Conceptualisation of Groundwater Recharge from Braided Rivers

Abstract

Braided rivers are a significant source of groundwater recharge for New Zealand’s gravel aquifers. However, their spatial and temporal complexity has made quantification of recharge and representation in numerical models particularly challenging. This presentation summarises the results of a research programme aimed to understand the structural controls on leakage from braided rivers and develop a conceptual model of how they function in the subsurface.Instrumentation and field campaigns were established in the main losing reaches of three New Zealand braided rivers, the Selwyn, Wairau, and Ngaruroro. A multi-method approach was undertaken to characterise three key physical aspects: longitudinal flow change, sediment structure, and subsurface saturation. The field methods applied were lidar, bathymetry, coring, grainsize analysis, earth resistivity, nuclear magnetic resonance sounding, thermal sensing, radon sampling, differential flow gauging, and hydrological monitoring. In addition, some hypothesis testing was carried out using Hydrus 2D.Subsurface saturation in all three study rivers was found to be associated with gravels of the contemporary braidplain. Due to repeated flood mobilisation, the gravels in the contemporary blaidplain have a smaller fraction of silt and clay, and are also less compacted than the underlying and adjacent gravels. Sediment mobility associated with flooding therefore enables a high permeability aquifer to form within these gravels, with shallow groundwater flow subparallel to the dominant river flow direction. This alluvial aquifer, which we are calling the ‘braidplain aquifer’ provides a storage reservoir for hyporheic and parafluvial exchange to occur. Water exchange between the river and regional aquifer is mediated by the braidplain aquifer (there is no direct exchange of water between the river and regional aquifer). The implication of this conceptualisation is that hydrological connectivity between a braided river and groundwater (e.g. as formulated by Brunner et al. 2009) occurs at two spatial scales; at the river-braidplain aquifer interface, and at the braidplain aquifer-regional aquifer interface. For assessing regional scale water balances, the latter spatial scales is most relevant. In a case where the braidplain aquifer is perched above the regional aquifer, recharge to the regional aquifer is regulated by vertical hydraulic conductivity in the underlying sediments, and the rate of recharge is fairly steady throughout the year. In a case where the braidplain aquifer is hydraulically connected to the regional aquifer, our results suggest that the exchange is controlled by horizontal conductivity of the sediments on the margins of the braidplain, vertical conductivity of the underlying sediments, and the hydraulic gradient. As such, flow losses can be highly variable throughout the year and appear to form a power-law relationship with flow (Woehling et al. 2018).ReferencesBrunner, P., Cook, P. G., and Simmons, C. T. (2009), Hydrogeologic controls on disconnection between surface water and groundwater, Water Resources Research 45: W01422Wöhling, Th., Gosses, M., Wilson, S., Wadsworth, V., Davidson, P. (2018). Quantifying river-groundwater interactions of New Zealand's gravel-bed rivers: The Wairau Plain. Goundwater 56: 647-