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[SCG59-02] Current status of the spring water from the deep bottom in Lake Biwa and its effect on the lake bottom environment
Keywords:Lake Biwa, Spring water from the lake bottom, Environment
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(Shiga Prefec., 2018). 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 (methan>99%) from the deep lake bottom in Lake Biwa in 2009 using AUV, sonic exploration, and temperature gradient measurements of the lake bottom (Fig.1). According to Kumagai et al.(2021), the benthic vents, which are outlets of the spring water and gas, lined up on a line about 10km north-south and the underwater acoustic anomalies caused by gas from the vents ( hereafter referred as the gas acoustic anomalies) also lined up along the vents. The vents and the gas acoustic anomalies existed where the lacustrin sediment is less than 500m. 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 and evaluate the effect of it on the environment. It is supported by the Grant-in-Aid for Scientific Research "20H01974: Elucidation of the relationship between the underground structure and deep bottom spring in Lake Biwa and evaluation of its effect on the lake bottom environment".
2.Methods
We carried out comprehensive sonic exploration, CTD measurement, and lake water quality survey including hydrogen and oxygen isotope ratio measurements at three depths (5m, 50m and bottom) 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 and 2022. Y1 is the point where the gas acoustic anomalies is 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 also carried out temperature gradient measurements of the lake bottom around Y1 and 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 detected at 36 points, and were more widely distributed in a north-south belt (Fig.1), than the linear distribution of them reported by Kumagai et al. (2021). 26 of the 36 points were where the lacustrine sediment thickness (basement depth) was less than 500m, while the remaining 10 points were where it was greater than 500m. These indicate that the location of gas eruptions at the bottom of the lake is influenced by the subsurface structure at a depth of around 500 m.
The results of the CTD survey measurement and the water quality survey were generally consistent between Y1 and T1, but they were sometimes different between Y1 and T1 at the bottom of the lake. The hydrogen and oxygen isotope ratios of the lake water sampled by a usual water sampler showed the lake water at T1 and Y1 was from precipitation. However those of the supernatant water from the sediment sampling at Y1 on December 2, 2021 revealed that it fell outside the range of normal precipitation sources. These indicate that the location of gas eruptions at the bottom of the lake is influenced by the subsurface structure at a depth of around 500 m. They also indicate that the amount of deep lake bottom spring water or the water quality of it is changing with time, and that it may not be derived from precipitation.
The methane concentration in the sampled gas was about 30-60%, and the methane was found to be of organic origin derived from the lake bottom sediments from the isotope ratio of carbon and hydrogen in the methane. This result indicates that the origin of methane is shallower than that of the deep lake spring water.
It is said that around 10-20% of the water flowing into Lake Biwa is spring water from lake bottom in Lake Biwa(Shiga Prefec., 2018). 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 (methan>99%) from the deep lake bottom in Lake Biwa in 2009 using AUV, sonic exploration, and temperature gradient measurements of the lake bottom (Fig.1). According to Kumagai et al.(2021), the benthic vents, which are outlets of the spring water and gas, lined up on a line about 10km north-south and the underwater acoustic anomalies caused by gas from the vents ( hereafter referred as the gas acoustic anomalies) also lined up along the vents. The vents and the gas acoustic anomalies existed where the lacustrin sediment is less than 500m. 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 and evaluate the effect of it on the environment. It is supported by the Grant-in-Aid for Scientific Research "20H01974: Elucidation of the relationship between the underground structure and deep bottom spring in Lake Biwa and evaluation of its effect on the lake bottom environment".
2.Methods
We carried out comprehensive sonic exploration, CTD measurement, and lake water quality survey including hydrogen and oxygen isotope ratio measurements at three depths (5m, 50m and bottom) 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 and 2022. Y1 is the point where the gas acoustic anomalies is 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 also carried out temperature gradient measurements of the lake bottom around Y1 and 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 detected at 36 points, and were more widely distributed in a north-south belt (Fig.1), than the linear distribution of them reported by Kumagai et al. (2021). 26 of the 36 points were where the lacustrine sediment thickness (basement depth) was less than 500m, while the remaining 10 points were where it was greater than 500m. These indicate that the location of gas eruptions at the bottom of the lake is influenced by the subsurface structure at a depth of around 500 m.
The results of the CTD survey measurement and the water quality survey were generally consistent between Y1 and T1, but they were sometimes different between Y1 and T1 at the bottom of the lake. The hydrogen and oxygen isotope ratios of the lake water sampled by a usual water sampler showed the lake water at T1 and Y1 was from precipitation. However those of the supernatant water from the sediment sampling at Y1 on December 2, 2021 revealed that it fell outside the range of normal precipitation sources. These indicate that the location of gas eruptions at the bottom of the lake is influenced by the subsurface structure at a depth of around 500 m. They also indicate that the amount of deep lake bottom spring water or the water quality of it is changing with time, and that it may not be derived from precipitation.
The methane concentration in the sampled gas was about 30-60%, and the methane was found to be of organic origin derived from the lake bottom sediments from the isotope ratio of carbon and hydrogen in the methane. This result indicates that the origin of methane is shallower than that of the deep lake spring water.