Hydrogeochemical, multiple isotopic approaches to investigate seawater mixing of groundwater in volcanic Jeju Island, Korea

Abstract:

Groundwater is a sole resource for water supply in Jeju Island which is composed of various formations of porous volcanic rocks. Therefore, preservation of the groundwater resource is an essential issue. Due to its geological features of the island, seawater has been intruded landward, mainly in the eastern region, which restricts groundwater use in the area. In the western region, severe nitrate contaminations of groundwater have been occurred by heavily performed agricultural activities, and moreover deterioration of groundwater quality by seawater intrusion has been observed in recent years. In this study, to delineate the mixing process related to seawater intrusion into groundwater from Gosan (western region) and Pyoseon (eastern region) of Jeju Island, hydrogeochemical and multiple isotopic approaches were applied. Also, fractionation ratios of each factors (fresh groundwater, nitrate contaminated groundwater, and seawater) which affect the groundwater quality from the study areas were estimated by using the MIX_PROGRAM. The effect of seawater was observed at the groundwater wells located inland up to 1.5 km from the coast and showed to be enlarged landward during a dry season. The fractionation ratios of seawater had the minor range (0.1~1.2%) for the Pyoseon area and 0.4~3.7% of seawater was mixed with fresh groundwater in the Gosan area. Differences in hydrogeological properties between Gosan and Pyoseon areas made dissimilar occurrences of seawater mixing into groundwater in the island.