Arsenic-treated wood may have a long toxic legacy
Although residential use of a common arsenic-based wood preservative was phased out in 2003, new research indicates that leaching from products treated with it could impact soils and groundwater for decades to come.
Since the 1940s, wood treated with a preservative called chromated copper arsenate (CCA) has been used to erect hundreds of thousands of decks, docks, and fences. Although health and environmental concerns inspired industry to phase out CCA products for residential use in 2003, the agreement with the U.S. EPA did not apply to existing residential structures or to many industrial applications, such as electricity and telephone poles or railway ties. Now, two research articles posted today on ES&T’s Research ASAP website (10.1021/es0514702, 10.1021/es051471u) indicate that arsenic leaching from these structures will continue to contaminate the environment for many decades.
In their two-part study, researchers from the University of Miami, the University of Florida, and Florida International University looked at the arsenic leaching from wood under actual field conditions. Because of its warm, wet climate, Florida serves as the home for as much as 15% of the CCA-treated wood used in the U.S. Consequently, it is ground zero for studying possible impacts to soils and groundwater, says John Schert, director of the Florida Center for Solid and Hazardous Waste Management. “Nobody realized [CCA-treated wood] leached as much as it did and caused soil contamination problems, before this work,” Schert says.
During a one-year period, Helena Solo-Gabriele, an environmental engineer at the University of Miami and the corresponding author of the study, and her colleagues found average arsenic concentrations of 600 micrograms/liter (µg/L) in rainwater runoff from a CCA-treated deck, more than 100 times higher than in runoff from an untreated deck. Both of the element’s more toxic inorganic forms, As(III) and As(V), were present; As(V) accounted for roughly 90% of the total arsenic released in the runoff.
A layer of sand beneath the treated deck appeared to capture a significant portion of the arsenic runoff. There, the researchers found average arsenic levels of 30 milligrams/kilogram (mg/kg), which were 15–30 times higher than background levels of 3 µg/L.
The researchers also found arsenic at levels up to 18 µ/L in the runoff water that percolated through the sand layer. By the close of the study, they had seen no evidence that maximum arsenic concentrations had been reached. Even more troubling, given Florida’s high groundwater table, is the fact that they found a higher ratio of the more soluble, mobile form, As(III), in the infiltrated rainwater. This finding suggests that microbial activity in the sand layer is converting some of the As(V) to As(III).
After examining annual production statistics for CCA-treated wood, the researchers estimate that by 2000 Florida had imported 28,000 metric tons of arsenic, 4600 of which have already leached into the environment. They predict that as much as 40% of the arsenic in this wood will leach out over its entire in-service lifespan, which varies from 9 to 13 years for products such as decks and is 40 years or more for utility poles.
“Only a small fraction leaches in any given year [roughly 5%],” Solo-Gabriele explains. But because of the cumulative effects of all the in-service years, “the impacts can be significant, especially given the high concentrations of arsenic in the actual wood itself,” which on average is about 3100 mg/kg.
That leaves 60% of the arsenic remaining in the wood when it’s discarded, typically in unlined construction and demolition (C&D) debris landfills. And when the researchers conducted a series of simulated landfill studies, they found that “the concentration of arsenic is much higher” than what leaches out during use, Solo-Gabriele says.
The research is fueling “a lot of concern that over the long term, arsenic will start showing up in the groundwater beneath these landfills,” Schert says, adding that “there’s some evidence of that already.”
CCA-treated wood qualifies as a hazardous waste under the federal Toxicity Characteristic Leaching Procedure, but unlike the EU, EPA exempted it from this classification, allowing the wood to be disposed of as a regular solid waste. In 2004, however, the agency did recommend that CCA-treated wood be disposed of in lined landfills where the leachate is collected and treated, and it could make this a requirement, according to Ross Elliott, an environmental protection specialist with EPA’s Office of Solid Waste.
These and previous findings by the Florida researchers have “been very useful in helping us understand the issues,” Elliot says. EPA recently initiated a groundwater fate and transport modeling project to determine where the risks may lie at landfills, their magnitude, and what types of liners might ameliorate the problem.
Florida’s Department of Environmental Protection (DEP) is already drafting regulatory changes to divert CCA-treated wood products directly to lined landfills and reduce the amount being recycled as mulch or wood fuel or disposed of in unlined C&D landfills. “It’s clear from [Solo-Gabriele’s] research that CCA-treated wood leaches arsenic in levels that are above the standards, and we think it supports changes to rules governing the way we manage treated wood in Florida,” says Richard Tedder, program administrator of the DEP’s Solid Waste Section.
