Natural tracers of water: (A) Temperature, electrical conductivity and chemical composition

Chapters 5 and 6 are devoted to the description of the most important natural tracers of water that are commonly used to obtain information on the origin (i.e., whether the emerging water is exclusively formed by the reservoir water) and the localisation of infiltration points in lakes and reservoirs. More concretely, natural tracers of water are used to obtain information on the followings aspects: Determination of the origin of the groundwater emerging at the discharge points located downstream from the dam as well as groundwater flowing through the abutments. As shown in the following chapters through the presentation of case studies, groundwater emerging at these springs does not always entirely correspond to reservoir water. Very frequently, some springs located close to the dam site represent the discharge of adjacent aquifers or a mixture of groundwater recharged by local precipitation and reservoir water. For this reason, when a leakage study is initiated, the first aspect to be tackled should be the confirmation of the origin of the emerging water, that is, whether the origin can be proven to be exclusively related to the reservoir, even in cases where it appears to be obvious. Experience proves that nature frequently reserves unexpected surprises. Investigation of the dynamic behaviour of groundwater flows emerging downstream. These studies should be based on the analysis of certain changes produced as a function of time, specifically the concentration of determined characteristic chemical or isotopic components contained in the reservoir water. Initially, the monitoring work should focus on the study of the trends in the concentration of certain parameters in the reservoir water and in the emerging water, and in an affirmative case, on the determination of the delay with which any concentration pulse is transmitted. This time lag provides information on the transit time of water between the reservoir and the springs, as well as between the reservoir and the boreholes located close to or downstream from the dam. If various springs discharge downstream from the dam, the transit time can be different for each flow path, and therefore, individual surveys for each spring should be carried out. Obviously, the first aspect to check in such cases is to determine whether groundwater feeding these springs and the reservoir water have the same chemical and isotopic composition.