Analytics in laboratory effect studies with soil invertebrates—technical challenges and implications for soil risk assessment of plant protection products

The inclusion of analytics in soil invertebrate laboratory studies is gaining increasing attention in the European risk assessment of plant protection products (PPPs). Analytics in soil were recently requested for fast-dissipating compounds in the revised Central Zone Working Document. However, the Working Document, as well as the technical OECD testing guidelines, lack clarity on 1) how to design the laboratory studies to reliably fulfill this requirement, 2) how to consider the analytically measured values to derive robust ecotoxicological endpoints, and 3) how to use endpoints that consider time-variable exposure in the test, in the risk assessment of PPPs. A hypothetical case study is presented to show the impact on the risk assessment when ecotoxicological endpoints that are expressed as time-weighted average (TWA) concentrations are compared with maximum predicted environmental concentrations (PEC) in soil to calculate a Tier 1 toxicity-exposure-ratio (TER). The persistent compound would pass the critical TER trigger of 5 whereas the fast-dissipating compound fails the risk assessment. However, a fast dissipation of a compound is, from an environmental perspective, a favourable substance property and especially inherent for biological products. This sets the wrong motivation for the development of new PPPs. The suitability of using TWA-PECs in the risk assessment instead of maximum PECs is discussed by comparing temporal exposure scenarios in the test system with scenarios that may occur under realistic field situations. This analysis shows that potential underestimation of the risks may occur only for specific situations where the PEC in soil temporally exceeds the regulatory acceptable concentration over time. In such cases the use of TWA-PECs in soil may be applicable in the risk assessment provided the assumption of reciprocity is fulfilled. A reciprocity check can be performed via tailored ecotoxicological testing and/or effect modeling to justify the use of TWA-PECs in the risk assessment.