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Mulch matters

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11:12am, August 16, 2006
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Mulch made from recycled construction and demolition wood can release arsenic into the environment, a study finds.

Outdoor structures such as decks typically contain wood treated with chromated copper arsenate (CCA) to make it resistant to termites and other pests. Since the end of 2003, the Environmental Protection Agency has banned this wood in residential structures over concerns that CCA exposure could cause cancer (SN: 1/31/04, p. 74: Danger on Deck?).

Some of the treated wood ends up at recycling facilities and gets shredded into mulch, says Helena M. Solo-Gabriele of the University of Miami in Coral Gables, Fla. She and her colleagues measured how much arsenic the mulch releases. They also investigated whether the arsenic-binding dye iron oxide affects the rate of release.

The researchers shredded recycled wood that contained 100 percent, 5 percent, or no CCA-treated stock. Each batch of mulch was further separated into two groups, one colored with iron oxide and one not. The researchers placed the samples outdoors and monitored the runoff for a year.

By year's end, the mulches that contained 5 and 100 percent CCA-treated wood had leached 10 to 15 percent of their original arsenic content. On average, the iron oxide-colored samples had leached about 25 percent less arsenic than the uncolored samples did.

Regardless of the coloration, all the mulches that contained CCA-treated wood released amounts of arsenic that exceed Florida's groundwater guidelines, the researchers report in the Aug. 15 Environmental Science & Technology.

"We aren't saying, 'Don't make mulch out of recycled wood,'" says Solo-Gabriele. "The emphasis should be on making sure you minimize contamination with CCA-treated wood."

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