제주도 동부 중산간 교래리 및 해안 한동리지역 대수층특성 연구

Abstract

Hydrogeological structure in Jeju Island is complex and especially difficult to determine from the surface because of the uncertainly of volcanic geology where little of the internal structure visible. Lush vegetation masks the underlying lava flows of the island. Rainfall is abundant, surface water runoff is meager and dependent upon intense rainfall, facts that indicate that the groundwater resource is large. Details of its occurrence and size are not precisely known and would benefit from further investigation. The customary coring of tube wells in Jeju Island provides excellent opportunities to study composition and physical character of the volcanic formations. Geophysical logging of the wells provides evaluation of a larger sample of the formation properties outside the borehole for potential water. The high percentage of core recovery is due to massive character of the lavas. The massive character has other affects such as higher particle velocities of groundwater flow in the permeable zones, narrowed mixed zones in basal groundwater aquifers and higher heads. The purpose of this study is to understand the characteristics of each aquifer and phenomenon of seawater intrusion in Gyourae-ri middle mountainous area and Handong-ri coastal area in eastern Jeju Island. For this purpose, I made aquifer test for each aquifer using packer and change characteristics of electric conductivity by pumping test at aquifer of two monitoring well in the study area. Seawater intrusion is a common problem in coastal areas and many Islands, especially Jeju Island. Seawater intrusion into an aquifer depends on the characteristic of the local geology. Aquifer tests were performed to determine the hydraulic properties of aquifer systems. These properties may be needed to design municipal water supply well fields, to predict rates and directions of groundwater flow. Aquifer tests may be pumping test mainly. In pumping test, groundwater is pumped from a pumping well and water levels are measured in the pumping well and in one or more observation wells. The results obtained from this study are summarized as follows : It is formed by 5th major geological layer in study area, volcanic rock, sedimentary, SGF(Seogwipo formation), hyaloclastites and UF(Unconsolidated formation). Geological structure of aquifer is consisted of joint and fracture, hyaloclastites, fine sand, sand and gravel, scoria. The aquifer in JG-site is made of three aquifer and these aquifer is formed upper sea level. Eespecially, Perched aquifers are formed in this area. Aquifer in JH-site is made of three aquifer and first aquifer is formed -10～-40m below sea level and second aquifer is consisted of sand and gravel middle of lava. Last aquifer is formed in hyaloclastites of 51m layer. During a pumping test, the tidal effect is different in each aquifer and the range is about 1m.