Monitoring programs throughout America and Europe have demonstrated the common occurrence of herbicides in surface water. Nevertheless, mixtures are rarely taken into account in water quality regulation. Taking mixtures into account is only feasible if the water quality criteria (WQC) of the single compounds are derived by a common and consistent methodology, which overcomes differences in data quality without settling on the lowest common denominator but making best use of all available data. In this paper, we present a method of defining a risk quotient for mixtures of herbicides with a similar mode of action (RQ m ). Consistent and comparable WQC are defined for single herbicides as a basis for the calculation of the RQ m . Derived from the concentration addition model, the RQ m can be expressed as the sum of the ratios of the measured environmental concentration and the WQC for each herbicide. The RQ m should be less than one to ensure an acceptable risk to aquatic life. This approach has the advantage of being easy to calculate and communicate, and is proposed as a replacement for the current limit of 0.1 µg/L for herbicides in Switzerland. We illustrate the proposed approach on the example of five commonly applied herbicides (atrazine, simazine, terbuthylazine, isoproturon, and diuron). Their risk profile, i.e., the RQ m as a function of time for one exemplary river, clearly shows that the single compounds rarely exceeded their individual WQC. However, the contribution of peaks of different seasonally applied herbicides, whose application periods partially overlap, together with the continuously emitted herbicides from nonagricultural use, results in the exceedance of the RQ m threshold value of one upon several occasions. IntroductionPesticides, including herbicides, differ from most industrial organic compounds in being introduced into the environment with the explicit intention of exerting effects on one or more target organisms. Unfortunately, they do not exert their toxic action only where they are applied, but can, through persistence and transport, reach other compartments of the ecosystem. Monitoring programs throughout North America and Europe have demonstrated the widespread presence of pesticides in various freshwater bodies (1-6). Over the past decade, as public concern has focused on the possible impacts of pesticides on the environment, several European and North American countries (7)(8)(9)(10)(11)(12)(13)(14) have defined specific water quality criteria (WQC) for each pesticide in surface waters. Within the EU, these WQC are often equivalent to the predicted no-effect concentration (PNEC), which aims to ensure the overall protection of aquatic life (9,12,14). This parameter is usually estimated by finding the lowest reliable aquatic effect concentration and applying a safety factor to account for various uncertainties, such as interspecies differences in sensitivity, acute-to-chronic ratios, and laboratory-to-field extrapolations (for review see refs 15-17). The drawback of this approach is...

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