Refining the Conceptual Model for Flow in the Edwards Aquifer—Characterizing the Role of Fractures and Conduits in the Balcones Fault Zone Segment
|Author||S Hovorka, T Phu, J P Nicot, and A Lindley|
|Description||Hydrogeologic study using existing data on role of fractures and conduits in groundwater flow in the recharge and artesian zones of the Edwards Aquifer Balcones Fault Zone|
|Publisher||The University of Texas at Austin Bureau of Economic Geology|
|Location||Edwards Aquifer, Balcones Fault Zone|
The purpose of this study was to evaluate the influence of fractures and conduits on groundwater flowpaths in the recharge and artesian zones of the Edwards Aquifer. Hydrologic and geologic data were evaluated for evidence of karst aquifer flow regimes and specific areas are delineated for focused data collection in the future. The study was completed in January 2004 by the University of Texas – Bureau of Economic Geology, Austin, Texas.
The Balcones Fault Zone Edwards aquifer of South Texas exhibits multimodal permeability. High matrix porosity and permeability are overshadowed by high permeability developed in structurally influenced karstic conduit systems. High permeability is developed in the confined part of the aquifer at depths greater than 4,000 feet below sea level, as well as in the unconfined zone. Optimization of use of this heavily subscribed aquifer requires accurate quantification and realistic mapping of the relationships between the limestone matrix, which stores most of the water, and the conduit system, which transmits water into, through, and out of the aquifer. This balance between storage and drainage is a key variable needed for predicting sustainability of flow during periods of low recharge and heavy use.
The purpose of this study is to interpret and integrate a selection of the existing diverse data to better characterize the conduit system. Data sets selected to feed into this interpretation include (1) water-level data, (2) structural information, (3) cave maps, (4) existing water-chemistry data, and (5) well hydrographs. It is intended that analysis of the existing data will supply hypotheses that will then be tested using field-based followup studies. These might include injected tracers; high frequency, closely spaced water level monitoring; conductivity monitoring; and well testing to better define flow systems.
A regionally extensive system of high-permeability zones is defined by broad troughs in the potentiometric surface in the confined zone. Indications of connections of the confined aquifer to the recharge zone are weakly defined by troughs in the available water-level data. Intersection of the Edwards/Trinity merged potentiometric surface maps with base Edwards structure maps to show that the Edwards Formation is saturated over only a part of the Edwards outcrop zone. This fact suggests that recharge flows through the Edwards into the upper beds of the underlying Glen Rose Formation before returning to the Edwards. Steep gradients of 100 ft/mi are mapped where flow crosses between the Glen Rose and the Edwards Formations, showing that the cross-formational flowpaths are much less transmissive than those in the Edwards confined zone. Spring discharge at the Edwards-Glen Rose contact suggests that perched water tables or flood-stage perched conduits might have a significant role in transmitting water through the recharge zone.
Cave maps show a persistent overprint of vadose-zone karst development, forming near-vertical shafts; however, in the horizontal segments, cave elongation shows that about half the caves are elongated along the principal fault direction. This orientation documents the role of opening-mode fractures parallel to the Balcones Fault Zone trends in controlling permeability. The other half of the horizontally elongated caves trend in other directions, reflecting multiple fracture systems in this structurally complex system.
Local gradient toward a local discharge point is interpreted to be a possible contribution to elongations of some cave patterns.
Water chemistry from Texas Water Development Board (TWDB) files is used as an indicator of flowpath geometry. High-salinity water (>3000 mg/L total dissolved solids [TDS]) indicative of long residence times is found in the deep part of the Edwards aquifer and in parts of the Glen Rose (Trinity) aquifer. These areas are therefore interpreted as regions bypassed by low-TDS recharge water moving through conduits.
Other Trinity samples have salinities in the same range as that of average Edwards water (200 to 500 mg/L) and may indicate faster flowpaths that communicate from the Trinity to the Edwards and from the Edwards recharge zone through the Trinity and back into the Edwards. About 5% of the Edwards waters in the confined Edwards are strongly undersaturated with respect to calcite, interpreted as an indicator of rapid conduit flow that has limited reactions between rock and water. Undersaturated samples are not strongly clustered in the major conduits zones defined by troughs in the potentiometric surface, suggesting that a network of conduits is active, with groundwater velocities adequate to impact chemistries. Samples with reported high (more than 30 mg/L) nitrate in the confined aquifer are tentatively interpreted as indicators of fast flowpaths from surface sources of nitrate; however, well leakage and sample contamination are alternative explanations.
Separation of representative well hydrographs recession curves shows variability in the ratio of matrix to fracture storativity. However, strong rapid drainage characteristic of conduits was not observed in the small population of hydrographs analyzed.