2:15 PM - 2:30 PM
[AHW19-03] Estimation of Groundwater Contribution to the Fuji Five Lakes Using Trace Elements and Water Stable Isotope Tracers from Lake Bottom Springs
★Invited Papers
Keywords:Groundwater, Trace elements, Water stable isotope, Fuji Five Lakes
Mt. Fuji is one of the largest active volcanoes in Japan. Numerous springs, including the Kakitagawa springs, which rank among the largest springs in Japan (1 million m3/s), are scattered across its foothills. However, the distribution of these springs is regionally biased, with a higher concentration on the southern side of the mountain. This disparity is usually attributed to reduced precipitation on the northern foot of Mt. Fuji, influenced by the rain shadow effect, and a lack of knowledge regarding springs distributed within the Fuji Five Lakes (Lakes Yamanaka, Kawaguchi, Sai, Shoji, and Motosu). Indeed, scant data exist on the lake-bottom springs in Fuji Five Lakes; therefore, our understanding of groundwater flowing into these Lakes is very limited. In this study, we conducted surveys of lake-bottom springs in Lake Kawaguchi and analyzed the trace element compositions and stable isotope ratios of spring water. The objective of this study is to elucidate the origin and volume of groundwater inflow into Fuji Five Lakes.
The water quality survey conducted using a CTD meter identified an area with an anomalous water quality depth profile in the central part of Lake Kawaguchi. Additionally, underwater surveys utilizing sonar and drones revealed previously undiscovered springs approximately 10 m in diameter at a depth of approximately 8 m. To ascertain the origin of these springs, we directly collected water samples from them using a seepage meter and analyzed their geochemical characteristics. A comparison of the spring water stable isotope ratios with those of the groundwater around the lake indicated that the spring water mainly originated from groundwater sourced from the Misaka mountains rather than from Mt. Fuji. Previous studies have estimated the contribution of groundwater to the lake to be approximately 4% (Koshimizu and Tomura, 2000), based on the assumption that the vanadium in the lake water primarily originates from groundwater from Mt. Fuji. However, the vanadium concentration in the spring water at the bottom of Lake Kawaguchi is significantly lower than that of the groundwater from Mt. Fuji (up to 5.5 µg/L), yielding an estimated groundwater contribution of 34 to 53%. These estimates are consistent with those based on radiocarbon concentrations in lake waters (33–47%; Ota et al., 2021) and those calculated based on water balance equations (47%). These results indicate the significance of trace element and water isotope analyses in estimating groundwater quantities, particularly in lakes lacking direct input from inflowing rivers. Additionally, the radiocarbon concentrations in water samples emerge as a useful tool for estimating water budgets and groundwater inflows in the Fuji Five Lakes areas.
The water quality survey conducted using a CTD meter identified an area with an anomalous water quality depth profile in the central part of Lake Kawaguchi. Additionally, underwater surveys utilizing sonar and drones revealed previously undiscovered springs approximately 10 m in diameter at a depth of approximately 8 m. To ascertain the origin of these springs, we directly collected water samples from them using a seepage meter and analyzed their geochemical characteristics. A comparison of the spring water stable isotope ratios with those of the groundwater around the lake indicated that the spring water mainly originated from groundwater sourced from the Misaka mountains rather than from Mt. Fuji. Previous studies have estimated the contribution of groundwater to the lake to be approximately 4% (Koshimizu and Tomura, 2000), based on the assumption that the vanadium in the lake water primarily originates from groundwater from Mt. Fuji. However, the vanadium concentration in the spring water at the bottom of Lake Kawaguchi is significantly lower than that of the groundwater from Mt. Fuji (up to 5.5 µg/L), yielding an estimated groundwater contribution of 34 to 53%. These estimates are consistent with those based on radiocarbon concentrations in lake waters (33–47%; Ota et al., 2021) and those calculated based on water balance equations (47%). These results indicate the significance of trace element and water isotope analyses in estimating groundwater quantities, particularly in lakes lacking direct input from inflowing rivers. Additionally, the radiocarbon concentrations in water samples emerge as a useful tool for estimating water budgets and groundwater inflows in the Fuji Five Lakes areas.