Measurement of the 36Cl deposition flux in central Japan: natural background levels and seasonal variability.
Essential parameters for the applications of (36)Cl as a tracer in groundwater studies include the initial (36)Cl/Cl ratio, at the time of recharge, and/or the natural background deposition flux of (36)Cl in the recharge area. To facilitate the hydrological use of (36)Cl in central Japan, this study aimed to obtain a precise estimate of the long-term average local (36)Cl flux and to characterize its seasonal variability. The (36)Cl in precipitation was continuously monitored in Tsukuba, central Japan over a period of >5 years. The (36)Cl flux showed a clear seasonal variation with an annual peak during the spring, which was attributed to the seasonal variability of tropopause height. The long-term average (36)Cl flux (32±2atoms m(-2)s(-1)), estimated from the measured data, was consistent with the prediction from the (36)Cl latitudinal fallout model scaled using the global mean production rate of 20atoms m(-2)s(-1). The initial (36)Cl/Cl ratio was estimated to be (41±6)×10(-15), which is similar to that of pre-bomb groundwater in the Tsukuba Upland. An observation period covering an 11-year solar cycle would yield more accurate estimates of the values, given the increased (36)Cl flux during the solar minimum.
Crustal Fluid Research Group, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan. yuki.tosaki@aist.go.jp
Yuki
Y
Tosaki
Norio
N
Tase
Kimikazu
K
Sasa
Tsutomu
T
Takahashi
Yasuo
Y
Nagashima
D001417:Background Radiation; D002713:Chlorine / Q000032:analysis*; D007564:Japan; D011834:Radiation Monitoring; D011848:Radioactive Pollutants / Q000032:analysis*; D011868:Radioisotopes / Q000032:analysis*; D012621:Seasons; D014887:Weather
D011848:Radioactive Pollutants; D011868:Radioisotopes; D002713:Chlorine