A radioecological model with moisture-dependent K_d: Application to ¹²⁹I and ⁷⁹Se natural release to a grassland

A radioecological model previously developed to simulate chlorine cycling in a Scots pine forest was modified to examine the effect of soil hydrochemical conditions on the fate of ¹²⁹I and ⁷⁹Se released to a grassland through natural discharge of contaminated groundwater. To this end, the constant solid-liquid distribution coefficient (K_d) in the original model was replaced by a parametric equation to estimate ¹²⁹I and ⁷⁹Se K_d values from soil saturation - as a proxy for soil redox potential - and a set of K_d values determined experimentally under oxic and anoxic conditions. Additionally, the multi-compartment Scots pine tree module was replaced by a two-compartment module to represent ¹²⁹I and ⁷⁹Se cycling in grass. Simulations undertaken with the model indicated a considerable effect of soil redox conditions on ¹²⁹I and ⁷⁹Se accumulation in the soil column, especially in the saturated subsoil above the water table. The constant K_d overestimated ¹²⁹I accumulation in the soil in relation to the parametric K_d. In contrast, the constant K_d underestimated ⁷⁹Se accumulation in the soil. These results have implications for radiological impact assessments, specifically regarding the degree of conservatism in the K_d used in the assessment. In respect of bioavailability to grass, the simulated soil-to-plant transfer factors of ¹²⁹I and ⁷⁹Se compared favourably with values reported in the literature for similar soils and plant species, giving confidence in the model performance. The model presented here is a step forward in radioecological modelling as it includes the key processes that drive radionuclide transfers in soil-plant systems and the effect of soil redox conditions on sorption. The model can be readily extended to other cultivated lands and release scenarios to predict radionuclide transfer up the food chain.