Jeju is the biggest volcanic island in Korea, which was entirely formed from volcanic activities approximately 1.88 million years ago.
Jeju consists of volcanic rocks, Seogwipo Formation, U Formation and Basement.
Volcano thickness is about 2km(above sea level 1,950m and below sea level 150m)
Most of the streams developed on the north-south slopes and streasm length is short and steep slopes, runoff duration is short too.
Because Jeju consist of high permeable volcanic rocks, the streams flow only when the rainfalls is over 50 mm per day.
Groundwater occurrence type in Jeju is divided into three ; basal groundwater, parabasal groundwater and high level groundwater,
according to the relation between fresh water and sea water, the characteristics of subsurface geologic structure and groundwater level.
- High-level groundwaterHigh-level groundwater is occurred when rainfall coming into below the surface of the earth through water permeable stratum by the force of gravity, which is unable to go down any further due to impermeable rock stratum with low water permeability, and moves through low water impermeable stratum.
- Basal groundwaterBasal groundwater is occurred when fresh water stays above salty water in a shape of lens due to specific gravity difference between them(Ghyben-Herxberg principle).
- Parabasal groundwaterParabasal groundwater is occurred when fresh water is not able to make direct contact with sea water by Seogwipo stratum with low water permeability, to which G-H principle is not applied.