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

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1 Karst Water Resources (Proceedings of the Ankara - Antalya Symposium, July 1985). IAHSPubl.no. 161 HYDROLOGICAL INVESTIGATION IN KARST TERRAIN UTILIZING GEOPHYSICAL AND GEOCHEMICAL METHODS Chester Summers and James W. Gero, Cooper Consultants inc., # 1 Carlisle Drive, Bella Vista, Arkansas, 72714, USA Leslia E. Mack, Water Resources Research Center, University of Arkansas, Fayettevilla, Arkansas, 72701, USA Abstract Man - made reservoirs in karst terrain are subject to leakage even though the dam itself is properly designed and constructed with minimum seepage. Leakage may occur through the limestone abutments, wells and floor of the reservoir. If the recharge to the reservoir from precipitation runoff and groundwater seepage is greater than the reservoir leakage, the problem will hardly be noticed as the reservoir will remain full. If the reservoir leakage is greater than the replenishment, the result is very noticeable as the reservoir will not fill to design capacity. As precipitation is naturally acidic and made worse by man - made activities, reservoirs in karst terrain will become more leaky with time. Blind grouting is not the answer. It is believed that a blend of geophysical, geochemical, geological methods and satellite photos can be used to locate leaky zones and confine engineering treatment to limited areas by grouting or other methods. In conjunction with standard methods such as well logging and water well monitoring, shallow seismic reflection or refraction methods were used to pick up fracture zones and caves in karst terrain. Comparing the seismic data with electrical resistivity data using the Wenner array and the Carpentor method can confirm the location of leaky zones. A basic knowledge of the local geology is necessary. Superimposing geochemical data upon geophysical data is even more revealing. Water quality parameters measured from test wells confirm the relationship between the water quality in the reservoir and the local groundwater. This study has shown that geophysical and geochemical methods can complement engineering techniques and save substantial money. Description of Problem The purpose of this study is to determine the effectiveness of resistivity, geochemical, and seismic studies in locating zones of leakage from a man - made impoundment in karstic terrain. 261

2 262 L. E. Mack, C. Summers, J.W. Gore Location The man - made impoundment is named Lake Brittany which is located in Nortwestern Arkansas in the central portion of the United States. (Fig. 1). Fig. 1 Location of Lake Brittany in Northwestern Arkansas. The study area is essentially flat with steeply incised valleys and a regional slope of the surface sedimentary layers of one to two degrees to the north. Boone Limestone and St. Joe Limestone of the Mississippian Period form the surface outcrops. Based on the unconformable Mississippian - Devonian contact, the local area has a dip of 1/2 degree to the west. The region has a North 10 degree East fracture pattern with a congruent pattern at approximately 87 degrees to the first set of fractures. Lake Brittany is impounded within the Boone and St. Joe Limestones. Hydrology After a dam is constructed and the runoff is restricted, the slope of the water table and the permeability of the soil and rock become the controlling factors, increasing the time that the water is in contact with the rocks of the abutment and the head that the water in the lake exerts on the water in the abutments. By increasing the head on the ground water in the lake abutments, the time that the water is in contact with the rock is decreased because flow increases with an increase in head.

3 Hydrological investigation in karst 263 Britt a^ne tnt' d T* thc adjacent t0 a ^ ^ **** man " made lake - * as Lake ^ face y, t wl av Lt. "T?*' 6 T '" the 3bUtmentS ab0ve the elevatio " of the lake airface. It was determined that m both abutments some of the monitoring wells water eve ls acted, the expected manner. However, water levels in the eastern abutment we have consistently been lower than the lake level. The maximum lake level differ! e occurred,n one well which had a water level 20 feet below the lake level. These areas of ower water leveu indicate zones that are ground water troughs.there were.ower con ctivities in the ground water troughs adjacent to Lake Brittany. The fractures draining the aquifer are responsible for the troughs. Recharge from the presence of Lake Brittany sustains the ridges in the study area. The significance of the low water troughs,n the vicinity of Lake Brittany is that these features are avenues of discharge through which water travels to leave the basin. Forty - seven openings, solution channels or solution zones of bedding planes were ocated m the lake bottom as the lake was drained. These features ranged from four inches to three feet w.de and two to eighteen inches high. The flow from these openings ranged from less than one gallon per minute to 400 gallons per minute. The significance of the flow from the openings is that these features indicate zones of discharge from the ground - water table. The water enters the walls of the impoundment through the surface cover of soil with the controlling factors being the head on the water, the permeability of the soil, and the thickness of the soil. The water then enters the system of fractures and bedding planes as -SOIL - LIMESTONE --PIEZOMETRIC SURFACE - LIMESTONE - SHALE - Fig. 2 - Three dimensional perspective drawing of a hypothetical fracture syste

4 264 L. E. Mack, C. Summers, J.W. Gore The water flows along the fractures in a system of near right angles, which has an orientation of North 10 degrees East and North 80 degrees West in the study area. The water that flows in this system can change directions very rapidly and is considered to be conduit flow. Water also flows along bedding planes and through pore spaces in the rock. This flow is concentrated in the cave or void system located in the area when the fractures intersect them. Where the lake intersects the fracture the water can enter the fracture directly and the permeability approaches that of a pipe with inlet control. Geophysical - Resistivity Carpenter Method The electrical resistivity method is based on the pysical property of earth material to resist current flow. Resistivity is a constant for a given material. The measured response will differ in a heterogenous earth due to different materials, different moisture contents, and amounts of void spaces. Current flows in one dimension obeying Ohm's Law : R = V/i (1) where R is resistance, V is voltage and I is current. Because current flows in three dimensions between two electrodes in the earth,ohm's Law is modified to R = (2)(pi)(ER)(L/A) (2) where R is apparent resistivity, ER is the true electrical resistivity in ohm - meters or ohm - feet, L is the length between the current electrodes, and A is the cross sectional area. True resistivitiy is defined as the resistivitiy of a locally homogeneous medium (Glossary of Geology, 1980). A more comprehensive treatment of the mathematical theory may be found in Carpenter and Habberjam (1956), Kirk (1976) and Dobrin (1976). The geometry or configuration factor is the result of the spacing and configuration of the electrodes (Fig. 3)., The configuration factor is a constant based on the distance between electrodes and is calculated by using the formula : 1/(1/R!-1/R 2-1/R 3 + 1/R 4 ) (3) Fig. 3 General orientation of current (C) and potential (P) electrodes. Rj, R 2, R3 and R 4 are distances used to determine the geometrical factor. I and V indicate where current and potential measurements, respectively, are made (after Kirk, 1976).

5 Hydrological investigation in karst 265 Ogden and Eddy (1983) state that by using three different electrical configurations, CPPC (A), CCPP (B) and CPCP (C) air water filled fractures and caves can be differentiated by using the following criteria : 1. Water - filled fractures : the apparent resistivity decreases in the CPPC (A) and CPCP (C) configurations, but it increases in the CCPP (B) configuration. 2. Air - filled fractures : the apparent resistivity increases in the CPPC (A) and CPCP (C) configurations, whereas it decreases in the CCPP (B) configuration. 3. Water - filled caves : in general, the apparent resistivity decreases for the CPPC (A) and CPCP (C) configurations and increases for the CCPP (B) configuration. The response in the arrays will vary with depth to the cave, the size of the cave, the A - spacing, and the angle of the cave passage with respect to the traverse. At shallow depths, three peaks form with the center and lowest peak being directly over the cave. At greater depths, it is impossible to distinguish between a water - filled fracture and a water - filled cave due to similar responses (one peak) for both features. 4. Air - filled caves : As the traverse crosses an air - filled cave, the apparent resistivity for all three configurations will increase. The responses of the arrays will vary due to the same factors that affect water - filled caves. Ogden and Eddy's criteria were used to locate the features that were interpreted as channels through which the water was flowing, out of Lake Brittany. The grout curtain was then considered for identified areas. However, due to the length of grout curtain required to seal the lake a more economical method of sealing the lake, was ultimately devised. Although the grout curtain method was not chosen,the resistivity data did confirm suspected zones of leakage and locate leakage zones that had not been suspected prior to this investigation. Radial Method The purpose of this portion of the investigation was to see if resistivity could be used to better orient a grout curtain by delineating the fracture orientation. The Soil - test Strata - Scout R40C with Tri - potential alteration was used to conduct the measurements. Wells were chosen that were located in relatively flat areas with water table depth less than 100 feet so that a current electrode could be placed directly in the saturated zone. A 50 foot A - spacing was utilized because this is close to the maximum length of electrical cable available. By placing probe CI into the water of a well, the array becomes asymmetrical. This asymmetry requires that the configuration factor be calculated for each well (Fig. 4).

6 266 L. E. Mack, C. Summers, J.W. Gore Fig. 4 Determination of the configuration factor of the CPPC array with CI 73 feet in a well. By placing the probe at a depth h in the well, the depth of investigation is expanded by the value of h. By placing the probe into the well the current source and therefore the current density is closer to the anomalous zone. The current density, from surface measurements, would be less in these zones. The resistivity data are weighted toward the CI value because of the extra volume that is added at CI due to the increased geometry factor due to the increased lengths of R1 and R3. The fractures located using this method correlate with the regional fracture pattern of 10 degrees and 280 degrees suspected of being fracture The resistivity method can be used to locate features that are channels through which the water is flowing. This method did confirm suspected zones of leakage and locate leakage zones that were not suspected prior to this investigation. Geophysical - Seismic The seismic method is based on acoustic wave theory. The basic information gained is stratigraphie and structural in nature (Dobrin, 1976).Porosity may also be interpreted from seismic data (Maureau and Van Wijhe, 1979). During this investigation both reflection and refraction seismic methods were attempted. The Boone Formation is a complex layering of chert and limestone beds which scatters the signal; furthermore, the surface red clay attenuates the seismic signal. First break signals were difficult to record, and distinctive reflections were hard to identify. Figure 5 has an interesting feature. A void of 25 feet was encountered at this location while the well was being drilled. The presence of the void could be interpreted from the trough.

7 Hydrological investigation in karst 267 LAKE BRITTANY 3EI3MIC SKIS CAVE Fig. 5 - Seismic section showing a cave. Geophone spacing is 5 meters. The seismic method is useful in determining the gross characteristics of the shallow subsurface within the study area. This method can locate areas of suspected higher porosity, and can define structure. In areas with a thick clay soil, more energy is required to insure an adequate signal return from the strata in the subsurface. The water table was not identifiable in this area with seismic data. Geochemical The possibility of utilizing geochemistry to locate fractures near a lake is clearly evident. The water table adjacent to the lake will show variations in the chemical properties of the water due to the differing residence time near fractures, or other avenues of water movements. What seems as important as the well's proximity to the lineament is the chemistry of the recharge water. For instance, if rain water recharges a zone that is fractured, we would expect to see a decrease in the specific conductance of that water. This change is brought about by the dilution of the subsurface water by the rain water of lower specific conductance. These lower specific conductance values will be followed by a period when the specific conductance wili increase to the equilibrium stage previously held. If the change in specific conductance shows a marked decrease this would indicate a direct contact of the ground water with the surface rain water and would also indicate conduit flow in the region of the well. This marked decrease in specific conductance was observed in five wells. If the change in specific conductance is less pronounced, then the water is also 'exhibiting diffuse flow. Other wells had less pronounced decreases than the decreases found in the wells classified as diffuse flows, based on the results of following wells through a storm event. Increases in specific conductance following a storm event would indicate that water from a diffuse flow region is being forced into a conduit.

8 268 L. E. Mack, C. Summers, J.W. Gore The main controlling factor of alkalinity values is the contact time of the water with the limestone. Another contributing factor is the ph of the water as it enters the system. However, the ph will be neutralized rapidly. In addition, the ph of the recharge water should be constant over the short distances with which we are dealing unless a marked change in vegetation or soil is encountered. Over short distances (hundreds of feet) the acidity of the water should be constant. The lower the permeability the more time the water has to reach a chemical equilibrium by the water reacting with the limestone. Data show that the groundwater, adjacent to a man - made lake in a karstic terrain is different from normal groundwater. Measurement of specific conductance or alkalinity can be used to locate and identify groundwater troughs.. Conclusions No one method can be used to answer all the questions about an area. Each of the three methods used to investigate the area adjacent to man - made lake in a karstic terrain provided useful information about the subsurface. Resistivity, in this investigation, was found to identify fractures. In addition, fracture orientation can be determined by pivoting the array about a point. By placing the CI probe in a well, the fracture orientation becomes more obvious. When using the Tri - potential method the array layout should have the same or similar orientation throughout a study and a cross pattern should be used for best results. Results which are more easily correlated can be expected when similar array orientations are used. It became readily apparent that the curve matching was not a useful method for this study, and; therefore, some questions still remain unanswered. The depth of the water table and the depth to a known stratigraphie horizon were not determined utilizing geophysical methods. Chemical variation in the groundwater is a useful tool with which to study the flow of groundwater. Variations in specific conduction of the water were used to locate chemical differences in water and flow paths of the water. The seismic method provided a method to investigate the shallow subsurface. This method was used to tentatively locate variations in porosity and major structural features. However, fractures and caves were not located by direct evidence. The only definite method to determine what is below the surface is to drill. The methods described in this paper show that more information can be gained by using geophysical and geochemical methods in a mutual supporting role than any single geophysical method can alone. A multi - discipline approach insures that fewer surprises occur later. The best approach to locating avenues of leakage is to use geophysical methods to site monitoring wells and to confirm the suspected problem area using the change in the water's chemical properties as the lake fills.

9 REFERENCES Hydrological investigation in karst 269 Carpenter, E.W., and Habberjam, G.M., A Tri - Potential Method of Resistivity Prospecting, Geophysics Prosp., Vol 21, pp Dobrin,B., Introduction to Geophysical Prospecting :McGraw - Hill, Inc.622p. Glossary of Geology, Kirk, George Keith, Evaluation of The Tri - potential Resistivity Technique in locating Cavities, Fracture Zones, and Aquifers : Unpubl. M.S. Thesis, West Virginia University, 81pp. Maurea, G.T.F.R., and Van Wijhe, D.H., The Prediction of Porosity in the Permian Zechstein 2 Carbonate of Eastern Netherlands Using Seismic Data : Geophysics, Vol 44, No 9, pp Ogden, A.E., and Eddy, P.S. Jr., The Use of Tri - Potential Resistivity to Locate Fractures and Caves for Siting High Yield Water Wells. In Press.

Soils, Hydrogeology, and Aquifer Properties. Philip B. Bedient 2006 Rice University

Soils, Hydrogeology, and Aquifer Properties Philip B. Bedient 2006 Rice University Charbeneau, 2000. Basin Hydrologic Cycle Global Water Supply Distribution 3% of earth s water is fresh - 97% oceans 1%