Evaluating the potential of constructed groundwater systems to improve the water supply operations of Auckland City, New Zealand
A range of constructed groundwater systems are evaluated for augmenting the potable water supply of Auckland City, New Zealand. The term ‘constructed groundwater systems’ encompasses any constructed means of recharging or extracting water from an aquifer. This may for example include infiltration wells and basins, injection wells, aquifer storage and recovery wells, and horizontal wells. The study motivation is to increase the sustainable yield from Auckland’s existing groundwater resources to help meet Auckland’s increasing water demand. Presently, 97% of the water supply comes from 10 storage reservoirs in the Hunua and Waitakere Ranges, and up to three percent from the Onehunga - Mt Wellington aquifer. The Onehunga - Mt Wellington aquifer is a highly fractured basalt aquifer and has the potential to store water in its unsaturated zone. Various methods of artificially increasing the recharge of the Onehunga - Mt Wellington aquifer to increase its sustainable summer yield were investigated using a FEMWATER numerical model. Three sources of recharge water were identified in the Auckland Region: winter spillage from Watercare’s 10 reservoirs, groundwater from the dewatering of the Three Kings quarry, and wastewater from the Mangere Wastewater Treatment Plant. Artificial storage and recovery (ASR) wells, and soakage wells were simulated as a means of increasing the aquifer’s yield. The greatest simulated increase in yield was from winter injection of reservoir water via ASR wells, in conjunction with the summer extraction of the stored reservoir water. This simulation resulted in the average summer aquifer yield increasing from 9,100 to 44,100 cubic metres per day. A hydrogeological review of the both the Hunua and Waitakere Ranges showed that there is little opportunity for utilising constructed groundwater systems below the regolith zone in those two locations. However, the storage capacity of the unconfined aquifers in the regolith layer of the Waitakere and Hunua Ranges could potentially be used to increase the efficiency of the existing water supply reservoirs presently losing water to seasonal spillage. To increase the efficiency of the existing water supply reservoirs it is proposed that horizontal wells can be used to control the water table elevation in the reservoir catchments, permitting a degree of control over the discharge of the streams flowing to the reservoirs. The use of horizontal wells provides a range of control options, including increasing stream discharge during summer and reducing and delaying peak discharges from rainfall events during winter. The use of horizontal wells in this manner is most suited to catchments where the magnitude of stream quickflow discharge is dependent on the depth of the surrounding water table. That is, for similar-sized rainfall events, a water table near the ground surface results in more quickflow stream discharge compared to a deeper water table. The Upper Nihotupu water supply reservoir catchment in the Waitakere Ranges was selected as a study site to gather hydrological data to simulate use of horizontal wells. In the Upper Nihotupu catchment, there is a fourfold increase in quickflow discharge for similar sized rainfall events for a water table 0.5 m below the ground surface, compared to a water table 2.7 m below the surface. Based on this, a MODFLOW numerical groundwater model was utilised to simulate the operation of hypothetical horizontal wells within the Upper Nihotupu catchment, and to evaluate if the manipulation of stream discharge will increase the reservoir’s storage capacity. Simulation results suggest there is potential for a significant increase in summer stream flow by draining the surrounding aquifer with the horizontal wells. During the 30 day period that the wells are draining the aquifer, the corresponding average reservoir delivery increased from 10,000 to 25,000 cubic metres per day. During the winter, the draining of the aquifer using horizontal wells results in reduced and delayed quickflow stream discharge. This in turn resulted in less spillage from the Upper Nihotupu reservoir. The increased control of the stream hydrograph by the simulated horizontal wells increased the average annual delivery of water from the Upper Nihotupu reservoir by seven percent.
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