The environmental mobility of newly deposited radionuclides in surface soil is driven by complex biogeochemical relationships, which have significant impacts on transport pathways. The partition coefficient (Kd) is useful for characterizing the soil-solution exchange kinetics and is an important factor for predicting relative amounts of a radionuclide transported to groundwater compared to that remaining on soil surfaces and thus available for transport through erosion processes. Measurements of Kd for 238U are particularly useful because of the extensive use of 238U in military applications and associated testing, such as done at Los Alamos National Laboratory (LANL). Site-specific measurements of Kd for 238U are needed because Kd is highly dependent on local soil conditions and also on the fine soil fraction because 238U concentrates onto smaller soil particles, such as clays and soil organic material, which are most susceptible to wind erosion and contribute to inhalation exposure in off-site populations. We measured Kd for uranium in soils from two neighboring semiarid forest sites at LANL using a U.S. Environmental Protection Agency (EPA)-based protocol for both whole soil and the fine soil fraction (diameters<45 microm). The 7-d Kd values, which are those specified in the EPA protocol, ranged from 276-508 mL g-1 for whole soil and from 615-2249 mL g-1 for the fine soil fraction. Unexpectedly, the 30-d Kd values, measured to test for soil-solution exchange equilibrium, were more than two times the 7-d values. Rates of adsorption of 238U to soil from solution were derived using a 2-component (FAST and SLOW) exponential model. We found significant differences in Kd values among LANL sampling sites, between whole and fine soils, and between 7-d and 30-d Kd measurements. The significant variation in soil-solution exchange kinetics among the soils and soil sizes promotes the use of site-specific data for estimates of environmental transport rates and suggests possible differences in desorption rates from soil to solution (e.g., into groundwater or lung fluid). We also explore potential relationships between wind erosion, soil characteristics, and Kd values. Combined, our results highlight the need for a better mechanistic understanding of soil-solution partitioning kinetics for accurate risk assessment.

译文

新沉积的放射性核素在表层土壤中的环境迁移率是由复杂的生物地球化学关系驱动的,这些关系对运输途径有重大影响。分配系数 (Kd) 可用于表征土壤溶液交换动力学,并且是预测输送到地下水的放射性核素与残留在土壤表面的放射性核素的相对量相比的重要因素,因此可用于通过侵蚀过程进行运输。238U的Kd测量特别有用,因为238U在军事应用和相关测试中广泛使用,例如在洛斯阿拉莫斯国家实验室 (LANL) 进行的。需要对238U进行特定地点的Kd测量,因为Kd高度依赖于当地土壤条件,也高度依赖于精细的土壤分数,因为238U集中在较小的土壤颗粒上,例如粘土和土壤有机材料,这些颗粒最容易受到风蚀的影响并有助于异地人群的吸入暴露。我们使用基于美国环境保护局 (EPA) 的协议,针对整个土壤和细土部分 (直径 <45微米),测量了LANL两个相邻的半干旱森林站点土壤中铀的Kd。EPA协议中指定的7-d Kd值的范围为整个土壤的g-1为276-508 mL,而细土部分的g-1为615-2249 mL。出乎意料的是,为测试土壤溶液交换平衡而测得的30 d Kd值是7-d值的两倍以上。使用2组分 (快速和慢速) 指数模型得出溶液中238U对土壤的吸附率。我们发现LANL采样点之间,整个土壤和细土之间以及7-d和30d Kd测量值之间的Kd值存在显着差异。土壤之间的土壤溶液交换动力学和土壤大小的显着变化促进了使用特定地点的数据来估算环境迁移速率,并暗示了从土壤到溶液 (例如,进入地下水或肺液) 的解吸速率可能存在差异。我们还探讨了风蚀,土壤特征和Kd值之间的潜在关系。结合起来,我们的结果强调了需要对土壤溶液分配动力学进行更好的机械理解,以进行准确的风险评估。

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