A water firm shut a pumping station to protect an underground water source after fly-tipped metal drums were found to be leaking unknown chemicals.
Portsmouth Water said 174 containers were found dumped on Friday in Lovedean, Hampshire.
The chemicals "posed a real risk to the water supply" in Lovedean, Cowplain and Waterlooville, the firm said.
Lovedean pumping station, which extracts water from a borehole, was returned to service on Saturday.
Portsmouth Water said the incident was "by the worst of its kind we've ever seen".
Chief executive Bob Taylor said: "This is the single biggest risk we face as a business.
"I worry about the fact that these incidents are happening and that people don't seem to understand that illegal dumping of chemicals like these can lead to longer term impacts on the underground aquifers."
The pumping station was turned back on when soil samples showed little contamination, the firm said.
It said customers were at "minimal risk" because its treatment works would have detected any pollution.
The water supply to the local area was reconfigured so that customers were unaffected, it added.
The chemicals, found in paint and solvent containers, are undergoing tests.