Occurance and Movement of Ground Water in Austin Chalk and Eagle Ford and Ozan Formations at the Superconducting Super Collider (SSC) Site, Ellis County, Texas

This report defines hydrologic properties and describes rates and modes of groundwater flow in weathered and unweathered Austin Chalk and Ozan (lower Taylor Marl) and Eagle Ford Formations at the Superconducting Super Collider (SSC) site in Ellis County, Texas. Fractures probably are the primary conduit of groundwater flow in these formations because unfractured bedrock has low hydraulic conductivity. Distribution and fabric of detrital versus authigenic clays, rather than total clay content, influence the mechanical properties and log-response character of chalk and marl. The middle Austin Chalk typically has lower fracture intensity and abundance, greater ductility, lower porosity, and lower average hydraulic conductivity than the upper and lowermost Austin.

Precipitation over upland drainage divides percolates into the ground and moves downward through the soil zone and weathered bedrock to the water table. Water levels in both weathered and unweathered bedrock generally mimic topography, reflecting a dynamic balance between the rate of recharge from precipitation and rates of discharge by evapotranspiration, flow to springs and seeps, and pumping of wells. Groundwater percolates along vertical fractures and horizontal bedding-plane joints and through the more permeable sedimentary layers. Flow paths are generally eastward but bend toward discharge points in springs and seeps in the valley bottoms and stream banks. Vertical movement is retarded by unfractured, low-permeability beds. Only a small amount (<1 percent) of the groundwater moving through the surficial weathered bedrock moves downward into unweathered, low-permeability bedrock. Vertical circulation of groundwater in fractured zones locally is deep. At the eastern side of the SSC, groundwater moves downward from the Ozan into the Austin beneath the upland drainage divides but upward from the Austin through the Ozan beneath stream valleys.