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United States Department of Agriculture

Agricultural Research Service

Research Project: RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS Title: Soil Metal Bioavailability to Plants

Author
item Chaney, Rufus

Submitted to: Partners in Environmental Technology Technical Symposium and Workshop
Publication Type: Abstract Only
Publication Acceptance Date: October 3, 2005
Publication Date: November 28, 2005
Citation: Chaney, R.L. 2005. Soil metal bioavailability to plants. Partners in Environmental Technology Technical Symposium and Workshop [abstract]. Proceedings of Partners in Environmental Technology Technical Symposium and Workshop, Washington, DC, No. D-15 (T420).

Technical Abstract: Soils at DOD locations have become contaminated with a number of elements from several sources. Remediation decisions require information on risk of these elements in soils, and effects of treatments to remediate the potential toxic effects of the contaminating elements. Metals can be in highly phytoavailable forms in soils, and high levels of Zn, Cu, Ni, and some other elements in acidic soils can kill most plants. Besides effects on cover vegetation, metals can accumulate in plant biomass and injure wildlife which occur at the site. Thus the phytoavailability of elements in soils is a key part of any risk assessment or soil remediation decision. Risks from soil ingestion is another process that must be considered in soil risk assessment and remediation decisions. Elements such as Pb and As are poorly accumulated by plants, and depending on the chemistry of the soil, the elements may be bioavailable to wildlife which ingest soil. Thus for some elements, treatments to remediate risk of phytotoxicity must take into consideration of risk from soil ingestion. Assessment of the potential for food-chain transfer and toxicity of elements accumulated in plants has shown that for nearly all elements, phytotoxicity limits plant yield before the element residue in plants can cause chronic toxicity to small mammals or birds which live on the site. At alkaline pH (calcareous soils), Mo and Se can be bioaccumulated into plants, and plant concentrations can exceed threshold levels which cause harm to ruminant livestock or wildlife. Tungsten may be included in this category if soils are contaminated. Plant accumulation of these anionic elements can be reduced by maintaining soil pH at low levels, well below pH 6.5. Accumulation of cationic elements in plants seldom causes risk to food-chains. Elements such as Zn, Cu, Ni and Mn in strongly acidic contaminated soils can accumulate in plants but they injure the plants and cause substantial yield reduction before the concentration in aboveground biomass is a hazard to wildlife. When Cd contaminates without the normal much higher level of Zn, plants can accumulate hazardous levels of Cd in shoots. Most Cd uses by DOD have ceased, so most sites with Cd above background will have more than 100-fold higher Zn. Under this situation Cd accumulation in plants is limited by competition with Zn, by Zn induced phytotoxicity. And when Zn is 100-fold Cd in plants, the Zn inhibits Cd absorption by animals. Thus most DOD soils with Cd contamination will not have risk from that Cd because of co-contaminating Zn. Co is theoretically a food-chain poison because plants can accumulate Co concentrations that are harmful to ruminant livestock. However, Co is seldom a significant soil contaminant. In cooperation with E-US-EPA Superfund, we have demonstrated methods to remediate risks from severely Zn phytotoxic contaminated soils near smelters or mines. Soils or mine wastes are rich in Zn, Cd, Pb, and possibly other elements, and the soil is strongly acidic from pyrite oxidation. Long barren soils have been persistently revegetated with plants that are safe for consumption by wildlife in several demonstration projects. An amendment of limestone or alkaline byproduct mixed with biosolids or biosolids compost is surface applied on the acidic phytotoxic soil. The alkalinity makes the soil alkaline, and supplies both N and P to improve soil fertility, organic matter to aid soil structure and water holding, and microbes to re-establish an effective microbial ecology. After a few days to allow equilibration, the site is then seeded with appropriate locally adapted cover species. Small mammals at several sites where long barren smelter killed soils were remediated are not harmed by the metal residues in plants under these conditions. The high phosphate in biosolids aids in reducing the bioavailability of soil Pb. High Fe in the same biosolids increases specific metal adsorption by the amended soil, aiding remediation of higher metal levels than is possible with lower Fe in the amendment.

Last Modified: 8/29/2014