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Polluted by sewage 3,948 hours in 2025 , monitoring offline 2,005 hours
What's a potential overflow? If the CSO goes 'Offline' while its Overflowing, we don't know if it stopped overflowing until we get a 'Stop' event.
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Donate Now| Company | Site Name | Receiving Waterway | Overflow Days | Total Duration (hours) | Offline/Potential Pollution |
|---|---|---|---|---|---|
| Thames Water | Markyate STW | River Ver | 153 | 3,522h 30m | - |
| Thames Water | Berkhamsted STW | Grand Union Canal | 32 | 202h 15m | - |
| Thames Water | Wheathampstead SPS | River Lee | 14 | 68h 44m | 1,758 h |
| Thames Water | Caddington STW | Gw Via Boreholes | 8 | 60h 15m | - |
| Thames Water | Harpenden STW | Lee | 6 | 58h 45m | - |
| Thames Water | Wheathampstead SPS | River Lee | 6 | 15h 18m | 247 h |
| Thames Water | Harpenden STW | Lee | 2 | 10h | - |
| Thames Water | Berkhamsted STW | Grand Union Canal | 1 | 6h | - |
| Thames Water | Markyate STW | River Ver | 1 | 3h 45m | - |
| Thames Water | Watery Lane (Marsworth) SPS | Tring Bourne | 1 | 0h 30m | - |
| Thames Water | Tring STW | Into Guc/Reservoir | 0 | 0h | - |
| Thames Water | Caddington STW | Gw Via Boreholes | 0 | 0h | - |
EDM (Event Duration Monitoring) data and Consented Discharges to Controlled Waters with Conditions from Environment Agency & Data Map Wales, and Scottish Water
Experimental derived data from Thames Water API, Some data may be missing or inaccurate. Live sewage data from Stream Water API
Annual figures up to and including 2025 come from official water company returns. The 2026 figure is our own estimate, derived continuously from the companies' live data feeds.
Uses Environment Agency rainfall data from the real-time data API (Beta) - Rainfall data is delayed by up to two days.
This website is intended to provide an accurate representation of the Environment Agency data. The data is hard to use, and thus some errors may have been made. If you find something that is incorrect, please raise an issue at the GitHub issues page and we'll endeavour to fix it quickly.
This project is not associated, approved or endorsed by any of the companies whose logos you see here.
Get in touch if you have queries or want to discuss our data.
press [at] top-of-the-poops.org