Pesticide degradation in subsurface soils: Risk assessment and regulatory considerations

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