Deriving site-specific soil clean-up values for metals and metalloids: Rationale for including protection of soil microbial processes
Authors: Kuperman Roman G , Siciliano Steven D , Römbke Jörg , Oorts Koen 
Published: December 2013

Abstract
Although it is widely recognized that microorganisms are essential for sustaining soil fertility, structure, nutrient cycling, groundwater purification, and other soil functions, soil microbial toxicity data were excluded from the derivation of Ecological Soil Screening Levels (Eco-SSL) in the United States. Among the reasons for such exclusion were claims that microbial toxicity tests were too difficult to interpret because of the high variability of microbial responses, uncertainty regarding the relevance of the various endpoints, and functional redundancy. Since the release of the first draft of the Eco-SSL Guidance document by the US Environmental Protection Agency in 2003, soil microbial toxicity testing and its use in ecological risk assessments have substantially improved. A wide range of standardized and nonstandardized methods became available for testing chemical toxicity to microbial functions in soil. Regulatory frameworks in the European Union and Australia have successfully incorporated microbial toxicity data into the derivation of soil threshold concentrations for ecological risk assessments. This article provides the 3-part rationale for including soil microbial processes in the development of soil clean-up values (SCVs): 1) presenting a brief overview of relevant test methods for assessing microbial functions in soil, 2) examining data sets for Cu, Ni, Zn, and Mo that incorporated soil microbial toxicity data into regulatory frameworks, and 3) offering recommendations on how to integrate the best available science into the method development for deriving site-specific SCVs that account for bioavailability of metals and metalloids in soil. Although the primary focus of this article is on the development of the approach for deriving SCVs for metals and metalloids in the United States, the recommendations provided in this article may also be applicable in other jurisdictions that aim at developing ecological soil threshold values for protection of microbial processes in contaminated soils. Integr Environ Assess Manag 2014;10:388–400. © 2014 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of SETAC.
Key Points
This article presents an overview of relevant test methods for assessing microbial functions in soil, and examines soil microbial toxicity data sets for Cu, Ni, Zn, and Mo that have been incorporated into regulatory frameworks High natural spatial and temporal variation of microbial biomass, and its potential for functional redundancy and adaptation to elevated contaminant concentrations, did not compromise the relevance and utility of functional endpoints for ecosystem protection. Incorporating microbial endpoints into regulatory frameworks resulted in calculated hazardous concentration values similar to those calculated without, increased the confidence associated with these values, and provided a statistically more robust determination of an ecological soil standard. Recommendations are provided on how to integrate the best available science into method development for deriving site-specific soil clean-up values that account for bioavailability of metals and metalloids in soil for protection of critical soil microbial processes.