HYDROGEOLOGIC CHARACTERISTICS OF GRANITE IN AN UNDERGROUND PETROLEUM STORAGE AREA

This study concerns hydraulic properties of an underground crude oil storage located in Geoje Island and to detect the leakage of crude oil components from the storage. The storage is composed of unit 1, unit 2 and unit 3. The crude oil storage is located at 30 m below the sea level with 30 m of the cavern height. Units 2 and 3 are operated as using water curtain system, while unit 1 utilizes natural water pressure only. Hydrogeological investigation was conducted to clarify crude oil leakage from the storage. Cretaceous sedimentary rock and andesitic rock are intruded by Cretaceous Granite in the study area. Acidic dikes, intermediate dykes and basic dykes intruded the above-mentioned rocks. Fractures trending NNE, SE and NE are developed in the study area. Groundwater levels were measured at monitoring wells in units 1- 3 from August 9, 2002 to January 30, 2004. In unit 3 area, the water level of August 9, 2002 declined about nine meters at monitoring wells drilled in lower fractured aquifer (confined aquifer) and about one meter at monitoring wells drilled in upper fractured aquifer (unconfined aquifer) compared to the water level of January 30, 2004 during the period of the construction. This demonstrates that groundwater inflow rate to the unit 3 caverns was greater than the amount supplied by the water curtain system. Groundwater level declined more strongly in the lower aquifer than the upper cue to cavern excavation. On the contrary, groundwater level increased in unit 1 and unit 2 areas during the period. This indicates steady-state condition in unit 1 and unit 2 areas. BTEX were detected at six wells in unit 1, ranging 0.3-64.1 µ¶/L with average 23.8 µ¶/L. The PAH concentration in unit 1 area ranges 0.046-7.29µ¶/L with average 0.63µ¶/L. In unit 2 area, BTEX were detected at only two monitoring wells with the concentration of 0.63-1.9 µ¶/L. The average PAH concentration (0.16 µ¶/L) is lower than that in unit 1. BTEX and PAH concentration in unit 1 area without water curtain system is higher than in unit 2 area. Hence, water curtain system is necessary to enhance the safety of underground crude oil storage as protecting leakage of crude component from the storage with carefully monitoring of the water curtain. This study was financially supported by the Second Stage of the BK21 Project, Ministry of Education, Korea.