Maitai’s OK, but needs aeration
In the fourth part of a series, Bill Moore learns that while Nelson’s Maitai River doesn’t look the best, it’s not in as bad a shape as might be supposed.
No dam is ever built without having an effect on the river it interrupts. But the changes can be minimised, says Cawthron Institute scientist David Kelly.
Dr Kelly, a Canadian immigrant who earned his PhD at the University of Alberta, specialises in lakes and their effects on rivers. For two years he’s been studying the river and the 32-hectare reservoir at the head of Nelson’s Maitai Valley and co-authoring reports for the Nelson City Council as it prepares to seek renewal of its water supply resource consents in 2017.
Most of the city’s water is drawn from the Maitai’s South Branch. The reservoir, filled from the North Branch, is used to keep the river topped up so that it doesn’t fall below a consented minimum flow and fish life is kept safe.
When the river is dirtied by floods, the reservoir is used to feed the water treatment plant.
It’s a scheme that works well for the city, guaranteeing a steady water supply even in midsummer, but it comes at a cost to the health of the Maitai.
That’s plain to see when the South Branch is viewed above and below the point about 800 metres from the dam where the water is extracted for the scheme and replaced by tannin-stained reservoir water.
Upstream it’s a clear river with little algal growth on the rocks – pretty much fitting the ideal of a mountain stream, its bush-lined banks adding to the picture. Downstream the water has a yellow-brown tinge and the rocks are coated in the dark-brown slime of heavy algal growth.
Dr Kelly says there’s an unmistakable correlation between the reservoir water and the degradation of the riverbed – but that for several kilometres downstream from the dam the Maitai is actually in remarkably good health, with lots of macroinvertebrates – aquatic insects and other stream life essential to the food chain.
‘‘There are things going on that are caused by the reservoir, but in terms of the macroinvertebrate community I would rate it in the upper 25th percentile and certainly in terms of rivers that are in urban areas, I would say it’s probably higher than that,’’ he says.
Another indicator of river health is the percentage of caddis flies, mayflies and stoneflies in the insect mix. Typically in degraded streams it’s under 50 per cent, and above that ‘‘you’re starting to get into the ‘good range’ for macroinvertebrates. I think the Maitai is up around 60 or 70’’.
Dr Kelly’s methods include fish surveys carried out with ‘‘electric fishing’’ to stun fish for counting and by spotlight to more easily count secretive native fish, sediment core sampling and taking water samples at various depths to study the levels of dissolved oxygen, nutrients and heavy metals.
With other Cawthron scientists he has found juvenile eels and native koaro – good climbers – scaling partway up the dam spillway and is attempting to establish if they can reach the top and get into the reservoir. A fish passage isn’t part of the dam design so far.
He hasn’t got a final conclusion on what causes the marked tannin staining in the reservoir water. Pictures from the 1980s during the construction show that the bottom was largely if not totally cleared of vegetation before it was flooded. The staining is most likely to result from the decomposition of plant matter washed into the reservoir during floods, he says, but is also present in the water from the North Branch.
‘‘When you put a big storage basin in, when it comes up in flood, a lot of material that’s in the catchment comes out. Normally that would just flush through to the sea and that would be the end of it. But when you’ve got a reservoir that’s then storing that material. It all ends up rotting.’’
The bigger issue is that during the hottest part of the summer the water at the bottom of the reservoir loses all its oxygen, a state known as anoxic. That makes it unsuitable for use as drinking water, and inhospitable to aquatic life. But water can’t be fed into the river from the upper layers, because it’s too warm and would lift the temperature too much.
So it’s the bottom layer that mainly supplies the river – and which contributes to the growth of the brown slime, much of which is cyanobacteria, the algae that can turn highly toxic. It has killed a number of dogs and forced the council to issue seasonal warnings against swimming, the last of them lifted just before Christmas.
Cyanobacteria is the subject of a long-running separate study led by Cawthron specialist Susie Wood, an internationally-recognised expert in the field.
But Dr Kelly says the reservoir is ‘‘definitely having an effect’’.
He has found that the bottom waters of the reservoir has higher nutrient concentrations than much of the river.
‘‘We think that particularly the higher nitrate concentrations are fuelling some of that growth that occurs. A lot of that nitrate will be taken up and potentially a few kilometres downstream you’d see that effect disappearing.’’
Work at the moment is looking at how far down-river the micronutrient fertilisation is having an effect.
‘‘It is definitely one of the aspects between Sharland Creek and the reservoir outlet – you shouldn’t have that black-brown slime that’s growing in the river.’’
Further down, where Sharland Creek and other tributaries join the Maitai, logging and sediment input play a bigger part and fuel the same sort of algal growth problems, he says.
Heavy metals are mainly bound to sediment and have very little toxicity risk, but in anoxic water can become unbound and dissolved.
‘‘Iron and manganese are the two most common ones to come out of reservoirs that go anoxic. Iron in particular has a real strong link to algal production as well,’’ he says.
It’s not Dr Kelly’s job to pass judgment on the council. He says the Maitai’s water quality has declined, but is good compared with other waterways around the country or even within the region.
But he is hoping that the council will adopt the suggestion in the Cawthron report on options to improve water quality that it spend approximately $70,000 on an aeration system for the reservoir, with annual running costs of around $20,000.
That would pump air through the water column and prevent the bottom layer from becoming anoxic. It would make more reservoir water available for the water scheme, improve the quality of what goes into the river and possibly also reduce the tannin staining.
It’s standard practice to aerate reservoirs around New Zealand and the cost isn’t a lot when put up against the improvement in river health that is likely to result from it, Dr Kelly says.
‘‘In my view, it’s unacceptable to put anoxic water that would be rich in nutrients or metals into a waterway. We’re just trying to solve a problem that’s pretty solvable.’’