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IMPROVED RISK ASSESSMENT AND REMEDIATION OF SOIL METALS BASED ON BIOAVAILABILITY MEASUREMENTS
Citation:
Ryan*, J A., R. L. Chaney, P. G. Reeves, J. G. Halfrischand, AND S. L. Brown. IMPROVED RISK ASSESSMENT AND REMEDIATION OF SOIL METALS BASED ON BIOAVAILABILITY MEASUREMENTS. Presented at Technical Symposium & Workshop, sponsored by SERDP and ESTCP, Arlington, VA, 11/28-30/00.
Description:
Heavy metals in soils can comprise risk through plant uptake or soil ingestion. Recent research results and progress in understandings of risks and methods for soil metal remediation will be presented. Beneficial use of composts/bosolids plus limestone to remediate metal killed ecosystems wll also be summarized. Progress has been made in understanding bioavailability of Cd in plants grown on contaminated soils - soil Cd comprises insignificant risk to humans or the environment under nearly all soil contamination situations. Soil Cd has harmed humans only where subsistence farmers consumed rice "home-grown" on flooded soils contaminated by mining or smelting of metals in Japan and China. Soils with as little as 2-10 mg Cd plus 200-1200 mg Zn/kg soil caused Cd-disease after 30-50 years exposure. But where garden foods were grown in aerobic soild highly contam9inated with Zn plus Cd, no Cd-adverse effects on humans were found at three cities even where sols contained up to 100 mg Ca and 10,000 mg Zn/kg. This difference is believed to result from the unique properties of rice regarding Cd flow in bood chains and bioavailability. When grown in flooded soils, rice accumulates Cd to danger0ous levels in grain, but grain Zn is not increased - very different from crops grown in aerobic contaminated soil. We had hypothesized that the lack of Zn movement to grain, coupled with malnutrition of zinc, iron and calcium in subsistence rice consumers, caused much greater Cd adsorption than other foods - allowing soil Cd to cause disease. but when lettuce or Swiss chard were grown on compost or biosolids amended soils, even when Cd and Zn were increased 5-fold on acidic treatments, feeding tests showed no increase in Cd in kidney or lever of the test animals - zero increase in bioavailable Cd when these crops were grown on amended soils. We have not conducted feedings tests with rats fed sunflower or rice with 109Cd and about 0.5 mg Cd/kg grain using a factorial experimental design: diet Zn, Fe, and Ca were provided at marginal or adequate levels. Although low dietary Zn normally increases Cd uptake by animals, sunflower supplied enough Zn to prevent Zn-deficiency and gave low Cd uptake. With the multiple deficient treatment (low Zn, Fe, and Ca), 2.7% of rice 109Cd but only 0.78% of sunflower 109Cd were adsorbed to the kidney and lever of the test rats; with adequate supply, 0.35% of rice 109Cd and 0.22% of sunflower kernel 109Cd were adsorbed to liver and kidney. This study confirms the remarkable difference between rice and other crops, and the very low feed-chain transfer, bioavailability and risk of soil Cd in the common contamination conditions in which soil Zn is about 100-fold higher than soil Cd.