An Estimate of Local Bomb-Produced 36Cl Fallout Using the Depth Profile of Groundwater in the Tsukuba Upland, Central Japan

Abstract

Abstract:The depth profile of 36 Cl/Cl ratio in groundwater was investigated in the Tsukuba Upland of central Japan. The obtained results clearly show the influence of bomb-produced 36 Cl; the highest 36 Cl/Cl ratio is about one order of magnitude greater than the natural background ratio (1 × 10 13 ). The vertical distribution of 36 Cl is consistent with previous observations using 3 H and Darcy's law. From the profile, the total bomb-produced 36 Cl fallout in the upland is 2.3 × 10 12 atoms/m 2 after the correction for surface runoff (c.f. 2.4 × 10 12 atoms/m 2 at the Dye-3 site, Greenland) and a scaling factor of 0.96 was obtained (c.f. 2.5 based on the simplified latitudinal fallout distribution model). We then reconstructed the local fallout history of 36 Cl based on the Dye-3 data (scaled with a factor of 0.96 for the Tsukuba Upland) and the mean 36 Cl flux, produced in the atmosphere from cosmic rays and measured 30 atoms m 2 s 1 in the upland. The ratio of the maximum bomb-peak fallout to the average natural background flux of meteoric 36 Cl is consistent with that of measured data in Nepal. The result implies that the simplified latitudinal distribution model for 36 Cl deposition is not easily applicable for the prediction of the bomb-produced 36 Cl fallout pattern.

Estimation of Groundwater Residence Time Using the 36Cl Bomb Pulse

“…In our previous study (Tosaki et al 2008), the total bomb‐produced 36 Cl fallout in Tsukuba, Central Japan (36°04′N, 140°08′E), was estimated to be 2.3 × 10 12 atoms/m 2 , based on depth profiling of groundwater in an upland area. A revision of the assumed values of parameters used in the calculation (i.e., assumed layer porosities with 10% uncertainties, compare Yasuhara et al 1991 and a surface runoff rate of 90 ± 10% in upland areas) yields an estimated fallout of (2.4 ± 0.3) × 10 12 atoms/m 2 .…”

“…Reference 36 Cl data are also required to estimate the residence time. These data were obtained as follows: (1) reduction of Dye‐3 fallout data (Synal et al 1990) using the background 36 Cl flux at the Dye‐3 site (Synal et al 1994) to derive a time series of the above‐ background 36 Cl flux; (2) linear scaling of the above‐background 36 Cl flux using the estimated total bomb‐produced 36 Cl fallout in Tsukuba (Tosaki et al 2008) after correcting for the difference in precipitation amount between the two sites (i.e., between the Tsukuba and Mt. Fuji areas); and (3) conversion of the scaled above‐background 36 Cl flux to the above‐background 36 Cl concentration by using a mass balance equation for the recharge area (Andrews et al 1994).…”

Anthropogenic changes in a confined groundwater flow system in the Bangkok Basin, Thailand, part II: how much water has been renewed?

“…In addition, very low 3 H concentration worsens the relative accuracy. The use of alternative tracers such as chlorofluorocarbons (CFCs), 85 Kr (Ekwurzel et al , 1994; Clark and Fritz, 1997) or bomb‐produced 36 Cl (Tosaki et al , 2008) may be more useful for analysing the mixing of modern and submodern groundwaters. On the other hand, 14 C should be more effective if it is applied in experiments over longer time‐scales, as attempted by Sanford and Buapeng (1996).…”

Estimation of groundwater residence time using environmental radioisotopes (14C,T) in carbonate aquifers, southern Poland

“…Modern water is water younger than 50-70 years [80,81]; therefore, it is water which can be recognised, for example, by high tritium content. Sometimes this term is mixed up with young water.…”