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2.4.         Dealing with mixtures of chemicals

For establishing causal relationships between chemical pollution and ecological effects, it has to be appreciated that in the real world there are no cases where only a single substance occurs in the environment. Emissions data and research show that the aquatic environment has to deal with mixtures of chemicals, which contain many more substances than just the priority substances. Nutrients from urban point sources, agricultural diffuse pollution, metals from stormwaters from cities and atmospheric deposition, as well as many potentially harmful organic chemicals from urban waste water and agriculture, have been shown to be present in freshwater systems simultaneously. Indeed, scientific monitoring approaches highlighted the co-occurrence of hundreds of chemicals in different freshwater bodies (e.g. Loos et al. 2009 & 2013, Moschet et al. 2014). This complexity mismatches with the single substance approach of current chemicals assessment under the WFD.

The occurrence of chemical mixtures in freshwater systems is the result of different sources and different patterns in time, space and concentration (e.g. Baker & Kasprzyk-Hordern 2013, Beckers et al. 2018) and so does the respective risk for the ecosystems. The challenge is to figure out which of the many substances present are most important for the toxicity of a mixture.

Efforts exist to simplify this complicated picture. In essence, these aim to separate and categorize the issues of pollution, impact and identification of key chemicals to achieve a problem-targeted assessment (figure 2.5). Statistical methods are used to characterise complex pollution situations and relate these to sources (Posthuma et al. 2017). This approach offers the potential for identifying categories of mixture: “typical” i.e commonly-occurring, or “priority” i.e. containing substances which are of particular concern in a mixture, for instance because they promote toxicity. This is particularly relevant for the diverse and numerous organic micropollutants for which single representative candidates on lists of regulated substances are often outdated or may be substituted by substances with potentially similar toxicity when regulation comes into play. The combined action of similar compounds occurring together is not captured at all (Altenburger et al. 2015).

Examples for co-occurrence of similar compounds comprise the neonicotinoid insecticides imidacloprid, thiacloprid, acetamiprid which have been shown to occur simultaneously in water bodies but also antibiotic drugs such as azithromycin, erythromycin, and clarithromycin or the herbicides (e.g. diuron and isoproturon).

Figure 2.5: Dealing with mixtures in water management through differentiation into pollution (priority mixtures), effect (impact of mixtures) and risk (drivers of mixture toxicity) issues (modified from Altenburger et al. 2015)

A study by Busch et al. (2016) described the diversity of potential molecular targets for contaminant-biosystem interactions. In this study 426 organic chemicals were summarized to be detected in European freshwaters, containing 173 pesticides, 128 pharmaceuticals, 69 industrial chemicals and 56 other compounds. These targeted more than 100 different biological molecules known to exist in aquatic organisms. This complicated picture was simplified by considering the ways in which the chemicals acted upon organisms – “modes-of-action”. 30 mode-of-action categories were identified for freshwater contaminants (figure 2.6), so that even with a potentially unlimited number of chemicals, there is a limited range of adverse biological effects. This approach could be used to simplify toxicity assessment.

Figure 2.6 Modes of biological action of organic micropollutants in European freshwaters

 Source: Busch et al. 2016)

Notes: Abbreviations: GABA – gamma-Aminobutyric acid (chief inhibitory neurotransmitter in the mammalian central nervous system); nAChR - nicotinic acetylcholine receptor (see table 3.1); ATP – adenosine triphosphate (energy carrier in the cells of all known organisms), DPP4 - Dipeptidyl peptidase-4 (an enzyme)