When a broken pipeline spills oil into a residential neighborhood, the most immediate health concerns are those caused by volatile chemicals—airborne toxins that leave people complaining of symptoms like headaches and nausea and worrying about long-term problems like cancer.
But crude oil also contains small amounts of heavy metals that rarely evaporate into the air. Instead, they stay with the oil as it spills onto the ground and into waterways. These compounds, which include mercury, manganese, nickel and chromium, are toxic at high doses, and some, like arsenic and lead, can damage the nervous system even at relatively low doses. Yet little is known about the potential health risks to people who live near oil spill sites.
In Arkansas, regulators are testing for heavy metals in the city of Mayflower, where more than 210,000 gallons of Canadian oil spilled on March 29. But at this point there are still more questions than answers.
Although most of the visible oil has been cleaned up, residual oil remains in local soils and waterways, including a cove of Lake Conway, a popular fishing area. The lake isn't used for drinking water, and residents have been barred from the cove since the spill occurred.
The Arkansas Department of Environmental Quality and the pipeline operator, ExxonMobil, have found that most of the heavy metals in the cove and the main body of the lake are below levels of concern. Their testing is incomplete, however, because so far they’ve sampled only the water, not the soils or lake sediment.
Even when all the tests are done, health experts say it will be almost impossible to predict the long-term effects on residents, because little is known about how mixtures of heavy metals break down and change in the environment over time.
Joseph Graziano, a professor of environmental health sciences at Columbia University, said that in addition to determining the concentrations of heavy metals, scientists also must study if and how residents come into contact with the contaminants. "Sure, heavy metals have serious health effects," he said. "But only if exposure takes place."
Graziano and other experts say it's important to know, for example, if the metals are seeping into groundwater and reaching basements or backyard gardens, and if they're becoming more concentrated—and therefore more toxic—as they make their way up the food chain in Lake Conway.
These questions are particularly important in Mayflower because the type of oil that spilled—diluted bitumen from Alberta's oil sands region—has far higher concentrations of heavy metals than conventional crude oil. Diluted bitumen, or dilbit, is the same type of oil that contaminated Michigan’s Kalamazoo River in 2010, creating the most expensive oil pipeline spill in U.S. history. It's also the type of oil that will be carried from Alberta to Nebraska on the Keystone XL pipeline if the Obama administration approves the project.
A 2009 report prepared for the oil industry by the Alberta Research Council found that samples of bitumen had 10 times as much chromium as Alberta conventional crude and more than 38 times as much manganese. Chromium is a carcinogen that weakens the immune system, and manganese is associated with tremors and cognitive problems.
The same report said bitumen contains such high concentrations of nickel, vanadium and mercury that companies are considering recovering these commercially valuable metals from the waste generated by bitumen processing. Nickel can lead to kidney failure, vanadium affects the respiratory system and mercury can cause neurological problems.
In 2010, a peer-reviewed paper in the research journal Proceedings of the National Academy of Sciences reported elevated levels of heavy metals in areas of Canada’s Athabasca River downstream from oil sands production sites. Study co-author David Schindler said some of the metals came from natural sources such as the erosion of bitumen-rich geologic deposits.
"However, our study showed that contrary to industry advertising, mining and extraction are adding to this burden," he said in an interview.
Schindler, an ecology professor at the University of Alberta, said any heavy metals found in Mayflower would probably come from a mix of sources including the oil spill, pre-existing industrial emissions and naturally-occurring trace metals in the water and soil.
That appears to be the case in the Kalamazoo River, where cleanup of the million-gallon dilbit spill in July 2010 continues. Nicole Zacharda, an enforcement specialist at the Michigan Department of Environmental Quality, said the arsenic the agency found in its tests is naturally occurring, due to the local geology. But the agency determined that elevated levels of nickel and vanadium are from the spill.
Zacharda said there isn’t enough nickel or vanadium to trigger health concerns or fish advisory warnings, but the agency is continuing to sample the water and soil. Testing will also continue in the riverbed, which is contaminated with bitumen that sank after the spill.
The bitumen that spilled in Arkansas was lighter, so most of it remained on the surface of the water, according to Jennah Durant, a spokeswoman for the U.S. Environmental Protection Agency. Some oil sank after it "mixed with sediment during recovery activities and through the weathering process," she said in an email.
Mixed Water Sampling Results