5:15 PM - 6:45 PM
[AHW24-P01] Mixing of highly saline deep-seated water in streams and groundwater in Yashio, Gunma and Wataruse, Saitama, Japan
Keywords:stream and shallow groundwater, highly saline deep-seated water, mixing, chloride ion, dissolved load, mineral spring
Cold mineral springs with high Cl- concentrations are found in the Sambagawa Belt in the Yashio district of Fujioka City, Gunma Prefecture and they affect the water quality of streams flowing eastward through the district (Takase, 1967). According to Takase (1967), the water quality of these mineral springs is NaCl-type (Cl- concentration: 10,430-12,560 mg/L), and Matsubaya et al. (1985) attribute the origin of the water to fossil seawater incorporated into the Neogene sedimentary rocks. On the other hand, shallow groundwater from the wells in river terrace deposits overlying the Sambagawa metamorphic rocks in the Wataruse district of Kamikawa Town, Saitama Prefecture, is also characterized by high Cl- concentrations, with the highest concentration of 1,720 mg/L (Takase, 1967). The locations and fluxes of the Cl--enriched ‘deep-seated fluid’ rising from deep underground and mixing with surface water and shallow groundwater systems, which are often observed in the Sambagawa Belt, are not yet known, and more information is needed to make a better understanding of the phenomena.
In this study, we measured Cl- concentrations and calculated Cl- loads (Cl- concentration multiplied by stream flow rate) at the downstream ends of the Minamisawa River and nine other streams, which are all about 1 km long and flow eastward through the Yashio district, eventually joining the Kanna River. The same study was also conducted in four streams flowing westward into the Kanna River in the Wataruse district. The Cl- flux from deep-seated fluids to the shallow groundwater system in the Wataruse district was estimated by hydraulic calculations based on the Cl-concentration contour map of shallow groundwater, water table map (hydraulic gradient), and hydraulic conductivity obtained from pumping tests. Field investigations were conducted in July-August 2018 (wet summer season) and December 2018-February 2019 (dry winter season).
The Cl- concentration in the Yashio mineral springs in August 2018 ranged from 4,798 mg/L to 11,892 mg/L, and that in December 2018 ranged from 5,162 mg/L to 11,032 mg/L. The hydrogen isotopic ratio (δD) of the Minamisawa River in December was -54.5‰ at the headwaters and -46.7‰ at the downstream end, an increase of about 8‰ within 1 km; the oxygen isotope ratio (δ18O) increased from -8.0‰ at the headwaters to -7.3‰ at the downstream end, a 0.7‰ increase. This is thought to be due to isotopically heavy deep-seated fluids (δD = -29.8 to -10.8 ‰ and δ18O = -1.4 to 4.3 ‰) mixing into the stream water through stream beds.
The Cl- concentration in the downstream end of the Minamisawa River was 632.5 mg/L in July 2018 and 213.7 mg/L in December 2018. The Cl- load in the Minamisawa River just before the confluence with the Kanna River was approximately 44 kg/day and 45 kg/day in July and December, respectively, and no seasonal changes in the Cl- load were observed. The overall Cl- load of the streams in the Yashio district, including the Minamisawa River, before the confluence with the Kanna River was about 50 kg/day in December, indicating that the Minamisawa River is responsible for 90% of the total stream Cl- load during the dry winter season. On the other hand, the Cl- load of streams in the Wataruse district during the dry winter season was estimated to be 0.2 kg/day, and the Cl- load mixed into the shallow groundwater system was 76 kg/day. These results indicate that there is a Cl- flux of about 130 kg/day from the deep underground in the Yashio and Wataruse districts during the winter season. Assuming the Cl-concentration of deep-seated fluids to be 12,000 mg/L, the Yashio and Wataruse districts receive about 11 m3/day of highly saline water from deep underground in winter.
In this study, we measured Cl- concentrations and calculated Cl- loads (Cl- concentration multiplied by stream flow rate) at the downstream ends of the Minamisawa River and nine other streams, which are all about 1 km long and flow eastward through the Yashio district, eventually joining the Kanna River. The same study was also conducted in four streams flowing westward into the Kanna River in the Wataruse district. The Cl- flux from deep-seated fluids to the shallow groundwater system in the Wataruse district was estimated by hydraulic calculations based on the Cl-concentration contour map of shallow groundwater, water table map (hydraulic gradient), and hydraulic conductivity obtained from pumping tests. Field investigations were conducted in July-August 2018 (wet summer season) and December 2018-February 2019 (dry winter season).
The Cl- concentration in the Yashio mineral springs in August 2018 ranged from 4,798 mg/L to 11,892 mg/L, and that in December 2018 ranged from 5,162 mg/L to 11,032 mg/L. The hydrogen isotopic ratio (δD) of the Minamisawa River in December was -54.5‰ at the headwaters and -46.7‰ at the downstream end, an increase of about 8‰ within 1 km; the oxygen isotope ratio (δ18O) increased from -8.0‰ at the headwaters to -7.3‰ at the downstream end, a 0.7‰ increase. This is thought to be due to isotopically heavy deep-seated fluids (δD = -29.8 to -10.8 ‰ and δ18O = -1.4 to 4.3 ‰) mixing into the stream water through stream beds.
The Cl- concentration in the downstream end of the Minamisawa River was 632.5 mg/L in July 2018 and 213.7 mg/L in December 2018. The Cl- load in the Minamisawa River just before the confluence with the Kanna River was approximately 44 kg/day and 45 kg/day in July and December, respectively, and no seasonal changes in the Cl- load were observed. The overall Cl- load of the streams in the Yashio district, including the Minamisawa River, before the confluence with the Kanna River was about 50 kg/day in December, indicating that the Minamisawa River is responsible for 90% of the total stream Cl- load during the dry winter season. On the other hand, the Cl- load of streams in the Wataruse district during the dry winter season was estimated to be 0.2 kg/day, and the Cl- load mixed into the shallow groundwater system was 76 kg/day. These results indicate that there is a Cl- flux of about 130 kg/day from the deep underground in the Yashio and Wataruse districts during the winter season. Assuming the Cl-concentration of deep-seated fluids to be 12,000 mg/L, the Yashio and Wataruse districts receive about 11 m3/day of highly saline water from deep underground in winter.