Karst Water Resources (Proceedings of the Ankara - Antalya Symposium, July 1985). lAHSPubl. no. 161

APPLICATION OF DYE - TRACING TO DAM - SITE EVALUATION IN A KENTUCKY KARST AREA, U.S.A.

James F. Quinlan National Park Service Box 8 Mammoth Cave, Kentucky 42259, U.S.A.

Abstract

Leakage beneath or around a dam is always a potential problem in limestone terranes. Accordingly, when a small, low - cost flood - control dam on Short Creek, about 20 km west of Leitchfield, Kentucky, was proposed by the Soil Conservation Service, the hydro-geology of the dam site and impoundment area required study. The dam site is about 2 km downstream from the complexly faulted Rough Creek Fault Zone, in gently dipping Mississippian limestone and minor sandstone. The dam was to be 14 meters high with a permanent pool height of 5 meters. It would impound water from a 39 km2 area, 12 km2

of which is part of a 15 km2 fault - bounded polje. The permanent pool would have covered highly fractured sandstone and limestone. The flood > pool would have covered: 1) a usually dry group of sinkholes which are estavelles (an estavelle is a swallet which sometimes reverses flow direction to function as a spring), and 2) the perennial swallet of Short Creek which is located 100 meters upstream from them. The estavelles and swallet are in the faulted zone Dye tests were run : 1) to confirm the hypothesis that there would be leakage into the major estavelle and into numerous west - trending, solutionally - enlarged fractures and faults, and 2} to determine how much of the polje (30 % of the drainage area above the dam site) contributes to the discharge of the estavelle and to the volume of viates to be impounded by the dam and/or contributes to the discharge of the spring to which the estavelles flow. Subsurface flow from the swallet and the major estavelle was traced to Mahurin Spring, a large spring 3.5 km to the west, which is also fed by flow from two additional sinking streams south of it. This spring comprises the headwaters of Spring Fork, to which the creek to be dammed is a major tributary.

Given enough money, an engineer can do almost anything! But the cost of preventing leakage of impounded water in the faulted zone would have been far more than the benefits derived from doing so. The project was cancelled because the flood - control goals could not be met by the proposed design unless extensive, prohibitively expensive grouting was done. The flood waters would have been only temporarily impounded by the dam and their release could not be controlled. The flood waters would still reach Spring Fork, but via the spring at its headwaters rather than via the Short Creek tributary which joins it 2.2 km downstream from the spring.

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Introduction

All dams should leak, but only a little bit. If they don't, they were over - designed. Nevertheless, leakage beneath or around a dam is always a potential problem in limestone terranes. Leakage is difficult to predict; it can accelerate and be dangerous.

In 1978, the Soil Conservation Service (SCS), an agency of the Federal government, proposed to build a small flood - control dam on Short Creek, about 20 km west of Leitchfield, in western Kentucky, and 50 km northwest of Mammoth Cave. The proposed dam site is shown in Figure 1. A local farmer opposed construction of the dam because it would cause flooding of his land. He called the agency's attention to some sinkholes in and adjacent to the bed of Short Creek. He also stated that :

1) Sometimes the sinkholes function as springs and discharge flood waters. (These sinkholes were, and are, estavelles.)

2) Water that sank in Short Creek probably emerged at Mahurin Spring, the major spring 3.5 km to the west.

3) After a heavy rain in the Pine Knob Creek basin, south of but not in the Short Creek basin, the estavelles discharge large quantities of water. The discharge abates rapidly and concomitantly with the rapidly decreasing water levels in the sinkholes (swallets) into which Pine Knob Creek sinks.

4) After a heavy rain in both basins, the water levels in the Pine Knob Creek swallets subside slowly rather than rapidly.

The relative locations of these features are shown in Figure 1. Their elevations are shown in Figure 2.

In response to the farmer's opposition, the SCS decided to evaluate the geology and hydrogeology of the area. The published U.S. Geological Survey geologic maps show that the dam site is about 2 km downstream from the Short Creek swallet and sinkholes which are developed in a complexly faulted area that is part of the Rough Creek Fault Zone, as shown in Figures 3 and 4. The SCS then regonized that these sinkholes would be covered by the impounded flood water, that leakage would occur, and that the agency had a problem. I was asked to evaluate the karst hydrogeology.

This paper is written as a brief I description of the problem and as a summary of the results of its investigation. The dam project is interesting, but not important. The study procedures, however, would be applicable and efficient in many other karst areas. They are inadvertent results of learning acquired during research done on behalf of the National Park Service to determine the hydrologie relations between Mammoth Cave National Park and the surrounding area (Quinlan and Ray, 1981 ; Quinlan et al., 1983) and during consulting projects. A summary of other serendipitous practical results of dye-tracing, which are readily applicable to solving environmental problems in areas far beyond the sites studied, has been published by Quinlan and Ewers (1985).

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Fig. 2 - Elevation of proposed dam and its features, various swallets of creeks, and Mahurin Spring (based on Soil Conservation Service file notes).

Application of dye tracing to dam site 537

Description of problem

A flood - control dam about 14 meters high was to be built on Short Creek, a tributary of Spring Fork. All of the base flow of Short Creek sinks through gravel at the swallet 100 meters upstream from the group of estavelles shown in Figures 1 and 3. The major estavelle leads to a cave about 100 m long which ends in two sumps. Groundwater flow was believed to be westward 3.5 km to Mahurin Spring, the major source of Spring Fork, where base flow is estimated to be 250 liters per second and flood flow is several cubic meters per second. Although the 5—meter permanent pool behind the dam would not flood the swallet or estavelles on Short Creek, the flood pool would cover them with 8 meters of water and cause water to back up in the three ponors on Pine Knob Creek. This would still allow most of the flood water to flow to Spring Fork, but by a different route. There was no sense in building a dam that couldn't do what it was supposed to do, a dam that would cause itself to be by - passed.

Study of the geologic maps led to prediction of the existence of a major spring where the fault zone is crossed by the north - flowing stream at the east end of the area shown in Figure 3. The spring was expected because this crossing is the intersection of a probable zone of maximum permeability with local base level. The spring was there, as expected and as shown. Its base flow is estimated to be about 20 liters per second. Confirmation of this prediction made it necessary to locate the groundwater divide between waters that flow to the west and waters that flow to the east. The location of the groundwater divide would affect the amount of the recharge to the estavelles and thus the amount of "extra" water discharged into Short Creek at the estavelles and impounded by the proposed dam.

The structure shown in Figures 3 and 4 seems to be an anticline which has been faulted parallel to its axis. Actually, the anticlinal structure is along the north margin of a large, west - trending graben that is 100 km long and about 20 km wide. The north edge of the graben is bounded by the Rough Creek Fault Zone and comprises the south half of the map. Although a graben characteristically has younger rocks preserved in the downthrown block, some of the older rocks adjacent to the graben, the Fort Payne Siltstone and Limestone, have been squeezed and injected upward, as shown in Figure 4.

The cross section also shows one of the few American poljes. Part of this polje is occasionally covered by floodwater, but most of it is a sinkhole plain rimmed by an escarpment about 30 meters high. The polje is about 9 km long and is locally almost 2 km wide (Figs. 5 and 6).

Field work was done in order to locate all springs, caves with streams, swallets, and other possible dye - input sites. This included verification of information supplied by local farmers. The results are shown in Figure 5. They were needed for planning the dye - tracing program.

The easternmost headwaters of Short Creek (Dry Creek) sink in the middle of the map area (Fig. 5). One of the questions to be answered was, "Do these headwaters flow to the spring in the east or to the spring in the west ?"

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The results of the dye - tracing program are summarized in Figure 6. They shown that:

1. All water from the swallets of Short Creek and Dry Creek flows to Mahurin Spring, at the head of Spring Fork. This spring is also fed by water from the swallets of the two other sinking streams draining from the south.

2. Eighty percent of the 15 km2 polje area contributes to the flow of Mahurin Spring and the estavelles. Only 20 % of the polje drains to the spring in the east.

The reported flow from the swallets of Pine Knob Creek to the estavelles when rain falls in the south but not in the east was not verified I consider these reports to be plausible and reliable.

The amount of flood water to be impounded by a dam affects its designed size. For the proposed dam the accuarcy of this calculation would be greatly complicated by :

1. loss of surface water into the soil profile, underlying caves, and holes in the bed of short creek.

2. flood - stage discharge, both at the estavelles and at Mahurin spring, of groundwater from the polje which is not part of the surface - water basin,

3. flood-stage discharge at Mahurin Spring of water that sinks at the Short Creek swallet and in the estavelles when they function as swallets, and

4. flood - stage discharge at the estavelles of water sinks in the two streams in the south and also flows to Mahurin Spring. Discharge from the south would impede flow from the east, thus increasing from the estavelle.

This complex, perhaps impossible, calculation was not attempted.

The groundwater divide shown near the east end of the polje is farther east than I had expected. I believe this divide is probably shifting westward. The hydraulic gradient from the ground surface at the divide to the springs is 16 m/km to the east and 5 m/km to the west. The gradient to the east is 3.2 times steeper than it is to the west. The presence of a steeper gradient favors headward shifting of a divide in the direction of the drainage basin with the lower gradient. The 3.2 ratio leads me to conclude that spring discharge along the fault zone in the east began much more recently than in the west. I don't know why.

There are many aspects of the hydrology and water balance of this area that would have been fascinating to study. Once it was obvious that the dam should not, and would not, be built, the non - existent research budget was slashed to zero and study of the area ceased.

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Application of dye tracing to dam site 543

Summary

The dam site described is interesting but of no great consequence. However, the efficient strategy by which it was evaluated is applicable to the study of other sites and to similar problems in karst areas throughout the world. This strategy consists of five phases. They are :

1. Compilation of geologic and structural data.

2. Prospecting for springs. The efficiency of this phase is much higher if local farmers are interviewed.

3. Prospecting for swallets and other sites that can be used as dye - inputs. Wet -weather dye - input sites can be used during dry weather by injecting dye carried by a tank - truck full of water.

4. Asking oneself for answers to the following five questions :

A. Where is the flow of sinking streams going?

B. What are the possible recharge areas of the springs ?

C. Which swallets are also estavelles, and when?

D. Where and when did breaching of caprock first take place?

. E. Where are the structurally - controlled or stratigraphically - controlled "spillover" points of the groundwater basins?

Hypotheses are constructed in an attempt to answer these five questions. Water -level data, if they exist or if they can be obtained readily and contoured, can be extremely useful in answering some of them.

5. Testing and revising the hypotheses as dye - trace data become available. Repetition of phases 2,3, and 4 may be necessary.

Aerial photographs and topographic maps may be useful in applying this strategy, but in the United States most of them show fewer than 10% of the springs and swallets which exist in an area. In conclusion, I cannot over - stress the importance of field work, map study, an imagination in the hydrogeological analysis that should precede dye - tracing.

Acknowledgements

The invaluable, dedicated', assistance and resourcefulness of Joseph A. Ray in doing much of the field work for this study is greatefully acknowledged. Illustrations were drafted by him and by Tim Schafstall. The guidance, knowledge, and file notes of Paul Howell of the Soil Conservation Service greatly facilitated completion of the field work. A. Richard Smith's editorial talents improved this paper.

544 ,' J. F. Quintan

REFERENCES

Quinlan, J. F., and Ewers, R. O., 1985. Groundwater flow in limestone terranes : Strategy rational and procedure for reliable, efficient monitoring of groundwater quality in karst areas : National Symposium and Exposition of Aquifer Restoration and Ground Water Monitoring (5th,Columbus, Ohio),, Proceedings, p. 197—234.

Quinlan, J. F., Ewers, R. O., Ray, J. A., Powell, R. L , and Krothe, N. C , 1983. Ground­water hydrology and geomorphology of the Mammoth Cave Region, Kentucky, and of the Mitchell Plain, Indiana, in Shaver, R. H., and Sunderman, J. A., ends, Field Trips in Midwestern Geology : Bloomington, Ind., Geological Society of America and Indiana Geological Survey, v. 2, p. 1—85.

Quinlan, J. F., and Ray, J. A., 1981. Groundwater basins in the Mammoth Cave Region, Kentucky : Friends of the Karst, Occasional Publication, No. 1.