil was more toxic than physically-dispersed oil. About half of these found that the cause for this was the increased PAHs (typically about 5 to 10 times) in the water column. Others noted the increased amount of total oil in the water column. Two researchers noted the damage to fish gills caused by the increased amount of droplets.

The prime motivation for using dispersants is to reduce the impact of oil on shorelines. To accomplish this, the dispersant application must be highly successful and effectiveness high. As some oil would come ashore, there is much discussion on what effectiveness is required to significantly reduce the shoreline impact. A major issue that remains is the actual effectiveness during spills so that these values can be used in estimates for assessment and models.

The second motivation for using dispersants is to reduce the impact on birds and mammals on the water surface. As the NAS committee (2006) on dispersants notes, little or no research on this has been carried out anytime since the 1980′s. The benefits or deleterious effects of using dispersants to reduce impacts on wildlife still remain unknown.

“Of additional concern is the effect of dispersed oil and dispersants on the waterproof properties of feathers and their role as thermal insulators. One of the recommendations of the NRC (1989) report was that studies be undertaken to ‘assess the ability of fur and feathers to maintain the water-repellency critical for thermal insulation under dispersed oil exposure conditions comparable to those expected in the field’. This recommendation is reaffirmed because of the importance of this assumption in evaluating the environmental trade-offs associated with the use of oil dispersants in nearshore and estuarine systems because it has not been adequately addressed”

TOXICITY

In 2005 Scarlett, et al, compared the toxicity of two dispersants: Corexit 9527 and Superdispersant-25 (SD-25). Results:

The results were consistent with the hypothesis that dispersants act physically and irreversibly on the respiratory organs and reversibly, depending on exposure time, on the nervous system. Superdispersant-25 was found overall to be less toxic than Corexit 9527 and its sublethal effects more likely to be reversible following short-term exposure.

A 2003 study tested toxicity in freshwater marsh microcosms containing South Louisiana Crude (SLC) or diesel fuel, and treated with either a cleaner (Corexit 9580) or dispesant (Corexit 9500). RESULTS:

The crude was less toxic than diesel, chemical additives enhanced oil toxicity, the dispersant was more toxic than the cleaner.

This next section is important because Nalco and BP have maintained that Corexit is completely safe. Notice they don’t say that Corexit combined with SLC (South Louisiana Crude) is safe:

The results of dispersant toxicity testing are similar to that found in previous years, namely that dispersants vary in their toxicity to various species, however, dispersant toxicity is typically less than the toxicity of dispersed oil, by whatever tests.

Effectiveness remains a major issue with oil spill dispersants. It is important to recognize that many factors influence dispersant effectiveness, including oil composition, sea energy, state of oil weathering, the type of dispersant used and the amount applied, temperature, and salinity of the water. The most important of these is the composition of the oil, followed closely by sea energy. It is equally important to note that the only thing that is important is effectiveness on real spills at sea.

Of the recent toxicity studies of dispersed oil, most researchers found that chemically-dispersed oil was more toxic than physically-dispersed oil.

Recent studies have also raised the issue of much-increased concentrations of PAHs (polyaromatic hydrocarbons) in the water column caused by the use of dispersants. These studies also show increased toxicity as a result of the PAHs. Long-term effects of chemically-dispersed oil are poorly-studied and relatively unknown at this point in time. Again little has changed from the first review in 2002, but it is very clear now that the toxicity of dispersed oil is greater than that of physically dispersed oil, primarily because of the large increase (5 to 50 times) the amount of aromatics and PAHs in the water column.

A recently-released report on effectiveness during the Exxon Valdez spill, shows that there was little to no effectiveness after dispersant application on this actual spill.

The second important issue when discussing dispersants is toxicity, both of the dispersant itself and of the dispersed oil droplets. Toxicity became an important issue in the late 1960s and early 1970s when application of toxic products resulted in substantial loss of sea life. For example, the use of dispersants during the Torrey Canyon episode in Great Britain in 1968 caused massive damage to intertidal and sub-tidal life (Fingas, 2002). Since that time, dispersants have been formulated with lesser aquatic toxicity. Although, the issue may not be the toxicity of the dispersant itself but the large increase in the oil droplets in the water and the large increase in PAHs in the water column as a result of dispersant use.

For some organisms, dispersed droplets are also an important route of exposure, either through droplet/gill interactions or through ingestion. Studies show that some organisms accumulate PAHs differently via particulate or dissolved routes. Organisms may also be exposed to oil by contamination of their food. Many oil constituents, such as the monoaromatics and PAHs, are narcotics, that is substances which causes a state of arrested activity of protoplasmic structures.

Several researchers have noted that oil and especially dispersed oil has greater toxicity when exposed to UV or UV components of natural sunlight. Brief exposure to sunlight of about 2.5 hours/day for 2 days, increased toxicity from 1.5 to 48-fold over control lighting.

Photo-enhanced toxicity consists of two mechanisms, but the most important one is photosensitization. This occurs when a PAH absorbs energy from the light and then transfers this to dissolved oxygen. This results in enhanced toxicity to many organisms.

Recent data, however, show that chemically-dispersed oil is more toxic than physically dispersed oil because dispersants will temporarily entrain significant amounts of aromatics and PAHs in the water column. Studies show that this is increase typically ranges from 10 to 50 times the amount compared to that of physically-dispersed oil.

Wolfe et al. (1997, 1998a,b, 2001) studied the passage of naphthalene through the food chain by a primary producer and a primary consumer. It was found that naphthalene passed through the food chain at much higher rates when the oil was dispersed chemically. Chemically dispersing the crude oil resulted in five times higher concentrations of TPH in the water column, compared to the water soluble fractions alone.

(Note: TPH = Total Petroleum Hydrocarbons – a measure of total hydrocarbons in a sample.)

In the [fish] gills, the dispersed oil treatment resulted in the most pronounced biological response, suggesting that in the short term the use of dispersants on an oil slick might cause the most perturbations to fish metabolism.

The addition of dispersant caused a two- and fivefold increase in the concentrations of total PAH and high-molecular-weight PAH (HMWPAH) with three or more benzene rings. Highest mortality rates (89%) were observed in larvae exposed to DWAF.

(Note: DWAF = Dispersed Water Accommodated Fraction)

A 2005 study found:

The percentage of gill lesions was generally greater with the dispersed oil. The authors note that the increase in gill lesions was probably as a result of dispersant-enhanced toxicity.

Ramachandran et al. (2004) conducted an experiment to measure whether oil dispersion increases or decreases the exposure of aquatic species to the toxic components of oil. These experiments suggest that the use of oil dispersants will increase the exposure of fish to hydrocarbons in crude oil.

Shafir et al. (2007) employed a nubbin assay on more than 10,000 coral fragments to evaluate the short- and long-term impacts of dispersed oil fractions (DOFs) from six commercial dispersants, the dispersants and water-soluble-fractions (WSFs) of Egyptian crude oil, on two Indo Pacific branching coral species. The dispersed oil and the dispersants were significantly more toxic than crude oil WSFs.

Here are a few of the recommendations for more studies that need to be conducted:

The committee’s recommendations for further studies include: quantifying the weathering and fate of chemically-dispersed oil compared to undispersed oil; obtain data on dissolve-phase PAH and particulate/oil-droplet phase PAH concentrations in test tanks or ideally, at spills-of opportunity; assess the ability of fur and feathers to maintain water-repellency under dispersed oil exposure conditions; and conduct a series of focused toxicity studies to provide data on photo-enhanced toxicity, estimate the contributions of dissolved and particulate oil phases to toxicity and expand toxicity tests to include delayed effects. Of particular concern is the actual toxicity of the dispersed oil – compared to physically dispersed oil.

Next post will explore the Biodegradation of chemically-dispersed oil:

It is clear, on the basis of current literature that the surfactants in some of the current dispersant formulations can inhibit biodegradation. No enhancement of biodegradation was clearly shown in any recent studies.