1. Groundwater velocity (m/s)
Groundwater-Surface Water Connectivity in a Karst System: Dye-tracing Morrell Cave, Bluff City, TN
Jana Archer and Ingrid Luffman, Department of Geosciences, East Tennessee State University, Johnson City, TN 37614
Abstract
The purpose of this project was to examine surface and groundwater connectivity in the Morrell Cave springshed to identify the hydrologic connection between Dry Creek and Morrell Cave Stream using dye tracings. Two fluorescent dyes (Fluorescein and Rhodamine WT) were injected in to Dry Creek at two different locations. Surface water and activated charcoal samplers were used to identify and quantify dye movement. Fluorescein was detected midway through the cave system, and was detected 136 hours after injection. Rhodamine WT was not detected in the cave, but was detected at Morrell Spring (as was Fluorescein). Assuming a straight line distance from injection point to cave, groundwater velocity from the Dry Creek injection point to Morrell Cave is calculated at a minimum of 0.05 m/s. This research indicates that Dry Creek is one of multiple water sources for the Morrell Cave stream.
Introduction
Karst landscapes form by carbonic acid solution of soluble rocks by groundwater flow through joints. Over time solutional landforms (sinkholes, caves, springs, and swallets) form and a complex surface-subsurface drainage system develops. Dye tracing, the injection of non-toxic fluorescent dye into a sinking stream or sinkhole, followed by downgradient monitoring for dye resurgence, is a technique used to delineate surface and groundwater connectivity in karst landscapes. This project used dye tracing in the Morrell Cave springshed to identify a hydrologic connection between Dry Creek and Morrell Cave Stream in Bluff City, TN (Fig. 1).
Dye first detected at 136 hours
Figure 6: FL was detected at the ISCO sampler 136 hours after injection, peaked 2 days later, and returned to background levels two weeks later.
Table 1: Groundwater velocity estimated from injection point to cave sump. In-cave travel time (4 h), calculated from cave stream velocity ( m/s), was subtracted from total travel time to estimate groundwater flow velocity.
Dye
Total travel time (h)
Cave travel time (h)
Travel distance
(m)
Groundwater velocity (m/s) FL
136 4
2,340
0.005
RWT
(not recovered at ISCO)
stream flow
Cave
Figure 3: Study area hydrology, cave and sampler locations, and dye injection points. FL was detected in the cave and at the ISCO sampler. RWT was detected at Morrell Spring only, near its outlet to Holston River. A B
Lower entrance
Figure 7: FL detected in bugs 4, 6, and 7, no dye detected in bug 5 (deployed in a side spring). RWT was not detected in the cave (after Adams et al., 1973).
Field and Analytical Methods
Thirteen activated charcoal samplers (bugs) were deployed in the cave and surface water streams. Eight bugs were placed in the cave from to Five were placed in surface streams from to (replaced once).
Dye was injected on into Dry Creek (Fig. 2); Fluorescein (FL) was injected at Dry Creek and Chinquapin Grove Road (126 g) and Rhodamine WT (RWT) was injected at the sink (105 g) (Fig. 3).
Water samples were collected using an ISCO sampler located 71 m from the cave entrance (nine rounds of 24 samples were collected) and fluorescence spectra were captured using a Shimadzu spectrofluorophotometer.
Charcoal from each bug was eluted in a solution of 70% isopropyl alcohol, 30% deionized water, and 10 g/L NaOH (Fig. 4) and processed in the spectrofluorphotometer.
Fourteen salt traces were used to determine stream discharge (twelve at the cave entrance and two at the back of the cave) (Fig. 5).
Figure 1: Morrell Cave Location (Archer, 2014).
Discussion and Conclusion
Water from Dry Creek enters joints in the stream bed and flows northwest towards Morrell Cave, where it enters the cave stream between two sumps (bug locations 3 and 4, Fig. 7).
FL reached the ISCO sampler five days after injection, and groundwater flow velocity is calculated to be m/s. The Morrell Cave stream velocity during the trace was an order of magnitude higher at m/s.
Increased stream discharge associated with a 0.55” rainfall event on (CoCoRaHS) may have helped to push dye through the system (FL was detected at the ISCO shortly after the precipitation event).
The Morrell Cave stream is fed by multiple sources including Dry Creek. Bugs deployed in small side springs (Bugs 5 and 8) did not detect FL, and similarly FL was not detected in the deepest part of the cave (Bugs 1 – 3) indicating that Morrell Cave stream is fed by multiple sources that include Dry Creek. Previous dye traces have linked nearby swallets to the rear cave sump (Burnham, 2014). The water source for the in-cave springs has yet to be determined.
Both RWT and FL were detected at Morrell Spring indicating that Dry Creek is a water source for Morrell Spring.
Geologic and Hydrologic Setting
The study area is characterized by Cambro-Ordovician Knox Group carbonates which are overlain by the Ordovician Sevier Shale. Jointed Knox Group limestones are exposed across the springshed with the Sevier Shale acquiclude occuring around the margins and as isolated knobs.
Groundwater flow, through joints in the limestone, developed a three-level network of passages locally known as Morrell Cave or Worley’s Cave. A stream exits through the lower cave entrance and sinks underground some 300 m from the cave.
The study area is bounded by two surface streams: Indian Creek to the west and Dry Creek to the east. Both streams are tributaries of the Holston River. Dry Creek loses a portion of its flow though joints in the stream bed and its flow regularly sinks completely before reaching Holston River (Fig. 1). A B
Figure 4: Bug processing A) charcoal extraction; B) charcoal elution.
Acknowledgements
We acknowledge the valuable contributions of graduate assistant Taylor Burnham, and students, Nicolas Barnes, Michelle Bradburn, Crystal Johnson, Hannah Miltier, Tim Spiegel, and Trevor Wilson. Special thanks go to Ben Elrod for cave guiding.
References
Adams, M., Anderson, T., Booth, C., Bowery, R., Cox, J., Harrison, T., Mire, D., Powers, A., Powers, D., and Powers, J., Morrill Cave Map, Sullivan County, Tennessee. Unpublished map.
Burnham, T., 2014, Hydrogeology and Groundwater Flow of the Morrell Cave Springshed, Sullivan County, Tennessee. Master’s Thesis. East Tennessee State University
CoCoRaHS Community Collaborative Rain, Hail & Snow Network. Station Precipitation Summary. Oak Grove Road/Boone Lake Station (TN-SL-1). Accessed at http// on
Figure 2: Dye injection A) Fluorescein; B) Rhodamine WT.
Figure 5: Morell Cave stream discharge (in m3/s), measured using salt traces. Stream flow at the cave entrance decreased throughout the experiment. Flow at the back of the cave remained consistent.