Markert et al. Environ Sci Eur (2020) 32:51 https://doi.org/10.1186/s12302-020-00326-5 RESEARCH Open Access Mixture toxicity in the Erft River: assessment of ecological risks and toxicity drivers Nele Markert1* , Stefan Rhiem1, Michael Trimborn2 and Barbara Guhl1 Abstract Background: A vast number of chemical substances are released into the aquatic environment, leading to complex chemical mixtures in surface waters. Current water quality assessments, however, are based on the risk assessment of single substances. To consider potential mixture efects in water quality assessments, the North Rhine Westphalian State Agency for Nature, Environment and Consumer Protection (LANUV), Germany started a project assessing mix- ture toxicity in surface waters. This article summarises the mixture evaluation of chemical data collected by the Erft- verband during a water sampling campaign in the Erft River in 2016/2017. Altogether, 153 substances were included in the analysis, of which 98 were detected. Two diferent approaches based on the concept of concentration addition were used to analyse the data. The results were compared to fndings based on datasets from LANUV surveillance monitoring according to the EU Water Framework Directive. Results: Acute and chronic mixture risk calculations indicated risks for 32% and up to 90% of the samples, respec- tively. The greatest acute toxic pressure was identifed for the aquatic fora due to continuous exposure to varying pesticides, whereas the greatest chronic mixture risk was identifed for fsh as result of a ubiquitous presence of the pharmaceuticals diclofenac and ibuprofen. Overall, only a limited number of substances signifcantly contributed to the calculated mixture risks. However, these substances varied seasonally and regionally. When mixture risks were cal- culated based on diferent datasets, the monitoring design markedly afected the outcome of the mixture risk assess- ment. Data gaps of both ecotoxicological and exposure data lead to high uncertainties in the mixture risk assessment. Conclusions: Ecotoxicological efects on aquatic organisms caused by chemical mixtures can be expected along the Erft River throughout the year. Both mixture risk assessment approaches can be used for a conservative assessment of mixture risks and characterise the aquatic pollution in the river more realistically than single substance assessments. For the integration of mixture risk evaluations into the assessment and management of the chemical water quality, a two-staged assessment combining both approaches is suggested. To improve future risk assessments, the accessibil- ity and exchange of high-quality ecotoxicological data should be enhanced. Keywords: Mixture efects, Toxic units, Surface waters, Chemical monitoring, Toxicity drivers Background mixtures, i.e. seasonally and regionally varying combina- A variety of chemical substances originating from difer- tions of multiple chemicals that may contribute to a joint ent point and difuse sources of pollution such as agricul- mixture toxicity for aquatic organisms [1]. Regulatory ture, industry and municipal wastewater treatment plants risk assessment of chemicals, however, is mainly based (WWTP) are released into the aquatic environment on the assessment of single substances. In particular, daily. Tese chemical substances are present in complex unintentional mixtures of chemicals originating from dis- charge during production, transport, usage or disposal of chemicals are yet only rarely addressed in regulation [2]. *Correspondence: [email protected] Several publications from both the scientifc and the reg- 1 LANUV NRW, Wallneyer Str. 6, 45133 Essen, Germany Full list of author information is available at the end of the article ulatory communities have stated concern about the risks © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Markert et al. Environ Sci Eur (2020) 32:51 Page 2 of 13 chemical mixtures might pose, even when all substances between the biological groups of algae, macrophytes, occur at low concentrations at or below each respective aquatic invertebrates and fsh as well as substances poten- predicted no efect concentration (PNEC) [3–9]. Fur- tially acting as drivers of mixture toxicity was identifed. thermore, they emphasised the need for harmonised Te results of both approaches are compared with regard scientifc and regulatory approaches for mixture risk to the information generated and with respect to the assessment and the identifcation of substances acting as applicability to data from routine monitoring. main drivers of mixture efects [4–7, 9]. Potential mixture efects have so far not explicitly been Methods considered in water quality assessments in North Rhine Sampling site and sampling campaign Westphalia (NRW), Germany. To implement scientifc Te Erft River is located in the Southwest of NRW, Ger- approaches on mixture toxicity, and thereby, improve many with its source in the low mountain range of the Eifel the current water quality assessments, the North Rhine close to the city Bad Münstereifel and its confuence into Westphalian State Agency for Nature, Environment and the river Rhine south of the city Neuss. Te Erft River has Consumer Protection (LANUV) started a project assess- a length of 104 km and a total catchment area of 1918 km2. ing mixture toxicity in surface waters using data from Te upper part of the catchment area is mainly character- chemical monitoring, on behalf of the Ministry for Envi- ised by forest and grassland (43% and 29%, respectively), ronment, Agriculture, Conservation and Consumer Pro- whereas the middle and lower reaches are increasingly tection of the State of North Rhine-Westphalia (MULNV). infuenced by intensive agriculture (47–57%) as well as Tere are two mathematical models that have com- urban and industrial areas (17–22%) [22, 23]. All in all, monly been used for the assessment of ecotoxicological the Erft catchment area has a high population density of efects of defned chemical mixtures: (1) concentration 665 inhabitants per square kilometre [24]. An additional addition (CA) assuming a similar mode of action (MoA) infuencing factor is active and former lignite mining in of the individual compounds of a mixture, and (2) inde- the lower reaches and the associated discharge of drainage pendent action (IA) based on the assumption of dis- water from the mines. Terefore, the Erft River and its trib- similar MoAs [10–13]. In general, both models predict utaries are exposed to a wide range of chemical substances. mixture efects in a similar order of magnitude with From March 2016 to March 2017, surface water sam- CA being assumed to be a more conservative approach ples were taken in 13 sampling campaigns at 39 sampling [14–16]. Te CA model usually enables predictions close sites along the Erft River and seven tributaries (Veybach, to observed mixture efects and can be used for a con- Swist, Rotbach, Nefelbach, Finkelbach, Gillbach and servative estimation of mixture toxicity even if mixtures Norfbach; Fig. 1). Four samplings were performed after contain substances with dissimilar or unknown MoA rain events and nine during dry weather conditions. For [16–18]. Furthermore, CA only requires data on the envi- the Erft River, sampling sites were located at the river ronmental concentration and the ecotoxicological efect source, up- and downstream of seven WWTP, and up- concentration (ECx), whereas for IA information on the and downstream of the tributaries mentioned above. Fur- MoA of the compounds as well as detailed knowledge on thermore, samples were taken in each of the tributaries the complete dose–response curves are imperative [10, mentioned above as well as up- and downstream of three 11]. Stronger deviations from both models, such as those additional WWTP located in tributaries of the Erft River. caused by synergism, are rarely expected to occur at envi- ronmentally relevant concentrations [19]. Terefore, the Chemical analyses of surface water CA model can be used as a pragmatic and protective Surface water samples were analysed for a total of model assessing the efects of environmental mixtures 153 substances (Additional fle 1: Table S1 1–3), includ- [11, 20, 21]. ing 100 pesticides, 41 pharmaceuticals and 12 other sub- In the present study, two CA-based risk assessment stances, by high-performance liquid chromatography approaches were applied to datasets from the Erft River: coupled with tandem mass spectrometry (HPLC–MS/ (1) summation of Toxic Units (TU) and (2) summation of MS) as well as gas chromatography coupled with tan- risk quotients (RQ) of the individual mixture components dem mass spectrometry (GC–MS/MS). Some substances (c.f. Backhaus and Faust [10]). Two chemical datasets