Risk Assessment of Heavy Metals in Soil of a Susceptible Agro-ecological System Amended with Municipal Solid Waste Compost

Abstract

Environmental pollutants, such as heavy metals, must be controlled because of their toxicity to animals, plants and microorganisms. Although soils have considerable immobilizing capability, large-scale metal build-up can adversely affect different components of the environment. The current study was carried out to establish the limits of Cd, Cr, Cu, Ni, Pb, and Zn levels permissible in soil, which would help in protecting animals, plants, and microorganisms. Separate sets of pot experiments were carried out for each of these heavy metals in graded dose levels (Cd at 0.02–20 mg kg^-1, Cr at 0.4–200 mg kg^-1, Cu at 1.6–800 mg kg^-1, Ni at 0.5–250 mg kg^-1, Pb at 0.4–150 mg kg^-1, and Zn at 4.6–1000 mg kg^-1) added to an acidic, light-textured, alluvial soil. Spinach crop was grown for 50 days on these treated soils after a stabilization period of two months. Maximal protective concentration limits of these metals were determined in the soil through three different approaches, namely, ‘phytotoxicity’, ‘food contamination,’ and ‘soil microbial activity diminution’. Adverse effects of the heavy metals on microbial activity and contamination of the food chain were observed much before their adverse effects on plant growth became evident. The ED20 (ecological dose corresponding to a 20% decrease in soil microbial activity) values were found to be 392
mg kg^-1 for Zn, 179 mg kg^-1 for Cu, 9.5 mg kg^-1 for Cd, 81 mg kg^-1 for Pb, 51 mg kg^-1 for Ni, and 31 mg kg^-1 for Cr. Considering the lowest values of these maximal permissible soil concentrations of the heavy metals determined by three different approaches to be protective for all target organisms, these levels were found to be: 392 mg Zn kg^-1, 179 mg Cu kg^-1, 0.34 mg Cd kg^-1, 81 mg Pb kg^-1, 30.7 mg Ni kg^-1 and 31 mg Cr kg^-1