Mark ChildreNatural Sciences and Kinesiology Department
Laredo Community College West End Washington Street Laredo, TX
AbstractThe hydrogeologic controls, flow velocities,
flow direction, groundwater delineation, and physical
characteristics in a joint controlled dendritic conduit- spring
system are characterized. The known conduit extends from Magic
Spring to and past CM Cave with 4,475 m of measured passages
and tributaries. Four storm events were measured characterizing
the system's hydrodynamics. The rise time and half flow period
time (t0.5) occur in less than a day. The volume of ground
stored in conduits is approximately one half million m3. Storm
flows into the conduit-spring system drain within 3.7 to 7.5
days. This system is thermally ineffective with little radial
heat flux into the conduit walls. The field components of this
study include a karst feature density survey, four dye traces,
continuous monitoring of dynamic parameters, stage height, and
discharge at Magic Spring. Hydrographs and chemographs show
patterns interpreted as pulses of water recharging through
caves, sinkholes, and a stream sink. These pulses are
superimposed on baseflow from the joint controlled dendritic
conduit- spring system. The dye tracing results identified
groundwater piracy across surface water divides. The storm flow
velocities at Magic Spring ranged between 8,700 and 15,120 m/d
with baseflow characteristics below 3,000 m/d.
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 1Texas, USA. It was chosen as a study location based on history and previous work by Veni (1994). Veni completed his dissertation on the hydrogeology of the lower member of the Glen Rose formation (Lower Glen Rose) as a stratigraphic setting. Veni (1994) focused his research south of the Guadalupe River. This investigation was conducted north of the river in the same formational setting. Similar composition and texture occur in both areas. Karst landscapes in limestone terrain, such as the Magic Spring drainage basin, are the result of bedrock dissolution where recharged water dissolves calcite and dolomite, enlarging fractures and joints, forming sinkholes and caves (Palmer, 2007). Surface streams interact complexly with karst groundwater systems. Locally at the research site, runoff is intercepted by karst features and drains through the conduit system to Magic Spring. The purpose of this study is to determine where the recharge points located, the interconnections of the gained from this study will be useful for improved management of this groundwater resource. Techniques used to meet the goals of this study included dye tracing, and water quality and quantity monitoring. Water quality sondes, pressure transducers, and autosamplers were used during a test period from January 29, 2012 until August 22, 2012.Study Area and Hydrogeologic SettingThis Spring Branch Creek watershed is bounded by the Guadalupe River to the south and the Twin Sisters hills and related highlands to the north (Figure 1). The local topography of the study site is a gently rolling landscape Abstract direction, groundwater delineation, and physical characteristics in a joint controlled dendritic conduitspring system are characterized. The known conduit extends from Magic Spring to and past CM Cave with 4,475 m of measured passages and tributaries. Four storm events were measured characterizing the time (t0.5) occur in less than a day. The volume of ground stored in conduits is approximately one half million m3. to 7.5 days. This system is thermally ineffective with little density survey, four dye traces, continuous monitoring of dynamic parameters, stage height, and discharge at Magic Spring. Hydrographs and chemographs show patterns interpreted as pulses of water recharging through caves, sinkholes, and a stream sink. These pulses are superimposed on spring system. at Magic Spring ranged between 8,700 and 15,120 m/d IntroductionThe Spring Branch Creek drainage basin is located in the eastern part of the Edwards Plateau in Comal and Kendall counties, near the town of Spring Branch, Mark Childre Natural Sciences and Kinesiology Department Laredo Community College West End Washington Street Laredo, TX 78040 email@example.comHYDROGEOLOGIC CONTROLS ON THE OCCURRENCE AND MOVEMENT OF GROUNDW A TER DISCHARGED A T MAGIC SPRING IN THE SPRING BRANCH CREEK DRAINAGE BASIN, SPRING BRANCH, TEXAS
NCKRI SYMPOSIUM 20th National Cave and Karst Management Symposium2resulted in thin stony soils. Residential development is becoming much more common.HydrogeologyThis study site is stratigraphically in the lower member of Glen Rose Limestone (Lower Glen Rose) atop the contact with the Hensel Member of the Pearsall Formation. The Spring Branch area (Hammond, 1984). The bedding dips approximately 0.5o and strikes 130o along the contact of the Lower Glen Rose and Hensel, as calculated using that is dissected by steep and narrow drainages over composed of eight tributary watersheds covering the surface drainage. Its main channel is a 14.4 km long limestone-bedded waterway that drains north to south. Originally covered by juniper-oak savanna and mesquite-oak savanna, most of the drainage basin is used for grazing beef cattle, sheep, goats, and wildlife. Hunting leases are a major source of income. Erosion and the environmental climate in the Spring Branch area Figure 1. Spring Branch Creek drainage basin and research site.
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 3Rainfall at US Geological Survey station 08167347 Honey Creek Site 1C near Spring Branch, Texas. A daily record of rainfall was also documented at Magic Spring throughout the research period.Magic Spring Discharge and Hydraulics A rating curve was constructed by measuring the springs meter and a wading staff were used. Multi-Parameter TROLL 9000 and YSI 556 MPS (Multi-Probe System) pressure, and temperature.Dye TracingCharcoal receptors and an Isco 6712 full-size portable auto-sampler were used to sample for dyes at the monitoring points. The methods and test procedures for tracing were adopted from Schindel & Johnson (2007). Dye injection and monitoring Four caves were chosen as dye injection locations to establish direct paths to the main groundwater conduit system. Sattlers Deep Pit is within the Spring Branch Creek surface drainage basin, as is Echo River in CM Cave. Cave Crack and No La Vie Cave are in the Cypress injection locations, dye, dye quantity, when injected, and when recovered. Monitoring for dye occurred at Magic Spring, with an ISCO 6712 automatic water sampler, and at 11 other locations along Spring Branch and bordering creeks to the east and west with activated charcoal packets. The the dye peaks were calculated using analytical software.ResultsKarst DensityTwo areas with high densities of karst features were discovered. One is in the area targeted by this study around CM Cave. The second is about 1 km to the northeast and surveyed by a team of cavers led by Terry as at least 20 karst features/0.16 km2. The combined survey area covered 3.52 km2 and was found to have 146 sinkholes, pits, caves, and a stream sink. previously released stratigraphic mapping and the threepoint method. Spring Branch Creek dissects the upper and lower members of the Glen Rose Limestone and the underlying Hensel and Cow Creek members of the Pearsall Formation. Groundwater development of the Lower Glen Rose Aquifer began about 1.2 Ma when the Lower River drainage basin. Spring Branch is a tributary to the Guadalupe near the downstream limit of the Lower Glen Rose outcrop (Veni, 1994). Since 1.2 Ma to the present, the water level in the aquifer has declined because of the incising of the Guadalupe River. Cave SystemsThe Magic Spring-CM Cave system is the primary conduit system in the study area. Magic Spring has been known for decades but could not be explored until recently with diving equipment. Rambo (1990) discussed cave exploration north of the Guadalupe River and when a sinkhole was excavated in 1989 to reveal the entrance pit of CM Cave. A second short pit followed and at a depth of 27.2 m below the surface led into a (OMG). This 16-m long passage is a tributary to Echo River, the main conduit that feeds Magic Spring about 1.3 km downstream; upstream Echo River has been explored over 2.4 km. Additionally, 720 m of tributary passages, have been surveyed to date. The main stream ends in a sump; exploration continues by divers. The Magic Spring-CM Cave system is hydrologically perched on the Hensel. It passages are typically guided by joints. Recharge mostly occurs through overlying sinkholes and caves that have not yet been physically connected into the larger cave system. The conduit network has a dendritic pattern, although this is only subtly seen in map view due to limited exploration of most tributary passages.MethodsKarst Density SurveyA karst density survey was performed in order to features. The survey was completed after a 200 x 200 meter grid system was established around the CM Cave entrance, and 49 sinkholes and 43 smaller caves were recorded.
NCKRI SYMPOSIUM 20th National Cave and Karst Management Symposium4Cave and Cave Crack are located east of Echo River, respectively about 100 m and 200 m downdip in the Cypress Creek surface water drainage basin. Dye from No La Vie Cave was detected from a tributary passage at survey marker MM5, located 68 m upstream of the detected in Echo River from a tributary conduit at survey marker CA18, over 300 m downstream of the CM Cave entrance (Figure 2). DiscussionHydrodynamic Response: Two Pulse Recharge EventMagic Springs hydrographs show bimodal behavior as (Figure 3). In contrast, dye tracing from within Echo River and from No La Vie Cave indicate a unimodal response. This bimodal behavior indicates at least two distinct recharge paths upstream of the entrance to CM Cave. This behavioral response supports the hypothesis that recharge enters the groundwater system at multiple focused locations. The mapping of CM Cave revealed a joint controlled dendritic pattern bearing approximately 45o and 315o in the subsurface. The survey of karst surface features revealed that most bear approximately 300o.Hydrogeologic DataFour storm events were recorded between February and May 2012. The May storm event occurred after a dry period resulting in a logarithmic decline in discharge from evapotranspiration.Dye Tracing ResultsHydrograph data and sampling results for each of the four traces were evaluated. Groundwater velocities, hydraulic connectivity between different creek drainage basins is assessed. All four dyes were detected at Magic Spring, two dyes showed up in water samples from the auto-sampler and the other two dyes from the charcoal packets. that conduit to Magic Spring. Dye injected at the three surrounding caves entered Echo River as follows. Dye from Sattlers Deep Pit was detected in the furthest upstream sampled location at survey marker VB1, about 600 m upstream of the CM Cave entrance. No La Vie Test # Injection Point Injection Date Dye Dye Quantity grams Dye Recovery Location Arrival Time 1 CM Cave 6/3/2012 13:38 Uranine (10.85g) 10.85 Magic Springs 6/4/2012 4:19 2 No La Vi Cave 6/24/2012 12:10 Eosin (168g) 168 Magic Spr-MM3 6/25/2012 5:30 3 Cave Crack 6/30/2012 12:22 SRB (146g) 146 Marker Ca18 6/24 8/19/12 Cave Crack 6/30/2012 12:22 SRB (146g) 146 Magic Springs 7/4 7/15/12 4 Sattlers Deep Pit 7/1/2012 13:06 Uranine (254g) 254 Marker vb1 6/24 8/19/12 Sattlers Deep Pit 7/1/2012 13:06 Uranine (254g) 254 Magic Springs 7/4 7/15/12 Test # Distance (apparent) meters (m) Distance (actual) meters (m) Travel Time from Inject to LOD days (d) Apparent Velocity (m/d) Actual Velocity (m/d) Sinuosity 1 893 1816 0.61 1441 2929 2.03 2 837 1391 0.745 1123 1865 1.66 3 829 1209 charcoal 1.46 89 1209 charcoal 1.46 4 2410 5417 charcoal 2.25 2410 5417 charcoal 2.25Table 1. Dye tracing results from Magic Spring-CM Cave.
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 5 downstream toward the spring.Hydrodynamics velocities ranged between 8,400 m/d and 15,120 m/d sites (Figure 4). The primary recharge source for the second pulse is from Cool Creek Cave, a stream sink about 1.3 km upstream of Magic Spring that takes water from Spring Branch Creek. That reach of the during a storm event was observed to enter Cool Figure 2. Magic Spring-CM Cave dye injection and monitoring locations.
NCKRI SYMPOSIUM 20th National Cave and Karst Management Symposium6 three recharge points: the two high density karst areas and Cool Creek Cave (Figure 6). The karstic terrain, stream sink, topography, and conduit characteristics are the primary controls on the rapid bimodal response. Magic Springs hydrologic response is characterized based on its hydrographs rising limb, falling hydraulic events that happen within this rise time period would be the same and invariant with maximum discharge. and maximum diffusivity are known and will be the same for all four events. Historically, there is a 50% ratio for Atkinson, 1977). An analysis of the four storm events monitored for this study shows the ratio between the focused karst areas and the stream sink ranges from about 30-83% during maximum discharges that respectively range from about 425-1,160 L/s. The thermodynamic response of the cave system is ineffective during storm events, such that 85-87% of the temperature change is transmitted over 1.3 km. Figure 3. Four storm event hydrographs and chemographs: Magic Spring, 2012. Figure 4. Response of SpC exhibiting two-pulse behavior.
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 7For each storm event in Table 2, the dynamics for both the rising limb and the recession limb may be calculated. 0.5, rising and is invariant with values around 6 hours while the t0.5 ranges between 12.9 to 15.7 hours depending on total discharge. The storm events are superimposed and plotted in Figure 7 for comparison. The sum of the rise 3.8 to 7.5 days. Surface drainage from Cypress Creek basin is partially pirated by the groundwater discharged at Magic Spring. As has been well established in the literature, karst groundwater drainage basins cannot be reliably delineated based on surface water drainage boundaries. Dye tracing, karst feature surveys, and spring hydrograph data should also be considered. In this study, two of the four dye injection points were in the Cypress Creek surface drainage basin. Their detection in CM Cave and Magic Spring demonstrates groundwater piracy from the Cypress Creek area, increasing the size of the springs groundwater drainage basin to the east. Additionally, the sinking of Spring Branch Creek into Cool Creek Cave the conduit system and suggests a possible decrease in groundwater basin size between Cool Creek Cave and Magic Spring (Figure 8). However, north of Cool Creek Cave the groundwater drainage was greatly increased to Aquifer Volume and Mass Balance of the aquifer. Tracer mass recovery at Magic Spring was measured for a rough estimate of the maximum conduit volume. If a single discharge value is used as a mean spring discharge then the volume of groundwater stored in conduits at the time of the tracer test may be estimated by: where Q is mean spring discharge and V is the Shape and Characteristics of Hydrologic Response Using DischargeThe rate of withdrawal of water from storage, from the springs, or from pumping; is indicated by the slope of of a spring is a function of the volume of water held in A t0.5 discharge. Substitution into previous equation gives: where The parameter t0.5 change, and is a direct measure of the rate of recession and therefore can be used as a means of characterizing There is a linear relationship between hydraulic head conditions), and the curve can be expressed as a straight in Table 2 may be determined from: Figure 5. Conduit flow velocity at Magic Spring-CM Cave, and time delay between recharge and discharge. = = = . = = ( ) = =
NCKRI SYMPOSIUM 20th National Cave and Karst Management Symposium8 Figure 6. Two pulse recharge system to CM Cave and Magic Spring.
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 9 Storm Event Rising Response Initial Response Final Response Time from (bf) to bf+1 cfs days Average cfs Rising Time hrs Rising Rate cfm/m 2/18/2012 1:17 3/9/2012 17:29 3/20/2012 1:56 5/10/2012 20:00 2/21/2012 19:57 3/15/2012 0:54 3/27/2012 12:53 5/15/2012 0:22 3.78 5.31 7.46 4.18 14.08 14.31 26.30 7.77 6.33 6.00 6.50 6.08 2.93 2.58 2.74 1.71 1/hr Time bf to bf+1 cfs days Time t0.5 hrs cfs cfs cfs 3.78 5.31 7.46 4.18 12.75 13.92 15.17 12.92 19.95 26.22 41.17 15.41 10.18 13.69 21.70 8.56 0.42 1.19 2.29 1.71 0.052 0.047 0.042 0.046Table 2. Magic Spring discharge characteristics during peak flows. Figure 7. Magic Spring combined storm flow recession hydrographs.
NCKRI SYMPOSIUM 20th National Cave and Karst Management Symposium10 pulse were calculated and the ratio between the pulses established. This ratio correlates best with the maximum discharge and shows the possibility of the stream sink at Cool Creek Cave dominating the second pulse as discharge increases. The conduit system is thermally ineffective with 85% hydrograph pulse following a storm event. It has a rise time (t0.5) between 12.8 and 15.2 hours. The total time from storm event to t0.5 is less than one day. The storm velocities were measured between 8,400-15,120 m/d for over 1.3 km of the cave stream. The groundwater drainage basin was preliminarily are expected as more tracer tests are completed under RecommendationsPresently, there is no groundwater management authority for this system. While much of its drainage basin is in Cow Creek Groundwater Conservation District, that district manages the Cow Creek Aquifer, not the Lower Glen Rose. A groundwater conservation district should be formed similar for the Lower Glen Rose Aquifer, but based on hydrogeologic boundaries otherwise each time period, 33.6% of the dye injected into Echo River was recovered (% R) at Magic Spring. An estimate may be calculated: It is important to stress that this volume does not represent humanly accessible cave-size conduits but Simply dividing the calculated volume by the channel cross section shown in Figure 9 suggests similar size passages in excess of 97 km in length along the traced Cave demonstrated exceptionally high storage volumes due to the honeycomb conduit porosity of basal Glen ConclusionsThe Magic Spring-CM Cave system has over 4.5 km of mapped joint controlled passages organized into a dendritic pattern. It is overlain by 146 known karst recharge features, many of which occur in the two high Figure 8. Groundwater drainage basins for Magic Spring-CM Cave. Figure 9. Cross section of CM Cave passage at Magic Spring. = ( ) % = ( ) =
20th National Cave and Karst Management Symposium NCKRI SYMPOSIUM 11References limestone terrain in the Mendip Hills, Somerset (Great Britain): Journal of Hydrology, 35(1-2), 93-110. Ford, D. and Williams, P.W., 2007, Karst Hydrogeology and Geomorphology: Chichester, England, John Wiley & Sons. Hammond, W., 1984, Hydrogeology of the Lower Glen Rose Aquifer, South-Central Texas: Ph.D, Austin, University of Texas. Palmer, A.N., 2007, Cave Geology: Dayton, OH, Cave Books. Rambo, Bill, 1990, Eisenhauer Ranch Caves: Texas Caver, v. 35, no. 5, p. 96-98. Schindel, Geary M. and Johnson, Steven, 2007, Tracer Test Work Plan for Kinney and Uvalde Counties: Edwards Aquifer Authority, 21p. Veni, G., 1994, Geomorphology, hydrogeology, geochemistry, and evolution of the karstic Lower Glen Rose aquifer, south-central Texas. Ph.D., Pennsylvania State University. management of this and other groundwater systems would be split between multiple agencies. A meeting of the property owners in the Magic Springs drainage basin from both counties should be held to discuss their mutual water resource. The Magic Spring-CM Cave system requires additional the characteristics of this system, potentially giving evidence to other possible discharge points. That work should include Cool Creek Cave. Although the regional dip has been established using the three point method on geologic surface maps, the local dip should be surveyed. Local variations in the dip and orientation may have impact on the overall recharge characteristics. The passages at the junctions near survey markers AA1 and MM5 (Figure 2) should be closely investigated. Veni current drainage basin boundaries.AcknowledgementsWithout the support, assistance, and access granted by the property owners, this study would not have been possible. I was granted access to Magic Spring by Will McAllister IV, who also provided rainfall data for the spring site. The entrance to CM Cave and unending support was given by Joe Eisenhauer. David Oden allowed dye injection at Sattlers Deep Pit. Special thanks must be offered to Wally Henderson who owns Cool Creek Cave. Funding was provided by the South Texas Geological Society Jones-Amsbury Research Grant, and The University of Texas at San Antonios Department of Geological Sciences and the Center for Water Research. The Edwards Aquifer Authority provided the dyes, lab support, and the lab for analysis of the traced water samples. Special thanks are extended to Bexar Grotto, CM Cave survey teams, and Terry Holsinger for their continued technical and physical support.
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