Tests explained

Water quality tests

Water quality is monitored continuously at all our treatment works and, in the unlikely event of a problem arising at a site, it is shut down automatically.

Each year we analyse around 50,000 samples of water taken from treatment works, reservoirs and customers’ taps.

Analysis is carried out using sophisticated laboratory instruments and many results are close to the lowest detectable levels.

The analysis falls into three main categories:

  • physical which examines the basic character and behaviour of water
  • chemical which measures the level of specific substances 
  • microbiological which ensures that no harmful micro-organisms are present.

The following substances are tested:

1, 2- Dichloroethane

A chemical intermediate and solvent. It can contaminate groundwater beneath industrial sites. Where necessary, special treatment to remove solvents is used. Limit: 3µg/l


Alkalinity, like hardness, comes from the rocks through which water has passed. It is an indication of the natural hardness and pH of the water. It can be altered by softening. Limit: None specified


Aluminium occurs naturally in some water sources and is removed during water treatment processes such as coagulation and filtration. As aluminium sulphate it can be used as a treatment chemical to remove particles or cloudiness. Its use is closely controlled. Limit: 200µg/l


Occurs naturally as ammonium salts in the environment and may be present in trace amounts in water. They are used in fertilisers and can indicate possible contamination, such as, in surface waters. Ammonium salts do not themselves cause health problems. They can interfere with disinfection processes although generally treatment removes them. Limit: 0.5mg/l


Antimony is rarely present in water although trace levels have been associated with impurities from plumbing. There are no problems with antimony in drinking water we supply. The standard for antimony is stringent to reflect its potential toxicity. Limit: 5µg/l


Arsenic is rarely present in water although trace levels have been associated with impurities from plumbing. The standard for arsenic is very stringent to reflect its potential toxicity. There are no problems with arsenic in drinking water we supply. Limit: 10µg/l


Benzene is not present in uncontaminated drinking water. It is an organic solvent and is used in industry and as an additive in petrol. It has been identified as a carcinogen. Limit: 1µg/l


A PAH compound (see total polycyclic aromatic hydrocarbons) that has a standard as a individual substance based on its toxicity. It does not generally cause a problem in our supply area. Limit: 0.01µg/l


Boron at low concentration is often naturally present and is not a risk to health. Significant concentrations may be present in some surface sources where it derives from detergent residues. It is widely used by industry in chemical, medical, cosmetic and agricultural preparations. Limit: 1mg/l


A byproduct of disinfection formed by the reaction of naturally occurring bromide with strong oxidants, particularly ozone. A stringent standard has been set based on toxicity and best water treatment practice. Ozone is not used in our region. Limit: 10µg/l


Cadmium is rarely present in water although trace levels have been associated with impurities from plumbing. The standard for cadmium is stringent to reflect its potential toxicity. There are no problems with cadmium in drinking water we supply. Limit: 5µg/l

Calcium and magnesium

Calcium and magnesium are naturally occurring minerals found, for example, in limestone and chalk rock. Both, but particularly calcium, contribute to the natural hardness of water. They are essential elements for health. Limit: None specified


Chloride in drinking water originates from natural mineral salts. The most common are hardness salts - calcium. It may gain access to raw water sources from local use of de-icing salt. Chloride is a component of table salt and in low quantities is part of a normal diet. At these concentrations, it is not harmful to health. Limit: 250 mg/l

Chlorine, free and total

It is a legal requirement to disinfect water during treatment to kill harmful bacteria. In our region disinfection with chlorine is a critical part of the treatment process. Dosing is monitored continuously at every treatment works. The amount dosed is intended to ensure protection as treated water travels to your home. Chlorine reacts readily with other substances in water so that total chlorine is the total amount of chlorine present in the water while free chlorine is the proportion that remains unreacted. Limit:  No limit. World Health Organisation (WHO) guidance for free chlorine between 0.2 and 0.5 mg/l


Chromium is rarely present in water although trace levels have been associated with impurities from plumbing. The standard is stringent to reflect its potential toxicity. There are no problems with chromium in drinking water we supply. Limit: 50 µg/l

Clostridium perfringens (including spores)

Clostridium perfringens is a spore forming bacterium present in the gut of warm-blooded animals. It is commonly found in soils and the environment. The spores remain viable for long periods and can survive disinfection. The presence of spores in drinking water indicates a remote or intermittent source of contamination that requires investigation. Tests for clostridia in treated water are applied at sources influenced by surface water, when proving new or repaired mains and when investigating potential microbiological incidents. Limit: 0 per 100ml

Coliform bacteria

Coliforms are bacteria that are distributed widely in the environment, often as a result of human and animal activity but also in association with soil and vegetation. Their presence in water supplies indicates a quality problem and a need to investigate for a source of contamination or reason for deterioration. They do not themselves generally cause harm to health. Coliform bacteria indicate whether water treatment, especially disinfection, is satisfactory. The standard applies as an absolute value at treatment works and must be met by 95% of samples taken at storage reservoirs. At the customer's tap, they are an indicator parameter. Limit: 0 per 100ml

Colony counts: 2-day at 37C; 3-day at 22C

Colony counts test for background levels of bacteria usually of environmental origin. These bacteria are generally harmless and occur naturally in soil, air and water. Results from these tests are collated to provide an overall picture of microbial water quality and assist in maintaining the efficiency of treatment and integrity of distribution networks. Limit: No regulatory limit. Any significant change is investigated.


Colour occurs naturally in surface water sources and is removed during water treatment processes. Treated water is required to be clear, bright and free from colour. The standard is set primarily for aesthetic reasons. Limit: 20º Hazen


This measure is obtained by passing an electric current through the water. It is a measure of the amount of mineral salts the water contains. Limit: 2,500 µS/cm at 20C


Copper occurs naturally in foods and the environment. In drinking water copper arises mostly as a result of copper pipes and fittings in household plumbing. This is especially true of new pipework where copper can result in blue or green staining of bathroom fittings and impart a sharp taste to water. This problem usually disappears after a short time. Limit: 2mg/l


A protozoan parasite that can infect humans and a variety of animals and birds causing gastroenteritis. The parasite is shed into the environment as oocysts. The oocysts are very resistant to the chlorine used routinely for disinfection. Where there is a significant risk from their presence in water supplies, treatment processes are required to remove or inactivate them. Sound hygiene procedures are required to prevent their entry after treatment. Limit: No specific standard, risk assessment required. Water must be free from any parasite posing a risk to human health.


Rarely present in water although trace levels have been associated with impurities from plumbing. The standard for cyanide is very stringent to reflect its potential toxicity. There are no problems with cyanide in drinking water we supply. Limit: 50µg/l

Escherichia coli (E. coli)

E. coli is a bacterium present in the gut of all warm-blooded animals. They should not be present in drinking water and immediate action is required to identify and remove the source of faecal contamination. They are sometimes found in untreated water, especially surface water reservoirs that provide wildlife habitats. Whilst on their own these bacteria generally cause no harm to health, their presence in water supplies indicates the potential presence of pathogens. Limit: 0 per 100ml


These are bacteria found in the gut of all warm-blooded animals. They should not be present in drinking water and immediate action is required to identify and remove the source of faecal contamination. Tests for enterococci are applied at customers' taps when proving new or repaired mains and when investigating potential microbiological incidents. Limit: 0 per 100ml


Can occur as a result of traces present in water treatment chemicals. It is controlled by means of product specification. Limit: 0.1µg/l


Traces of fluoride occur naturally in many water sources, particularly groundwater. Fluoride is not added to any of the drinking water supplied by us but one source in Wiltshire has natural levels of around 1,000µg/l. Fluoride is not removed by conventional water treatment. Responsibility for a decision to add fluoride as a protection against tooth decay rests with the local health authority. You are advised to contact your local health authority for further information. Limit: 1,500µg/l

Geosmin and 2-methyl isoborneol

These are organic compounds sometimes present in surface water sources due to the activity of certain micro-organisms and algae. When present they cause musty or earthy odours and tastes at very low concentrations. They can be removed by appropriate activated carbon contact during treatment and are not harmful to health. Limit: No regulatory standard


Present naturally in many water sources and removed during water treatment, iron in water supplies may be derived from the corrosion of iron mains or customers' galvanised iron pipes. Iron is not harmful to health and standards are primarily set for aesthetic reasons. Under certain conditions, rust sediments in the mains can be disturbed and result in 'brown water'. This is usually cleared by flushing the water main and taps. Limit: 200µg/l


Lead is rarely present in water sources but was widely used as a material in plumbing - for both pipe construction and solder. There are no longer any lead mains in use in our region but a proportion of customer properties, particularly older ones, may still contain lead pipework. If the water supply tends to dissolve lead, water companies are required to treat the water to protect the health of consumers. Lead levels well above the standard can be harmful to health, especially if consumed consistently over many years. Customers wishing to replace their pipework can contact us about our policy regarding service pipe replacement. In brief, we will replace our portion of the service pipe when customers replace their section. Limit: 10µg/l


Present naturally in many water sources, it is usually removed during water treatment. Rarely, disruption to water mains can cause sediment, containing manganese, to be stirred up and cause 'black water'. It is not harmful to health. Flushing the tap generally clears the problem. Limit: 50µg/l


Mercury is rarely present in water although trace levels have been associated with impurities from plumbing. The standard is very stringent to reflect its potential toxicity. There are no problems with mercury in drinking water we supply. Limit: 1µg/l


A trace metal occurring widely in the environment. Rarely present in water although coatings on some taps and other plumbing fittings may contain it. These fittings can impart very low levels of nickel when water is left to stand in contact with them for a prolonged period. A brief period of flushing will usually clear this from the system. Limit: 20µg/l


Present naturally in all source waters, although higher concentrations are more commonly found in agricultural areas where nitrogen-based fertilisers are used on the land. Where necessary, nitrate levels are reduced to acceptable levels by water treatment. Liaison with the Environment Agency, farmers' groups and landowners is important when addressing issues in the short and long term. Limit: 50mg/l (Reg 4 gives a combined nitrate/nitrate wholesomeness standard)


Traces of nitrite may be produced when chlorine and ammonia are used together in the disinfection process. We do not use this process. Limit: 0.5mg/l


Pesticides - aldrin, dieldrin, heptachlor and helptachlor epoxide

These are persistent organo-chlorine chemical compounds no longer used in the UK. Generally they are not found in water sources but a more stringent standard has been set on the basis of their toxicity. Limit: 0.03μg/l 

Pesticides - other compounds

This group includes organic chemicals with a range of uses such as weedkillers, insecticides and fungicides. Many water sources contain traces of pesticide residues as a result of both agricultural and non-agricultural uses of pesticides on crops, and for weed control. Some of these chemicals persist in the environment. Where necessary, additional treatment is installed to remove pesticides. Limit: 0.1μg/l

Pesticides - total

This is the sum of all the individual pesticides detected. Limit: 0.5μg/l


Also called hydrogen ion, this is a measure of the acidity or alkalinity of the water. Water having a pH of seven on a number scale is described as neutral. Lower values occur if the water is acidic. Waters are usually maintained at a pH slightly higher than neutral, sometimes by the addition of alkali during treatment to minimise corrosion. Limit: 6.5 - 9.5


Phosphorus occurs naturally in the environment and is a major component of fertilisers. It is not harmful to health and is often added to water, as low levels of phosphate, to form a protective layer on the inside of lead pipes, thereby reducing lead levels in tap water. Limit: No regulatory limit

Qualitative odour, qualitative taste

These are subjective assessments at the time of sampling or in the laboratory, using the sensitivity of the human palate and nose to detect any discernible odours and tastes. Most treated water has no distinctive odour or taste although customers' plumbing can contribute to a range of tastes such as disinfectant, plastic or metal if water has been standing in pipework for periods of time. Limit: All samples are checked and any significant finding is investigated

Quantitative (dilution or threshold) odour and taste

In addition to the qualitative assessments described above, a proportion of treated water samples are subjected to more rigorous tests. These involve panels of assessors tasting and smelling the water under strictly controlled conditions to provide a more representative assessment. Limit: Acceptable to consumers and displaying no abnormal change


Radon is a naturally occurring radioactive gas found in small amounts in all rock and soil. It seeps out of the ground in some locations and can collect in enclosed spaces where it primarily poses a risk through inhalation. Radon may dissolve in ground water where this originates in an area usually already recognised as being at risk of Radon in air. When present at all in drinking water concentrations are generally low.  Limit: 100Bq/l


Selenium is rarely present in water although trace levels have been associated with impurities from plumbing. The standards for these are generally very stringent to reflect their potential toxicity. There are no problems with selenium in drinking water we supply. Limit:  10 µg/l


Not present in source waters and not usually present in drinking water but traces may be present when certain types of domestic filter are used. Limit:  No regulatory limit


Sodium is a component of common salt. It is present in seawater and brackish groundwater. Concentrations in drinking water are normally very low but some water softeners can significantly add to the sodium concentration. In moderation, sodium is a normal and essential part of a healthy diet, in excess, sodium can be harmful, particularly to infants but also to adults over the longer term. Limit: 200mg/l


Occurs naturally in the environment, as mineral deposits. Sulphate is not harmful to health and is not removed during treatment. Limit: 250mg/l


The temperature of surface waters varies according to the season while in groundwater it tends to remain relatively constant throughout the year. High temperatures can cause the water to taste less palatable. Limit: No regulatory limit

Tetrachloromethane, trichloroe-thene and tetrachloroethene

These solvents are not normally present in water. They may occur in low concentrations in groundwater affected by industry where they are often used as de-greasants. Where necessary, they are removed from drinking water by special treatment. The standard for the latter two compounds relates to the sum of their detectable concentrations. Limit: 3µg/l, 10µg/l

Trihalomethanes (THMs)

These organic substances are by-products of chlorination, occurring as chlorine reacts with organic substances in drinking water. They are not considered harmful to health at the levels found in water. Limit: 100µg/l


A radioactive isotope of hydrogen. It is not normally found in water sources. Discharge of radioactivity to the environment is closely monitored by the Environment Agency. Limit: 100Bq/l

Total indicative dose (radioactivity)

A measure, obtained by calculation, of the effective dose of radiation the body would receive from consumption of treated water. All water sources in our region are assessed periodically to show compliance. Limit: 0.1mSv/yr (indicative)

Total hardness

Hardness is a natural characteristic of water. It is formed when rainwater dissolves calcium and magnesium salts while passing through rock such as chalk. Hardness is not harmful to health although customers may notice a difference in the amount of lather from soaps - hard water lathers less than soft water. Most of the water supplied by us falls in the range moderate to very hard - the only naturally soft water in the region is found in parts of Somerset. Limit: None specified

Total polycyclic aromatic hydro-carbons (PAHs)

PAHs are components of a material which, in the past, was used to line water mains. There are four individual PAH compounds that are monitored regularly. The sum of these four is calculated and regulated by one overall standard. PAHs are not harmful to health at the levels likely to be found in water, and do not generally cause a problem in our region. Limit: 0.1μg/l


Turbidity measures the presence of tiny particles that may make the water appear cloudy. Air bubbles can sometimes cause the water to look cloudy although air will clear from the bottom upwards if aerated water is left to stand. Limit: 4 NTU

At treatment works turbidity is monitored continuously and readings above a pre-set limit will cause the site to stop operating. Limit: 1 NTU

Units of measurement

mg/l = milligrams per litre or parts per million

µg/l = micrograms per litre or parts per billion

Bq/l = becquerel per litre, unit of radioactivity (equal to one disintegration or nuclear transformation per second)