5:15 PM - 7:15 PM
[SCG56-P03] Generation mechanism of spring water and gas from the deep lake bottom of Lake Biwa and its effect on the lake bottom environment
Keywords:Lake Biwa, Spring water from the lake bottom, Environment, Methane
1. Introduction
It is said that around 10-20% of the water flowing into Lake Biwa is spring water from lake bottom in Lake Biwa. The spring water is considered to have a non-negligible effect on the environment of Lake Biwa. However spring water from the deep lake bottom has not been well clarified.
Kumagai et al.(2021) firstly found the spring water with gas (methane>99%) from the deep lake bottom in Lake Biwa in 2009, using AUV, sonic exploration, and temperature gradient measurements of the lake bottom. According to Kumagai et al.(2021), the benthic vents, which are outlets of the spring water, and the gas acoustic anomalies, which are the underwater acoustic anomalies caused by the gas from lake bottom, mostly lined up on a line about 10km north-south. However the positions of the gas anomalies were scatterd more than those of the vents. Kumagai et al.(2021) were concerned about the effect of the spring water on the environment of Lake Biwa. However, the deep bottom spring water of Lake Biwa has not been sufficiently investigated since 2013.
Therefore we conducted this research to know current conditions of the deep bottom spring water from 2021 and evaluate the effect of it on the environment.
2.Methods
We carried out comprehensive sonic exploration in 2022-2024. We also carried out CTD measurement, and lake water quality survey including hydrogen and oxygen isotope ratio measurements at three depths (5m, 50m and bottom(90-100m) at Y1(35°20.22-20.25'N, 136°6.09-6.13'E) and T1 (35°22.19'N, 136°05.83'N ) in 2021-2024. Y1 is the point where the gas acoustic anomalies are usually detected. T1 is the reference point, which is a periodical observation point of the University of the Shiga Prefecture. The water depth of Y1 and T1 is 90-100 m and 90 m, respectively. We carried out long-term temperature monitoring in lake bottom sediment at Y1 from October 2022 to March 2023 and from October 2023 to July 2024. We also carried out carbon and hydrogen isotope ratio measurements of the methane in the gas sampled by overwater displacement at the surface of the lake at Y1.
3.Result and Discussion
The gas acoustic anomalies were widely distributed and not lined up as reported by Kumagai et al. (2021). The results of the CTD survey measurement and the water quality survey were generally consistent between Y1 and T1, but they were sometimes a little different between Y1 and T1 at the bottom of the lake. The methane concentration in the sampled gas was about 10-60%, which is much higher than that in the air. The methane was found to be of organic origin in the lake bottom sediments from the isotopic analysis. During the observation periods from October 2022 to March 2023 and from October 2023 to July 2024, the temperature gradient at Y1 was estimated to be about 150 mK/m (temperature gradient expected for a normal Lake Biwa bottom: about 50-70 mK/m). This high temperature gradient can be explained by heat supply from the groundwater gushing from the lake bottom as the spring water. The heat supply possibly warms the lake sediments under the lake bottom, activates methanogenic bacteria and produce the methane.
It is said that around 10-20% of the water flowing into Lake Biwa is spring water from lake bottom in Lake Biwa. The spring water is considered to have a non-negligible effect on the environment of Lake Biwa. However spring water from the deep lake bottom has not been well clarified.
Kumagai et al.(2021) firstly found the spring water with gas (methane>99%) from the deep lake bottom in Lake Biwa in 2009, using AUV, sonic exploration, and temperature gradient measurements of the lake bottom. According to Kumagai et al.(2021), the benthic vents, which are outlets of the spring water, and the gas acoustic anomalies, which are the underwater acoustic anomalies caused by the gas from lake bottom, mostly lined up on a line about 10km north-south. However the positions of the gas anomalies were scatterd more than those of the vents. Kumagai et al.(2021) were concerned about the effect of the spring water on the environment of Lake Biwa. However, the deep bottom spring water of Lake Biwa has not been sufficiently investigated since 2013.
Therefore we conducted this research to know current conditions of the deep bottom spring water from 2021 and evaluate the effect of it on the environment.
2.Methods
We carried out comprehensive sonic exploration in 2022-2024. We also carried out CTD measurement, and lake water quality survey including hydrogen and oxygen isotope ratio measurements at three depths (5m, 50m and bottom(90-100m) at Y1(35°20.22-20.25'N, 136°6.09-6.13'E) and T1 (35°22.19'N, 136°05.83'N ) in 2021-2024. Y1 is the point where the gas acoustic anomalies are usually detected. T1 is the reference point, which is a periodical observation point of the University of the Shiga Prefecture. The water depth of Y1 and T1 is 90-100 m and 90 m, respectively. We carried out long-term temperature monitoring in lake bottom sediment at Y1 from October 2022 to March 2023 and from October 2023 to July 2024. We also carried out carbon and hydrogen isotope ratio measurements of the methane in the gas sampled by overwater displacement at the surface of the lake at Y1.
3.Result and Discussion
The gas acoustic anomalies were widely distributed and not lined up as reported by Kumagai et al. (2021). The results of the CTD survey measurement and the water quality survey were generally consistent between Y1 and T1, but they were sometimes a little different between Y1 and T1 at the bottom of the lake. The methane concentration in the sampled gas was about 10-60%, which is much higher than that in the air. The methane was found to be of organic origin in the lake bottom sediments from the isotopic analysis. During the observation periods from October 2022 to March 2023 and from October 2023 to July 2024, the temperature gradient at Y1 was estimated to be about 150 mK/m (temperature gradient expected for a normal Lake Biwa bottom: about 50-70 mK/m). This high temperature gradient can be explained by heat supply from the groundwater gushing from the lake bottom as the spring water. The heat supply possibly warms the lake sediments under the lake bottom, activates methanogenic bacteria and produce the methane.