HYDROGEOLOGIC SIGNIFICANCE OF DEPOSITIONAL SYSTEMS ON GROUNDWATER QUALITY AND WATER RESOURCES IN THE CISCO GROUP, UPPER PENNSYLVANIAN AND LOWER PERMIAN FACIES, ROLLING PLAINS, TEXAS

The complex hydrogeology of the Cisco Group, Eastern Shelf of the Midland Basin, Texas, determines the large heterogeneity in groundwater quality and resources across 14 counties of the Texas Rolling Plains. Depositional systems that compose the Cisco are characterized by 16 cyclic terrigenous and carbonate sedimentary systems. This project integrates geologic, hydrochemical and hydrologic data in a GIS and 3-D visualization environments and uses PHREEQC for geochemical analysis.

Texas’ population is expected to double within the next 50 years. Groundwater demand for the Rolling Plains may exceed recharge rates by 44%. Firm geologic controls on groundwater quality must be evaluated in order to maximize the yield of current groundwater supplies and develop regional water plans.

A recognized problem in the area is the natural pollution of groundwater and surface water from salt springs and seeps from the Upper Pennsylvanian and Lower Permian. Whereas formational dip is west, toward the Midland Basin, the prevalent direction of groundwater flow is east, following the regional slope of the southern Great Plains. Discharge of brine from the Midland Basin and from salt-dissolution zones affects salinity in shallow aquifers and rivers across the Rolling Plains.

ArcGIS is used to create digital coverages of depositional facies, total dissolved solids, hydrochemical facies, hydraulic head, and topography. A 3-D visualization model of the Cisco stratigraphy was used to assign water-quality data to depositional layers and determine the influence of sand channels on fresh or saline water transmission. A total of 777 water quality samples from Cisco formational units were extracted from the TWDB groundwater database. TDS concentrations are less than 3000 mg/l in 93% of these samples. Sixteen Cisco saline water samples from the USGS produced water database are used in PHREEQC as an end member for inverse geochemical modeling. Inverse modeling is used to test hypotheses of how water quality in the depositional facies is controlled using end member waters. Water quality in the limited amount of potable water is found to be controlled by reacting recharge water with formation minerals (mainly silicates). The other main reaction is the mixing of locally recharged freshwater and saltwater moving eastward from the Eastern Basin.