5:15 PM - 7:15 PM
[AHW30-P03] Elucidation of the groundwater flow system in the Kumagawa River basin using multi-isotope tracers
In Japan, torrential rain frequently occurs due to the generation of linear precipitation zones by the stagnation of the baiu front. As a result, floods have caused great damage in some areas. One of the areas is the Kumagawa River basin - Hitoyoshi Basin in Kumamoto Prefecture. Due to the stagnant baiu front from July 3, 2020, to 31st, heavy rain continued over a wide area from western to eastern Japan, and 65 people in Kumamoto Prefecture, including 50 people in the Kumagawa River basin, were killed by floods, landslides, and other disasters. Therefore, more advanced monitoring of water circulation system is necessary for disaster prevention, and visualization of groundwater flow system can be an important factor. It is also useful for evaluating the amount of water available for agricultural and industrial use.
While the ion concentrations of groundwater are considered to fluctuate greatly while it migrates from the recharge area of precipitation to the discharge area, there are some tracers that keep the information of precipitation. Therefore, we’ve been trying to clarify the flow system in the Kumagawa River basin by appropriately combining these tracers.
Water sampling was carried out in the Kumagawa River basin in the Hitoyoshi basin to elucidate the groundwater flow system mainly by following three methods. The first is "Estimation of recharge area by stable isotope ratio measurement of oxygen and hydrogen in water," the second is "Estimation of residence time by 3 H-3He dating method," and the third is "Discrimination of flow system by main dissolved ion analysis."
The first method, "Estimation of recharge area by stable isotope ratio measurement of oxygen and hydrogen in water," was carried out by measuring the isotope ratio of oxygen and hydrogen in water samples from spring water, wells, and rivers. Since these isotope ratios vary with elevation, it is possible to estimate the recharge area of sampled groundwater. This is based on the fact that light water molecules evaporate relatively easily while heavy water molecules preferentially condense. If precipitation occurs from the same vapor through continuous cooling and condensation processes, the heavy isotopes are gradually depleted in the vapor.
Regarding the second method, "Estimation of the residence time by the 3H-3He dating method," 4He concentration is considered to be constant during groundwater migration if there is no influence from the earth's crust or mantle. In contrast, 3He concentration in the groundwater is considered to increase due to the decay of 3H (half-life of 12.3 years). First, the helium isotope ratio (3He/4He) and 4He concentration of spring water and well water were measured. Next, after the gaseous components were completely removed, the sample water was stored under vacuum, waiting for the 3H in the sample to decay to 3He. Several months later, a similar analysis was performed on the water sample to determine 3He, thereby obtaining the 3H contained at the time of sampling. From these results, the residence time and the initial concentration of 3H are determined.
Regarding the third method, "Discrimination of flow system by main dissolved ion analysis," the concentrations of dissolved Na+, K+, Mg2+, Ca2+, Cl-, HCO3-, CO32-, NO3-, and SO42- in sampled water are measured since groundwater from the same flow system is generally considered to share similar ion concentrations at each discharge site.
Based on the preliminary results, some groundwater samples showed 3He/4He ratios, which are too high to be explained by the addition of 3He of tritium origin, suggesting a contribution of mantle-derived He. In addition, the concentrations of ions in the groundwater were different from that in the nearby river water, which suggests that it has flowed deep underground. The above three methods are continuously carried out to clarify the groundwater flow system of the Kumagawa River basin in the Hitoyoshi basin.
While the ion concentrations of groundwater are considered to fluctuate greatly while it migrates from the recharge area of precipitation to the discharge area, there are some tracers that keep the information of precipitation. Therefore, we’ve been trying to clarify the flow system in the Kumagawa River basin by appropriately combining these tracers.
Water sampling was carried out in the Kumagawa River basin in the Hitoyoshi basin to elucidate the groundwater flow system mainly by following three methods. The first is "Estimation of recharge area by stable isotope ratio measurement of oxygen and hydrogen in water," the second is "Estimation of residence time by 3 H-3He dating method," and the third is "Discrimination of flow system by main dissolved ion analysis."
The first method, "Estimation of recharge area by stable isotope ratio measurement of oxygen and hydrogen in water," was carried out by measuring the isotope ratio of oxygen and hydrogen in water samples from spring water, wells, and rivers. Since these isotope ratios vary with elevation, it is possible to estimate the recharge area of sampled groundwater. This is based on the fact that light water molecules evaporate relatively easily while heavy water molecules preferentially condense. If precipitation occurs from the same vapor through continuous cooling and condensation processes, the heavy isotopes are gradually depleted in the vapor.
Regarding the second method, "Estimation of the residence time by the 3H-3He dating method," 4He concentration is considered to be constant during groundwater migration if there is no influence from the earth's crust or mantle. In contrast, 3He concentration in the groundwater is considered to increase due to the decay of 3H (half-life of 12.3 years). First, the helium isotope ratio (3He/4He) and 4He concentration of spring water and well water were measured. Next, after the gaseous components were completely removed, the sample water was stored under vacuum, waiting for the 3H in the sample to decay to 3He. Several months later, a similar analysis was performed on the water sample to determine 3He, thereby obtaining the 3H contained at the time of sampling. From these results, the residence time and the initial concentration of 3H are determined.
Regarding the third method, "Discrimination of flow system by main dissolved ion analysis," the concentrations of dissolved Na+, K+, Mg2+, Ca2+, Cl-, HCO3-, CO32-, NO3-, and SO42- in sampled water are measured since groundwater from the same flow system is generally considered to share similar ion concentrations at each discharge site.
Based on the preliminary results, some groundwater samples showed 3He/4He ratios, which are too high to be explained by the addition of 3He of tritium origin, suggesting a contribution of mantle-derived He. In addition, the concentrations of ions in the groundwater were different from that in the nearby river water, which suggests that it has flowed deep underground. The above three methods are continuously carried out to clarify the groundwater flow system of the Kumagawa River basin in the Hitoyoshi basin.