WHAT BOREHOLE GEOPHYSICAL LOGGING AND WELL TESTING TELLS US ABOUT GROUNDWATER PRESENCE AND FLOW IN HEADWATER, FRACTURED GRANITE AQUIFERS?

摘要

In the U.S. Critical Zone Observatories (CZO) and similar headwater CZOs, there are few boreholes and even fewer connected wellfields to study groundwater flow and storage. The Blair-Wallis (WY) CZO (Wyoming Center for Environmental Hydrology and Geophysics - WyCEHG) and the Calhoun (SC) CZO are currently the only sites that contain boreholes with depths ranging up to 70m. These deep boreholes allow us to observe and understand the factors controlling groundwater transmissivity in fractured granite which consists of three main subsurface zones: Unconsolidated (soil/saprolite), Fractured/weathered Bedrock, and Protolith bedrock (i.e. less fractured bedrock). Additionally, the Blair-Wallis site contains a hydraulically connected wellfield that not only has geophysical logs but pump testing and in situ transmissivity & reverse head measurements by a FLUTe liner system. These well testing data provide an empirical understanding of the hydraulic condition within the fractured and protolith bedrock of these boreholes and granite wellfield. In this presentation, we combine the data of the most commonly used geophysical logging tools for groundwater analyses (i.e. normal resistivity (8, 16, 32, 64)/IP/SP/SPR, electromagnetic induction, fluid temperature & fluid conductivity, impeller flowmeter, heat-pulse flowmeter, and Magnetic Resonance Imaging (NMR)) with the in situ transmissivity and hydraulic conductivity testing to look at the controls on hydraulically connected fractures within these boreholes. The in situ pump tests and FLUTe measurements provide a unique means to calibrate traditional geophysical logs for flow rate, water content, transmissivity and hydraulic conductivity. Since fracture permeability is the dominant mechanism for groundwater transport in these environments, a better understanding of geophysical logging method sensitivities and limitations can be useful for future groundwater investigations in igneous and metamorphic units.