Norfolk study proves how farmland ‘traps’ can clean up our polluted rivers

Silt traps alongside farmland on the Salle Estate. Picture: Wensum Alliance

Silt traps alongside farmland on the Salle Estate. Picture: Wensum Alliance - Credit: Wensum Alliance

Following a damning report into the health of the nation's rivers, Norfolk researchers have proved that digging special ditches could reduce harmful run-off from farm fields and roads by more than half.

Silt traps alongside farmland and a road on the Salle Estate. In this aerial photo, the Blackwater r

Silt traps alongside farmland and a road on the Salle Estate. In this aerial photo, the Blackwater river runs underneath the road. Picture: Wensum Alliance - Credit: Wensum Alliance

The Rivers Trust says 86pc of rivers in England are failing to meet the 'good' ecological standard set by the European Water Framework Directive, due to factors including chemicals and sediments from farms, industrial abstraction, sewage discharge and pollutants from roads and urban areas.

In Norfolk, rivers rated only 'moderate' include the Wensum and its tributary the Blackwater – which has been the subject of study by the University of East Anglia and the Wensum Alliance.

Three sediment traps were dug at Salle, near Reepham, to prevent soil being washed off farm fields and roads, which could choke gravel beds with sediment and carry harmful pesticides and fertiliser residues which could endanger fish and other aquatic life.

When they tested the water downstream they found a 58pc reduction in sediment, averaged over the year, with one of the roadside ditches trapping seven tonnes of material in 12 months.

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Prof Andrew Lovett, of the UEA's School of Environmental Sciences, said although the results varied due to weather conditions and river flows, data such as this could prove vital for farmers if they wanted to claim funding under the government's principle of 'public money for public goods'.

'I think it is very important in terms of what sorts of measures the government might want to support in agri-environment schemes in the future,' he said.

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'If we can show there is a cost of doing this, but we can demonstrate the effectiveness of it, there would be a stronger case for making more support available for this sort of project. It benefits the aquatic quality in the stream and there are societal benefits to that, and in the long term it is helping to improve water quality and water supply. There are pretty obvious public benefits.'

The three sediment traps were funded by Norfolk Rivers Trust and the Broadland Catchment Partnership at a cost of about £15,000.

Prof Lovett said the site was chosen because it was alongside agricultural land and a road – both major sources of sediment pollution – and it allowed the results to be monitored on existing equipment already in use by the wider Demonstration Test Catchment (DTC) project at the Salle estate.

He said his UEA colleague Dr Richard Cooper has also calculated that the cost of the roadside trap would pay for itself in eight years, by avoiding the 'damage costs' from sediment, phosphate and nitrates.

'These kinds of traps are becoming much more common in the last few years, but the most important reason one was put at Salle was that we knew we had an issue with one particular area where we have a road running down a slope in two directions to a bridge that crossed the stream,' said Prof Lovett.

'And we had some examples in previous years of just how much mud and sediment can come down that road at certain times when it is raining heavily and there is lots of mud about.

'That particular site is a good one because we know the road is acting as a pathway for things to get into the stream.

'In this part of the world, one of the things that puts mud on the road is harvesting sugar beet in winter. That was one of the factors at Salle. There was a sugar beet pad off a field further up the road and it was surprising how much material came from that pad.'

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