Constraining Fault-Zone Hydrogeology through Integrated Hydrological and Geoelectrical Analysis

Permeability heterogeneity introduced by faults can substantially impact groundwater flow.  However, because faults are often poorly exposed at the surface, the architecture of the fault is frequently unobservable and understanding the hydrologic impact of a specific fault is particularly challenging.  To improve our ability to estimate fault-zone permeability structure and to document the impact of a major, inactive fault on groundwater flow, we supplemented traditional hydrogeologic measurements with electrical resistivity and self-potential data at the Elkhorn fault in South Park, Colorado. 

Water levels taken in four wells across the fault indicate that permeability generally decreases from the fractured granitic hanging wall to the sedimentary footwall.  Permeability estimates from slug tests and single-well pumping tests are consistent with this decreasing pattern and vary over a few orders of magnitude across the site.  However, the lack of outcrop or detailed documentation of the spatial extent and nature of the fault zone hinders our ability to understand these hydrogeologic data in the structural context of the fault.  Resistivity tomography was used in combination with available geologic maps, drill-core lithologic descriptions, and water-level/permeability data to determine the fault location and geometry.  Self-potential measurements co-located with the resistivity data were used to interpret groundwater-flow patterns in the immediate vicinity of the fault and to create a high-resolution interpretation of the hydraulic-head distribution in transects across the fault.  The hydrogeologic measurements and geoelectrical data were used to make interpretations about the presence and permeability structure of fault-zone components at the meter to tens-of-meters scale.