The Water Corporation believe they accidentally killed nearly one hundred fish after discarding contaminated water into a small lake in South Perth.

From last Friday a lake on Mill Point Road was being used by
the Water Corporation as a dumping ground for excavated groundwater, dug up as
part of routine maintenance works.

But four days after works began about 80 black bream fish washed up dead and
initial studies indicate excess acid in the groundwater may have stripped
oxygen levels in the pond, the Water Corporation said.

Lake water drains into the Swan River near the Narrows Bridge and officials
fear some of the contaminated  groundwater may have also escaped into
the river before drainage channels were blocked.

Works at the site, which were expected to take about three weeks, have now been
suspended while further investigations are carried out by both the Water
Corporation and The Swan River Trust.

Prosecution under the Environmental Protection Act may be carried out, a
spokesperson from The Swan River Trust said.

A spokesperson from the Water Corporation Steven MacKenzie said this type of
thing has never happened before.

“The Water Corporation had a permit from the Swan River Trust to dispose of the
groundwater to the pond, but the impacts of the disposal were underestimated,”
he said.

“The Water Corporation had considered disposal to sewer, road drainage, the
pond and directly into the river.

“In this case, the sewer and road drainage did not have sufficient capacity to
take all of the dewatering water, and disposal to the pond and directly to the
river were considered to be environmentally acceptable disposal locations.

"As the pond was closest, it was chosen as the preferred disposal
location.''

The department of Fisheries fish health laboratory is analysing fish samples
and the Swan River Trust said it’s too soon to know what the environmental
impact on the lake and river will be.

The Water Corporation believe it is unlikely the river would be affected by the
incident as the contaminated groundwater will be massively diluted by such a
large flowing water body.

Sue Oldham from the Department of Health said people are advised not to swim in
the lake until investigations reveal the exact cause of the fish deaths.