Background In Switzerland the assessment of surface water quality with respect to contaminations by plant protection products (PPP) is based on the Swiss water protection law (Schweizer Bundesgesetz vom 24. Januar 1991 über den Schutz der Gewässer, Gewässerschutzgesetz, GSchG, SR 814.20), as well as on the Swiss water protection ordinance (Gewässerschutzverordnung vom 28. Oktober 1998, GSchV, SR 814.201). According to GSchV a general quality criterion of 0.1 μg/l exists for single organic pesticides (plant protection products and biocidal products) in Swiss running waters. Other values remain subject to reserve on the basis of individual substance assessments in the context of the authorisation procedure. In the European Union, the protection of water bodies is regulated under the Water Framework Directive (2000/60/EC, WFD). In addition, the aquatic risk of PPP is already being assessed during the PPP authorisation procedure under the ordinance on the placing of plant protection products on the Swiss market (Verordnung vom 18. Mai 2005 über das Inverkehrbringen von Pflanzenschutzmitteln, Pflanzenschutzmittelverordnung, PSMV, SR 916.161) and in the EU under the Council Directive 91/414/EEC of 15 July 1991 concerning the placing of plant protection products on the market. Aims and Approaches The goal of this project was to propose an appropriate method for the derivation of Environmental Quality Standards (EQS) for active substances (AS) from PPP in Swiss surface waters, consistent with the aims of the Swiss legislation on water protection (GSchG and GSchV). For this purpose the approaches currently applied in the EU were (i) identified by query and review, (ii) reviewed with regard to their underlying protection goals and methods and (iii) applied in case studies to six AS currently used in PPPs in Switzerland: the herbicides Diuron, Mecoprop and Terbuthylazine, the fungicide Carbendazim and the insecticides Diazinon and Imidacloprid. The resulting EQS values were compared with EQS values derived for the same AS by other authorities as well as with the regulatory acceptable concentrations (RACs) that can be calculated from the PPP authorisation documents. Results The review demonstrated that minor differences exist between the ecotoxicological protection goals underlying the three guidance documents currently used for deriving EQS under the Water Framework Directive (WFD), the draft Technical Guidance Document for EQS (TGD for EQS), its predecessor („Lepper method“) and the Dutch guidance document for the implementation of the WFD, and those described in the Swiss water protection law (GSchG) and in the Swiss Water Protection Ordinance (GSchV). With some additional provisions concerning the data selection however, the approaches of the WFD are compatible with the GSchG, therefore the selection of relevant toxicity data should be guided not only by their relevance under the WFD but also by the protection goals from the GSchG and the GSchV. A review of the methods showed that the three approaches are very similar. The few minor distinctions, led to small differences regarding the EQS values derived in the case studies for the selected AS. However, EQS values never differed by more than a factor of 3, if the use of different data sets and/or different “expert judgement“ in the evaluation of data could be ruled out. The influence of this - so called - “expert judgement” on resulting EQS values was significant, mostly due to differences in weighting the available data and/or the selection of the assessment factor (AF). Differences in “expert judgement” resulted in EQS values that differed by a factor up to 20. The largest difference was found between the EQS derived according to the WFD and RACs calculated from the PPP authorisation documents. RACs were always higher than the EQS, by up to a factor of 230. The key cause seems to be the consideration of recovery during PPP authorisation, which is neither compatible with the protection goals of the GSchG and the GSchV nor with the WFD. Conclusions and recommendations The EQS derived for the selected AS ranged between 0.01 μg/l and 1000 μg/l. Clearly, EQS are significantly higher than the general value of 0.1 μg/l for some AS, while being significantly lower for other AS. Consequently, effect based EQS for PPP are needed for a comprehensive ecotoxicological water quality assessment in Switzerland. For EQS derivation PPP authorisation data are highly relevant and should be included in the data set where appropriate. The hazard assessment procedures of the PSMV, however, have not shown to be compatible with the GSchG. It is recommended that EQS should be derived according to the TGD for EQS, but the protection goals underlying the GSchG and the GSchV should be used as additional guidance during the selection of relevant data. It is further recommended that the Klimisch scoring system for the assessment of data reliability should be revised or amended, since it does not seem to ensure consistent validation between different hazard assessors. In addition, the importance of implementing mixture toxicity approaches into regulatory frameworks has been recognised internationally. Hence, it should be evaluated in the future, how this can be done with respect to EQS. Finally, a scheme for EQS derivation for AS from PPP has been proposed, that is specifically designed to balance the influence of “expert judgement“.