Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2004
Publication Date: 3/20/2005
Citation: Farfel, M.R., Orlova, A.O., Chaney, R.L., Lees, P.S., Rohde, C., Ashley, P. 2005. Biosolids compost amendment for reducing soil lead hazards: a pilot study in urban yards. Science of the Total Environment. 340(1-3):81-95.
Interpretive Summary: Many urban soils have become enriched in Pb from automotive emissions and paint; especially near painted walls, surface soil Pb is high enough that parents are advised to keep children away. Hand-to-mouth play can transfer soil and dust into mouths of children, and children are susceptible to excessive Pb intakes. Because there are no government programs to remove and replace high Pb urban soils, we have examined alternative methods to reduce the potential for risk to children who may ingest those soils. Two soil treatments have been shown to substantially reduce the bioavailability of soil Pb to animals, phosphate additions which induce formation of a very low solubility Pb phosphate, pyromorphite; and composted biosolids rich in Fe which increases Pb binding and helps promote formation of pyromorphite. After laboratory and field testing at hazardous waste sites confirmed the effectiveness of in situ treatment of soils to reduce soil Pb bioavailability, we conducted the present test to examine the benefit of treating urban soils with biosolids compost. At nine locations soils were sampled before any treatment, and the site was rototilled well and sampled again. In some cases, lack of any tillage after Pb was deposited on the soil surface may allow substantial reduction in total Pb concentration by mixing surface contaminated soil with subsurface soil. After tillage, 110-180 t compost dry weight/ha was applied and thoroughly tilled into the tillage depth. Samples were collected after tilling and one year after tillage. Untreated well mixed subsamples were maintained as check samples. The bioaccessibility of soil Pb was measured using the 0.1 M HCl-glycine buffered extraction at pH 2.5; pH 2.2 was used because earlier testing by Brown et al. had showed that extraction at pH 1.5 did not measure reductions in bioavailability demonstrated by rat feeding tests. The bioaccessibility of Pb in the soil samples taken nearer to the housesides was significantly reduced by the treatments. And the compost supported effective vegetative cover which inhibits soil ingestion by children. Thus this first inner-city test of using high Fe biosolids compost to reduce the risk of soil Pb to children confirmed that this approach can inexpensively reduce excessive Pb risk to children and meet HUD guidance by the combination of increasing soil binding of Pb and providing plant cover. Other field tests have shown that the treatment becomes increasingly effective over time. Compost offers great promise for people to help themselves protect their children at low cost.
Technical Abstract: In situ inactivation of soil Pb is an alternative to soil removal and replacement that has been demonstrated in recent years at industrial sites with hazardous soil Pb concentrations. Most children exposed to elevated soil Pb, however, reside in urban areas, and no government programs exist to remediate such soils unless an industrial source caused the contamination. Because modern regulated biosolids composts have low Pb concentrations and can improve grass growth on urban soils, and because high Fe biosolids composts have reduced the bioavailability and bioaccessibility of soil Pb and can aid in establishing vegetation that would reduce soil transfer into homes, we conducted a field test of their use to reduce Pb bioaccessibility in urban soils in Baltimore, Maryland USA. We chose biosolids compost for its expected reduction in bioaccessible Pb fraction, ease of use by urban residents, and ability to beautify urban areas. Nine urban yards with mean soil Pb concentrations >800 mg Pb/kg were selected and sampled at several distances from the house foundation before soil treatment. The soils were rototilled to 20 cm depth to prepare the sites and resampled. The yards were then amended with 6-8 cm depth of Orgro® biosolids compost (110-180 dry t/ha) rich in Fe and P, mixed well by rototilling, and resampled. Kentucky bluegrass (Poa pratensis) was seeded and became well established. Soils were resampled one year later. At each sampling time, total soil Pb and bioaccessible Pb fraction (using glycine buffered HCl at pH 2.2) were measured. Samples of untreated mixed soils served as controls over time for the field treated soils. At one-year post-treatment, grass cover was healthy and reductions in bioaccessible Pb concentrations were 64% (from 1655 to 595 mg/kg) and 67% (from 1381 to 453 mg/kg) at the sampling lines closest to the houses. Little or no reduction in bioaccessible Pb concentration was found in sampling lines more remote from the house that also had the lowest bioaccessible Pb concentrations at pre-tillage (620 and 436 mg/kg, respectively). For the control soil, changes over time in total Pb and bioaccessible Pb concentrations and the bioaccessible Pb fraction were insignificant. This study confirms the utility of in situ remediation of soils in urban areas where children are at risk of high Pb exposure from lead in paint, dust and soil.