Composted biosolids bind lead in soil, reducing danger of poisoning. (EH Update).

The addition of composted biosolids rich in iron, manganese, and organic matter to a lead-contaminated home garden in Baltimore appears to bind the lead, according to a study published in the January/February 2003 issue of the Journal of Environmental Quality As a result, the lead is less likely to be absorbed into the bloodstreams of children who dirty their hands playing outside or are tempted to taste mud pies they have "baked" in the backyard.

The bioavailability of lead in the soil was lowered by 20 percent to 38 percent after composted biosolids were mixed with the contaminated garden soil. The best mixture for reducing bioavailability was made from Baltimore biosolids that contained more iron and manganese than the others tested.

Biosolids are the organic residuals produced during wastewater treatment. Once composted, biosolids look like other commercially available composts and are approved by the U.S. Environmental Protection Agency (U.S. EPA) for use as a soil amendment.

The study, funded by the nonprofit Water Environment Research Federation, tested seven different biosolids and composted biosolid treatments, adding 3 inches of each to different areas of the garden, then thoroughly mixing the soils weekly for 30 days. Soil samples and laboratory rats exposed to the soils were tested for changes in lead levels.

The garden soil in the study is similar to potentially hundreds of thousands of yards contaminated with lead in Baltimore and other inner cities, according to Sally L. Brown, lead author of the article and assistant professor of forest resources at the University of Washington. A pilot program that adds composts to home gardens in Baltimore and East St. Louis, Illinois, appears to confirm the findings of the study Researchers still need to find out how long the effects last and if similar results can be obtained with compost that does not come from biosolids. Brown and co-authors also want to detennine exactly why composted biosolids change the nature of lead so that it is not so readily absorbed by the body They hypothesize that biosolids, which are generally more than 50 percent organic matter often contain high concentrations of iron, as well as high levels of phosphorous and manganese. Studied singly by other groups of scientists for studies published in 1999 and 2000, each of these substances has been sh own to reduce lead availability in soils.