Ground-water flow and solute transport in a fractured chalk outcrop, North-Central Texas

It is important to understand hydrogeologic controls on ground-water flow and solute transport in weathered and faulted zones because these zones are common in outcrops all over the world, relied on for ground water in many locales, and susceptible to contamination due to their proximity to the land surface. The weathered and faulted zones of the Austin Chalk in Texas are especially susceptible to contaminants from municipal, industrial, and agricultural sources because it underlies the Interstate 35 growth corridor that extends from Dallas through Austin to San Antonio. This study shows that the distribution and properties of fractures affect ground-water flow and solute transport in weathered, unweathered, and faulted chalk. Characterization data reveal that the weathered zone has: (1) a limited thickness (about 2.5 m); (2) a fracture intensity 300 times greater and a permeability 150 times greater than unweathered and unfaulted VII bedrock; and (3) water levels that respond rapidly to rainfall. Characterization data also show that faulted zones have: (1) permeability as much as 160 to 200,000 times greater than permeability of the weathered zone and unfractured, unweathered chalk, respectively; (2) connectivities that extend over vertical and lateral distances at least as great as 90 and 1,300 m; and (3) different hydraulic behavior depending on their connection with the land surface and topography. Numerical models based on conceptual models developed from the characterization data suggest that: (1) over 99 percent of ground-water flow occurs in the weathered zone; (2) faulted zones can greatly increase the depth and rate of transport in the chalk; and (3) ground-water travel times in the weathered zone depend on vertical variations in hydraulic conductivity, water-table position, and point of entry into the flow system and can be 10 times faster at higher than at lower water table positions. Geometric mean residence time for water in the weathered zone under wet conditions is about 16 days with a geometric mean velocity of 48.2 m d⁻¹. Small-scale numerical studies indicate that matrix heterogeneity may lead to longer back-diffusion times when attempting to remediate contaminated sites.