Camelford water pollution incident
The Camelford water pollution incident involved the accidental contamination of the drinking water supply to the town of Camelford, Cornwall, in July 1988. Twenty tonnes of aluminium sulphate was inadvertently added to the water supply, raising the concentration to 3,000 times the permissible level.
Source: Wikipedia — Camelford water pollution incident (CC BY-SA 4.0)