Submitted to: Soil Dynamics International Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2001
Publication Date: 9/5/2001
Citation: Interpretive Summary: This paper is a review of progress in understanding the risks from metal contaminated soils, and new approaches to remediation of soil Cd and Pb. Great progress has been made in the last 30 years in understanding the soil, plant, and animal processes which allow transfer of metals from soils to cause risk to humans, wildlife, livestock, plants or soil biota. Two metals are reviewed here, Cd and Pb. Cd risk results when individual human over their lifetimes ingest or inhale enough bioavailable Cd to cause Cd to accumulate in the kidney cortex to levels which cause tubular dysfunction. Subsistence rice consumers in Asia were harmed by lifetime consumption of polished rice grown on soils contaminated by zinc-mine wastes; thus much research has been conducted to understand and prevent food-chain Cd risks. The paper stresses that one must consider the nature of the source of Cd which contaminated a soil, the nature of the soil, the properties of the crop in accumulating Cd and other elements, and the effect of the crop on the nutritional status of the consumer, in order to understand how soil Cd can reach kidney and cause harm. Rice accumulates Cd but excludes Zn from grain, different from other crops. Subsistence rice consumers experience deficiency of Zn, Fe and Ca, and those deficiencies promote Cd absorption up to 20-fold higher than from other foods. Malnutrition induced by subsistence rice consumption caused Cd in rice to be absorbed, accumulated in kidney cortex, and to harm these consumers. In Europe and the US, individuals with gardens on highly contaminated soils suffered no Cd harm because the foods supplied enough Fe, Zn and Ca to inhibit Cd absorption. Thus, regulations to achieve Soil Protection need to consider the crop, presence of Zn along with Cd, and nature of diet.
Technical Abstract: This paper was invited for a Symposium on Soil Protection in Europe. Because the authors have been conducting research to more fully understand and prevent risks from soils or foods contaminated with Cd or Pb, this paper reviews progress in understanding how soil Cd or Pb move into humans and cause harm, and how such risk can be prevented. In the case of Cd, subsistence rice consumers in Asia grew their lifetime supply of rice on flooded soils contaminated by Zn mining and smelting. In villages with contaminated soils, a high fraction of citizens over 50 years of age had accumulated enough Cd in their kidney cortex to cause dysfunction, the first adverse health effect of excessive Cd absorption. After nearly 30 years of study, we have now shown that rice is unique among food crops in transferring soil Cd to human kidney. Subsistence on rice promotes Zn, Fe and Ca deficiency, and these deficiencies promote Cd absorption into animals. Further, during grain production, rice excludes Zn from the grain while accumulating Cd. No other food has this combination of factors which promote Cd risk, and even individuals who consume home grown garden foods on much more highly contaminated soils do not experience excessive Cd absorption because their varied diets have adequate Fe and Ca, and 100-times more Zn than Cd is accumulated in the grain. Fuller understanding of how soil Cd is transferred to kidney show that soil protection is achieved for Cd when Zn accompanies Cd, for crops other than rice. Similar extensive progress has been made in understanding sol Pb risk to children which ingest soil. By adding phosphate and Fe to adsorb or precipitate soil Pb, we can convert Pb to forms which are not bioavailable to children, thereby achieving inexpensive remediation.