Select Committee on Environmental Audit Minutes of Evidence

Supplementary note from Ofwat addressing leakage from sewerage systems


  In England and Wales the majority of sewerage systems are of the combined design. This is where a single sewer pipe network is used to collect and convey foul sewage and surface water flows. In all sewer systems the raw sewage flows tend to be supplemented by groundwater seepage into the sewers (infiltration) and there may be some loss of sewage through leakage into the surrounding ground (exfiltration). The rate of infiltration is far in excess of that of exfiltration with some sewers acting as land drains well below ground levels.

  Having considered the mechanisms that might create a public health hazard Ofwat is convinced that direct ingress of raw sewage leaked from sewers into nearby water mains is unlikely in the extreme. This is because:

    —  in all but the most exceptional circumstances the water pressures within the distribution system greatly exceed groundwater pressures;

    —  water mains are generally laid at a relatively shallow depth and above any sewers. This orientation greatly reduces the chance of leaked raw sewage coming in contact with even the outside of a water main.

  Contamination of groundwater by sewer leakage is a more likely scenario. It should not normally cause a problem because water extracted from ground water supplies is treated and tested before consumption. A number of recent reports published by reputable organisations address this issue.

  In 1996 the Construction Industry Research and Information Association (CIRIA) published the report "Reliability of sewers in environmentally vulnerable areas"[4]. Based on the evidence available, this concluded that "groundwater contamination originating from leaking sewers occurs with limited frequency. Questionnaire and literature surveys in England and Wales uncovered a total of 60 incidents which broadly equates to one per water service company region every seven years".

  The vast majority of these incidents did not lead to any adverse effect on public health as potable supplies are generally chlorinated. The few outbreaks of waterborne disease associated with sewage-contaminated groundwater sources in this country have all coincided with the breakdown or lack of disinfection of potable water supplies. The most recent incident reported as having adverse public health effects occurred at Bramham, Yorkshire in 1980 when an outbreak of gastro-enteritis was linked to faecal contamination of boreholes caused by a nearby blocked sewer.

  The report noted that existing groundwater monitoring is generally inadequate to quantify the scale of sewer-related contamination.

  In 1997 the Groundwater Protection and Research Group at Sheffield University[5] reported that although there was a "strong suspicion (even expectation)" that sewer leakage has an impact on groundwater quality, no proof had been found. A fundamental problem is that exfiltration from sewers, whilst being as easy to cure as infiltration once found, is far harder to identify.

  The Environment Agency's document "Policy and Practice for the Protection of Groundwater"[6] (1998) does not highlight sewer leakage as a particular problem but notes that the sewerage system is one of many potential sources of contamination which can pose a risk to groundwater quality. The document sets out some guidance relating to development within Source Protection Zones and makes clear the Agency's opposition to the laying of new mains sewers in Zone 1[7], although the use of pipework which is less vulnerable to leakage would be considered on a case by case basis.

  Although leaking sewers can be a contributory factor to the poor quality of many urban groundwaters, there is no quantitative evidence that they are the main cause. Greater prominence is given in the Agency's document to other potential sources of contamination such as municipal landfills, the application of liquid effluent, sludges and slurries to land, discharges to underground strata and diffuse pollution.

March 2001

4   B Misstear, M White, P Bishop, G Anderson (1996). Reliability of sewers in environmentally vulnerable areas. Construction Industry Research and Information Association (CIRIA) Project Report 44 prepared under contract by Mott MacDonald Limited. Back

5   MH Barrett, Y Yang, DN Lerner, MJ French, JH Tellam (1997). The impact of cities on the quantity and quality of their underlying groundwater. Groundwater Protection Research Group, University of Sheffield. Back

6   Policy and Practice for the Protection of Groundwater, 2nd Edition (1998) Environment Agency. Back

7   The area is defined by a 50-day travel time from any point below the water table to the source and as a minimum of 50 metres radius from the source. Back

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