Conceptualization of groundwater recharge from a braided river: The Wairau Fan
5th International Conference on Alluvial Fans, Christchurch, New Zealand
Wilson, S.R., Davidson, P., Wöhling, T.
The braided Wairau River is the main source of recharge to the highly productive Wairau Aquifer. Concurrent flow gauging measurements at low flow indicate that a 7 cumec loss occurs as the river traverses the Wairau alluvial fan, a distance over 15 km. Half of this loss occurs in the upper to middle 4 km of the fan, although the nature of this loss is not well understood. Theoretically, the relationship between a river and groundwater can be considered as being hydraulically connected (gaining or losing), disconnected (perched), or transitional (Brunner et al. 2011). A disconnected river is distinguished from a hydraulically connected river by the presence of a partially saturated zone between the river bed and the aquifer. The aim of this study is to improve our conceptual understanding of how flow losses occur, and to test a new hypothesis that much of the river is perched above the aquifer.
It is practically difficult to make direct observations beneath a river bed. However, indirect methodologies can be employed to characterize the nature of the river-aquifer exchange, and we have employed a number of these approaches. LiDAR imaging across the Wairau River has enabled the inclusion of representative river level elevations in piezometric surveys. Tracer data can also be used to understand fluxes adjacent to the river. For example, radon analyses (Close, 2014) indicate that the groundwater flux is higher in the upper fan. We have reviewed existing 18O samples from the river and adjacent aquifer (Stewart, 2008) to understand the various recharge sources within the aquifer. Temperature loggers have also been installed in bores adjacent to the river. The available observational data have been used to develop a new numerical model of the Wairau Aquifer so that we can better understand river-aquifer exchange dynamics.
Revised piezometric surfaces that include river elevations show a consistent sharp disjuncture between the river surface and the water table in the upper half of the fan. Water levels in recently drilled riparian bores support this observation. Streams on the north bank of the river also lose flow and may be ephemeral in the vicinity of the Wairau River, indicating that the water table may dip southwards beneath the river. 18O data for the upper fan show high values considering the proximity of wells to the river, and the abundance of river recharge expected. This suggests that river recharge signature may be diluted by north bank groundwater flowing underneath the river. Temperature data from riparian bores show a response to individual flow events in the uppermost gravels, with only a seasonal cycle being evident at greater depths.
The combined evidence from all data sources supports the hypothesis that the dominant recharge reach of the river may be hydraulically disconnected from the aquifer. Results from aquifer tests indicate that the hydraulic conductivity of the alluvial gravels is highly anisotropic. We suggest that this anisotropy inherently creates a perched river setting. The numerical model confirms our hypothesis, as we have found that the aquifer cannot be simulated without a hydraulically disconnected river and strong anisotropy (Wilson and Wöhling, 2015).
Brunner, P, Cook, PG, & Simmons, CT, 2011. Disconnected surface water and groundwater: From theory to practice. Ground Water, vol. 49, no. 4, pp. 460-467.
Close, M, 2014. Analysis of Radon data from the Wairau River and adjoining Wairau Plains Aquifer. ESR Client Report CSC14001.
Stewart, M, 2008. Age and source of Wairau Plains groundwater. GNS Science Report 2008/18.
Wilson, S, Wöhling, T, 2015. Wairau River-Wairau Aquifer Interaction. Envirolink Report 1514-MLDC96.