A rubber chemical factory has been ordered to pay £16,800 in fines and costs after pleading guilty to two pollution charges.
Flexsys have to pay nearly £17,000
Almost a quarter of a million gallons of industrial effluent poured into the River Dee in north Wales following the incident in September last year.
It was reported at the time that a number of fish were killed and extraction points for household water were closed as a precaution.
At Wrexham magistrates on Thursday, Flexsys Rubber Chemical plant was fined £5,000 on each charge and ordered to pay £6,800 in costs.
At the time of the incident Cefn Mawr firm Flexsys admitted diluted waste liquid from a storage tank had leaked from the site into the River Dee, via Tref-y-Nant Brook, a tributary of the Dee at 1420BST on 22 September 2002.
"This was a major incident in a very sensitive location," said an Environment Agency spokesman.
"To this end we are working with the company to ensure these types of problems do not give rise to risks elsewhere on the site."
The River Dee is one of the most closely monitored waterways in Europe.
It is part of a network of lakes and rivers that supplies water to more than two million people in north Wales and along the border with England.
It is also an extremely important wildlife habitat for salmon and other species.
Flexys is not the only company to have polluted the River Dee.
In recent years the river has suffered a number of other incidents, including one in July 2000 when 100,000 fish suffocated, but the source of that leak was never traced.