Characterising surface water and groundwater interactions in braided rivers using hydraulics and environmental tracers: The Waikirikiri Selwyn river : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

Quantifying surface water – groundwater exchanges is vital for ensuring the sustainable management of freshwater resources. Although braided rivers are known to contribute significant recharge amounts to alluvial aquifers, there are still uncertainties regarding groundwater recharge rates and their temporal variability. This gap exists mainly due to the complex nature of braided rivers and the field challenges associated with their study. Applying multiple approaches simultaneously may help improve the hydrogeological conceptual understanding of these types of systems. This study employed several techniques to characterise water exchanges between the Waikirikiri Selwyn River, located in the South Island of New Zealand, and the underlying aquifer. To achieve the study’s aim of determining the river water infiltration dynamics from the Selwyn River into the underlying braidplain groundwater system, two reach-scale surveys were conducted between July 2021 and January 2022. During this time, hydrochemical and hydraulic measurements were collected from the Selwyn River, and from a network of 34 monitoring wells installed along the river. In situ 222Rn measurements in the shallow braidplain aquifer provided information on residence times and infiltration velocities of infiltrated river water. 222Rn concentrations from laboratory-based analyses of samples obtained before and after purging the wells were used to quantify groundwater velocities in the vicinity of the wells. A spatial in-river 222Rn survey, along with TIR imagery acquired from a concurrent study, helped in locating hyporheic discharges within the river. Time series stream and groundwater level measurements provided insight into water movement from the river to the shallow aquifer, the direction of horizontal groundwater flow within the shallow aquifer, and vertical flows between the three groundwater bearing layers at the study site. 222Rn data, supported by the groundwater flow map, demonstrates that the Selwyn River infiltrates the aquifer in both the northern and southern banks of the river. 222Rn concentrations in the wells ranged between 1.5 to 20.1 Bq/L depending on their proximity to the river and the depth of the well screen. Estimated mean aquifer residence times in the shallow braidplain aquifer ranged from between 1 to 13 days, and the infiltration velocities ranged from between 10 to 180 m/d. Vertical differences in hydraulic heads in the three aquifer layers, supported by a downward increase in radon concentrations, strongly suggest downward groundwater flow from the shallow braidplain aquifer to the regional aquifer. The spatial distribution of aquifer residence times and groundwater velocities in the shallow braidplain aquifer revealed preferential flows through paleochannels. In addition, high 222Rn concentrations obtained from low resolution in-river 222Rn surveys indicated hyporheic discharges which correlated with some of the areas identified by the TIR survey. These results underscore the complexity of the water exchange interface at the study site. This study provides river infiltration velocities and additional insights valuable for modelling exchange processes at the site, without the need to inject artificial tracers into the aquifer. In general, it shows that the quality and quantity of groundwater in the shallow aquifer is most likely sensitive to the quality and quantity of water in the Selwyn River.