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Stormwater poses beach health risk

Stormwater that potentially contains hydrocarbons and heavy metals is lingering off Perth's beaches for up to 30 days after draining into the ocean, according to a world-first study.

Modelling from the UWA Centre for Water Research has shown that stormwater stagnates near the shore for up to a month - raising the risk of an algal breakout - rather than being flushed away quickly as previously thought.

The problem is compounded by tidal flow from the Swan River, which is contaminated by run-off from hundreds of suburban drains.

Modelling, developed to measure the spread of material stirred up by the Fremantle harbour-deepening project, showed the outflow settled near the coast from the river mouth to Scarborough.

Previous studies have found that stormwater drains in Perth can contain heavy metals, general litter, nitrogen, phosphorus, sewage overflow and hydrocarbons, including oil.

UWA centre director Jorg Imberger said the new information, from sensors placed at Mindarie, Garden Island, Rottnest and in the Swan and Canning rivers, had wide implications for the health of Perth's beaches and, potentially, for swimmers.

Professor Imberger said the risk of algal blooms increased when water stagnated for 30 or 40 days and did not get flushed out.

While there was currently no danger to swimmers, he said that as Perth's population increased, beaches would be affected by potential pollution. "Because if that water hangs around there, if you have any pollution in there, say pathogens from people swimming, some sewage inflow, stormwater inflow, or some nutrients, you will get problems," he said.

Professor Imberger said the modelling was on the internet and in the future would allow beachgoers to identify what was in the water and where.

Water Department acting director for water resource management, Greg Davis, said the pollutants, and their levels, found in stormwater depended on the catchment area. "If there's something that water can carry, that it goes past, it will pick it up," he said.

Reforms being implemented by the Water Department include retaining and restoring natural drainage lines, minimising pollutant inputs via planning, minimising run-off and decreasing the volume of stormwater.

The Health Department advises people to avoid swimming near stormwater drains after significant rainfall. Its monitoring has shown coastal waters comply with Australian guidelines except when affected by one-off events such as heavy rainfall or illegal discharges.

A Department of Water study in 2007 of 65 stormwater drains found high levels of heavy metals at beaches in Scarborough, Cottesloe, Stirling and Joondalup and bacteria from faeces at levels six times higher than acceptable at Cottesloe, Rockingham, Stirling and Safety Bay.