Treatment urged to aid ailing river
The Cawthron Institute says that aerating the Maitai Reservoir is the best option to improve the declining water quality of Nelson's Maitai River, and that it could be done for $70,000 or less.
The city council's resource consents for the reservoir expire in 2017, 30 years since it built the controversial earth dam on the river's North Branch to ensure a reliable water supply for Nelson.
Last year it commissioned a series of reports from Cawthron, the independent Nelson-based science institute with an international reputation for its work on water quality.
This dovetails with growing concerns around the greater presence of deadly cyanobacteria in the river, continuing problems with sewage intrusion in the lower reaches and the formation of a new group aiming to help restore and protect the river, Friends of the Maitai.
Cawthron's scientists conclude in the reports that the effects of forestry and faecal contamination from human waste are the two biggest threats to the river.
They say the impact of the Maitai Dam is likely to be "comparatively minor" compared to the other pressures.
But their report on the dam's impact lists three "specific ecological issues" arising from the dam: higher concentrations of naturally occurring heavy metals, restricted passage for trout and native fish, and changes in water chemistry that potentially favour cyanobacteria, the cause of a number of dog deaths and repeated warnings against swimming.
A report on options for improving water quality says the reservoir could be aerated with a United States system costing $60,000-$70,000, plus installation and running costs, with the possibility of more cost-effective options within New Zealand.
Such a system would mix the layers of water in the reservoir, re-oxygenate the lower levels and alleviate the problem of micronutrients being released from the reservoir sediments in times of low oxygen, to be eventually discharged into the river.
Cyanobacteria is known to have killed dogs exposed to it in the Maitai, Waimea and Takaka rivers, and around the country. They are drawn to the pungent mats it forms when it blooms and becomes toxic, and there is concern that people, especially children, could be badly affected if they ingest any while swimming.
It has been a particular problem in the Maitai since 2009, with the city council forced to issue regular summer warnings against using the river's string of popular swimming holes dotted from within the city to near the 32-hectare reservoir, which is permanently closed to public access apart from viewing.
Faecal contamination of the lower reaches has been occurring for longer, with a warning against swimming in the stretch from the Collingwood St Bridge to the outlet in Nelson Haven in place since 2007.
Leaks in the old sewers of the Wood and city are considered to be the main cause, with bird droppings sometimes also implicated.
In 2009 Consumer magazine labelled the former Collingwood St bridge swimming hole as one of the 30 most polluted in the country.
Friends of the Maitai, formed at the end of last year, is working on practical measures to improve the river's health, such as riparian plantings and voluntary monitoring.
Spokeswoman Ami Kennedy said the group, so far mainly comprising residents of the Nile St-Cleveland Tce neighbourhood, wanted to offer help rather than criticise the council.
"This is our commons, we should all be working towards caring for it and not feeling that it is out of our hands."
The Maitai's wellbeing is also of keen interest to Fish & Game, the statutory organisation, funded by licence-holders, which represents anglers and hunters and provides co-ordination of the management, enhancement and maintenance of sports fish and game.
Regional manager Neil Deans said the river was "a treasure", an urban stream that until 20 years ago had traditionally been abundant with small trout.
"We may never be certain but there's a coincidence in time between the operation of the dam and the demise of the fishery - I have to confess to being somewhat suspicious," Mr Deans said.
The reservoir is filled from the Maitai's tannin-stained North
Branch and is mainly used to supply a consistent minimum flow to the river downstream of the dam.
Most of the city's water is drawn from the river's South Branch just upstream of the dam, and piped to the council's water treatment plant.
When the river floods and is discoloured, the city temporarily switches to water from the reservoir, which is also used to top up the system during peak demand.
Nelson City Council communications manager Angela Ricker said trials were under way on options for varying the source of dam water intake to improve water quality in the reservoir and river.
"Mechanical aeration may be further investigated depending upon the results achieved from the variations to operating processes currently being trialled."
She said the council had a programme to track stormwater drains for cross-connections to the sewers.
"Those found so far have been fixed but we suspect there are more, so that work is ongoing."
There was continued concern about the contamination in the Collingwood St Bridge area, Ms Ricker said.
- The Nelson Mail
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