Policy Analysis Including Mixtures in the Determination of Water Quality Criteria for Herbicides in Surface Water N A T H A L I E C H EÅ V R E , * CHRISTIAN LOEPFE, HEINZ SINGER, CHRISTIAN STAMM, KATHRIN FENNER, AND BEATE I. ESCHER Swiss Federal Institute for Aquatic Science and Technology (Eawag), 8600 Duebendorf, Switzerland
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 (RQm). Consistent and comparable WQC are defined for single herbicides as a basis for the calculation of the RQm. Derived from the concentration addition model, the RQm can be expressed as the sum of the ratios of the measured environmental concentration and the WQC for each herbicide. The RQm 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 RQm 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 RQm threshold value of one upon several occasions.
Introduction Pesticides, 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 * Corresponding author e-mail:
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 2, 2006
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 (7-14) 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 that the PNEC is derived from the lowest value in the data set and therefore depends strongly on the latter’s quality and quantity. Currently, large differences in PNEC values for a given pesticide can be found in the literature (see for example refs 9, 12, 14). More recently, hazardous concentrations (HCs) have been proposed for use as WQC instead of PNEC (for review see ref 18). They are derived from the species sensitivity distribution (SSD), a statistical function describing the variation in toxicity (generally the no-observed-effect concentration; NOEC) of a certain compound among a set of species. The species set should represent natural diversity and may be composed of several species from a specific taxon, a selected species assemblage, or a natural community (19). This SSD facilitates the calculation of an HC that is assumed to protect a given percentage of the species in a given environmental compartment. For example, HC5 represents a concentration that protects 95% of the species. However, a pesticide rarely occurs as a single contaminant in the ecosystem and organisms are typically exposed to mixtures of pesticides. Even if each single compound of a mixture is present at or below its NOEC level, acting together they may exhibit a significant effect as has been shown by several laboratory (20-23) and field (24) studies with herbicides and other compounds. This is particularly relevant for herbicides with similar modes of action (22, 25-27). For this reason, several authors have discussed the necessity of including mixtures of pesticides in the water quality regulation (18, 22, 28-32). However, so far only Canada (13) has proposed WQC for pesticide mixtures, and then only for the short-term occurrence of herbicides (acute WQC). One obstacle in considering mixtures of herbicides in a regulatory framework is that the ecotoxicological information available on herbicides varies greatly in terms of quality and quantity. Consequently, the independent calculation of WQC for single herbicides causes these values to be mutually inconsistent. For example, in the case of two herbicides A and B (where A is more toxic than B, A > B), B may have a lower WQC than A (B > A) if a more extensive data set is available for A, resulting in a smaller extrapolation factor or a better approximation of the HC5. Such inconsistent values are too incoherent for use in calculating WQC for pesticide mixtures. In this study, we propose a methodology to determine consistent WQC for single substances, which can be combined to calculate a risk quotient for mixtures of herbicides with similar modes of action (RQm). This RQm must be less than one to ensure an acceptable risk to aquatic life and can thus be considered as a risk indicator for the mixture in a regulatory framework as discussed later. Note that we focused 10.1021/es050239l CCC: $33.50
2006 American Chemical Society Published on Web 12/13/2005
this study on mixtures of active ingredients with similar mode of action. We did not consider the metabolites, which can also have a similar mode of action as the parent compounds (33) but for which very limited toxicity data are available. The methodology developed and proposed in this paper must satisfy three main criteria: (i) it should be based on best available scientific evidence, (ii) it should allow consistent WQC to be calculated for herbicides with similar modes of action, and should therefore allow the risk of mixtures to be consistently assessed, and (iii) since it is intended for application in Swiss regulations, this methodology should also be easy to calculate and communicate to facilitate its implementation. In the following two sections we present the concept for the derivation of the RQm and the individual WQC values. In the Concept section a recipe-style summary of the concept is given. In the Materials and Methods section, all equations are derived and all assumptions are stated. The details on the datasets, statistics, and modeling are given in the Supporting Information. In the Results and Discussion section, we illustrate the concept using the example of two families of photosystem II inhibitors: the triazines and the phenylureas. These herbicides target photosystem II, where they compete with plastoquinone, the electron transporter at the reducing site (34). We apply the derived WQC to assess the risks in a river of a small Swiss catchment area with intensive agriculture where five commonly used herbicidess atrazine, simazine, terbuthylazine, diuron, and isoproturons have been regularly detected by chemical analysis. We conclude with a detailed critical discussion of each of the assumptions and by highlighting further research needed to improve the applicability of the methodology. In addition, a sensitivity analysis is performed to better understand to what extent the assumptions of concentration addition of mixtures of compounds with similar mode of action influences the results and conclusions.
Concept Concept for the Derivation of RQm. For a single compound i, the site-specific risk for aquatic organisms is normally evaluated by comparing the measured environmental concentration (MEC) in the aquatic system under consideration with the WQC of this compound. In this paper, the WQC was taken as equivalent to HC5-95%, which represents the lower 95% confidence limit of the HC5 determined on the basis of the species sensitivity distribution of the NOEC data (SSDNOEC), (19, 35). The ratio MECi to HC5-95%i (RQi, eq 1) must be less than one to ensure that a given chemical presents an acceptable risk to the environment.
RQi )
MECi
