Temporal and spatial patterns of metaldehyde in semi-agricultural catchments in Essex, UK: The influence of application regimes and hydrological conditions.

Abstract:

Metaldehyde, a widely used molluscicide, is one of the most commonly detected pesticides in aquatic environments in the UK. The following study is the first analysis of long-term metaldehyde concentration data, where pesticide concentrations and fluxes in stream water over a ten-year period (2008–2018) were investigated at a catchment scale, and the impact of hydrological conditions and application regime on metaldehyde levels assessed.

Patterns of seasonal and annual presence of metaldehyde were assessed in two areas in Essex, southeast England, with contrasting catchment-based approaches to management of water quality: the Ardleigh catchment, where farmers are incentivised to use slug control methods as an alternative to metaldehyde since 2015; and the River Colne catchment with on-going seasonal applications of metaldehyde.

Metaldehyde concentration peaks displayed a seasonal pattern, mainly occurring during the application season between September and December, and were associated with a hydrological regime of high intensity (stream flow up to 18 m³ s^-1, daily rainfall >10mm). Metaldehyde concentrations during time periods when the molluscicide is not applied to agricultural land and when applied to spring and summer crops were considerably lower (0.01–0.03 µg L^–1). Peaks in metaldehyde concentration typically did not exceed 0.5 µg L^–1.

Consistent increase of metaldehyde concentrations in watercourses during the autumn application period in the Ardleigh catchment each year highlights a limited effect of a catchment-based voluntary approach vs. a statutory approach. Hydrological conditions of high intensity during no-application months were not associated with increase in metaldehyde levels above 0.05 µg L^–1. Findings suggest that metaldehyde is short-lived in the environment, and metaldehyde levels are likely to be under the regulatory standards of 0.1 µg L^–1 (the European Union Drinking Water Directive) should metaldehyde ban be introduced the UK.