Source analysis of radiocesium in river waters using road dust tracers

Following the Fukushima Dai-ichi Nuclear Power Station accident, regional road dust, heavily contaminated with radiocesium, now represents a potential source of radiocesium pollution in river water. To promote effective countermeasures for reducing the risk from radiocesium pollution, it is important to understand its sources. This study evaluated the utility of metals, including Al, Fe, and Zn as road dust tracers, and applied them to analyze sources of ¹³⁷Cs in rivers around Fukushima during wet weather. Concentrations of Zn in road dust were higher than agricultural and forest soils, whereas concentrations of Fe and Al were the opposite. Concentrations of Zn were weakly but significantly correlated with benzothiazole, a molecular marker of tires, indicating Zn represents an effective tracer of road dust. Al, Fe, and Zn were frequently detected in suspended solids in river water during wet weather. Distribution coefficients of these metals and ¹³⁷Cs exceeded 10⁴, suggesting sorptive behavior in water. Although concentrations of Al, Fe, Zn, and ¹³⁷Cs were higher in fine fractions of road dust and soils than in coarse fractions, use of ratios of ¹³⁷Cs to Al, Fe, or Zn showed smaller differences among size fractions. The results demonstrate that combinations of these metals and ¹³⁷Cs are useful for analyzing sources of radiocesium in water. These ratios in river water during wet weather were found to be comparable with or lower than during dry weather and were closer to soils than road dust, suggesting a limited contribution from road dust to radiocesium pollution in river water.