Preventing pollution of drinking water rather than trying to treat it could help reduce metaldehyde levels in surface waters. Robert Harris reports

Water companies are set to work much more closely with growers to reduce key pollutants, including metaldehyde, in water supplies.

It follows new emphasis by water industry regulators on understanding and mitigating risks in catchment areas, rather than relying on water treatment to manage high concentrations of pesticide in water used for public supply.

The move is potentially good news for farmers, suggesting the authorities favour closer co-operation to manage the metaldehyde problem rather than an outright ban, at least in the short term.

This key slug killer is almost impossible to remove by conventional treatment so it is vital to keep raw water levels below the EU drinking water standard of 0.1 parts per billion. However, concentration peaks above that level are commonly found in surface water in arable catchments during the autumn.

When tests became widely available in 2008 the extent of the problem was uncovered, throwing metaldehyde’s future into doubt.

Slug pellet manufacturers and suppliers joined ranks to form the Metaldehyde Stewardship Group, promoting good pelleting practice on farms and liaising with the authorities to avoid a ban.

Its efforts have certainly helped. Although metaldehyde levels continue to exceed the legal maximum in some areas, generally readings have dropped significantly.

However, the two autumns since 2008 have been much drier, so whether this improvement is down to better practice or more benign weather remains to be seen.

As such, it is crucial to keep ahead of the problem, says Paul Fogg, spokesman for the Metaldehyde Stewardship Group. He welcomes the shift of emphasis to prevention rather than cure.

“By focusing on cleaning up water catchment areas, water companies are working closely with all stakeholder groups, including the MSG.

“The water companies have a better understanding of their catchments than anyone, and can run relevant models to identify areas of risk, for example heavy soils or steep slopes, allowing for more targeted and effective measures. We are very hopeful that this new approach will deliver the required results.”

It could also engage many more farmers in the process, he adds. “You typically need at least 95% of growers to implement pollution-reducing measures to make the required difference. Just one or two mistakes can affect an entire catchment.”

One of the key drivers behind the switch to catchment monitoring is the World Health Organisation’s promotion of Drinking Water Safety Plans. The Drinking Water Inspectorate has incorporated this plan in to its regulations, says principal inspector, Sue Pennison.

“Water companies are responsible for ensuring drinking water complies with the drinking water directive. We now require them to provide a risk assessment for their supply system to comply with legal limits.”

The undertakings for metaldehyde run until 2015 and cover water supplies from 80 treatment works.

“Some companies with a more widespread problem will really need to get into the detail and understand the issues thoroughly to make the necessary differences,” she notes.

One such company is Anglian Water, which obtains half of its drinking water supply from surface water.

Three years of data shows metaldehyde commonly exceeds the drinking water standard in the catchments of all 13 of the company’s surface water treatment works during peak autumn use, says the company’s source protection manager Simon Eyre.

“Although levels vary widely across catchments, exceedances were widespread in 2008. In general 2009 was much better as we had a drier autumn and farmers were more aware of the problem. Last year’s levels were somewhere in between.”

The problem is worse in pumped storage reservoirs, he explains. “If abstracting directly from a river, it might be possible to temporarily avoid abstraction until the problem passes. That’s not the case with a reservoir, as the main refill period coincides with autumn metaldehyde peaks and there are no alternative supplies.”

With 10,000sq km of diverse catchments affected by metaldehyde, Anglian Water faces a tough challenge and a tight timescale. Remedial plans need to be written into the next five-year plan, which runs from 2015-20, so drafts need to be drawn up by 2013.

In that time the company needs to assess the problem, identify potential solutions and run cost benefit analyses.

The company initiated a pilot scheme last autumn in the Pitsford reservoir catchment, which has the hallmarks of a problem area – winter wheat and oilseed rape are widely grown on sloping, heavy soils, and all the water within the catchment ends up in a large reservoir.

“We are currently at the awareness-building stage, and we are trying to develop working relationships with stakeholders (see panel) so we can assess what steps might work, how acceptable these would be and how we can help,” says Mr Eyre.

“An obvious one might be to stop using metaldehyde, though we are well aware of its importance. This could be done through product substitution or change of land use in problem areas.”

Other potential measures include reducing rates of active ingredient, constructing wetland areas or buffer strips to capture drain outflows and surface water run-off and good pelleting practice.

“We are also developing models which, with detailed observations, will identify better how metaldehyde moves from field to catchment abstraction points under different scenarios.”

The aim is to produce a list of practical and cost-effective mitigation measures that can rolled out across Anglian Water’s entire catchment, says Mr Eyre.

Anglian Water stakeholder engagement

Key stakeholders include Natural England, Environment Agency, Catchment Sensitive Farming, NFU, CLA, farmers, Farming and Wildlife Advisory Group, Metaldehyde Stewardship Group, agronomists, ADAS, The Rivers Trust and RSPB.

Close working will:

• Raise awareness of metaldehyde problem

• Help identify catchment risks and influence actions

• Monitor impact of voluntary measures

• Build knowledge of catchments

• Understand stakeholder roles, needs, and interests

• Develop effective working relationships.

Case study: Wessex Water

While most companies have only just started catchment area assessment, Wessex Water has been using it for six years with considerable success.

The main focus has been on nitrates and pesticides in general. “Although there is no particular health issue with the level of metaldehyde currently found in drinking water, the premise is it shouldn’t be there, hence the EU limit which was based on the limit of detection at the time,” says Luke de Vial, head of water resources at Wessex Water.

“We are fortunate in that much of our water supplies come from groundwater in arable areas. We found traces of metaldehyde in some surface supplies in autumn 2008; only in one case was it significantly above the limit.”

The problem catchment fed the Durleigh reservoir in Somerset. “Fortunately it is not very large, so we drained it to 20% capacity and refilled with better quality water.”

The small catchment contained with only six arable farmers, one of whom had been using metaldehyde to protect oilseed rape.

“We were probably the heaviest user in the catchment,” says Edward Tarr, who farms about 200ha from Rexworthy Farm, near Bridgewater. “We had sown about 32ha of oilseed rape and applied 3kg/ha of metaldehyde to the seed-bed.”

Despite Mr Tarr following guidelines to the letter, Wessex Water found metaldehyde in the drain flows. “We felt awful – it seems to be more soluble than anyone thought,” says Mr Tarr.

“It was really worrying – we have to use pellets but the Environment Agency didn’t like methiocarb because of possible impacts on wildlife. Ferric phosphate was the other option, but at the time it cost 10 times as much as metaldehyde.”

Wessex Water offered to pay 80% of the price, and the results of the switch were dramatic. Metaldehyde levels fell from a peak of 0.65ppb at the start of October 2008 to below 0.1ppb by the end of the year, remaining there since (see graph).

Ferric phosphate also proved effective against slugs, says Mr Tarr. “Numbers seem to have fallen overall. A couple of drier autumns may have helped, but I have no doubt the pellets will work in a wet season, too.”

Luke de Vial believes the catchment approach will succeed, even for companies with a widespread metaldehyde problem. “We have been doing a lot of work with pesticides and nitrates and it is possible to speak to lots of farmers and get results.