Risk assessment and remediation of soils contaminated by mining and smelting of lead, zinc and cadmium.

Milling and smelting of Pb, Zn and Cd ores have caused widespread soil contamination in many countries. In locations with severe soil contamination, and strongly acidic soil or mine waste, ecosystems are devastated. Research has shown that Zn phytotoxicity, Pb-induced phosphate deficiency, Cd risk through uptake by rice or tobacco, and Pb risk to children, livestock or wildlife which ingest soil are the common adverse environmental effects at such contaminated sites. Improved understandings of soil metal risks to the environment have been developed which examine risk to all possible exposed organisms through soil, plants, animals, or water exposures. This review summarizes present information about soil Cd risk to food-chains, explaining that when Cd is present at the usual 0.005-to-0.02 ratio to Zn in the contaminated soil, only rice and tobacco allow Cd to be transferred from the soil in amounts which can harm humans over their lifetime. Zn inhibits plant uptake of Cd, and inhibits intestinal absorption of Cd, protecting animals from Cd in most situations. Pb risk to children or other highly exposed organisms results from ingestion of the contaminated soil, and absorption of Pb from the soil into the blood where adverse health effects occur at l0-to-15 µg Pb/dL blood. Soil Pb has much lower bioavailability than water Pb, and if ingested with food has even lower bioavailability. Research has shown that if high phosphate levels are added to Pb contaminated soils, an extremely insoluble Pb compound - chloropyromorphite - is formed in soils from all known chemical species of Pb which occur in contaminated soils. It had earlier been learned that adding adsorbents and phosphate to Pb contaminated soils inhibited Pb uptake by crops, and combined with the evidence that these materials could reduce the bioavailability of soil Pb to children, feeding tests were conducted with rats and pigs in several laboratories. A new approach to remediation of severely disturbed Pb/Zn/Cd contaminated soils has been developed which uses mixtures of limestone equivalent from industrial byproducts such as woodash (to make soil calcareous and prevent Zn phytotoxicity), phosphate and Fe from biosolids and byproducts (to precipitate Pb and with Fe, increase Pb, Zn and Cd adsorption), organic-N from biosolids and manures and other beneficial components which correct the infertility of contaminated and eroded soils. Highly effective revegetation has resulted at four field test locations where this approach was tested, Palmerton, PA; Katowice, Poland; Bunker Hill, ID; and Leadville, CO. All plants tested were readily grown on the amended soil even with soil contained over 1% Zn and 1% Pb. Plant analysis indicates that these plants may be consumed safely by wildlife and livestock, although soil ingestion should be minimized at such sites. The potential use of metal hyperaccumulator plants to phytoextract soil metals is a new method of remediation under development. Combining improved cultivars of these accumulator plants, agronomic management practices to maximize yield and metal accumulation, burning the biomass to generate power, and recovery of metals from the ash appear to offer an economic technology compared to soil removal and replacement. Although mining and smelting contamination has caused severe environmental harm in many locations, these methods of soil metal remediation allow effective and persistent remediation at low cost, and should be applied to prevent further dispersal of the contaminated soil materials at many locations.