Dropping aquifer levels in Marlborough linked to falling riverbed levels

Groundwater levels in Marlborough's largest aquifer have been dropping for more than 50 years, with new research linking the decline to falling riverbed levels.

The Wairau River was studied as part of a national gravel bed rivers research project, which found that since 1973, aquifer levels have fallen by about a metre.

The findings are detailed in the Marlborough District Council's State of the Environment report, which brings together monitoring and research between 2016 and 2025 to show what condition the region's environment is in, how it has changed over time and the pressures that are affecting it.

It found freshwater availability was a critical risk for Marlborough now and into the future, given the community's reliance on the region's freshwater resources.

Senior groundwater scientist Peter Davidson said the decline in aquifer levels was due to a combination of natural processes and human influences, and it was significant because the region relied on it for drinking water and irrigation.

The falling riverbed levels had been linked to historic gravel extraction and flood control works, along with straightening and narrowing of the river that had unintentionally slowed the speed at which the aquifer refilled.

"These are essential things, otherwise people couldn't live here," he said.

"Straightening out rivers and things like that are necessities if you want to have agriculture and settlement on a floodplain."

A hydroelectric dam in the Waihopai River, a tributary of the Wairau, had also starved the main river of gravel since the 1930s, he said.

He said aquifer levels were expected to fluctuate with the seasons, falling in summer and recovering through winter and spring.

While climate volatility and high water allocation through the summer months affected groundwater levels, they did not explain the long-term decline.

But the research found that the long-term downward trend was linked to riverbed levels and had also been observed in the Ngaruroro River in Hawke's Bay and the Selwyn/Waikirikiri River in Canterbury.

"We really do depend on these groundwater resources for all of our economic wealth, all our municipal supplies for Blenheim and Renwick that depend on that aquifer.

"[It's] very high quality water, we essentially just pump it out of the ground with minimal treatment."

He said the council were exploring ways to halt the falling levels, that included active management of gravel extraction to improve the recharge rate and stop the decline, and rewatering to recharge the aquifer.

It was a balancing act to ensure gravel did not build up and contribute to flooding, but that there was enough to allow the aquifer to recharge and the groundwater to stabilise.

"We're optimistic that over time when we manage these things, especially the gravel extraction and those really sensitive recharge reaches, that these measures will work and stabilise things."

He said the council would continue to monitor the Wairau River using remote sensing including LiDAR, and would assess gravel accumulation after major floods to actively manage how much could be removed from the recharge-sensitive reaches.

Key achievements and pressures for Marlborough detailed in report

The State of the Environment, Te Āhua o te Taiao report gives a snapshot of the region's unique environment, the trends of the last 10 years and the challenges and opportunities ahead.

It found there has been a significant increase in monitored biodiversity sites, with 70 new ecologically significant marine sites and subsites and over 1300 mapped wetlands.

There has been large-scale habitat protection and restoration, with more than 138,000 hectares covered by its significant natural areas programme.

Marlborough now has a substantial and comprehensive freshwater monitoring network, with most monitored rivers and groundwater showing stable or improving trends.

Advanced environmental monitoring, including pioneering eDNA sampling revealed nearly 3,000 freshwater species. Catchment Care programmes have achieved extensive native planting and riparian fencing and led to improved water quality in some catchments.

The council also has stronger climate data, hazard mapping, and resilience planning support decision-making and adaptation.

The district was facing ongoing biodiversity decline due to pests, habitat loss and environmental pressures.

Sedimentation was impacting estuaries and coastal habitats, particularly in Pelorus Sound / Te Hoiere Sound.

Invasive species such as wilding conifers, Mediterranean fanworm and wild ungulates (like goats and deer) continued to threaten ecosystems.

Climate change was amplifying future risks: rising temperatures, floods, wildfire and drought risk, and sea-level rise.

There was an increased frequency and severity of natural hazards, including floods, landslides and earthquakes.

There was also increased pressure from ageing infrastructure and the high cost of recovery and maintenance following major weather and seismic events.