| Proposal | Pilot Study | Geology and Hydrogeology of the Fault-Partitioned Hickory Aquifer |
|---|---|---|
| Executive Summary | Pilot Study Objectives and Results | Abstract, Introduction |
| Stratigraphy | ||
| Proposal | Structure | |
| Hydraulic Compartmentalization | ||
| Hydraulic Conductance of Faults |
The Cambrian-age Hickory Sandstone is a major aquifer in Central Texas. The Hickory Sandstone is a 450 ft (137 m) thick, high porosity, siliciclastic unit that unconformably overlies Precambrian crystalline basement. A system of N- and NE-trending normal and oblique-slip faults structurally partition the Hickory Sandstone aquifer within the study area in northern Mason County. Typically, the Hickory Sandstone is only partially offset across the faults and portions of the saturated interval of the aquifer usually are juxtaposed against each other across the faults. Exposures of shear zones of faults in sandstone horizons exhibit significant grain comminution and porosity reduction; shear zones containing deformed clay interbeds, however, are not exposed.
The observed spatial and temporal variations of water levels in wells in the study area clearly show that faults impede the lateral flow of groundwater and influence both the short- and long-term hydraulic responses of fault-defined regions. The existence of a low conductance fault is indicated by one or more of the following features: 1) an anomalously large hydraulic-head change across the fault, 2) a significant variation of the hydraulic gradient in proximity to and on either side of the fault, and 3) poor or indiscernible hydraulic communication between wells on either side of the fault. In general, the greater the displacement of a fault, the greater is the effect of the fault on the groundwater system. Discernible effects are observed for discrete faults with displacements as small as 50 to 75 ft (15-23 m). Regions with numerous small faults dramatically reduce production rates of wells.
The study area can be subdivided into at least three major hydraulic compartments with boundaries defined by faults with at least 100 ft (30 m) of displacement. Poor hydraulic communication exists between wells in neighboring major hydraulic compartments, such that irrigation pumping in one compartment induces in the neighboring compartment an anomalously small drawdown relative to that expected in a laterally uniform aquifer. The major hydraulic compartments are further subdivided into subcompartments by faults across which there is relatively good hydraulic communication, yet the faults still influence the short-term water-level variations within the subcompartments. Following a long period of sustained pumping, residual drawdowns after a short time of recovery (5-12 days) differ significantly from compartment to compartment. Pumping-induced drawdown and recovery characteristics are similar for wells within the same major hydraulic compartment, but often differ from those of wells in adjacent compartments. These differences reflect, in part, the differences in the areal extent of the compartment and the rate of groundwater flow into and out of the compartment. Annual and longer-term water-level declines differ significantly among the hydraulic compartments, which reflects a limited flow of groundwater from a hydraulic compartment positioned up-gradient into an adjacent down-gradient hydraulic compartment.
Water-level data for wells straddling faults permit the calculation of the hydraulic conductance of two faults in the study area. The hydraulic conductance, Cf, is a pragmatic, simple, 1-D measure combining the effects of fault-rock hydraulic conductivity, Kf, and net gouge thickness, t, (Cf =Kf/t) that relates the head change, dh, across a fault to the normal component of the specific discharge, vn; (vn = Cf dh). A simple, 1-D, steady-state analysis of well data indicates that a normal fault with a displacement of 50-75 ft (15-23 m) has a hydraulic conductance of 0.0068 dy-1. Assuming a net gouge thickness of 1 ft (0.3 m), the average permeability of the fault gouge associated with this fault is about 2 millidarcies, which is approximately 250 times smaller than that of the adjacent aquifer sandstone. An oblique-slip fault with approximately 150 ft (46 m) of dip-slip displacement has a hydraulic conductance of 0.00058 dy-1, which is about 10 times smaller than the smaller-displacement normal fault. This smaller hydraulic conductance reflects a combination of a smaller permeability of the fault gouge and a greater net thickness of the fault gouge.
The Hickory Sandstone is a major aquifer in Central Texas. The Cambrian-age Hickory Sandstone is a 450 ft (137 m) thick, high porosity, quartzose sandstone that unconformably overlies Precambrian crystalline basement. Normal and oblique-slip faults cut the Hickory Sandstone in many areas in the region. These faults usually do not totally offset the unit. Instead, a lower stratigraphic interval of Hickory Sandstone is faulted against a higher stratigraphic interval of Hickory Sandstone. Faults between juxtaposed Hickory Sandstone are characterized by narrow shear zones with fault gouge produced by cataclasis; locally, the gouge is cemented with secondary quartz. The fault gouge has a significantly lower permeability than that of the adjacent undeformed sandstone. Consequently, flow of groundwater across a fault is likely to be impeded compared to flow parallel to the fault, and the fault perturbs the local groundwater flow directions and/or magnitudes. If the fault or a zone of faults is laterally extensive, the effect on the groundwater system could be significant. In addition, a system of interconnected faults may result in hydraulic compartmentalization of the aquifer with groundwater flow into and out of these domains controlled, in part, by the magnitude and spatial variation of the hydraulic conductance of the bounding faults. Hydraulic communication between adjacent hydraulic compartments is inhibited and the compartments exhibit differing pumping-induced short- and long-term water-level variations.
The objectives of this study were: 1) to document the temporal and spatial variations of the water table/piezometric surface, as reflected in water levels in uncased wells, in an area where the Hickory Sandstone aquifer is partially offset by faults and structurally partitioned; 2) to determine to what degree faults affect the groundwater system and hydraulically compartmentalize the aquifer; and 3) to determine the hydraulic conductance of specific faults.
The study area is located in the Hickory Sandstone outcrop belt just northeast of the town of Katemcy in northern Mason and southern McCulloch Counties in Central Texas (Figure, 58K). The study area was selected on the basis of the geology, the number and spatial distribution of accessible water wells and the existence of an comprehensive well water-level data measured in 1987.
