Run-off not causing lake bloom, say experts

Expert advice has dismissed suggestions that stormwater run-off from recent residential subdivisions on St John's Hill is the root cause of algal bloom which continues to foul Virginia Lake.
That opinion was outlined in a report presented to the Wanganui District Council meeting on Monday.
Julian Reweti, council's infrastructure manager, said National Institute of Water Atmospheric Research (NIWA) reports prepared in 2004 rejected the ascertain that stormwater runoff from the expanding subdivisions above the lake was the major cause of algal bloom.
There is a holding pond in Rotokawau subdivision that takes excess stormwater that eventually discharges into the lake but the last discharge was in the July 2006 storm.
The scientific research showed that levels of phosphorous and nitrogen ? key ingredients in fostering algal growth ? were considerably less in Rotokawau subdivision than in Virginia Lake itself. And it went on to say that most nutrients find their way into the lake through natural ground water seepage.
The report concluded that long-term farming practice in the area was a major contributor to this accumulation of nutrients in the ground that could seep into the lake.
Mr Reweti said one of the major problems was that water in Virginia Lake takes abouty 16 years to recycle. Niwa had suggested introducing more stormwater was a way of accelerating this flushing process.
Scientists now say high winds in December stirred the lake's surface layer and brought phosphorous in lake sediment to the surface and calm weather and warm temperatures since then have been ideal for algal growth.
Cr Sue Westwood said council needed to be assured that the new subdivision being prepared on the other side of Virginia Rd was "self-contained" and stormwater overflow would not be directed into the Rotokawau holding pond and ultimately into Virginia Lake.
Mayor Michael Laws said there had been "much ill-informed comment".
Mr Laws said while stormwater runoff was being blamed by some, the scientific data suggested just the opposite.
"The question is how is council going to find a short-term solution and what are our long-term options. It's going to take considerable effort by council to resolve it."
Council CEO David Warburton said there were three options that council officers had looked at ? aerating the lake "but to do that properly is a big issue"; flush the nutrients out "but that's difficult because the top of the lake is warm"; or use flocculants (which bind the phosphorous in lake sediment "but that needs resource consents and some long-term regular use and we're working with Horizons Regional Council about this option".
"We've don some trials and it looks like it (flocculation) will work very effectively," Dr Warburton said.
NIWA was going to carry out national studies over winter which may provides methods of containing the bloom in future.
Meanwhile, the algal bloom which has turned Virginia Lake a deep green colour shows little sign of abating and the latest water tests show the "nasties" in the lake have increased as well.
Testing by Horizons water quality staff a week ago showed counts of the bacteria E. coli had risen to 2900 parts per 100 millilitres.
According to Barry Gilliland, water quality leader at Horizons Regional Council in Palmerston North, said this latest E. coli sampling was "significantly higher" than the swimming guideline of 550 per 100 millilitres.
"Until this latest reading all we had seen was a trend downwards," Mr Gilliland said. "It seems to be the most likely source is probably dependent on the amount of water foul that were in the vicinity a few minutes before the tests were taken," he said.
Mr Gilliland said the sample contained mostly the blue-green algae Anabaena, and this was a shift from previous samples where two types of blue-green algae have been observed.
"Whether this is an indication that things might be starting to change in the lake only time will tell," he told the Chronicle.