[1] A cross-hole tracer test involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients (bromide and pentafluorobenzoate) and a weakly sorbing solute tracer (lithium ion) was conducted in a fractured granite near an underground nuclear test cavity in central Nevada. The test was conducted to (1) test a conceptual radionuclide transport model for the site and (2) obtain transport parameter estimates for predictive modeling. The differences between the responses of the two nonsorbing tracers (when normalized to injection masses) are consistent with a dualporosity transport system in which matrix diffusion is occurring. The large concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggests that diffusion occurs primarily into matrix pores, not simply into stagnant water within the fractures. The relative responses of the tracers at late times suggest that the diffusion-accessible matrix pore volume is possibly limited to only half the total volume of the flow system, implying that the effective retardation factor due to matrix diffusion may be as small as 1.5 for nonsorbing solutes in the system. The lower end of the range of possible sorption K d values deduced from the lithium response is greater than the upper 95% confidence bound of K d values measured in laboratory sorption tests using crushed granite from the site. This result suggests that the practice of using laboratory sorption data in field-scale transport predictions of cation-exchanging radionuclides, such as 137 Cs + and 90 Sr ++ , should be conservative for the SHOAL site.

Plutonium (Pu) is one of the primary actinides of concern for long-term disposal and storage of nuclear waste. Strong sorption of Pu onto colloids of iron oxide, clay, and silica could result in colloid-facilitated transport of this actinide in groundwater systems. However, fundamental data on Pu sorption to colloids is sparse, resulting in large uncertainties in long-term predictions of colloid-facilitated Pu transport. This sparseness of data and the potential to significantly reduce uncertainties in predictive models served as a motivation for this study. The authors investigated the sorption and desorption behaviors of

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.

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither thle United States Government nor any agency thereof, nor amy of their employees, make any warranty, express or impliedl, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any informationl, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Referencie herein t o any specific commercial product, process, olr service by trade name, trademark, manufacturer, olr otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ACKNOWLEDGMENTSMany individuals and organizations are gratefully acknowledged for their financial, technical, and emotional support, which enabled me to complete this dissertation. Financial support was provided by the U. S. Department of Energy through the Yucca Mountain Site Characterization Project at Los Alamos National Laboratory. Special thanks are due to Bruce Robinson of Los Alaimos, who had direct oversight and ultimate responsibility for the work. He supported the effort from inception to completion, including serving as a key dissertation committee member. I also wish to thank my advisor, Eric Nuttall of the University of New Mexico, for his guidance, gentle pushing, and flexibility in accommodating unconventional circumstances in a dissertation project. The participation and contributions of all my dissertation committee members are greatly appreciated.Others at Los Alamos whom I would like to thank include: Joe Ladiish and John Hererra for making the research possible by establishing a new Los Alamos program for Ph.D. candidates; Phil Thullen, Harold Sullivan, Tom Hirons, Julie Canepa, and Wes Myers for advocating and/or approving my participation in the Ph.D. program; Ed Essington, Darlene Linzey, Everett Springer, and Wayne Hansen for providing laboratory space for the tracer experiments as well as laboratory training, orientation, and advice; Brent Newman for his assistance in obtaining the two fractured core samples from the Nevada Test Site; Robb Habbersett for providing training and assistance in the use of the flow cytometer for fluorescent microsphere analyses; Barbara Carlos for conducting a qualitative mineralogical characterization of the fracture surfaces; Ines Triay for providing technical advice and reviews; Chuck Cotter and Robert Lopez for obtaining the J-13 well water for the tracer experiments, Jose Olivares for conducting the inductively coupled plasma-mass spectrometry analyses of cations in the J-13 water; Jay Thorne for machining the aluminum parts for the fracture flow systems; Dave Mann for offering the use of his rock saws and grinders to prepare the fractures for the experiments; Kelly Tapia and Cindy Sandoval f...

Understanding colloid transport in ground water is essential to assessing the migration of colloid‐size contaminants, the facilitation of dissolved contaminant transport by colloids, in situ bioremediation, and the health risks of pathogen contamination in drinking water wells. Much has been learned through laboratory and field‐scale colloid tracer tests, but progress has been hampered by a lack of consistent tracer testing methodology at different scales and fluid velocities. This paper presents laboratory and field tracer tests in fractured rock that use the same type of colloid tracer over an almost three orders‐of‐magnitude range in scale and fluid velocity. Fluorescently‐dyed carboxylate‐modified latex (CML) microspheres (0.19 to 0.98 μm diameter) were used as tracers in (1) a naturally fractured tuff sample, (2) a large block of naturally fractured granite, (3) a fractured granite field site, and (4) another fractured granite/schist field site. In all cases, the mean transport time of the microspheres was shorter than the solutes, regardless of detection limit. In all but the smallest scale test, only a fraction of the injected microsphere mass was recovered, with the smaller microspheres being recovered to a greater extent than the larger microspheres. Using existing theory, we hypothesize that the observed microsphere early arrival was due to volume exclusion and attenuation was due to aggregation and/or settling during transport. In most tests, microspheres were detected using flow cytometry, which proved to be an excellent method of analysis. CML microspheres appear to be useful tracers for fractured rock in forced gradient and short‐term natural gradient tests, but longer residence times may result in small microsphere recoveries.

Abstract. 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 x 10 -•5 and 7.8 x 10 -•9 m 2, 5 and 9 orders of magnitude less than the respective fracture permeabilities. This suggested that the cores could be modeled as dualporosity 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. IntroductionBecause solute transport is controlled by both the physical nature of the flow system and the characteristics of the solutes (velocity, dispersivity, water saturation, sorption behavior, and diffusion coefficient), it is often difficult to determine the im- Separate samples of each core were used for matrix diffusion coefficient, porosity, and permeability measurements. These experiments provided independent estimates of the matrix diffusion coefficients of the different solutes used in the fractured core experiments.The dual-porosity conceptual model accurately described solute transport through the fractured cores and captured the importance of fracture/matrix interactions. The solute breakthrough curves follow a -1.5 slope in log space, which is probably due to diffusive mass transfer between the fracture and the surrounding porous matrix. However, the independent matrix diffusion coefficient measurements for Br-and PFBA in the same rock types were smaller than those calculated from 3547

In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7 m of core of the formerly the ISR-mined Smith Ranch-Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (N 1 wt.%) and clay (N6-17 wt.%) contents yielded higher 234 U/ 238 U activity ratios (1.0-1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374-11,534 ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234 U/ 238 U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining. EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U-O distances of~2.05 Å were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from U_O at 1.79 Å, which indicates that U is partially oxidized. There is no apparent U-S or U-Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite. Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of t...

No abstract