Relief driver John Stephens said he let himself into the Lowermoor treatment works, near Camelford, north Cornwall, in 1998, with a key he had been given after finding no staff there. He then spent 20 minutes looking for the right tank.
When no one from the South West Water Authority, which ran the works, turned up to assist him he opened the manhole cover for what he thought was the liquid aluminium tank and emptied his load.
It affected the drinking water of about 20,000 homes in the area, causing rashes, diarrhoea, and mouth ulcers.
The inquiry at Shire Hall, Taunton, is being held into the death of Carole Cross, who died 16 years after the incident. A coroner found she had "abnormally high levels of aluminium" in her brain after death.