Depth-distributions and migration of fallout radionuclides in mountain soils from Chréa National Park (Algeria): The role of rhizospheres

Abstract

The distribution and migration of artificial fallout radionuclides in natural soils has been profusely studied for assessing radioecological impacts and predicting their long-term behaviour, among other topics. Despite the standardized use of the analytical solutions of a simplified convection-diffusion equation (CDE), there are still some concerns and open questions. This work is aimed at contributing to the understanding of basic processes governing the distribution of fallout radionuclides in vegetated soils with rhizospheres. It studies ²¹⁰Pb and ¹³⁷Cs in soil cores and vegetal samples from Chréa National Park, in Algeria, along with other natural radionuclides and some major and trace elements. Results include surficial and depth distributions of radionuclide concentrations, and site and plant-specific concentration ratios (CR). Inventories of ¹³⁷Cs (3620 ± 120 Bq m^-2) and ²¹⁰Pb_exc (9000 ± 900 Bq m^-2) in soils are typical from global fallout in high precipitation areas in the Northern Hemisphere. A simple model of a polyphasic soil, including rhizospheres, provides a realistic description in the studied case, where plant roots occupy about 45% of the volume in the 0-10 cm interval, with a high porosity around rhizomes. This composite soil matrix explains the different patterns observed in the depth distribution of the studied elements. The depth-distributions of ¹³⁷Cs and ²¹⁰Pb_exc have been modelled with different approaches: i) analytical solution of the CDE with mean annual convection and large observation times; ii) as before, but with convection representing infiltration events and short observation times; iii) numerical modelling of the ¹³⁷Cs profile in the mineral phase using CDE with fast initial distributions. The three approaches fit the empirical data, but they predict different time evolutions. The approach iii) provides a more realistic description. Results are questioning the common accepted analysis and its predictive use.

Keywords: CDE models; Chréa park; Depth distribution; Fallout radionuclides; Mountain soils; Rhizospheres.