Like most of us, Dom Di Toro cringes when he sees photos of oil-slicked pelicans in the Gulf of Mexico and tar balls in the sugary sand along the shoreline. But he is just as worried about what he can't see -- the toxic effects of oil on the water and sediment environments.
“It's easy to see the direct, or physical, effects,” Di Toro says, “while the chemical effects tend to be invisible. However, what's going on below the surface can be just as devastating as the oil slicks that we can see on the surface.”
An expert in water quality and sediment quality criteria models for organic chemicals, metals and mixtures, Di Toro is the Edward C. Davis Professor in the Department of Civil and Environmental Engineering at the University of Delaware.
The U.S. Environmental Protection Agency (EPA) relies on researchers like Di Toro to help develop methodologies for predicting toxicity so that criteria for water and sediment quality can be developed and implemented. Such criteria are used to assess risk and guide cleanup efforts.
“At first glance it would seem that it is difficult to determine just how toxic oil is,” Di Toro says. “Petroleum is made up of tens of thousands of hydrocarbons, and it's not feasible to perform toxicity tests on that many chemicals.” The issue is further complicated by the fact that the chemicals involved are combined into mixtures, suggesting the potential for incremental toxicity.
As it happens, though, all oil compounds are narcotic chemicals-that is, they all exert their toxicity by the same mechanism. Their similar mode of action means that their toxicities can be added using a method called toxic units, and it also means that 30 or 40 compounds-- rather than 10,000 -- can be measured to provide a fairly accurate picture of what is happening.
“Just how clean is clean?” Di Toro asks. “At what level of concentration do you have to remediate the water or the sediment that has come in contact with it?” This is the central problem in assessing the damages caused by such a spill, and it’s the problem that water quality and sediment quality criteria are designed to solve.
But water quality is not the only issue. Sediment, an area of particular interest to Di Toro, comprises an ecosystem in itself. Toxic effects in this region affect not only sediment-dwelling organisms but also other creatures who feed on them.
Di Toro and his research group have developed a procedure for deriving equilibrium partitioning sediment benchmarks, an approach that enables the toxicity of nonionic chemicals in sediments to be predicted mathematically using only chemical measurements. The procedure is now an EPA methodology that, Di Toro says, is being applied to help in both establishing baseline information about conditions in the Gulf and in assessing changes in those conditions as the spill progresses.
Di Toro and his colleagues have also carried out studies to determine the effect of weathering on the toxicity of oil components. Common wisdom would suggest that because lighter compounds are less toxic than the heavier components of oil and they evaporate more quickly, toxicity would increase over time. But it turns out that the heavier compounds are so much less soluble in water that the overall toxicity of water that is in contact with oil actually decreases over time.
“Quantity is everything in this business,” Di Toro says. “Zero concentration is a meaningless idea in an industrial society. The fact is that contaminants are in our environment. What matters is the amount and their bioavailability --whether there is enough to harm us or the wildlife or the plant life. In toxicology, the dose makes the poison.”
Article by Diane Kukich