|Budget Amount *help
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 2002: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 2001: ¥3,100,000 (Direct Cost: ¥3,100,000)
The objective of the study is to elucidate heterogeneity of the soil composition within a pertinent soil horizon by measuring variability of the vertical profiles of environmental radionuclides (^<40>K, ^<226>Ra, ^<137>Cs and ^<210>Pb) in the soil, and to obtain data on partition coefficient of heavy metals (Mn(II) and Zn(II)) using a radiotracer technique. For these purposes, two types of the location were selected : one is a site of alluvial deposits down to a depth of 3.5 m on the campus of Hokkaido University. The samples were collected from November 2000 to March 2001 by the courtesy of Prof. Kasahara (Graduate School of Science, Hokkaido University). And the other is several sites covered with conifer and beech stands of different age at Wetzstein, Tharandt, Hainich and Leinefelde in Germany. The samples were collected in November 2001 (Wetzstein) and in March, 2002 (Tharandt, Hainich and Leinefelde) by the help of Prof. T. Paces (Geological Survey, Czech Republic) and Dr. Reiner
Zimmermann (Max Plank Institute, Germany).
Gamma spectrometry was used to obtain activity concentrations of these nuclides for the alluvial sediment to know their background values. The results also show that there appeared a minimum of ^<40>K activity at certain depth within a deposition layer of the same origin. Such an anomaly was also observed in other properties of the sediment at this depth such as moisture and organic contents. It is likely that depositional and/or post-depositional situation would have been changed probably by water action like heavy rain, percolating groundwater or melting snow.
Investigation of the forest soils in Germany gave the results that vertical, profiles of ^<40>K depend on local geology (400-1100 Bqkg^<-1>). Its activity concentration increases with soil depth, and with increasing density. Radium-226 activity is varied with depth ranging from 20 to 55 Bqkg^<-1> for all the soil samples investigated, which suggests that the soil is rather heterogeneous in composition within a horizon of interest. An activity ratio between ^<210>Pb and ^<226>Ra could be a measure of gaseous ^<222>Rn emanation at each depth. The ratio gave values between 0.3 and 0.5 for most of the depths except for the uppermost part (<10 cm in depth) where ^<210>Pb activity is quite high due to dry- and wet deposition from the atmosphere. High values of the ^<137>Cs activity were observed only in the upper portion of some sites. This nuclide was mostly derived from the Chernobyl accident in Russia (April, 1986), and its vertical profile has been used extensively for estimating deposition rate of the surface soil so far. The deposition rate obtained is 1.8 and 2.7 mm/y for the Wetzstein coniferous forest soils of 40 and 120 years old in stand age. The result seems to be related with mean stand age of the forest. However, there was no distinct relationship between deposition rate (1.2 to 2.2 mm/y) and stand age at other forest sites. It must be investigated further in detail that all the sites did not give an exponential decay of ^<137>Cs activity with depth. Concerning the ^<137>Cs profile with depth, there appeared two more types of ^<137>Cs profiles for the forest soils investigated in this study : One is the sites where radiocesium was little present even in the uppermost portion of the soil. Such a profile might be resulted from surface runoff. Canopy effects are also possible to affect the ^<137>Cs distribution on top of the soil. In addition, there were several sites where ^<137>Cs disappeared at certain depth and appeared again at much deeper depth. Such a profile may be due to biological activities like some type of micro- and macro-organisms inhabiting there. It must be added that there remain several facts not to be explained yet in this study.
On the partition of divalent metals to the forest soils, Zn(II) is always sorbed to a greater extent than Mn(II). Partition coefficient of Zn(II) (K_d(Zn)) is in the range between 100 and 200 (L/kg) at a depth deeper than 10 cm, and about a half of the K_d(Zn) value appeared on the Mn(II) sorption (K_d(Mn)). Partition coefficients of Mn(II) and Zn(II) were quite high at the surface portion of the soil. These values decreased remarkably after a heat treatment of the samples at 450℃. The results indicate that the soil organic matter is likely to provide sites for the metal(II) sorption, which is supported by a positive correlation between organic content and cation exchange capacity (CEC) of the soil.
In summary, change in K_d values with soil depth suggests relative contribution of inorganic and organic components to uptake metal ions.
Present study also gave applicability of environmental radionuclides and their variability of vertical profiles for estimating possible changes in soil environment. The line of this work should be continued further to evaluate consequences of bioturbation often observed in various soils. Less