The ecotoxicity of binary mixtures of Imazamox and ionized ammonia on freshwater copepods: Implications for environmental risk assessment in groundwater bodies

Groundwater bodies are impacted by substances such as pesticides and N-fertilizers, which usually occur in the environment as complex mixtures rather than isolated pollutants. The threat that these mixtures pose to groundwater-dwelling organisms is still poorly understood. The aims of the present study were to test the acute effect of a binary mixture of a herbicide (Imazamox) and NH4+ on epigean (Eucyclops serrulatus) and hypogean (Diacyclops belgicus) freshwater copepod species. In addition, to evaluate if the effect of the mixture can be explained by referencing non-interaction models or by more complex interaction models; and the implications for groundwater risk assessment. Compared with the action of the compounds evaluated separately, the effects of Imazamox and NH4+ in the binary mixture were more than additive or synergistic for both species. MixTox models evidenced a dose ratio and dose level deviations from concentration addition and independent action traditional models. The hypogean species was three times more sensitive to NH4+ that the epigean species when assayed as a single chemical. However, D. belgicus was only 1.13 times more sensitive than E. serrulatus when NH4+ was assayed in the mixture. The use of an integrated approach for substances that are known to interact in groundwater, should include copepods species as test organisms.