Modeling evaluation of the impact of residual source material on remedial time frame at a former uranium mill site

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.