Metaldehyde Transport Processes in a Water Abstraction Catchment in Essex, Southeast England

Abstract

This paper addresses the issue of pesticide loss from agriculture and its impact on the aquatic environment. Specifically, this study assesses the transport of the relatively water-soluble polar molluscicide compound metaldehyde in a small (14 km2) water abstraction catchment in Essex, southeast England during a 14-month period (January 2019–February 2020). A rise in metaldehyde concentrations was observed at all monitoring points, to varying extents, in the months of seasonal metaldehyde applications. The highest increases in metaldehyde concentrations (up to 0.05 µg L–1) across the catchment were observed in the autumn months and in December. These findings reinforce the mobile nature of the pesticide, with surface and field drain runoff likely to be the predominant mode of transfer to surface waters. The scale of individual sources of metaldehyde influx to the Ardleigh surface storage reservoir in the catchment showed that the highest contribution to the total flux to the reservoir was from water transferred from the adjacent River Colne catchment. Atmospheric deposition of metaldehyde followed a similar seasonal pattern to that observed in metaldehyde levels in surface water and field drain runoff, and in certain months accounted for a higher input than influx from surface runoff (May, August–October 2019). Monthly values of metaldehyde mass in the reservoir ranged from 27.7 to 47.4 g. An increase in mass was associated with elevated levels of flux from individual sources. Relatively stable levels of metaldehyde total mass in the reservoir are probably due to the aqueous solubility and decreased degradation rates of metaldehyde in the aquatic environment.