Analysing the behaviour of ⁹⁰Sr and stable Sr in highly weathered soils: Soil to plant transfer factor and geochemical partitioning

Public concerns over environmental protection have increased after Fukushima accident. The soil-plant transfer factor (Fv) is a critical parameter for environmental risk assessment. ¹³⁷Cs Fv values determined in acid Brazilian soils could be two orders of magnitude higher than Fv values measured in soils affected by the Chernobyl accident. This paper studied the behaviour of ⁹⁰Sr in Brazilian soils where very few ⁹⁰Sr Fv data is available. All classes of studied soils (Ferralsol, Acrisol and Nitisol) showed that more than 80% of total ⁹⁰Sr in soils remains potentially mobile, mainly at the bioavailable phase, more than 2 years after soil contamination in an experiment conducted in lysimeters. Higher ⁹⁰Sr Fv for maize and cabbage was observed in the acid soils with low content of exchangeable Ca, lower Fv values occurred at Nitisol, the subtropical soil type. It seems that the behaviour of ⁹⁰Sr in these soils are mainly controlled by ionic competition mechanisms occurring for root uptake and sorption sites, with stable Sr and major nutrients (Ca, Mg and K). The mineralogy also seems to play a relevant role, since goethite rich clay soil presented ⁹⁰Sr Fv values higher one or two orders of magnitude than ⁹⁰Sr Fv values observed at clayed soil goethite rich with trace of vermiculite, for all studied crops. The high rate of ⁹⁰Sr migration down the soil profiles observed for the studied soils during the 2 first years after contamination suggests a high rate of transfer to groundwater, even transfer to leafy crop group can be also a relevant pathway. In the present study, maize, among other plant species, presented the lowest Fv values, even when compared with maize cultivated in temperate soil. Our results suggest that stable Sr may not be a very good analogue of ⁹⁰Sr for determining plant Fv values: geometric mean of stable Sr Fv for maize was Fv_maize = 6.3E-1 ± 2.1E0 (n = 8) and for ⁹⁰Sr was Fv_maize = 1.8E-2 ± 2.5E0 (n = 16), geometric mean of stable Sr Fv for cabbage was Fv_cabbage = 1.4E0 ± 3.9E0 (n = 17) and for ⁹⁰Sr was Fv_cabbage = 7.9E-1 ± 4.4E0 (n = 18).