Risk Assessment of Herbicide Mixtures in a Large European Lake Nathalie Che` vre, 1 Patrick Edder, 2 Didier Ortelli, 2 Elisa Tatti, 1 Suren Erkman, 1 Franc¸ ois Rapin 3 1 Faculte´ des ge´ osciences et de l’environnement, Universite´ de Lausanne, CH-1015 Lausanne, Switzerland 2 Service de protection de la consommation, Quai Ernest-Ansermet 22, CH-1211 Gene` ve 4 Plainpalais, Switzerland 3 Secre´ tariat de la commission pour la protection des eaux du Le´ man (CIPEL), AVW-Changins, Rte de Duilier, CP 1080, CH-1260 Nyon 1, Switzerland Received 5 April 2007; revised 5 September 2007; accepted 6 September 2007 ABSTRACT: Lake Geneva is one of the largest European lakes with a surface area of 580 km 2 . Its catch- ment area covers 7400 km 2 , of which 20% is arable land. Monitoring campaigns have been carried out in 2004 and 2005 to determine the contamination of the lake by pesticides. The results highlight the wide- spread presence of herbicides in water, the measured concentrations for most substances remaining con- stant in 2004 and 2005. However, for some individual herbicides the concentrations increased drastically (e.g., the herbicide foramsulfuron). We assessed the environmental risk of the herbicides detected in the lake using water quality criteria recently determined for the Swiss environmental protection agency. Fur- thermore, we assessed the risk of herbicide mixtures, grouped based upon their mode of action. Gener- ally, the risk estimated for all single substances is low, except for some sulfonylurea compounds. For these substances, the measured concentrations are higher than the predicted no-effect concentration. Impact on the flora of the lake can therefore not be excluded. When mixtures of pesticides with similar mode of action are taken into account, the risk remains lower than the mixture water quality criteria for all groups, but can reach as high as one third of this quality criteria. A further step would therefore be to assess the risk of the total pesticide mixture, including similar and dissimilar modes of action. # 2008 Wiley Periodicals, Inc. Environ Toxicol 23: 269–277, 2008. Keywords: herbicides; mixture; risk assessment; water quality criteria; surface water INTRODUCTION Lake Geneva is one of the largest European lakes with a surface area of 580 km 2 . Its catchment area covers 7400 km 2 , of which 20% is arable land. The lake plays an im- portant role for both France and Switzerland: it is a place for recreative activities and fisheries. Furthermore, it is the source of drinking water for more than 500,000 inhabitants. It is therefore crucial to survey its water quality, as well as to ensure long-term preservation of aquatic life. Because of their inherent ecotoxicity, (e.g. they have been developed to exert a toxic action) pesticides are im- portant to monitor in surface water. Monitoring campaigns conducted in several countries have shown their widespread presence in the aquatic environment (Gilliom et al., 2006; Tre´goue¨t and Nirascou, 2006) and their effects on aquatic life have been highlighted (see for example Giddings et al., 2005). To protect surface waters against the deleterious effects of these substances, many countries have defined specific water quality criteria (WQC) for pesticides Correspondence to: Nathalie Che`vre; e-mail: nathalie.chevre@unil.ch Published online 23 January 2008 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/tox.20337 C 2008 Wiley Periodicals, Inc. 269