Metaldehyde is a hazardous chemical compound, toxic to humans, which is widely used as a pesticide against slugs and snails. Metaldehyde is normally used in pellet form by farmers and gardeners to protect a wide variety of crops. Farmers apply Metaldehyde to agricultural land in varied amounts depending on weather conditions and crop types.
In recent years this has ranged from 250 – 500 tonnes across Great Britain, though data for 2010 shows a considerable reduction to 160 tonnes. Metaldehyde helps to protect cereal, oil seed rape and other crops from slug damage.
There have been several reports of metaldehyde levels exceeding the safe levels cited by the regulations. Even very small traces are at levels above the European and UK standards set for drinking water.
Following initial reports of Metaldehyde since 2007, through River Monitoring it is now known that levels typically increase during the autumn months when metaldehyde is applied to crops. In addition to agricultural use, metaldehyde slug pellets are available to the general public for use in their gardens and allotments. Despite that, it is generally considered that garden and allotment use is not a significant contributor to the problem.
Is there any health risk?
According to EU legislation, virtually zero pesticides should be present in drinking water. However, the Health Protection Agency has stated there is no risk to health from the levels currently being detected in some water supplies. Based on the toxicological studies they have undertaken,
"To get to that level the average size person would have to drink more than 1000 litres of water – that’s over 1 tonne of water – each and every day of their life."
The Drinking Water Directive (98/83/EC) has set a limit of 0.1μg/l for any individual pesticide in our tap water.
Metaldehyde concentrations in some catchments are putting the UK at risk of not complying with these directives.Can metaldehyde be removed from drinking water?
Due to the characteristics of metaldehyde, it is not effectively removed by the current methods water undertakers employ in the treatment of our water supplies. It cannot be broken down by other water treatment processes using chlorine or ozone and is therefore a very difficult compound to remove. For this reason, water companies are working extensively to prevent the issue as it present significant risk to their organizations, both financially and from a compliance perspective.
According to the study "Metaldehyde removal from aqueous solution by adsorption and ion exchange mechanisms onto activated carbon and polymeric sorbents." by Tao B and Fletcher AJ in the Journal of Hazardous Materials [2013, 244-245:240-250]
"Metaldehyde removal from aqueous solution was evaluated using granular activated carbon (GAC), a non-functionalised hyper-cross-linked polymer Macronet (MN200) and an ion-exchange resin (S957) with sulfonic and phosphonic functional groups. Equilibrium experimental data were successfully described by Freundlich isotherm models. The maximum adsorption capacity of S957 (7.5 g metaldehyde/g S957) exceeded those of MN200 and GAC. Thermodynamic studies showed that sorption of metaldehyde onto all sorbents is endothermic and processes are controlled by entropic rather than enthalpic changes. Kinetic experiments demonstrated that experimental data for MN200 and GAC obey pseudo-second order models with rates limited by particle diffusion. Comparatively, S957 was shown to obey a pseudo-first order model with a rate-limiting step of metaldehyde diffusion through the solid/liquid interface. Results obtained suggest that metaldehyde adsorption onto MN200 and GAC are driven by hydrophobic interactions and hydrogen bonding, as leaching tendencies were high since no degradation of metaldehyde occurred. Conversely, adsorption of metaldehyde onto S957 occurs via ion-exchange processes, where sulfonic and phosphonic functionalities degrade adsorbed metaldehyde molecules and failure to detect metaldehyde in leaching studies for S957 supports this theory. Consequently, the high adsorption capacity and absence of leaching indicate S957 is promising for metaldehyde removal from source water."
What will remove Metaldehyde from my water?
Based on these conclusion, the best way to filter water with trace levels of Metaldehyde is to use a Reverse Osmosis Water Filter System. The Spectrum Premier Reverse Osmosis System has two very highly rated carbon blocks which are effective at reducing pesticides and would give you the best chance to completely removing Metaldehyde. Once the system is in place, we would advise that the incoming water be tested before filtration and again after filtration to prove the system has filtered the Metaldehyde out.