Case studies on the fresh-saline water interactions in volcanic rock with numerical modeling

General Geology in volcanic island, such as Ohau and Jeju Island in South Korea, is highly complicated. And the complexity of geology can provide the difficulties when we try to identify the hydrogeological systems in the volcanic island. For 4 years since January 2004, we have surveyed the hydrogeological features on Jeju Island; the spatial distribution of springs, discharge rates, water budget including direct runoff and fresh-saline groundwater interaction in near coastal area. Among these features, the fresh-saline groundwater interaction in near coastal area is one of the important factors to establish the public water supply management in Jeju Island. Numerical studies were conducted to investigate the fresh-saline groundwater interaction, considering channeling effect in Jeju Island with FEFLOW package. We applied 8 cases of conceptual models and 4 types of inland boundary conditions; constant head/constant flux, time-varying head/constant flux, constant head/time- varying flux and time-varying head/time varying flux. But in this study, we did not consider the oceanic tide. In cases of four conceptual models among 8, we assign the orthogonal channel network and different hydraulic conductivities to channel and matrix block respectively; 10 times higher in channel, 100 times, 1,000 times and impermeable matrix block. And in the other four, it was assumed that the whole media are homogeneous. The cases considering channeling effect with constant head and constant flux boundary condition show some different features in the fresh-saline groundwater interface. In 4 homogeneous cases, the stepwise shape of the interface did not observed while, in 4 channel-considering cases, the sharp and stepwise interface can be observed. When time-varying boundary conditions were applied, homogeneous cases showed the small amplitude of interface variations and slow responses. But 4 channel-considering cases showed totally different features. System responses are relatively fast and the amplitude of interfacial response is higher. These results from channel-considering cases are well matched with the monitoring data in Jeju Island.