Multicomponent transport of 226Ra in agricultural soils following the application of phosphogypsum

In this study we provide an analysis of the subsurface transport of radionuclides released from phosphogypsum (PG) when used as a fertilizer or amendment in agriculture. It is important to assess the environmental risks caused by the long-term use of PG in agricultural operations. Our study focused on the transport of 226Ra when PG is applied annually to a typical soil profile in the Brazilian Cerrado. Expected water flow and solute transport processes were estimated first using the HYDRUS-1D software package assuming linear equilibrium transport, followed by similar simulations using the HP1 multicomponent transport module of HYDRUS-1D. Analyses with HP1 considered a range of geochemical and physical parameters affecting 226Ra transport in the vadose zone. We investigated especially the effects of varying calcium concentrations on 226Ra transport, showing that this variable is relatively important when making predictions. Several aspects related to the modeling approach, such as the use of daily versus monthly weather data and the influence of assuming different transport regimes in the vadose zone (e.g., transient versus long-term steady-state flow) are discussed also.