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Structural Influence on Groundwater Flow as Evidenced by Groundwater Dye Tracing in the Barton Springs Segment of the Edwards Aquifer, Central Texas: Implications for Modeling Conduits

  • April 2005
  • Conference: South Central Geological Society of America
  • At: San Antonio, Texas
Authors:
Brian B. Hunt at University of Texas at Austin
Brian B. Hunt
Brian A. Smith at Caves and Karst LLC
Brian A. Smith
  • Caves and Karst LLC
Joseph Beery
Joseph Beery
Joseph Beery
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Nico Hauwert at City of Austin
Nico Hauwert
  • City of Austin
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Abstract

Major groundwater flow routes in the Barton Springs segment of the Edwards Aquifer closely follow the structural trends of the Balcones Fault Zone. Locations of these groundwater flow routes and velocities of groundwater flowing along the routes were determined by injecting fluorescent, non-toxic, organic dyes (Fluorescein and Eosine) into two caves within Onion Creek, the largest capacity losing stream in the Barton Springs segment of the Edwards Aquifer. Cripple Crawfish and Antioch caves are located about 17.5 mi and 14.0 mi south of Barton Springs, the primary discharge point from the aquifer, respectively. First arrival of dyes from each of these caves to Barton Springs occurred after about 3.5 and 7.0 days, indicating minimum groundwater velocities under high spring flow and water-level conditions of 5.0 and 2.0 mi/day, respectively. Dye detections at water-supply wells indicate a karst system composed of multiple diverging flow routes from these caves, which appear to re-converge as groundwater flows northeast discharging at Barton Springs. Interpreted flow paths are oriented normal or parallel to potentiometric contours, indicating a highly anisotropic flow system consisting of conduits. Conduit flow within the aquifer appears to be strongly influenced by the bimodal fault and fracture system with trends of N40E (dominant) and N45W (secondary). Groundwater flow was traced in wells along paths that are parallel to the dominant fault trend presented on geologic maps. Groundwater flow was also traced parallel to the secondary structural trend, at a high angle to the dominant mapped fault trend. Results indicate that conduits are an important component of flow, and that the bimodal structural grain has influenced conduit flow in this aquifer. Future numerical groundwater models incorporating conduit flow should consider, among other data, structural grain for modeling the distribution and orientation of conduits.
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Structural Influence on Groundwater Flow as Evidenced by Groundwater Dye
Tracing in the Barton Springs Segment of the Edwards Aquifer, Central Texas:
Implications for Modeling Conduits
Brian B. Hunt, P.G., BSEACD; Brian A. Smith, Ph.D., P.G., BSEACD; Joseph Beery,
BSEACD; Nico Hauwert, P.G., COA; David Johns, P.G., COA
Major groundwater flow routes in the Barton Springs segment of the Edwards Aquifer
closely follow the structural trends of the Balcones Fault Zone. Locations of these
groundwater flow routes and velocities of groundwater flowing along the routes were
determined by injecting fluorescent, non-toxic, organic dyes (Fluorescein and Eosine)
into two caves within Onion Creek, the largest capacity losing stream in the Barton
Springs segment of the Edwards Aquifer. Cripple Crawfish and Antioch caves are located
about 17.5 mi and 14.0 mi south of Barton Springs, the primary discharge point from the
aquifer, respectively. First arrival of dyes from each of these caves to Barton Springs
occurred after about 3.5 and 7.0 days, indicating minimum groundwater velocities under
high spring flow and water-level conditions of 5.0 and 2.0 mi/day, respectively. Dye
detections at water-supply wells indicate a karst system composed of multiple diverging
flow routes from these caves, which appear to re-converge as groundwater flows
northeast discharging at Barton Springs. Interpreted flow paths are oriented normal or
parallel to potentiometric contours, indicating a highly anisotropic flow system consisting
of conduits. Conduit flow within the aquifer appears to be strongly influenced by the
bimodal fault and fracture system with trends of N40E (dominant) and N45W
(secondary). Groundwater flow was traced in wells along paths that are parallel to the
dominant fault trend presented on geologic maps. Groundwater flow was also traced
parallel to the secondary structural trend, at a high angle to the dominant mapped fault
trend. Results indicate that conduits are an important component of flow, and that the
bimodal structural grain has influenced conduit flow in this aquifer. Future numerical
groundwater models incorporating conduit flow should consider, among other data,
structural grain for modeling the distribution and orientation of conduits.
... Groundwater flow paths in the Barton Springs segment of the Edwards Aquifer have been conceptualized to be focused along solutioned (karstic) NE-trending fractures and faults (Hunt et al., 2005). The primary flow path in the Barton Springs segment of the Edwards Aquifer, determined from potentiometric maps and dye tracing, is called the Manchaca Flow route (labeled "MF" on Figure 5;Hauwert et al., 2004). ...
... Dye tracing results summarized from Hauwert et al., 2004 and Trinity Aquifers (Ferrill et al., 2004). Faults have generally been the primary structure cited in the literature to influence groundwater flow (Maclay and Small, 1986;Hunt et al., 2005). Aquifer permeability is reported to generally be enhanced parallel to faults and decreases perpendicular to faults in the Edwards Aquifer (Ferrill et al., 2004;Ferrill et al., 2008). ...
Relay Ramp Structures and Their Influence on Groundwater Flow in the Edwards and Trinity Aquifers, Hays and Travis Counties, Central Texas
Conference Paper
Full-text available
  • Oct 2015
The Cretaceous Edwards and Middle Trinity Aquifers of central Texas are critical groundwater resources for human and ecological needs. These two major karst aquifers are stratigraphically stacked (Edwards over Trinity) and structurally juxtaposed (normal faulting) in the Balcones Fault Zone (BFZ). Studies have long recognized the importance of faulting on the development of the karstic Edwards Aquifer. However, the influence of these structures on groundwater flow is unclear as groundwater flow appears to cross some faults, but not others. This study combines structural and hydrological data to help characterize the potential influence of faults and relay ramps on groundwater flow within the karstic Edwards and Middle Trinity Aquifers. Detailed structure contour maps of the top of Walnut Formation in the study area were created from a geologic database (n=380) comprised of primarily geophysical and driller’s logs. The data were then contoured in Surfer® (Kriging) with no faults. Structure contour surfaces revealed detailed structural geometries including linear zones of steep gradients interpreted as faults) with northeast dipping zones of low gradients (interpreted to be ramps) between faults. Hydrologic data (heads, dye trace, geochemistry) were overlaid onto the structure contour maps in GIS. Results for the Middle Trinity Aquifer suggest relay ramps provide a mechanism for lateral continuity of geologic units and therefore groundwater flow from the Hill Country (recharge area) eastward into the BFZ. Faults with significant displacement (>100 m) can provide a barrier to groundwater flow by the juxtaposition of contrasting permeabilities, yet flow continues across fault zones where ramps exist, or where permeable units are juxtaposed with other permeable units. In the Barton Springs segment of the Edwards Aquifer the primary flow path defined by dye tracing and heads is coincident with the Onion Creek relay ramp dipping to the northeast. This work addresses the lateral continuity (intra-aquifer flow) of the Edwards and Trinity Aquifer systems, which has importance for conceptual models and ultimately resource management.
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... The Edwards Aquifer is geologically and hydraulically heterogeneous and anisotropic, both of which strongly influence groundwater flow and storage (Slade et al., 1985;Maclay and Small, 1986;Hovorka et al., 1996;Hovorka et al., 1998;Hunt et al., 2005). Karst aquifers such as the Barton Springs segment are commonly described as triple porosity (and permeability) systems consisting of matrix, fracture, and conduit porosity (Ford and Williams, 1992;Quinlan et al., 1996;Palmer et al., 1999). ...
... Groundwater generally flows west to east across the recharge zone, converging with preferential groundwater flow paths subparallel to major faulting, and then flowing northward toward Barton Springs. Rates of groundwater flow along preferential flow paths, determined from dye tracing, can be as fast as 4 to 7 miles per day under high-flow conditions or about 1 miles per day under low-flow conditions (Hauwert et al., 2002;Hunt et al., 2005). ...
A Comparison of the 1950s Drought of Record and the 2009 Drought, Barton Springs Segment of the Edwards Aquifer, Central Texas
Article
Full-text available
The Barton Springs segment of the Balcones Fault Zone Edwards Aquifer is an im-portant resource for water supply and environmental flows; however extreme droughts can limit the amount of available water. The Barton Springs/Edwards Aquifer Conser-vation District has developed its drought management policies specifically for a recur-rence of the 1950s drought of record (DOR). A comparison was made between the DOR and the 2009 drought using springflow, streamflow, rainfall, and water-level data. Gen-erally, the values of these parameters for the DOR are slightly lower than those during the 2009 drought. The 24-month rainfall total for the 2009 drought was almost the same as the value for the last 2 years of the DOR, 35.2 and 34.8 inches, respectively. However, from a water-budget perspective, there was almost twice the amount of water being dis-charged (by pumping and springflow) at the end of the 2009 drought compared to the DOR. Several possible explanations for the difference in water budgets between the DOR and the 2009 drought are: (1) the DOR was considerably longer so the amount of water in storage was more depleted; (2) there was a long-term shift to wetter conditions after 1957 leading to more water in storage during the 2009 drought; and (3) increased pumping since the 1950s could have been offset by an increase in flow from adjacent and underlying aquifers, or even urban recharge such as leaking water pipes. However, it is likely that during a recurrence of the DOR, springflow will decrease below rates ob-served during the DOR owing to higher rates of pumping.
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... More recent water balance estimates of the Barton Springs segment suggest that more water could be recharged in the upland or intervening areas (Hauwert, 2009; Hauwert, 2011; Hauwert, 2012). The Edwards Aquifer is inherently heterogeneous and anisotropic, characteristics that strongly influence groundwater flow and storage (Slade et al., 1985; Maclay and Small, 1986; Hovorka et al., 1996 and 1998; Hunt et al., 2005). The Edwards Aquifer can be described as a triple porosity and permeability system consisting of matrix, fracture, and conduit porosity (Hovorka et al., 1995; Halihan et al., 2000; Lindgren et al., 2004) reflecting an interaction between rock properties, structural history, and hydrologic evolution (Lindgren et al., 2004). ...
Cover-Collapse Sinkhole Development in the Cretaceous Edwards Limestone, Central Texas
Conference Paper
Full-text available
  • May 2013
Sudden cover-collapse sinkhole (doline) development is uncommon in the karstic Cretaceous-age Edwards limestone of central Texas. This paper presents a case-study of a sinkhole that formed within a stormwater retention pond (SWRP) in southwest Austin. Results presented include hydrogeologic characterizations, fate of stormwater, and mitigation of the sinkhole. On January 24, 2012, a 11 cm (4.5 in) rainfall filled the SWRP with about 3 m (10 ft) of stormwater. Subsequently, a sinkhole formed within the floor of a SWRP measuring about 9 m (30 ft) in diameter and 4 m (12 ft) deep. About 26.5 million liters (7 million gallons) of stormwater drained into the aquifer through this opening. To determine the path, velocity, and destination of stormwater entering the sinkhole a dye trace was conducted. Phloxine B was injected into the sinkhole on February 3, 2012. The dye was detected at one well and arrived at Barton Springs in less than 4 days for a minimum velocity of 2 km/day (1.3 mi/day). Review of pre-development 2-foot topographic contour and geologic maps reveals that the SWRP was built within a broad (5,200 m2; 6 acre), shallow depression bounded by two inferred NE-trending fault zones. Photographs taken during SWRP construction showed steep west-dipping bedrock in the northern SWRP wall. Following collapse of the sinkhole, additional hydrogeologic characterization included excavation to a depth of 6.4 m (21 ft), surface geophysics (resistivity), and rock coring. Geologic materials consisted mostly 89of friable, highly altered, clayey limestone consistent with epikarst in-filled with terra rosa providing a cover of the feature. Dipping beds, and fractured bedrock support proximity to the mapped fault zone. Geophysics and surface observations suggested a lateral pathway for stormwater flow at the junction between the wet pond’s impermeable geomembrane and compacted clay liner for the retention pond. The collapse appears to have been caused by stormwater down-washing poorly consolidated sediments from beneath the SWRP and into a pre-existing karst conduit system. Mitigation of the sinkhole included backfill ranging from boulders to gravel, a geomembrane cover, and reinforced concrete cap. Additional improvements to the SWRP included a new compacted clay liner overlain by a geomembrane liner on the side slopes of the retention pond.
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... The Edwards Aquifer is geologically and hydraulically heterogeneous and anisotropic, both of which strongly influence ground-water flow and storage (Slade et al., 1985;Maclay and Small, 1986;Hovorka et al., 1996;Hovorka et al., 1998;Hunt et al., 2005). Karst aquifers such as the Barton Springs segment are commonly described as triple porosity (and permeability) systems consisting of matrix, fracture, and conduit porosity (Ford and Williams, 1992;Quinlan et al., 1996;Palmer et al., 1999). ...
SUSTAINABLE YIELD OF A KARST AQUIFER IN CENTRAL TEXAS
Conference Paper
Full-text available
  • Jan 2007
In recent years, various hydrogeologic studies have been conducted to characterize the Barton Springs segment of the Edwards Aquifer and to support sustainable yield determination. These studies include: geologic mapping, pumping tests, tracer tests, evaluation of historical data, numerical ground-water modeling, well-impact analysis, and biological studies. A broad definition of sustainable yield considers that water can be extracted from an aquifer only to the extent that no undesired results take place. The Barton Springs/Edwards Aquifer Conservation District has determined that undesired results for the aquifer are sole-source water-supply wells' going dry and jeopardy of the endangered salamander population at Barton Springs. Historical data show that during the 1950's drought of record, flow from Barton Springs reached a low of 9.6 cubic feet per second (cfs). Currently, permitted and exempt annual pumpage from the aquifer is 11.3 cfs, and actual pumpage from the aquifer in 2006 is estimated to have been 9.6 cfs. If drought-of-record conditions recur, and actual pumpage is 9.6 cfs or greater, it is likely that flow from Barton Springs will cease for some period of time. Considering that water quality is also expected to decrease as spring flow decreases, it is not likely that the endangered salamanders will survive. Additional studies based on historical water levels, ground-water modeling, and well-completion data indicate that as many as 20% of the wells in the District will experience yield problems under these conditions. To address these concerns, the District has implemented a conditional permit policy and has set drought trigger levels for the aquifer. Additional studies are being conducted on potential impacts to the endangered salamanders and on possible use of alternative water sources. When results are available from these studies, the District's ground-water management plan will likely be further revised to reflect changes to our definition of sustainable yield.
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