The Hydrological Regime in a Highly Acid Crater Lake: Lake Katanuma in Naruko Volcano, Japan

Abstract

In order to clarify the effects of the volcanic activity on hydrological, thermal and chemical features of a crater lake, Lake Katanuma, Japan, temperature and electrical conductivity at 25 °C (EC25) of lake water were monitored at two maximum-depth points in 2023−2025. Under closed condition of the lake without river outflow, the lake is characterized by a high acidity of pH = 2 on average and the existence of the surrounding geothermal area. A seasonal variation in the thermal and chemical features of the lake was shown by sporadic vertical profiling (0.1 m pitch) of temperature, EC25 and dissolved oxygen (DO) at the two points. The groundwater flow system around the lake was specified by quantifying the groundwater inflow Gin and groundwater outflow Gout in the lake from the estimate of hydrological and chemical budgets under no precipitation. During the pycnal stratification of May–September, the lake exhibited a four-layer structure with different temperature and EC25 values, probably due to the lower intrusion of groundwater passing through three different pathways. The four-layer structure disappeared due to vertical mixing in October–December and March–April and inverse stratification in January–February. The Gout and Gin values were estimated for three budget periods in the mixing season. Utilizing Darcy’s law for the Gout values as a confined groundwater flow, it is found out that Gout has a linear relationship with lake volume. The relatively high Gin and Gout values estimated are probably caused by the high permeability of lacustrine sand and gravel deposited as bedrock, which furnishes the high adjustability to lake level, i.e., its small variation at ca. 0.5 m in amplitude.