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In montane environments, tree roots often extend into fractured bedrock beneath thin soils. The extent to which water is dynamically stored in fractured bedrock and how much of it is available to trees is largely unknown. This uncertainty stems from the challenge of making hydrologic measurements in highly heterogeneous fractured bedrock environments in steep upland terrain. Here, we present the results of a hydrogeophysical study of fractured Mancos shale bedrock in the Upper Slate River, near Crested Butte, Colorado. We use a combination of electrical resistivity tomography, laboratory porosimetry, and borehole nuclear magnetic resonance (NMR) to estimate unsaturated water storage below the soil. Our drilling and road cut characterization revealed a fractured bedrock vadose zone to at least 4 m depth underlying the conifer forest, composed predominantly of Engelmann spruce and subalpine fir. Borehole NMR surveys conducted in Aug 2018 indicate that volumetric water contents in the bedrock vadose zone range from 0.18-0.26 spatially. Resistivity profiles indicate a continuous 1.5-4.5 m thick resistive zone near the surface that may represent a decline in water content and/or increase in porosity (i.e. fracture density) towards the surface. We did not encounter drilling in our boreholes (up to 4 m depth), but resistivity profiling did not rule out the possibility of a shallow (10 m deep) water table. NMR T2 relaxation times and helium porosimetry on bedrock matrix chips support the inference that water may be stored in fractures. Our study demonstrates the utility of geophysical methods for investigating water storage in the bedrock vadose zone as a source of water for transpiration in uplands forests. Presentation Date: Tuesday, September 17, 2019 Session Start Time: 8:30 AM Presentation Start Time: 11:00 AM Location: 301B Presentation Type: Oral
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