SOLUTIONS proposes a multiple lines-of-evidence (LOE) approach for the identification of priority mixtures presenting significant risks and drivers of mixture toxicity dominating the overall risks (Fig. 1). The suggested methodology is applicable at all scales (EU, river basin, and site-specific level).
The approach merges evidence from
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chemical monitoring, in combination with so-called component-based approaches for mixture risk assessment and driver identification,
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integrated modelling of co-exposure and resulting mixture risks,
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effect-based monitoring, in combination with so-called effect-directed analyses or related methods for the identification of causative (groups of) pollutants,
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ecological monitoring, (field observations on so-called biological quality elements), in combination with possible indications on causative (groups of) pollutants.
The multiple LOE approach is detailed in a public SOLUTIONS deliverable [8]. Explanations of individual techniques are given in dedicated policy briefs on chemical screening [9] and associated component-based methods [10], modelling of co-exposures [11] and resulting mixture risks [6], effect-based methods [12], and ecological tools [13].
For developing the approach, SOLUTIONS thoroughly examined all available concepts and methods for both (i) the regulatory assessment of risks from chemical mixtures and (ii) the integration of such mixture risk assessment methods into prioritization procedures. No single method was found to provide a comprehensive solution for the complex problem of assessing risks from pollutant mixtures in the aquatic environment. Every option has some advantages but also suffers from specific limitations. As the best possible way forward, SOLUTIONS, therefore, proposes a framework which integrates all available LOEs on significant risks.
The advanced framework does not replace existing procedures for single substance prioritisation but integrates them with novel methodological elements into the suggested multiple LOE approach. Where one or more lines of evidence identify groups of pollutants presenting a significant risk, these should be included in ranking procedures for risk reduction measures. Criteria for mixture risk ranking may be essentially the same as those which have been established for single substance prioritization, including the frequency and the extent of threshold exceedances [14]. Where appropriate, large groups of dozens or hundreds of co-occurring pollutants may be reduced to few mixture components or even one single component which can be demonstrated to explain most of the overall risk, so-called drivers of mixture risks.
Wherever conclusive evidence on significant risks and resulting needs for risk reduction cannot be provided because all LOEs suffer from significant knowledge-gaps, mixture components of potential concern are not left unaccounted for but are prioritised for further research and testing. This principle is adopted from the NORMAN approach for the prioritisation of individual substances of emerging concern [15].