Spatial and temporal trends in radiocesium and organic matter fraction in sediments from Lake Chuzenji, central Honshu Island, Japan

Abstract

　Radiocesium (¹³⁷Cs) was measured in sediments from 68 sampling points of Lake Chuzenji, Honshu Island, Japan using the Minasoko in situ measurement system. The ¹³⁷Cs inventory was higher in sediments from the deeper area than from the shallow area, and the ¹³⁷Cs inventory varied widely in sediments from the relatively shallow area (≤2 m in depth). The ¹³⁷Cs concentrations in near-shore sediments did not show any clear tendency to decrease from 2012 to 2016. We determined the organic content of sediments from 20 sampling points using sequential chemical extraction and found that the ratio of organic content fraction to bulk sediment was positively correlated with the ¹³⁷Cs concentrations in bulk sediment. The ratio of the ¹³⁷Cs concentrations in the organic matter fraction and in the bulk sediment ranged from 0.1 to 18.1. Radiocesium in the sediments from Lake Chuzenji had a greater affinity for mineral substances than sediments collected previously from areas off-shore from Fukushima. The ratio of the ¹³⁷Cs inventory in the organic matter fraction and the bulk sediment was between 1.5 and 32.8%. Spatial variations in ¹³⁷Cs in Lake Chuzenji sediments were mostly attributable to water depth and heterogeneity of the organic matter fraction in sediments.