A series of batch experiments were performed to assess the uranium sorption capacity of four mineralogically distinct lithologies from the Negev Desert, Israel, to evaluate the suitability of a ...potential site for subsurface radioactive waste disposal. The rock specimens consisted of an organic-rich phosphorite, a bituminous marl, a chalk, and a sandstone. The sorption data for each lithology were fitted using a general composite surface complexation model (GC SCM) implemented in PHREEQC. Sorption data were also fitted by a non-mechanistic Langmuir sorption isotherm, which can be used as an alternative to the GC SCM to provide a more computationally efficient method for uranium sorption. This is because all the rocks tested have high pH/alkalinity/calcium buffering capacities that restrict groundwater chemistry variations, so that the use of a GC SCM is not advantageous. The mineralogy of the rocks points to several dominant sorption phases for uranyl (UO22+), including apatite, organic carbon, clays, and iron-bearing phases. The surface complexation parameters based on literature values for the minerals identified overestimate the uranium sorption capacities, so that for our application, an empirical approach that makes direct use of the experimental data to estimate mineral-specific sorption parameters appears to be more practical for predicting uranium sorption.
The scale dependence of the matrix diffusion coefficient (Dm) for fractured media has been observed at variable scales from column experiments to field tracer tests. In this paper, we derive an ...effective Dm for multimodal heterogeneous fractured rocks using characteristic distributions of matrix properties and volume averaging of the mass transfer coefficient. The effective field‐scale Dm is dependent on the statistics (geometric mean, variance, and integral scale) of laboratory‐scale ln(Dm) and on the domain size. The effective Dm increases with the integral scales and is larger than the geometric mean of ln(Dm). Monte Carlo simulations with 1000 realizations of heterogeneous Dm fields were conducted to assess the accuracy of the derived effective Dm.
•Li+ and Cs+ cation exchange parameters were estimated for an EGS demonstration site.•Data aid interpretation of single-well tests to interrogate fracture surface area.•Li+ transport shows strong ...temperature dependence.•Cs+ transport indicates weaker temperature dependence.
A series of column transport experiments ranging from 25°C to 275°C, as well as batch sorption experiments at 25°C, were conducted to estimate cation exchange parameters for lithium and cesium at the Newberry Crater Enhanced Geothermal System demonstration site. The experiments were designed to facilitate interpretation of single-well field tracer tests to interrogate fracture surface area. Lithium column transport from 125°C to 275°C showed a strong temperature dependence, with much greater cation exchange at higher temperatures than in 25°C experiments. Cesium column transport at 225°C indicated a weaker temperature dependence, and unlike Li+, its exchange decreased at higher temperatures.
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, phi(m)/b radicalD(m) (sec(-1/2)) (where phi(m) is the matrix porosity, b is the effective fracture half-aperture, and D(m) 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 D(m) values measured at different timescales in laboratory experiments using the same rocks. Also, the overall variability in phi(m) and D(m) in the laboratory experiments is not large enough to account for the order-of-magnitude smaller field-scale values of phi(m)/b radicalD(m) compared to lab-scale values. We conclude that experimentally observed trends of phi(m)/b radicalD(m) 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 phi(m)/b radicalD(m) from laboratory and field tracer experiments are to be reliably extrapolated to time and distance scales in risk assessment models.
A cross‐hole tracer test involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients (bromide and pentafluorobenzoate) and a weakly sorbing solute ...tracer (lithium ion) was conducted in a fractured granite near an underground nuclear test cavity in central Nevada. The test was conducted to (1) test a conceptual radionuclide transport model for the site and (2) obtain transport parameter estimates for predictive modeling. The differences between the responses of the two nonsorbing tracers (when normalized to injection masses) are consistent with a dual‐porosity transport system in which matrix diffusion is occurring. The large concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggests that diffusion occurs primarily into matrix pores, not simply into stagnant water within the fractures. The relative responses of the tracers at late times suggest that the diffusion‐accessible matrix pore volume is possibly limited to only half the total volume of the flow system, implying that the effective retardation factor due to matrix diffusion may be as small as 1.5 for nonsorbing solutes in the system. The lower end of the range of possible sorption Kd values deduced from the lithium response is greater than the upper 95% confidence bound of Kd values measured in laboratory sorption tests using crushed granite from the site. This result suggests that the practice of using laboratory sorption data in field‐scale transport predictions of cation‐exchanging radionuclides, such as 137Cs+ and 90Sr++, should be conservative for the SHOAL site.
The disposal of high-level radioactive waste in deep geological repositories is a critical environmental issue. The presence of bentonite colloids generated in the engineering barrier can ...significantly impact the transport of radionuclides, but their effect on radionuclide sorption in granite remains poorly understood. This study aimed to investigate the sorption characteristics of strontium (Sr) on granite as well as on the coexistence system of granite and colloids under various hydrogeochemical conditions, through batch experiments. Fourier transform infrared spectroscopy was employed to analyze the sorption forms of Sr on granite before and after sorption. Several hydrogeochemical factors were examined, including contact time, pH, ionic strength, coexisting ions, and bentonite and humic acid colloid concentration. Among these factors, the concentration of bentonite colloids exhibited a significant effect on Sr sorption. Within a specific range of colloid concentration, the sorption of Sr on the solid system increased linearly with the bentonite colloid concentration. pH and ionic strength were also found to play crucial roles in the sorption process. At low pH, Sr sorption primarily occurred through the outer sphere's surface complexation and Na+/H+ ion exchange. However, at high pH, inner sphere surface complexation dominated the process. As the ionic strength increased, electrostatic repulsion gradually increased, resulting in fewer binding sites for particle aggregation and Sr sorption on bentonite colloids. The results also indicate that with increasing pH, the predominant forms of Sr in the solution transitioned from SrHCO3+ and SrCl+ to SrCO3 and SrCl+. This was mainly due to the ion exchange of Ca2+/Mg2+ in plagioclase and biotite, forming SrCO3 precipitation. These findings provide valuable insights into the transport behavior of radionuclides in the subsurface environment of the repository and highlight the importance of considering bentonite colloids and other hydrogeochemical factors when assessing the environmental impact of high-level radioactive waste disposal.
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•Batch experiments determined impact of bentonite colloids on Sr sorption on granite.•As Na+ ionic strength increases, Sr sorption on bentonite colloids decreases.•As pH increases, Sr sorption shifts from ion exchange to inner sphere complexation.•FTIR was used to explore Sr sorption form on granite before and after sorption.
Colloids have the potential to enhance the mobility of strongly sorbing radionuclide contaminants in groundwater at underground nuclear test sites. This study presents an experimental and numerical ...investigation of colloid-facilitated plutonium transport in fractured porous media to identify plutonium reactive transport processes. The transport parameters for dispersion, diffusion, sorption, and filtration are estimated with inverse modeling by minimizing the least-squares objective function of multicomponent concentration data from multiple transport experiments with the shuffled complex evolution metropolis algorithm. Capitalizing on an unplanned experimental artifact that led to colloid formation, we adopt a stepwise strategy to first interpret the data from each experiment separately and then to incorporate multiple experiments simultaneously to identify a suite of plutonium–colloid transport processes. Nonequilibrium or kinetic attachment and detachment of plutonium–colloid in fractures were clearly demonstrated and captured in the inverted modeling parameters along with estimates of the source plutonium fraction that formed plutonium–colloids. The results from this study provide valuable insights for understanding the transport mechanisms and environmental impacts of plutonium in groundwater aquifers.
Colloids play a crucial role in influencing the mobility of radionuclides in high-level radioactive waste repositories. However, the co-transport behavior of radionuclides and colloids in geological ...media remains insufficiently understood. This study investigated the transport of Strontium (Sr) in four types of granite minerals (quartz, biotite, K-feldspar, and plagioclase) in the presence and absence of Na-bentonite colloids (Na-BC) using column experiments. Employing a stepwise modeling strategy, this study first determined the basic flow and transport parameters through breakthrough curve analysis of the conservative tracer (Br). Then, the experimental data of Sr in the colloid-free Sr transport experiments and Na-BC in the Sr-colloid co-transport experiments were quantitatively explained using a two-site sorption model and a two kinetic sites model, respectively. Finally, the co-transport behavior of Sr and Na-BC was fitted using the colloid-facilitated solute transport model. The stepwise modeling allowed quantification of the kinetics of Sr sorption onto mobile and immobile Na-BC and highlighted the role of straining in Na-BC retention. In the absence of Na-BC, Sr transport experienced the greatest retardation in biotite, followed by plagioclase, K-feldspar, and quartz, respectively, which positively correlated with the specific surface area of the minerals. Moreover, Na-BC enhanced Sr transport with the same retardation order in all tested minerals, with recovery rate increments of 68.61%, 19.21%, 6.67%, and 4.33%, respectively. The transport of Sr in K-feldspar was found to be less affected by Na-BC compared to the other tested minerals, likely due to the strong cation exchange capacity of K+ among the cation components of minerals, making hydrated K+ more likely to exchange with Sr2+ on K-feldspar surfaces. These findings hold significant implications for assessing the risks associated with the transport of radionuclides in deep geological repositories.
The co-transport behavior of radionuclides and colloids in geological media, as demonstrated in this study, highlights the significant influence of colloids on strontium (Sr) mobility within granite minerals. The observed enhancement of Sr transport in the presence of Na-bentonite colloids emphasizes the potential role of colloids in facilitating radionuclide migration. These findings offer valuable insights into radionuclide transport and underscore the necessity for comprehensive assessment and management strategies for radioactive waste in the context of geological disposal.
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•Co-transport of Sr and Na-BC in four granite minerals were investigated.•Stepwise modeling was employed to describe the co-transport of Sr and Na-BC.•Na-BC showed greatest facilitation of Sr migration in biotite.•Na-BC had a weak impact on Sr transport in K-feldspar due to cation exchange.
Matrix-diffusion parameters deduced from an infiltration tracer test at Idaho National Laboratory (INL), USA, are combined with other site information in an analysis involving two dimensionless ...lumped parameters to assess the effects of matrix diffusion on contaminant transport at the INL over longer distance and time scales than were evaluated in the test. Matrix diffusion was interrogated in the test by comparing, in three different observation wells, the breakthrough curves of two simultaneously injected nonsorbing solutes that have different diffusion coefficients. The matrix-diffusion parameters deduced from the different breakthrough curves were in good agreement, suggesting that the parameters may be broadly applicable at the INL. With this in mind, the uncertainties in the individual parameters that make up the two lumped parameters were estimated, and the resulting ranges of parameter values were used to assess matrix diffusion over larger scales. Assessments of the effects of flow transients, spatial heterogeneity in transport parameters, and sorption on solute transport in the shallow subsurface flow system were also conducted. The methods presented here should be generally applicable to other settings for making bounding assessments of the effects of matrix diffusion while honoring the information obtained from tracer tests and other supporting data.
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