A month ago the Perth Seawater Desalination Plant in outer suburban Kwinana had to cut its production (from 130 million litres of drinking water a day to just over 21 million litres) for about a week because low levels of oxygen were measured in Cockburn Sound .
The saltwater outlet of the plant is located in Cockburn Sound, a sheltered bay with marine life feeding grounds in seagrass meadows which are under ecological stress from industry on the Sound . The outlet was not in the open ocean where water movement would disperse the hypersaline water faster, because the cost of a pipeline that far was considered too expensive. Two detailed field experiments were conducted under calm conditions (when oxygen levels are lowest) by the UWA Centre for Water Research and found that the hypersaline outlet is not effecting the environment of Cockburn Sound.
The combination of experimental and numerical modelling results demonstrate that the saline discharge is diluted to such an extent by the action of the diffuser and natural environmental mixing processes that the Perth Seawater Desalination Plant has no measurable impact on the oxygen levels in Cockburn Sound .
The plant’s operating conditions specify that oxygen monitors in Cockburn Sound measure the impact of the plant’s hypersaline discharge on the health of marine life. There were initially three monitors, but one was removed in January 2007 and the Water Corporation wants the other two removed and oxygen monitoring to become part of the Cockburn Sound Management Council’s regular program. The Water Corporation has offered the council a one-off $10,000 donation to do this. This proposal is currently being considered by the Environmental Protection Authority .
If the EPA agrees to this, the current licensing condition that production automatically decrease when oxygen levels fall below a certain level would cease and the plant could ignore oxygen levels. I wonder why they suggested Cockburn Sound Management Council monitor oxygen levels, but I guess they have to seem to be doing something and $10,000 is less than they lose when production decreases.
The light winds of autumn cause oxygen levels to drop naturally. In 2005 when the Plant was in the planning stage the Water Corporation said,
Routine maintenance activities that require production to be reduced will be scheduled for autumn when dissolved oxygen levels are likely to be at their lowest due to naturally prevailing calm conditions. 
Obviously they’ve got sick of routine maintenance in autumn and the Water Corporation’s environment manager David Luketina now says that because the Desalination Plant is not to blame for the drop, production should not be curtailed. He said,
It’s [decreasing production is] just an unnecessary risk now. There is no point having that risk to production if there is no environmental protection to be gained. 
A University of NSW study undertaken in the planning stages found that the operation of the Plant will increase the salinity in Cockburn Sound by up to 1% . The Water Corporation believes,
There will not be salt ‘build-up’ in Cockburn Sound and there will not be any adverse ecological effects associated with salinity. 
Phillip Jennings from the Conservation Council of WA says more research is needed . Continuing research for the three year period that was initially planned  will ensure the increase in salinity of 1% really does have no adverse ecological effects . The Water Corporation wants to end research and any conditions on operation after only 18 months of operation.
The Kwinana plant produces 45 gigalitres of drinking quality water per year which provides 17% of Perth’s water needs . It is thought that decreasing of rainfall could mean dams would stop supplying drinking water to Perth within seven years , but up to 60% of Perth’s water is supplied from groundwater, also recharged through rainfall and steadily declining .
The Public Environment Review for the second proposed Southern Seawater Desalination Plant in Binningup is currently open for public comment. Go to the Water Corporation website to read the Review and make a submission.
- Banks, Amanda (2008) Pollution worries spark desal cuts The West Australian, 18 April 2008.
- Craig & Wild-Ellen (2005) Peer Review of Studies for Desalination Plant Discharge, Cockburn Sound. CSIRO Marine Research.
- Antenucci, Jason (2007) The impact of the Perth Seawater Desalination Plant discharge on Cockburn Sound. Perth: UWA Centre for Water Research.
- Water Corporation (2005) The Perth Seawater Desalination Plant, April 2005.
- Van Senden & Miller (2005) Stratification and Dissolved Oxygen Issues in Cockburn Sound Pertaining to Discharge of Brine from Desalination. Sydney: University of New South Wales.
- ABC News (2008) Pollution fears force output cut in desal plant, 18 April 2008.
- Spigel, Davies-Colley, Spooner & Cooke (2005) Peer Review of Studies for Desalination Plant Discharge, Cockburn Sound. Brisbane: NIWA.
- WA Dept of Water (2008) Gnangara Mound: a unique water resource.