Sorbing Tracers Research Articles

Numerical simulation of anomalous observations from an in-situ long-term sorption diffusion experiment in a rock matrix

At the in-situ Long Term Sorption Diffusion Experiment (LTDE-SD), performed in the Äspö Hard Rock Laboratory (Sweden), anomalous penetration profiles of tracers were observed. The concentration of sorbing tracers, particularly 137Cs, sharply decreased around the rock surface but was still quite high in the distance, which was known that it is difficult to model with the classic Fick’s law. To understand the mechanism of matrix diffusion and sorption of solute by reproducing the observed penetration profiles in the LTDE-SD test, three cases for the matrix were proposed and evaluated: (1) homogeneous medium with a disturbed zone; (2) heterogeneous medium considering mineral grains with intragranular porosities and their intergranular pores; and (3) heterogeneous medium considering vein and microfractures as well as porous mineral grains and intergranular pores. The modelling domains for Cases 2 and 3 were established using 2-D microstructure characterization results from analyses including chemical staining of minerals and 14C-PMMA autoradiography. The numerical models for Cases 1–3 were calibrated by minimizing the discrepancies between simulations and observations of the penetration profiles and the changes in concentration in the reservoir for the tracers, 137Cs and 22Na. The results show that only the calibrated transport parameter values for Case 3 were within the reported ranges, and we concluded that the sharp decrease in concentrations in the near-surface zone in the observed penetration profiles was likely to result from disturbed biotite grains and the long tails in the observed penetration profiles might originate from diffusion along the vein and microfractures transecting minerals.

137Cs transport in crushed granitic rock: The effect of bentonite colloids

The paper focuses on the study of caesium transport through crushed granite in the presence of bentonite colloids in a dynamic column arrangement with the continuous input of the liquid phase. The aim of the experiments was to investigate the effect of the presence of bentonite colloids on the behaviour of caesium in granitic rock. The behaviour of the tracer was studied under aerobic conditions applying a constant tracer concentration (activity) and constant flow rate. Defined volumes of the liquid phase were sampled periodically at the outlet of the column for the measurement of tracer concentrations (activity). Transport was described by means of breakthrough curves. The stepwise approach included the following steps: 1) the non sorbing tracer 3H was used to evaluate the hydrodynamic properties of the column; 2) the conducting of a column experiment with bentonite colloids in deionised water; 3) the migration of 137Cs in two liquid phases (deionised and synthetic granitic water); 4) the study of the transport of the radiocolloid suspension in deionised water. The results revealed the completely different behaviour of the bentonite colloids and the caesium in the granitic column and the effect of colloid particles on caesium sorption on granite. The bentonite colloids behaved as conservative non-sorbing tracers whereas, conversely, caesium sorbed strongly on the granitic material. It was determined that the behaviour of the caesium in the granitic rock was influenced by the strong sorption of caesium with respect to the bentonite colloids. A minor part of the caesium irreversibly sorbed on the bentonite colloids and passed through the granite immediately together with the bentonite colloids, and a major part of the caesium was desorbed from the bentonite colloids and was retained in the granite.

Pore structure, wettability and tracer migration in four leading shale formations in the Middle Yangtze Platform, China

Deposited in different sedimentary settings, four leading shale formations (Late Ordovician Wufeng, Early Silurian Longmaxi, Late Permian Dalong, and Early Jurassic Dongyuemiao Shales) are currently the most promising zones for shale gas development in the Middle Yangtze Platform of South China. Based on complementary tests [low pressure gas physisorption, mercury injection capillary pressure (MICP), contact angle measurement, fluid imbibition into initially dry shale, and tracer diffusion into initially fluid-saturated shale followed by tracer mapping with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)], this work investigates their pore structure (both geometry and connectivity) and wettability characteristics, as well as the coupled effects of these characteristics on fluid flow and tracer migration.Pores <50 nm account for the majority of the pore volume in these organic shales. The shapes of hysteresis loop from gas physisorption show that the pores are mostly inkbottle-shaped in Wufeng, Longmaxi and Dongyuemiao Shales, while they are mostly narrow plate- or slit-like shaped pores in Dalong Shale. These four organic shales are strongly oil-wetting and moderately water-wetting. According to the imbibition behaviors toward aqueous (deionized water) and oleic (n-decane) phases, hydrophobic pores are better connected than hydrophilic pore networks, which is consistent with the measured contact angles. Diffusion of nano-sized nonsorbing (perrhenate [ReO4−]) and sorbing (cesium [Cs+]) tracers into brine-saturated shales indicates a high spatial variability and limited pore connectivity in these organic shales. The effective diffusion coefficient values are on the order of 10−13 m2/s with an associated geometric tortuosity ranging from 11.1 to 41.4. Due to the limited edge-connected pore spaces and low diffusion coefficients, the migration of hydrocarbons will be very slow from the shale matrix to hydraulically created fractures, with the initially high production of hydrocarbons from connected pore spaces near the sample edge (e.g., fracture face).

Organic molecules as sorbing tracers for the assessment of surface areas in consolidated aquifer systems

Based on the assumption that the specific surface area to volume ratio Asurf/V of consolidated rock materials is proportional to the surface area available for sorption, several inorganic cations were recently proposed as sorbing (cation exchanging) tracers for estimating these ratios in aquifers (e.g., for deriving the efficient heat exchange area of geothermal reservoirs). The main disadvantages of inorganic ions, however, are the limited number of suitable ions, their potential geogenic background, and their challenging online detection at trace concentrations. In this work, the spectrum of chemical substances used as sorbing tracers expands by considering fluorescent organic compounds that are cationic. They have the advantage of being highly water soluble and easy to measure online at very low concentrations. Results from systematic lab column experiments with seven selected organic cations under various conditions (different salinities and temperatures) are presented, emphasizing the potential of organic molecules as alternative sorbing tracers especially in consolidated aquifer systems. This work is a first stepping stone in identifying suitable molecular structures that can be selected or even individually adapted to the requirements of the tracer tests and prevailing aquifer conditions.

The effect of stagnant water zones on retarding radionuclide transport in fractured rocks: An extension to the Channel Network Model

An essential task of performance assessment of radioactive waste repositories is to predict radionuclide release into the environment. For such a quantitative assessment, the Channel Network Model and the corresponding computer program, CHAN3D, have been used to simulate radionuclide transport in crystalline bedrocks. Recent studies suggest, however, that the model may tend to underestimate the rock retarding capability, because it ignores the presence of stagnant water zones, STWZs, situated in the fracture plane. Once considered, the STWZ can provide additional surface area over which radionuclides diffuse into the rock matrix and thereby contribute to their retardation.The main objective of this paper is to extend the Channel Network Model and its computer implementation to account for diffusion into STWZs and their adjacent rock matrices.In the first part of the paper, the overall impact of STWZs in retarding radionuclide transport is investigated through a deterministic calculation of far-field releases at Forsmark, Sweden. Over the time-scale of the repository safety assessments, radionuclide breakthrough curves are calculated for increasing STWZ width. It is shown that the presence of STWZs enhances the retardation of most long-lived radionuclides except for 36Cl and 129I.The rest of the paper is devoted to the probabilistic calculation of radionuclide transport in fractured rocks. The model that is developed for transport through a single channel is embedded into the Channel Network Model and new computer codes are provided for the CHAN3D. The program is used to (I) simulate the tracer test experiment performed at Äspö HRL, STT-1 and (II) investigate the short- and long-term effect of diffusion into STWZs. The required data for the model are obtained from detailed hydraulic tests in boreholes intersecting the rock mass where the tracer tests were made.The simulation results fairly well predict the release of the sorbing tracer 137Cs. It is found that over the short time-scale of the tracer experiment, the effect of diffusion into STWZs is not as pronounced as that of matrix diffusion directly from the flow channel, and the latter remains the main retarding mechanism. Predictions for longer time-scale, tens of years and more, show that the effect of STWZs becomes strong and tends to increase with transport time. It is shown that over the long times of interest for safety assessment of radioactive waste repositories, STWZs can substantially contribute to radionuclide retardation, though for the short time-scales the impact is not very strong and is not expected to affect the results of short-term field experiments.

A field-scale long-term study on radionuclide transport through weathered granites at a site in southern China

A regional strategy for safety disposal of low- and intermediate-level radioactive wastes (LILW) has been implemented in China to protect humans and the environment. A joint onsite and laboratory investigation was conducted for a field site in southern China to assess the probability for safe disposal of LILW, which requires the understanding of long-term radionuclide transport behaviors under field conditions. This study presents the field-scale modeling of radionuclide transport through weathered granites for a conservative, a weakly sorbing and a strongly sorbing tracer by incorporating laboratory and field data. The field-scale radionuclide transport model was developed on the basis of a validated long-term groundwater flow model and field-measured dispersion coefficient, as well as laboratory-characterized strontium and cesium distribution coefficients in the weathered granites. The model was then used to perform the long-term transport prediction and risk assessment of radionuclide pollution for both the natural site setting and the graded site setting. Model simulation reveals that the numerical sensitivities of calculated concentrations are tracer dependent and changing with time. The conservative radionuclide is most sensitive to changes in hydraulic conductivity (K) while slightly sensitive to changes in effective porosity (φ), specific yield (μ) and longitudinal dispersion coefficient (D L), indicating advection is the main transport process of conservative radionuclide. The weakly and strongly sorbing tracers, on the other hand, are most sensitive to changes in the distribution coefficient (K d) and less sensitive to changes in the rest of model parameters, revealing that sorption is the main process for controlling the transport of sorbing tracers. A conservative radionuclide plume moves at an average velocity of about 54 m/a, which is too fast to be considered as safe disposal under the natural site setting. However, the plume of the conservative radionuclide could be slowed down to a velocity around 5.3 m/a due to the reduction of the hydraulic gradient under the graded site setting. Therefore, the conservative radioactive wastes could be disposed at the mid-eastern part of the site under the graded site setting because the transport path has been prolonged and thus no conservative radionuclides could migrate out of the site in a reasonable timeframe. For the sorbing tracers, however, results of the computed transport distance are 40 and 2 m at 500 years, respectively. Therefore, they can be disposed safely at the site under both natural and graded site settings. This study provides an insight to the field-scale long-term behaviors of radionuclide transport. The integrated modeling method presented in this study is most useful for the environmental impact assessment of the site conditions relevant to the safe disposal of hazardous wastes.

Anisotropic diffusion at the field scale in a 4-year multi-tracer diffusion and retention experiment – I: Insights from the experimental data

Claystones are considered worldwide as barrier materials for nuclear waste repositories. In the Mont Terri underground research laboratory (URL), a nearly 4-year diffusion and retention (DR) experiment has been performed in Opalinus Clay. It aimed at (1) obtaining data at larger space and time scales than in laboratory experiments and (2) under relevant in situ conditions with respect to pore water chemistry and mechanical stress, (3) quantifying the anisotropy of in situ diffusion, and (4) exploring possible effects of a borehole-disturbed zone. The experiment included two tracer injection intervals in a borehole perpendicular to bedding, through which traced artificial pore water (APW) was circulated, and a pressure monitoring interval. The APW was spiked with neutral tracers (HTO, HDO, H2O-18), anions (Br, I, SeO4), and cations (Na-22, Ba-133, Sr-85, Cs-137, Co-60, Eu-152, stable Cs, and stable Eu). Most tracers were added at the beginning, some were added at a later stage. The hydraulic pressure in the injection intervals was adjusted according to the measured value in the pressure monitoring interval to ensure transport by diffusion only. Concentration time-series in the APW within the borehole intervals were obtained, as well as 2D concentration distributions in the rock at the end of the experiment after overcoring and subsampling which resulted in ∼250 samples and ∼1300 analyses. As expected, HTO diffused the furthest into the rock, followed by the anions (Br, I, SeO4) and by the cationic sorbing tracers (Na-22, Ba-133, Cs, Cs-137, Co-60, Eu-152). The diffusion of SeO4 was slower than that of Br or I, approximately proportional to the ratio of their diffusion coefficients in water. Ba-133 diffused only into ∼0.1m during the ∼4a. Stable Cs, added at a higher concentration than Cs-137, diffused further into the rock than Cs-137, consistent with a non-linear sorption behavior. The rock properties (e.g., water contents) were rather homogeneous at the centimeter scale, with no evidence of a borehole-disturbed zone. In situ anisotropy ratios for diffusion, derived for the first time directly from field data, are larger for HTO and Na-22 (∼5) than for anions (∼3–4 for Br and I). The lower ionic strength of the pore water at this location (∼0.22M) as compared to locations of earlier experiments in the Mont Terri URL (∼0.39M) had no notable effect on the anion accessible pore fraction for Cl, Br, and I: the value of 0.55 is within the range of earlier data. Detailed transport simulations involving different codes will be presented in a companion paper.

Diffusion–reaction studies in low permeability shale using X-ray radiography with cesium

An X-ray radiography method for the determination of diffusion coefficients in rock has been modified to provide an estimation of diffusion–reaction parameters for the sorbing tracer, cesium (Cs+). Cesium tracer diffusion and sorption on cation exchange sites can be monitored in intact rock samples using data extracted from radiographs to plot time-series relative-concentration profiles of Cs+ as a function of transport distance. Cesium was found to be a very good tracer for these experiments because of the sensitivity of the X-ray attenuation measurements to Cs+ concentration.Reactive-transport modeling coupled with parameter estimation software was used to match experimental data and estimate pore diffusion coefficients for Cs+ (Dp-Cs=7.6×10−11m2/s), single-site selectivity coefficients for Cs+ exchange (logKCs+/Na+=1.5) and cation exchange capacity (CEC=8.4meq/100g) for drill core samples of Queenston Formation shale, from the Michigan Basin in Ontario, Canada.

KURT 암반 단열에서 현장 용질이동 실험: 실험 장치 설치 및 현장 용질 이동 실험

An in situ solute migration system was designed and installed in KAERI Underground Research Tunnel (KURT) constructed in the site of Korea Atomic Energy Research Institute (KAERI) in order to investigate the migration and retardation of non-sorbing and sorbing tracers through a rock fracture. The system is composed of three main parts including injection, extraction, and data treatment. For the selection of a water-conducting fracture, boreholes were drilled. The fractures in the drilled boreholes were investigated using borehole image analysis using borehole image processing system (BIPS). The results of BIPS analy -sis showed that borehole YH 3-1 and YH 3-2 were connected each other. Moreover, hydraulic tests were carried out to determine the test section with connectivity for the in situ experiments. The in situ solute migration experiments were accomplished to understand the migration of solutes through fractures in KURT using non-sorbing tracers which were fluorescein sodium, eosin-B, bromide and sorbing tracers which were rubidium, nickel, zirconium, and samarium.

Modeling of Cs+ diffusion and retention in the DI-A2 experiment (Mont Terri). Uncertainties in sorption and diffusion parameters

In the DI-A2 experiment several non-reactive and reactive tracers were injected as a pulse in a packed-off borehole in the Opalinus Clay. Unlike the previous DI-A1 test, the design of the Teflon filter in the injection borehole forced the water to flow through the filter and the open space between the filter and the borehole wall (the filter itself did not act as a diffusion barrier between the circulating solution and the rock). The decrease in tracer concentration in the liquid phase was monitored during a period of a year. Afterwards, the borehole section was overcored and the tracer profiles in the rock were analyzed. A main interest of this experiment was to understand the chemical behavior of sorbing tracers: Cs+ (stable), 85Sr2+, 60Co2+ and Eu3+ (stable). The complete dataset (except for Eu3+ because of strong sorption to experimental equipment) was analyzed in a previous study with a 2D diffusion–reaction model and the derived diffusion and sorption parameters were compared with laboratory data. As in DI-A1, a difference by a factor of about 2 for sorption (magnitude of the Freundlich isotherm) was obtained between in situ and laboratory batch sorption experiments.Recent experimental and modeling studies have shown equivalent Cs+ sorption on intact and disaggregated Opalinus Clay samples. In view of these developments, new modeling of Cs+ diffusion and retention in the DI-A2 experiment has been performed using CrunchFlow. The calculations include transport by diffusion and a multisite cation exchange model to account for the retention of Cs+. The new results show that upscaling of Cs+ sorption from laboratory to field is no longer required. However, a difference in sorption by a factor of about 2 is still explained by the use of different versions of the same cation exchange model (a small difference in the selectivity coefficient for one type of site). This uncertainty in sorption leads to an uncertainty in the effective diffusion coefficient (De) for Cs+, also by a factor of 2 (2–4×10−10m2/s). Clearly, the values of De obtained are correlated with the strength of sorption in the model, with stronger sorption leading to larger De values. Discrimination between the two versions of the exchange model is not possible when using only the results of the in situ test. Additionally, during early times (t<10days) the drop in Cs+ concentration in the circulation system is slower than expected. Due to the experimental setup, this slow decrease in concentration cannot be caused by the filter in the contact between borehole and rock. Poor mixing in the circulation system could explain this effect.

Inverse Estimation of the Effective Diffusion of the Filter in the In Situ Diffusion and Retention (DR) Experiment

Porous filters are often used in laboratory and in situ diffusion and retention experiments. The proper interpretation of these experiments requires knowing the effective diffusion, De, of the filter which is commonly determined from laboratory diffusion experiments or estimated from the filter porosity. The De of the filter in the in situ experiment may differ from the De of the filter measured in the laboratory due to pore clogging. Here, we present an inverse method to estimate the De of the filter of in situ diffusion experiments. The method has been tested for several sampling schemes, numbers of synthetic data, N, and standard deviations of the noise, σ. It has been applied to the following tracers used in the in situ diffusion and retention (DR) experiment performed in the Opalinus clay at Mont Terri underground research laboratory: HTO/HDO, Br−,I−, 22 Na+,133 Ba2+,85 Sr2+, Cs+/137Cs+, and 60Co2+. The estimation error increases with the standard deviation of the noise of the data and decreases with the number of data. It is smallest for sorbing tracers. The De of the filter can be properly estimated from 12 data collected within the first 3 days for conservative tracers as long as σ ≤ 0.02 and for sorbing tracers as long as σ ≤ 0.05. The estimate of De for conservative tracers is poor when data are collected from a 10-day experiment with daily sampling. The convergence of the estimation algorithm for conservative tracers improves by starting with a value of the De smaller than the true value. The choice of the initial value of De does not affect the convergence of the estimation algorithm for sorbing tracers. Filter clogging and vertical flow though the filter can influence the tracer transport through the filter. The use of the De of the filter obtained from a laboratory test for the in situ experiment may result in large errors for strongly sorbing tracers. Such errors can be overcome by estimating the equivalent De of the filter with the proposed inverse method which will be useful for the design of in situ diffusion experiments.

Experimental and modeling analysis of coupled non-Fickian transport and sorption in natural soils

We present experimental breakthrough curve (BTC) data and a modeling investigation of conservative and sorbing tracer transport in natural soils. By analyzing the data using the continuous time random walk (CTRW) model, we probe the traditional approach of using conservative tracer model parameters as a basis for quantifying the transport of sorbing solutes in the same domain when non-Fickian transport is present. Many known contaminants in groundwater are sorbed to the host solid porous medium, to varying extents, while being transported; this enhances the long tailing of BTCs which often already occurs because of the inherent non-Fickian nature of the transport. The CTRW framework has been shown to account very well for non-Fickian conservative (nonsorbing) transport. Here, we examine two BTC data sets in laboratory columns packed with natural soils; the first (previously analyzed by Mao and Ren (2004)) comprises transport of (conservative) bromide and (sorbing) atrazine tracers, while the second presents new data with bromide and tribromoneopentyl alcohol (TBNPA), a key flame retardant, as a sorbing solute. TBNPA has received little attention in the past, and is shown to be sorbed onto Bet Dagan soil in a nonlinear manner. We find that the transport behavior of bromide is non-Fickian in all cases, which is caused by the heterogeneity of the soil. Comparative model analysis of the non-Fickian BTCs of the conservative, and sorbing tracers and examination of the fitting parameters, exemplify the coupling between transport and adsorption/desorption processes. The difference in transport parameters used to match the conservative and sorbing data sets shows that conservative tracer parameters (average velocity and dispersion coefficient) are not valid for the transport of reactive tracers.

Solute transport and retention in three‐dimensional fracture networks

Resolving the hydrodynamic control of retention is an important step in predictive modeling of transport of sorbing tracers in fractured rock. The statistics of the transport resistance parameter β [T/L] and the related effective active specific surface area sf [1/L] are studied in a crystalline rock volume on a 100 m scale. Groundwater flow and advective transport are based on generic boundary conditions and realistic discrete fracture networks inferred from the Laxemar site, southeast Sweden. The overall statistics of β are consistent with statistics of the water residence time τ; the moments of β vary linearly with distance, at least up to 100 m. The correlation between log τ and log β is predominantly linear, however, there is significant dispersion; the parameter sf strongly depends on the assumed hydraulic law (theoretical cubic or empirical quadratic). Fast and slow trajectories/segments in the network determine the shape of the β distribution that cannot be reproduced by infinitely divisible model over the entire range; the low value range and median can be reproduced reasonably well with the tempered one‐sided stable density using the exponent in the range 0.35–0.7. The low percentiles of the β distribution seems to converge to a Fickian type of behavior from a 50 to 100 m scale.

Single-well and large-scale cross-hole tracer experiments in fractured rocks at two sites in Sweden

As part of the Swedish site investigations and research associated with the disposal of spent nuclear fuel, tracer experiments in deep boreholes have been employed in order to characterize solute transport and hydraulic connectivity in the fractured bedrock at two sites—Forsmark and Laxemar-Simpevarp. Performance and analytical results are presented from a suite of tracer experiments in varying scales, from single-hole injection-withdrawal tests, intermediate-scale tests with sorbing tracers, to large-scale connectivity tests over distances up to several hundred meters in major fracture zones or networks of zones. In addition to demonstrating transport connectivity over large distances, a general result is that the single-hole tests, as well as the cross-hole tests with sorbing tracers, have clearly demonstrated the process of solute retention of water-conductive features in the rock at different scales.

Modeling and analysis of the migration of HTO and237Np in a fractured granite core at the Äspö hard rock laboratory

AbstractMigration characteristics of HTO and237Np were analyzed through experiments performed with a fractured granite core in the Chemlab2 probe at the Äspö hard rock laboratory in Sweden. These experimental data covered the elution of HTO and237Np as well as information of retained actinides onto the fracture surfaces. The flow field was simulated with a two-dimensional variable aperture channel model. The mass transport process was described with a random-walk method. Three types of basic migration processes were considered: advection, sorption and matrix diffusion. By a combination of these processes, specific effects of each process on transport were evaluated. The retardation of the neptunium were analysed quantitatively by the model. The results were compared to the experimental findings in terms of the elution curves and of distribution patterns on the fracture surface. The elution curves of HTO were used to determine the hydraulic propert ies in the fractured core such as the extent of a dispersion effect according to the flow rates. After termination of the migration test, the rock core was opened, and the retained radioactivities onto the fracture surfaces were measured to investigate an extend of237Np retardation and the retardation mechanism. A retardation factor was obtained by comparing the simulated results to the distribution of sorbed tracers along the flow path. The validity of sorption data from batch tests was also discussed for estimating migration of sorbing tracers.

HTO as a conservative tracer used for characterization of contaminant migration in porous rock environment

Methodology for diffusion coefficient determination was applied on sandstone samples, using conservative non sorbing tracer. The results proved that methodology, through-diffusion cell design and GoldSim diffusion module can be used for sandstone samples in order to determine important migration parameters, necessary for transport model input. However, rock heterogeneity factor has to be taken into account in any case as it can potentially change rock properties, even within centimeter scale. Effective diffusion coefficient De for rock samples was determined. The values fell into the range of 1.0–6.17 × 10−10 m2 s−1. Discrepancies in measured and simulated porosity were observed. Statistical analyses revealed that values of diffusion coefficient De were in close interrelation to primary mineral (quartz) and cement forming minerals (kaolinite and organic matter).

Transport and retention from single to multiple fractures in crystalline rock at Äspö (Sweden): 1. Evaluation of tracer test results and sensitivity analysis

We evaluate the breakthrough curves obtained within a comprehensive experimental program for investigating the retention properties of crystalline rock, referred to as Tracer Retention Understanding Experiments (TRUE). The tracer tests were conducted at the Äspö Hard Rock Laboratory (Sweden) in two phases jointly referred to as TRUE Block Scale (TBS); the TBS tests comprise a total of 17 breakthrough curves with nonsorbing and a range of sorbing tracers. The Euclidian length scales are between 10 and 30 m, compared to 5 m for the earlier tests TRUE‐1. The unlimited diffusion model is consistent with measured breakthrough curves and is adopted here for evaluation. The model has four independent parameters, two of which are related to advection and dispersion, one which is related to diffusion‐sorption, and one which is related to surface sorption; the individual retention parameters or properties cannot be inferred from breakthrough curves alone and require additional constraints. The mean water residence times for the TBS tests are in the range 15–250 h, whereas the coefficient of variation of the water residence times is in the range 0.4–0.6. A consistent trend is found in the calibrated retention parameters with the sorption affinities of the tracers involved. Using Bode sensitivity functions, it is shown that sensitivity increases for the retention parameter with increasing sorption affinity; for nonsorbing tracers, diffusion and hydrodynamic dispersion are shown to “compete,” exhibiting similar effects; hence, their estimates are uncertain. The analysis presented here exposes a few fundamental limitations and sensitivities when evaluating diffusion‐controlled retention in the subsurface; it is general and applicable to any site with comparable tracer test data. In part 2, it will be shown how discrete fracture network simulations based on the hydrostructural information available can be used for further constraining individual retention parameters, in particular, the active specific surface area (sf) and the rock matrix porosity (θ).

Numerical model of a long-term in situ diffusion and retention (DR) experiment in Opalinus Clay

Determining diffusion parameters under real conditions is necessary for the performance assessment of a deep geological repository. Here we present a numerical model for the interpretation of a long-term in situ diffusion and retention (DR) experiment performed on Opalinus clay (OPA) at Mont Terri underground rock laboratory in Switzerland using a comprehensive set of tracers which includes neutral (HTO, HDO), anionic (Cl, Br, I), weakly-sorbing (22Na, 133Ba and 85Sr) and strongly-sorbing tracers (60Co, 137Cs). Compared to the model used for scoping calculations, our current model accounts for the presence of a sintered filter, a gap between the filter and borehole wall and an excavation disturbed zone (EdZ). Sensitivity analyses have been performed to identify the relevance of these non-ideal effects and evaluate identifiability of diffusion anisotropy and distribution coefficients. Results of sensitivity analyses indicate that tracer concentrations are not sensitive to De of gap, but very sensitive to De of filter except for HTO and HDO. Concentrations of HTO, HDO, I, Na, Sr and Ba are sensitive to De of EdZ while those of strongly-sorbing tracers such as Co and Cs are much less sensitive. All sorbing tracers are sensitive to changes in Kd of clay. The relevance of diffusion anisotropy increases with time for all tracers except strongly-sorbing tracers.

Scale dependence of sorption coefficients for contaminant transport in saturated fractured rock

A significant challenge in contaminant transport modeling is to obtain a mechanistic understanding of transport parameter scaling that accurately addresses the combined influence of physical and chemical heterogeneities at different scales. In this paper, we have developed a scaling methodology to upscale matrix sorption coefficients for fractured‐rock systems by characterizing both the tortuosity field (physical heterogeneity) and retardation factor field (chemical heterogeneity) in the rock matrix. We compute the effective tortuosity with a conservative tracer (e.g., tritium), and then using a sorbing tracer (e.g., uranium), we derive the equations for upscaling the sorption coefficients in a saturated, fractured rock system. The derived upscaling equations for the sorption coefficients are verified with Monte Carlo simulations, which are based on a generalized dual‐porosity model to enable highly efficient and accurate numerical simulations of diffusive concentration fronts moving between the fractures and matrix material. The scientific results from this study will provide a theoretical and practical link between controlled experimental results at scales increasing from the laboratory bench to the field scale at which risk assessment and contaminant remediation are actually conducted.

Sorbing tracer experiments in a crystalline rock fracture at Äspö (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, 5 m) at Äspö (Sweden). The analysis is based on the microscale characterization results as presented in part 1 of this series. Our main objective in this last part of the series is to provide an independent interpretation (or “prediction”) of the effective parameters as estimated from calibration in part 2 by combining the data presented in part 1 with analytical and numerical transport modeling. We show here that the independent information from microscopic characterization can be used for “predicting” the effective diffusion time td reasonably well; a discrepancy is to be expected given the uncertainties of microscale retention properties, in particular of the sorption coefficient.