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

The water sampling in Arkansas shows that the heavy metals in the bitumen haven't impacted the main body of Lake Conway, said Ryan Benefield, deputy director of the Arkansas Department of Environmental Quality (ADEQ). That conclusion is based on water sampling from inside and outside the cove as well as historical data on the lake's water quality.

The ADEQ hasn't sampled all areas of the cove for heavy metals, "so we cannot make any broad sweeping statements" about it, Benefield said.

But he said the cove was "significantly" affected by the spill, and the agency is concerned about all the contaminants in the oil—not just heavy metals.

InsideClimate News examined results for eight compounds that are particularly harmful: arsenic, manganese, nickel, vanadium, lead, chromium, selenium and mercury.

Mercury is of special concern because it can be transformed by bacteria into methylmercury, a compound that becomes increasingly toxic as it travels up the food chain. If the chemical begins to concentrate in Lake Conway fish, that could have serious impacts on local wildlife and residents who consume the fish.

The ADEQ did not test for mercury, but Exxon's mercury sampling results show that mercury concentrations remain at safe levels. ADEQ spokeswoman Katherine Benenati said the analytical procedure for mercury is different from the typical metals tests the agency's lab conducts, so the lab doesn't run it without a specific request

The results for arsenic, nickel, vanadium and selenium were generally below ecological screening levels—concentrations that might harm the ecology—and below federally established limits for drinking water. Concentrations of lead and chromium regularly exceeded ecological screening levels but stayed within acceptable drinking water limits.

Benenati said that exceeding the screening values doesn't necessarily indicate a problem but "merely indicates we want to take a closer look at the area."

The metal that most often exceeded both ecological and drinking water limits was manganese, a neurotoxin that's associated with tremors and cognitive problems. According to Tomás Guilarte, a toxicologist at Columbia University's Mailman School of Public Health, emerging research shows that manganese can amplify the toxicity of lead, so children exposed to both metals at once are at particular risk for neurological problems.

In some cases, manganese levels in the cove and in a nearby creek were 10, 20 or nearly 30 times above the EPA's safety standard for tap water.

Benefield, the ADEQ deputy director, said the manganese levels outside the cove may be naturally occurring, but there's no question that the cove was directly impacted by the spill. For now, the cove's aquatic community appears unharmed, he said.

More Tests for Soils, Sediment

So far, the agency's testing has been limited to water samples collected from the surface of the lake and near the base of the water column. The next step is to examine the soils and lake sediment, tests that experts say are crucial to determining the extent of heavy metal contamination in Mayflower. The spill was followed by several days of heavy rain, which helped spread the oil across the land.

The ADEQ recently approved a remediation plan for the lake and other water bodies impacted by the spill. The plan includes soil and sediment sampling, and was created by an Exxon contractor with input from the EPA and ADEQ.

Benefield said the results of those tests will guide the agency's long-term remediation plans. If alarming levels of heavy metals show up in the sediment, for example, ADEQ could start collecting tissue samples from fish to study bioaccumulation.

Those tests may be just the beginning. Health experts say it's impossible to gauge the public health risk from heavy metals without adequate data on human exposure. What concentrations were cleanup workers exposed to, and for how long? Are children ingesting the metals as they play in the dirt?

The ADEQ is aware of these challenges, Benefield said, and will work with state health officials to assess potential health risks.