Kay H. Birdsell

The effects of barometric pumping on contaminant transport

Variation in the ambient atmospheric pressure results in subsurface motion of air in porous and fractured earth materials. This is known as barometric pumping. We analyze this phenomenon for unfractured media and demonstrate that if hydrodynamic dispersivity is included, barometric pumping can significantly decrease the retention time of volatile subsurface contaminants. Formulae are derived which give analytically the dependence of the effect on the amplitude and frequency of the barometric pump as well as on the material properties. In addition, numerical modeling results using a method which carefully avoids spurious numerical dispersion are presented. Field data will appear to follow a standard diffusive transport model; but in the systems in which barometric pumping is significant, the value for D will need to be greater than that derived from isobaric tests. In addition to enhancing the diffusion, pumping also sweeps out pore gases near the surface and thus, reduces the distance a subsurface contaminant must diffuse before it mixes with the atmosphere. These barometric pumping mechanisms will be enhanced by the presence of fractures, which permit pressure variations to propagate deep into the ground. Because barometric pumping affects the rate of contaminant transport from subsurface, this process may play an important role in the environmental remediation of volatile organic chemicals in the vadose zone.

The influence of coatings and fills on flow in fractured, unsaturated tuff porous media systems

A numerical study of a single fracture embedded in a porous matrix was performed to investigate the role of fracture coatings and fills on water movement in permeable, fractured porous media. The variables considered were conductivity and continuity of fracture coatings; location, length, and conductivity of fracture fills; combinations of fills and coatings; initial matrix saturation; and inflow boundary conditions. Results from the simulations indicate that in low-saturation, high-capillarity tuff systems, the conditions under which fractures act as rapid flow paths are limited. These conditions include a continuous coating with conductivity several orders of magnitude lower than that of the neighboring matrix, and large inflow rates. However, as initial matrix saturation increases, the amount of fracture flow also increases. Discontinuities in coatings substantially reduce their effectiveness in preventing matrix imbibition. The presence of any coating, however, does produce increased infiltration depths. Fills appear to be effective barriers to fracture flow.

Groundwater flow and radionuclide transport calculations for a performance assessment of a low-level waste site☆

Abstract Predictions of subsurface radionuclide transport are used to support the groundwater pathway analysis for the performance assessment of the low-level, solid radioactive waste site at Los Alamos National Laboratory. Detailed process-based models rather than higher-level performance-assessment models are used to perform the transport calculations. The deterministic analyses predict the fate of the waste from its source, through the vadose zone, into the saturated zone and, finally, the potential dose to humans at the accessible environment. The calculations are run with the finite-element code FEHM, which simulates fluid flow, heat transport, and reactive, contaminant transport through porous and fractured media. The modeling approach for this study couples realistic source-term models with an unsaturated-zone flow and transport model, which is then linked to the saturated-zone flow and transport model. The three-dimensional unsaturated-zone flow and transport model describes the complex hydrology associated with the mesa-top and volcanic geology of the site. The continued migration of nuclides into the main aquifer is calculated using a three-dimensional, steady-flow, saturated-zone model that maintains the spatial and temporal distribution of nuclide flux from the vadose zone. Preliminary results for the aquifer-related dose assessments show that doses are well below relevant performance objectives for low-level waste sites. A general screening technique that compares the nuclides half-life to its unsaturated-zone travel time is described. This technique helps to decrease the number of transport calculations required at a site. In this case, over half the nuclides were eliminated from further consideration through this screening.

Modeling tracer diffusion in fractured and unfractured, unsaturated, porous media

Abstract A proposed tracer diffusion test for the Exploratory Shaft Facility at Yucca Mountain, NV, is modeled. For the proposed test, a solution containing conservative tracers will be introduced into a borehole in the geologic medium of interest. The tracers will diffuse and advect from the saturated source region into the unsaturated matrix in the surrounding tuff. After some time, the borehole is to be overcored, and tracer concentrations in the fluid will be measured in the core as a function of distance from emplacement. The data will be used to evaluate diffusive behavior and to derive effective diffusion coefficients for the tracers in the specific tuff. Numerical simulations are used to study the effects of effective diffusion coefficient, porosity, saturation, and fracturing on tracer transport. Results are reported for numerical simulations of tests in the Topopah Spring Member and the Tuff of Calico Hills, which have significantly different porosities and saturations. The simulations make the following predictions: The spread of tracer during the test will be sensitive to the effective diffusion coefficient of the tracer. Tracer will diffuse farther in the Topopah Spring Member than in the Tuff of Calico Hills because of the formers lower porosity and saturation. Tracer transport by advection into the Topopah Spring Member will be greater than that into the Tuff of Calico Hills because of capillary effects. While advection will be a significant mechanism for tracer penetration into the Topopah Spring tuff, it will be less significant for tracer penetration into the Calico Hills tuff. The proximity of a single vertical fracture to the source region determines its effects on tracer transport, especially if the fracture diverts fluid flowing from the source region into the matrix.

Comparison of experimental methods for estimating matrix diffusion coefficients for contaminant transport modeling

Diffusion cell and diffusion wafer experiments were conducted to compare methods for estimating effective matrix diffusion coefficients in rock core samples from Pahute Mesa at the Nevada Nuclear Security Site (NNSS). A diffusion wafer method, in which a solute diffuses out of a rock matrix that is pre-saturated with water containing the solute, is presented as a simpler alternative to the traditional through-diffusion (diffusion cell) method. Both methods yielded estimates of effective matrix diffusion coefficients that were within the range of values previously reported for NNSS volcanic rocks. The difference between the estimates of the two methods ranged from 14 to 30%, and there was no systematic high or low bias of one method relative to the other. From a transport modeling perspective, these differences are relatively minor when one considers that other variables (e.g., fracture apertures, fracture spacings) influence matrix diffusion to a greater degree and tend to have greater uncertainty than effective matrix diffusion coefficients. For the same relative random errors in concentration measurements, the diffusion cell method yields effective matrix diffusion coefficient estimates that have less uncertainty than the wafer method. However, the wafer method is easier and less costly to implement and yields estimates more quickly, thus allowing a greater number of samples to be analyzed for the same cost and time. Given the relatively good agreement between the methods, and the lack of any apparent bias between the methods, the diffusion wafer method appears to offer advantages over the diffusion cell method if better statistical representation of a given set of rock samples is desired.

Discrete Fracture Network Modeling and Simulation of Subsurface Transport for the Topopah Springs and Lava Flow Aquifers at Pahute Mesa, FY 15 Progress Report

This progress report for fiscal year 2015 (FY15) describes the development of discrete fracture network (DFN) models for Pahute Mesa. DFN models will be used to upscale parameters for simulations of subsurface flow and transport in fractured media in Pahute Mesa. The research focuses on modeling of groundwater flow and contaminant transport using DFNs generated according to fracture characteristics observed in the Topopah Spring Aquifer (TSA) and the Lava Flow Aquifer (LFA). This work will improve the representation of radionuclide transport processes in large-scale, regulatory-focused models with a view to reduce pessimistic bounding approximations and provide more realistic contaminant boundary calculations that can be used to describe the future extent of contaminated groundwater. Our goal is to refine a modeling approach that can translate parameters to larger-scale models that account for local-scale flow and transport processes, which tend to attenuate migration.

Natural and Engineered Barriers in a Romanian Disposal Site for Low and Intermediate Level Waste

The operational waste generated by the Cernavoda Nuclear Power Plant will be disposed in a near-surface facility. The low and intermediate level wastes, containing particularly large concentrations of C-14 and H-3, are treated and conditioned in steel drums, which will be placed in the disposal cells and then immobilized in concrete. The Saligny site has been proposed for LIL waste disposal. Geologically, the main components of this site are the quaternary loess, the Precambrian and pre-quaternary clays, and the Eocene and Barremian limestones. Hydrologically, the site can be divided into a vadose zone down to 45–50m and three distinct aquifers, two of them in the limestone beds and the third into the lenses of sand and limestone existing in the pre-quaternary clay layer. Preliminary performance assessments, presented in this paper, indicate that the geologic layers are efficient natural barriers against water flow and radionuclide migration from the vadose zone to the Barremian aquifer. The semi-arid climate and the low precipitation rate prevent contaminant transport from the disposal site to the Eocene aquifer. FEHM simulations of transient groundwater flow showed that seasonal variations influence the moisture content profile in the top of the vadose zone, but the influence over the long term is not significant for contaminant transport. The Danube River level variations control water movement in the Barremian aquifer, especially in the upper part where the limestone is highly fractured and water moves toward the river when its level is low and toward the site when the river level is high. The disposal concept tries to combine the natural and engineered barriers in order to ensure the safety of the environment and population. Therefore, the concrete filling the disposal cells surrounds the waste with a medium that facilitates C-14 retention by precipitation, thus reducing the C-14 releases in the atmosphere and geosphere.Copyright

Simulations of Groundwater Flow and Radionuclide Transport in the Vadose and Saturated Zones beneath Area G, Los Alamos National Laboratory

Numerical simulations are used to predict the migration of radionuclides from the disposal units at Material Disposal Area G through the vadose zone and into the main aquifer in support of a radiological performance assessment and composite analysis for the site. The calculations are performed with the finite element code, FEHM. The transport of nuclides through the vadose zone is computed using a three-dimensional model that describes the complex mesa top geology of the site. The model incorporates the positions and inventories of thirty-four disposal pits and four shaft fields located at Area G as well as those of proposed future pits and shafts. Only three nuclides, C-14, Tc-99, and I-129, proved to be of concern for the groundwater pathway over a 10,000-year period. The spatial and temporal flux of these three nuclides from the vadose zone is applied as a source term for the three-dimensional saturated zone model of the main aquifer that underlies the site. The movement of these nuclides in the aquifer to a downstream location is calculated, and aquifer concentrations are converted to doses. Doses related to aquifer concentrations are six or more orders of magnitude lower than allowable Department of Energy performance objectives for low-level radioactive waste sites. Numerical studies were used to better understand vadose-zone flow through the dry mesa-top environment at Area G. These studies helped define the final model used to model flow and transport through the vadose zone. The study of transient percolation indicates that a steady flow vadose-zone model is adequate for computing contaminant flux to the aquifer. The fracture flow studies and the investigation of the effect of basalt and pumice properties helped us define appropriate hydrologic properties for the modeling. Finally, the evaporation study helped to justify low infiltration rates.