Identification of Source of Leakage in Drainage Gallery of Tehri Dam Using Isotopic Techniques

The present study is carried out using stable isotopes (δD and δ¹⁸O) to identify the sources of leakage in the drainage galleries of the Tehri Dam, which is constructed on the river Bhagirathi at Tehri. The water samples from leakage points, springs and the reservoir (surface) were analysed for δD and δ¹⁸O. The δD and δ¹⁸O signatures of the samples obtained from the reservoir and leakage water revealed the connectivity of five leakage points out of six with the reservoir when the reservoir level (RL) rises above 770 m during the monsoon and post-monsoon months. The analysis of isotopic data indicates that leakage water is a mixture of reservoir water and groundwater. However, reservoir water contributes the major component in the leakage during the monsoon period when the RL rises above 770 m. At one site (D9), leakage is only due to local recharge of precipitation. Analysis of RL and leakage discharge data of two drainage galleries (i.e. AGIR and AGR-3) corroborates the findings of isotopic techniques.