Quantifying River-Groundwater Interactions of New Zealand's Gravel-Bed Rivers: The Wairau Plain

New Zealand's gravel-bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river-groundwater exchange processes in gravel-bed rivers, we investigate the Wairau Plain Aquifer using a three-dimensional groundwater flow model which was calibrated using targeted field observations, "soft" information from experts of the local water authority, parameter regularization techniques, and the model-independent parameter estimation software PEST. The uncertainty of simulated river-aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null-space Monte-Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³ /s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³ /s, the net exchange flow rarely exceeds 12 m³ /s and seems to be limited by the physical constraints of unit-gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low-flow periods in the river. We hypothesize that the new insights into the river-groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.