Kotaro Nakata

An Evaluation of the Long-Term Stagnancy of Porewater in the Neogene Sedimentary Rocks in Northern Japan

A groundwater dating for very old porewater using 36Cl and 4He was applied to the Koetoi and Wakkanai formations distributed in the northernmost part in Japan. Measured 36Cl/Cl in the Koetoi Formation was 2.6 ± 2.0 × 10−15 and that in the Wakkanai Formation was 8.1 ± 2.5 × 10−15. These values are similar to 36Cl/Cl in situ secular equilibrium calculated from chemical compositions of core suggesting that Cl− ions and porewater have remained in the formations for much longer than half-life of 36Cl . He concentration in porewater ranged from 1.1 × 10−6 to 2.6 × 10−5 ( ) and it is much higher than water saturated with air indicating that both formations contain very old porewater. However, the possibility of mixing of young water was indicated because He concentration was lower than that calculated by multiplication of in situ He production and time after the uplift. This possibility was also supported by Cl−, δD, and δ18O data. After combining information on 36Cl/Cl, 4He, and δD and δ18O, it was inferred that the porewater in the deep part of the Wakkanai Formation might have been stagnant since the uplift. The porewater in the Koetoi Formation and the shallow part of the Wakkanai Formation were found to be affected by young surface water.

Comparison of 14 C Collected by Precipitation and Gas-Strip Methods for Dating Groundwater

Dissolved inorganic carbon (DIC) in groundwater is used to estimate the residence time based on radiocarbon concentration. DIC is usually extracted by a gas-strip or precipitation (SrCO 3 or BaCO 3 ) method. In this study, the gas-strip and precipitation methods of DIC were applied to both artificially prepared NaHCO 3 solutions and natural groundwater to estimate the certainty of the two methods for 14 C dating. 14 C values obtained by the gas-strip method for NaHCO 3 solutions with distinct salinity, DIC, and 14 C concentrations were close to the theoretically predicted 14 C value based on the 14 C value of NaHCO 3 powder. Conversely, the 14 C value obtained by the precipitation method always showed higher values than the predicted values. The difference in 14 C value between the gas-strip and precipitation methods was assumed to be caused by the contamination of modern carbon in the NaOH solution used in the precipitation method. The contamination of modern carbon derived from the NaOH solution during precipitation was found to range from less than 1 mg/L to about 1 mg/L. The applicability of the precipitation method for groundwater should be considered carefully according to the DIC, 14 C concentration of groundwater, and purpose of the study being conducted.