By Maia Hart, Local Democracy Reporter
New research suggests historic work to narrow the Wairau River could be contributing to declining levels in the recharge aquifer - one of Marlborough's main water sources.
Narrowed Wairau River could be contributing to a decline in the Wairau aquifer, a key source of irrigation, drinking and stock water. Photo: Supplied/ Local Democracy Reporting - Stuff
The Wairau aquifer is the main groundwater system underlying the Wairau Plain and a source of irrigation, drinking and stock water.
Water seeping from the Wairau River into the aquifer is the main way it is recharged. But its levels have dropped since 1973, at rates unable to be explained by irrigation.
A new report on the aquifer was presented to the Marlborough District Council's environment committee last week.
The council's Wairau Aquifer project leader Peter Davidson said "without exception" levels of the aquifer were decreasing.
Years of staff investigations had looked into how changes in the landscape could have influenced the aquifer recharging, including historic flood control measures to contain the Wairau River and prevent flooding, and possible natural changes in the channel bed elevation.
A report prepared by Davidson suggested one of the "prime reasons" for the ongoing decline in the Wairau aquifer was due to there being less water in the Wairau River available for recharge - this was influenced by the narrowing of channels over time.
Council hydrologist Val Wadsworth told councillors that works on the Wairau River in 1960, which narrowed the channel, were aimed at protecting property and increasing the amount of land available for farming.
Wadsworth said it restricted the river to a relatively narrow area, instead of meandering across large areas of the Wairau Plains, as it had historically.
"Stopbanks prevented overflows, and inside those stop banks there were rock armoured training banks directing the river in a series of controlled meanders from side to side," he said.
"This technique causes the river to develop a single thread channel, instead of pre-existing braided channels."
A groundwater flow cycle illustration, showing shallow, medium depth and deep aquifers in Marlborough and how they are recharged. Photo: Supplied/ Local Democracy Reporting - Marlborough District Council
The report said the Wairau aquifer recharge relied on a gradient to drive water away from the Wairau River.
"The decline in Wairau River water levels means that over time the gradient has reduced significantly and under low flows in some reaches, groundwater drains back from the aquifer to the Wairau River, reversing the recharge process," the report said.
Furthermore, deeper gravels beneath the river were much less permeable and could not transmit as much recharge water vertically or horizontally.
Before: Ariel footage of the Wairau River shows it had multiple braids in 1948. Photo: Supplied/ Local Democracy Reporting - Marlborough District Council
Meanwhile, Scott Wilson from Lincoln Agritech Ltd in Canterbury explained that "highly permeable gravels" in the Wairau River had become thinner.
The report said the saturated thickness of these gravels was now only four or five metres thick, and thinner gravels had less ability to transfer water away from the river into the aquifer.
"The thickness of the river gravels is thought to be the main limiting factor for groundwater recharge rates.
"Because the river gravels have been thinned, for the equivalent flow conditions, the Wairau River cannot physically lose as much water to the wider Wairau Aquifer now as has occurred historically.
After: Ariel footage of the Wairau River shows a single channel in 2016. Photo: Supplied/ Local Democracy Reporting - Marlborough District Council
The study was timely, given the Wairau aquifer Freshwater Management Unit (FMU) allocation status had just changed from "over allocated" to having allocation available.
In a statement on 1 June, the council said the change in allocation was a result of water take permits expiring, and the applying of "reasonable use calculations" through the Proposed Marlborough Environment Plan (PMEP).
Councillors were told at least two applications were waiting; one new application, and one that sought changes to its current consent that was being processed as a result of the new allocation.
That prompted Climate Karanga member Budyong Hill to ask if there was any other option to hold-off on water allocation.
"Can you go to the minister and say ... we've got some very good models now and indications of what our problems are, can we hold off allocating any water that becomes available until we've got an understanding of what a full allocation level should be?"
But council's consents and compliance manager Gina Ferguson said it was important to note from the meeting that the aquifer level was not being driven by the extraction, but by the recharge.
"From a planning focus ... that may be not about just looking at allocation limits, but also about some of our other controls ... including land use activity, flood protection works and gravel extraction," she said.
The council's next steps would be to workshop specific river management scenarios, including whether to raise or lower mean river bed levels, to change transmissivity and storage, or to increase the river width to reduce scouring.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air
