Deep-water lake tests to try to find a new source of drinking water for Wanaka residents that is free of problematic invasive algae have been unsuccessful.
Queenstown Lakes District Council water contractors sampled water at a depth of about 40m in Lake Wanaka recently in a bid to source an uncontaminated water intake.
The council's 3 Waters manager, Gerry Essenberg, said sample tests at two intake sites near the eastern and western shores of the lake had returned results showing algae were still present at that depth.
The results were puzzling, Mr Essenberg said.
Algae should not be present in the water at the "critical" depth of about 40m, because sunlight - which is required by most typical algal blooms to survive - could not penetrate to such a depth, he said.
"At 40m in the lake - under winter conditions - there was no difference in the amounts of algae found, compared with the existing town water supply intakes," Mr Essenberg said.
Sunlight "usually" only penetrated water to a depth of about 30m, he said.
He speculated the algae found might be sinking, or dying in the water.
More tests would be carried out because the results "may" improve during summer conditions in the lake, he said.
"We've identified we won't have a solution to this problem before summer and are preparing information brochures for Wanaka households which will continue to be affected by the algal presence," Mr Essenberg said.
The mysterious Lake Wanaka algae were first noted in 2003 and their increasing presence continues to frustrate affected residents.
The invasive algae block and clog filter systems, affecting drinking supplies and water pressure, particularly in the town's older homes.
Council contractors have tested water quality at alternative sites to the town's existing intakes at Bremner Bay and near Rippon Vineyard in a bid to find an algae-free source.
There might be a scientist "out there" with an answer to how the algae had come to be present in Lake Wanaka at such depths, "or even in the first place at all", Mr Essenberg said.
He speculated that climate change might have affected or led to an increase in nutrients, which could have given rise to an upsurge in algal blooms.
Nutrients from nitrogen run-off into Lake Wanaka from fertilisers could also have had that effect.
