January 2011
Water UK briefing paper
Metaldehyde briefing paper1) What is metaldehyde?
Metaldehyde is a selective pesticide used by farmers and gardeners to control slugs and snails in a wide variety of crops. (Technically it is known as a ‘molluscicide’ and its action is very specific to slugs and snails.) It is sold under a variety of brand names as blue coloured pellets.
2) What is the current position?
Some water companies have recently been finding traces of metaldehyde in the raw water they abstract from rivers or reservoirs and treat to produce drinking water. These traces are at levels above the European and UK standards set for drinking water. Initial findings in late 2007 followed the development of new advanced analytical techniques for detecting very low levels of trace substances in water.
More recently, other companies in the Midlands, eastern, south east and south of England have detected metaldehyde in raw waters. It is however a seasonal issue, with levels increasing mainly in the autumn, when metaldehyde is applied to crops.
Current treatment methods designed to remove a range of pesticides are not effective at reducing the levels of metaldehyde in water. Some companies are therefore now detecting very low levels of metaldehyde in treated drinking water. These levels are extremely low – the highest being around 1 part in 1 billion (1 part per 1,000,000,000) and mostly much lower. However the levels are above the European and UK standards for pesticides in drinking water that is set at 0.1 micrograms per litre (1 part per 10,000,000,000).
3) Is there any risk to health?
No. Although the levels being detected are above the standard for drinking water, the standard for individual pesticides is not set on a health basis, but as a near-zero value reflecting European Union legislation that pesticides should not be present in drinking water. This is really a technical and political issue about meeting a technical standard set on a “one size fits all” basis.
The water companies are focused on maintaining the very high quality of drinking water in the UK – some of the best in the world – to ensure that water is safe to drink and to meet the national and European standard.
The Health Protection Agency has confirmed that there is no risk to health from the levels currently being detected in some water supplies.
There is a published “acceptable daily intake”1, based on toxicological studies, for all substances like this. 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 generally recommended amount of water to drink for proper hydration is two litres – but most people don’t even drink this amount.
HPA statement
The maximum level of metaldehyde in treated drinking water reported to DWI to date is 1.03 μg/l. Expert advice from the Health Protection Agency has confirmed that at this concentration, the intake from drinking water would be well below the acceptable daily intake and therefore there would be no reason to expect adverse effects.
4) Why can’t metaldehyde be removed from drinking water?
The “treatability” of pesticides is dependent upon their physical and chemical properties. The characteristics of metaldehyde mean that it is not effectively removed by adsorption onto activated carbon – the normal treatment for removing any pesticides that may be in raw water.
In addition, the relatively simple structure of metaldehyde means it cannot be broken down by other water treatment processes using chlorine or ozone. It is therefore a very difficult compound to remove even using existing advanced water treatment processes. Further research is being carried out into other treatment methods but early indications are that even if they work they would be prohibitively expensive and energy consuming to implement.
The only and most sustainable solution is to stop metaldehyde getting into watercourses and rivers in the first place. This is best achieved by changing the way metaldehyde is used in the catchment.
5) What is the industry doing about it?
The water companies have agreed a detailed programme of work with the Drinking Water Inspectorate to reduce the non compliance risks for each affected waterworks. In particular companies have been working closely with the producers and distributors of metaldehyde (the Metaldehyde Stewardship Group) to provide a major national programme of advice and training to farmers and their suppliers. The aim is to make them aware of this problem and ensure they follow best practice when applying metaldehyde pellets to crops, to avoid pellets getting into ditches and watercourses to reduce the quantity of metaldehyde that is used, and the frequency they apply it.
The Metaldehyde Stewardship Group has a nationwide plan of communication to farmers, agronomists and the supply chain which started in February 2009 and updated in May 2010. They will be monitoring take up of the advice through follow up surveys in the farming community. The water companies will continue to monitor raw and treated waters for metaldehyde levels and are looking for significant reductions in levels found in raw water. If we do not see such reductions in levels in rivers then we will need to consider calling for restrictions in the use of metaldehyde to protect our essential water supplies. For example as set out in our recent response the Government Consultation document on the transposing of new EU pesticide legislation into UK law.
The water industry is also working closely with the regulator for drinking water quality in England and Wales, the Drinking Water Inspectorate, the regulator for pesticides, the Chemicals Regulation Directorate , the Environment Agency and Natural England.
For more information please contact:
Jim Marshall
1 Acceptable Daily Intake (ADI) is a measure of the quantity of a particular chemical in food which, it is believed, can be consumed on a daily basis over a lifetime without harm. The acceptable daily intake for metaldehyde is given as 0.02 milligrams per kilogram of body weight per day. Thus for a 60kg erson the ADI is 1.2 milligrams per day. At the maximum levels being found in tap water (1.03micrograms per litre l) this is equivalent to 1100 litres of water per day. (ADI source – the National Centre for Environmental Toxicology; PSD / Defra evaluation)
