Use of Biochar Amendments to Mitigate Soil Pb Toxicity to People and Waterfowl.

Biochar can improve plant growth and reduce metal bioavailability to plants in metal-contaminated soil but the ability of biochar to reduce gastrointestinal bioavailability of heavy metals to people and waterfowl remains unknown. In this project, Pb-contaminated soil and sediment from three sites with distinct contaminant legacies were amended with 2% biochar (17 unique chars) and incubated for six months. The soils and sediment were analyzed for changes in in vitro Pb bioaccessibility post incubation according to four variations of EPA’s standard soil Pb bioaccessibility assay (EPA Method 1340 at pH 1.5 and 2.5, dried and sieved and wet and not sieved), the Avian Ohio State University In Vitro Gastrointestinal bioaccessibility assay, and the Mehlich 3 soil nutrient test. Results from one upland and one wetland site are currently available. Biochar amendments to the upland soil contaminated by aerial smelter deposits reduced Pb bioaccessibility (EPA Method 1340, pH 2.5, dry) by up to 13% relative to control. In the wetland sediment contaminated by alluvial mining deposits, all pots including the control increased in Pb bioaccessibility after six months. This increase is likely due to oxidation of reduced Pb compounds over the course of the experiment and illustrates the influence of redox conditions in determining Pb toxicity. Remediation strategies that disturb reduced sediments, including incorporating biochar, may inadvertently exacerbate Pb bioaccessibility by converting sparingly soluble reduced Pb compounds to more soluble, oxidized species. Results also illustrate that the drying and sieving step in the standard operating procedure for determining soil Pb bioaccessibility to humans (EPA Method 1340) increases Pb extractability by an average of 48% compared to sediment samples processed wet and not sieved. To avoid overestimating Pb risk to organisms exposed to Pb in reduced form (e.g. waterfowl), bioaccessibility metrics must preserve relevant Pb redox conditions for the target organism.

Impact/Purpose

Biochar can improve plant growth and reduce metal bioavailability to plants in metal-contaminated soil but the ability of biochar to reduce gastrointestinal bioavailability of heavy metals to people and waterfowl remains unknown. In this project, Pb-contaminated soil and sediment from three sites with distinct contaminant legacies were amended with 2% biochar (17 unique chars) and incubated for six months. Results from one upland and one wetland site are currently available. In the upland soil, biochar amendments reduced Pb bioaccessibility to people up to 13% relative to control treatments. In the wetland soil, all treatments as well as the control increased in Pb bioavailability after six months. This increase is likely due to oxidation of reduced Pb compounds over the course of the experiment and illustrates the influence of redox conditions in determining Pb toxicity. As illustrated by this experiment, remediation strategies that disturb reduced sediments may inadvertently exacerbate Pb bioavailability by converting sparingly soluble reduced Pb compounds to more soluble, oxidized species. Results also illustrate that for the sediment, the drying and sieving step included in the standard operating procedure for determining soil Pb bioavailability increased Pb extractability by an average of 48% compared to samples processed wet and not sieved. Using the standard Pb bioavailable assay (EPA Method 1340) to estimate Pb toxicity to organisms that are exposed to Pb in wet, not-sieved sediment may therefore greatly overestimate overall risk. This research seeks to aid federal, state and tribal agencies charged with cleaning up Pb-contaminated sites, including the EPA, by illustrating the remedial potential as well as the limitations of using biochar to reduce soil Pb toxicity, and by informing the selection of appropriate Pb bioavailability metrics for a given site.

Citation

Plunkett, S., Todd P Luxton, Chris S Eckley, AND Mark G Johnson. Use of Biochar Amendments to Mitigate Soil Pb Toxicity to People and Waterfowl. ASA-CSSA-SSSA International Annual Meeting, Virtual, OR, November 08 - 11, 2020.