Distribution of ¹³⁷Cs in soil along Ta-han River Valley in Tau-Yuan County in Taiwan

Environmental ¹³⁷Cs contamination was suspected from accidents at spent fuel storage pits of a research reactor site in the Ta-han River valley in Taiwan. In order to further characterize this contamination, soil samples were collected and measured by a gamma-spectroscopy system in 1999. It was found that ¹³⁷Cs contamination is distributed up to 4km from the reactor in an area covered mostly by rice and plant fields. ¹³⁷Cs concentration in the topsoil ranged up to about 1000Bqkg^-1, as compared with soil beyond the contaminated area, which does not exceed 15Bqkg^-1. Spatial distribution of ¹³⁷Cs was characterized by strong non-uniformity, which complicated our understanding of the distribution pathway of the radionuclides. The highest concentrations of ¹³⁷Cs, up to more than 1000Bqkg^-1, were found within a few rice fields. The relative location of these rice fields and the water supplies from local streams suggested that the ¹³⁷Cs was distributed along water pathways in the valley.