Abstract
The average concentrations of 226Ra and 228Ra in the water in hot springs in and around metropolitan Tokyo were 39 mBq L-1 and 55 mBq L-1 for NaCl-type water and 0.6 mBq L-1 and 0.8 mBq L-1 for simple alkaline-type water, respectively. From the chemical characteristics, the hot spring waters from the Kazusa and Miocene formations are classified as NaCl and that from the Shimanto and Chichibu formations is simple alkaline. This indicates that the concentrations of Ra isotopes are high for the NaCl type and low for the simple alkaline type and are correlated with the relevant sedimentation layer from which the hot spring water is welled up. Good correlation was observed for the ratios in the concentrations of Ra/Na and Ra/Cl in both types of hot spring water. The correlation patterns of Ra/Na and Ra/Cl for the simple alkaline type and sedimentary rock were different from those for the NaCl type and seawater, suggesting that the origin of the Ra isotopes may be different. The 228Ra/226Ra activity ratio was found to be related with Si concentration, showing that the activity ratio decreased with an increase of H2SiO3 for both the NaCl and the simple alkaline type. Assuming that the Ra isotopes in hot spring water were supplied from the relevant rock in the aquifer together with Si, the activity ratio of 228Ra/226Ra in hot spring water might be different between the NaCl and simple alkaline type, suggesting that the (228Ra/226Ra) t=0 activity ratio for residence time at t = 0 might be different for the two types.
