Drinking water in the South East mainly comes from rainwater. When it rains, water flows into streams, rivers, lakes and reservoirs – this is called 'surface water'.
If water seeps through the ground until it reaches rock that it can’t penetrate, it forms underground aquifers. The water in these aquifers is called ‘groundwater’ and is often very high quality, because as it seeps through the ground, many contaminants are naturally filtered out. It also tends to be hard water.
Water supplied for drinking may contain some of the substances listed below. These substances can:
- occur naturally in raw (untreated) water
- enter water during the treatment process
- be deliberately added to safeguard public health.
Why we test water quality
It's our duty to ensure a wholesome supply of drinking water for our customers.
The Water Supply (Water Quality) Regulations set the standards required to produce this. They explain, in detail, the levels of certain characteristics, elements and substances that are allowed in drinking water to protect public health, and how much of each substance should be in the water supply.
Usually the limit is a maximum level but occasionally a minimum value is also set (for example, the pH, or acidity levels) – this permissible level is known as the Prescribed Concentration or Value (PCV).
How we test water quality
We regularly sample drinking water to monitor its quality at our water supply works, service reservoirs and customer taps.
We also continuously monitor some water quality measures at our water supply works. Hundreds of thousands of samples are analysed across our region each year to comply with the water quality standards. We also carry out checks every time we do something that might affect water quality, for example when we’ve responded to a burst supply pipe or installed new water mains.
When we receive complaints about water quality we take samples – more than 99% of all samples we check comply with the water quality standards.
If a test fails
If a sample fails a test, it doesn’t necessarily mean the water is unfit to drink. Sometimes the water in mains, pipes and neighbouring properties meets all the required standards, but there's an issue with the householder’s plumbing system. In this case, we'll let the customer know and give them advice on what action to take next.
We have specialist teams that deal with all samples that fail, and we quickly record, investigate and act upon any water quality failures to make sure any problems are resolved as soon as possible. We also report failures to the Drinking Water Inspectorate, health professionals (Public Health England) and the local environmental health services.
What we test for
We test for the parameters and substances listed below. The common units of measurement for the Prescribed Concentration or Value (PCV) of a substance are:
- one milligram per litre (mg/l) is one part per million
- one microgram per litre (ug/l) is one part per billion or thousand million
- one nanogram per litre (ng/l) is one part in a million million
- NTU = Nephelometric Turbidity Units (to measure turbidity – the level of cloudiness caused by particles in water)
- °H = Degree Hazen (for colour measurement)
- uS/cm – microSiemens/centimetre (to measure conductivity)
- E. coli or enterococci = number per 100 ml.