Environmental quality standards for mixtures: A case study with a herbicide mixture tested in outdoor mesocosms

Traces of pesticides are frequently detected in surface waters. As a consequence, specific environmental quality criteria (EQS) for a set of single pesticides in surface waters were defined by the environmental authorities in several countries. In this context, the aim of this study was to investigate if the sum of the five percentile hazard concentration (ΣHC5–95percent, meaning that 5 percent of the aquatic assemblage remains affected considering a 95 percent confidence interval) of three herbicides with the same mode of action derived from a species sensitivity distribution based on acute toxicity data (EC50 values) of the most sensitive taxonomic group is a suitable EQS for surface water addressing the occurrence of herbicide mixtures as common exposure scenario. Therefore, an outdoor mesocosm study was performed with three replicates per treatment for a period of 173 days. Results demonstrated that a constant long-term exposure over 35 days to the HC5–95 percent of a mixture of three PSII inhibitors did not lead to adverse effects on the aquatic community in this field mesocosm study. Neither adverse effects on very sensitive functional endpoints such as photosynthesis measurements of algae and macrophytes nor adverse effects on structural endpoints such as abundance data and species composition were determined. In contrast and as a positive control, the HC30 treatment affected statistically significant all investigated endpoints and it was demonstrated that the PSII inhibitors acted additive on various level of organization (Knauert et al., 2008). This study is filling the gap that no empirical evidence is published indicating that the chronic exposure at the HC5–95 percent estimate is leading to no adverse effects for the aquatic community and is therefore a suitable EQS for surface waters in the agriculture landscape.

► The HC5–95 percent of a herbicide mixture did not lead to effects on the aquatic community.
► Macrophytes recovered faster than phytoplankton during exposure to PS II inhibitors.
► The HC5–95 percent is protective for short and long term exposure.
► The HC5–95 percent is a suitable quality standard for freshwater.