Wairau Aquifer recharge pathways

Abstract

The Wairau Aquifer is almost entirely river-recharged, although the distribution of recharge groundwater flow paths is poorly constrained. To improve conceptualisation and inform numerical flow modelling, the stratigraphy of the Holocene alluvial fan was reviewed. Previous authors have noted subtle variability within the gravel assemblage. Our aim was to identify spatial patterns of deposition informed by climate-mediated changes in the energy of the river environment. We used a three-stage process: an initial scan of deep well logs, 3D visualisation using GMS software, and a final refinement by plotting intercepts of distinctive facies in GIS. Drillers’ well logs were referred to throughout this approach. The lithologies were lumped into four depositional environments, from high-energy (proximal channel gravels) to low-energy (distal overbank flow). Lithological information was supplemented by qualitative comments made by the drillers on the water-bearing nature of the sediments encountered. The alluvial fan consists of upper and lower members which are separated by low-permeability matrix-supported gravel. The upper member shows lateral facies changes which have distinct hydraulics properties. Three areas for preferential recharge can be recognized: 1) 35% via a 3km reach in the fan apex where lower member is in contact with the active river channel gravels. 2) 20% via a shallow historical overbank flow channel. 3) 45% via a 3–4km reach in the fan toe where the river traverses a thickening high-permeability upper member gravels. Recharge from this reach provides a shallow groundwater pathway to downgradient springs.