Ecological risk assessment of heavy metal-contaminated soil using the triad approach

摘要

Purpose Heavy metal soil-pollution is of concern due to its adverse effects on the ecosystem and human health. However, conventional ecological risk assessments considering only chemical concentration have limitations because of an increased uncertainty in assessing soil-risk. Therefore, this study applied the Triad approach, using a tier system, for assessing site-specific ecological risk in heavy metal-contaminated soil. In addition, risk-level was determined to analyze the necessity of remediation in a contaminated site. Materials and methods Soil samples from six different sites, including heavy metal-contaminated regions (S1-S5) and a control site (C, unpolluted with heavy metals) were collected. Soil properties were assessed based on a tier system to evaluate the ecological risk of each site. The first step tier-1, involved calculating the soil pollution index (SPI) and the second, tier-2, involved assessing site-specific ecological risk based on three lines of evidence (LoEs), viz., chemistry (ChemLoE), ecotoxicology (EcotoxLoE), and ecology (EcoLoE). The total and bioavailable fractions of heavy metals in soil and bio-accumulated heavy metals in earthworms (Eisenia fetida) were measured for ChemLoE. A bioassay involving mortality and weight reduction rates of earthworms, and a root-elongation test of lettuce (Lactuca sativasp.) was conducted for EcotoxLoE. Soil biological properties, soil basal respiration, and soil enzyme activities, were evaluated for EcoLoE. Final integrated risk (IR) was obtained by combining the risk-values of each LoE, determined using IR values and deviation of each LoE. Results and discussion Although three out of the five sampling sites had SPI 1 in tier-1, all five sampling sites were subjected to tier-2 to evaluate site-specific ecological risk. In tier-2, ChemLoE (0.53-0.99) was higher than EcotoxLoE (0.05-0.43) and EcoLoE (0.00-0.29), indicating that chemical aspects impacted risk evaluation more than the toxicological and ecological aspects. The final IR ranged between 0.24 and 0.85; risk-level was classified as low (S2, S3, S5), moderate (S1), and high (S4) according to the IR value. Based on the risk-level, site S4 could require remediation for agricultural land-use. Conclusions The IR of each sampled site varied depending on the impact of heavy metals. Based tier-1 and tier-2 results, a thorough site-specific evaluation is required to understand the adverse effects of heavy metals on soil ecosystems, and the triad approach could be useful for managing heavy metals in soil.