Understanding Your Water Company's Quality Report (2026)

Updated April 2026 — TapWater.uk editorial team

Every year, every water company in England and Wales is required to publish a detailed compliance report showing how its water performed against 48 regulated parameters. These reports exist because of a legal framework built on the 1989 Water Act, the 2000 Water Supply (Water Quality) Regulations, and their subsequent amendments — a framework designed to give the public a right to know what is in their water. In practice, the data is published in formats that require some effort to interpret. This guide explains how the regulatory system works, what the numbers mean, and how to use TapWater.uk to find the information that matters for your area.

In Scotland, drinking water quality is regulated by the Drinking Water Quality Regulator for Scotland (DWQR), operating under broadly similar principles to the DWI but with its own reporting structure. In Northern Ireland, the Drinking Water Inspectorate for Northern Ireland (DWI-NI) performs the equivalent function. The data that TapWater.uk presents covers England and Wales via the DWI; we are working to expand coverage to Scotland and Northern Ireland.

Who regulates UK drinking water?

The Drinking Water Inspectorate was established in 1990, shortly after the privatisation of water companies in England and Wales, specifically to provide independent oversight of the new commercial operators. It sits within the Department for Environment, Food and Rural Affairs (Defra) but operates independently. Its core functions are to inspect water companies, investigate failures and consumer complaints, prosecute where legal standards have been breached, and publish the annual Chief Inspector's report on drinking water quality across England and Wales.

The DWI does not set the regulatory standards itself — those come from EU-derived legislation now retained in domestic law via the Water Supply (Water Quality) Regulations 2016. What the DWI does is enforce them. It employs a team of qualified water quality professionals who carry out on-site inspections of treatment works and distribution infrastructure, review the testing data that water companies are legally required to submit, and have the power to issue Notices of Intention requiring companies to improve their performance.

In Scotland, DWQR performs the same oversight role for Scottish Water, the publicly owned utility that serves the entire country. DWI-NI operates similarly for NI Water in Northern Ireland. All three regulators publish annual reports, and their data ultimately underpins what you see on TapWater.uk.

What gets tested?

The regulations specify 48 parameters that must be monitored at the tap. They fall into three broad groups.

Microbiological parameters are the most immediately safety-critical. The primary indicators are E. coli and total coliform bacteria. Their presence in treated drinking water is a regulatory failure with a prescribed concentration or value (PCV) of zero: any detection requires immediate investigation and, if confirmed, remedial action. Intestinal enterococci and Clostridium perfringens are also monitored as additional indicators of potential faecal contamination. The frequency of microbiological testing is proportional to the size of the supply zone and typically involves multiple samples per zone per month.

Chemical parameterscover the widest range of substances and are where most of the technical complexity lies. The regulated list includes lead (a PCV of 10 micrograms per litre, tightened from 25 μg/L in 2013), nitrate (50 mg/L, primarily a concern in agricultural areas), pesticides (individual substances at 0.1 μg/L, aggregate at 0.5 μg/L), trihalomethanes (total at 100 μg/L, a by-product of chlorination), arsenic, fluoride, copper, and polycyclic aromatic hydrocarbons. PFAS — per- and polyfluoroalkyl substances — have recently been added as regulated parameters under updated guidance, reflecting growing concern about their presence in source water catchments.

Physical and indicator parameters include turbidity (cloudiness, measured in nephelometric turbidity units), colour, odour, taste, pH, and conductivity. These are not primarily health parameters but are important for treatment efficiency and consumer confidence. Very high turbidity, for example, can protect pathogens from disinfection by chlorine.

A PCV (Prescribed Concentration or Value) is the legal maximum for each parameter. For some parameters, such as hardness, colour, and temperature, the regulations specify an acceptable range or target rather than an absolute maximum. Where no PCV exists, the regulations may still require that water is “wholesome” — a somewhat elastic legal standard that the DWI interprets on a case-by-case basis.

What compliance rates mean

The headline compliance figure published by the DWI for England and Wales is typically around 99.96 percent. This number appears reassuring, and in the context of what it represents — the proportion of individual test results that fall within their legal limit — it is genuinely impressive. But it requires careful interpretation.

Water companies collectively carry out several million individual regulatory tests each year. At 99.96 percent compliance, that leaves roughly 4,000 test failures annually across England and Wales. Most of these failures are minor, short-lived, and caught quickly by the testing regime. A test failure does not automatically mean consumers were exposed to unsafe water; in many cases, the water company identifies and resolves the cause before the supply is affected. But the raw compliance percentage obscures the actual number and nature of individual failures.

The DWI's annual report breaks down compliance by parameter and by water company, which gives a more useful picture than the headline aggregate. Parameters with the most frequent failures in recent years have been lead (driven by legacy lead service pipes in older properties), coliform bacteria (typically isolated incidents associated with distribution system disturbance), and certain pesticides in agricultural catchments. A company with 99.99 percent compliance may be performing significantly better than one at 99.90 percent, even though both numbers look high in isolation.

Compliance figures are also averaged across large supply zones. A zone covering a million people will have far more tests than a small rural zone, and failures in a small zone may be statistically invisible in the aggregate. This is why postcode-level data — where available — is more informative than regional or company-wide averages.

How to read your area's data

TapWater.uk maps your postcode to a DWI supply zone and presents the compliance data for that zone alongside the water company's published readings for each regulated parameter. Here is what you are looking at on a typical postcode page.

The safety score at the top summarises how far each measured parameter sits from its regulatory limit, weighted by the health significance of the parameter. A score of 9 or above means all measured parameters are well within their limits; a score below 7 flags that one or more parameters are closer to their PCV than the national average. The score is a signal, not a verdict — you should read the parameter table below it to understand which substances are driving the result.

The parameter table shows each regulated substance alongside its measured concentration, the legal limit, and a simple visual indicator of the margin. Lead, bacteria, nitrate, and PFAS are highlighted separately because they carry the highest health weighting in our scoring model. If a parameter shows a reading close to its PCV, the table will flag it.

To find your area's data, enter your postcode on the TapWater.uk homepage or go directly to a postcode such as SW1 (central London) or M1 (central Manchester) to see how the data is structured.

When to be concerned

Persistent failures in a single parameter, particularly lead, bacteria, or nitrate, merit attention. A one-off exceedance followed by a return to compliance is typically the result of a transient issue — a distribution disturbance, a sampling error, or a brief treatment anomaly. Repeated failures in the same zone over multiple reporting periods suggest a structural problem that the water company has not resolved.

The DWI publishes a list of formal enforcement actions taken against water companies, which includes Notices of Intention (essentially formal warnings), Undertakings (legally binding improvement commitments), and prosecutions. These are relatively rare — the regulator estimates that around 99 percent of supply zone non-compliances are resolved informally without formal action — but they represent the most serious end of the compliance spectrum. The DWI's enforcement register is publicly accessible on its website.

If you have noticed a change in the taste, smell, or appearance of your tap water, or if you have received a notice from your water company about a boil water advisory or supply interruption, report it directly to your water company. For incidents that your water company has not resolved satisfactorily, you can escalate to the DWI directly by contacting its consumer team.

Your rights as a consumer

Under the Water Industry Act 1991 and the Water Supply (Water Quality) Regulations 2016, you have a statutory right to receive water that is wholesome and compliant with all prescribed concentrations and values. You also have a right to information: water companies must provide details of water quality in your supply zone on request, free of charge.

The Consumer Council for Water (CCW) is the independent consumer body for the water sector in England and Wales. If you are dissatisfied with your water company's response to a complaint about quality, you can escalate to CCW, which has powers to investigate and make recommendations. CCW also publishes annual reports on water company customer service performance, which include quality complaint data.

In Scotland, Consumer Scotland handles water-related consumer complaints alongside its broader remit. In Northern Ireland, the Consumer Council for Northern Ireland performs an equivalent function for NI Water customers.

If you have old lead pipes in your property — most common in homes built before 1970 — your water company is required to have a policy for replacing lead communication pipes (the section of pipe between the water main and your property boundary). The replacement of supply pipes within your property is your responsibility, though some companies offer subsidised replacement schemes. Contact your water company directly to find out whether your street has lead communication pipes and whether a replacement programme is active in your area.

Find your supplier

The United Kingdom has around 20 licensed water supply companies, ranging from the very large (Thames Water serves approximately 15 million customers) to the very small (Portsmouth Water serves around 700,000). Your supplier depends on your address — unlike energy, you cannot switch water companies. The links below go to each supplier's page on TapWater.uk, where you can see its compliance summary and find the supply zones it operates.