摘要：Positive and negative roles of dam reservoirs in radiocesium behavior in a river catchment were examined based on the results of comprehensive, long-term hydrological monitoring from 2013 to 2017 at Matsugabou Dam on the upstream reaches of the Uda River, which is one of the main rivers in the northern coastal region of Fukushima Prefecture. An estimation of the annual mass balance of Cs-137 associated with suspended solids (SS) in the reservoir showed that Matsugabou Dam had the effect of reducing the migration of radiocesium associated with SS from the dam reservoir catchment to the downstream reaches by more than 85% by storing SS in the reservoir bed. Moreover, proactive discharge control for preventing flood inundation in the downstream region had the ability to make this storage function perform even more effectively. On the other hand, the estimated annual mass balance of dissolved Cs-137 in the reservoir showed that the amount in the water discharged clearly tended to exceed the amount in the inflow, suggesting that substantial amounts of dissolved radiocesium were presumably produced by dissolution/elution from the stored bottom sediments in the dam reservoir. This negative role of a dam producing dissolved radiocesium may contribute to long-term contamination of freshwater ecosystems in downstream water bodies.
