Assessing the fate of Metaldehyde Applied to Arable Soils

Pesticide pollution is a major challenge currently facing many water companies in the UK. Effective management of pesticide transfers from agricultural land to surface waters requires an understanding of the environmental fate of the pesticide active ingredients applied and of their transport pathways. One of the most challenging pesticides for the UK water industry is metaldehyde which seasonally exceeds drinking water standards in many supplies. It is especially problematic because there is currently no economical way of removing it using conventional water treatment processes. In this thesis, aspects of the physical disintegration of slug pellets and the fate of metaldehyde in soils were investigated. Metaldehyde is a molluscicide used in 80% of slug pellets. Three separate studies were performed. The first focused on determining metaldehyde leaching from intact soil cores, assessing differences between loam and clay soil, and between wet-processed and dry-processed slug pellets. The second study compared the half-lives of pelletised and non-pelletised metaldehyde in a laboratory incubation experiment. The final study was split into three sub-experiments focussing on the physical disintegration of slug pellets. The impact of soil moisture content and combined environmental processes was assessed through visible surface area and colour changes over time. The impact of kinetic rainfall energy on pellet visible surface area and weight changes was assessed using a rainfall simulator. The main findings were: 1. Soil moisture is the primary driver of changes in pellet integrity. 2. Clay soils leached more metaldehyde than loam soils after being subject to relatively dry environmental conditions. 3. High soil moisture content led to an increase in the rate of visible surface area reduction and colour change over time. 4. When subject the same environmental conditions, no statistical differences were found between wet-processed and dry-processed pellets in any of the experiments.