Title: Colloid-Facilitated Radionuclide Transport in Fractured Carbonate Rock from Yucca Flat, Nevada National Security Site

In the Yucca Flat basin of the Nevada Test Site (NTS), 747 shaft and tunnel nuclear detonations were conducted primarily within the tuff confining unit (TCU) or the overlying alluvium. The TCU in the Yucca Flat basin is hypothesized to inhibit radionuclide migration to the highly transmissive and regionally extensive lower carbonate aquifer (LCA) due to its wide-spread aerial extent, low permeability, and chemical reactivity. However, fast transport pathways through the TCU by way of fractures may provide a migration path for radionuclides to the LCA. Radionuclide transport in both TCU and the LCA fractures is likely to determine themore » location of the contaminant boundary for the Yucca Flat/Climax Mine Corrective Action Unit (CAU). Radionuclide transport through the TCU may involve both matrix and fracture flow. However, radionuclide migration over significant distances is likely to be dominated by fracture transport. Transport through the LCA will almost certainly be dominated by fracture flow, as the LCA has a very dense, low porosity matrix with very low permeability. Because of the complex nature of reactive transport in fractures, a stepwise approach to identifying mechanisms controlling radionuclide transport was used. The simplest LLNL experiments included radionuclide transport through synthetic parallel-plate fractured tuff and carbonate cores. These simplified fracture transport experiments isolated matrix diffusion and sorption effects from all other fracture transport processes (fracture lining mineral sorption, heterogeneous flow, etc.). Additional fracture transport complexity was added by performing induced fractured LCA flowthrough experiments (effect of aperture heterogeneity) or iron oxide coated parallel plate TCU flowthrough experiments (effect of fracture lining minerals). Finally naturally fractured tuff and carbonate cores were examined at LLNL and LANL. All tuff and carbonate core used in the experiments was obtained from the USGS Core Library, Mercury, Nevada. Readers are referred to the original reports ''Radionuclide Transport in Tuff and Carbonate Fractures from Yucca Flat, Nevada Test Site'' (Zavarin et al., 2005) and ''Radionuclide Sorption and Transport in Fractured Rocks of Yucca Flat, Nevada Test Site'' (Ware et al., 2005) for specific details not covered in this summary report.« less

In recent years, numerous laboratory and field experiments have been conducted to assess and parameterize colloid and colloid-facilitated radionuclide transport for the Yucca Mountain Project and the Nevada Test Site (NTS) Environmental Restoration Project. Radionuclide contamination of ground water currently exists within or near underground nuclear test cavities at the NTS, and the proposed Yucca Mountain high-level nuclear waste repository represents a potential future source of radionuclide contamination of ground water at the NTS. Furthermore, recent field observations have indicated that small amounts of Plutonium, which normally adsorbs very strongly to mineral surfaces in aquifers, can transport quite rapidly andmore » over significant distances in ground water when associated with inorganic colloids (Kersting et al., 1999). Groundwater samples from all over the Nevada Test Site have been analyzed for colloid concentrations and size distributions, and it is clear that there are significant mass loadings of colloids in the ground water at some locations. These colloids represent mobile surface area for potentially transporting strongly-adsorbed radionuclides. Field transport experiments have involved the use of fluorescent-dyed carboxylate-modified latex (CML) microspheres in the 250- to 650-nm diameter size range as surrogates for natural colloids in forced-gradient tracer tests. These experiments have indicated that effective colloid filtration coefficients appear to decrease as time and length scales increase. They suggest that a small fraction of colloids may be able to transport significant distances in groundwater systems. Laboratory experiments have been conducted to determine radionuclide sorption and desorption parameters onto inorganic colloids present in the groundwater systems and also to determine transport parameters for inorganic colloids in both fractured and porous media present at the Nevada Test Site. More recent laboratory experiments have involved injecting inorganic colloids with radionuclides adsorbed onto them into fractured or porous media to determine the ability of the colloids to facilitate the transport of the radionuclides through the media. Recent experiments have also involved comparing the transport behavior of CML microspheres and inorganic colloids so that more defensible inferences about inorganic colloid transport can be made from CML microsphere transport observations in field tracer tests. All of this experimental information has been collectively used to develop a modeling framework for evaluating sensitivities of predicted colloid-facilitated radionuclide transport to various colloid-transport and radionuclide-colloid-interaction parameters. This modeling framework is helping to focus future experimental efforts on processes and parameters that have the greatest potential impact on colloid-facilitated radionuclide transport at the Nevada Test Site.« less

In the Yucca Flat basin of the Nevada Test Site (NTS), 747 shaft and tunnel nuclear detonations were conducted primarily within the tuff confining unit (TCU) or the overlying alluvium. The TCU in the Yucca Flat basin is hypothesized to reduce radionuclide migration to the regional carbonate aquifer (lower carbonate aquifer) due to its wide-spread aerial extent and chemical reactivity. However, shortcuts through the TCU by way of fractures may provide a migration path for radionuclides to the lower carbonate aquifer (LCA). It is, therefore, imperative to understand how radionuclides migrate or are retarded in TCU fractures. Furthermore, understanding themore » migration behavior of radionuclides once they reach the fractured LCA is important for predicting contaminant transport within the regional aquifer. The work presented in this report includes: (1) information on the radionuclide reactive transport through Yucca Flat TCU fractures (likely to be the primary conduit to the LCA), (2) information on the reactive transport of radionuclides through LCA fractures and (3) data needed to calibrate the fracture flow conceptualization of predictive models. The predictive models are used to define the extent of contamination for the Underground Test Area (UGTA) project. Because of the complex nature of reactive transport in fractures, a stepwise approach to identifying mechanisms controlling radionuclide transport was used. In the first set of TCU experiments, radionuclide transport through simple synthetic parallel-plate fractured tuff cores was examined. In the second, naturally fractured TCU cores were used. For the fractured LCA experiments, both parallel-plate and rough-walled fracture transport experiments were conducted to evaluate how fracture topography affects radionuclide transport. Tuff cores were prepared from archived UE-7az and UE-7ba core obtained from the USGS core library, Mercury, Nevada. Carbonate cores were prepared from archived ER-6-1 core, also obtained from the USGS core library, Mercury, Nevada.« less

Quantitative modeling of the potential for contaminant transport from sources associated with underground nuclear testing at Yucca Flat is an important part of the strategy to develop closure plans for the residual contamination. At Yucca Flat, the most significant groundwater resource that could potentially be impacted is the Lower Carbonate Aquifer (LCA), a regionally extensive aquifer that supplies a significant portion of the water demand at the Nevada National Security Site, formerly the Nevada Test Site. Developing and testing reasonable models of groundwater flow in this aquifer is an important precursor to performing subsequent contaminant transport modeling used to forecastmore » contaminant boundaries at Yucca Flat that are used to identify potential use restriction and regulatory boundaries. A model of groundwater flow in the LCA at Yucca Flat has been developed. Uncertainty in this model, as well as other transport and source uncertainties, is being evaluated as part of the Underground Testing Area closure process. Several alternative flow models of the LCA in the Yucca Flat/Climax Mine CAU have been developed. These flow models are used in conjunction with contaminant transport models and source term models and models of contaminant transport from underground nuclear tests conducted in the overlying unsaturated and saturated alluvial and volcanic tuff rocks to evaluate possible contaminant migration in the LCA for the next 1,000 years. Assuming the flow and transport models are found adequate by NNSA/NSO and NDEP, the models will undergo a peer review. If the model is approved by NNSA/NSO and NDEP, it will be used to identify use restriction and regulatory boundaries at the start of the Corrective Action Decision Document Corrective Action Plan (CADD/CAP) phase of the Corrective Action Strategy. These initial boundaries may be revised at the time of the Closure Report phase of the Corrective Action Strategy. (authors)« less

The Underground Test Area (UGTA) Corrective Action Unit (CAU) 97, Yucca Flat/Climax Mine, in the northeast part of the Nevada National Security Site (NNSS) requires environmental corrective action activities to assess contamination resulting from underground nuclear testing. These activities are necessary to comply with the UGTA corrective action strategy (referred to as the UGTA strategy). The corrective action investigation phase of the UGTA strategy requires the development of groundwater flow and contaminant transport models whose purpose is to identify the lateral and vertical extent of contaminant migration over the next 1,000 years. In particular, the goal is to calculate themore » contaminant boundary, which is defined as a probabilistic model-forecast perimeter and a lower hydrostratigraphic unit (HSU) boundary that delineate the possible extent of radionuclide-contaminated groundwater from underground nuclear testing. Because of structural uncertainty in the contaminant boundary, a range of potential contaminant boundaries was forecast, resulting in an ensemble of contaminant boundaries. The contaminant boundary extent is determined by the volume of groundwater that has at least a 5 percent chance of exceeding the radiological standards of the Safe Drinking Water Act (SDWA) (CFR, 2012).« less