Quantification of long-term fate and health-risk of remobilized radiocesium through porous and fractured aquifers below a dam

Abstract

Understanding the long-term fate and transport of radiocesium (137Cs) through porous and fractured aquifers is critical for the risk assessment of nuclear accidents. In particular, characterizing 137Cs transport below a dam storing potable water is critical for assessing the risk of 137Cs migration. In this study, a 2D cross-sectional aquifer model was developed based on Paldang Reservoir in South Korea, and transport of desorbed 137Cs beneath a dam was investigated systematically. Various scenarios investigated 137Cs transport within the reported ranges of distribution coefficient (Kd) and local temperature conditions (basal heat flow and reservoir-bottom temperature) affecting the 137Cs desorption rate and hydrogeologic heterogeneity (hydraulic conductivity and fracture networks). The characteristics of the 137Cs plume represented by the average plume concentration and the migration rate of the mass center were assessed, and the health risk of chronic exposure for humans was predicted. In base-case (K=8.64×10−2 m/day and Kd=10 mL/g), approximately 0.06 % of initially released 137Cs was transported downstream over 300 years; the maximum migration rate was 5.1×10−4 m/day, and a maximum annual radioactive dose was 4.5 mSv/y after 230 years. The notable effect of Kd on the migration rate (5.1×10−5 ∼ 9.9×10−4 m/day) and annual dose (9.1×10−11 ∼ 130 mSv/y) indicates that the Kd is a critical parameter that controls the transport of 137Cs in the subsurface aquifer. The effect of temperature on 137Cs transport was relatively insignificant, but it still had a noticeable effect, especially on the desorption rate. In addition, 50 realizations of heterogeneous K and Kd were generated to evaluate the influence of physical and chemical heterogeneity on 137Cs transport. The migration rate and annual dose of 137Cs plume showed large variability when K and Kd were correlated. Finally, 137Cs transport was evaluated in fractured aquifers and assessed the importance of fracture orientation and connectivity.