Scientific concepts and methods for moving persistence assessments into the 21st century

Abstract

The evaluation of a chemical substance's persistence is key to understanding its environmental fate, exposure concentration, and, ultimately, environmental risk. Traditional biodegradation test methods were developed many years ago for soluble, nonvolatile, single-constituent test substances, which do not represent the wide range of manufactured chemical substances. In addition, the Organisation for Economic Co-operation and Development (OECD) screening and simulation test methods do not fully reflect the environmental conditions into which substances are released and, therefore, estimates of chemical degradation half-lives can be very uncertain and may misrepresent real environmental processes. In this paper, we address the challenges and limitations facing current test methods and the scientific advances that are helping to both understand and provide solutions to them. Some of these advancements include the following: (1) robust methods that provide a deeper understanding of microbial composition, diversity, and abundance to ensure consistency and/or interpret variability between tests; (2) benchmarking tools and reference substances that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles; (3) analytical methods that allow quantification for parent and metabolites at environmentally relevant concentrations, and inform on test substance bioavailability, biochemical pathways, rates of primary versus overall degradation, and rates of metabolite formation and decay; (4) modeling tools that predict the likelihood of microbial biotransformation, as well as biochemical pathways; and (5) modeling approaches that allow for derivation of more generally applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data. We also identify that, while such advancements could improve the certainty and accuracy of persistence assessments, the mechanisms and processes by which they are translated into regulatory practice and development of new OECD test guidelines need improving and accelerating. Where uncertainty remains, holistic weight of evidence approaches may be required to accurately assess the persistence of chemicals. Integr Environ Assess Manag 2022;1-34.

Moving persistence assessments into the 21st century: A role for weight‐of‐evidence and overall persistence

“…Screening and higher tier test methods are not broadly applicable to all chemical types (Davenport et al, 2021; Shrestha et al, 2019).…”

“…However, unless the environmental conditions are appropriate, (bio) degradation may not take place. Some of these environmental conditions are further discussed below (also see Davenport et al, 2021).…”

“…It is questionable whether these criteria can be readily applied to other substances without considering their possible different intrinsic properties (Whale et al, 2021). For example, ionic substances, volatile and poorly soluble substances, substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs), and polymers have special properties that can require adapted methods (Davenport et al, 2021; ECETOC, 2020) and dedicated frameworks for persistence evaluations.…”

“…; Boethling et al, 2009), which may be heterogeneous. Test design should minimize artifactual impacts of environmental factors (Davenport et al, 2021), for example, limited active biomass (Ott et al, 2019(Ott et al, , 2020 or limited bioavailability (Ortega-Calvo et al, 2015;Schäffer et al, 2018) to understand the intrinsic degradation potential for a given substance. Overall, the current general approach to regulatory persistence (P) assessments, with notable exceptions (ECCC, 2016), follows a simplistic approach in which many important influencing factors are not satisfactorily considered, and anthropogenic chemical substances of interest can therefore be falsely assessed to be intrinsically persistent.…”

“…This narrow focus excludes several important environmental processes such as resuspension, dynamic partitioning, and photodegradation processes (Solomon et al, 2013) and adaptation of microbial communities (Poursat et al, 2019). The significance of these processes for persistence assessment is discussed in the companion paper (Davenport et al, 2021). Further, existing criteria were developed based on structurally specific classes of legacy pollutants and do not reflect the behavior of a wide range of other chemical classes (Matthies et al, 2016).…”