Groundwater flow in a mountain headwater catchment - how Important is that fracture zone?

Diana M. Allen, Alex H. Nott

Dans les comptes rendus d’articles de la conférence: GeoNiagara 2021: 74th Canadian Geotechnical Conference; 14th joint with IAH-CNC

ABSTRACT: Mountain streams often coincide with the location of fracture zones, where preferential erosion allows the stream to more deeply incise the bedrock. However, fracture zones can be difficult to map, particularly in forested areas where tree cover obscures land surface features. This study is a continuation of our previous research in the Upper Penticton Creek (UPC) 241 watershed, a small (4.74 km2) snowmelt-dominated headwater catchment in the Okanagan Basin, British Columbia. In the original study, four lineaments were mapped in the catchment, but none were in proximity to the bedrock well drilled at the bottom of the catchment, despite the borehole geophysical logs indicating more intense fracturing, the well exhibiting a linear flow response during a pumping test, and the well flowing artesian during the spring freshet. A newly acquired bare earth model based on LiDAR data clearly highlights a linear erosion zone that passes nearby the well. Closer examination of the major ion chemistry and stable isotope data of the groundwater from this well reveals a slightly different chemical composition and a more depleted isotopic signature compared to the geochemically and isotopically similar compositions of water sampled at different depths in the other bedrock wells at high elevation, shallow groundwater from the network of soil piezometers, and stream water at different locations along the stream network. Collectively, there is more substantial evidence of a fracture zone near this well. Therefore, the original integrated land surface - subsurface model has been updated to include a weathered bedrock layer and vertical fracture zones. The recharge and baseflow estimates for the new conceptualization of the catchment differ significantly from the original model, pointing to the importance of including a weathered zone and fracture zones in these small headwater catchments.

Please include this code when submitting a data update: GEO2021_244

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