sequence age and source of silicic fallout tuffs in middle to late miocene basins of the northern basin and range province the latest cenozoic 6 ma ash beds in the western united states have been intensively studied for several decades the more widespread of these ash beds are well documented event horizons that are of great value in studies of the timing and pace of geological climatological and biological events throughout the region because explosive volcanism was not restricted to latest neogene time in this region many older ash beds are likely to prove as useful as younger beds as event horizons once they are located characterized and dated as a first step in developing a useful chronology of older cenozoic ash beds in the western united states we have sampled and analyzed silicic fallout tuffs in middle to late miocene sedimentary basins across the northern basin and range province the northern basin and range basins ideally situated in the vicinity of major coeval silicic volcanic centers contain numerous relatively unaltered silicic fallout tuffs we have correlated tuffs between all sampled sections on the basis of glass shard composition the composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs including 59 widely distributed tuffs termed correlative tuffs the tuffs vary widely in composition but most are in one of two compositional groups gray metaluminous vitric tuffs gm tuffs or white metaluminous vitric tuffs wm tuffs distribution patterns compositional characteristics and correlation with ash flow tuffs show that the source for most gm tuffs was the snake river plain volcanic province along the northern edge of the northern basin and range and the source for most wm tuffs was the southwestern nevada volcanic field in the southern part of the northern basin and range the northern basin and range tuffs range in age from ca 16 6 ma the ages of individual tuffs are determined variously by direct isotopic dating by correlation to previously dated fallout and ash flow tuffs or by interpolation age estimation ages for most tuffs are known to within 0 25 m y 1 or less and for many tuffs to within 0 1 m y or less the sequence and ages of tuffs established in this study provide insights into the evolution of the northern basin and range basins and patterns of explosive volcanism in coeval volcanic centers and contribute to the development of a high resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western united states

the hayes tephra deposits and upper holocene marker horizon in south central alaska abstract the most widespread of all holocene tephra deposits in the cook inlet region of south central alaska is a set of deposits from hayes volcano because of their unique phenocryst content biotite in rare amounts and a high proportion of amphibole to pyroxene the deposits are readily identifiable at all but the most distant sites where they are very fine grained eighteen radiocarbon dates from eight upland sites limit the age of the tephra set to between about 3500 and 3800 yr the set originated at hayes volcano in the tordrillo mountains 150 km northwest of anchorage seven or possibly eight closely succeeding deposits low silica dacite in composition compose two main lobes that extend northeast for 400 km and south for at least 250 km from the vent we estimate the total tephra volume to be 10 km 3 multiple layers imply four to six larger and two or three smaller eruptions the deposits are a nearly isochronous marker horizon that should be useful in future archeologic geologic and palynologic studies in the region

volcanic ash retrieved from the grip ice core is not from thera 1 tephra found in an ice core from greenland grip has been claimed to be from the minoan eruption of thera santorini greece if true this would date the eruption thereby resolving a decades long debate in chronology herein it is shown that the methods used to match the greenlandic tephra with thera are flawed and that the geochemical data imply the tephra is not from thera

understanding the anomalous temperature data from the large block test at yucca mountain nevada 1 the large block test lbt at yucca mountain nevada is unique because of its size which is large enough to include a number of large fractures while still small enough so that boundary conditions and heterogeneities can be adequately controlled or characterized extensive mapping of the test block has established the presence of many small and large fractures preheat air injection testing has also revealed that the block is highly heterogeneous with fracture permeability varying more than four orders of magnitude we hypothesize that these large fractures and the resulting heterogeneity play a significant role in the development of coupled thermal hydrological th processes in the lbt a large volume of th data including temperature and saturation measurements has been collected from the lbt some of these temperature data from the lbt particularly those recorded by sensors tt1 14 and tt2 14 appear anomalous we show that these anomalous temperature data can be understood only if heterogeneity is invoked as an example we show that the heat pipe signature an indication of the extent of th coupling in tt2 14 is significantly longer in duration than that in tt1 14 we show that such a difference can be explained only if one includes a high permeability layer the existence of which has been indicated by air injection testing through the location of sensor tt2 14 similarly the oscillating temperature pattern in all sensors of borehole tt2 during 4470 4500 hours of heating is shown to be the result of rainwater flowing down a high permeability fracture the irregular temperature pattern in borehole tt1 happens because of rainwater reaching the source of heat quickly through a highly permeable inclined fracture and vapor rising upwards through a less permeable vertical fracture we also demonstrate that the anomalous temperature pattern recorded by tt1 14 during 2500 3500 hours of heating can be explained in terms of continuous upward movement of vapor and the downward flow of condensate through the fractures we conclude that the temperature data from the lbt are the results of coupled th processes occurring in a heterogeneous environment

a continuum model for water movement in an unsaturated fractured rock mass the movement of fluids in a fractured porous medium has been the subject of considerable study this paper presents a continuum model that may be used to evaluate the isothermal movement of water in an unsaturated fractured porous medium under slowly changing conditions this continuum model was developed for use in evaluating the unsaturated zone at the yucca mountain site as a potential repository for high level nuclear waste thus its development has been influenced by the conditions thought to be present at yucca mountain a macroscopic approach and a microscopic approach are used to develop a continuum model to evaluate water movement in a fractured rock mass both approaches assume that the pressure head in the fractures and the matrix are identical in a plane perpendicular to flow both approaches lead to a single flow equation for a fractured rock mass the two approaches are used to calculate unsaturated hydrologic properties i e relative permeability and saturation as a function of pressure head for several types of tuff underlying yucca mountain using the best available hydrologic data for the matrix and the fractures rock mass properties calculated by both approaches are similar

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

heterogeneity correlatives and proposed stratigraphic nomenclature of hayes tephra set h alaska abstract several holocene tephra deposits of hayes volcano constitute a marker horizon in southern and east central alaska their identification is aided by high amphibole pyroxene ratio and biotite in trace amounts unique among holocene tephra deposits of the region however correlations are obscured by chemical heterogeneity of the glass which occurs at a scale less than the size of a lapilli single shard analyses confirm that the heterogeneity is due neither to fractionation nor to plagioclase microlites the heterogeneity may be due to mixing of magmas prior to eruption it is proposed that the deposits be informally called hayes tephra set h

identification of aniakchak alaska tephra in greenland ice core challenges the 1645 bc date for minoan eruption of santorini pearce nicholas westgate j a eastwood w j preece s j 2006 identification of aniakchak alaska tephra in greenland ice core challenges the 1645 bc date for minoan eruption of santorini geochemistry geophysics and geosystems 5 pp q03005 rae2008

approaches to modeling coupled thermal hydrological and chemical processes in the drift scale heater test at yucca mountain abstract a large scale underground thermal test drift scale test dst in fractured volcanic tuff resulted in changes to water and gas chemistry as well as mineral precipitation and dissolution in fractures thermal hydrological and chemical thc processes in the dst were modeled by lawrence berkeley national laboratory lbnl and japan nuclear cycle development institute jnc as part of the international working group decovalex predictions of thc processes in the dst for the 4 year heating and 4 year cooling periods were initially performed by the lbnl group with the current model reflecting a revised heater operation history and model jnc used primarily the original data from the prediction and created a new model to evaluate a selected set of data the approaches taken by the groups differed in several ways and a comparison of the methodologies and results of the simulations allow for a better understanding of modeling coupled processes in unsaturated fractured rock the lbnl model represented the fractures and rock matrix as a fully interacting dual continuum in terms of fluid heat and chemical transport with the local mineral water gas reactions treated by kinetic and equilibrium reactions the jnc model represented the fractures and matrix as a single effective continuum with equilibrium mineral water reactions controlling the chemical evolution both models considered aqueous species transport with gas phase co 2 transport only considered in the lbnl model comparisons to data collected from the dst illustrate the behavior of the models and their ability to capture the relevant thc processes overall both models capture the temperature evolution in the rock quite closely although the jnc model gave a closer match to the initial temperature rise in the rock likely owing to the use of site specific thermal data as opposed to average properties used for the lbnl model both models showed the contrasting solubility effects of increasing temperature on calcite and silica solubility yet the dual continuum approach better represented the effects of boiling and condensation on aqueous species chemistry and the distribution of mineral precipitation

multiphysics processes in partially saturated fractured rock experiments and models from yucca mountain 1 the site investigations at yucca mountain nevada have provided us with an outstanding data set one that has significantly advanced our knowledge of multiphysics processes in partially saturated fractured geological media such advancement was made possible foremost by substantial investments in multiyear field experiments that enabled the study of thermally driven multiphysics and testing of numerical models at a large spatial scale the development of coupled process models within the project have resulted in a number of new advanced multiphysics numerical models that are today applied over a wide range of geoscientific research and geoengineering applications using such models the potential impact of thermal hydrological mechanical thm multiphysics processes over the long term e g 10 000 years could be predicted and bounded with some degree of confidence the fact that the rock mass at yucca mountain is intensively fractured enabled continuum models to be used although discontinuum models were also applied and are better suited for analyzing some issues especially those related to predictions of rockfall within open excavations the work showed that in situ tests rather than small scale laboratory experiments alone are essential for determining appropriate input parameters for multiphysics models of fractured rocks especially related to parameters defining how permeability might evolve under changing stress and temperature a significant laboratory test program at yucca mountain also made important contributions to the field of rock mechanics showing a unique relation between porosity and mechanical properties a time dependency of strength that is significant for long term excavation stability a decreasing rock strength with sample size using very large core experiments and a strong temperature dependency of the thermal expansion coefficient for temperatures up to 200 c the analysis of in situ heater experiments showed that fracture closure opening caused by changes in normal stress across fractures was the dominant mechanism for thermally induced changes in intrinsic fracture permeability during rock mass heating cooling and that fracture shear dilation appears to be less significant significant effort was devoted to predicting the long term stability of underground excavations under mechanical strength degradation and seismic loading perhaps one of the most challenging tasks within the project we note that such long term strength degradation is actually an example of a chemically mediated process governed by underlying microscopic stress corrosion and chemical diffusion processes in the yucca mountain project such chemically mediated mechanical changes were considered implicitly through model calibrations against laboratory and in situ heater experiments at temperatures anticipated to be experienced by the rock a possible future research direction would be to simulate such processes mechanistically in a complete coupled thmc framework where c denotes chemical processes

modeling of thermally driven hydrological processes in partially saturated fractured rock 1 this paper is a review of the research that led to an in depth understanding of flow and transport processes under strong heat stimulation in fractured porous rock it first describes the anticipated multiple processes that come into play in a partially saturated fractured porous volcanic tuff geological formation when it is subject to a heat source such as that originating from the decay of radionuclides the rationale is then given for numerical modeling being a key element in the study of multiple processes that are coupled the paper outlines how the conceptualization and the numerical modeling of the problem evolved progressing from the simplified to the more realistic examples of numerical models are presented so as to illustrate the advancement and maturation of the research over the last 2 decades the most recent model applied to in situ field thermal tests is characterized by 1 incorporation of a full set of thermal hydrological processes into a numerical simulator 2 realistic representation of the field test geometry in three dimensions and 3 use of site specific characterization data for model inputs model predictions were carried out prior to initiation of data collection and the model results were compared to diverse sets of measurements the approach of close integration between modeling and field measurements has yielded a better understanding of how coupled thermal hydrological processes produce redistribution of moisture within the rock which affects local permeability values and subsequently the flow of liquid and gases the fluid flow in turn will change the temperature field we end with a note on future research opportunities specifically those incorporating chemical mechanical and microbiological factors into the study of thermal and hydrological processes

simultaneous inversion of air injection tests in fractured unsaturated tuff at yucca mountain air injection tests are being used to characterize the flow characteristics of the fractured volcanic tuffs at yucca mountain nevada the proposed site for a high level nuclear waste repository as the air component flows mainly in the heterogeneous fracture system air injection tests can be used to determine the hydrological properties and parameters of the fracture networks in situ air injection tests have been carried out in 30 boreholes drilled in a fractured rock block of 13 21 5 18 m3 in the underground facility at yucca mountain these in situ field tests consist of a constant rate flow injection in one of the boreholes while the pressure response is monitored in all 30 boreholes of the rock block this paper presents a simultaneous inversion for 21 air injection tests in 21 separate boreholes using tough2 a three dimensional numerical code for multiphase multicomponent transport pruess 1991 pruess et al 1996 spatially variable fracture permeability is used as an adjustable parameter to fit the measured pressure responses for most of the pneumatic experiments the calculated pressure changes match the measured data well estimated permeabilities range over 5 orders of magnitude from 10 15 to 8 10 11 m2 indicating large spatial variability in permeability of the heterogeneous fracture system

correlation of the holocene jarvis creek tangle lakes cantwell and hayes tephras in south central and central alaska abstract the geochemistry petrography and distribution of the jarvis creek ash pewe 1965 1975a indicate that this tephra from the lower delta river area of central alaska is correlative with vol volcanic ash from sites in south central alaska near tangle lakes upper delta river area and the cantwell ash from hayes volcano found in the upper nenana river area riehle et al 1990 volcanic glass compositions of distal jarvis creek and tangle lakes tephra samples are compositionally restricted while several discrete glass populations are present in some samples are compositionally collected nearer hayes volcano these correlations extend the known distribution of hayes volcano tephras across the alaska range and into central alaska a distance of more than 650 km new geochronologic data for the jarvis creek ash suggest it was deposited ca 3660 125 yr b p consistent with previous age estimates of tephra eruptions at the hayes volcano the name jarvis creek ash has well established priority with respect to cantwell ash or other local names for this tephra layer from the hayes volcano

modeling the thermal hydrologic processes in a large scale underground heater test in partially saturated fractured tuff the drift scale test dst is being conducted in an underground facility at yucca mountain nevada to probe the coupled thermal hydrological mechanical and chemical processes likely to occur in the fractured rock mass around a potential high level nuclear waste repository thermal hydrological processes in the dst have been simulated using a three dimensional numerical model the model incorporates the realistic test configuration and all available site specific measurements pertaining to the thermal and hydrological properties of the unsaturated fractured tuff of the test block the modeled predictions were compared to the extensive set of measured data collected in the first year of this 8 year long test the mean error between the predictions and measurement at 12 months of heating for over 1600 temperature sensors is about 2 degrees c heat pipe signature in the temperature data indicating two phase regions of liquid vapor counterflow is seen in both the measurements and simulated results the redistribution of moisture content in the rock mass resulting from vaporization and condensation was probed by periodic air injection testing and geophysical measurements good agreement also occurred between the model predictions and these measurements the general agreement between predictions from the numerical simulations and the measurements of the thermal test indicates that ourmore fundamental understanding of the coupled thermal hydrologic processes at yucca mountain is sound however effects of spatial heterogeneity from discrete fractures that are observed in the temperature data are not matched by simulations from the numerical model which treat the densely spaced fractures as a continuum less

oligocene crustal anatexis in the tethyan himalaya southern tibet abstract recent studies in the xiaru and malashan gneiss dome of the tethyan himalaya southern tibet identify that xiaru and paiku tourmaline bearing leucogranite dike formed at 28 29 ma together with 28 ma kuday garnet bearing leucogranites it is demonstrated that the himalayan orogenic belt experienced a major episode of crustal melting in the mid oligocene geochemical data indicate that three suites of leucogranite are characterized by large variations in the major and trace element compositions as well as sr nd isotope systematics which could be explained by combined fractional crystallization and relative contributions of micas and accessory phases dissolved into a crustal melt during decompressional melting of metapelitic rocks documentation of oligocene partial melting of crustal rocks could indicate that the exhumation of deep crustal rocks in the himalayan orogenic belt could have started as early as oligocene

modeling gas water processes in fractures with fracture flow properties obtained through upscaling 1 many environmental systems are driven by complex gas water processes in fractured aquifers one example is degassing processes occurring in fractures in the vicinity of radioactive waste disposal sites these fractures can represent a potential fast track for radioactive substances to reach the surface due to buoyancy effects the created gas phase can move relatively quickly to the surface the aim of this paper is to investigate and model the involved processes first a model for a single fracture is developed based on statistical properties of real fractures an aperture distribution raster element model is created a percolation renormalization model yields effective properties such as relative permeabilities and capillary pressure and provides the size of a representative elementary volume these effective relationships represent the basis for modeling the migration of the created gas phase through the fractured aquifer of the subsurface an example modeling degassing and flow in a single fracture is shown and compared to experimental data the presented model represents a conceptual method for degassing processes and the subsequent migration of the gas phase in fracture matrix systems improving predictions on the fate of radioactive substances

chemical differences between the island and submarine pumice layers of thera abstract investigations on tephra layers of deep sea cores from near the island of santorini showed that only the pumice clasts from the west of the island can be attributed to the minoan eruption the tephra layers within the caldera are of uncertain origin

coupled thermal hydrological mechanical analyses of the yucca mountain drift scale test comparison of field measurements to predictions of four different numerical models abstract the yucca mountain drift scale test dst is a multiyear large scale underground heating test designed to study coupled thermal hydrological mechanical chemical behavior in unsaturated fractured and welded tuff as part of the international cooperative code comparison project development of coupled models and their validation against experiments four research teams used four different numerical models to simulate and predict coupled thermal hydrological mechanical thm processes at the dst the simulated processes included heat transfer liquid and vapor water movements rock mass stress and displacement and stress induced changes in fracture permeability model predictions were evaluated by comparison to measurements of temperature water saturation displacement and air permeability the generally good agreement between simulated and measured thm data shows that adopted continuum model approaches are adequate for simulating relevant coupled thm processes at the dst moreover thermal mechanically induced rock mass deformations were reasonably well predicted using elastic models although some individual displacements appeared to be better captured using an elasto plastic model it is concluded that fracture closure opening caused by change in normal stress across fractures is the dominant mechanism for thermal stress induced changes in intrinsic fracture permeability at the dst whereas fracture shear dilation appears to be less significant this indicates that such changes in intrinsic permeability at the dst which are within one order of magnitude are likely to be mostly reversible

a study of thermally induced convection near a high level nuclear waste repository in partially saturated fractured tuff we present simple estimates for velocities of buoyancy driven flow of liquid and gas phases near a hypothetical high level nuclear waste repository in partially saturated fractured tuff these estimates indicate that gas phase convection could take place with appreciable velocity of the order of 20 m year while liquid convection is expected to be very slow of the order of 0 1 mm year detailed numerical simulations using an effective continuum approximation to represent fracture effects confirm these estimates and demonstrate a sensitivity of gas convection to the strength of binary diffusion

the thermo hydro mechanical behavior of the argillaceous cobourg limestone the cobourg limestone is a low permeability argillaceous rock that forms a part of the paleozoic sedimentary sequence found in southern ontario canada the limestone has a heterogeneous fabric consisting of nodular regions of calcite and dolomite and argillaceous partings of a similar composition but with a low clay content which gives the appearance of nominal stratifications the thermo hydro mechanical thm behavior of the rock is of interest to the proposals for using sedimentary formations as candidate rocks for siting deep geological repositories for the storage of heat emitting nuclear fuel waste the paper presents the results of experiments where thm processes were initiated in an intact cylindrical sample of the cobourg limestone containing a central cylindrical fluid filled cavity biot s classical theory of poroelasticity extended to include thermal effects is used to examine the thm response of the fluid cavity due to boundary heating of the cylinder the rise and decay of thermally induced cavity fluid pressure is used to examine the applicability of the thm modeling the experiments were conducted on cylindrical samples of the cobourg limestone with their axes either along or normal to the nominal planes of the argillaceous partings

age extent and climatic significance of the c 3400 bp aniakchak tephra western alaska usa tephra deposits at whitefish lake and cape espenberg on the northernmost seward peninsula of western alaska were derived from the aniakchak caldera on the alaska peninsula more than 1500 km to the south radiocarbon dates on the climactic caldera forming aniakchak eruption and on proximal and distal tephra indicate the aniakchak tephra was deposited about 3435 40 bp 3614 3815 cal bp and forms an isochronous marker horizon across much of western alaska the aniakchak tephra eruption and several other targe eruptions during the seventeenth century bc may have had widespread effects on climate

revised 14c age for st helens y tephra at tonquin pass british columbia the age of a mount st helens set y tephra at tonquin pass british columbia previously assumed to be 4300 years bp is revised to ca 3400 years bp based on bracketing 14c dates of 3140 70 beta 13558 and 3680 80 years bp beta 13559 the implications of this revision for tephrostratigraphy of this area are discussed

estimating liquid fluxes in thermally perturbed fractured rock using measured temperature profiles a new temperature profile method was recently developed for analyzing perturbed flow conditions in superheated porous media the method uses high resolution temperature data to estimate the magnitude of the heat driven liquid and gas fluxes that form as a result of boiling condensation and recirculation of pore water in this paper we evaluate the applicability of this new method to the more complex flow behavior in fractured formations with porous rock matrix in such formations with their intrinsic heterogeneity the porous but low permeable matrix provides most of the mass and heat storage capacity and dominates conductive heat transfer fractures on the other hand offer highly effective conduits for gas and liquid flow thereby generating significant convective heat transfer after establishing the accuracy of the temperature profile method for fractured porous formations we apply the method in analyzing the perturbed flow conditions in a large scale underground heater test conducted in unsaturated fractured porous tuff the flux estimates for this test indicate a significant reflux of water near the heat source on the order of a few hundred millimeter per year much larger than the ambient percolation flux of only a few millimeter per year

on thermohydrologic conditions near high level nuclear wastes emplaced in partially saturated fractured tuff 2 effective continuum approximation this paper presents an effective continuum approximation for modeling of fluid and heat flow in fractured porous media the approximation is based on the thermohydrologic behavior observed in detailed simulations with explicit consideration of fracture effects see the companion paper part 1 the crucial concept in the development of an effective continuum approximation is the notion of local thermodynamic equilibrium between rock matrix and fractures where applicable it provides a substantial simplification of the description of fluid and heat flow in fractured porous media we derive formulas for effective continuum characteristic curves relative permeabilities and capillary pressures in terms of the properties of fracture and matrix continua respectively numerical simulations demonstrate that under favorable conditions the effective continuum approximation closely matches predictions obtained from an explicit modeling of fracture effects it is also demonstrated that the approximation breaks down under unfavorable conditions very tight rock matrix a simple criterion for the applicability of an effective continuum approximation is derived from consideration of diffusive processes a quantitative evaluation shows the criterion to be consistent with results of our numerical simulations

a probabilistic analysis of episodic preferential flow into superheated fractured rock the amount of water seeping into waste emplacement tunnels is important for the long term performance of the proposed geologic nuclear waste repository at yucca mountain nevada the repository site is located in thick partially saturated fractured tuff that will be heated to above boiling temperatures as a result of heat generation from the decay of nuclear waste since water infiltrating down towards the repository will be subject to vigorous boiling for a significant time period the superheated zone i e rock temperature above the boiling point of water may form an effective vaporization barrier that reduces the possibility of water arrival at emplacement drifts in this paper we analyze the behavior of episodic preferential flow events that penetrate down into the hot fractured rock zone and evaluate the impact of such flow behavior on the effectiveness of the vaporization barrier the characteristic features of episodic preferential flow are estimated from laboratory experiments and described by appropriate probability distributions a semi analytical solution is utilized to determine the complex flow processes in the hot rock environment the solution is applied at several discrete times after emplacement in order to cover the time period of strongly elevated temperatures at yucca mountain

penetration of liquid fingers into superheated fractured rock 1 water infiltrating down a fracture in unsaturated rock experiences complex fluid flow and heat transfer phenomena when entering above boiling rock temperature regions such conditions are expected for example after emplacement of heat generating nuclear waste in underground repositories a new efficient semianalytical method is proposed in this paper that simulates the flow processes of infiltration events subject to vigorous boiling from the adjacent hot rock it is assumed that liquid flow forms in localized preferential flow paths and that infiltration events are typically short in duration but large in magnitude relative to the average net infiltration the new solution scheme is applied to several test cases studying sensitivity to a variety of input parameters sample simulations are performed for conditions representative of the potential nuclear waste repository at yucca mountain nevada a characteristic parameter is introduced that provides a quick estimate of the relative significance of boiling at a given location of interest

on thermohydrologic conditions near high level nuclear wastes emplaced in partially saturated fractured tuff 1 simulation studies with explicit consideration of fracture effects we have performed modeling studies on the simultaneous transport of heat liquid water vapor and air in partially saturated fractured porous rock formation parameters were chosen as representative of the potential nuclear waste repository site in the topopah spring unit of the yucca mountain tuffs the presence of fractures makes the transport problem very complex both in terms of flow geometry and physics the numerical simulator used for our flow calculations takes into account most of the physical effects believed to be important in multiphase fluid and heat flow it has provisions for handling the extreme nonlinearities that arise in phase transitions component disappearances and capillary discontinuities at fracture faces we model a region around an infinite linear string of nuclear waste canisters taking into account both the discrete fractures and the porous matrix thermohydrologic conditions in the vicinity of the waste packages are found to depend strongly on relative permeability and capillary pressure characteristics of the fractures which are unknown at the present time if liquid held on the rough walls of drained fractures is assumed to be mobile strong heat pipe effects are predicted under these conditions the host rock will remain in two phase conditions right up to the emplacement hole and formation temperatures will peak near 100 c if it is assumed that liquid cannot move along drained fractures the region surrounding the waste packages is predicted to dry up and formation temperatures will rise beyond 200 c a substantial fraction of waste heat can be removed if emplacement holes are left open and ventilated as opposed to backfilled and sealed emplacement conditions comparing our model predictions with observations from in situ heater experiments reported by zimmerman and coworkers some intriguing similarities are noted however for a quantitative evaluation additional carefully controlled laboratory and field experiments will be needed