The regulation of chemicals in water is focused on concentrations of a single substance. Such assessments do not consider the ‘real world’ experience, where organisms are typically exposed to many different chemicals. If these chemicals act similarly on an organism, they can lead to harmful effects at concentrations that are harmless for each chemical alone.
We used a new method to explore the risk posed by multiple chemicals at monitoring sites. Concentrations of each chemical measured in the environment were compared with the safety thresholds available for the single substances. While single substances were found to exceed thresholds at up to 82% of the monitoring sites, the method identified up to 12% more sites where exposure to multiple chemicals could additionally impact sensitive species.
Figure shows the sites where thresholds were exceeded by single substances (single substance risk), those where no risk was identified (low risk) and those where the presence of multiple chemicals led to potential toxicity despite no single substance threshold being exceeded (mixture risk).
This method can help identify monitoring sites where a mixture of chemicals may present a risk to aquatic organisms, even though regulatory thresholds for each individual chemical are not exceeded.
Please consult the relevant indicators and signals below for a more comprehensive overview on the topic.
Chemicals reach rivers, lakes and seas through direct discharges (such as urban wastewater treatment plants) and diffuse sources (such as runoff from agricultural land and roads, and deposition of atmospheric pollutants). Many different substances are typically present at the same site, thus exposing organisms to multiple chemicals. The possibility of combination/mixture effects should therefore be considered both in the case of independently-acting chemicals and for chemicals with similar modes of action (EC, 2012).
This study (Schneeweiss et al., 2023) investigated a new approach based on monitoring data for chemicals in European rivers and lakes. Concentrations were derived from data reported by each country to EEA’s Waterbase Water Quality database (EEA, 2023) and compared with European and national thresholds for chemicals in water. National-level thresholds were obtained from across Europe and may not apply in the countries reported here. Calculations were carried out for data from three European countries: Estonia (EE), France (FR) and the Netherlands (NL) for the year 2020.
In total, 144 substances exceeded their regulatory threshold value at least at one monitoring site in the three selected countries. There were 28 of these in EE, 89 in NL and 106 in FR. 42 sites were monitored in EE, 163 in NL and 793 in FR. Of these, 64% in EE, 82% in NL and 80% in FR were affected by single substance risk.
Furthermore, 12%, 5% and 8% of these monitoring sites were affected by mixture risk only in EE, NL and FR, respectively. This proportion of monitoring sites is not captured by single substance assessments.
We also wanted to explore the data for substances where legally-binding thresholds were unavailable. We used modelled toxicity thresholds (e.g. NORMAN, 2023) in combination with the regulatory thresholds already described. This expanded testing/assessment did not identify significantly higher numbers of monitoring sites at chemical risk.
