Metaldehyde and its relationship with groundwater contamination and landfill leachate

Increasing attention is directed towards many chemicals and microbial constituents which have not been historically considered as contaminants. These “emerging contaminants” are commonly derived from municipal, agricultural and industrial wastewater sources and pathways. Among others, metaldehyde, a widely used molluscicide, has been detected in groundwater at levels exceeding the 0.1 μg L-1[microgram per liter] limit required by the Water Framework Directive. This has raised concerns for drinking water treatment, as the compound cannot be removed with conventional granular activated carbon or ozonation treatment processes. Further to this, some recent case studies on groundwater quality reported metaldehyde concentrations > 0.1 μg L- 1 [microgram per liter] in the vicinity of landfill sites. In all cases, there is no evidence or record of metaldehyde disposal. This growing evidence raises the question about the relationship between the characteristics of leachate generated from the landfill sites, the presence of metaldehyde and its impact on surrounding groundwater resources. In the present study, two landfill sites located in the UK are being investigated as potential sources of contamination to groundwater by metaldehyde. In both cases, metaldehyde concentrations exceeded the levels expected from a point- source contamination site. For this reason, landfill leachate chemistry was assessed to determine whether an in situ synthesis of metaldehyde can occur within landfill sites. A critical review found that potential reagents such as acetaldehyde, catalysts such as acetic acid and environmental conditions are present in groundwater aquifers. However, lab-based experiments have suggested that metaldehyde cannot be synthesised in the environment despite the presence of a wide range of pH values, cold temperatures, a high concentration of acetaldehyde, and the presence of a calcium bromide catalyst. This finding, combined with data collected from the two case study sites over a 7 month period suggest that a point source of the pollutant is the most likely explanation. The in situ testing also highlighted that the acetaldehyde reagent contained significant contamination by metaldehyde, of which the supplier was unaware. Two independent laboratories also confirmed the presence of metaldehyde in the reagent. Acetaldehyde of a purer grade from other manufacturers was also tested and found to contain significant levels of metaldehyde. These findings demonstrate another potential source of metaldehyde which is not agricultural. Finally, leachate from another UK landfill site was also analysed for metaldehyde to determine how prevalent metaldehyde contamination could be across landfill and was found to be present well above the limit for drinking water.