Sequential Extraction of ^239+240Pu and the Vertical Distribution of ^239+240Pu, ¹³⁷Cs, and Heavy Metals in Chang-Bai Mountains’ Grassland Soil

To establish a ^239+240Pu, ¹³⁷Cs, and heavy metal reference soil site in Northeast China, three grassland soil cores were collected from the Lao-Ye-Ling Mountain, which is a branch of the Chang-Bai Mountains. The activities were in the range of 0.041–2.752 mBq/g for ^239+240Pu and in the range of 0.2–57.6 mBq/g for ¹³⁷Cs. The ²⁴⁰Pu/²³⁹Pu atomic ratios were 0.161–0.186, showing that ^239+240Pu in the study area originated from the global fallout. Also, ^239+240Pu, ¹³⁷Cs, and excess ²¹⁰Pb were enriched in the surface soil. The convective–dispersion equation model provided the lower apparent dispersion coefficients (D) and apparent convection velocities (v) for both ^239+240Pu and ¹³⁷Cs. The soil erosion rate in this study area was estimated by an erosion model, which was lower than that in the other area of Northeast China. ^239+240Pu in the different soil fractions was analyzed by a sequential extraction method, and the result showed that ^239+240Pu was mainly adsorbed by Fe/Mn oxides and their associated organic matter (NOM_{Fe/Mn‑oxides} fraction). Except for Fe, all other heavy metals from the anthropogenic source were enriched in the surface soil. The pollution level of heavy metals in this study area was lower. In general, the alpine grassland soil of the Lao-Ye-Ling Mountain can be used as a reference soil site to study environmental pollution and soil erosion and provided some reference baselines for future multidisciplinary studies in Northeast Asia.