Biosolids compost amendment for reducing soil lead hazards: a pilot study of Orgro® amendment and grass seeding in urban yards

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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. Modern regulated biosolids composts have low Pb concentrations and low bioaccessible Pb fractions and can improve grass growth on urban soils. High Fe and P biosolids composts can reduce the bioavailability and bioaccessibility of soil Pb and can aid in establishing vegetation that would reduce soil transfer into homes. For these reasons, we conducted a field test of their use to reduce Pb bioaccessibility in urban soils in Baltimore, MD USA. We chose biosolids compost for its expected reduction in the bioaccessible Pb fraction of urban soils, ease of use by urban residents, and ability to beautify urban areas.

Nine urban yards with mean soil Pb concentrations >800 mg Pb kg−1 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 1 year later. At each sampling time, total soil Pb was measured using a modified U.S. EPA nitric acid hotplate digestion method (SW 846 Method 3050) and bioaccessible Pb fraction was measured using the Solubility/Bioaccesibility Research Consortium standard operating procedure with modifications, including the use of glycine-buffered HCl at pH 2.2. Samples of untreated soils were collected from each yard and mixed well to serve as controls for the Pb bioaccessibility of field treated soils over time independent of positional variance within yards.

At 1-year post-treatment, grass cover was healthy and reductions in bioaccessible Pb concentrations compared to pre-tillage were 64% (from 1655 to 595 mg kg−1) and 67% (from 1381 to 453 mg kg−1) at the sampling lines closest to the houses. Little or no reduction in bioaccessible Pb concentration was observed at sampling lines more remote from the house that also had the lowest bioaccessible Pb concentrations at pre-tillage (620 and 436 mg kg−1, respectively). For the control soils, changes over time in total Pb and bioaccessible Pb concentrations and the bioaccessible Pb fraction were insignificant. This study confirms the viability 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.

Introduction

Urban soils are commonly contaminated with Pb from multiple sources, principally automotive emissions and exterior paint (ATSDR, 1988). Although interior Pb sources predominate in exposure and risk compared to contaminated soil (Jacobs et al., 2002, U.S. HUD, 2001, Weitzman et al., 1993), high concentrations of Pb in soils presents a risk to young children who may ingest soil directly or may ingest house dust that has been enriched in Pb from soil sources. Examination of the pattern of Pb distribution in major cities has shown that inner-city soils have become generally contaminated from automotive and stack sources, while exterior paint can cause very high local enrichment of soil Pb, even exceeding 5% in the surface 2 cm depth (Chaney et al., 1984, Chaney and Ryan, 1994, Mielke et al., 2000).

Although it is possible to remove all contaminated soil and replace it with clean soil, the cost and degree of disturbance have made soil replacement prohibitive in most situations. Only grossly contaminated soil associated with housing adjacent to Superfund sites (e.g., smelters, mines, battery recycling smelters) has been subject to the soil replacement recommended by U.S. EPA and U.S. HUD for highly contaminated soils because this program can fund soil Pb remediation only for industrial point sources. A number of alternative methods to address soil Pb risk have been investigated to determine if less expensive methods including sandboxes, shrubs as barriers to humans, and raised boxes with clean soil could be used by homeowners, tenants and local groups to reduce community soil Pb risks (Hynes et al., 2001, U.S. EPA, 2001a, Litt et al., 2002). These are the types of interim control hazard reduction measures that are currently being used by local lead hazard control programs funded through grants from U.S. HUD.

Investigation has also identified two methods which can reliably reduce the bioavailability of soil Pb to animals fed the soils: (1) application of high levels of phosphate (phosphate fertilizer, rock phosphate, phosphoric acid, and bone phosphate sources) to promote formation of pyromorphite, a form of soil Pb with low solubility and low bioavailability (Ryan et al., 2001); and (2) application of biosolids compost products rich in phosphate and hydrous Fe oxides which both promote formation of pyromorphite and increase the strength of adsorption of Pb to ingested soil particles (Brown et al., 2003a). Biosolids composts rich in P and Fe also reduced bioavailability of Pb to rats in a feeding study of soil amendments (Ryan et al., 2004). Urban soils are often infertile or even Zn phytotoxic from Zn that co-contaminates with Pb (Chaney et al., 1984, Mielke et al., 1999, Mielke et al., 2000), resulting in weak plant cover that is easily disturbed by play activities. Zn phytotoxicity is more severe in contaminated urban soils which are strongly acidic (Chaney et al., 1984) so remediation needs to prevent Zn phytotoxicity to minimize soil/dust Pb exposure for children.

Incorporation of composted biosolids rich in P and Fe offers a soil amendment method which can correct most soil infertility and metal toxicity problems of urban soils, provide the P and Fe which reduce bioavailability of soil Pb, and mix surface soil (usually richer in Pb) with subsurface soil materials. Although incorporating compost may dilute soil Pb somewhat, the principle effects are obtained by reducing bioavailability of the Pb and mixing surficial Pb with less contaminated subsurface soil. It can also yield a fertile soil which will support a strong turfgrass cover that can further reduce children's ability to come in contact with the soil, potentially reducing the transfer of lead-contaminated soil into houses via tracking and blowing. Thus, the composted biosolids amendment approach has the potential to reduce risk from Pb in urban soils by reducing direct contact with bare soil, transfer of soil into the house, and the bioavailability of Pb in ingested soil.

Biosolids composts have been successfully used to reduce soil Pb hazards at several industrial sites and residential communities associated with smelters, including a lead smelter in Joplin, MO (Brown et al., 2004), a lead smelter and mine pile in Katowice, Silesia, Poland (Stuczynski et al., 2000), and multiple Superfund sites in the U.S. (Bunker Hill, ID and Palmerton, PA) (Brown et al., 2003b, U.S. EPA Remediation Technology Demonstration Forum Action Team).

Bioavailability is determined by measuring the fraction of Pb that is absorbed following ingestion by a test animal. With the development of a chemical test of the potential bioavailability of soil Pb to animals (Ruby et al., 1993), the term bioaccessibility was introduced to avoid confusion with Pb bioavailability to test animals measured by tissue Pb residues from feeding soils. Lead bioaccessibility is assessed with a chemical extraction method shown to give results significantly correlated with bioavailability results from the feeding of Pb enriched soils to pigs and rats (Ruby et al., 1999, Ryan et al., 2004)). A laboratory-based study using lead-contaminated garden soil from Baltimore, MD showed reduced Pb bioaccessibility and bioavailability after amendment with biosolids composts, especially those rich in iron and phosphorous (Brown et al., 2003a). In that study, Baltimore composted biosolids (Orgro®) rich in Fe and P provided the greatest reductions in Pb bioavailability and Pb bioaccessibility in rats.

The ability of composted biosolids to reduce bioaccessible Pb, however, has not been field tested in an urban setting where children's exposure to Pb in residential soil can be particularly acute due to the presence of older lead-painted housing. This study, conducted in 2000–2001, investigated the effectiveness of in situ treatment by incorporation of composed biosolids and grass into contaminated urban residential soils to reduce lead hazards.

Section snippets

Biosolids compost and soil amendment method

Eckology High Organic Compost (Orgro®) was selected for use as a soil amendment in this study. To create Orgro®, municipal biosolids (sewage sludge) produced by the Baltimore City wastewater treatment plant are composted with woodchips and sawdust at the Baltimore City composting facility. This biosolids compost contains approximately 1% nitrogen (USFilter, 2000a) and is moderately rich in Fe (mean=17.2 g kg−1) and P (mean=16.4 g kg−1) (personal communication, David Hill, USFilter, for the

Results

A total of 126 soil probe composite samples were collected at the different stages of biosolids compost treatment, including 30 soil probe composite samples at 1-year follow-up. One yard could not be sampled at 1 year. Nine bulk soil samples and 18 cyclone dust samples were collected before biosolids compost amendment was conducted.

Discussion

Approximately 7% of U.S. dwellings (6.46 million units) have soil Pb concentrations above U.S. EPA and U.S. HUD standards (400 mg kg−1 for bare play area soil and 1200 mg kg−1 for bare soil in the rest of the yard) (Jacobs et al., 2002). Biosolids compost application is one of a number of methods (e.g., sandboxes and raised beds with clean soil) that can be used individually or in combination to reduce soil lead hazards. For example, biosolids compost application could be combined with the use

Acknowledgments

This research was supported by a cooperative agreement (No. MDLHR0066-99) with U.S. HUD. We thank the USFilter (formerly Professional Service Group), for donating the Orgro® compost. We also thank our community partners Lucille Gorham (Middle East [Baltimore] Community Organization); Bea Gaddy (Bea Gaddy's Women and Children's Center); Jeff Thompson (Historic East Baltimore Community Action Coalition); Leon Pernell (The Men's Center); and Justine Bonner (Open Space Committee,

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