Estimation of Groundwater Residence Time Using the 36Cl Bomb Pulse

We propose a methodology for estimating the residence time of groundwater based on bomb-produced Cl-36. Water samples were collected from 28 springs and 2 flowing wells located around Mt. Fuji, Central Japan. Cl-36/Cl ratios in the water samples, determined by accelerator mass spectrometry (AMS), were between 43 x 10(-15) and 412 x 10(-15). A reference time series of the above-background (i.e., bomb-derived) Cl-36 concentration was constructed by linearly scaling the background-corrected Dye-3 data according to the estimated total bomb-produced Cl-36 fallout in the Mt. Fuji area. Assuming piston flow transport, estimates of residence time were obtained by comparing the measured bomb-derived Cl-36 concentrations in spring water with the reference curve. The distribution of Cl-36-based residence times is basically consistent with that of tritium-based estimates calculated from data presented in previous studies, although the estimated residence times differ between the two tracers. This discrepancy may reflect chlorine recycling via vegetation or the relatively small change in fallout rate, approximately since 1975, which would give rise to large uncertainties in Cl-36-based estimates of recharge for the period, approximately since 1975. Given the estimated ages for groundwater from flowing wells, dating based on a Cl-36 bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s.