In this study, seepage phenomena through the left abutment of Marun dam are investigated. The Marun dam is a 170 m high rock fill dam, which regulates the waters of the Marun River, serves power generation, and flood control and provides irrigation needs. The dam site lies in the Zagros Mountains of southwest Iran. This region presents continuous series of mainly karstic limestone, marl, shale and gypsum ranging in age from Cretaceous to Pliocene. The region has subsequently been folded and faulted. All underground excavations are sited in the left abutment. The spacing of the diversion tunnels and pressure tunnel is considered to be acceptable, meaning relatively short, thus requiring 2 row grouting curtain into both embankments. Prior the reservoir impoundment, the concrete plug was constructed into the middle section of second diversion tunnel. Upstream section of tunnel was not concreted. During the first reservoir impounding, the old karst channels along ‘Vuggy Zone’ cut by the second diversion tunnel were reactivated and leakage occurred. The total amount of water leakage through the left bank of Marun dam was about. The unlined second diversion tunnel had a key role in connecting reservoir with karst conduit system. On the basis of detailed engineering geological analysis, the concept of remedial works was carried out. The main points of this concept are one of row grout curtain extension up to the section with shaly interbeds declared as watertight Asmari sequence (close to the watertight Pabdeh formation) and plugging of accessible section of main karst channel by concrete. In order to determine the seepage direction and karstification pattern, solubility studies were done. Also pinhole, XRD and XRF tests were carried out. The major joint system and interbedding cracks have predominant role in karst evolution process. Hydrogeological role of joints, perpendicu-lar to geological structure, is not negligible. As a result of these studies, seepage paths have been identified in the karstic limestone in the left abutment of the dam.