Fault Zone Control on Rapid Groundwater Flow in Cretaceous Carbonates of the Hidden Valley Fault Zone, Texas

Abstract Understanding geological controls on subsurface fluid flow is critical for groundwater resource management and aquifer protection and is particularly challenging in faulted carbonate aquifers due to dissolution and conduit development. We conducted an integrated hydrologic study of the Hidden Valley fault zone in mechanically layered Glen Rose Formation (Trinity Aquifer) exposed in the Canyon Lake gorge in central Texas. Initial hydrologic assessment included geochemistry of water from springs, water wells, and adjacent Canyon Lake, and water temperature, conductivity, and flow monitoring. Geochemistry results indicate spring sourcing from Canyon Lake rather than the Trinity Aquifer. Water temperature and conductivity monitoring indicated complex water movement in the fault zone including spring discharge, surface flow, water infiltration (recharge) along the fault zone, and subsurface flow via secondary porosity (e.g. faults, fractures, vugs). We conducted tracer tests with three distinct dyes (Uranine, Eosin, Phloxine B) at three locations in the fault zone and monitored water at (i) down-gradient springs within the fault zone, (ii) wells in the hanging wall and footwall of the Hidden Valley fault zone, and (iii) the Guadalupe River upstream and downstream of confluence with the gorge outflow. One or more dyes were detected at all down-gradient spring, seep, and stream monitoring sites, but none were detected at monitored water wells or upstream river sites. Uranine injected into a recharge feature (sink) along the fault core travelled laterally underground along the fault zone up to 1993 m/d. Eosin injected in the footwall travelled across the footwall fault damage zone (approximately perpendicular to the fault) at 987 m/d and parallel to the fault at up to 2596 m/d. Fault zone deformation features, including normal faults, extension fractures, and associated dissolution, produced strongly anisotropic hydraulic conductivity that is >7 times larger parallel to the fault compared with perpendicular to the fault.