Estimate the contribution of water-derived 137Cs in the total 137Cs in brown rice using water-to-brown rice transfer parameters and the ratio of 137Cs/133Cs

ABSTRACT The purpose of this study is to estimate the contribution of water-derived 137Cs in the total activity of 137Cs in brown rice collected from Minamisoma, Fukushima. Three different methods used for the estimation were: 1) the concentration ratio of 137Cs from water to brown rice (CR–water–rice), 2) mass balance ratio of the total activity of 137Cs from water to brown rice (MB–water–rice), and 3) the ratio of 137Cs/133Cs. To obtain water-to-brown rice transfer parameters (CR–water–rice and MB–water–rice), rice plants were grown in pots filled with soil collected from Minamisoma, Fukushima and irrigated with three types of water (tap water free of 137Cs containing 0.39 mg L−1 K+, tap water free of 137Cs containing 1.0 mg L−1 K+, and tap water containing 5.6 Bq L−1 137Cs and 1.0 mg L−1 K+). The activity concentrations of 137Cs in irrigation water, soils, and brown rice collected from Minamisoma, Fukushima were 0.032 Bq L−1, 1.04, and 0.0012 Bq g−1 dry wt, respectively. Water-derived 137Cs in brown rice only accounted for 1.9% estimated from the CR–water–rice parameter. It was 2.8% if estimated with the MB–water–rice parameter. The ratio of 137Cs/133Cs in brown rice was 0.00162 Bq ng−1, similar to that in the exchangeable fraction of soils (0.00173 ± 0.00007 Bq ng−1, n = 3). Thus, the contribution of 137Cs from irrigation water to brown rice was negligible. The results estimated by three different methods were in good agreement. Therefore, these methods will be useful tools for assessing the contribution of water-derived 137Cs in the total activity of 137Cs in brown rice collected from Minamisoma, Fukushima. The estimation using 137Cs/133Cs ratio can also be applicable for rice in other regions.