Soil processes affecting the bioavailability of radiocaesium

The Chernobyl accident in 1986 has contaminated a vast area of European soils with radiocaesium (¹³⁷Cs). The uptake of ¹³⁷Cs from soil into food crops was found to be high variable and largely unpredictable. This work elaborates the concept that the soil solution composition can be used to predict the availibility of ¹³⁷Cs in soil. This concept is used in plant growth conditions of increasing complexity, viz. solution culture, pot trials and lysimeters. Preliminary experiments were set-up to improve predictions of the ¹³⁷Cs concentration in soil solution. Reducing soil water content increased pore water ¹³⁷Cs concentrations along with increasing potassium (K) concentrations in nine different soils. The relative changes in ¹³⁷Cs and K concentrations showed a consistent relationship with relative soil water content among all soil types. Concentrations of ¹³⁷Cs in soil solution could be predicted from the ¹³⁷Cs concentrations in a CaCl₂ 0.01 M soil extract (1:1 solid:liquid ratio). This soil extract can overcome ¹³⁷Cs detection limits in small samples of pore water. Solution cultures studies were set-up to measure the uptake of ¹³⁷Cs in different species (beans, lettuce, barley, ryegrass and bentgrass) at varying K concentrations (0.025-1.0 mM) maintained during plant growth. Dry weight based ¹³⁷Cs concentrations varied maximally 4-fold among species when grown at similar K supply. Increasing K concentrations decreased shoot ¹³⁷Cs concentrations 17- to 81-fold, illustrating the marked effect of K competing with the ¹³⁷Cs uptake process. Additional scenario's were set-up in which K concentrations were allowed to deplete from initially 0.010 to 0.25 mM to finally 137Cs concentration in grass could be predicted from data obtained at constant solution K and from the time course of solution K concentration during transient K supply. The same species were grown in pot trials containing two soils homogenously spiked with ¹³⁴Cs activities and differing in K supply. The shoot ¹³⁴Cs activities were up to 15-fold larger in the soil with lowest K supply and varied 4.5 fold (large K supply) and 19-fold (lowest K supply) among species. Uptake of ¹³⁷Cs was finally measured in these crops (excluding bentgrass) grown in 5 undisturbed lysimeters covering major European soil types. Plants were grown during 3 successive years starting 3 years after soil contamination. The plant-soil ¹³⁷Cs concentration ratios varied maximally only 11-fold between soils and maximally 35-fold among species when grown on the same soil. Models were constructed predicting crop radiocaesium concentrations with soil solution data and radiocaesium uptake data from the solution culture experiments. The soil solution (or CaCl₂ soil extract) was sampled from bulk soil during or after plant growth. A solute transport model (steady state or dynamic model) was developed to estimate the composition of soil solution in the rhizosphere. Predicted K concentrations in the rhizosphere were well below that in bulk soil whereas corresponding ¹³⁷Cs concentration gradients were always less pronounced. The predictions based on rhizosphere soil solution compositions were generally closer to observations than those based on bulk soil solution composition. Predicted shoot ¹³⁷Cs concentrations in crops were 11- to 45 below observations based on the bulk soil solution composition and within a factor 7 of observations when based on the rhizosphere soil solution composition in the pot trial. Predicted ¹³⁷Cs concentrations in crops grown in the lysimeters matched observations or were up to 17-fold below observations (bean and lettuce) when based on the rhizosphere soil solution composition. The model failed to predict ¹³⁷Cs uptake in ryegrass grown on the lysimeters where uptake of the 6-years old ¹³⁷Cs from 3 soils was about 40-fold larger than predicted. The model generally underpredicted crop ¹³⁷Cs concentrations at soil solution K concentration below about 1.0 mM. It is concluded that ¹³⁷Cs uptake can be predicted from the soil solution composition at adequate K nutrition but that significant uncertainties remain when soil solution K in the rhizosphere drops below about 0.025 mM.