Ecological risk assessment of polymetallic sites using weight of evidence approach

Highlights

• The application of quantitative weight of evidence approach in ecological risk assessment is effective and comprehensive.

• Severely contaminated soil caused the higher overall risk, while slight contamination of soil had slight hazard.

• Earthworm biomarkers are suitable and sensitive indicators for ecological risk assessment of contaminated soil.

Abstract

Ecological risk assessment (ERA) of polymetallic contamination in soils has caused extensive solicitude. However, objective and feasible methods suitable for soil ERA are limited. Therefore, in this study, a multidisciplinary and quantitative weight of evidence approach (WOE) specific to soil ecosystems was developed based on the previous WOE for aquatic ecosystems. The framework consisted of four lines of evidence (LOEs): DTPA-extractable heavy metal in soils, bioaccumulation in earthworms, integration of biomarker responses and expected community effect (multi-substance Potentially Affected Fraction, msPAF). These four LOEs were initially evaluated by each hazard quotient (HQ) of them based on the ratio to the reference (RTR) of each parameter. Then, Environmental risk index (EnvRI) integrated by HQs with different weights was calculated. At last, three sites, one for reference (N1) and two for contaminated soils (N2 and N3) were chosen to apply the modified WOE approach. Results showed that heavily contaminated site, N3 had higher HQ classification for each LOE and its EnvRI was classified as Major levels, while the EnvRI of N2 was assigned into Moderate. What's more, HQ of biomarker response (HQ_biomarker) integrated by RTRs of biomarkers increased gradiently with the increase of heavy metal levels in soils though irregular changes were observed for most of those biomarkers. Overall, our results indicated that the quantitative WOE framework specific to soil ERA had the advantage of obtaining a comprehensive and objective risk assessment.

Introduction

In traditional risk assessment of contaminated sites, assessment of chemical and toxicological risks has been conducted separately (Piva et al., 2011). However, either chemical or toxicological assessment solely is not robust and sufficient to evaluate the integrative ecological risk as a whole. Thus, ecological discipline, including contamination level and integration of external and internal responses at genetic, biomarker, individual and community levels is becoming a significant tool compensating for the deficiency of the traditional risk assessment (Dagnino et al., 2008). Many previous works have also confirmed that multidisciplinary methods based on chemical and ecotoxicological measurements represent an added value in risk assessment of contaminated soils by using the weight of evidence (WOE) framework (Semenzin et al., 2008).

The WOE framework is used to assess the possible ecological risk by synthesizing individual line of evidence (LOE, such as, chemistry, toxicology or ecology) to form a comprehensive and scientific conclusion concerning the degree of impairment or risk (Semenzin et al., 2008). Various WOE approaches are diverse in their integration methodologies, ranging from qualitative (examining distinguishing attributes) to quantitative (measuring aspects of magnitude) (Chapman et al., 2010). In general, current WOE applications tend to use qualitative methods, such as listing the evidence, best professional judgment and logic and causal criteria. However, those qualitative WOE methods are more or less deficient in their objective, certainty, transparency, repeatability and consistency (Linkov et al., 2011). In comparison, semi-quantitative WOE methods, such as indexing and scoring methods, make progress in transferring individual LOEs into numerical values for further interpretation and integration. Compared to the qualitative methods, semi-quantitative WOE methods are better at achieving their objective, and have better certainty and consistency. However, they are also short of transparency in decision making and repeatability in consensus building (Linkov et al., 2009). Quantification, which combines numerical information into several measures of risks (Chapman et al., 2010), is the most quantitative assessment and preserves a rigorous evaluation process that simultaneously considers the transparency and repeatability in risk assessment (Linkov et al., 2009). However, in spite of their unparalleled advantages compared to qualitative and semi-quantitative methods, quantitative methodologies are rarely used in WOE frameworks because of their high requirements for data and professional knowledge of statistical analyses and specific discipline expertise (Chapman et al., 2010).

A successful application of the quantitative method was carried out by Piva et al. (2011). They proposed a conceptual and software-assisted quantitative method for characterizing the sediment quality, based on the information about chemistry, tissue chemistry (bioaccumulation), sub-lethal effects (biomarkers) and acute toxicity (bioassays). However, the method was practical to assess the ecological risk in aquatic ecosystem rather than the complicated terrestrial ecosystem.

With the expansion of urbanization, soil in urban green space is becoming rare resource for organism habitats and attenuating pollution in urban ecosystem. However, heavy metals, especially for Cu, Cd, Pb and Zn were reported as the important contaminants in urban soils, originating from multiple sources including industrial and traffic emission as well as fertilizer application due to anthropic activities (Wang et al., 2012). As the significant proportion in soil biomass, earthworms are regarded as “keystone species” in pedogenesis and highly responsive to soil quality and sensitive receptors of multiple environmental pollutants, especially heavy metals (Rombke et al., 2005). Earthworms could accumulate heavy metals mainly by ingestion of metals bound to soil components and direct dermal uptake with dissolved ions (Becquer et al., 2005). For both pathways, the distribution and solubility of metal-bearing minerals are of important for earthworm metal uptake (Centofanti et al., 2016). Earthworms could also regulate, detoxify and excrete excess heavy metals from their tissue by accumulative immobilization and homeostatic control (Kamitani and Kaneko, 2007).

Therefore, in order to make our WOE approach more suitable for terrestrial ecosystem, we modified LOEs based on the principle of Piva et al. (2011). Earthworm was used as the toxic target during the risk assessment using the modified WOE framework in this study. The purpose of this study was to provide a feasible WOE approach in ecological risk assessment specific for terrestrial ecosystem.