GROUNDWATER FLOW IN CRETACEOUS CARBONATES OF THE HIDDEN VALLEY FAULT ZONE, COMAL COUNTY, TEXAS

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 formation along fault zones. We conducted an integrated hydrologic study of the Hidden Valley fault zone, a normal fault within the Balcones Fault zone in mechanically layered Glen Rose Limestone (Trinity Aquifer) exposed in Canyon Lake spillway gorge in central Texas. Initial hydrologic assessment included (i) geochemical analysis of water samples from springs, water wells, and adjacent Canyon Lake; (ii) surface-water temperature, conductivity, and flow measurements; and (iii) groundwater modeling. Chemistry and groundwater modeling results indicate sourcing of springs 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, surface water infiltration (recharge) along the fault zone, and subsurface flow via secondary porosity (e.g., faults, fractures, vugs). We conducted three tracer tests by injecting distinct dyes (Uranine, Eosin, Phloxine) at three locations in the fault zone on 14 February 2012, and monitoring water at down-gradient springs within the fault zone, water wells in the hanging wall and footwall, and the Guadalupe River (upstream and downstream of confluence with spillway outflow). One or more dyes were detected at all down-gradient spring, seep, and stream monitoring sites. No dyes were detected at monitored water wells or upstream river sites as of the last samples collected in June 2012. Uranine injected into a recharge feature (sink) along the fault core travelled laterally underground along the fault zone 357 and 710 m to monitored springs at 1.70 and 1.99 km/day, respectively. Eosin injected in the footwall travelled 74 m across the footwall fault damage zone to a monitored spring at 0.99 km/day, and 375 and 411 m along the fault zone to monitored springs at 2.37 and 2.60 km/day, respectively. Fault zone deformation features and associated dissolution produced pronounced permeability anisotropy in the fault core and fault damage zone, resulting in maximum transmissivity parallel to the fault strike.