Fractures and groundwater flow in a triassic sandstone aquifer, Colorado City, Texas
The abandoned Col-Tex tank farm site, in Mitchell County, Texas, sits atop fractured sandstone and mudstone belonging to the Trujillo Sandstone of the Triassic Dockum Group. From the late 1920s to the early 1970s, the site was used as a storage facility for petroleum and associated products ranging from aviation fuel to asphalt. Tar, asphalt, and oily water remain in on-site impoundments. The presence of perched water tables in the upper 5m to 7m of the Trujillo Sandstone suggests that vertical groundwater flow is inhibited by a shale-siltstone layer within the formation. As this site overlies the Santa Rosa aquifer which is used for municipal and irrigation water in this area, the possibility of contamination to local groundwater has been investigated. In addition, the location of the site on a 30-meter bluff adjacent to the nearby Colorado River represents potential surface water contamination. A definite orthogonal fracture pattern in the upper 5 to 7 meters of the Trujillo Sandstone is documented at the site. A prominent fracture trend of approximately N50E is found. These fractures likely formed due to far-field E-NE directed compressional stresses associated with the Late Cretaceous to Eocene Laramide Orogeny in southwest Texas and south central New Mexico. Similar fracture systems likely penetrate the lower part of the Trujillo Sandstone and the underlying Santa Rosa Sandstone. Although fractures in the Upper Trujillo Sandstone directly beneath fluid-filled impoundments may be partially clogged with mud and tar, the presence of these fractures affects water and contaminant flow through the Upper Trujillo Sandstone, as evidenced by the presence of cliff face seeps associated with these fractures. Eventual remediation of this site should take into account the likelihood of anisotropic groundwater flow direction caused by these fractures in the Trujillo Sandstone.