Trends in metaldehyde concentrations and fluxes in a lowland, semi-agricultural catchment in the UK (2008–2018)

Highlights: • Peaks in metaldehyde concentration mainly occurred during the autumn-winter application season. • Concentrations above the EU DWD limit of 0.1 μg L⁻¹ in the river water were short-lived. • Metaldehyde is highly mobile, runoff-driven transport of metaldehyde serves as a major pathway. • Losses from point of application to surface water varied between 0.01 and 0.25%, maximum of 1.18% (2012). • Concentrations is watercourses are likely to be below 0.1 μg L⁻¹ following metaldehyde ban in the UK. Metaldehyde, a widely used molluscicide, is one of the most commonly detected pesticides in aquatic environments in the UK. In this study, metaldehyde concentrations and fluxes in stream water over a ten-year period (2008–2018) are reported for the River Colne catchment (Essex, southeast England), and the influence of hydrological conditions and application regimes are assessed. In general, peaks in metaldehyde concentration in river water occasionally exceeded 0.25 μg L⁻¹, and concentrations did not typically exceed the European Union Drinking Water Directive (EU DWD) regulatory limit of 0.1 μg L⁻¹. Metaldehyde concentration peaks displayed a seasonal pattern. Metaldehyde concentrations during periods when the molluscicide was not applied to agricultural land (January, July) and during the spring-summer application period (February to June) were generally low (0.01–0.03 μg L⁻¹). Peaks in metaldehyde concentration mainly occurred during the autumn-winter application season (August to December), and were typically associated with high intensity hydrological regimes (daily rainfall ≥10 mm; stream flow up to 18 m³ s⁻¹). Where metaldehyde concentrations exceeded the EU DWD regulatory limit, this was short-lived. The annual flux at the top of the Colne catchment (0.2–0.6 kg a⁻¹) tended to be lower than in the middle of the catchment (0.3–1.4 kg a⁻¹), with maximum flux values observed at the bottom of the catchment (0.5–25.8 kg a⁻¹). Metaldehyde losses from point of application to surface water varied between 0.01 and 0.25%, with a maximum of 1.18% (2012). Annual flux was primarily controlled by the annual precipitation and stream flow (R² = 0.9) rather than annual metaldehyde use (kg active applied). Precipitation explained 37% and 81% of variability in metaldehyde concentration and flux, respectively. Annual ranges in metaldehyde concentration were greater in the years 2012 and 2014 with an overall reduction in the range of metaldehyde concentrations evident over the period 2015–2018. It is the expectation that metaldehyde concentrations in stream water will continue to decrease following the withdrawal of metaldehyde for outdoor use in the UK from March 2022.