Paul W. Reimus

Using multiple experimental methods to determine fracture/matrix interactions and dispersion of nonreactive solutes in saturated volcanic tuff.

The objective of this research was to investigate the effects of matrix diffusion on solute transport in fractured volcanic tuff. Two tuff cores were studied, one with a matrix porosity of 0.27 and the other with a porosity of 0.14. The matrix permeabilities of the cores were 4.7×10−15 and 7.8×10−19 m2, 5 and 9 orders of magnitude less than the respective fracture permeabilities. This suggested that the cores could be modeled as dual-porosity systems with no flow in the matrix but significant solute storage capacity. Two types of tracer tests were conducted in each fractured core: (1) iodide was injected in separate experiments at different flow rates and (2) two tracers of different matrix diffusion coefficients (bromide and pentafluorobenzoate (PFBA)) were injected in another test. A difference in the maximum concentrations of the solutes and the extended tailing of the breakthrough curves were assumed to be indicative of diffusive mass transfer between the fracture and the porous matrix of the cores. Interpreting the results from both methods allowed the identification of matrix diffusion and dispersion effects within the fracture by simultaneously fitting the data sets (with known constraints) using a relatively simple conceptual model. Estimates of mass transfer coefficients for the fractured cores were also obtained.

Testing and parameterizing a conceptual solute transport model in saturated fractured tuff using sorbing and nonsorbing tracers in cross-hole tracer tests

Two cross-hole tracer tests involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients (bromide and pentafluorobenzoate) and one weakly sorbing solute tracer (lithium ion) were conducted in two different intervals at the C-wells complex near the site of a potential high-level nuclear waste repository at Yucca Mountain, NV. The tests were conducted to (1) test a conceptual radionuclide transport model for saturated, fractured tuffs near Yucca Mountain and (2) obtain transport parameter estimates for predictive modeling of radionuclide transport. The differences between the responses of the two nonsorbing tracers and the sorbing tracer (when normalized to injection masses) were consistent with a dual-porosity transport system in which matrix diffusion was occurring. The concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggested that diffusion occurred primarily into matrix pores, not simply into stagnant water within the fractures. The K(d) values deduced from the lithium responses were generally larger than K(d) values measured in laboratory batch sorption tests using crushed C-wells cores. This result supports the use of laboratory-derived K(d) values for predicting sorbing species transport at the site, as the laboratory K(d) values would result in underprediction of sorption and hence conservative transport predictions. The tracer tests also provided estimates of effective flow porosity and longitudinal dispersivity at the site. The tests clearly demonstrated the advantages of using multiple tracers of different physical and chemical characteristics to distinguish between alternative conceptual transport models and to obtain transport parameter estimates that are better constrained than can be obtained using only a single tracer or using multiple nonsorbing tracers without a sorbing tracer.

Upscaling of reactive mass transport in fractured rocks with multimodal reactive mineral facies.

[1] This paper presents a methodology for upscaling matrix‐material transport parameters in fractured‐flow dominated systems with multimodal reactive mineral facies. The upscaling method provides a theoretical and practical link between controlled experimental results at the laboratory/bench scale and multikilometer field scales at which contaminant remediation and risk assessment are actually conducted. As sorption reactions in matrix are in part determined by mineral properties, a new conceptual model is developed to reflect the hierarchical structure of reactive mineral facies at the microform, mesoform, and macroform scales. The conceptual model of hierarchical reactive matrix mineral facies is integrated with a dual‐porosity model for simulating diffusion of solutes out of fractures and sorption onto the matrix minerals. By assuming that sorption reactions primarily occur in the rock matrix, we develop a multimodal spatial random function for characterizing both the tortuosity (physical heterogeneity) and sorption coefficient (chemical heterogeneity) at different scales in the rock matrix. The effective tortuosity at the field scale is derived by volume averaging of mass transfer coefficient for a conservative species. Subsequently, using a sorbing species (e.g., uranium), we derive the equations for upscaling the sorption coefficients in a saturated, fractured‐rock system for field‐scale simulations. The effective field‐scale tortuosity and sorption coefficient are related to their mean, variance, integral scale, and domain size along a pathway through a three‐ dimensional flow field. The upscaled values increase with the integral scale and are larger than their geometric mean. Simulations conducted with upscaled sorption coefficients and tortuousities are compared very well with high‐resolution Monte Carlo simulations capturing the parameter spatial variations. Results of this study can be extended to explore scaledependenceofotherimportant transportparametersforfractured‐rocksolutetransport. Citation: Deng, H., Z. Dai, A. Wolfsberg, Z. Lu, M. Ye, and P. Reimus (2010), Upscaling of reactive mass transport in fractured rocks with multimodal reactive mineral facies, Water Resour. Res., 46, W06501, doi:10.1029/2009WR008363.

Lithium sorption to Yucca Mountain tuffs

The Li ion has been used as a reactive tracer in field tests performed in the saturated and unsaturated-zone in volcanic tuffs at Yucca Mountain, Nevada. Lithium sorbs weakly by cation exchange and permits field-scale testing of laboratory-based predictions of reactive-solute transport. A series of laboratory studies show that Li sorption is nonlinear and varies with lithology in the different horizons of the tuff. In particular, both Li sorption and Li-specific cation-exchange capacity vary as functions of tuff mineralogy, and can be predicted given quantitative X-ray diffraction data. These results indicate that Li sorption is dominated by clay and zeolite minerals, and that sorption coefficients can be predicted given mineralogic analysis results.

The saturated zone at Yucca Mountain: An overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

The US Department of Energy is pursuing Yucca Mountain, Nevada, for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste, if the repository is able to meet applicable radiation protection standards established by the US Nuclear Regulatory Commission and the US Environmental Protection Agency (EPA). Effective performance of such a repository would rely on a number of natural and engineered barriers to isolate radioactive waste from the accessible environment. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the potential repository. The saturated zone (SZ) system is expected to act as a natural barrier to this possible movement of radionuclides both by delaying their transport and by reducing their concentration before they reach the accessible environment. Information obtained from Yucca Mountain Site Characterization Project activities is used to estimate groundwater flow rates through the site-scale SZ flow and transport model area and to constrain general conceptual models of groundwater flow in the site-scale area. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the scale required for total system performance assessment of the site. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale but is more detailed because more data are available at the site-scale level. The mathematical basis of the site-scale model and the associated numerical approaches are designed to assist in quantifying the uncertainty in the permeability of rocks in the geologic framework model and to represent accurately the flow and transport processes included in the site-scale conceptual model. Confidence in the results of the mathematical model was obtained by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those calculated by the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and those independently inferred from water-chemistry data. The general approach of the site-scale SZ flow and transport model analysis is to calculate unit breakthrough curves for radionuclides at the interface between the SZ and the biosphere using the three-dimensional site-scale SZ flow and transport model. Uncertainties are explicitly incorporated into the site-scale SZ flow and transport abstractions through key parameters and conceptual models.

EFFECTS OF MINERALOGY, EXCHANGE CAPACITY, SURFACE AREA AND GRAIN SIZE ON LITHIUM SORPTION TO ZEOLITIC ALLUVIUM NEAR YUCCA MOUNTAIN, NEVADA

The resistance to acidic and sulfate attack of Portland-pozzolan cement containing 35 wt.% of zeolite was compared with that of unamended Portland cement. Mortar specimens kept in 0.5% and 1.0% HCl solution, 5% Na2SO4 solution, and in reference water for 365 and 720 days were tested using a set of physical-mechanical and chemical techniques. The ability of mortars containing zeolitic cements with 15 to 50 wt.% of zeolite to protect steel against corrosion was verified by a potentiodynamic method. Mortar with zeolitic cement performs better when exposed to 1% HCl solution due to the presence of a finer pore matrix, a hydrate phase poorer in CaO-containing hydration products with lower leachability, and the high resistance of zeolite material itself to acidic attack, compared with Portland cement and siliceous sand. The improved sulfate resistance of the mortar with zeolitic cement is caused by the decreased C3A in the cement blend in comparison with that in Portland cement, a reduction in SO3 binding into the cement paste and decreased amount of CaO-containing hydration products capable of reacting with a sulfate solution forming voluminous reaction products, and consequent crack propagation, large expansion and structural disintegration. Passivation of steel in mortars with blends of Portland cement to zeolite percentage ratios of 85/15, 75/25 and 65/35 by weight is comparable to that of Portland cement mortar. This is particularly important because the mortar with zeolitic cement exhibits late strengths similar to that of Portland cement mortar. This confirms that zeolitic cement can replace Portland cement in many applications with the advantage of higher resistance to acidic and sulfate attack.The Li + ion is used frequently as an environmentally acceptable surrogate for sorbing radionuclides in field tracer tests, and experiments using Li are an important part of assessing the potential transport of radionuclides in saturated alluvium south of Yucca Mountain, Nevada, the site of a proposed nuclear waste repository. Equilibrium partition constants (Li + K d s) were measured using batch studies incorporating a wide range of Li + concentrations and two different grain-size fractions of alluvium samples from multiple depth intervals in two wells. Cation exchange capacity, surface area, bulk mineralogy from quantitative X-ray powder diffraction, and trace Mn- and Fe-oxyhydroxide mineralogy from extractive studies were evaluated as predictors for linearized Li + K d values (K lin ) in the alluvium. Many of the predictor variables are correlated with each other and this was considered in the analysis. Linearized K d values were consistently higher for fine particle-size fractions than for coarse fractions. Single and multivariate linear regression analyses indicated that the clinoptilolite + smectite content, taken together as a combined variable, was the best predictor for Li + sorption in the alluvium, although clinoptilolite content was clearly a better predictor when the two variables were considered separately in simple linear regressions. Even so, Li + K lin predictions based on clinoptilolite and smectite abundance were accurate only to within about ±100%. This uncertainty suggests that there is either a high inherent variability in Li + K lin values or that additional alluvium characteristics not measured or evaluated here may play an important role in simple Li + cation exchange in the alluvium.

Transport of a reactive tracer in saturated alluvium described using a three-component cation-exchange model

A weakly sorbing cation, lithium, will be used as a reactive tracer in upcoming field tracer tests in the saturated alluvium south of Yucca Mountain, Nevada. One objective of the field tests is to determine how well field-scale reactive transport can be predicted using transport parameters derived from laboratory experiments. This paper describes several laboratory lithium batch sorption and column transport experiments that were conducted using ground water and alluvium obtained from the site of the planned field tests. In the batch experiments, isotherms were determined over 2.5 orders of magnitude of lithium concentrations, corresponding to the range expected in the field tests. In addition to measuring equilibrium lithium concentrations, concentrations of other cations, namely Na(+), K(+), and Ca(2+), were measured in the batch tests to determine Li(+)-exchangeable equilibria. This information was used in conjunction with alluvium cation exchange capacity measurements to parameterize a three-component cation-exchange model (EQUIL) that describes lithium sorption in the alluvium system. This model was then applied to interpret the transport behavior of lithium ion in saturated alluvium column tests conducted at three different lithium bromide injection concentrations. The concentrations were selected such that lithium ion either dominated, accounted for a little over half, or accounted for only a small fraction of the total cation equivalents in the injection solution. Although tracer breakthrough curves differed significantly under each of these conditions, with highly asymmetric responses occurring at the highest injection concentrations, the three-component cation-exchange model reproduced the observed transport behavior of lithium and the other cations in each case with a similar set of model parameters. In contrast, a linear K(d)-type sorption model could only match the lithium responses at the lowest injection concentration. The three-component model will be used to interpret the field tests, with the expectation that it will help refine estimates of effective flow porosity, particularly if the lithium response curves are asymmetric.

Determining the random time step in a constant spatial step particle tracking algorithm

In some cases, the accuracy and efficiency of a particle tracking model may be greatly enhanced by determining the time for a particle to travel a specified distance rather than the distance traveled during some time interval. For instance, it may be desirable to know when a particle reaches a boundary or interface or a location where the system properties change. In this work, we derive an analytical expression in the form of a probability function describing the distribution of random times necessary for a particle to diffusively travel a specified distance. Upon substitution of the specified distance and a random number for the probability, the expression may be solved for the necessary time. Comparison of the results from this equation with an empirical expression determined by James and Chrysikopoulos (Chem. Eng. Sci. 56(23)(2001)6535) shows excellent agreement. Recommendations for implementing the analytical solution into particle tracking algorithms with or without a deterministic velocity component are included. This general equation is amenable to use in modeling both fractured and porous media systems.

Isotopic and Geochemical Tracers for U(VI) Reduction and U Mobility at an in Situ Recovery U Mine

In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual U(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers (238)U/(235)U (δ(238)U), (234)U/(238)U activity ratio, and (34)S/(32)S (δ(34)S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ(238)U in Rosita groundwater varies from +0.61‰ to -2.49‰, with a trend toward lower δ(238)U in downgradient wells. The concurrent decrease in U(VI) concentration and δ(238)U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U(VI) reduction. Additionally, characteristic (234)U/(238)U activity ratio and δ(34)S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U(VI) at Rosita and other similar ISR mining sites.

Analysis of tracer responses in the BULLION Forced-Gradient Experiment at Pahute Mesa, Nevada

This report presents an analysis of the tracer data from the BULLION forced-gradient experiment (FGE) conducted on Pahute Mesa at the Nevada Test Site from June 2, 1997 through August 28, 1997, for the Underground Test Area (UGTA) Program. It also serves to document the polystyrene microsphere data from the FGE. The FGE involved the injection of solute and colloid tracers into wells ER-20-6 No. 1 and ER-20-6 No. 2 while ER-20-6 No. 3 was pumped at approximately 116 gallons per minute (gpm). The experimental configuration and test design are described briefly in this report; more details are provided elsewhere (IT, 1996, 1997, 1998). The tracer responses in the various wells yielded valuable information about transport processes such as longitudinal dispersion, matrix diffusion and colloid transport in the hydrogeologic system in the vicinity of the BULLION nuclear test cavity. Parameter values describing these processes are derived from the semi-analytical model interpretations presented in this report. A companion report (IT, 1998) presents more detailed numerical modeling interpretations of the solute tracer responses.