Deriving site-specific clean-up criteria to protect ecological receptors (plants and soil invertebrates) exposed to metal or metalloid soil contaminants via the direct contact exposure pathway.
Bottom Line: Resulting site-specific SCVs that account for bioavailability may permit a greater residual concentration in soil when compared to generic screening limit concentrations (e.g., Eco-SSL), while still affording acceptable protection.Two choices for selecting the level of protection are compared (i.e., allowing higher effect levels per species, or allowing a higher percentile of species that are potentially unprotected).A case study for molybdate shows the large effect of bioavailability corrections and smaller effects of protection level choices when deriving SCVs.
Affiliation: US Army Edgewood Chemical Biological Center, Environmental Toxicology Branch, Aberdeen Proving Ground, Maryland, USA.Show MeSH
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Mentions: Soil quality guidelines to protect soil organisms exist for different jurisdictions, including Australia (Soil Ecological Investigation Limit [Soil EIL]) (NEPC 2011a), Canada (Provisional Soil Quality Guidelines [SQG]) (CCME 2006), the European Union (EU) (predicted no effect concentration-soil [PNECsoil]) (ECHA 2008b), and the United States (Eco-SSL) (USEPA 2005). Conceptually, all 4 approaches follow similar processes for deriving guidelines specific to protecting soil invertebrates and plants. Data selection and screening are compulsory in any approach, and compilation of ecotoxicological data yields effect concentrations for numerous species. Effect concentrations used for the derivation of limits include no observed effect concentrations (NOEC, highest concentration tested with no significant effect), lowest observed effect concentrations (LOEC, lowest observed concentration that causes a significant effect), and ECx (exposure concentrations yielding a specified percentage effect; e.g., EC50). Jurisdictions differ in the type of endpoints selected but typically select only one endpoint for each test, often the most sensitive one with known ecological relevance. NOEC and EC10 values have been used interchangeably in limit setting in the EU and elsewhere, and it is beyond the scope of this article to make a recommendation regarding whether it is internationally or scientifically defensible to completely eliminate NOEC thresholds from use in deriving soil limit values. Effect concentrations of metals in soils not only differ among different plant and invertebrate species but also among different soils (Roembke et al. 2006; Rooney et al. 2006; Van Gestel et al. 2011). Comparison of response data for tests conducted with the same species illustrates variability in response among soils (Figure 1A), ostensibly related to the difference in bioavailability of metals when added to different soils. To generate generic soil screening levels, as prescribed within Canada (SQG) and the United States (Eco-SSL) guidelines, all effect concentrations from available soil invertebrate and plant studies that meet both acceptance and data quality criteria are used. In Canada, these are also used to plot cumulative frequency distributions of effect concentrations (e.g., EC10, EC50; Figure 1C) on which basis soil limits are derived. However, the prescribed guidelines from Australia (Soil EIL) and the EU (PNECsoil) have advanced the approach to include procedures for normalization of the ecotoxicity data based on factors affecting bioavailability. This bioavailability normalization reduces the variation of effect concentrations among soils (Figure 1B). This process collapses all data for a single species and places each into a consistent response curve that can be used to estimate valid species-specific effect concentrations for a given metal. As a final step, the effect concentrations for all species can then be collated to construct frequency distributions (Figure 1D) termed the Species Sensitivity Distribution (SSD) (Posthuma et al. 2001; http://www.epa.gov/caddis/da_advanced_2.html, accessed 23 August 2013). A specific percentile is then selected for the derivation of the limit, the hazardous concentration for y% of the species (HCy), also known as the concentration protecting “100 − y” percent of the species. In addition, before construction of the SSD, data normalization for different soils can be carried out to correct for differences that affect bioavailability, which then allows the derivation of site-specific SCVs. The section below reviews which steps are included in the derivation of soil quality criteria within 4 major jurisdictions (Table1).
Affiliation: US Army Edgewood Chemical Biological Center, Environmental Toxicology Branch, Aberdeen Proving Ground, Maryland, USA.