Environmental fate assessments to understand surface water pollution from metaldehyde-based molluscicide

Abstract

The large-scale use of metaldehyde-based molluscicide to control slug populations in agricultural fields has led to pollution of surface waters and contamination of drinking water at levels exceeding the statutory limit. Regulatory environmental fate assessments and studies in the literature do not predict that metaldehyde would be persistent in the environment, contrary to observations from monitoring schemes. To understand the reasoning for this disparity, this study conducted a suite of batch degradation experiments, covering different incubation temperatures and soil moisture contents to examine whether degradation rates are underestimated by current risk assessments. The results display a range of DT50 values. The temperature dependency of metaldehyde degradation would not significantly affect model predictions, but metaldehyde had the potential to become persistent, subject to high soil moisture conditions. In the second part of the study, lysimeters were set up containing intact soil to recreate the field environment under controlled conditions. It was revealed that high concentrations of metaldehyde can escape in leachate and residues remained in soil longer than expected, suggesting that the standard laboratory-based risk assessments deviate from environmental realism.