2:15 PM - 2:30 PM
[HTT18-03] Understanding the water characteristics of groundwater and estimation of groundwater recharge area using a multi-tracer in Miyagino-ku, Sendai City
Keywords:coastal area of Sendai City, groundwater, water quality, 87Sr/86Sr, multi-tracer, groundwater flow
Objectives
The tsunami, caused by the 2011 off the Pacific coast of Tohoku Earthquake, flooded large area near the coastal area from Kanto district to Tohoku district, and water salinization occurred in many wells at those areas. Because the water salinization is influenced not only water use of human but also ecosystem, it is necessary to understand the current situation of water salinization. And also, it is important to determine the water quality and groundwater flow system for sustainable groundwater use. The objectives of this study are to understand the characteristics of water quality, groundwater recharge area, and water residence time and accumulate these data near the coastal area of Sendai City.
Methodology
The observation have occurred every month since December 2018 in Shinhama area (Nos. 1, 3, 4, 5, 6, 7, 8), where located near the coast in Miyagino-ku, Sendai City, and monthly observation have also started since February 2020 for three observation wells (F-1, F-2, F-3). The site of No.1 is channel, No.6 is shallow well (shallow groundwater), Nos. 3, 4, 5, 7, and 8 are deep well (deep groundwater). The well depth of No.6 is 3.1 m, Nos. 3, 4, 5, 7, and 8 are approximately 30 m. In terms of the observation well, well depth of F-1 is 29 m (well strainer was settled from 26 to 30 m), well depth of F-2 is 15 m (well strainer was settled from 7 to 15 m), and well depth of F-3 is 10 m (well strainer is settled from 2 to 10 m).
The EC, pH, water temperature, ORP, and water level were measured in the field, and water for chemical analysis were sampled at each site. The dissolved inorganic ions, trace elements, δ18O, δ2H were analyzed for all samples, and Sr isotope ratio (87Sr/86Sr), concentrations of CFCs and SF6 for estimating of water residence time were measured in some sites.
Results and discussion
Water quality of channel (No.1) was Na-Cl, and dissolved concentration was very high. This channel water was might be mixed with sea water, river water, rain water, and shallow groundwater. Water quality of shallow groundwater (No.6) was (Na+Ca)-HCO3 and dissolved concentration was relatively low. Water quality of deep groundwater (Nos.4, 5, 7, and 8) was Na-HCO3; however, of the well No3, which was located near the coast, was slightly different and Na-(Cl+HCO3). The dissolved concentration and stable isotope ratios of shallow groundwater (No.6) varied, so the shallow groundwater might be affected by the precipitation event. In the case of observation wells, water quality of F-1 was Na-HCO3 and similar to deep groundwater (Nos.4, 5, 7, and 8), F-2 was Na-Cl with Ca2+ and Mg2+, and F-3 was Na-Cl. The dissolved concentration of F-2 and F-3 was very high and water quality was similar to the sea water; therefore, sea water was likely mixed into the groundwater. These estimation result was agree with the result of δ18O, δ2H, and trace element.
87Sr/86Sr value of deep groundwater (Nos. 3, 4, 5, 7, and 8), F-1, and shallow groundwater (No.6) were relatively low (from 0.7073 to 0.7076); however, that of F-2 and F-3 were relatively high (approximately 0.7089). Because the 87Sr/86Sr value of sea water is 0.70918, this result also suggested that the F-2 and F-3 was affected by the mixing of sea water.
In addition, the residence time of deep groundwater (Nos. 3, 4, 5, 7, and 8) and F-1 estimated from the CFCs and SF6 was approximately 60–70 year.
In the future study, we will discuss and recognize the formation process of water quality, and groundwater flow system by using the multi-tracer.
The tsunami, caused by the 2011 off the Pacific coast of Tohoku Earthquake, flooded large area near the coastal area from Kanto district to Tohoku district, and water salinization occurred in many wells at those areas. Because the water salinization is influenced not only water use of human but also ecosystem, it is necessary to understand the current situation of water salinization. And also, it is important to determine the water quality and groundwater flow system for sustainable groundwater use. The objectives of this study are to understand the characteristics of water quality, groundwater recharge area, and water residence time and accumulate these data near the coastal area of Sendai City.
Methodology
The observation have occurred every month since December 2018 in Shinhama area (Nos. 1, 3, 4, 5, 6, 7, 8), where located near the coast in Miyagino-ku, Sendai City, and monthly observation have also started since February 2020 for three observation wells (F-1, F-2, F-3). The site of No.1 is channel, No.6 is shallow well (shallow groundwater), Nos. 3, 4, 5, 7, and 8 are deep well (deep groundwater). The well depth of No.6 is 3.1 m, Nos. 3, 4, 5, 7, and 8 are approximately 30 m. In terms of the observation well, well depth of F-1 is 29 m (well strainer was settled from 26 to 30 m), well depth of F-2 is 15 m (well strainer was settled from 7 to 15 m), and well depth of F-3 is 10 m (well strainer is settled from 2 to 10 m).
The EC, pH, water temperature, ORP, and water level were measured in the field, and water for chemical analysis were sampled at each site. The dissolved inorganic ions, trace elements, δ18O, δ2H were analyzed for all samples, and Sr isotope ratio (87Sr/86Sr), concentrations of CFCs and SF6 for estimating of water residence time were measured in some sites.
Results and discussion
Water quality of channel (No.1) was Na-Cl, and dissolved concentration was very high. This channel water was might be mixed with sea water, river water, rain water, and shallow groundwater. Water quality of shallow groundwater (No.6) was (Na+Ca)-HCO3 and dissolved concentration was relatively low. Water quality of deep groundwater (Nos.4, 5, 7, and 8) was Na-HCO3; however, of the well No3, which was located near the coast, was slightly different and Na-(Cl+HCO3). The dissolved concentration and stable isotope ratios of shallow groundwater (No.6) varied, so the shallow groundwater might be affected by the precipitation event. In the case of observation wells, water quality of F-1 was Na-HCO3 and similar to deep groundwater (Nos.4, 5, 7, and 8), F-2 was Na-Cl with Ca2+ and Mg2+, and F-3 was Na-Cl. The dissolved concentration of F-2 and F-3 was very high and water quality was similar to the sea water; therefore, sea water was likely mixed into the groundwater. These estimation result was agree with the result of δ18O, δ2H, and trace element.
87Sr/86Sr value of deep groundwater (Nos. 3, 4, 5, 7, and 8), F-1, and shallow groundwater (No.6) were relatively low (from 0.7073 to 0.7076); however, that of F-2 and F-3 were relatively high (approximately 0.7089). Because the 87Sr/86Sr value of sea water is 0.70918, this result also suggested that the F-2 and F-3 was affected by the mixing of sea water.
In addition, the residence time of deep groundwater (Nos. 3, 4, 5, 7, and 8) and F-1 estimated from the CFCs and SF6 was approximately 60–70 year.
In the future study, we will discuss and recognize the formation process of water quality, and groundwater flow system by using the multi-tracer.