Submitted to: Biocycle
Publication Type: Popular publication
Publication Acceptance Date: 6/1/2002
Publication Date: 6/15/2002
Citation: Interpretive Summary: At many historic lead-zinc mines and smelters, soils were contaminated and killed all vegetation. Our research team developed a method to revegetate such toxic soils using biosolids and alkaline by-products which has repeatedly been shown to achieve effective, diverse and persistent vegetation on sites which had been barren for decades. We undertook a literature review and risk assessment for wildlife which could consume the plant materials, live in the soils, or ingest soil either inadvertently or within soil macrofauna such as earthworms living on a site. If effective diverse vegetation is established on such a contaminated site, the soil contaminants are no longer present at phytotoxic levels in the plants. Further, for normal geogenic Cd+Zn contamination, Zn in plants prevents risks to herbivores from Cd in soil or plants by inhibiting availability to plants, and bioavailability of plant Cd to plant consumers. Pb uptake by plants is very limited because phosphate was added to hasten formation of insoluble Pb compounds. Thus risks thru soil ingestion are the most limiting for assessment of soil metal risks at remediated sites. Pb risks are reduced by formation of chloropyromorphite, a very insoluble Pb compound with phosphate which is formed in the remediated soil. Effective plant cover also limits soil ingestion by wildlife. Soil Cd risk is greatest to small mammals such as shrews which ingest earthworms; earthworms bioaccumulate soil Cd, and contain about 40% soil on a dry matter basis when ingested. Fe and Mn oxides in the amendments reduce Cd accumulation in earthworms and by small mammals which ingest earthworms. We conclude that remediated sites offer very low risk to attracted wildlife.
Technical Abstract: Historic Zn-Pb-Ag mine and smelter sites are often severely metal contaminated barren. Our team has demonstrated that such Superfund sites can be revegetated using a combination of organic matter and alkaline byproduct. The supply of organic-N, phosphate, alkalinity, organic matter, and soil microbes can cause inactivation of the metals present and make the soil calcareous so it will persistently limit metal solubility. Using this approach, diverse plant species can thrive on the remediated soils to prevent erosion, including legumes which can provide N for a sustainable vegetation. This paper reports a risk assessment for potential adverse effects of the remediated metal contaminated soil materials. Metal levels in the vegetation are within normal concentrations for lifetime ingestion by livestock and wildlife. Zn is usually 100 or more times higher than Cd in these sites; so plant Cd is limited to low levels in plants; further Zn reduces bioavailability of plant Cd. Thus herbivore pathways comprise no risk to wildlife. The second major Pathway for risk is soil ingestion; soil ingestion is greatest by small mammals and birds which consume soil macrofauna such as earthworms. Results from remediated contaminated soils indicate that soil metal bioaccumulation does not comprise Cd risk to earthworm consumers unless soil Cd is extreme; in such cases, inclusion of Mn and Fe oxides in the amendments can increase metal adsorption by the amended soil, which reduces Cd absorption by earthworms and consumers of earthworms. These results indicate that surface application of biosolids plus alkaline byproducts is sufficient to restore vegetative cover to metal mine wastes making them safe for use as wildlife habitat.