Sewage Spills Across South London & Surrey: The 2025 Data Map
How often did storm overflows discharge sewage into the rivers and brooks of our region last year? We mapped the official Environment Agency figures for every monitored overflow across seven constituencies in our service area.
Every one of the monitored storm overflows below is operated by Thames Water, which the Environment Agency rated the worst-performing water company in England in its 2024 assessment — the company discharged raw sewage for 298,081 hours nationally that year, up 51% on 2023. The 2025 figures below are the most recent full-year data, released in 2026.
Sewage spills by area — 2025
Ranked by total discharge duration. Bars show hours relative to the worst-affected area.
| Area (constituency) | Spills | Hours | Worst overflow site |
|---|---|---|---|
| Epsom & Ewell | 55 | 801 | Leatherhead WWTW → River Mole |
| Reigate & Redhill | 61 | 556 | Earlswood Brook (Earlswood WWTW) |
| Mitcham & Morden | 125 | 204 | Streatham & Balham Storm Relief → Graveney |
| Carshalton & Wallington | 5 | 13 | Beddington Effluent Carrier → River Wandle |
| Croydon East | 4 | 4 | South Norwood CSO → Chaffinch Brook |
| Croydon West | 0 | 0 | Canterbury Rd, Thornton Heath (monitored, no spills) |
| Sutton & Cheam | 0 | 0 | No monitored overflows in constituency |
Figures are counts of discrete spill events and total hours of discharge from Environment Agency Event Duration Monitoring (EDM), calendar year 2025.
The named hotspots
The individual storm overflows responsible for the most discharge in our region during 2025.
Why this matters for your own drains
Storm overflows spill when the sewer network is overwhelmed — most often during heavy rain, when rainwater and wastewater surge through pipes faster than they can be treated. The same pressures that push the public network past its limit also show up inside private drains: the River Mole and Hogsmill sit on ageing clay and chalk-stream catchments where tree-root ingress and collapsed joints are common, and blockages of fat, grease and wet wipes build up fastest in exactly the wet conditions that trigger overflows.
Your drains — the pipes inside your property boundary — are your responsibility, not the water company's, up to the point they meet the public sewer. If your drains gurgle, drain slowly or back up after heavy rain, it usually means a blockage is forming on your side of that boundary. A quick CCTV drain survey shows exactly what's happening before it becomes a flood.
Sources
- Environment Agency — Storm overflow (EDM) spill data, gov.uk
- Top of the Poops — constituency sewage data, 2025 (maps the Environment Agency EDM dataset)
- Thames Water — Storm discharge and flow data
- Environment Agency — Thames Water EPA data report 2024, gov.uk
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