APPLICATION OF ISOTOPE TRACER TECHNIQUES FOR ASSESSING THE SEEPAGE OFTHE HYDROPOWER DAM AT TRI-AN, SOUTH VIETNAM

Some symptoms of excessive seepage of water were observed at the earth fill dam of the 400 MW Tri An hydropower plant in South Viet Nam To pr ovide experimental information for assessing the situation, isotope tr acer experiments were performed using 24 existing piezometric borehole s encompassing a 1100 m long middle section of the dam. Filtration vel ocities measured were in the ranges (0.2-30) cm d(-1) in the dam body and (0.5-75) cm d(-1) in subsurface strata No abnormally permeable hor izons were detected Permeability coefficients were calculated and comp ared with that obtained in pre-project hydrogeological surveys and by field tests during dam construction. Tracer experiments yielded permea bility of the same range for the subsurface aquifer but from one to tw o orders of magnitude higher for the dam body. Such a dramatic change of permeability of the dam body media is interpreted as a result of in teraction with seeping water after the reservoir impoundment. A transi ent erosion process took place during several years. The losses of wat er by seepage through the dam body and infiltration to underground wat er were estimated as 1100 m(3) d(-1) and 15,000 m(3) d(-1) respectivel y, less than 1/2000 of the quantity of water consumed for power genera tion. A multi-well interconnection experiment was performed to determi ne the velocity and direction of subsurface flows.