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

The purpose of this study is to estimate the contribution of water-derived Cs-137 in the total activity of Cs-137 in brown rice collected from Minamisoma, Fukushima. Three different methods used for the estimation were: 1) the concentration ratio of Cs-137 from water to brown rice (CR-water-rice), 2) mass balance ratio of the total activity of Cs-137 from water to brown rice (MB-water-rice), and 3) the ratio of Cs-137/Cs-133. 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 Cs-137 containing 0.39 mg L-1 K+, tap water free of Cs-137 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 Cs-137 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 Cs-137 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 Cs-137/Cs-133 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 Cs-137 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 Cs-137 in the total activity of Cs-137 in brown rice collected from Minamisoma, Fukushima. The estimation using Cs-137/Cs-133 ratio can also be applicable for rice in other regions.