Cross-checking groundwater age by 4He and 14C dating in a granite, Tono area, central Japan

Abstract Groundwater dating was performed simultaneously by the 4 He and 14 C methods in granite of the Tono area in central Japan. Groundwater was sampled at 30 packed-off sections of six 1000-m boreholes. 4 He concentrations increased and 14 C concentrations decreased along a groundwater flow path on a topographic gradient. 4 He ages were calculated by using the in situ 4 He production rate derived from the porosity, density, and U and Th content of the rock, neglecting external flux. 14 C ages were calculated with a noncorrected model in which the initial 14 C content was 100 percent of the modern radiocarbon level ( C o  = 100 pmC), a statistical model using the average 14 C content of tritium-bearing samples ( C o  = 46.4 pmC), and a δ 13 C model based on the isotopic mass balance. Although the absolute 14 C ages calculated by the models were different, the relative 14 C ages were almost identical. The relative 14 C ages were considered reliable because dissolved inorganic carbon has no significant geochemical reactions in granite. The relation between the 4 He ages and the noncorrected 14 C ages was [ 4 He age] = 1.15 [ 14 C age] + 7200 ( R 2  = 0.81), except in the discharge area. The slope of this relation was equivalent to unity, which indicates that the 4 He accumulation rate is confirmed by the relative 14 C ages. Moreover, the accumulated 3 He/ 4 He ratio was equivalent to that derived from the 6 Li( α , n ) 3 H reaction in granite. These results show that the accumulated He is of crustal origin, produced in situ without external flux, except in the discharge area. The intercept value of 7200 a implies that the 14 C concentrations were diluted due to geochemical reactions. Tritium-bearing samples supported this result. Simultaneous measurements make it feasible to estimate the accumulation rate of 4 He and initial dilution of 14 C, which cannot be done with a single method. Cross-checking groundwater dating has the potential to provide more reliable groundwater ages. The circulation time of the groundwater flow in the Tono area may be several tens of thousands of years, and the groundwater age calculated from the 4 He and 14 C ages appears to be consistent with geochemical information such as groundwater types, δD and δ 18 O.