Flow and Salinity Patterns in the Low-Transmissivity Upper Paleozoic Aquifers of North-Central Texas

摘要

Paleozoic aquifers of north-central Texas may become an important source of groundwater, particularly for use in the production of natural gas from the Barnett Shale in areas where the more prolific Cretaceous Trinity aquifer does not exist. Relatively little is known about those aquifers. In order to further their characterization, there is a need to understand the regional flow system and to develop a conceptual groundwater-flow model. We collected flow-parameter information from data available in the public domain. Sandstone bodies likely to host groundwater in extractable quantities were identified by integrating outcrop sandstone maps and subsurface well-log data. Aquifer hydraulic properties were derived from mostly domestic-well specific-capacity data. Regional flow pattern was derived using multi-year wintertime ground-water-level observations. Surface-water and groundwater CI concentrations and CI/Br ratios were examined to assess their interaction. County estimates of distributed recharge were determined using a CI mass-balance approach. Hydraulic conductivity averages 0.5 to 1.5 ft/day. Higher-than-usual surface-water salinity (>1000 mg/L) originates further upstream; salinity decreases in the downstream direction to <100 mg/L and does not impact aquifers. Groundwater salinity in the Paleozoic aquifers increases northwest to southeast, from younger to older formations, with a reversal and fresher water in those formations in contact with the Trinity aquifer, suggesting that the Trinity aquifer recharges the Paleozoic system through cross-formational flow. Average distributed recharge into the Paleozoic aquifers amounts to 0.1 in/ yr. Overall, Paleozoic aquifers represent a shallow-flow system, mostly unconfined and discontinuous, with general flow toward the northeast, mostly discharging into streams and rivers, and receiving water input through limited distributed recharge and likely from the overlying Trinity aquifer, where present.