Risk evaluation of radionuclides contamination on soil and groundwater under different scenarios simulating by HYDRUS-1D

A large number of radionuclides, produced by nuclear accidents or nuclear waste, may cause radioactive contamination in the agricultural and aquatic ecosystems. Under these circumstances, it is necessary to optimize the remediation of agricultural areas polluted by radionuclides using innovative monitoring and prediction techniques. To mitigate radioactive contamination in farmland soil and effectively protect groundwater, some measures should be taken against on field investigation, laboratory experiment and model prediction. In this study, the HYDRUS-1D model was used to simulate the vertical migration of ¹³⁷Cs and ⁶⁰Co in farmland soil in northern China calibrating by the soil lysimeter experiment, and the scenario simulations of ¹³⁷Cs and ⁶⁰Co migration were conducted under different radioactive levels. Results showed that the order of sensitivity in saturated water content (θ_s), residual water content(θ_r), saturated hydraulic conductivity(K_s) and distribution coefficient (K_d) applied in HYDRUS 1D model was K_d > θ_s > θ_r >K_s. The simulated concentrations ​​of ¹³⁷Cs and ⁶⁰Co in Brown soil and Aeolian sandy soil on day 175 and 355 were significantly positively correlated with the measured values​​ (r>0.90, p<0.01). The verification results showed that the predictive values on the 577^th day were also significant positive correlated with the measured values ​​(r>0.90, p<0.01). The RMSE, CRM and NRMSE calculating by simulated and measured values ​​of ¹³⁷Cs and ⁶⁰Co in soil were very small, indicating that HYDRUS 1D can be used to simulate the migration of radionuclides in farmland soil. Scenarios simulation results revealed that radionuclides were concentrated in the surface layer within 5 cm, but the migration depth has exceed 10 cm soil depth, and even reaches up to 23.5 cm depth at high concentration level. The surface soil should be cleaned timely to protect groundwater with high level from radioactive contamination and further study should be done about horizontal transport and numerical simulation.