Water quality and water circulation formation of a deep lake on the basis of interaction between surface water and groundwater in a highly acidic hot spring region

Hydrological interactions between surface water and groundwater are key processes to describe the watershed water cycle including lake water hydrology. Understanding these processes is important for quality and quantity controls of water resources and their future management. Water originating from mining/hot spring regions gives significant impacts on local water resources. However, hydrological processes of how the discharged water with unique water quality affects different water bodies have not been fully understood. Therefore, the study aims to investigate features of surface water and groundwater interaction forming lake hydrology in a highly acidic Tamagawa hot spring region, in Japan. For this, the river water, spring water, and lake water at 51 locations in total were sampled. Lake water collection with depths of 0, 50, 100, 200, 300, and 400 m was undertaken at the center of Lake Tazawa. A variety of environmental tracers such as pH, EC, DO, inorganic ion concentrations, oxygen/hydrogen stable isotopes, CFCs, and SF₆ in collected water samples were measured.

The river water upstream of the Tamagawa River showed a pH of <3.0 and characteristic Ca-Cl type water quality due to the influence of a highly acidic hot spring. The other river and spring waters were almost neutral and of the Na-HCO₃ type water quality. Lake Tazawa water was weakly acidic (pH 5.5, Ca-Cl type water quality), suggesting that the water in the lake originated mainly from the Tamagawa River. Vertical profiles of environmental tracers of lake water at the center of Lake Tazawa indicated that dissolved oxygen concentrations were above 87% saturation even at depths greater than 100 m, suggesting that a rapid lake circulation was occurring. The multiple uses of gas tracers (SF₆ and CFC-12) suggested that water age is 4 and 12-20 years in the water close to the lake surface and water deeper than 50 m, respectively. The binary plot of SF₆ and CFC-12 concentration indicated that the exponential mixing primarily governs lake water circulation processes. Moreover, calculations of the water balance of Lake Tazawa for the five-year period from 2011 to 2015 inferred that groundwater inflow into Lake Tazawa from the surrounding area may have occurred at a rate of at least 5-12 mm/day. These findings suggest that the inflow of groundwater and river water into Lake Tazawa is responsible for the lake's water quality and a part of rapid lake water circulation.