A driver involved in the UK's worst mass water poisoning has told an inquest he had to guess which tank to put 20 tonnes of aluminium sulphate in.
It affected the drinking water of about 20,000 homes in Camelford, Cornwall, causing rashes, diarrhoea, mouth ulcers and other health problems.
Driver John Stephens gave evidence to the inquest of Carole Cross, 59, who died 16 years after the poisoning.
Large amounts of aluminium were found in her brain after her death in 2004.
Mrs Cross and her husband Doug moved from Camelford to Dulverton in Somerset two years after the poisoning.
The original inquest into her death was adjourned two years ago, when the coroner asked for more tests to be carried out.
When it resumed in Taunton earlier, relief driver Mr Stephens said the Lowermoor water works were unattended when he arrived with his delivery in 1988.
He told the inquest he had let himself into the works with a key given to him by the regular driver Barry Davey.
Unknown to Mr Stephens the former South West Water Authority, which ran the works, used the same key at all its plants.
Mr Stephens said he had believed the key would let him into the site and open one tank.
He told the inquest, with no-one to show him, he spent about 20 minutes looking for the correct tank, before seeing a manhole cover he thought was the right one.
With no phone available to ring anyone, Mr Stephens said he emptied the aluminium sulphate into the tank.
"You saw some liquid and what did you think that was," West Somerset Coroner Michael Rose asked.
"Liquid aluminium," the driver, from Bristol, replied.
Aluminium sulphate is used to remove solid particles from cloudy water.
Four days later, when he was told there was a problem, Mr Stephens returned to Lowermoor and pointed out which tank he had emptied his delivery into.
The inquest was also read a statement from Barry Atkinson, who was the South West Water Authority's regional water treatment scientist in 1988 but has since died.
In it he said: "Had we known the true cause at the time we would have had to shut down the reservoir and done all sorts of things.
"It was unthinkable to assume something like that had happened.
"We told the consumers the water would not do them any harm. I'm not sure that was thought through enough."
After the water poisoning those who drank or bathed in it reported suffering from stomach cramps, skin rashes, diarrhoea, mouth ulcers and aching joints.
In 1991 the South West Water Authority was fined £10,000 with £25,000 costs for supplying water likely to endanger public health.
But an independent inquiry report, published in January 2005, said it was unlikely the chemicals would have caused any persistent or delayed health effects.
In 2005, the coroner Mr Rose said Mrs Cross had had a neurological disease "usually associated with Alzheimer's", but complex DNA tests had ruled out any genetic origin for the disease.
He said the "abnormally high level of aluminium" in her brain could have caused the disease and adjourned the inquest for further medical research.
The inquest, which is scheduled to last for two weeks, is due to hear from former South West Water Authority staff, including district manager John Lewis who was the only staff member to face disciplinary action over the poisoning.
Other witnesses include workers who dealt with the contamination at Lowermoor, medical and environmental health officers and scientists.
Mrs Cross's husband, a scientist and long-term Lowermoor campaigner, who now lives in Lowick Bridge, Cumbria, will also give evidence.