European Gas Turbines, Lincoln's biggest employer, was also ordered to pay pounds 2,750 costs.
Peter Carter, for the prosecution, told Grimsby Crown Court that the company's discharge into Lincoln's Pike Drain had a 'catastrophic' effect on wildlife, killing fish, eels and other creatures.
Tests showed 146 milligrams per litre of chromium was discharged from the plant through a cleaning process, the safe level being 0.25 milligrams, he said. Deposits of the manufacturing detergent Ardrox were also found after an unsuspecting employee poured it down a drain. Between 6.5 and 7.5 milligrammes per litre was discharged, compared with the safe limit of 0.4 milligrammes.
Judge Richard Jenkins said that the company could not be blamed for human error, but added that the matter was of 'considerable public concern'.