• June 4, 2012
• By Shawn Wright

High concentrations of lead found in Boston's compost site have city officials and scientists perplexed.

Since 2005, lead levels have more than doubled in the compost that is collected every year from Boston-area household yards, the Boston Globe reported.

"Right now we have no conclusive explanation for why lead levels have been rising, but we do know that we've certainly been collecting a lot more yard waste as we make an emphasis on reducing solid waste,'' Jim Hunt, the city's chief of environmental and energy services, told the newspaper.

The most recent tests of the compost this spring found mean lead concentrations of 260 parts per million, the newspaper said. That is below the state's safety limit of 300 parts per million and the federal standard of 400 parts per million.

But the findings this spring were more than double what city officials found in 2005. Some samples taken this year showed concentrations of as much as 480 parts per million of lead, the newspaper said. The city's mean lead concentration last year was 299 parts per million.

Among the potential reasons for the increased lead levels is more of the compost has come from yards of older homes that are shedding flakes of lead paint, the newspaper said. Other sources include fishing-line sinkers, bullets, stained glass and remnants of insecticides once sprayed on plants.

This is the third year in a row that the city has withheld its compost from gardeners for safety reasons, Hunt told the newspaper. City officials said gardeners have never been at risk from the yard waste collection and composting program.

The city first began collecting yard waste and other compostable material in 1995. Since then, the amount of leaves, branches, grass and other material increased from 638 tons to more than 11,000 tons last year, the newspaper said.

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