Environmental Fate Assessments to Understand the Legacy of Metaldehyde in Agricultural Fields and Surface Water

Abstract

There are increasing concerns about the hazards posed to drinking water resources by persistent, mobile and toxic (PMT) substances in the environment. For example, the extensive use of metaldehyde-based molluscicide to control slug populations in agricultural fields has frequently 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 did not predict that metaldehyde would be persistent in the environment, contrary to observations from monitoring schemes. To understand the reasons for this disparity, this study conducted a suite of degradation experiments, covering different soil types and environmentally realistic conditions, and generated a distribution of DT50 values for metaldehyde to examine whether degradation rates are underestimated by current risk assessments. The results were found to vary, showing a range of DT50 values (3.6-4150 d), which indicated that metaldehyde had the potential to become persistent, subject to high soil moisture conditions. Additionally, leaching and dissipation assessments were conducted in lysimeters, using representative soils to understand the legacy of metaldehyde in the field. Metaldehyde concentrations were detected in leachate and soil after a period of 120 days, suggesting that its persistence in the environment could be greater than the predictions made during the regulatory environmental fate assessments. Lastly, molecular microbiology was employed to elucidate how the soil microbial population characteristics relate to the degradation potential for metaldehyde. Molecular techniques, such as qPCR and MinION sequencing revealed that the known metaldehyde degraders were very rare members of the soil communities. Hence, the trends in metaldehyde degradation may not be wholly attributed to these species and other organisms might be utilising metaldehyde that have yet to be characterised. While this research has identified environmental conditions that may lead to metaldehyde persistence in the field, further research needs to be done to understand the microbiology of metaldehyde degradation in soil. . Improved management strategies can then be developed to prevent the pollution of drinking water with metaldehyde.