2020 Volume 129 Issue 5 Pages 697-724
Quaternary volcanoes and their surrounding terrains typically have excellent water resources and generally are more abundant in groundwater than other areas, because aquifers consist of very permeable volcanic materials, such as lava flows and fall deposits. However, the groundwater-flow systems and storage processes are complex, due to the geological setting and volcanic histories. Fuji Volcano is the most famous Quaternary volcano in Japan, and is one of the most active. Volcanic activity at Fuji Volcano began about 100,000 years ago. Fuji Five Lakes (Fuji-Goko), consisting of Lake Motosu, Lake Shoji, Lake Sai, Lake Kawaguchi and Lake Yamanaka, lying in the northern foot area, were dammed by pyroclastic flows from Fuji Volcano. The groundwater-flow system at the northern foot area of Fuji Volcano is summarized based on the results of previous hydrogeological research and joint research on the groundwater-flow system and volcanic geology. The results of aquifer unit classification and hydrogeological characterization at the northern foot of Mt. Fuji are summarized as follows with the aquifer unit classified into five divisions. The aquifers are subdivided into Saq-1 and 2, Faq-1 and 2, and Haq-2 in descending order of depth. Haq-1 is a volcanic mudflow deposit that was formerly known as the Old Fuji Mudflow, and is considered an impervious layer. The timing of the formation of Fuji Five Lakes is as follows: Lake Yamanaka has two formation times, about 5 ka and 1.4 ka. Lake Kawaguchi was dammed up about 20 ka and took almost its present shape in 10 ka. Lake Motosu was prototyped in 30 ka and remains to the present day. In the process, the “Lake Se (Se no Umi)” was formed, and about 1,200 years ago, the Jogan eruption divided Se no Umi into Lake Sai and Lake Shoji. This timeline was confirmed by observations of the groundwater level, indicating that the water flows out from Lake Kawaguchi to the south through lava flow Faq-1 and 2.