The Wairau Aquifer: a precious resource under pressure
Marlborough's Wairau Aquifer, a vital regional water source, faces unprecedented challenges. Based on 52 years of monitoring at Conders well near Renwick, observations show water levels are at their lowest ever for this time of year.
While water abstraction for irrigation and town use does impact aquifer levels, it's not the primary cause of the decline.
That is attributed to reduced recharge from the Wairau River, a recent national study has shown.
Hydrologist Charlotte Tomlinson said it was a complex picture, and several factors had contributed to less recharge.
“Wairau riverbed levels have fallen over the past 50 years relative to the aquifer, meaning water can’t flow into the aquifer as easily,” she said.
“This was caused by less gravel arriving from the hill catchment and/or local extraction, together with the side effects of essential flood protection works. Consented abstraction from the Wairau River also reduces summer flows with less water available for aquifer recharge than historically.”
“Climate patterns are also playing a role, with Blenheim receiving only two thirds of its average rainfall since June 2023. The 2024/25 irrigation season started wet but quickly turned dry, with December 2024 being the warmest since 1930. These conditions mean that when it does rain, more water is absorbed by dry soils, resulting in less runoff to replenish rivers and aquifers.”
“The consequences of declining aquifer levels are becoming visible. Spring Creek and other spring-fed streams on the Wairau Plain show reduced flows and this trend is likely to continue.”
Senior Groundwater Scientist Peter Davidson said Council was actively seeking solutions.
“Council’s science and engineering teams are working together on potential solutions to stabilise groundwater levels by increasing aquifer recharge rates,” he said.
“One option being discussed is making the Ruakanakana Stream channel ‘leakier’ by removing silt, so more water diverted from the Wairau River infiltrates into the underlying aquifer.”
“We are also looking to have more targeted gravel extraction and trial localised flood management approaches in the river reach where most Wairau River water is naturally lost to the aquifer.”
While climate change isn't the primary factor in the aquifer's decline, projections are for more frequent and prolonged droughts. Adapting water management practices and aligning river floodway approaches now will be essential to mitigate these effects and ensure the long-term sustainability of the aquifer.
