Coble, M.A. 2023 Using Radon-222 and concurrent flow gauging to understand groundwater and surface water interaction in the Lower Kauru River, North Otago. Lower Hutt: GNS Science. GNS Science report 2023/35. 25 p.; doi: 10.21420/D2PV-SH32
Abstract:
Understanding groundwater–surface water (GW–SW) interaction processes is integral for comprehending the hydrological characteristics of a catchment. Knowledge of the GW–SW relationship is also required for effective management of freshwater resources, such as setting of minimum flows and GW–SW allocation limits. This report details a combined approach using Radon-222 and concurrent flow gauging to investigate GW–SW interaction between the Kakanui-Kauru Alluvial Aquifer and the Kauru River, North Otago. This survey was undertaken collaboratively by the Otago Regional Council and GNS Science in March 2023 along a 5.7 km reach of the Lower Kauru River. Results confirms that the Lower Kauru River is a losing reach, with flow decreasing by~35% over 5.7 km from the Kauru Hill Road Bridge to the Kakanui Valley Road Bridge. Radon concentrations generally increased along the investigated Lower Kauru River reach. The presence of elevated radon (>0.5 Bq/L) in the Kauru River along a significant proportion of the surveyed reach suggests groundwater discharge from the Kakanui-Kauru Alluvial Aquifer to river flow. Groundwater discharge to the Kauru River increases downriver based on increased radon concentrations, despite the overall reduced river flow in this reach. The hydrochemical composition of the Kakanui-Kauru Alluvial Aquifer at bore J41/0763 overlaps the composition of the Kauru River surface water measured in three locations along the investigated reach. This, in addition to increased radon concentration at bore J41/0763, suggests that the hyporheic zone (the region of sediment beneath and alongside a riverbed where shallow groundwater and surface water are mixing) plays an important role by facilitating the mixing of shallow groundwater and surface water. Overall, these results show that the surface water and groundwater are highly connected along the Lower Kauru River. (auth)
