Influence of Soil Production Processes on Differential Mobility of Fallout Radionuclides in Taiwan.
Narasimham L. Vemuri and Chih-An Huh. Institute of Earth Sciences, 128 Academia Road, PO Box 1-55, Nankang, Taipei, Taiwan
The depth distribution pattern for the fallout radionuclides, 210Pbexcess and 137Cs in apparently undisturbed soil profiles, depends upon the past soil production processes within a landscape position. The magnitude of variance in the function of exponential depth distribution due to differential mobility of the nuclide(s) is governed by the time-dependent maturity of the landscape. Soil profiles produced on the mountainous backslopes through erstwhile diffusive creeping, followed by advanced diagenesis have shown similar activities at shallow depths after an initial exponential decrease pattern. In contrast, a classical exponential decrease pattern from surface to deeper depths was observed for consolidated and significantly matured soils, derived from weathering of the underlying bedrock. Hand-driven shallow soil cores (length < 40 cm and i.d. 3.4 cm) were collected, a) along the coastal gemorphological landforms, b) on the complex hinterland forest hillslopes, c) on anthropogenically undisturbed ridges and plains, d) along the coastal lateritic plains, and e) on cultivated terraced backslopes - of different mountainous counties of north and eastern parts of Taiwan. Depth distributions for 210Pbexcess and 137Cs activities were determined for ≤ 2 mm size fractions of the soil after slicing at equal thickness intervals of the two adjacently-taken cores, and by compositing them. Three HPGe detectors (EG&G ORTEC GMX-100230), each interfaced to a stand-alone digital gamma-ray spectrometer (DSPec PlusŪ), were used in determining the activities of fallout nuclides. The analytical methodology is similar to the procedure described by Huh and Su, (2004). Findings show that in spite of the highest total inventories (395.1 ± 3.6 dpm/cm2 and 95.3 ± 1.8 dpm/cm2 for 210Pbexcess and 137Cs, respectively) compared to other locations, nuclides were mired at very shallow depths in the consolidated weathered soil profiles derived from the underlying sedimentary bedrocks. The permeable lateritic soils formed through weathering of underlying andesitic lava flows of the coastal plains have shown classical exponential depth distribution with correlation coefficient, r2 = 0.98 and 0.95 for 210Pbexcess and 137Cs, respectively, and also found that the mobility was into greater depths. Secondly, for diagenitically-derived rock-free soils of the hinterland mountainous backslopes, a noticeable shift towards higher activities from the initial exponential decrease was observed at the middle depth intervals, but original exponential decrease was continued at the bottom depths. The pattern observed could be the result of simultaneous diagenesis of nuclide(s) and dilute soil suspension at two or more depth intervals of the soil column. Thirdly, the effective soil volume samples, (average rock content 51.27% ± 11.61, n=32), from the undisturbed soil ridges have shown mixed trends but have maintained an exponential decrease (r2=0.98 and 0.80 - 210Pbexcess and 137Cs, respectively). The studies conclude that in the soil profiles that have had a prolonged chemical weathering of the underlying bedrock, mobility of the radionuclides is largely governed by matured soil physical properties, irrespective of aerial difference in the sampling locations, or in the estimated activities. At the hinterland forest slopes where rapid physical processes must have happened, passive dynamic soil solute was a conduit for the nuclide mobility, and diagenesis seems to have followed. The studies show, i) the possibility of differentiating soil genesis from the diagenetic and the pedo-geomorphological, processes, ii) negation and behaviour of the undisturbed sites, and iii) a potential mechanism of reactive transport in different landscape conditions.