Researchers from Lincoln Agritech and Technische Universität Dresden in Germany are collaborating with Marlborough District Council staff to understand how water from the Wairau River interacts with the underlying aquifer. As the river flows across the upper part of the Wairau Plain, water is drained to the underlying aquifer, a process that provides over 96% of the groundwater recharge. Water level surveys across the river suggest that the river is hydraulically perched above the regional water table in its upper reaches, which supports theoretical predictions.
Monitoring of groundwater levels in the recharge sector of the Wairau Aquifer began in 1982, and records indicate that groundwater levels in the recharge sector of the aquifer have declined by 780mm since then. A corresponding decline in flow within lowland springs such as Spring Creek is also apparent in the records.
Our research indicates that the aquifer gravels consist of a shallow and deeper horizon that is separated by a lower permeability gravel. Modelling the water chemistry indicates that 50% of the groundwater flowing into Spring Creek is sourced from the deeper horizon during winter. This contribution drops to less than 10% during summer when groundwater pressures are low, making the springs highly vulnerable to lower groundwater recharge rates when river flows are low during the summer period.
A 3D model of the river and aquifer system was developed at TU Dresden to better understand the river-groundwater exchange process, groundwater flow pathways, and travel times of water between the river and Spring Creek. The model can predict dynamic river recharge rates to the aquifer, which are as low as 5 m3/s, but rarely exceed 12 m3/s even during high flow events. This suggests that the saturated footprint of the river reaches a maximum width at high flows and the rate of drainage beneath the river becomes limited by the vertical hydraulic conductivity.
Tracking flow particles within the aquifer shows that the mean residence time is less than a year, indicating the aquifer is highly dynamic and drains rapidly. The implication of this is that storage within the aquifer, and associated spring flows, depends on the frequency and duration of low flows in the Wairau River.
The model-based investigations have helped to explain the long-term decline in groundwater levels as a combination of climate variability, long-term climate oscillations, and changes of the Wairau River channel. We now understand that aquifer storage is particularly vulnerable to a succession of dry summers with prolonged low river flow periods. This research was funded by the Ministry of Business, Innovation and Employment (Transfer Pathways Project, LVLX1502) with additional funding from Marlborough District Council.