Suburban Canberrans are dumping chemicals down stormwater drains and feeding Lake Tuggeranong's blue-green algae blooms, which could already take decades to diminish, new data suggests.
Environment Minister Mick Gentleman said two University of Canberra reports, released on Wednesday, showed wetlands were "not enough" to solve the lake's potentially toxic plague.
Instead, the government was committed to tracking down the source of "excessive nutrient loads" coming into the lake. Nutrients feed algal blooms, along with light, heat, and the right water conditions.
The reports' lead authors, Dr Fiona Dyer and Dr Rod Ubrihien, ran preliminary tests as part of their government-funded Lake Tuggeranong project to determine where the most nutrients were coming from.
"What we're seeing is there's not very much coming from the farmland, so it is coming from the suburbs of Canberra," Dr Dyer said.
While Village Creek was the main contributor of nutrients overall, the university team detected very high concentrations of nutrients in Tuggeranong Creek, directly below Calwell Shopping Centre.
Despite the expectation that nutrient concentrations would always be higher when it's raining, data showed levels in dry conditions sometimes outweighed levels during rainfall.
"[That] suggests that sometimes there's some contamination events happening and we don't know what they are," Dr Dyer said.
"It's like somebody tipping some fertiliser into the creek, or washing something into the creek."
As well as gardening products, high nutrient loads could be coming into the lake through bathroom products, cleaning products, and leaves and grass clippings, Dr Dyer said.
The experts were surprised by the amount of dissolved phosphorus lingering in the surface of the lake, rather than sitting in its sediment at the bottom. Phosphorous was the key nutrient determining algae's abundance.
Normally, researchers would expect about 90 per cent of the mineral to be attached to clay when it rains, but in Lake Tuggeranong's case, it was only about 50 per cent.
"The critical part for algae is that [it] can easily access [phosphorus] if it's dissolved; it's like applying liquid fertiliser," Dr Dyer said.
Because there was such a massive amount of dissolved phosphorus moving around in Lake Tuggeranong, the ACT government couldn't get away with just treating the sediment where it was stored, Dr Dyer said.
Instead, it would have to invest in stopping nutrients from flowing in from urban Canberra for at least 10 or 15 years. The university had government funding to look more at the sources of nutrients until June 2020.
A solution that killed the algae in the lake would aid that effort but finding one should be approached with caution, Dr Dyer said. The university team recently tried several products, only for the most promising to "knock back" the blue-green algae for a few days.
Dr Ubrihien said when "algaecides" destroyed blue-green algae cells, they could release the dangerous toxins within them, including neurotoxins that were destructive to nerve tissue.
He noted one review found it could take up to 10 years to understand a lake's nutrient processes, and between five and 20 years for the lake to respond to a solution.
"If you were to treat an algal bloom and release a whole heap of toxins into the water column and then say, 'I'm going to go open the lake', and you didn't know those toxins were gone, it's super high risk," Dr Ubrihien said.
Minister Gentleman said the government would continue to work with the university to mitigate high nutrient flows into Lake Tuggeranong. It would also fund additional projects.
"I'm committed to addressing algal blooms so Lake Tuggeranong can be used year-round," he said.
The University of Canberra's project was funded as part of the joint ACT-Australian government Healthy Waterways program.