William E. Glassley

Reactive transport modeling of plug-flow reactor experiments: quartz and tuff dissolution at 240°C

Abstract Extension of reactive transport modeling to predict the coupled thermal, hydrological, and chemical evolution of complex geological systems is predicated on successful application of the approach to simulate well-constrained physical experiments. In this study, steady-state effluent concentrations and dissolution/precipitation features associated with crushed quartz and tuff dissolution at 240°C have been determined experimentally using a plug-flow reactor (PFR) and scanning electron microscopy (SEM) techniques, then modeled with the reactive transport simulator GIMRT ( Steefel and Yabusaki, 1996 ) using a linear rate law from transition state theory (TST) . For quartz dissolution, interdependence of the specific surface area (Am) and reaction rate constant (km) predicted from the modeling agrees closely with that obtained from an analytical solution to the reaction–transport equation without diffusion/dispersion, verifying the advection-dominant nature of the PFR experiments. Independently-determined Aqtz and kqtz from the literature are shown to be internally consistent with respect to the model and analytical interdependence, implying appropriateness of the linear TST rate law and adequacy of BET-determined Am for use in modeling PFR experiments. Applications of this integrated approach for monomineralic dissolution include assessment of internal consistency among independent Am and km data, estimation of km from BET-determined Am, and rapid evaluation of alternative rate laws. For tuff dissolution, accurate simulation of the experimental steady-state effluent concentrations (to within 3% for Na, Al and K; to within 15% for Si and Ca) and dearth of alteration phases (

Mineralogical heterogeneity in fractured, porous media and its representation in reactive transport models

Abstract Reactive transport models that simulate processes in porous media have, generally, required abstracted representation of porosity, permeability, and mineralogy. This study compares abstracted, homogeneous representations of porosity and permeability, mineral surface areas and distributions, to discrete distribution representation of these same properties. Discretization was accomplished by high-resolution (ca. 1 μm2) characterization of fractured tuffaceous rock from Yucca Mountain, Nevada, using optical microscopy and X-ray fluorescence spectroscopy. A sample area of 106 μm2 was mapped in detail, and the resulting element and porosity maps were digitized. The domain was decomposed into 12,208 cells that were 8.77 × 10−6 m on a side. Simulations were conducted in which a dilute fluid enters the discretized porous medium at modest flow rates. Simulation results using a discrete mineral distribution point to the conclusion that slow flow rates, in which fluid residence times are on the order of days, provide fluid composition results that are very similar to those obtained from the homogeneous mineral distribution representation. At higher flow rates, where fluid residence times are on the order of hours, contrasts in fluid composition persist throughout the flow domain. The results demonstrate that the fluid composition characteristics in the homogeneous and discrete mineral representations will be similar only when the bulk average contact times for the individual mineral phases along the flow paths are approximately equivalent (within a few percent) for the two cases.

The impact of climate change on vadose zone pore waters and its implication for long-term monitoring

Protecting groundwater is of growing interest as pressure on these resources grows. Recharge of groundwater takes place through the vadose zone, where complex interactions between thermal-hydrological-geochemical processes affect water quality. Monitoring processes in the vadose zone is an important means of evaluating the long-term health of aquifer systems, and has become an integral part of many subsurface engineering efforts. Monitoring such systems, however, may be affected by changes in climate that slowly propagate through vadose zone systems. We describe in this paper the use of NUFT-C, a reactive transport simulator designed to run on a high performance, massively parallel computer, to compare quantitatively the evolution of a deep vadose zone with changes expected from an engineered high-level nuclear waste repository. The results suggest that the impacts from waste emplacement are, in some instances, similar to those that would be observed as a result of climate change, whereas others are distinguishable from evolution of the natural system. Such simulations facilitate design of long-term monitoring programs that take account of these complex effects. The results emphasize the importance of developing long-term baseline measurements and control sites, in order to enhance confidence in interpretations of complexly evolving data sets that will be obtained from multidecade monitoring efforts.

Elemental composition of concentrated brines in subduction zones and the deep continental crust

Abstract It has been well established that fluids played an important part in determining chemical characteristics in many crustal terranes. Studies of fluid inclusions in eclogites have established that brines coexisted with the primary mineral assemblages during their metamorphic crystallization. These brines are currently multiply saturated in halide salts, carbonates, oxides, and sulfides. As a first step in quantitatively bounding the composition of the brines during metamorphism, the equilibrium compositions of the brines at room temperature were computed using the aqueous speciation codes EQ3/6. The results demonstrate that the brines are high density solutions (ca. 1.4 g/cm 3 ) that have ionic strengths of approximately 8 mol, and are approximately 40% dissolved solids, by weight. These are predominately Na- and K-rich brines, with subordinate Ca and Mg. The approximate Na:K:Ca:Mg molar ratios are 4:2:0.5:0.2, but are sensitive to the assumed HCO 3 − concentrations. Charge balance is primarily maintained by the very high Cl concentrations. These brines bear resemblance to brines analyzed from fluid inclusions in metamorphic rocks reported by Roedder (Roedder, E., 1972. Composition of fluid inclusions. US Geol. Surv. Prof. Paper 440JJ, p. 164). Although these fluids have the potential of acting as significant metasomatic agents in subduction zones and deep crustal environments, their impact will be mineralogically discernible only if the fluid release and movement is channelized.

The Impact of Climate Change on the Chemical Composition of Deep Vadose Zone Waters

is a product of those physical and chemical interactions that take place among water, mineral surfaces, and pore Chloride mass balance, and stable (deuterium and 18O) and radiogas in the unsaturated zone (Faybishenko, 2000). If genic (3H, 36Cl) isotope studies of deep vadose zone pore waters have generally concluded that variations in moisture flux can account for the moisture flux varies in the vadose zone as a result of observed variations in abundance of these approximately conservative variation in climate, it is likely that the dissolved load tracers. It can be inferred, on the basis of these observations and in vadose zone pore waters will also vary in sympathetic interpretations, that a climate change record is preserved in these ways. It is the purpose of this paper to evaluate the vadose zone waters. In arid regions where thick ( 100 m) vadose magnitude and extent of this potential sympathetic varizones persist, it has been concluded that this record may extend back ation using published data and simulations of reactive more than 100 000 yr. Consideration of the mechanisms that control transport in the vadose zone. The conclusion is reached reactive transport led to the conclusion that such climate-driven effects that significant changes in the composition of vadose will also be evident as chemical reactions involving dissolution and/ zone pore waters should occur in response to climateor precipitation of mineral phases along the flow pathway. As a result, forced changes in infiltration flux and surface temperathere should also be variations in the concentrations of nonconservative chemical species that correspond to changes in the concentrations ture. As a result, there is a potentially rich archive of of the conservative tracers. Simulations of this reactive transport, in regional-scale climate change data preserved within a regime typical of the arid U.S. Southwest, demonstrate that these large regions of most continental land masses. However, changes can modify pore water chemistry by factors of up to 200%, interpreting empirical pore water chemical composibut the changes take place slowly, requiring thousands of years to tions in terms of past climate change is currently probachieve steady-state conditions. This suggests that a very rich archive lematic because of uncertainties in key parameters that of climate change history is preserved in this type of setting. However, control the chemical kinetics and the hydrologic history. extracting that history is currently hampered by limitations in data and models (e.g., effective mineral reactive surface areas, fluid flow Background pathways, and quantified models of wetted fracture surface in unsaturated, fractured systems). This challenge may be overcome if coordiThat vadose zone pore waters may preserve a nonconnated efforts are undertaken that exploit the power of detailed studies servative chemical signature of climate change is implicit of isotope systematics, microscale rock characterization, and high in two lines of reasoning. One line of reasoning, which performance computing. considers time variation of water flux across the land surface boundary layer, suggests that changes in climatedriven moisture flux are preserved in the vadose zone D the last quarter century, research that as slowly migrating variations in concentration of the has examined the distribution of “conservative” solute load. The second line of reasoning, which considtracers in vadose zone pore waters has concluded that ers the kinetics of dissolution and precipitation reactions this geological setting preserves a record of local moisin the vadose zone, implies that the reactions that are ture flux (Allison et al., 1985; Barnes and Allison, 1988; responsible for the evolution of pore water chemical Cook et al., 1989, 1994; Dettinger, 1989; Scanlon, 1991; characteristics may be responsive to changes in the land Walker et al., 1991). In arid environments, where the surface temperature and infiltration flux boundary convadose zone can be hundreds of meters thick, and infilditions. Both of these considerations are discussed below. tration fluxes are low, it has been inferred that this record may extend as much as 100 000 yr into the past Variability of Moisture Flux Through the Vadose Zone (Tyler et al., 1996). If this conclusion is correct, the potential exists to reconstruct moisture flux records on The analysis of diverse chemical tracers in pore waters continental land masses in many parts of the world. If has allowed development of conceptual models useful moisture flux can be treated as a proxy for changes in for quantifying the temporal record of water movement precipitation, then the possibility exists that long-term rates and residence times in the vadose zone. Chloride climate records may exist in some vadose zone settings. was among the first tracers to support such an effort. The chemical composition of vadose zone pore water As the chemical characteristics of Cl-bearing aerosols became better understood (e.g., Winchester and Duce, W.E. Glassley, J.J. Nitao, and C.W. Grant, Lawrence Livermore Na1967), and deposition of chloride on land surfaces was tional Laboratory, Livermore CA 94550. Received 25 Jan. 2002. *Corconceptualized, use of chloride as a tracer for water responding author (glassley1@llnl.gov). movement in vadose zones became relatively common (e.g., Allison et al., 1985; Barnes and Allison, 1988; Published in Vadose Zone Journal 1:3–13 (2002).

Thermal-mechanical-hydrological-chemical responses in the single heater test at the ESF

The Single Heater Test (SHT) is conducted in the Exploratory Studies Facility (ESF) to study the thermal-mechanical responses of the rock mass. A set of boreholes were drilled in the test region for conducting a scoping test of the coupled thermal-mechanical- hydrological-chemical (TMHC) processes. The holes for the TMHC tests include electrical resistivity tomography (ERT), neutron logging/temperature, hydrological, and optical multiple point borehole extensometers. A 4-kW heater was installed in the heater hole, and was energized on August 26, 1996. Some observed movements of the water around the heater are associated with a possible dry-out region near the heater. The water that has been moved is more dilute than the in situ ground water, except for the concentration of Ca. This indicates that fractures are the major water pathways, and the displaced water may have reached an equilibrium with carbonate minerals on the fracture surfaces. No mechanical-hydrological coupling has been observed. The tests are on-going, and more data will be collected and analyzed.

Modeled Near-Field Environment Porosity Modification due to Coupled Thermohydrologic and Geochemical Processes

Heat deposited by waste packages in nuclear waste repositories can modify rock properties by instigating mineral dissolution and precipitation along hydrothermal flow pathways. Modeling this reactive transport requires coupling fluid flow to permeability changes resulting from dissolution and precipitation. Modification of the NUFT thermohydrologic (TH) code package to account for this coupling in a simplified geochemical system has been used to model the time- dependent change in porosity, permeability, matrix and fracture saturation, and temperature in the vicinity of waste-emplacement drifts, using conditions anticipated for the potential Yucca Mountain repository. The results show, within a few hundred years, dramatic porosity reduction approximately 10 m above emplacement drifts. Most of this reduction is attributed to deposition of solute load at the boiling front, although some of it also results from decreasing temperature along the flow path. The actual distribution of the nearly sealed region is sensitive to the time- dependent characteristics of the thermal load imposed on the environment and suggests that the geometry of the sealed region can be engineered by managing the waste-emplacement strategy.

Field-Based Tests of Geochemical Modeling Codes: New Zealand Hydrothermal Systems

Hydrothermal systems in the Taupo Volcanic Zone, North Island, New Zealand are being used as field-based modeling exercises for the EQ3/6 geochemical modeling code package. Comparisons of the observed state and evolution of the hydrothermal systems with predictions of fluid-solid equilibria made using geochemical modeling codes will determine how the codes can be used to predict the chemical and mineralogical response of the environment to nuclear waste emplacement. Field-based exercises allow us to test the models on time scales unattainable in the laboratory. Preliminary predictions of mineral assemblages in equilibrium with fluids sampled from wells in the Wairakei and Kawerau geothermal field suggest that affinity-temperature diagrams must be used in conjunction with EQ6 to minimize the effect of uncertainties in thermodynamic and kinetic data on code predictions.

Using Arrested Solid-Solid Multiphase Reactions in Geological Materials to Deduce the Rate of Crustal Uplift

The history geological terrains experience can be traced as a series of temperature and pressure changes. Each change drives the system toward a new state of thermodynamic equilibrium. The resultant overprinted rock fabrics, textures and chemical heterogeneities can be difficult to interpret. However, if carefully chosen, features from the scale of kilometers to nanometers can be used to reconstruct the history of mountain systems. Uplift of the Sri Lankan Central Highlands was rapid enough to preserve well-developed symplectite textures, some of which represent arrested solid-state diffusion-controlled reactions of garnet + O2 to form orthopyroxene + plagioclase + magnetite, as the rocks were exhumed from over 30 km in the earth’s crust. Our objective has been to determine the reaction mechanisms responsible for symplectite development, and to establish the time interval over which these reactions occurred, to constrain the rate of mountain uplift. Considering that the most rapid mechanism is solid st te grain-boundary diffusion of oxygen, the reaction time can be constrained by bounding the rate of oxygen supply to the reaction site. The solid state grain boundary diffusion rate of oxygen has been inferred to be ca. 10−14 m2-sec (Farver and Yund, 1991), but is sensitive to inferred grain boundary width. The range of rates thus determined allows the distinction between rapid uplift similar to that of the Himalayan Mountains, and the slow and progressive erosion of a less dramatic terrain. Further constraints on diffusion control and energetic relationships are determined from crystallographic relationships between the reactant and product phases, and submicron scale microstructures.

Geothermal areas as analogues to chemical processes in the near-field and altered zone of the potential Yucca Mountain, Nevada repository

The need to bound system performance of the potential Yucca Mountain repository for thousands of years after emplacement of high-level nuclear waste requires the use of computer codes. The use of such codes to produce reliable bounds over such long time periods must be tested using long-lived natural and historical systems as analogues. The geothermal systems of the Taupo Volcanic Zone (TVZ) in New Zealand were selected as the site most amenable to study. The rocks of the TVZ are silicic volcanics that are similar in composition to Yucca Mountain. The area has been subjected to temperatures of 25 to 300 C which have produced a variety of secondary minerals similar to those anticipated at Yucca Mountain. The availability of rocks, fluids and fabricated materials for sampling is excellent because of widespread exploitation of the systems for geothermal power. Current work has focused on testing the ability of the EQ3/6 code and thermodynamic data base to describe mineral-fluid relations at elevated temperatures. Welfare starting long-term dissolution/corrosion tests of rocks, minerals and manufactured materials in natural thermal features in order to compare laboratory rates with field-derived rates. Available field data on rates of silica precipitation from heated fluids have been analyzed and compared to laboratory rates. New sets of precipitation experiments are being planned. The microbially influenced degradation of concrete in the Broadlands-Ohaaki geothermal field is being characterized. The authors will continue to work on these projects in FY 1996 and expand to include the study of naturally occurring uranium and thorium series radionuclides, as a prelude to studying radionuclide migration in heated silicic volcanic rocks. 32 refs.