PERMEABILITY-DEPTH OBSERVATIONS IN A MINERALIZED SEDIMENTARY MOUNTAIN BLOCK FROM GEOPHYSICAL, GEOLOGICAL, AND IN-SITU HYDRAULIC TEST DATA

Permeability places substantial control on groundwater flow and transport. In mountain watersheds, direct measurements of permeability are limited, in part because wells are rare at high elevations to facilitate hydraulic testing. As part of a study of metal transport in the Redwell Basin, a watershed with hydrothermally altered and mineralized sedimentary rocks in the Elk Mountains of Colorado, two high-elevation boreholes were recently drilled along the mountain valley axis and completed as multi-depth monitoring wells with 3-4 vertically discrete well screens in each borehole: MW1 was drilled in the upper basin in the recharge zone to a depth of 81 m, and MW2 was drilled in a mid-basin discharge zone to a depth of 46 m. While drilling, in-situ permeability measurements were made using packer tests: stepped-pressure injection tests were performed over 10 m intervals to depth. Both wells were continuously cored and logged for lithology, alteration, and structure. Borehole geophysical logs were collected to characterize the geology, hydraulic properties, alteration, and fracture space including acoustic televiewer, full waveform sonic, gamma, formation conductivity, fluid temperature/conductivity, borehole flowmeter, and caliper. MW2 also included optical televiewer, magnetic susceptibility, induced polarization, spectral gamma, and borehole nuclear magnetic resonance. Significant permeability (K = 10^-7 to 10^-6 m/s) exists in both boreholes; permeability generally decreases with depth, but both boreholes exhibited increasing permeability at the bottom of the hole. Groundwater flow appears to be primarily hosted by fractures, and the most significant water-bearing fractures occur along bedding planes and at abrupt lithologic transitions, such as between shale and sandstone units. Surface geophysical data, including resistivity and nuclear magnetic resonance, provide some insight into the how the borehole observations can be scaled to larger parts of the watershed. These preliminary findings highlight the likely hydrogeologic significance of lithology and structure to groundwater flow in the Redwell Basin and suggest that permeability controls in sedimentary-rock mountain blocks may differ from more commonly studied crystalline mountain settings.