Hydrological and chemical budgets in a volcanic caldera lake: Lake Kussharo, Hokkaido, Japan

The contribution of groundwater output and input to lake chemistry was examined by estimating the hydrological and chemical budgets of a volcanic caldera lake, Lake Kussharo, Hokkaido, Japan. The lake level, meteorology, river water discharge and water properties were measured in the ice-covered period of February–March and in the open-water period of June–October in 2000. The inorganic chemistry was then analyzed for sporadically sampled surface water and hot spring water. The chemistry of lake water at pH of 6.91–7.57 and EC25 (electric conductivity at 25 °C) of 29.2–32.7 mS/m appears to be controlled by the input of two types of hot spring water: the inflowing Yunokawa River (pH of 2.27–2.54 and EC25 of 197.8–258.0 mS/m) and groundwater discharging directly on the shore (pH of 7.13–8.32, water temperature of 35.0–46.5 °C and EC25 of 53.1–152.0 mS/m). Excluding the days with rainfall or a great change in lake level, the water budget in June–October gave a net groundwater input of −7.41 to 2.97 m3/s. A combination of the water budget with the chemical budget of two solutes, Na+ and Cl−, led to the best estimate of groundwater output, Gout, at 3.82±3.02 m3/s, the total fresh groundwater input, ∑Gfresh, at 2.14±1.00 m3/s, and the total groundwater input of hot springs, ∑Gspa, at 0.46±0.05 m3/s. This is comparable to Gout=3.87 m3/s, ∑Gfresh=1.49 m3/s and ∑Gspa=0.41 m3/s during the ice-covered period. The chemical flux by the freshwater input plays an important role in the alkalinity of lake water, as does the chemical flux by the shoreline hot springs. The large groundwater output could occur by the leakage through the highly permeable, underground pumice, distributed from the east-to-south lake basin to southeast of the outlet.