Taeko Itono

Desorption of intrinsic cesium from smectite: inhibitive effects of clay particle organization on cesium desorption.

Fine clay particles have functioned as transport media for radiocesium in terrestrial environments after nuclear accidents. Because radiocesium is expected to be retained in clay minerals by a cation-exchange reaction, ascertaining trace cesium desorption behavior in response to changing solution conditions is crucially important. This study systematically investigated the desorption behavior of intrinsic Cs (13 nmol/g) in well-characterized Na-montmorillonite in electrolyte solutions (NaCl, KCl, CaCl2, and MgCl2) under widely differing cation concentrations (0.2 mM to 0.2 M). Batch desorption experiments demonstrated that Cs(+) desorption was inhibited significantly in the presence of the environmental relevant concentrations of Ca(2+) and Mg(2+) (>0.5 mM) and high concentrations of K(+). The order of ability for Cs desorption was Na(+) = K(+) > Ca(2+) = Mg(2+) at the highest cation concentration (0.2 M), which is opposite to the theoretical prediction based on the cation-exchange selectivity. Laser diffraction grain-size analyses revealed that the inhibition of Cs(+) desorption coincided with the increase of the clay tactoid size. Results suggest that radiocesium in the dispersed fine clay particles adheres on the solid phase when the organization of swelling clay particles occurs because of changes in solution conditions caused by both natural processes and artificial treatments.

Deposition records in lake sediments in western Japan of radioactive Cs from the Fukushima Dai-ichi nuclear power plant accident

Sediment trap observations of lakes in western Japan before and after Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident indicate that changes in the radioactive Cs discharge are influenced by fluctuations in precipitation and geomorphological conditions in each catchment. Contributions of FDNPP-derived (134,137)Cs to the sediment decreased from 19-48% to 10-15% within a few months, implying that the major transport processes of FDNPP-derived (134,137)Cs have changed from direct fallout to transport by soil particles.

Present Earth-Surface Processes and Historical Environmental Changes Inferred from Lake-Catchment Systems

Environmental changes in lake-catchment systems due to climatic, tectonic, and anthropogenic activities (processes) have been imprinted on lacustrine sediments. Long continuous observation in a small pond-catchment system at Kobe, Japan following the 1995 Kobe earthquake provides good data on environmental changes. These records are available for establishing a mathematical model in which the rate of sedimentation varies in two stages before arriving at the pre-earthquake stage. The model’s calculations are fairly consistent with observational results, indicating that the model is acceptable. The 1995 earthquake is faintly recorded in the physical properties of sediments obtained in the system (increase in density and slight change in grain size). A combination of grain density (or mineral content) and grain size may be useful as an indicator for the occurrence of comparatively large earthquakes in tectonically active and climatically humid areas. The Tong-hai earthquake of 1970 in Yunnan, China was detected in the combination (density increase and grain size decrease) for the core sediments obtained from Qilu-he. The Kanbun earthquake of 1662 in Lake Biwa catchment was also detected in the corresponding data from some cores obtained from Lake Biwa, Japan.

Recent Eutrophication and Environmental Changes in the Catchment Inferred from Geochemical Properties of Lake Onuma Sediments in Japan

This study investigated the continuous record of eutrophication in Lake Onuma based on the geochemical properties of two lake sediment cores obtained from the deepest part of the lake in 2011. Based on a tuff layer deposited during the eruption of Mt. Komagatake, and on the correlation between fluctuations in δ13C and δ15N values, two sediment cores, ON11-2-2 and ON11-6, were dated to the 1920s and 1890s, respectively. The δ13C value and C/N ratio for the lake sediments show values within the ranges for planktonic material and river sediment, suggesting that the lake sediment is a mixture of these sources and that their mixture ratio was almost constant since the 1920s. On the other hand, the δ15N of two cores show a similar trend with increasing δ15N from the 1950s–1960s to the present time. It is attributed to the increase in the δ15N value of planktonic material reflecting anthropogenic nitrogen inflow to the lake.

Reconstructing Modern Hydro-Environmental Fluctuations Inferred from Lacustrine Sediment in Lake Onuma, Hokkaido

Surface core sediments obtained from Lake Onuma, Hokkaido, were used to reconstruct hydro-environmental changes during the past 100 years. The physical properties of lacustrine sediments record both physical environmental and artificial changes in the lake-catchment system.

Possible Age Models for Lake Onuma Lacustrine Sediments Based on Tuffs Recovered in Three Cores

Lake Onuma, Hokkaido, Japan, is located south of Hokkaido-Komagatake volcano. The present Lake Onuma was formed by an intensive eruption and partial collapse of the volcanic cone in 1640 AD, which caused a debris flow that dammed the drainage. Three cores (ON12A, C, and D) were recovered from Lake Onuma to examine the environmental change in the region. These three cores are correlated based on lithological descriptions and water content fluctuation. Volcanic deposits Ko-a (1929 AD), Ko-c1 (1856 AD), Ko-d (1640 AD), and possibly Ko-c2 (1694 AD) were successfully identified with the help of radiocarbon age dating, 137Cs radioactivity measurement, and chemical analyses, to construct an age model of these cores.

Accumulation of accident-derived radiocesium in lake and coastal sediments at 300–700 km distance from Fukushima area

The accumulation of accident-derived radiocesium was investigated in nine water bodies located 300-700 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). (134)Cs from the accident was detected in surface sediment of five water bodies. The (134)Cs concentration, corrected to the time of the accident in 2011, was generally lower than that of (137)Cs, and its spatial pattern does not fully correspond to that of (137)Cs. These results suggest that radiocesium derived from both FDNPP and past global fallout can be separately observed and that the contributions of both sources are non-uniform within these sites. The (134)Cs inventory in surface sediments is smaller than its deposition, suggesting that almost all deposited (134)Cs remains within the catchment and/or a part has been discharged from the saline and brackish water bodies.

Comparison of Luminescence Dating Methods on Lake Sediments from a Small Catchment: Example from Lake Yogo, Japan

When applying luminescence dating to sediment deposited in aquatic environments, a key issue for accurate age determination is resetting of acquired luminescence in sediment by surface exposure (bleaching) before burial. The time needed for bleaching is known to vary among the signals used in three methods: optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL), and post-infrared IRSL (pIRIR). A comparison of luminescence ages from these different signals is therefore useful to assess whether a sample was fully bleached before burial. In a comparison of OSL, IRSL50/225 and pIRIR225 ages of eight samples of fine-grained sediment from a 294-cm-long sediment core from Lake Yogo, a small-catchment lake in central Japan, the IRSL50/225 and pIRIR225 ages were much older than the OSL ages. The IRSL50/225 residual signals were close to zero, and the difference between pIRIR225 and OSL signals was much larger than the pIRIR225 residual signals. Thus, IRSL50/225 and pIRIR225 signals were not completely bleached, which we attribute to the short sediment transport distance in this small catchment. Five corrected bulk radiocarbon (14C) ages agreed with the OSL ages, except for two intervals in which OSL ages were about 500 and 1,900 years older than the corrected 14C ages. These discrepancies are attributable to incomplete bleaching related to sediment transport, whereas the rest of the OSL ages show no evidence of incomplete bleaching. This study shows that even in samples in which the pIRIR225 and IRSL50/225 signals are not well-bleached, OSL dating yields accurate age estimates because of the faster bleaching rate.

Sediment transport processes in a reservoir–catchment system inferred from sediment trap observations and fallout radionuclides

Sediment transport processes and their responses to hydrological conditions were investigated based on sediment trap observations in a reservoir–catchment system in Japan. The annual discharge rate of 134Cs from the catchment was much higher than that of 137Cs. This difference suggests that 134Cs freshly derived from the Fukushima Dai-ichi Nuclear Power Plant accident accumulated in the surface soil, which acted as a major sediment source. The seasonal changes in the 137Cs/210Pbex ratio in the trap samples corresponded well to the precipitation changes. This suggests that the precipitation intensity affects the source of eroded sediment in the catchment.