Investigating the Influence of Groundwater Transport at LANL Area G: Can We Disable a Negligible Dose Pathway rather than Neglect It? - 23463

At United States Department of Energy (US DOE) radiological waste disposal sites, the movement of water and radionuclides from the disposal unit to the underlying groundwater must be simulated to calculate future dose to the aquifer and to potential groundwater users. This can be a complex task, involving analysis of multiple interacting transport processes, which may hinder required sensitivity or uncertainty analyses (SA or UA). At the Los Alamos National Laboratory Area G site, a primarily low-level waste disposal site in northern New Mexico, net infiltration is modeled using steady-state background flux to represent average inputs and withdrawals, such as from rainfall and evapotranspiration. Transient analysis is also used for time-variable inputs such as water used during active disposal. Infiltrating water passes through buried waste and into an approximately 1000- to 1200-ft (300- 370 m) unsaturated zone comprised of low-water-content Bandelier Tuff underlain by thick fractured basalt that extends into the regional aquifer. Groundwater receptor scenarios are used to calculate radiological doses for future members of the public (MOPs) who might drink and water their gardens and livestock from wells piercing the aquifer. In addition, the potential future dose to the groundwater resource itself is calculated. Receptor scenarios are generated following regulatory guidelines for performance assessments (PAs) found in US DOE Order 435.1. In models of Area G, groundwater receptors receive the lowest projected dose of any of the PA receptors considered due to the vertically extensive unsaturated zone separating waste from the aquifer below the site and the low net infiltration rates assumed for the semi-arid site. Two scenarios are required for the PA annual reporting: the All Pathways - Groundwater and Groundwater Resource Protection scenarios. In FY2021, projected peak mean dose for the All Pathways - Groundwater Scenario at Area G was 6.6 E-3 mrem/yr or less than 0.03% of the applicable 25-mrem/yr performance objective. Projected peak mean dose for the Groundwater Resource Protection scenario was 1.1 E-2 mrem/yr or less than 0.3% of the applicable 4-mrem/yr performance objective. Simulating unsaturated and saturated groundwater transport of radionuclides from Area G is a computationally intensive task. A sensitivity analysis, in which the groundwater calculations were disabled in groundwater-user dose projections, reduced the model run time by approximately half, with a change from over 34 processor hours to just under 17 processor hours. The sensitivity analysis also assessed whether disabling the groundwater pathway may have the unintended consequence of increasing dose to other receptors at the ground surface due to retention of radionuclides that would otherwise be transported downwards by groundwater. Here we examine the change in projected doses received by other receptors when the groundwater pathway is disabled. We find that disabling the groundwater pathway does not impact other receptors within 0.1% of performance objectives. Therefore, disabling the groundwater pathway for Area G is a viable method for simplifying sensitivity and uncertainty analyses of projected doses to other potential receptors. Disabling a negligible pathway is a potentially precedent-setting action, which could lead other sites to consider, justify, and document the case for their own pathway omissions. (authors)