Persistence of plant protection products in soil; a proposal for risk assessment

Persistence in soil is one of the evaluation aspects of plant protection products. However, except for trigger values indicating persistence in soil, there is no broadly accepted evaluation procedure at the European level and member states use different approaches for the evaluation of persistence in soil at the national level. Until recently, the Netherlands used a cut-off criterion, but the Netherlands Court of Appeal for Trade and Industry (CBb) ruled this to be in contravention of Directive 91/414/EEC. This report proposes tiered procedures for the assessment of persistence in soil. The system considers three protection goals: 1) protection of soil functions relevant to agricultural production, 2) protection of the structure of agro-ecosystems, and 3) protection of the structure of soil ecosystems in general. The procedure distinguishes three trigger values for the half-life for dissipation (DT50) from soil. Substances having a DT50 > 30 days are assessed according to the Functional Redundancy Principle (FRP); i.e. it is evaluated whether soil functions, for example mineralization of organic matter, are affected. Substances having a DT50 > 90 days are assessed also according to the Community Recovery Principle (CRP); i.e. whether the community structure is affected at two years post application. Finally, substances having a DT50 > 180 days are assessed additionally according to the Ecological Threshold Principle (ETP); i.e. whether concentrations in the soil at seven years post last application potentially allow the development of natural ecosystems. The report proposes separate decision schemes for each of the protection goals. In these schemes both the Predicted Environmental Concentrations (PEC) and the ecotoxicological endpoints can be determined using tiered approaches. Exposure concentrations in test systems are essential for deriving ecotoxicity endpoints. Only rarely, all essential information on environmental conditions and substance properties is available for these test systems. The report describes procedures to derive conservative estimates for the exposure concentration. For this, both pore water concentrations and total contents in soil are considered