tapwater.uk

Our Data Sources

TapWater.uk is built entirely on publicly available data from UK government and regulatory bodies. We do not collect our own water samples or conduct laboratory analysis. This page explains exactly where our data comes from, what it covers, and how it is kept up to date — because we believe full transparency about data provenance is essential for a project like this.

Drinking Water Inspectorate (DWI)

This is our primary source. The Drinking Water Inspectorate is the independent regulator for drinking water quality in England and Wales. It publishes the most comprehensive and authoritative dataset on the quality of treated tap water supplied to homes and businesses.

The DWI dataset covers 48 regulated parameters — including microbiological indicators (E. coli, total coliform), heavy metals (lead, arsenic, copper), organic compounds (pesticides, trihalomethanes, PFAS), and aesthetic parameters (turbidity, pH, hardness, chlorine, iron). Sampling takes place at treatment works, service reservoirs, and at consumers' taps, giving a broad picture of water quality throughout the distribution system.

Data is published on both an annual and a quarterly basis depending on the parameter. Annual reports cover the full set of 48 parameters at the supply zone level; quarterly releases focus on the most health-critical parameters. We ingest both schedules and use the most recent available result for each parameter when calculating scores.

Water Company Postcode Lookups

England and Wales are served by approximately 20 licensed water companies, each responsible for one or more supply zones. To map a postcode to the correct set of DWI data, we maintain a regularly updated lookup table sourced directly from each water company's published supply zone information.

This water company layer provides the most granular geographic mapping available — more precise than regional or local authority boundaries — because supply zones are drawn to reflect the physical infrastructure of the water distribution network. A single postcode district can sometimes span two different supply zones with meaningfully different water characteristics; the water company lookup ensures we assign each postcode to the correct zone.

Environment Agency Water Quality API

This is a supplementary source. The Environment Agency (EA) operates one of the most extensive environmental monitoring networks in Europe. Their open data API provides access to over 72 million water quality observations from more than 58,000 sampling points across rivers, lakes, estuaries, and groundwater in England.

The EA dataset includes measurements for 50+ PFAS (per- and polyfluoroalkyl substance) compounds, nitrates, pesticides, heavy metals, biological oxygen demand, and many other determinands. The coverage of PFAS is particularly notable given growing scientific interest in this class of compounds.

Important distinction: EA data measures environmental water, not treated drinking water. A river or groundwater source with elevated contaminants does not mean your tap water contains those contaminants — water treatment is specifically designed to remove or reduce them. We use EA data as contextual background (weighted at 20% in our overall score) to indicate the environmental quality of source waters in your area, not as a direct measure of drinking water safety.

The EA Water Quality API is publicly accessible at environment.data.gov.uk/water-quality/api.

Postcode Mapping

Translating a postcode search into the correct water supply zone involves two reference datasets:

  • Water UK postcode lookup — maps postcodes to water company and supply zone identifiers, maintained by the industry body Water UK.
  • ONS Postcode Directory (ONSPD) — the Office for National Statistics' definitive postcode reference, used to validate postcodes, resolve geographic coordinates, and link to local authority and region boundaries for contextual display.

Update Frequency

Different data sources are refreshed on different schedules, reflecting the cadence at which upstream data is published:

  • Environment Agency API — checked daily for new observations; new measurements are incorporated within 24 hours of publication.
  • Water company postcode lookups — re-scraped weekly to capture supply zone boundary changes and new postcode assignments.
  • DWI drinking water data — ingested on a quarterly basis, aligned with the DWI's own publication schedule. Annual reports are processed as soon as they are released, typically in the spring following the reference year.

Open Data

The Environment Agency water quality data is published under the Open Government Licence and is freely available to anyone. DWI drinking water reports are public documents available on the DWI website. Water company postcode mapping data is published by each company as part of their regulatory obligations.

TapWater.uk does not claim ownership of any underlying data. We are a presentation and analysis layer on top of publicly funded, publicly available information. Our value lies in combining, normalising, and making this data accessible — not in the data itself.