Anglian Water spilled 3,500 tonnes of sewage into the Mardyke; CEO lost £1.05m bonus

On August 25, 2025, almost 3,500 tonnes of untreated sewage — about one and a half Olympic pools — entered the River Mardyke in Grays, Essex, killing at least 4,723 fish across 11 species, including the critically endangered European eel. The Environment Agency rated it a Category 1 incident, its highest classification. Sewage discharged at an average 55 litres per second for at least 18 hours; suspended solids read more than five times the permitted limit at one point, and ecological effects were visible some 5km downstream.

The cause was operational. A valve controlling sewage flow shut automatically when it should not have, triggering an alarm at the Corran Way pumping station in Stifford, Thurrock. The alarm was ignored: investigators found an oversight meant operators had been instructed to disregard alerts from that site.

The BBC revealed the consequence for the boardroom: Anglian Water confirmed chief executive Mark Thurston's £1.05m bonus was withheld for 2025-26 under the Water (Special Measures) Act 2025, which lets Ofwat ban bonuses for water bosses after environmental failures. He was the second consecutive Anglian CEO to lose a bonus over pollution — the previous one was denied after 900 fish died at King's Dyke, near Peterborough, in September 2024, an incident the Environment Agency is prosecuting.

The company pointed out that Thurston still received a £500,000 retention payment and a £451,440 Medium Term Alignment Plan payment the same year, which it said were not linked to performance, were permitted under the regulations and were funded by shareholders, not customers. Campaign group River Action called that 'a worrying pattern' of rising base pay as bonus restrictions tighten.

CEO Mark Thurston's £1.05m 2025-26 performance bonus withheld; at least 4,723 fish killed across 11 species.

Operators had been instructed to disregard alarms from the Corran Way pumping station, so nobody responded when the valve failed.

A valve controlling sewage flow shut automatically when it should not have, diverting untreated sewage into the river.

The discharge ran for at least 18 hours before action reduced its impact, long enough to kill thousands of fish.

Under the Water (Special Measures) Act 2025, a Category 1 pollution incident now strips performance bonuses from water company bosses.

Alarm fatigue is a design choice: telling operators to ignore a site's alerts turns one stuck valve into 18 hours of undetected pollution.

The Environment Agency's investigation into the Mardyke spill is ongoing; it said it had ensured Anglian Water took swift action to reduce the impact. Anglian said no performance bonus was paid to its CEO in 2025-26. Ofwat noted companies were barred from paying roughly £4m of potential bonuses in the last year alone; the government published a white paper in January 2026 setting out plans to overhaul the water system in England and Wales.