Isotopic Approach To Understanding The Groundwater Flowsystem Within An Andesitic Stratovolcano In A temperate Humid Region: Case Study Of ontake Volcano, Central Japan

摘要

We used isotope tracer methods to clarify the groundwater flow system within Mt Ontake: a stratovolcano consisting of andesite lava and pyroclastic rock, located within a temperate humid region. Precipitation was collected monthly at 11 sites on the south, east, north, and west slopes of the volcano for a period of 26 months. The weighted mean δ-value in precipitation shows a clear decrease with increasing elevation (altitude effect) on all slopes and is relatively low on the leeward slope (rain-shadowing effect). The springs collected over the entire area of the volcano show δ-values that are controlled by the altitude effect and the rain-shadowing effect on precipitation. The average recharge elevations of the individual springs were calculated from their δ~(18) O-values and the equation of the altitude effect of groundwater for corresponding slope. The average recharge elevation and tritium concentration in springs indicate that the large-scale groundwater flow systems (vertical drop: ca 800 m) with a relatively long residence time are maintained along the lava flow in the younger volcano north zone, whereas relatively small-scale groundwater flow systems (vertical drop: ca 400 m) are distributed across a wide range of elevations in the younger volcano south zone. In the older volcano zone at the foot of the volcano body, it is inferred that local groundwater flow systems (vertical drop: less than 200 m) dominate and that these flow systems are not connected to those in the younger volcano zone. Consequently, the scale of the groundwater flow system decreases with increasing age of the volcano body. This contraction of the flow system with time might reflect the progressive erosion of the volcano body, especially within the younger volcano zone.