Importance of Chemical Speciation and Bioavailability in Hazard Assessment, Risk Assessment and Regulation of Metals in Multi-Stressor Environmental Conditions

Raewyn M. Town, Universiteit Antwerpen; Adam Peters, wca consulting; Stephen Lofts, UKCEH; and Chris Cooper, International Zinc Association

Trace metals exhibit a wide range of chemical and biological reactivity, in terms of both the thermodynamic and kinetic features of their interactions with environmental and biological ions, molecules and particles. Despite decades of research, latest scientific understanding is not yet implemented in hazard assessment, environmental risk assessment or regulation of metals. Metal speciation, bioavailability and potential toxicity are affected by many factors, including interactions with a wide range of complexants, other chemical stressors with different modes of action, as well as general stressors such as temperature, major ion concentrations, pH and redox conditions. Also, some metal species may contribute to antibiotic resistance, neuro and endocrine disruption, immunosuppression and gut dysbiosis. A mechanistic description of multi-stressor scenarios – in terms of reactivity in the exposure medium and within organisms – is fundamental for environmental risk assessment. Empirical strategies have aimed to describe the effects of mixtures of metals, but the combination with other factors remains largely unaddressed. The nature of toxicological endpoints, as well as the timescale and threshold conditions for manifestation thereof, may be modified in multi‑stressor scenarios. Physiological factors, such as temperature tolerance, energy metabolism, as well as ion- and osmoregulatory capacity, may play an important role in determining biological species sensitivity towards metal species in multi-stressor conditions.

Our session invited contributions that addressed advances in understanding the speciation, bioavailability, ecotoxicological effects and the regulation of metals. We paid particular attention to mechanistic approaches that consider interactions between metal species and other factors in the exposure media and within organisms, and the timescales thereof. The scope included the following:

1. The effects of temperature, redox conditions, water stress and electrolyte composition on metal ion reactivity
2. The influence of complexants on metal ion speciation, bioavailability and effects
3. The role of metal species in exacerbating antimicrobial resistance, neuro and endocrine disruption, immunosuppression, and gut dysbiosis
4. Physiological factors that render organisms more sensitive to adverse effects in multi-stressor scenarios
5. Approaches that enable bioavailability and background concentrations of trace metals to be taken into account in the setting and implementation of environmental quality standards

The focus was on mechanistic understanding of the interactions and effects and whether they are of relevance in a metal risk assessment and regulatory context.

Our session attracted 12 oral presentations and 16 posters and included laboratory and field studies on chemical speciation, bioavailability, (eco)toxicological effects and modeling approaches. Many contributions addressed multi-stressor scenarios, including mixtures of metals, metals and nanoparticles, as well as the effects of wildfires on metal mobilization. The online discussion session attracted about 83 participants.

The online discussion focused on overarching issues of interest, and the main points of discussion encompassed:

• Climate change-related stressors. A range of phenomena associated with climate change influence metal speciation and bioavailability, such as wildfires, increased meltwater, increased mineral weathering. The relative importance of the various phenomena will be region‑specific, for example, in the UK excess rainfall may lead to leaching of metals from mining waste, whilst in Southern Europe, droughts and wildfires may be of primary concern. Understanding and evaluating the relative importance of the implications for metals compared to the broader implications of climate change is likely to remain very challenging.
• Mixture and combination toxicity. Descriptors for mixture toxicity remain largely empirical and even metal mixture effects remain difficult to properly describe using existing biotic ligand model (BLM) approaches. In addition, there are no sound means to predict the potential combined toxicity of metals and organic pollutants, except for chelating agents such as ethylenediaminetetraacetic acid (EDTA) where the interactions with metals in the environment are relatively well understood. Analysis of mixture effects must consider both the influence of modified chemical speciation in the exposure medium as well as altered biological responses. Although it was suggested that the available literature for metal plus organics toxicity was relatively low, the “toxic unit” principle may work for metal plus organics toxicity. Given the vast number of possible mixture scenarios, suggested strategies to tackle mixture toxicity include identification of the most important combinations of compounds which occur in the environment as well as groups of compounds with similar modes of action, e.g., redox effects. Risk assessments may need to include a “safety factor” to account for additional uncertainties associated with mixtures.
• Regulatory applications of bioavailability models. Some regulators are relatively comfortable with the use of bioavailability-based approaches for regulating metals, despite the additional complexity that they introduce. The principal challenge for increasing the utilization of bioavailability-based approaches for metals in regulation lies with providing practical approaches that are scientifically robust and that also can be implemented readily and without introducing additional costs compared to more simplistic approaches. Efforts continue to make the tools that are available to regulators more robust as well as easier and simpler to use and understand, in order to increase levels of uptake and implementation, thereby increasing the ecological relevance of metals regulation.
• Compliance assessment. Taking account of bioavailability enables the most and least sensitive sites to be readily identified and enables limited resources to be focused on those locations where the risks are greatest. Many sites have significant background concentrations of metals arising from weathering of rocks and soils; however, surprisingly, there is little information on weathering rates. There is also limited data on the extent to which organisms acclimate and adapt to elevated background metal concentrations. There is no scientifically agreed means by which background concentrations can be properly taken into account in assessment of compliance with EQS, meaning that this must currently be considered on a site-by-site basis if necessary.

Conclusions From the Session

There is a need to critically revise current empirical strategies for dealing with metal mixtures as well as mixtures of metals together with organic pollutants and in combination with climate change‑related phenomena. In compliance assessment, robust (site-specific) strategies are needed to account for background concentrations of metals.

Authors’ contact information: [email protected], [email protected], [email protected] and [email protected]