Pesticide degradation in subsurface soils: Risk assessment and regulatory considerations

The Acetochlor Registration Partnership (ARP) has undertaken an extensive programme to determine subsoil degradation rates under undisturbed field conditions and traditional laboratory incubation conditions, using acetochlor as a test compound. The results show rapid and comparable rates of degradation of acetochlor in field- and laboratory-incubated subsoils, that often exceed surface degradation rates. The leaching models used to perform groundwater risk assessments (e.g. PELMO, PESTLA, MACRO-DB, PRZM and the FOCUS EU leaching scenarios), have provision for inclusion of subsoil degradation rates. However, conservative or zero default subsoil degradation values are typically used as no other alternative is available. Results presented in this paper show that these default values may significantly underestimate the true subsoil degradation contribution and therefore not accurately predict pesticide concentrations in groundwater. This paper reviews the latest findings in subsoil degradation research, summarises the subsoil degradation data generated under the ARP programme and demonstrates the importance of the inclusion of subsoil degradation data in mathematical models and thereby in the registration of pesticides in Europe.