[AHW24-P01] Elucidation of groundwater discharge mechanism after the Kumamoto earthquake in northwest Aso
Keywords:Kumamoto earthquake, northwest Aso, new spring water
The new spring water was classified into two (2); the mountain spring water at the foot of the outer ring mountain, and the plain spring water confirmed at the plain spreading across paddy fields. These waters showed different quality compositions. The mountain spring water was of low concentration Ca-HCO3 type (the water type of common mountain stream waters), originating from the Somma. On the other hand, the plain spring water showed a high concentration of Ca-SO4 type originating from the central volcanic hill. It is speculated that this is underground water influenced by volcanic activity. Both waters were of the same quality as spring waters reported so far of the Aso caldera, as well as the distribution area.
Sulfur isotopic ratios were used as tracers to clarify the mechanism for the high concentrated water found only on the plain. The δ34S value varied depending on the site. One point showed relatively high δ34S value (10 ‰ to 17 ‰) and another showed relatively low δ34S value (2.5 ‰). The new spring water in the Kario area has the same isotopic composition as that of hot spring water in the surrounding area, and this may be due to the influence of a heat source similar to that of the hot spring water during the springing process. In a previous study, the δ34S of spring water recharged from the Aso volcano, which is a typical heat source in the Aso caldera, was approximately 5 to 10 ‰. Relatively, spring water in the northwestern part in this study have higher δ34S values, suggesting the possible presence of a heat source different from that of the central crater in the area.
In identifying reasons for the groundwater discharge after the earthquake, different factors were considered for both the mountain spring water and plain spring water based on flow system and water quality characteristics. The release of groundwater reserved beneath the mountain due to seismic motion and the possible formation of “water pathways” from the earthquake could be the reason for the occurrence of the new springs. New spring waters in the paddy fields of the plain whose discharge rate show no decreasing tendency even a year after the earthquake, taking into consideration seasonal variation, indicates that major underground structural changes have occurred. It can be inferred that new discharge route for groundwater has been created.