Sewage Spills

Water companies

Dwr Cymru Welsh Water’s figures here are only the overflows listed in the Environment Agency’s return for England.

How these figures are made

England: every storm overflow in the Environment Agency’s 2025 annual return (14,303 overflows). Each one is placed in its council area and Westminster constituency from the grid reference of its outlet, using Office for National Statistics boundaries (July 2024 constituencies, May 2026 council areas). 921 outlets lie below the high-tide line and are placed using the boundaries that extend out to sea. 279 lie further out and are counted with the nearest council area and constituency within 15 km; two of these were checked by hand and placed by the location of their treatment works instead (Ryde, Isle of Wight, and Hayle, Cornwall), because the nearest shore was a different area. 1 overflow could not be placed because its published grid reference is not valid (FILEY WASTE WATER TREATMENT WORKS); it is included in the national and company totals but not in any area. 1 overflow in the English return sits just over the border in Scotland (KERSHOPEFOOT VILLAGE STW); it is counted in the Scottish tables with Scottish Water’s overflows. 1 overflow in the English return sits in Wales; it is in the company totals but not in any table until Welsh figures are added.

Hours are the total time all spills lasted. “Hours per overflow” and “Spilled” use only overflows with figures for the year, so an overflow with no monitoring data is not counted as one that did not spill. Spills are counted the Environment Agency’s way: any discharge in the first 12 hours counts as one spill, and each further 24-hour block counts as another. 64 English overflows have no figures for 2025, so some totals may be lower than the true amount.

Scotland: Scottish Water’s overflow event data for 2025, 1,556 overflows, 1,287 of them with figures (the others had no monitoring data for the year). Each one is placed from its coordinates using the same boundaries as England. Scottish Water counts each separate spill, not the Environment Agency’s 12 and 24-hour blocks, and for some overflows publishes only spills longer than 15 minutes 30 seconds, so Scotland is shown in its own tables, not ranked against England. Volume is published only for overflows that report to the Scottish Environment Protection Agency (SEPA): 22,662,601 cubic metres from 49 overflows in 2025. Scottish Water’s figures for earlier years cover far fewer overflows, so the tables use 2025.

Northern Ireland: NI Water’s Event Duration Monitor data, with the council area and constituency as NI Water gives them. In 2025, 476 of 500 overflows reported figures; the rest are counted as overflows but add nothing to the totals. NI Water’s figures are recorded differently from England’s and Scotland’s, so each nation is shown in its own tables, not ranked together.

Wales is not included yet. Natural Resources Wales and Dwr Cymru Welsh Water publish their own figures, which Open Govt is working to add.

Sources: Environment Agency, Event Duration Monitoring storm overflow annual return 2025, and Office for National Statistics boundaries, both licensed under the Open Government Licence v3.0 (contains OS data © Crown copyright and database right 2026). Scottish Water, Reported Overflow Event Data to SEPA and Non-Reported Overflow Event Data (2026), licensed under the Open Government Licence. NI Water, Event Duration Monitor data, as published by NI Water.