| Popis: |
Metaldehyde is the molluscicide with a long legacy of use in the UK and globally. Due to its physico-chemical characteristics and common use, this pesticide is commonly detected in the aquatic environment. At times its concentration in surface water exceeds the EU Drinking Water Directive (DWD) limit of 0.1 µg L–1 for a single pesticide. Since metaldehyde cannot be removed from water using conventional water treatment methods, best management practices and catchment-based initiatives are especially important to control metaldehyde transfer to aquatic systems. Yet, studies on agricultural land management practices and outcomes for metaldehyde diffuse pollution abatement remain limited. To address this research need, this project investigated the drivers, pathways and sources of agrochemical diffuse pollution of metaldehyde in the Ardleigh catchment, Essex where farmers are incentivised to use alternative slug control methods in place of metaldehyde. A 14C respirometry method was applied to provide insights into the ubiquity of soil microbes to degrade metaldehyde in dissimilar soils. The influence of soil attributes (texture, pH, soil organic matter content) on metaldehyde biodegradation capacity was examined. Using a range of quantitative techniques, the annual trends and seasonal variations in metaldehyde concentrations and fluxes over a decadal period (2008-2018) were assessed. Long-term water chemistry, hydrological and agronomical datasets were analysed to investigate what factors control pesticide levels observed in surface water in two lowland, semi-agricultural catchments in the UK. A 14-month fieldwork period was completed and included sampling of surface water, field drain runoff and precipitation at a sub-catchment scale. Physico-chemical attributes of these media were assessed in situ and in the laboratory. Measured data were combined with climatic/terrestrial variables to develop a conceptual model of metaldehyde transport in the study area. A metaldehyde mass budget and the scale of individual sources of metaldehyde influx in the catchment reservoir were quantified, including atmospheric deposition of metaldehyde. Collectively, this research provided new insight into the understanding of metaldehyde transport and fate in the freshwater environment at a catchment scale. The findings suggest that metaldehyde has a ubiquitously high biodegradation potential in soils (14C-metaldehyde mineralisation values across all soils ranged from 17.7–60.0% range). The high mobility of the pesticide and its transfer via surface and sub-surface runoff are the key factors affecting metaldehyde occurrence in water resources. Metaldehyde fluxes were primarily controlled by streamflow and precipitation (R2=0.9). The rise in metaldehyde concentration and flux displayed a seasonal pattern, mainly occurring during the autumn-winter application period (August-December), typically within the range of 0.03–0.1 µg L–1. Concentration exceedances above the EU DWD regulatory limit in stream water were short-lived, and a continuous decrease in metaldehyde concentrations in surface water is expected to be observed in the UK following the withdrawal of metaldehyde for outdoor use from March 2022. Future monitoring of metaldehyde in soil, sediment, water and rainwater, following the outdoor use ban in the UK, and the benchmarking of these data with that contained herein will provide insights of metaldehyde legacy. |
