Metal Bioavailability Models: Current Status, Lessons Learned, Considerations for Regulatory Use, and the Path Forward

Abstract

Since the early 2000s, biotic ligand models and related constructs have been a dominant paradigm for risk assessment of aqueous metals in the environment. We critically review 1) the evidence for the mechanistic approach underlying metal bioavailability models; 2) considerations for the use and refinement of bioavailability-based toxicity models; 3) considerations for the incorporation of metal bioavailability models into environmental quality standards; and 4) some consensus recommendations for developing or applying metal bioavailability models. We note that models developed to date have been particularly challenged to accurately incorporate pH effects because they are unique with multiple possible mechanisms. As such, we doubt it is ever appropriate to lump algae/plant and animal bioavailability models; however, it is often reasonable to lump bioavailability models for animals, although aquatic insects may be an exception. Other recommendations include that data generated for model development should consider equilibrium conditions in exposure designs, including food items in combined waterborne-dietary matched chronic exposures. Some potentially important toxicity-modifying factors are currently not represented in bioavailability models and have received insufficient attention in toxicity testing. Temperature is probably of foremost importance; phosphate is likely important in plant and algae models. Acclimation may result in predictions that err on the side of protection. Striking a balance between comprehensive, mechanistically sound models and simplified approaches is a challenge. If empirical bioavailability tools such as multiple-linear regression models and look-up tables are employed in criteria, they should always be informed qualitatively and quantitatively by mechanistic models. If bioavailability models are to be used in environmental regulation, ongoing support and availability for use of the models in the public domain are essential.

A Generalized Bioavailability Model (gBAM) for Predicting Chronic Copper Toxicity to Freshwater Fish

“…An overview of the final parameter values included in the chronic Cu gBAM for juvenile rainbow trout and a comparison with the existing chronic fish BLMs are given in Table 2. The log‐linear pH effect in the newly developed gBAM for juvenile rainbow trout (parametrized via S pH ) is assumed to integrate in a nonmechanistic manner all possible mechanisms by which pH can affect the toxicity expressed as the free Cu 2+ ion activity (Mebane et al 2020). These can, next to the competition between H + and Cu 2+ at the biotic ligand sites, also include the potential toxicity contribution of complexes like CuOH + and CuCO 3 (Erickson et al 1996; Grosell et al 2004), influences of pH on the membrane permeability (Lavoie et al 2012), and/or differences between bulk pH and gill microenvironment pH and mucus production that exert a buffering effect on Cu 2+ ion activities in the gill microenvironment (Tao et al 2002).…”

“…The gBAMs are an alternative to the existing BLMs to predict chronic effect concentrations for Cu (and other metals) in freshwater organisms (Mebane et al 2020), such as fish (e.g., nickel [Ni]; Deleebeeck et al 2007), D. magna (e.g., for Cu and zinc [Zn]; Van Regenmortel et al 2015, 2017), and algae (e.g., for Cu; De Schamphelaere and Janssen 2006). The chronic Cu Daphnia gBAM combines a log‐linear pH effect on free Cu 2+ ion toxicity with conventional linear protective effects of major cations.…”

Functional Feeding Groups of Aquatic Insects Influence Trace Element Accumulation: Findings for Filterers, Scrapers and Predators from the Po Basin

“…Macroinvertebrates are exposed to metals through their gills and by dietary pathways through water filtration (filterers), grazing on periphyton (scrapers), or preying on other invertebrates (predators) [81]. In our study, although the scrapers accumulated higher amounts of trace elements (Al, As, Bi, Co, Cd, Cr, Ga, Fe, In, Mn, Pb, Ni, and Sr) compared to the other FFG, the concentrations of certain elements were higher for the predators (Ba, Hg, Li, Se, V, Ti, and Zn) and for the filterers (Mo and Cu) compared to the scrapers.…”

Demonstrating the Reliability of bio‐met for Determining Compliance with Environmental Quality Standards for Metals in Europe

“…Over the past 2 decades, several bioavailability models of this type have been developed for a variety of different metals and aquatic organisms (e.g., European Copper Institute 2008; Van Sprang et al 2009; Peters et al 2011a; Mebane et al 2019). These models, which are generally developed from data for a single species, are referred to as individual species BLMs in the present study.…”

“…Currently, BLMs are used for the derivation of the environmental quality standard (EQS) for nickel (Ni) under the Water Framework Directive (WFD) in Europe (European Commission 2013). Some countries have also applied the BLM approach to derive EQS values for copper (Cu) and zinc (Zn; European Commission 2018; Mebane et al 2019). The present study considers the bioavailability‐based EQS values for Cu, Ni, and Zn.…”