Contribution of spectral coherency analysis and tracer test to study leakage at the Doosti Dam reservoir, Iran and Turkmenistan

The Doosti Dam was built across the Harirood River on the border between Turkmenistan and Iran. During the reservoir impounding, leakage occurred as new springs through the sandstone layers of the Pesteleigh Aquifer and limy sandstone and limestone layers of the Neyzar–Kalat Aquifer, at the right abutment of the dam. To evaluate the grout curtain operation, a tracer test was carried out by injection of Uranine in a borehole located at the upstream of grout curtain in the Pesteleigh Aquifer. Tracer test results demonstrated a diffuse flow component through the grout curtain in the Pesteleigh Aquifer, but no tracer was detected at the main leakage point, SP1 Spring, emerged downstream of grout curtain from the Neyzar–Kalat Aquifer. Using the spectral coherency function, the lag time between changes in the water level of the injection borehole and water level in the tracer detected boreholes or discharge of the tracer detected springs was determined. Linear regression analyses indicated that the estimated lag time by the time series analysis was close to the first arrival time of the tracer. Therefore, the estimated velocities based on the lag time of time series could be considered close to the calculated velocities based on the first arrival time of the tracer. The estimated groundwater velocities based on the time series analysis ranged from 3.91 to 20.31 m h⁻¹, showed that diffuse flow dominated pathways from the reservoir toward the downstream boreholes in the Neyzar–Kalat Aquifer, while conduit flow was present within the pathways toward the SP1 Spring. Regarding the reservoir volume, the negligible amount of leakage at the maximum water level of the reservoir confirmed well overall operation of the grout curtain at the dam site.