11:00 〜 11:15
[HCG21-07] Evaluation of deep fluid injection to shallower groundwater by Li-B isotope systematics
キーワード：地下水、深部流体、リチウム-ホウ素同位体系
Lithium (Li) and boron (B) are regarded to be a useful indicator to detect natural contaminations to shallow groundwater aquifer because seawater and hydrothermal fluids contain high Li and B concentrations compared to those in the fresh groundwater. The conservative behavior of Li and B in groundwater regardless of the redox conditions in aquifer systems is also an important chemical signature to use it as a tracer to estimate their origins (Bassett et al., 1995). Application of Cl-B-Li ternary diagram was suggested first to detect the influence of geothermal fluids contributing into meteoric-originated groundwater systems (Giggenbach, 1991). This ternary diagram was further developed to distinguish the origin of mixed-waters with high solute concentrations into natural groundwaters in Japanese island arc among seawater, fresh and stagnant groundwater, and hydrothermal fluids (Amita et al., 2014).
In addition to these concentrations, their stable isotope ratios (7Li/6Li and 11B/10B) are useful for their source determination. These isotope ratios show wide natural variation owing to their relatively large mass differences like 13C/12C and 18O/16O, for instance, the highest values in seawater, and lower values in volcanic rocks (Hoefs, 2009). Recent analytical developments in mass spectrometry especially by the introduction of ICP ionization source allow their precise detection with a simple column separation technique to isolate each element (Tomascak, 2004; Foster et al, 2018). These geochemical tools will be applicable to studies investigating changes in groundwater chemistry.
In this presentation, Li-B isotope systematics are applied to two groundwater systems to detect the contribution of deep-seated fluid to shallower groundwater. One is to detect the supply of deep fluid to shallow groundwater in Eastern Kii Peninsula, Mie Prefecture. Quite low delta-7Li values were observed in this area along Median Tectonic Line and areas within 20 km southern part of MTL. Their quite low delta-7Li values indicate a small contribution of slab-dehydrated fluid from the Philippine Sea Plate to this forearc region without production of active volcanoes.
The other example is a duration of a shallow groundwater disturbance caused by large shallow inland earthquakes, Mw 7.0 and Mw 6.2 Kumamoto earthquakes in 2016. The change in water quality is characterized by the injection of hydrothermal fluid using Cl-B-Li ternary diagram and Li-B isotope systematics along the Futagawa-Fault, and this disturbance was recovered to the original condition within one year after the earthquake. It is demonstrated that these concentrations and the Li-B isotope systematics are useful to detect the contribution of the deep fluids including hydrothermal/slab-dehydrated fluids to shallower groundwater.
In addition to these concentrations, their stable isotope ratios (7Li/6Li and 11B/10B) are useful for their source determination. These isotope ratios show wide natural variation owing to their relatively large mass differences like 13C/12C and 18O/16O, for instance, the highest values in seawater, and lower values in volcanic rocks (Hoefs, 2009). Recent analytical developments in mass spectrometry especially by the introduction of ICP ionization source allow their precise detection with a simple column separation technique to isolate each element (Tomascak, 2004; Foster et al, 2018). These geochemical tools will be applicable to studies investigating changes in groundwater chemistry.
In this presentation, Li-B isotope systematics are applied to two groundwater systems to detect the contribution of deep-seated fluid to shallower groundwater. One is to detect the supply of deep fluid to shallow groundwater in Eastern Kii Peninsula, Mie Prefecture. Quite low delta-7Li values were observed in this area along Median Tectonic Line and areas within 20 km southern part of MTL. Their quite low delta-7Li values indicate a small contribution of slab-dehydrated fluid from the Philippine Sea Plate to this forearc region without production of active volcanoes.
The other example is a duration of a shallow groundwater disturbance caused by large shallow inland earthquakes, Mw 7.0 and Mw 6.2 Kumamoto earthquakes in 2016. The change in water quality is characterized by the injection of hydrothermal fluid using Cl-B-Li ternary diagram and Li-B isotope systematics along the Futagawa-Fault, and this disturbance was recovered to the original condition within one year after the earthquake. It is demonstrated that these concentrations and the Li-B isotope systematics are useful to detect the contribution of the deep fluids including hydrothermal/slab-dehydrated fluids to shallower groundwater.