What is Metaldehyde?

Metaldehyde is the active ingredient in many slug pellets, which are essential for protecting crops such as winter cereals and oilseed rape against slug damage. Unfortunately, metaldehyde can be washed into surface waters where it will not dissolve or break down and metaldehyde is very difficult for water companies to remove through the drinking water treatment processes. Although metaldehyde is not a proven health hazard, there is a legal limit for all pesticides in drinking water which is extremely low – only 0.1μg/l, equivalent to 1 grain of wheat in 390 tonnes!

Water companies have recorded multiple failures of this standard since analysis began in 2007. If a solution cannot be found then there will be political pressure to severely restrict the use of, or even to ban metaldehyde which will reduce the options available to farmers to control slugs and put greater pressure on the remaining alternative pesticides. Since treatment to remove metaldehyde from water is not a sustainable solution, water companies are working with farmers to solve the problem using catchment management.

Thames Water abstract water from the River Thames at Farmoor where it is treated prior to entering the drinking water supply. A number of the catchments within the Upper Thames catchment are known ‘hotspots’ for metaldehyde therefore Thames Water are offering financial incentives to farmers who will work with them to reduce levels of metaldehyde in surface waters.


What is an Ecosystem Service?

Ecosystem services are the benefits provided by ecosystems that contribute to making human life both possible and worth living. Examples of ecosystem services include products such as food and water, regulation of floods, soil erosion and disease outbreaks, and non-material benefits such as recreational and spiritual benefits in natural areas.

Please see the UK National Ecosystem Assessment website for further reference.


Protecting the Upper Thames services

The Upper Thames catchment constantly provides several services to its inhabitants. Therefore, protecting the Upper Thames Catchment ecosystem is vital for our future well-being and survival. To best protect these valuable services and to show the worth of an ecosystem, they have been given a monetary value. By putting an economic value on the service provided by an ecosystem, the service can be accounted for and protected for future benefit.

At FWAG, we work with our partners' by funding projects that promote an Ecosystem Service for the benefit of the Upper Thames catchment; these are called Payments for Ecosystem Services. Within the metaldehyde mitigaton projects, the service that is being paid for is ‘clean water’, i.e water that is within the drinking water standard for metaldehyde.


Payments for Ecosystem Services Projects

Metaldehyde Mitigation Project - What are we doing?

Working closely with Thames Water, we aim to see a reduction in Metaldehyde levels in surface waters and general diffuse water pollution. To do this, FWAG is providing advice to farmers on alternative slug control (such as ferric phosphate) and helping the implementation of mitigation measures. Payments will be made to farmers by Thames Water based on results of water testing downstream of the whole catchment (taken between October - December).


Waterbodies covered - FWAG are helping farmers in the following waterbodies

Source to Thames catchment:

The River Ray catchment:

The River Cole catchment:

Marston Meysey Brook Catchment

Ampney and Poulton Brooks Catchment

Product Replacement Project

In the Ampney Brook catchment there is an ongoing trial that Thames Water is carrying out in conjunction with the Royal Agricultural University, studying the efficacy of swales to reduce metaldehyde concentrations in water. This requires the ongoing use on metaldehyde in a certain area of the catchment, and therefore ppayments to farmers are based on the amount of alternative Ferric Phosphate used, rather than the results of water quality tests.


Map of Metaldehyde project areas

To download the map as a pdf, click here


Useful Links:


To read more about the various projects within the Upper Thames Catchment Partnership, click here.