Supplementary note from Ofwat addressing
leakage from sewerage systems
WATER PRICES AND THE ENVIRONMENT
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
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".
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
In 1997 the Groundwater Protection and Research
Group at Sheffield University
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"
(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,
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.
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
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
Policy and Practice for the Protection of Groundwater,
2nd Edition (1998) Environment Agency. Back
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