Distribution of ¹³⁷Cs and ⁶⁰Co in plough layer of farmland: Evidenced from a lysimeter experiment using undisturbed soil columns

ABSTRACT

Radionuclide fallout during nuclear accidents on the land may impair the atmosphere, contaminate farmland soils and crops, and can even reach the groundwater. Previous research focused on the field distribution of deposited radionuclides in farmland soils, but details of the amounts of radionuclides in the plough layer and the changes in their proportional distribution in the soil profile with time are still inadequate. In this study, a lysimeter experiment was conducted to determine the vertical migration of ¹³⁷Cs and ⁶⁰Co in brown and aeolian sandy soils, collected from the farmlands adjoining Shidaowan Nuclear Power Plant (NPP) in eastern China, and to identify the factors influencing their migration depths in soil. At the end of the experiment (800 d), > 96% of added ¹³⁷Cs and ⁶⁰Co were retained in the top 0–20 cm soil layer of both soils; very little ¹³⁷Cs or ⁶⁰Co initially migrated to 20–30 cm, but their amounts at this depth increased with time. The migration depth of ¹³⁷Cs was greater in the aeolian sandy soil than in the brown soil during 0–577 d, but at the end of the experiment, ¹³⁷Cs migrated to the same depth (25 cm) in both soils. Three phases on the vertical migration rate (v) of ⁶⁰Co in the aeolian sandy soil can be identified: an initial rapid movement (0–355 d, v = 219 ± 17 mm year^–1), followed by a steady movement (355–577 d, v = 150 ± 24 mm year^–1) and a very slow movement (577–800 d, v = 107 ± 7 mm year^–1). In contrast, its migration rate in the brown soil (v = 133 ± 17 mm year^–1) was steady throughout the 800-d experimental period. The migration of both ¹³⁷Cs and ⁶⁰Co in the two soils appears to be regulated by soil clay and silt fractions that provide most of the soil surface area, soil organic carbon (SOC), and soil pH, which were manifested by the solid-liquid distribution coefficient of ¹³⁷Cs and ⁶⁰Co. The results of this study suggest that most ¹³⁷Cs and ⁶⁰Co remained within the top layer (0–20 cm depth) of farmland soils following a simulated NPP accident, and little reached the subsurface (20–30 cm depth). Fixation of radionuclides onto clay minerals may limit their migration in soil, but some could be laterally distributed by soil erosion and taken up by crops, and migrate into groundwater in a high water table level area after several decades. Remediation measures, therefore, should focus on reducing their impact on the farmland soils, crops, and water.