Inter-annual variations in the chemical and thermal properties of well waters from the volcanic hydrothermal areas: Usu Volcano, Hokkaido, Japan

Through our monitoring of the volcanic characteristics of well waters around Usu Volcano, Hokkaido, Japan, we evaluated the relationships among dissolved carbon dioxide (D-CO₂), water temperature and pH in the Toya-ko and Sobetsu hydrothermal areas in 2000–2020 after the most recent eruption. The D-CO₂ concentrations of sampled waters from wells connected to the hydrothermal areas increased in 2000–2006 and decreased thereafter until 2020. The water temperature continued to fall from 2000 to 2010, and fell moderately further until 2020. The pH was constant from 2000 to 2015 and rose thereafter. Scatter diagrams were drawn with D-CO₂ concentration per unit water temperature on the x axis and D-CO₂ concentration per unit pH on the y axis for water samples from 12 to 16 wells each year. The approximate formula was represented as z = ax + by from the scatter diagrams using the principal component analysis. The value of b/a was predicted to have a high value for well water groups in the volcanic hydrothermal areas. The values of a, b and b/a might be applicable as distinct factors for volcanic or non-volcanic hot water based on the examples of 7 well water groups in the various hydrothermal areas. The b/a values rapidly decreased from 2000 to 2003 and showed a further decrease until 2020 for the well waters from the Toya-ko and Sobetsu hydrothermal areas. This suggests that the groundwater with no volcanic reaction increased in the well water.