Utilising nuclear techniques1 December 2004
Kamal Laksiri gives a brief report on the use of safe nuclear techniques in dam and reservoir seepage and leakage investigations
LEAKAGE incidents associated with dams and reservoirs have been reported frequently from projects around the world. These leakage incidents could be associated either with the reservoir banks, dam foundations or in the dam body itself, or may be found in a combination of these. The scale of a leakage could range from a small dripping coupled with wet patches to a stream flow with a flow rate of several cubic meters per second resulting in an economic loss by reducing the expected benefits of the project. Additionally a leakage could pose stability and safety problems due to the saturated ground as a result of activated ground water regime in the surrounding areas fed by the leakage ingress zones. There are also cases where reservoirs have been abandoned due to their inability to retain water.
In most of the cases the main cause for a reservoir leakage is due to adverse geological conditions in the surrounding ground structure, which forms and holds a reservoir. Furthermore, the unsuitability of the ground may be either due to unforeseen geological conditions or underestimation of the imperviousness of the geological features resulting in adoption of inadequate measures in improving the reservoir water tightness.
During the last four decades the use of safe nuclear techniques - basically using Isotopes, either naturally occurring (environmental) or intentionally injected (artificial Isotopes) - have proved valuable in studies related to water resources assessment, management and development in general, and specifically in leakage and seepage investigations associated with reservoirs. In addition to the applications in leakage and seepage studies, isotope techniques are used in the following areas:
• Source of recharge and estimation of recharge to ground water.
• Source of ground water salinity and pollution.
• Surface water - ground water salinity and interconnection.
• Aquifer interconnection.
• Dating of ground water.
• Efficacy of artificial recharge.
• Dynamics and sedimentation in lakes and reservoirs.
However, it is only the applications of Isotope techniques in dam and reservoir leakage and seepage investigations which are discussed in this paper.
Before getting into details it is worthwhile recalling the fundamentals of Isotopes. An atom consists of a nucleus surrounded by electrons, with most of the mass of the atom concentrated in the nucleus, which is formed by two kinds of particles: neutrons and protons. Protons are positively charged while neutrons carry no electric charge and hence behave neutrally. The number of protons is characteristic for each chemical element and is called the atomic number (Z). This number is equal to the number of electrons, which are negatively charged, so that the atom as a whole is neutral. Due to the repulsive electrical forces existing between protons, the presence of neutrons is required to stabilise the nucleus. In the case of light elements, the number of protons and neutrons is equal. The sum of protons and neutrons is called the mass number (A).
The term nuclide refers to each combination of neutrons and protons that can compose each element. The atomic nuclei containing different numbers of neutrons and protons are called Isotopes. All isotopes of a given element present the same chemical behaviour because the chemical properties are controlled by the external configuration of the electrons, which is the same for all isotopes of a given element. In nature, each element is constituted of a mixture of different isotopes. However, the tiny differences of mass of the atoms or molecules containing other isotopes besides the most abundant ones are responsible for a different physical behaviour, in which the structure and the characteristics of the nucleus are relevant. Isotopes are usually represented in the form AX, where X denotes the symbol of the element.
The environmental isotopes are those which occur in the environment in varying concentrations, and over which man has no direct control. Generally the environmental isotopes are classified into two categories: Stable isotopes and radioactive isotopes
Stable isotopes are those that are commonly found in natural elements. Some elements contain only one stable isotope. However, when several stable isotopes are present in the natural element, their relative proportions remain constant, although small differences can originate when element is involved in chemical reactions or phase changes.
The stable isotopes, which are commonly used in this type of study, are
• Deuterium (2H)
• Oxygen - 18 (18O)
• Carbon - 13 (13C)
• Sulphur - 34 (34S)
Radioactive isotopes are those which undergo spontaneous radioactive decay. That occurs mainly in the isotopes showing a large shortage or excess of neutrons compared with the stable isotopes of the same element. Details are not discussed in this paper.
Most commonly used radioactive isotopes in this type of study are
• Tritium (3H)
• Carbon - 14 (14C)
And following as artificial isotopes
• Iodine – 131 (131I)
• Gold –198 (198Au)
The three heavy isotopes of hydrogen and oxygen mentioned above viz. Oxygen - 18 (18O), Tritium (3H), Deuterium (H) constitute an integral part of the water molecule, in the form of HDO, H218O and THO. Water molecules containing any of the heavy isotopes mentioned above behave from the chemical point of view as any other molecule formed exclusively with the most abundant isotope (H216O). Therefore their behaviour is almost ideal when compared with other tracers whose properties might be or certainly are different when compared to those of water molecules. Water molecules containing these heavy isotopes are perfect tracers to study the mixing and behaviour characteristics of different water bodies in the water cycle.
Stable isotopes of water
Natural hydrogen is exclusively formed by two stable isotopes 1H (99.985%) and 2H or Deuterium (D)(0.0155). Similarly Oxygen is formed of three isotopes, 16O(99.759), 17O(0.0374) and 18O(0.2039). 1H and 16O are the most abundant species, and the usual form to express the composition of water is 1H216O. Out of the nine other isotopically different water molecules that can be formed with the indicated isotopes, only three viz. 1H216O, 1HD16O, 1H2 18O occur in nature in easily measurable concentrations. Taking the Ocean as the major reservoir of water, and therefore representing the typical water molecule, respective abundance of the three most abundant molecules are
• 1H216O 997.680 parts per million
• 1HD16O .320 parts per million
• 1H218O 2.000 parts per million
The common form to express the abundance of a given isotope is by using isotopic abundance ratios(R), defined as follows. For instance 2H/1H or 18O/16O. For practical reasons, instead of using the isotope ratio R, isotopic compositions are generally given as d values, the relative deviations with respect to a standard value, as defined by:
The accepted standard for the isotopes in water is VSMOW (Vienna Standard Mean Ocean Water). That is a sample prepared by mixing water from different oceans by the International Atomic Energy Agency in Vienna and used as the Standard water. There are also other standards developed later.
Thus the basis behind an Isotope study is to measure and establish the isotope ratio or the signature of different water bodies in a system and to compare their relationships.
Though a particular water source can have a unique isotopic signature, it can be affected by various other phenomenon such as evaporation, and condensation. It can also be affected by altitude and latitude effects but these aspects are not discussed here. For a detailed discussion the references given at the end are recommended.
In a study involving isotope techniques it is usual to establish the isotope signature patterns of constituent water bodies in a system by measuring for one or two years covering at least one full hydrological cycle in the particular area. The measurements can be done on a monthly basis.
In the case of artificial Isotopes applications they are used merely as tracers. The advantage being the small amount required and also the easiness in detecting. Thus it can be used in detecting the leakage paths and ingress zones.
In the case of a large reservoir if dye tracers are to be used, it may not be practicable due to the amount of dye required to maintain the dye in measurable levels against the dilution. However the main problem with the artificial isotopes is the difficulty in using them under normal site conditions.
For clarity the following simple case study could be presented. In the La Sierpe hydro power project in Panama, water started to leak into the power house through the cavern roof. The source of this leak was unknown. Reservoir water to the power plant is conveyed through an underground pressure pipeline and it was suspected that the tunnel, which is few kilometres long, could be feeding the leak. On the other hand a small creek, running close to the power plant, could also have been connected to the leakage. Isotope analysis was carried out and it gave the following results:
• Leakage water in the power plant dD = -72.5%, d18O= -10.65
• La Sierpe reservoir dD = -42.1%, d18O= - 6.76
• Chiriquicito Creek dD = -63.2%, d18O= - 9.19
These results clearly showed that the leaks were not connected to the reservoir but to the nearby creek. Continued measurements confirmed the above findings showing how the creek water isotopic composition varied with that of the leakage water.
There are number of projects where the technique involving environmental isotopes has been successfully used in studying leakage and seepage problems associated with water related structures. However in some cases the result interpretation has not been as straightforward as in the above example, and the technique has to be incorporated with other studies.
In such complex cases the use of artificial isotopes could be the best solution, though the application of them may not be convenient in some countries.
Kamal Laksiri BSc (Eng), MSc (Hydropower), MASCE, MASME, C.En, Ceylon Electricity Board – Sri Lanka. Email: [email protected]TablesTable 1