petrographic and trace element analysis of uranium rich tufa calcite middle miocene barstow formation california usa an integrated petrographic and spectroscopic x ray diffraction phosphor imaging and synchrotron x ray absorption spectroscopy study of tufas from the miocene barstow formation california relates sample morphology mineralogy and geochemical composition the tufas composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters have textures similar to those of microbially mediated terrestrial stromatolites and travertines the tufas have elevated concentrations of a number of trace elements including mn fe sr and u synchrotron x ray fluorescence analyses show that u concentrations can exceed 500 p p m x ray absorption spectroscopy indicates that the u in these samples is incorporated as u iv it is suggested here that alkaline lake waters had a high u ca ratio and tufa calcite formed where groundwater or possibly epithermal springs brought in ca and trace elements such as sr the rapid and possibly microbially mediated precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine scale inclusions

a study of flow wetted surface area in a single fracture as a function of its hydraulic conductivity distribution the contact area between flowing water and rock the flow wetted surface fws is a main factor controlling the rock matrix diffusion and sorption of flowing solute in a rock fracture flow channeli

sorbing tracer experiments in a crystalline rock fracture at aspo sweden 2 transport model and effective parameter estimation 1 transport and retention of sorbing tracers in a single altered crystalline rock fracture on a 5 m scale is investigated we evaluate the results of a comprehensive field study referred to as

sorbing tracer experiments in a crystalline rock fracture at aspo sweden 3 effect of microscale heterogeneity we study the impact of microscale 10 3 10 2 m heterogeneity in material and structural properties on sorbing tracer transport in a single crystalline fracture of the true 1 tests mesoscale

measurement and analysis of dissolution patterns in rock fractures 1 nuclear magnetic resonance imaging nmri is applied to noninvasively measure flow and dissolution patterns in natural rough walled water saturated halite fractures three dimensional images of water density and flow velocity acquired with nmri allow quantification of the developing fracture morphology and flow patterns the flow patterns are correlated strongly to the local apertures and the large scale wall roughness the correlations of the dissolution patterns to the fracture morphology flow patterns and mineralogical composition of the rock matrix are a function of the overall dimensionless damkohler number at high damkohler numbers the dissolution patterns are dominated by the flow structure in addition at high damkohler numbers buoyancy stratified flow becomes important in such cases the dissolution patterns also depend on the orientation and elevation of the fracture walls resulting in preferential upward dissolution at low damkohler numbers the dissolution patterns depend mainly on the mineralogical composition of the rock matrix these findings suggest that coupled flow and dissolution processes are much more complex and unpredictable than commonly assumed even under simplified conditions

matrix diffusion rates in fractured volcanic rocks at the nevada test site evidence for a dominant influence of effective fracture apertures 1 solute matrix diffusion in saturated fractured volcanic rock at the nevada test site was evaluated from field tracer tests conducted at two different locations and from laboratory scale transport experiments using core samples from the two locations the laboratory tests included 15 separate tracer transport experiments conducted in 8 fractured cores 4 from each location and 17 diffusion cell experiments conducted in matrix material adjacent to the fractures all of the experiments featured two nonsorbing tracers with free diffusion coefficients differing by a factor of approximately 3 to allow the effects of diffusion to be distinguished from the effects of advection dispersion and source term tailing in the experiments when considering all the laboratory and field transport experiments collectively the lumped mass transfer parameter that characterizes matrix diffusion rates sec 1 2 where m is the matrix porosity b is the effective fracture half aperture and dm is the matrix diffusion coefficient appears to decrease as time and length scales of observation increase however these decreasing trends largely disappear when the laboratory and field data are considered separately and there is little difference in dm values measured at different timescales in laboratory experiments using the same rocks also the overall variability in m and dm in the laboratory experiments is not large enough to account for the order of magnitude smaller field scale values of compared to lab scale values we conclude that experimentally observed trends of versus time or distance scale in saturated fractured rocks at the nevada test site are dominated by differences in effective fracture apertures in the various experiments with a tendency toward larger apertures in the field experiments these results underscore the importance of acquiring a better understanding of the factors that control effective fracture apertures as a function of distance scale in fractured media if values of from laboratory and field tracer experiments are to be reliably extrapolated to time and distance scales in risk assessment models

influence of aperture variability on dissolutional growth of fissures in karst formations the influence of aperture variability on dissolutional growth of fissures is investigated on the basis of two dimensional numerical simulations the logarithm of the fissure aperture before dissolution begins is modeled as a gaussian stationary isotropic random field the initial phase of dissolutional growth is studied up to the time when turbulent flow first occurs at a point within the fissure the breakthrough time the breakthrough time in variable aperture fissures is smaller than that in uniform fissures and decreases as the coefficient of variation of the aperture field increases in comparing uniform and variable aperture fissures in limestone the breakthrough time with 0 1 is about a factor of 2 smaller than that in a uniform fissure the breakthrough time is reduced by about an order of magnitude with 2 0 the mechanism leading to reduced breakthrough times is the focusing of flow into preferential flow channels which are enlarged at a faster rate than the surrounding regions of slower flow dissolution channels are narrower and more tortuous as increases investigations of the influence of reaction rate reveal that the influence of aperture variability is more pronounced in rapidly dissolving rock in uniform fissures in rapidly dissolving minerals breakthrough times are very long since water becomes saturated with respect to the mineral within a short distance of the entrance to the flow path however in variable aperture fissures breakthrough occurs rapidly because of selective growth along preferential flow channels which progressively capture larger fractions of the total flow these results partly explain why conduits develop rapidly in gypsum although previous one dimensional studies suggest that conduit growth will not occur

diffusion controlled tracer retention in crystalline rock on the field scale 1 tracer retention is a key process for the barrier function of crystalline rock to any contaminant here we investigate the nature of retention mechanisms and their field scale parametrization using results of a comprehensive tracer transport experiment in crystalline rock on the field scale aspo hard rock laboratory sweden a method for identifying dominant retention mechanisms and inferring key parameters on the site scale is presented taking advantage of multiple tracer tests with a wide range of sorption affinities retention is shown to be diffusion controlled for the considered site robust features of tracer migration can be reasonably well predicted within a rock volume on at least 200 m scale by combining independent information with a simple model

effective matrix diffusion in kilometer scale transport in fractured crystalline rock concentrations of tritium 3h and dichlorodifluoromethane cfc 12 in water samples taken from glacial drift and fractured crystalline rock over 4 km2 in central new hampshire are interpreted to identify a conceptual model of matrix diffusion and the magnitude of the diffusion coefficient dispersion and mass transfer to and from fractures has affected the 3h concentration to the extent that the peak 3h concentration of the 1960s is no longer distinguishable because of heterogeneity in the bedrock the sparsely distributed chemical data do not warrant a three dimensional transport model instead a one dimensional model of cfc 12 and 3h migration along flow lines in the glacial drift and bedrock is used to place bounds on the processes affecting kilometer scale transport arid model parameters are varied to reproduce the measured relation between 3h and cfc 12 rather than their spatial distributions a model of mass exchange to and from fractures that is dependent on the time varying concentration gradient at fracture surfaces qualitatively reproduces the measured relation between 3h and cfc 12 with an upper bound for the fracture dispersivity approximately equal to 250 m and a lower bound for the effective matrix diffusion coefficient equal to 1 m2 yr 1 the diffusion coefficient at the kilometer scale is at least 3 orders of magnitude greater than laboratory estimates of diffusion in crystalline rock the large diffusion coefficient indicates that diffusion into an immobile fluid phase rock matrix is masked at the kilometer scale by advective mass exchange between fractures with large contrasts in trarismissivity the measured transmissivity of fractures in the study area varies over more than 6 orders of magnitude advective mass exchange from high permeability fractures to low permeability fractures results in short migration distances of a chemical constituent in low permeability fractures over an extended period of time before reentering high permeability fractures viewed at the kilometer scale this process is analogous to the chemical constituent diffusing into and out of an immobile fluid phase

on the upscaling of chemical transport in fractured rock the impact of flow heterogeneity on chemical transport from single to multiple fractures is investigated the emphasis is on the dynamic nature of the specific surface area ssa due to heterogenei

ikaite precipitation by mixing of shoreline springs and lake water mono lake california usa abstract metastable ikaite caco3 6h2o forms abundantly during winter months along the south shoreline of mono lake where shoreline springs mix with lake water ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate ion inhibition of calcite and aragonite during the spring some of the ikaite is transformed to anhydrous caco3 and is incorporated into tufa but most is dispersed by wave action into the lake where it reacts to form gaylussite na2ca co3 2 5h2o spring waters have low ph values are dominantly ca na hco3 have low radiocarbon activities and are mixtures of deep seated geothermal and cold groundwaters chemical modeling reveals that precipitation of caco3 can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96 spring water and 4 lake water under these conditions all the ca and a significant fraction of the co3 of the precipitate is spring supplied a radiocarbon age of 19 580 years obtained on a natural ikaite sample supports this conclusion with the springs supplying a large and probably variable portion of the carbonate and with apparent 14c age of the carbonate varying from spring to spring tufa of similar actual antiquity may yield significantly different 14c dates making tufa at this location unsuitable for absolute age dating by the radiocarbon method

ikaite caco36h2o precursor of the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins western united states the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins of the western united states are identified as being pseudomorphous after crystals of the mineral ikaite caco36h2o ikaite is a metastable mineral in the sedimentary realm it crystallizes from solution at temperatures close to zero celsius but rapidly decomposes at warmer temperatures and becomes pseudomorphed in calcite by in situ redistribution of the calcium and carbonate ions the likely implication is that the thinolites record ice cold episodes during deposition of the tufas

significance of fracture rim zone heterogeneity for tracer transport in crystalline rock 1 conducting fractures of crystalline rock are typically altered over long periods of time the fracture rim zone a result of these alterations will as a rule have different physical and chemical properties from the unaltered fresh rock depending on various microscopic and macroscopic factors of the alterations in this paper we study the impact of rim zone heterogeneity exemplified by a decreasing porosity trend as inferred from the aspo hard rock laboratory site sweden on short and long term tracer transport our main finding is that this particular rim zone structure will have a dominant effect on transport of moderately to strongly sorbing tracers on experimental time scales and a notable effect on application time scales the findings of this work lend further support to the interpretation of the relatively strong retention reported by cvetkovic et al the fracture rim zone porosity structure may provide an additional safety margin for sorbing radionuclides in crystalline rock at sites where fracture alteration is prevalent

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

reactive transport modeling in fractured rock a state of the science review abstract the field of reactive transport modeling has expanded significantly in the past two decades and has assisted in resolving many issues in earth sciences numerical models allow for detailed examination of coupled transport and reactions or more general investigation of controlling processes over geologic time scales reactive transport models serve to provide guidance in field data collection and in particular enable researchers to link modeling and hydrogeochemical studies in this state of science review the key objectives were to examine the applicability of reactive transport codes for exploring issues of redox stability to depths of several hundreds of meters in sparsely fractured crystalline rock with a focus on the canadian shield setting a conceptual model of oxygen ingress and redox buffering within a shield environment at time and space scales relevant to nuclear waste repository performance is developed through a review of previous research this conceptual model describes geochemical and biological processes and mechanisms materially important to understanding redox buffering capacity and radionuclide mobility in the far field consistent with this model reactive transport codes should ideally be capable of simulating the effects of changing recharge water compositions as a result of long term climate change and fracture matrix interactions that may govern water rock interaction other aspects influencing the suitability of reactive transport codes include the treatment of various reaction and transport time scales the ability to apply equilibrium or kinetic formulations simultaneously the need to capture feedback between water rock interactions and porosity permeability changes and the representation of fractured crystalline rock environments as discrete fracture or dual continuum media a review of modern multicomponent reactive transport codes indicates a relatively high level of maturity within the yucca mountain nuclear waste disposal program reactive transport codes of varying complexity have been applied to investigate the migration of radionuclides and the geochemical evolution of host rock around the planned disposal facility through appropriate near and far field application of dual continuum codes this example demonstrates how reactive transport models have been applied to assist in constraining historic water infiltration rates interpreting the sealing of flow paths due to mineral precipitation and investigating post closure geochemical monitoring strategies natural analogue modeling studies although few in number are also of key importance as they allow the comparison of model results with hydrogeochemical and paleohydrogeological data over geologic time scales

flow dependence of matrix diffusion in highly heterogeneous rock fractures diffusive mass transfer in rock fractures is strongly affected by fluid flow in addition to material properties the flow dependence of matrix diffusion is quantified by a random variable transport resistance denoted as t l and computed from the flow field by following advection trajectories the numerical methodology for simulating fluid flow is mesh free using fup basis functions a generic statistical model is used for the transmissivity field featuring three correlation structures i highly connected non multi gaussian ii poorly connected or disconnected non multi gaussian and iii multi gaussian the moments of are shown to be linear with distance irrespective of the structure after approximately 10 integral scales of ln t percentiles of are found to be linear with the mean when considering all three structures taking advantage of this property a potentially useful relationship is presented between percentiles and the fracture mean water residence time that integrates all structures with high variability it can be used in discrete fracture network simulations where t statistical data on individual fractures are not available

deposition and diagenesis of mammoth hot springs travertine yellowstone national park wyoming u s a strata forming a 113 m long core through mammoth hot springs record the holocene evolution of this travertine accumulation from deposition as part of a lacustrine to a terraced mound environment the deposit is readily divided into four intervals 113 67 m carbonate cemented volcaniclastic with intercalated layers of travertine 67 60 m moderately pure travertine with some volcaniclastics 60 50 m carbonate cemented volcaniclastic rich interval and 50 0 m essentially pure travertine lithologic composition sedimentary structures and the rare ostracode fossils indicate that the lower 67 m predominantly accumulated in a lacustrine environment whereas the upper 40 m are terraced mound deposits all of the travertine is calcite some after aragonite layers of shrubs oncoids and peloids all bacterial in origin form the dominant allochems within the travertine stable isotopic carbon and oxygen values n 128 are strongly positively correlated and decrease up core 4 and 8 respectively reflec

simulation of solute transport in discrete fracture networks using the time domain random walk method the time domain random walk tdrw method has been developed for simulating solute transport in discrete fracture networks the following transport processes have been considered advective transport in fractures hydrodynamic dispersion along the fracture axis sorption reactions on the fracture walls and decay reactions the tdrw method takes advantage of both random walk and particle tracking methods it allows for the one step calculation of the particle residence time in each bond of the network while avoiding mass balance problems at fracture intersections with contrasted dispersion coefficients the accuracy of the tdrw method has been addressed by means of synthetic test problems into single fractures and into a 2d discrete fracture network in each case simulated and theoretical results compare very well

stochastic simulation of radionuclide migration in discretely fractured rock near the aspo hard rock laboratory we study the migration of sorbing tracers through crystalline rock by combining relatively simple transport measures with particle tracking in a discrete fracture network the rock volume is on a 1

large tufa mounds searles lake california mounds that have formed around spring vents occur in a variety of environmental settings many at sites generally difficult or inaccessible for sampling in contrast over 500 tufa mounds occur in the dry bed of searles lake california the mounds range from minor features to 45 m in height most are 5 to 12 m high these mounds composed of calcite and aragonite formed associated with spring vents in the pleistocene lake bottom thus analyses of these mounds in searles lake provide a model with regard to the origin and architecture of tufa mounds the mounds consist of four distinctive tufa facies the initial deposits consist of porous tufa including the innermost porous 1 and the outermost porous 2 deposits followed by nodular tufa then columnar tufa and laminated crusts there are two simple sequences of tufa deposition the first sequence is from porous 1 to nodular to laminated crusts and finally to porous 2 a second sequence consists of porous 1 to columnar to laminated crusts and lastly to porous 2 facies changes are a response to changes in environmental conditions from deep water porous 1 facies to an essentially dry lake phase during and after the formation of laminated crusts facies to deep water porous 2 facies and at the present time totally dry the primary constituents that comprise the tufa deposits include thin laminae pisoids spherulites peloids and stromatolite like crusts on the microscopic scale these constituents dominantly make up nano spheres micro rods and rod like crystals as well as other calcified bodies these constituents are interpreted to be the calcified remains of bacterial bodies these findings suggest that microbial participation in the construct of other mounds should be a major concern of investigation both for terrestrial and extraterrestrial spring fed mounds