Last reviewed 3 April 2013

Caroline Hand investigates the problem of heavy rainfall and water contamination.

Across the UK, 2012 was the second wettest year on record, and in England the wettest ever. June, in particular, broke the records, with rainfall 103% above the monthly average. Bad news for all the families huddling in seafront shelters or seeking refuge in the amusement arcades; but also for the serious swimmers and surfers who rely on clean, uncontaminated bathing waters in order to enjoy their sport without the risk of contracting an infection.

As a result of the heavy rainfall, popular beaches all over the UK failed to meet the EU bathing water standards. A total of 35 out of the 610 designated bathing beaches failed to meet the standards of the Bathing Water Directive (76/160/EEC), with the south west of England being particularly badly affected. An independent survey by the Marine Conservation Society found that one-third of the beaches it tested failed to meet water quality standards. “Wild swimmers”, who prefer open waters to the chlorinated lanes of the municipal baths, were warned to stay out of British rivers after heavy rainfall for fear of bacterial infection.

Polluted run-off

So how does heavy rainfall impact on water quality? It is not the rain itself that is the source of contamination, but the run-off from both agricultural and urban land surfaces. During an episode of intense rainfall, water streams off the land and into rivers and thence the sea, carrying with it a variety of contaminants.

In rural areas, contamination arises predominantly from nitrate and phosphate fertilisers, pesticide residues and bacteria from animal manure. This is just washed down the slopes and into rivers during episodes of heavy rainfall. In city streets, oil, grease and particulates from vehicle emissions, detergents from car washing, and dog waste are the main pollutants of concern. Bird and dog waste contain higher levels of bacteria than normal sewage, and are a particular problem in coastal towns.

Combined sewer overflows

All areas are affected by overflows of sewage that occur when the volume of stormwater overwhelms the sewerage system during episodes of heavy rainfall. These combined sewer overflows (CSOs) are a feature of older sewerage systems, some of which date back more than a century. Excess stormwater, combined with diluted sewage, flows into rivers as an alternative to flooding the streets or backing up in toilets. Storms that occur in late summer after a period of dry weather tend to carry the most pollutants. Newer systems avoid this problem by having separate sanitary and stormwater sewers.

The Marine Conservation Society calculated that there are 31,000 CSOs in the UK, only a quarter of which are monitored. This figure includes sewage treatment plants as well as sewers. These CSOs may discharge dozens or even hundreds of times each year. In the region covered by water company Southern Water, for example, CSOs near Bognor Regis and Littlehampton flooded 74 times in the 2011 season (1 May to 30 September), while CSOs in Cowes on the Isle of Wight overflowed 49 times. This data does not include the winter season when rainfall is higher.

Tackling the problem

The Department for Environment, Food and Rural Affairs (Defra) and the Environment Agency are very much aware of this problem and are tackling it on a range of fronts. Different strategies have been drawn up for urban and agricultural run-off, both of which are referred to in the 2011 Water White Paper, Water for Life.

In summary, the solutions being implemented are:

  • catchment-based water quality management

  • dissemination of better farming practice

  • sustainable urban drainage systems

  • investment in wastewater infrastructure.

The catchment-based approach

Pollution from a point source, such as a sewage treatment works, can easily be regulated by enforcing emission limits under the environmental permitting system, but diffuse pollution from run-off is much more difficult to regulate as there is no identifiable “polluter”.

For this reason Defra believes that additional legislation would be ineffective; instead, the way ahead is a semi-voluntary approach involving the local community. Under the EU Water Framework Directive, the Environment Agency is already obliged to take a catchment-based approach to improving water quality. England and Wales has been divided up into 100 catchments, within which the Agency has selected 25 smaller areas in England for pilot studies. The pilots officially closed in December 2012 and reports on the lessons learned are now available on the Agency website.

The projects varied according to the unique characteristics of the catchment. For example, in the Ecclesbourne catchment of rural Derbyshire the main problems are nutrient enrichment from fertiliser, plus excess silt which affects fish and wildlife. The project partners, which include the Environment Agency, Severn Trent Water and the National Farmers’ Union, as well as local authorities and community groups:

  • worked with farmers to help reduce phosphate inputs

  • encouraged landowners to plant trees and wetland meadows to reduce run-off and prevent flooding

  • upgraded the sewage treatment works to reduce the concentration of phosphate in treated effluent.

In contrast, the Irwell catchment in Greater Manchester is being polluted by dirty water from roads, badly connected sewers and old landfills. One small area, Moston Brook, is being improved through the construction of a new sewer that will better manage storm water and prevent sewage litter from entering the brook during times of heavy rainfall. This particular project extends beyond diffuse pollution prevention; the local community has been actively involved in cleaning up the area and improving the general amenities.

Better farming practice

Farmers can take simple, practical steps to reduce contaminated run-off from their fields.

  • Reduce stocking levels when it is wet.

  • Keep manure heaps away from watercourses.

  • Move feeders and troughs regularly.

  • Use clover instead of grass, or make more accurate calculations of the amount of fertiliser required.

  • Cultivate across the slope.

These and other measures are listed in Water for Life.

Sustainable urban drainage systems (SUDS)

SUDS use structures such as green roofs and swales (ditches) to slow down run-off and allow it to infiltrate into the soil. These systems both reduce the quantity of run-off and improve its quality. The Flood and Water Management Act 2010 contained a provision that will make it mandatory for developers to obtain approval for new drainage systems from a SUDS Approving Body (SAB). Initially, this requirement was set to come into force last year, but has been delayed for fear of SABs being overwhelmed by the number of applications. New legislation is now expected in 2014.

At the end of 2012 Defra published a consultation paper on urban diffuse pollution. It intends to promote local community action, taking a lead from projects such as the Moston Brook pilot described above. One expert commentator was unimpressed, telling the ENDS Report that the draft strategy contained “fine words that will lead to nothing” and instead pointed to engineering solutions.

Improvements to sewerage infrastructure

There are certainly various technical fixes that can eliminate the problem of CSOs, although of course they come at a price. Building new sewers is a very costly solution but there are other solutions that may be more practicable. Wastewater treatment expert Chris Day, writing on, suggested that distributed treatment at individual CSO hotspots can reduce the entry of pollutants such as hydrocarbons, organic contaminants and heavy metals into watercourses. Overseas, relatively low-cost technologies such as those using hydrodynamic vortex separation have proved effective.

In fact, water companies have already invested a great deal in meeting the standards of the Urban Wastewater Treatment Directive and have managed to keep the larger solids out of sewage outflows; the focus now is on tackling smaller particles such as silts. There is a need to keep the pollution problem in perspective: despite the effect of last year’s rainfall, our bathing water quality has improved dramatically over the past 20 years and the vast majority of beaches now meet the EU standards. The Environment Agency is already working with the water companies as they plan new investment to attain the even higher standards of the revised Bathing Water Directive, which requires all waters to be classed as at least “sufficient” by 2015.