Groundwater contamination at legacy uranium processing sites is an ongoing global challenge. Plumes at many uranium-contaminated sites are more persistent than originally predicted by groundwater modeling. Previous investigations of uranium plume persistence identified residual and secondary sources that contribute to plume longevity, but there is a remaining need to revise forecasted cleanup times using information about these ongoing sources. The purpose of this study is to investigate the quantitative impact of residual vadose zone sources of uranium on groundwater remediation time frame. This objective was approached by applying numerical uranium transport simulations and uncertainty analysis to a former uranium mill site in the southwestern United States. Information from recent site investigations provided details about the distribution and release characteristics of uranium accumulations in the vadose zone. The residual uranium characteristics were incorporated as decaying source terms in the transport model. A stochastic approach using an iterative ensemble smoother was applied for history matching, and the transport model was used to assess the impact of multiple remedial alternatives on forecasted time frame. The forecasted time frame to achieve the groundwater remediation goal for uranium by monitored natural attenuation is on the order of thousands of years, and treatment of the dissolved plume does not reduce the projected time frame. The large proportion of residual uranium mass remaining in the vadose zone and the gradual leaching rate due to the site's semiarid climate create a long-lived source that can sustain a dissolved plume for thousands of years despite an estimated 99% mass removal achieved during mill tailings disposal. Residual uranium in vadose zone sediments beneath former tailings impoundments could present comparable uranium plume persistence and remediation challenges at other legacy uranium mill sites in semiarid climates. Other remaining uranium-impacted sites are similarly complex, and forecasted remedial time frames are needed to effectively achieve compliance, manage risk, assess the benefits of additional treatment, manage and project costs, and support beneficial site reuse.

遗留铀加工场的地下水污染是一个持续的全球挑战。许多铀污染地点的羽流比地下水模型最初预测的更持久。先前对铀羽流持久性的调查确定了有助于羽流寿命的残留和次要来源，但仍然需要使用有关这些持续来源的信息来修改预测的清理时间。本研究的目的是调查残余渗流带源铀对地下水修复时间框架的定量影响。通过对美国西南部的一个前铀工厂进行铀输运数值模拟和不确定性分析，实现了这一目标。最近现场调查的信息提供了有关包气带铀积累的分布和释放特征的详细信息。残余铀特征作为衰变源项纳入输运模型中。使用迭代集成平滑器的随机方法用于历史匹配，并使用传输模型来评估多种补救方案对预测时间范围的影响。通过监测自然衰减实现铀地下水修复目标的预测时间范围约为数千年，并且溶解羽流的处理不会缩短预计时间范围。尽管在工厂尾矿过程中估计已去除了 99% 的质量，但包气带中残留的大部分残余铀质量以及由于该地点的半干旱气候而导致的逐渐浸出率创造了一个长期存在的来源，可以维持溶解的羽流数千年处理。前尾矿库下方渗流带沉积物中的残余铀可能会在半干旱气候下的其他遗留铀工厂中面临类似的铀羽持久性和修复挑战。其他剩余的受铀影响的场地也同样复杂，需要预测的补救时间框架来有效实现合规性、管理风险、评估额外处理的好处、管理和项目成本以及支持有益的场地再利用。