Including mixtures in the determination of water quality criteria for herbicides in surface water.

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 microg/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.