Microscale spatial variability of 7Be inventory at a reference site and its implications for soil erosion estimation

Quantitative assessment of soil erosion and deposition rates using fallout radionuclides, including Beryllium-7 (⁷Be), requires reliable reference inventory, a crucial parameter in the conversion models. However, little information is currently available on the microscale spatial variabilities of ⁷Be inventory at reference locations, producing less confidence in the accuracy of the estimated soil redistribution rates with ⁷Be measurements.

To address this need, 44 soil cores were sampled extensively at 1 m intervals on a 5×12 m² bare flat reference plot in each year of 2019 and 2021 in Southwestern China. Surface soil samples were collected using a stainless steel cylinder with an internal diameter of 10 cm and a height of 3 cm. The soils are purple soils characterised by a silt loam texture. ⁷Be activity concentrations in <2 mm particles were measured to explore potential variability of fallout inventory at the microscale within the reference area. To determine possible causes of ⁷Be variation in soils, physicochemical characteristics including organic matter content (OM), pH, cation exchange capacity (CEC) and grain size compositions were also analysed. 

In the case of 2019, ⁷Be mass activities in soil samples ranged from 2.5 to 10.9 Bq kg^-1 and the areal activities ranged from 82.7 to 417.6 Bq m^-2. The deposition of ⁷Be was higher in 2021, with mass activities ranged between 5.6 and 22.1 Bq kg^-1 and the areal activities between 213.9 and 775.6 Bq m^-2. The higher inventory of ⁷Be in 2021 (211.1 ± 66.2 Bq m^-2, mean ± 1SD) than that of 2019 (456.1 ± 145.5 Bq m^-2) can be explained by higher rainfall amounts of 384.0 mm in 2021 (January 1 - May 18) 2021, compared with 225.4 mm for 2019 (January 1 - May 15). The coefficient of variation (CV) analysis indicated that soil pH and CEC were the most stable properties at the study site with CVs ranged from 1.3 to 5.1%. In contrast, ⁷Be contents in soils, in terms of both mass and areal activities, exhibited almost the strongest variation with CVs around 30%. No significant correlations were noted between ⁷Be activities and the measured soil properties. The high degree of spatial viability in ⁷Be areal activities at the reference site indicates that the simple assumption of uniform distribution of ⁷Be across the reference site needs detailed examination. A spatially-integrated sampling design is recommended to improve the accuracy of reference inventory estimates and thus soil erosion assessment with ⁷Be technique.