TECHNIQUES FOR PREVENTION AND DETECTION OF LEAKAGE IN DAMS AND RESERVOIRS. Iván A. Contreras 1 Samuel H. Hernández 2 ABSTRACT

Transcription

1 TECHNIQUES FOR PREVENTION AND DETECTION OF LEAKAGE IN DAMS AND RESERVOIRS Iván A. Contreras 1 Samuel H. Hernández 2 ABSTRACT Leakage is a major safety issue that, if left unchecked, may result in dam failure by various mechanisms. There is enormous pressure on dam operators to repair leaks without significant delays. Frequently, the need to reduce the risk of failure or control water loss has led to costly remedial repairs that are planned and executed without a complete understanding of the problem. A lack of appropriate leakage investigation and monitoring can result in repairs that are unsuccessful in controlling or reducing leakage. In the last few decades, a series of new hydrological techniques have been developed to help in the assessment of leakage and seepage in dams. It is important to make these techniques available to the engineers responsible for dam construction and management so that they become aware of these tools. The available literature on dam leak studies is relatively limited with regard to the use of these techniques when assessing dam leakage. It is difficult to find case studies that discuss integrating the use of several of these techniques in comprehensive evaluations that lead to successful leakage mitigation. These techniques allow identification of recharge zones, preferential paths, and transit times, which aid in monitoring and mitigating the dam leakage. The paper includes description of the techniques and projects in the United States and abroad involving prevention and detection of dam and reservoir leakage, including leakage evaluation, analysis, design, construction, and post-construction verification of repairs. INTRODUCTION All water reservoirs and dams are expected to experience water losses to some extent. These water losses occur through the dam or through the geological formations located near the dam site. Water losses frequently appear through the geologic formations surrounding the dam site such as foundation or abutments. On the other hand, water losses through the dam are less frequent although somewhat common. As a result, certain degree of leakage or seepage is accepted as long as it does not compromise the safety of dam and surrounding facilities. However, it is sometimes difficult to assess the risks and consequences of excessive leakage and seepage. Leakage refers to concentrated water losses associates with structural or construction deficiencies at the contact between embankment and ground, abutment impermeabilization works, lack or deficient cutoff of 1 Vice President, Sr. Geotechnical Engineer, Barr Engineering Co., 4700 West 77 th Street, Minneapolis, MN 55435, icontreras@barr.com 2 Head of Project Management Division, Desurca, Centro Comercial El Pinar, San Cristóbal, Venezuela, samhleon@gmail.com Prevention and Detection of Leakage 785

2 alluvial materials, activation of perched old karstification levels, etc. In these cases, water flows in concentrated paths and emerges downstream creating streams. Seepage through and under the embankment is a process evaluated by investigating the permeability of the dam materials and geologic formations in its vicinity. Seepage is described as spread infiltration at the reservoir bottom through the permeable materials and embankment and can be predicted if the hydraulic parameters and geometry of the geologic formations and dam materials involved are known. The need to reduce the risk of failure or control water loss has led to costly remedial repairs that are planned and executed without a complete understanding of the problem. A lack of appropriate leakage investigation and monitoring can result in repairs that are unsuccessful in controlling or reducing leakage. The repair work is typically undertaken focusing exclusively on engineering aspects without sufficient hydrogeological information. The authors have encountered several cases in the United States and abroad where, in spite of large amount of money invested in remedial work such as grouting, concrete placement, cutoff, drainage curtains etc, the repairs have been unsuccessful in controlling or reducing leakage. It is commonly found that a large quantity of technical and economic resources are spent without positive outcomes due to the lack of adequate characterization to determine water origin, preferential paths, recharge zones, and transit time. In the last few decades, a series of new hydrological techniques have been developed to help in the assessment of leakage and seepage in dams. It is common to encounter engineers responsible for dam construction and management who are not familiar these techniques recently developed. The available literature on dam leak studies is relatively limited with regard to the use of these techniques when assessing dam leakage. It is difficult to find case studies that discuss integrating the use of several of these techniques in comprehensive evaluations that lead to successful leakage mitigation. These techniques allow identification of recharge zones, preferential paths, and transit times, which aid in monitoring and mitigating the dam leakage. This paper presents a description of these techniques including the relationship between reservoir level and flow rate at the leak, natural and artificial tracers, measurements in boreholes, interconnection tests, and tests within the reservoir. The paper also includes case histories where positive outcomes resulted from application of these techniques and subsequent remedial repairs. AVAILABLE TECHNIQUES FOR DETECTION OF LEAKAGE IN DAMS Leakage and preferential flow paths are often controlled by the geology of the site. Therefore, any leakage study should always include obtaining detailed geological and hydrogeological information as the first step. This initial information allows postulating a hypothesis about the investigation and the consequent strategy that led to the use of these techniques. Experience indicates that in general, a single technique is not always sufficient to investigate leakage in dams. Often a combination of techniques results in successful leakage detection and thus a technique should not be disregarded a priori 786 Collaborative Management of Integrated Watersheds

3 because sometimes the key information to solve the problem is found by applying the most unexpected technique. The following describes in more detail the commonly used techniques for investigating leakages from dams and reservoirs. Relationship between Reservoir Level and Flow Rate The relationship between the water level in the reservoir and the flow rate at the leak provides valuable information regarding the elevation interval within the reservoir where the infiltration is taking place. Utilization of this technique requires frequent measurements of the flow at the leak(s) and at different reservoir elevations. When more than one leak is present, separate flow-rate readings should be taken in each leak. Figure 1 shows a typical plot of the data obtained from this technique. It can be seen from Figure 1 that when all the water emerging from a leak comes from the same reservoir elevation, the points tend to align in a straight line. The flow rate in the leaks is proportional to the reservoir water level (i.e. hydraulic head). When the straight line in Figure 1 is extrapolated to a flow rate of zero, it provides the approximate elevation at the reservoir at which water infiltration takes place. This is very valuable information when evaluating leakage in dams and reservoirs. Figure 1 corresponds to South Creek leak at Boney Falls reservoir in Michigan. The intercept of the straight line with a flow rate equal to zero corresponds approximately to elevation ft. Figure 1. Relationship between Reservoir Level and Flow Rate at South Creek in Boney Falls, Michigan Prevention and Detection of Leakage 787

4 In other cases, the straight-line relating flow rate at the leaks and reservoir levels exhibit a slope change as shown in Figure 2. The slope change results from the activation of another infiltrating zone feeding that leak. This second infiltrating zone is located approximately at the elevation at which the break in the straight line takes place. In the example illustrated in Figure 2, which corresponds to West Flume leak at Boney Falls, two breaks are shown by the data and thus the other infiltrating zones are approximately at elevations ft and 904 ft. Figure 2. Relationship between Reservoir Level and Flow Rate at West Flume in Boney Falls, Michigan Data in Figures 1 and 2 do not include information on the time response of the flow rate at the leak and the reservoir level. This in fact can provide further information on the characteristics of the ground along the flow path between the reservoir and the leak. Sometimes the observed responses at the leak are different when the water level in the reservoir is increasing with respect to the response when the water level in the reservoir is decreasing. In other words, there is a delay in the flow rate peak at the leak at a constant reservoir level. Plata and Araguás (2002) present detailed discussion of how that information is used to compute the total volume of the cavities within the formation along the flow path. Natural Water Tracers In addition to the relationships between reservoir levels and flow rate at leaks downstream other investigating tools are required. Water has natural tracers, which can 788 Collaborative Management of Integrated Watersheds

5 be used to obtain information on the origin and location of the infiltrating points in the reservoir. In particular, natural tracers are used for determining the origin of the groundwater emerging at the discharge points located downstream of the dam as well as groundwater flowing through the foundation and abutments, and investigating the dynamics of the groundwater emerging downstream. The natural tracers most commonly used for this purpose and discussed herein are the temperature of water, conductivity, chemical composition, and stable isotopes. Water Temperature: Water temperature is a good natural tracer in the investigation of leakages in dams and reservoirs. This is because most reservoirs (depths greater than 10 m) develop thermal stratification. In shallow reservoirs (i.e. less than 10 m), the thermal stratification does not take place and they exhibit uniform temperature because of the mixing effect derived from wind action. The thermal stratification in deep reservoirs is the result of the heat exchange between the reservoir surface and the atmosphere. During the summertime, the water in upper most layer of the reservoir heats rapidly and this absorbed heat energy is transmitted to the deeper layers. On the other hand, during the wintertime, shallow water in the reservoir becomes colder resulting in a more or less uniform vertical temperature profile. Comparison of the temperature of the water emerging at the leaks emerging downstream of the reservoir and the vertical temperature profile measured in the reservoir provides valuable information on the depth at which the water emerging in the leak is leaving the reservoir. In the initial stages of the leak investigation, field campaigns should include systematic measurements of temperature and conductivity in the reservoir and leaks. These simple parameters can provide valuable information, which can be used to define other complementary techniques to be utilized. In those cases when the transit time of the water between the reservoir and the boreholes or leaks is less than a few days, the temperature of the water does not change significantly, as it flows through the ground. This is because the specific heat capacity of water is much higher than most natural materials found in the ground. As a result, the water associated with the flow path tends to maintain the same temperature as the reservoir at the infiltration zone. This is the case when the flow rates involved in the leak are high and transit times are short. However, water temperature may change when water infiltrated in the bottom of the reservoir mixes with groundwater of different origin. Vertical temperature and conductivity profiles measured in fully screened boreholes are commonly used for identification of zones where groundwater flow is significant. The zones of significant groundwater flow are characterized by having different values of temperature and conductivity than the rest of the column. It is very common to encounter zones of low flow that exhibit higher temperature and conductivity values than high flow zones. Figure 3 shows the temperature and conductivity profile measured in a fully screened borehole at a Dam Wyoming. It can be seen from Figure 3 that the zone of low temperature and conductivity is located approximately between elevations 6228 ft and Prevention and Detection of Leakage 789

6 ft. In this zone, the temperature and conductivity are lower indicating that this is a zone of significant groundwater flow. This corresponds to a zone of fractured bedrock immediately below the overburden soil. A soil mixed cutoff was built but it did not penetrate the fractured bedrock where most of the flow was taking placed. The low conductivity values at the bottom of the borehole are associated with presence of sediments left during drilling inside the pipe. Figure 3. Temperature and Conductivity Profile at Dam in Wyoming Conductivity: Similar to water temperature, conductivity is a good natural tracer that can provide valuable information when evaluating leaks in dams and reservoirs. This is because, as in the case of temperature, reservoirs also develop saline stratification and therefore deep waters exhibit higher salinity contents than water in upper layers. Measurements of conductivity in the reservoir, boreholes, and leaks are strongly recommended during the first stages of dam leakage investigations. It is common to obtain temperature and conductivity measurements simultaneously during the same campaign because both parameters are usually measured using the same instrument. The thermoconductivity probe is typically used to measure both the temperature and conductivity. The measurements of conductivity are performed in the reservoir, boreholes, and leaks located downstream of the dam. In the reservoir, the measurements of vertical profiles of conductivity should be included in the program to investigate the salinity stratification. When submerged springs discharge high salinity waters, they will probably accumulate in 790 Collaborative Management of Integrated Watersheds

7 the deep parts of the reservoir. However, the opposite situation can also be found. These possible scenarios are described in detail by Plata and Araguás (2002). Conductivity of the reservoir waters shows great variation with time and space. Frequently, reservoir waters show seasonal variations of conductivity with lower salinity water during the rainy period and higher salinity during the dry periods. These seasonal variations are often related to the origin of the water. During rainy periods, most of the water entering the reservoir is associated with surface runoff water and thus the salinity is lower. On the other hand, during dry periods the relative contribution of groundwater into the river will be higher and therefore, the salinity of the water entering the reservoir can be much higher. These variations can be of interest in the investigation of hydraulic connections between reservoir, boreholes, and leaks downstream of the dam. This is obtained by correlating the peaks in conductivity in the reservoir, boreholes, and leaks. Figure 4 shows the results of the conductivity survey obtained at Boney Falls in Michigan. The conductivity of the reservoir waters range between 145 μs/cm at the surface, and 152 μs/cm at the bottom. The conductivity at three springs (leaks) located downstream of the dam were 151 μs/cm, 147 μs/cm, and 154 μs/cm, respectively. The conductivity measurements provided valuable information allowing formulation of hypothesis for the interconnection of the reservoir water and the water emanating at the leaks. Figure 4. Results of Conductivity Surveys at Boney Falls, Michigan Conductivity measurements of saline tracers can also be used for determination of types and magnitudes of groundwater flows in boreholes, measurements of flow in open Prevention and Detection of Leakage 791

8 channels and springs, and the renewal rate of water in small water bodies. Details regarding these applications are described in detail by Plata and Araguás (2002). If important changes in conductivity are found between water samples collected in the reservoir, boreholes, and leaks, the next step is to perform chemical analyses to identify the ions responsible for those changes. Chemical analyses can provide valuable information regarding the change of the conductivity. Chemical Composition: The results obtained from temperature and conductivity measurements are improved by measuring the major chemical components of water. It is recommended to perform chemical analyses of major components by sampling the most representative waters in the study area during the initial stage of investigations as well as when important variations of the leakage regime occur and changes in conductivity are observed. Chemical composition of water provides additional information on its origin and geochemical evolution, facilitating the differentiation between reservoir and aquifer waters or a mixture of those waters. Chemical composition testing is often complemented with tools such stable isotopes which are also discussed herein. By comparing the chemical composition of the reservoir water and the water emerging at the leaks, valuable information is obtained regarding the lithological composition of the rock formation through which the emerging water is flowing. Additionally, seasonal changes in the chemical composition of the water can be used for investigating the dynamic characteristics of the groundwater feeding the leaks. This includes determination of transit times and evaluation of storage capacity of rock formation. The major dissolved components of water are: Anions: chloride (Cl ), sulphate (SO 4 = ), nitrate (NO 3 ), bicarbonate (HCO 3 ), and carbonate (CO 3 = ) Cations: sodium (Na + ), potassium (K + ), calcium (Ca ++ ), and magnesium (Mg ++ ) Special sampling collection techniques and quality checks of the laboratory tests results are required to validate the data. Details regarding the sampling collection techniques and quality checks on the laboratory data are discussed in detail by Plata and Araguás (2002). The results of the chemical analyses are typically illustrated in form of vertical column diagrams such as Piper, Schöller, and Stiff. Figure 5 shows the results of chemical composition on relevant samples at Borde Seco Dam in Venezuela in terms of the Stiff diagram. They correspond to samples taken at the reservoir and creeks or seeps downstream of the dam. It can be seen from Figure 5 that the shape of the Stiff diagram clearly indicates that samples M6, M8, and M9 have chemical compositions similar to the reservoir water whereas the rest do not. This information allowed differentiation in the 792 Collaborative Management of Integrated Watersheds

9 origin of the waters sampled downstream. It was determined that only leaks M6, M8, and M9 were associated with reservoir water. M1MARG DER M11 MARG IZQ M12 MARG IZQ Na Cl Na Cl Na Cl Ca HCO3 Ca HCO3 Ca HCO3 Mg SO4 Mg SO4 Mg SO (meq/l) (meq/l) (meq/l) Na Reservoir Cl Na M2 MARG DER Cl Na M4 MARG DER Cl Na M5 MARG DER Cl Ca HCO3 Ca HCO3 Ca HCO3 Ca HCO3 Mg SO4 Mg SO4 Mg SO4 Mg SO (meq/l) (meq/l) (meq/l) (meq/l) M6 MERG IZQ M8 MARG IZQ M9 MARG IZQ Na Cl Na Cl Na Cl Ca HCO3 Ca HCO3 Ca HCO3 Mg SO4 Mg SO4 Mg SO (meq/l) (meq/l) (meq/l) Figure 5. Chemical Composition of Water at Borde Seco Dam in Venezuela Stable Isotopes: The stable isotopes of water, deuterium ( 2 H or D) and oxygen 18 ( 18 O), offer unique possibilities for investigating leaks in dams and reservoirs. Since engineers and personnel involved in dam safety are typically not familiar with these tools, a brief theoretical background is presented before discussing the possibilities offered by these techniques. In addition to D and 18 O, tritium ( 3 H or T) as well as carbon isotopes carbon- 13 ( 13 C) and carbon-14 ( 14 C) also provide excellent possibilities. However, they are not discussed herein and the reader is referred to Plata and Araguás (2002) who describe their application in detail. Background: As is well known, the number of protons is a characteristic of each chemical element and is used to denote the atomic number. Within each element, there are different nuclides, which are characterized for having the same number of protons and a different number of neutrons. The sum of the neutrons and protons of the nuclide of a same element is called the mass number (A) of this element. The different nuclides of the same element are called isotopes of this element. The chemical behavior of all the nuclides of a given element is the same because it depends on the external configuration of the electrons, which does not change in the isotopes of the same element. However, they exhibit different behavior during kinetics phenomena in which the structure and energy bonds of the nucleus structure are involved. The difference in mass of the atoms makes Prevention and Detection of Leakage 793

10 them have different physical behavior. The isotopes of a given chemical element are represented by A X, where X is the symbol of the element and A the mass number. The atomic number of the element is incorporated on the element. The heavy isotopes of hydrogen and oxygen, D and 18 O (deuterium and oxygen 18), represent an integral part of the water molecule. This is a special characteristic, which differs from the dissolved tracers previously discussed. Water molecules containing any of these heavy isotopes behave (from the chemical standpoint) as any other water molecule containing the most abundant isotopes. As a result, their behavior is ideal when compared with other tracers whose properties are different from those of water molecules. Water molecules containing these heavy isotopes are perfect tracers to assess interaction of water bodies during the hydrologic cycle. Isotopic Ratios: The isotopes 1 H and 16 O are the main component of the molecular species most abundant in water molecules and are presented by ( 1 H 16 2 O). The ocean is the main reservoir of water on earth and thus represents the typical water molecule. The proportions of the three most important molecular species in the seawater are: 1 H 16 2 O: parts per million 1 HD 16 O: 320 parts per million 1 H 18 2 O: parts per million A common way of expressing the abundance of certain isotope is by using the isotopic ratio, which is defined as: R = Number of molecules of 1 HD 16 O or 1 H 2 18 O / (Number of molecules of 1 H 2 16 O) The isotopic ratios are modified by processes during the water cycle, which provides a characteristic labeling of the water involved in the process. This natural labeling is used as a fingerprint of water molecules that allows the study of the origin and dynamics of different water bodies. Isotopic Deviation: The isotopic ratios R of a given sample W are not reported as absolute number but rather they are expressed as a deviation of the same ratio from a standard S using the following equation: δ S (W) = (R W -R S )/ R S *1000 (1) The reasons to utilize isotopic deviation as a more appropriate notation include suitability of equipment to measure ratio variations rather than absolute values, avoiding the use of small numbers with too many decimals, and the relevance of evaluating relative changes in isotopic ratios. The delta value δ is adopted as a dimensionless number and expressed as per mil. Delta values can be negative or positive, depending on whether the isotopic ratio of a given sample is greater or lower than the standard. However, it is commonly found as 794 Collaborative Management of Integrated Watersheds

11 negative δ-values of waters because the standard is the seawater. The isotopic deviation for deuterium and oxygen 18 are denoted as δd and δ 18 O, respectively. Relationship between δd and δ 18 O in precipitation: Craig (1961) postulated a relationship between δd and δ 18 O of all the precipitation waters in the globe. The proposed relationship is known as the Global Meteoric Water Line (GMWL) and is given by the following equation: δd = 8 δ 18 O + 10 (2) Depending on local conditions, during leak studies at a particular dam site, it is necessary to define the so-called Local Meteoric Water Line (LMWL). This definition is required for meaningful interpretation of stable isotope data in precipitation and groundwater. Plata and Araguás (2002) present a detailed discussion of the factors controlling the isotope composition of meteoric waters. These factors include the following: temperature, conditions during cloud formation, mixing of atmospheric vapor of oceanic origin and local origin, latitude effect, altitude effect, partial evaporation of rainfall drops, evaporation from surface waters, continentality, and quantity effect. Application of Stable Isotope Technique in Leak Studies: Stable isotopes are mainly used in the identification of water from different origins. They are also applied in cases when there is isotopic stratification of deep reservoirs. In general, two waters with different hydrogeologic origin will have a different isotopic composition. This isotopic contrast is used as a natural water tracer, which allows identification of water of different origins and mixing process. The application of the technique in the study of dam leaks is valuable when waters with different isotopic composition exist at the site. Water from a particular recharge zone and flow path has a defined fingerprint, which is incorporated in the water molecule during precipitation and infiltration, for a water body, which allows differentiation of water from a different body or origin. The conservative nature of the isotopes allows labeling of the water to the origin. When there is an isotopic contrast between the waters in the study area this technique is very useful. In the majority of the cases the water bodies involved are: 1) the reservoir water and tributary rivers; 2) the groundwater in the area of the dam; and 3) the water seeping out at the leaks downstream of the dam. The water at the leak downstream of the dam may have different origins: 1) local groundwater; 2) water from the reservoir; or 3) a combination of reservoir and groundwater. There are cases when the presence of runoff from recent rains combined with emerging groundwater makes the determination more complex. Figure 6 shows the relationship between δd and δ 18 O for the waters involved at Borde Seco Dam in Venezuela. It can be seen from Figure 6 that the three groups of waters at the site are the groundwater, the river water, and the reservoir water. Figure 6 shows that the waters at the leaks at elevation 230 m and at the dam toe are completely associated with water from the reservoir. The reservoir water also deviates from (Global Meteoric Prevention and Detection of Leakage 795

12 Water Line) GMWL and shows a local deviation associated with the evaporation process that takes place in the reservoir. Figure 6. Relationship between δd and δ 18 O at Borde Seco Dam in Venezuela Artificial Tracers During investigation of leaks from dams and reservoirs, it is usually necessary to use artificial tracers. They are typically used after the chemical composition and the other natural tracers have been used. The techniques that utilize artificial tracers have had limited application in the past due to complexity of the hydrological framework, lack of information on the utilization of artificial tracers, technological limitations for detecting low concentrations, and the difficult interpretation of the results. It is very common to encounter the use of fluorescent dyes in interconnection experiments where the success/failure of the test is only based on the visual evaluation of the appearance of the dye at the leak without measurements. These applications frequently ignore the proper definition of the hydrogeological setting or the properties of the artificial tracer used. There are significant numbers of tracers, which can be used in leak studies. The most important tracers can be grouped as follows: suspended solids, ionic tracers, gaseousfluoro-compounds, fluorescent tracers, and radioactive tracers. The following discussion focuses only on suspended solids, ionic tracers, and fluorescent tracers because those tracers are the ones most extensively used and are most familiar to the authors. Plata and Araguás (2002) discuss in more detail the different tracer groups as well as the most appropriate tracers for a particular application. 796 Collaborative Management of Integrated Watersheds

13 Suspended Solids: Suspended solids can be applied only for investigation of groundwater flows through well-developed fractures, solution channels and cavities such as karstic terrain. In porous media, these tracers will not travel long distances from the injection point. The most important suspended solid tracers are spores, bacteria, fluorescent microspheres and starch. Käss (1998) provides a detailed description of the use of spores for hydrological purposes. Ionic Tracers: Strong ions are used as ionic chemical tracers. This includes elements present in ionic forms as cations and anions. One substance of great use in this group is salt NaCl. Salt has the advantage of having an almost ideal behavior and being easily encountered. The solubility of salt in water is about 350 g/l at 20 C. It can be injected in any water body as a saturated solution depending on the experiment. The main limitation for the use of salt as a tracer is that the chloride is always present in natural waters and sometimes in high concentration, which does not allow the use of salt. Salt can only be used with its concentration is less than 50 or 100 mg/l and the water volume to be labeled less than 1,000 m 3. Typical applications of the use of salt as tracer include tests measurements in boreholes, determination of small flows rates, and identification of groundwater discharge in small closed lakes. Fluorescent Tracers: Fluorescents are the most common tracers used in dam leak investigations. As previously mentioned, most of the time their detection is based on visual observation. However, today there are measuring tools that allow quantitative determination of the tracer concentrators. A large number of fluorescent tracers are available but the most common for water tracing are uranine, rhodamine B, and rhodamine WT. Rhodamine WT was specially developed for water tracings and is one of the most extensive used in practice because it is environmentally friendly. Fluorescent traces emit a characteristic fluorescence when they are excited by a light beam of certain wavelength. The wavelength of the excited light beam and the emitted fluorescence are characteristic for each product. The technology on the instruments used to measure fluorescence has improved significantly in the last decade and there is equipment to make measurements directly on a borehole. Plata and Araguás (2002) discuss in more detail the measuring techniques for fluorescent tracers and their application. Measurements in Boreholes Measurements in boreholes provide very useful information when investigating leaks in dams and reservoirs. The techniques described herein allow identification of the strata or zone with higher flow velocities and measurement of the magnitude of the flow velocities. The techniques described herein require that water can flow freely across the borehole. In other words, the borehole needs to be fully screened. In general, the slotted area of the casing needs to be at least between 1% and 3% of the casing surface area for these techniques to provide meaningful results. There are two basic water flow situations in a borehole. The flow may be either horizontal or with a vertical main component. The techniques for measurement in each case are different and both of them are explained Prevention and Detection of Leakage 797

14 herein. Salt is the most common tracer sued when applying these techniques. However, fluorescence tracers can also be used as described by Plata and Araguás (2002). Fully Labeled Water Column: Before labeling the water column, conductivity and temperature readings are taken along the borehole. This method consists of labeling the entire water column of the borehole with the tracer as shown in Figure 7a. A hose with inner diameter between 8 and 10 mm is inserted to the bottom of the borehole. The hose is provided with a weight at the end to facilitate insertion in the borehole. A volume of tracer equal to the hose volume between the bottom of the borehole and the phreatic surface is delivered at the upper end of the hose with a funnel. The hose is then slowly pulled out at an approximate steady rate, which allows uniform distribution of the tracer along the water column. Figure 7. Technique Used to Fully Label the Water Column After tracer injection, a sequence of vertical profile measurements of tracer concentrations is made at discrete elevations (usually at constant intervals) along the water column. The frequency of measurements is selected so that short intervals are taken at the beginning and longer intervals towards the end of the test depending on the dilution rate of the tracer. The test is conducted until most of the tracer has moved out of the borehole. 798 Collaborative Management of Integrated Watersheds

15 To interpret the result, the concentration values measured in each profile are plotted versus depth in a single graph as illustrated in Figure 8. Figure 8 shows the typical results of a test with a fully labeled water column at the Lake Bronson embankment in Minnesota. It can be seen from Figure 8 that below the depth of 55 ft the net conductivity rapidly decrease with time indicating a permeable stratum with predominantly horizontal flow and some vertical flow component. The net conductivity is defined as the difference between the background conductivity (before injecting the tracer) and the measured conductivity. The variation in conductivity with time can be plot versus time to compute the flow velocity. This procedure is shown in detail by Plata and Araguás (2002). Figure 8. Results of Fully Labeled Water Column Test at Lake Bronson Dam in Minnesota In those cases when very fast flows are present, the labeling has to be performed in successive stretches as shown in Figure 7b. This method is applied when most of the tracer escapes from the borehole during the period of time when the hose is being retrieved from the borehole. Measurements of Vertical Flows: In those cases when vertical flows are present in the borehole, the measurement of the temperature and conductivity profile is first performed. Typically, constant values of these parameters along the whole column or in a particular zone of the borehole will provide some hints about the possibility of vertical flow. The second step is to perform the test using the fully labeled procedure as previously described. This test will confirm the presence of vertical flow, or indicate if it is downward or upward. Prevention and Detection of Leakage 799

16 When the vertical flow is slow, the migration of the tracer cloud is used. This consists of a point injection of the tracer. The hose is inserted into the borehole with the lower end of the hose at the location where injection will be made. The tracer is injected at the top using a funnel. Immediately, after tracer injection, a series of vertical concentration profiles of the water column is initiated. The idea is to identify the location of the cloud of tracer with time along the borehole. When the cloud arrives at the zone where the water escapes, the velocity decreases and at that time, the tracer concentration will decrease. Figure 9 shows the results of a point release test measured at the Dayton Hollow embankment in Minnesota. The release was made near the top of the borehole at a depth of about 10 ft. It can be seen from Figure 9 that there is downward flow at this site. The cloud of tracer (peak conductivity) moves downward with time. This information can be used to compute the flow velocity as described by Plata and Araguás (2002). Figure 9. Results of Vertical Flow Measurements at Dayton Hollow Dam in Minnesota Plata and Araguás (2002) detail other techniques to measure vertical flows in boreholes. This includes the two-peak method, the movable measuring line, and point injection with 800 Collaborative Management of Integrated Watersheds

17 a peristaltic pump. These alternative procedures are useful when the vertical flow is fast and immediate measurement cannot be performed. Interconnection Tests Interconnection tests are an integral portion of leak investigations from dams and reservoirs. They provide proof of hydraulic connection between the point of injection and the measuring point(s). The test consists of the injection of a given tracer amount in the reservoir or boreholes and its subsequent measurement of arrival time and concentrations at boreholes or downstream leaks. In addition to providing the arrival time the interconnection tests allow determination of the passing curve, transit time, amount of tracer recovered, characteristics of the preferential flow path(s), and an estimate of the volume of fissures or cavities along the flow path. These tests required detailed planning and execution. These tests should be conducted after performing some of the other techniques previously described. Figure 10 shows an ideal passing curve for a point injection interconnection test. This is used to identify the most relevant values of flow velocities obtained from the passing curve. The time axis in Figure 10 corresponds to the elapsed time since tracer injection. Point 1 corresponds to the first appearance of the tracer at the measuring point and the time is used to compute the maximum velocity. Point 2 is associated with the time when the maximum tracer concentration is measured and is used to compute the dominant velocity. Point 4 corresponds to the time at which 50 percent of the tracer has been recovered and the time is used to compute the mean flow velocity. Point 3 corresponds to the real effective flow velocity and it is found after point 2 and before point 4. However, it cannot be determined precisely and typically is approximated as the midpoint between point 2 and point 4. Figure 10. Ideal Passing Curve for a Point Injection Test The transit time, t t, of the tracer in the rock formation between the injection and measuring points can be computed as the center of gravity of the passing curve, according to the following equation: Prevention and Detection of Leakage 801

18 0 t C( t) dt t t = (3) C( t) dt The transit time, obtained from the passing curve and the flow rate measured at the leak can be used to estimate the minimum volume of cavities associated with the flow path between the entry and exit points. This volume is computed as the product of the flow rate and the transit time. When continuous injection of tracer is performed, the passing curve differs from the one shown in Figure 10. The main difference is that when the tracer concentration reaches the maximum, the concentration remains constant showing a plateau as long as the tracer injection is maintained. As the injection stops, the concentration at the measuring point eventually will start to decrease gradually until it reaches zero. Interconnection tests are a very important part of leak studies. There are a series of details associated with interconnection tests such as selection of the injection point or zone, tracer preparation and injection, tracer amounts, and measurements. These aspects are further described in detail by Plata and Araguás (2002). 0 Figure 11 shows a passing curve obtained in an interconnection test at the Boney Falls reservoir in Michigan. The tracer used during the test was Rhodamine WT and the measuring point was the West Flume, which corresponds to a leak on the west side of the dam. It can be seen from Figure 11 that the tracer was detected only on the West Flume, which indicates no connection between the release point and Barney Creek and West Weir. The tracer arrival time was 30 minutes. The concentration then rises sharply up to 2,000 ppb indicating that the recharge point(s) is associated with karstic media. After reaching the peak, the concentration drops sharply with another smaller peak, indicating a rather concentrated flow condition. The shape of the passing curve suggests flow through a karstic media with relatively short path. The second peak suggests the presence of a second smaller recharge point sharing the same flow path. The passing curve was used to compute the transit time and an average flow velocity in the range of 1 to 2 ft/min. The storage volume of cavities was estimated as 1,150 yd Collaborative Management of Integrated Watersheds

19 Figure 11. Passing Curve Measured at West Flume in Boney Falls, Michigan Tests Performed in the Reservoir The main objective of the tests performed within the reservoir is identifying the point or area of infiltration. In most leakage studies, lowering the reservoir to search for the infiltration point or area is not an option. Therefore, techniques to search for the infiltration zone, with the reservoir full of water, have been developed. However, this is a very difficult task because most of the techniques are point oriented and the reservoir typically involves large areas. As a result, the techniques previously described (flow rate at the leaks, natural tracers, chemical composition, and stables isotopes) need to be applied first with the purpose of limiting or reducing the area where the infiltration point or area is going to be searched. Additionally, the geologic setting of the reservoir bottom and surrounding areas needs to be understood. Once these aspects have been evaluated and a relatively small area of potential infiltration is identified, the techniques for searching in the reservoir bottom can be applied. The methods used for location of the infiltration zones at the bottom of the reservoir include: 1) use of float drogues; 2) tracing of the reservoir water; 3) migration of tracer cloud; 4) filter tubes; 5) absorbed tracers; and 6) direct infiltration measurements. All these procedures are explained in detail by Plata and Araguás (2002). The majority of these methods require special equipment. The authors are most familiar with a direct infiltration method using a single point dilution. This method is relatively simple and does not require sophisticated equipment. Prevention and Detection of Leakage 803

20 The method was originally proposed by Plata and Iragüen (1992) using 131 I solution. However, it can also be performed using salt. The method consists of injecting a pulse of brine solution through a hose from the boat at the surface of the reservoir. The end of the hose is encased in a diffuser connected close to a conductivity meter. The diffuser is located up near the bottom of the reservoir. After injection of the brine pulse, measurements of the conductivity with time are performed. The relative time for the brine to move out of the diffuser provides an indication of the water velocity at the bottom of the reservoir. The tests are performed on a grid and then contours of velocities at the bottom of the reservoir can be developed. This information is then used to identify the zone of infiltration. Figure 12 shows the results of a single point dilution method at the bottom of the Boney Falls reservoir in Michigan. It can be seen from Figure 12 that in a high velocity zone the concentration curve moves out of the diffuser in about 1 minute. On the other hand, in a low velocity zone the concentration curve do not move as fast and takes more than 4 minutes to move out. Figure 12. Results of Single Point Dilution Method at Boney Falls GENERAL APPROACH FOR INVESTIGATION AND DETECTION OF LEAKS IN DAMS AND RESERVOIRS A general specified procedure describing each step of leaks investigations in dams is not available. Only general rules on how to approach the investigation and studies exists. One 804 Collaborative Management of Integrated Watersheds

21 aspect that has a major impact on the investigation program is the evaluation of the risk associated with the leak depending on their location and magnitude. An immediate action when a leak is detected involves the evaluation of the sediments that are in the leak. If the amount of sediments being carried by the leak is significant, emergency actions should be undertaken to preserve the integrity of the embankment. When the amount of sediments is not significant, the investigation process starts by collecting as much information as possible about the geology and hydrogeology of the impacted area. This includes history of the leaks (if available). Testimonial evidences from local residents or operators are of great help. Then an evaluation is made of the correlation between the leak flow rates and the reservoir level. This initial information allows the development of a preliminary investigation plan. The following action has the objective to identify the origin of the water emerging at the leaks. In many cases, it is directly assumed that the water at the leaks comes from the reservoir. However, this is not always the case and there are several cases where the origin of the water at the leak is not associated with the reservoir. In other cases, only part of the leak water comes from the reservoir because it is combined with another water source (i.e. groundwater). Field campaigns for measuring electrical conductivity and temperature on a regular basis should be initiated. The temperature and conductivity determinations are then combined with water sample campaigns for determination of chemical composition and stable isotopes. The information collected at this stage, allows formulating a flow hypothesis. This may then be verified with the use of interconnection tests and tests conducted in boreholes. When the results indicate the connection with the reservoir and areas have been delineated or identified as potential recharge zones, tests in the reservoir are then performed to locate such areas. These are general guidelines in the approach to leakage investigations. The investigation process has to be flexible and needs to allow modifications depending on the partial results as the investigation develops. CASE HISTORIES This section presents three case histories of application of the techniques previously described. The three cases illustrate how the use of these techniques led to the identification and subsequent repair of the leaks. La Honda Dam La Honda Dam is an earth embankment built over the Uribante River creating a reservoir with an approximate volume of 750 Million m 3 capacity in western Venezuela. The dam is part of the Uribante-Caparo hydroelectric complex to generate 300 MW utilizing an 8 km long tunnel from the reservoir to the powerhouse providing 354 m of static head. The dam is a 130 m tall zoned earth embankment with a central clay core and chimney Prevention and Detection of Leakage 805

22 drain system. The geology at the dam site consists of a sequence of fractured sandstone and shale in an active seismic zone. The fractured sandstone was identified as very permeable and sometimes erodible particularly on the left abutment. As a result, an intensive foundation treatment on the left abutment was undertaken during construction, which included the following four seepage barriers: grout curtain, concrete cutoff, drainage curtain, and construction of an upstream clay liner. During the first reservoir filling in 1987, when the reservoir reached the minimum operating level (i.e. elevations 1066 to 1080 m), a series of leaks were observed on the left abutment approximately 600 m downstream. These leaks, obviously, were not controlled by the foundation treatment and seepage barriers previously described. The total amount of flow from the leaks was 35 l/s and complementary concrete channels were built to collect and monitor the total leakage systematically. During more than 10 years, several studies were conducted and repair work was implemented with the purpose of reducing or stopping the leaks. The cost of these repairs was on the order of $6 Million. All of these repairs were implemented focusing only on the engineering aspects of the problem without a good understanding of the hydrogeology aspects such as determination of the origin of the water at the leaks, preferential flow paths, and transit times. Therefore, all of these repair works failed to reduce or eliminate the leaks. Figure 13 shows an aerial view of La Honda Dam and the relative location of the leaks. During the period from 1995 to 1997, an investigation methodology using the techniques described herein was implemented at the site. The methodology included the relationship between reservoir level and flow rate, systematic measurement of conductivity, temperature, stable isotopes, chemical composition, etc. The use of these techniques allowed determination of the recharge zone within the reservoir and the preferential flow path between the reservoir and the leaks. From the stable isotopes measurements, it was determined that the 100 m deep reservoir exhibited isotopic stratification. Figure 14 shows that the isotopic stratification of the reservoir between elevations 1050 and 1075 m are associated with the same isotopic composition as the water emanating at the leaks. This allowed determining that all the water at the leaks came from the reservoir with a recharge zone between elevations 1050 and 1075 m. 806 Collaborative Management of Integrated Watersheds