Heavy metal pollution, risk assessment and remediation in paddy soil environment: Research experiences and perspectives in Korea

This invited paper reviews the status of heavy metal pollution in the paddy soils andrice in Korea, the human health risk assessment for heavy metals, and the remediationapproaches to reduce the metal translocation from soil to rice grain. The Soil Environment Conservation Law (SECL) designates the soil pollution standard for As, Cd,Cu, Hg, Cr⁶⁺, Pb, Ni, Zn and F, and these are used as both the maximum permissiblelevels of such metals in agricultural soil. The extensive monitoring for heavy metals insoil and crop revealed that concentrations of metals in paddy soils, without an evidentanthropogenic source of contaminants, were mostly below the threshold levels designatedby the SECL. Major sources of metal pollution in paddy soils were however related withmining activities. The increased level of Cd and Cu in soil increased the activities ofcations (Ca>Mg>K) temporally, decreased the level of exchangeable cations, altered thesupply mechanisms and decreased the nutrient buffering capacity of soil. The mostwidely described effects of metal toxicity in plants were the stunted growth, leaf epinastyand chlorosis. The Korean Government implements various countermeasures to preventthe soil pollution by metals through legislation, monitoring networks, risk assessment andremediation. The potential risk of the adverse effects of metals on human health wasassessed based on the human exposure pathways to rice, groundwater and soil in threeabandoned mines where metal contents in soil and rice exceeded the safety guidelines.The hazard index (HI) values for As and Cd exceeded 1, representing a potential toxicrisk of As and Cd to the human health. The cancer risk for As via the rice andgroundwater consumptions exceeded one cancer case in ten thousand. Health riskassessment indicated that a long term exposure to rice grown in the metal contaminatedpaddy soils could pose a potential health threat. The soil and plant management optionshave been considered to prevent the heavy metal transfer to rice from the contaminatedpaddy soils. The soil management options include the uses of soil ameliorants, fertilizersand irrigation control, soil covering/dressing, reversing and soil layer mixing methods. Inthe plant management options, the 24 rice cultivars were screened to find theaccumulating or excluding variety. The Japonica cultivars were considerably lowaccumulating rice for Cd. These cultivars might be screened to cultivate in thecontaminated soil environment to have the metal concentration at a low enough for thesafe consumption. The continuous submersion of the soil was interacted better withfertilizer than the intermittent irrigation to retard the Cd uptake by rice. Based on theregulatory criteria of Cd for soil pollution and food safety, the quantity of Cd whichshould be remediate at most was estimated to be only 0.04% of Cd in the contaminatedsoils. Is it worthwhile to remove such a small quantity of Cd with effort and budgetwhich may be greater than land price? Are those criteria the risk-based or the concentration-based? At least limiting to rice, we need to devote to the development ofprotocols for pollution monitoring, risk assessment and remediation to cope with suchdilemmas in the paddy soil environment.