Issue Date: August 11, 2014
Digging Up The Dirt
To differentiate soils that, in their natural state, contain high levels of potentially hazardous elements from those that might be contaminated by human activity, scientists need data. So 10 years ago, the U.S. Geological Survey (USGS) launched a national program to measure the background levels of arsenic, lead, and 43 other potentially hazardous elements in soil.
Regulators are using these data as they consider how much cleanup is needed at contaminated sites and as they study possible reuse of industrial wastes.
David Smith, a USGS geologist who led the sampling program, compares an element’s background level to the typical range for cholesterol or other health indicators used in medicine. “You compare your value to the normal range, and if you fall within it you’re healthy, and if you don’t, you might have reason to be concerned,” he says.
After sampling nearly 5,000 locations, USGS released the raw data in spreadsheets last year. Then in June, the data were issued in a collection of interactive maps that show how the background levels change from region to region. The newly completed program updates USGS’s earlier data set for background levels of potentially hazardous elements. That set was compiled during the 1960s and ’70s with 1,300 sampling locations and using what are now antiquated analytical methods.
To collect the newest data set, field crews sampled the soil in each location at three depth intervals: an interval no more than 2 inches below the surface, another extending to about 16 inches deep, and a third reaching to where bedrock starts to weather, or break down. Bedrock, the solid rock underlying soil, is a natural source of elements in soil, and the deeper samples reflect its contribution, Smith explains. The samples taken nearer to the surface, meanwhile, show human influences. The soil samples were analyzed via inductively coupled plasma spectroscopy.
The data from USGS’s recent efforts reveal surprisingly abrupt regional variations, says Harvey Thorleifson, director of the Minnesota Geological Survey in St. Paul. Background arsenic levels, for instance, range from less than 1 ppm to 830 ppm, with the highest levels found in a belt that extends from Nevada to North Dakota and then east toward Maine.
Some of that arsenic could have originated in black shale deposits centered in North Dakota, which glaciers “smeared across the landscape,” Thorleifson says. Black shale also contains significant amounts of cadmium, zinc, and molybdenum, he adds, and levels of these elements in soil tend to go up or down as a group.
In Thorleifson’s view, the stark regional patterns suggest that geology—more so than human activity—is what controls the chemical makeup of soil. “That appears to be true to a much greater degree than we anticipated,” he says.
But human activities can also influence soil’s elemental makeup. Mining, for instance, contributed historically to the spread of mercury and arsenic, and the use of leaded gasoline in road transportation also boosted lead levels in soil.
Experts distinguish between the natural, geological background level of elements in soil—which is generally not found in most places—and anthropogenic background, which reflects an accumulation of human influences in otherwise uncontaminated areas. Some soils contaminated through human activities with lead or other potentially hazardous elements are often targeted by government agencies for cleanup. But regulators in most cases won’t set goals for these cleanups lower than the anthropogenic background for an element in a given location, says Deborah McKean, a toxicologist with the Environmental Protection Agency’s office in Denver.
McKean says she and others with EPA’s Superfund program, which addresses areas contaminated with hazardous waste, use it for first-cut approximations of whether a particular site might be contaminated.
“We could take a few samples, and if they’re high in arsenic, for instance, we could take a look at the USGS database to see if this is an area where arsenic is naturally high to begin with,” she explains.
But USGS’s sampling sites are separated by an average of 25 miles in any direction, which means there’s only a single data point for every 625 sq miles. Thus, the USGS data don’t have the density of sampling needed for risk management decisions at Superfund sites, McKean says. EPA and state regulators generally need site-specific background data if they’re setting targets for cleanups.
USGS data are adequate, however, for national-scale risk assessments conducted through EPA’s Resource Conservation & Recovery Act (RCRA) program for wastes, says Timothy Taylor, an environmental scientist with the agency’s headquarters in Washington, D.C. Taylor explains that RCRA staffers are sometimes called in to assess beneficial uses for industrial wastes, such as the sands left over after foundries use them to make casting molds for iron, steel, and aluminum. Heat and abrasion eventually render these sands unusable, and at that point they’re usually recycled or discarded in landfills.
Scientists with EPA and the U.S. Department of Agriculture are investigating potential uses for these spent foundry sands. One is adding them to manufactured soils for vegetable gardening applications. An essential question is whether the sands contain arsenic, manganese, and other elements at harmful levels, and EPA and USDA scientists are now evaluating that possibility. As part of the risk assessment process, they’re comparing the levels in the foundry sands with the USGS background data. Taylor is careful to note that EPA won’t make its decision solely on the basis of the comparison with background.
Complicating the issue of safety is that some elements have background levels higher than EPA’s corresponding risk-screening levels. These concentrations can trigger the agency to investigate a substance’s potential health or environmental risks. But these screening levels are based on extremely conservative assumptions, such as that a person could be exposed to an element in a homegrown food at the risk-screening level every day for a lifetime.
With a goal to limit potential human cancers caused by exposure, EPA’s risk-screening level for arsenic, a human carcinogen, in residential soils is 0.67 ppm. That’s about one-tenth the median U.S. background level of 5.2 ppm.
If the levels in a product being evaluated for beneficial uses fall below EPA screening levels, then there’s no problem, Taylor points out. Challenges occur when the levels fall between screening values and background.
Meanwhile, other applications for the USGS data are now emerging. For instance, Sarah C. Jantzi, a researcher at Memorial University, in St. John’s, Newfoundland, is exploring their use in criminal forensics. The premise is that soils have a regional fingerprint that could allow police to trace where shoes, tires, or shovels found at crime scenes have been.
USGS’s Smith says the data also provide a baseline for assessing the impact of storms, volcanoes, climate change, and human activity on soil composition.
Because winds and other natural forces can transport human-caused pollution over broad areas, “pristine soils are hard to find now,” Smith says. The database that USGS has generated, he adds, “is a current snapshot in time against which future changes can be measured.”
Charles Schmidt is a freelance writer based in Portland, Maine.
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