by Sara E. Pratt Tuesday, August 19, 2014
A new study in which all 842 soil samples taken in Ohio had more arsenic than recommended by the Environmental Protection Agency (EPA) raises the question of what to do when natural background levels in the environment exceed limits set to protect ecosystems and human health.
Arsenic in the environment is a public health concern; in the body, it replaces phosphate and can cause neuromuscular damage, skin diseases, cancer and even death. In the past few decades, new toxicological studies led to the lowering of screening standards for arsenic, particularly in drinking water. In 2001, the EPA lowered the maximum contaminant level of arsenic allowed in public drinking water supplies from 50 parts per billion (ppb) to 10 ppb. For soils, federal EPA guidelines for standards of concern begin at 390 ppb, or 0.39 parts per million (ppm).
In the new study, published in the Journal of Environmental Quality, Erik R. Venteris, a geologist with the Department of Energy’s Pacific Northwest National Laboratory, and colleagues analyzed soil samples collected by the U.S. Geological Survey as part of the National Geochemical Survey.
Soil samples were collected from either or both of two depth ranges — 15 to 30 centimeters and 30 to 60 centimeters below the surface — at 492 different sites, including streambeds, in a 116,000-square-kilometer area in Ohio. Arsenic levels in the samples ranged from 2.0 to 45.6 ppm, with most containing 10 ppm or more. No samples were below the lowest EPA soil screening guideline of 0.39 ppm, which is the level of concern for soil ingestion.
To determine statewide baseline levels, Venteris' team developed a map based on interpolation of the sample set and then looked for patterns to distinguish soil arsenic levels derived from the bedrock geology from soil arsenic contamination caused by human activities such as construction, mining and the use of pesticides.
“We found that the patterns were related to underlying bedrock and the overlying glacial sediments and soil processes,” says Venteris, who worked at the Division of Geological Survey of the Ohio Department of Natural Resources at the time the research was conducted. In other words, the soil arsenic levels in Ohio “correspond to the geology.”
“The presence of detectable levels of inorganic arsenic in soils in Ohio is not unexpected,” says geologist Robert Frey, director of health assessment within the Bureau of Environmental Health at the Ohio Department of Health. But that does not “in itself mean that Ohioans are at increased risk from adverse health effects due to exposure to inorganic arsenic.”
Arsenic is an infamous poison at high doses, but it exists in a variety of forms with many pathways of exposure, so determining safe levels can be difficult. Health guidelines address long-term, chronic exposure to lower levels of inorganic arsenic in the environment, Frey says, and the consequences of exposure depend on the dose, duration, frequency and pathway of that exposure.
Unlike drinking water exposure, which mainly is a risk with private wells drawing groundwater, exposure to arsenic through surface soils can come from direct skin contact, during activities like landscaping and gardening, from soil ingestion, which is more common in children, and from eating plants grown in arsenic-contaminated soils.
Currently, there are multiple screening levels for soil arsenic set by various state and federal agencies — ranging from less than 0.1 ppm to 20 ppm.
In Ohio, the health department uses guidelines developed by the federal Agency for Toxic Substances and Disease Registry (ATSDR) to assess the risks from contaminated soil, water and air. The ATSDR guideline for chronic exposure to soil arsenic for a child is 15 ppm.
One reason for the wide-ranging guidelines is that most human-health arsenic studies upon which the guidelines are based involved drinking water. Arsenic is more bioavailable, or able to be absorbed by humans and animals, in water than it is in soil. It would take frequent exposure to heavily contaminated soils, ingested or inhaled in large quantities, to result in similar adverse health effects, Frey says. “This scenario is unlikely due to the aversion of most people to eating large quantities of dirt and the normal reaction of sneezing back out inhaled dust,” Frey says.
In addition, Frey says that from a public health perspective, the uppermost soil, from zero to 15 centimeters, is of most concern, and anything deeper is unlikely to pose an exposure threat to people. “Our concerns regarding inorganic arsenic in the environment in Ohio are focused more on the groundwater/drinking water pathway than on the levels of arsenic in Ohio soils,” he says.
Although the findings are not cause for immediate public health concern, Venteris says, they do raise a question about regulations regarding remediation of contaminated soils.
“There’s toxicology and there’s geology, and what we found in this study is that there’s a disconnect” between the two, Venteris says. “We do not resolve this disconnect, but we ask the fundamental question: Is it fair to ask a company to clean up an area to a concentration that is less than what we expect naturally?”
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