17:15 〜 18:30
[AHW20-P06] Geochemical signature of trace elements in lake water at Lake Tazawa
キーワード：田沢湖、水質、微量元素、希釈、沈殿
Lake Tazawa is located at the eastern part of Akita Prefecture in Japan. The lake has the maximum depth of 423.4 m and is known as the deepest lake in Japan. Various kinds of fishes including endemic species had lived in the lake. However, an acidic river water of Tama River has been introduced to the lake for electricity and agriculture developments. As a result, numerous kinds of fishes and endemic species were died out because the lake water quality changed to acidic dramatically. The pH of the lake water decreased from 6.7 in 1920s to 4.6 in 1970s (Kato, 1989). Since 1991, a neutralization treatment of the acidic river water was started and the neutralized river water has been introduced to the lake. The pH of the lake surface water was 5.6 in August, 2020 (this study).
To investigate how the lake water quality be able to improve, understanding of details of the lake water quality and mechanism of lake water circulation are essential. Previous studies have pointed out that the lake water quality is homogeneous from shallow to deep parts and the circulation of the lake water occurs in a limited period (Boehrer et al., 2008; Ishiyama et al., 2013). In addition, Chikita et al. (2019) suggested that there are turnovers of the lake water in cold seasons around March. This study aims to clarify the geochemical signature of the lake water in Lake Tazawa from June to November.
The samplings of the lake water were carried out from June to October in 2019 and from August to November in 2020. The lake water samples were collected from the depths of 400 m to the lake surface at five locations in the lake (P1 to P5 in the map). In addition, the river waters of Tama Rriver (A) and Sendatsu Rriver (B) that inflow into the lake and an outflow of the lake water to Tama River (C) were also collected. Water temperature, pH, Eh, EC, major dissolved ions and trace elements were measured. Trace elements were analyzed separately in dissolved and particulate fractions.
Concentrations of Na, Cl and Al in the lake water increase from the lake surface to the depth of 400 m in August, 2019 (Na: 4.7 ppm (0 m), 4.9 ppm (50 m), 5.1 ppm (400 m); Cl: 17.3 ppm (0 m), 18.8 ppm (50 m), 19.3 ppm (400 m); Total Al: 269 ppb (0 m), 437 ppb (50 m), 522 ppb (400 m)). On the other hand, the concentration of total Fe (T-Fe) in the lake water decreases from 14.5 ppb (0 m) to 8.5 ppb (100 m), then gradually increases to 25.8 ppb (400 m). The ratio of the amount of Al particulate fraction in the total amount of Al is 0.04. While, the ratio of the amount of Fe particulate fraction in the total amount of Fe is 0.42. Therefore, approximately half of Fe is present as particulate fraction in the lake water. There is a possibility that the Fe particulate fraction scavenges other metals by adsorption. Boron-normalized values (NB), which defined as NB=(M/B)X(total) / (M/B)Base point(total), where (M/B)X(total) and (M/B)Base point(total) denote the ratio of total mobile amounts (the sum of dissolved and particulate fractions) of the target elements M and B at the certain point (X) and the base point, respectively. The base point is acidic river water of Tama River at the Kawasaki bridge immediately before inflow of the river water to Lake Tazawa (Point A in the map). The NB of Al of the surface water is 0.6 and the value increases around 0.7 toward the depth of 400 m. While, the NB of Fe of the surface water is around 0.5 and the value decrease to around 0.2 at the depth of 50 m. Then, the value increases to 0.5 towards at the depth of 400 m. These facts that the surface water has low NB of Al and Fe between 0.5 and 0.6 suggests that there is a process of dilution of acidic river water of Tama River by the lake water having low Al/B and Fe/B ratios in the lake. In addition, NB of Al and Fe in the lake water from the depth of 50 m to 400 m range from 0.2 to 0.7. This fact suggests that Al and Fe in the lake water from the depth of 50 m to 400 m are precipitated on the lake bed. According to the profiles of chemical compositions of lake sediments (Ishiguro et al., 2017), the concentrations of Al and Fe in the sediments of Lake Tazawa increase from prior to posterior to the introduction of the acidic river water of Tama River. The estimation on Al and Fe based on the NB is consistent with variations of Al and Fe contents in the sediments of the lake.
To investigate how the lake water quality be able to improve, understanding of details of the lake water quality and mechanism of lake water circulation are essential. Previous studies have pointed out that the lake water quality is homogeneous from shallow to deep parts and the circulation of the lake water occurs in a limited period (Boehrer et al., 2008; Ishiyama et al., 2013). In addition, Chikita et al. (2019) suggested that there are turnovers of the lake water in cold seasons around March. This study aims to clarify the geochemical signature of the lake water in Lake Tazawa from June to November.
The samplings of the lake water were carried out from June to October in 2019 and from August to November in 2020. The lake water samples were collected from the depths of 400 m to the lake surface at five locations in the lake (P1 to P5 in the map). In addition, the river waters of Tama Rriver (A) and Sendatsu Rriver (B) that inflow into the lake and an outflow of the lake water to Tama River (C) were also collected. Water temperature, pH, Eh, EC, major dissolved ions and trace elements were measured. Trace elements were analyzed separately in dissolved and particulate fractions.
Concentrations of Na, Cl and Al in the lake water increase from the lake surface to the depth of 400 m in August, 2019 (Na: 4.7 ppm (0 m), 4.9 ppm (50 m), 5.1 ppm (400 m); Cl: 17.3 ppm (0 m), 18.8 ppm (50 m), 19.3 ppm (400 m); Total Al: 269 ppb (0 m), 437 ppb (50 m), 522 ppb (400 m)). On the other hand, the concentration of total Fe (T-Fe) in the lake water decreases from 14.5 ppb (0 m) to 8.5 ppb (100 m), then gradually increases to 25.8 ppb (400 m). The ratio of the amount of Al particulate fraction in the total amount of Al is 0.04. While, the ratio of the amount of Fe particulate fraction in the total amount of Fe is 0.42. Therefore, approximately half of Fe is present as particulate fraction in the lake water. There is a possibility that the Fe particulate fraction scavenges other metals by adsorption. Boron-normalized values (NB), which defined as NB=(M/B)X(total) / (M/B)Base point(total), where (M/B)X(total) and (M/B)Base point(total) denote the ratio of total mobile amounts (the sum of dissolved and particulate fractions) of the target elements M and B at the certain point (X) and the base point, respectively. The base point is acidic river water of Tama River at the Kawasaki bridge immediately before inflow of the river water to Lake Tazawa (Point A in the map). The NB of Al of the surface water is 0.6 and the value increases around 0.7 toward the depth of 400 m. While, the NB of Fe of the surface water is around 0.5 and the value decrease to around 0.2 at the depth of 50 m. Then, the value increases to 0.5 towards at the depth of 400 m. These facts that the surface water has low NB of Al and Fe between 0.5 and 0.6 suggests that there is a process of dilution of acidic river water of Tama River by the lake water having low Al/B and Fe/B ratios in the lake. In addition, NB of Al and Fe in the lake water from the depth of 50 m to 400 m range from 0.2 to 0.7. This fact suggests that Al and Fe in the lake water from the depth of 50 m to 400 m are precipitated on the lake bed. According to the profiles of chemical compositions of lake sediments (Ishiguro et al., 2017), the concentrations of Al and Fe in the sediments of Lake Tazawa increase from prior to posterior to the introduction of the acidic river water of Tama River. The estimation on Al and Fe based on the NB is consistent with variations of Al and Fe contents in the sediments of the lake.