Data FAQ's

How do you know which area I'm in?

When you enter your postcode, we look it up using Postcodes.io, a free, open service built on official Office for National Statistics (ONS) data. It tells us which parliamentary constituency you live in, using the new boundaries from the 2024 general election. Every constituency has an official government code, called a GSS code, and we use that code to take you to the right page. We use the same codes to match each area to its sewage spill data, so the figures you see always belong to the right place. In Wales, we take you to your Senedd constituency instead, because each one covers two Westminster seats and elects six Members of the Senedd. Some areas record too little sewage data to give a fair local picture. If a constituency had fewer than 501 hours of sewage spills in the year, we show figures for the wider county instead, so the numbers reflect what's happening in the waters around you. We don't keep your postcode: it's only used to find your area and to fill in the email to your MP.

How accurate is the postcode/constituency data?

There will be pockets of anomalies so treat it as a strong guide rather than an exact match.

Matching a CSO to a postcode or constituency isn't an exact science. Some CSOs are offshore, so they don't technically sit within any postcode or constituency boundary. To match these, we expanded the search radius around the CSO until we found the nearest match — which means some matches will be approximate, especially in these offshore or edge cases.

What is the data area you are showing me?

When you enter your postcode, the system works out which constituency you're in and returns the results for the entire constituency not just your postcode.

Where does the data come from?

CSO data comes from the National Storm Overflow Hub (NSOH), the official platform for surfacing CSO data. We matched each CSO to a postcode and constituency using postcodes.io, based on the latitude and longitude reported by the NSOH.

Welsh Water CSO data comes directly from Welsh Water by agreement.

Is the underlying overflow data itself verified?

The NSOH data is near real-time rather than fully audited. Water companies are required to publish discharge data within an hour of it happening, but this hasn't been through the same verification process as the Environment Agency's official annual EDM return.

We aren’t using published EDM data because it’s not available for 2026 yet, only for 2025.