Sydneysiders are being urged to exercise caution when returning to swim in the city’s beaches and waterways after weeks of torrential rain and flooding.
Sediment and debris swept downstream by heavy rain turned Sydney Harbour brown last week, with swimmers warned to avoid the water.
With a few sunny days forecast to provide some respite in the coming week, how can you tell whether it’s safe to return for a dip?
Wait after rain
The New South Wales government’s Beachwatch program advises people avoid swimming at ocean beaches for one day after heavy rainfall, or for as long as storm water is present. For more sheltered harbour areas and bays, the suggested wait time stretches to three days.
The most obvious signs of storm water and flood water pollution to look for are discolouration of the water, and debris both floating in the water and washing up on the tide line.
A spokesperson for Beachwatch said flood waters were still flowing into the ocean from the Hawkesbury River, Sydney Harbour and Botany Bay, “turning the ocean brown from the Central Coast to Wollongong and up to 8km out to sea”.
Floodwaters are still flowing into the ocean from the Hawkesbury River, Sydney Harbour and Botany Bay turning the ocean brown. Signs of pollution seen while collecting water samples from Sydney Harbour today. pic.twitter.com/iW7K8DkN4I
— Beachwatch NSW (@BeachwatchNSW) March 17, 2022
Forecasts issued on Friday advised of possible pollution at Sydney’s ocean and harbour beaches. Daily forecasts are available on the Beachwatch website.
Prof Stuart Khan, a water quality expert and the director of the graduate school of engineering at the University of NSW, said the Beachwatch guidance was based on “more than a decade of water quality monitoring and understanding of how the beaches are impacted by rainfall”.
Swimming in polluted water is a health risk due to the pathogens from raw sewage, Khan said. “When it rains in big cities like Sydney, water gets into our sewers, because the sewers are not sealed.”
The sewers are designed to overflow in cases of extreme rain, running into storm water pipes that discharge directly into open waterways, instead of being directed to sewage treatment plants.
“Therefore you have all of the pathogenic microorganisms that you would associate with raw sewage: bacteria, viruses as well, and protozoan organisms like cryptosporidium and giardia,” Khan said. Exposure to these microorganisms can result in gastroenteritis, and ear and skin infections.
“The threat with those sorts of organisms is short term because most of them don’t survive well in the environment,” Khan said.
Given how extreme the recent rainfall has been, he suggests exercising caution and waiting for even longer than the guidelines recommend.
“For something like this where the whole system is flushed, I think you’d want to be pretty careful and leave a bit of extra time,” agreed Phillip Birtles, the integrated water cycle manager for Sydney Water.
Other swimming hazards
“Even as microbial levels return to normal there still may be a large amount of debris or other hazards, such as murky water, which should be considered,” said a Beachwatch spokesperson.
Experts have previously warned beachgoers to avoid swimming in murky water due the risk of shark encounters following weeks of heavy rain.
Earlier this month, the NSW Department of Primary Industries said turbid water “can make conditions ideal for ambush predators, like sharks, to hunt in”.
“Run-off and nutrients from the land run into our waterways and can attract fish and other animals to feed – this can then attract sharks,” a DPI spokesperson said.
How does heavy rain affect marine life?
Storm water can be surprisingly detrimental to both freshwater and saltwater environments, Khan said.
During heavy rainfall, organic carbon matter – such as leaves and sticks – is transported through storm water systems into waterways. Once there, bacteria biodegrade these materials, consuming oxygen in the process.
“When there are low oxygen concentrations in water, that’s when … fish suffocate,” Khan said. “There have been a number of reports of significant fish kills around NSW following the floods we had a couple of weeks ago.”
An increase in dissolved carbon compounds can also darken the water, leading to “blackwater events”.
The good news, says Birtles, is that water quality in the tributaries of Sydney Harbour has been improving since the 1970s – the result of stricter regulations and a reduction in industry along the foreshore.
“We’ve got a whole lot of mangroves and oysters coming back,” he said. “In general, the biodiversity is improving and we’re seeing dolphins and whales and seals moving back into the harbour.”
Modelling shows that as the city’s population density increases, “we could go backwards”, Birtles said. “As we get more development occurring in the catchment, more hard surfaces, there is risk that that will deliver more pollutants more efficiently to the waterway.”
He emphasises the need for well-designed storm water systems, and says Sydney Water is working with local councils to open new swimming sites along the Parramatta River.
