Geochemical evaluation of different groundwater–host rock systems for radioactive waste disposal

The geochemical suitability of a deep bedrock repository for radioactive waste disposal is determined by the composition of geomatrix and groundwater. Both influence radionuclide solubility, chemical buffer capacity and radionuclide retention. They also determine the chemical compatibility of waste forms, containers and backfill materials. Evaluation of different groundwater–host rock systems is performed by modeling the geochemical environments and the resulting radionuclide concentrations. In order to demonstrate the evaluation method, model calculations are applied to data sets available for various geological formations such as granite, clay and rocksalt.The saturation state of the groundwater–geomatrix system is found to be fundamental for the evaluation process. Hence, calculations are performed to determine if groundwater is in equilibrium with mineral phases of the geological formation. In addition, corrosion of waste forms in different groundwater is examined by means of reaction path modeling. The corrosion reactions change the solution compositions and pH, resulting in significant changes of radionuclide solubilities. The results demonstrate that geochemical modeling of saturation state and compatibility of the host formation environment with the radioactive waste proves to be a feasible tool for evaluation of various sites considered as deep underground repositories.     

The influence of colloid formation in a granite groundwater bentonite porewater mixing zone on radionuclide speciation

In the context of deep geological storage of high level nuclear waste the repository will be designed as multiple barrier system including bentonite as buffer/backfill material and the host rock formation as geological barrier. The engineered barrier (bentonite) will be in contact with the host rock formation and consequently it can be expected that bentonite porewater will mix with formation groundwater. We simulate in this study the mixing of Grimsel groundwater (glacial melt water) with synthetic Febex porewater (assuming already saturated state) in a batch-type study and investigate the formation of colloids by laser-induced breakdown detection (LIBD) and SEM-EDX as well as the changes in radionuclide (U, Th, Eu) speciation via ultrafiltration or via time-resolved laser fluorescence spectroscopy (TRLFS) analysis in the case of Cm(III). Based on PHREEQC saturation index (SI) calculations a precipitation of calcite might be expected at low Febex porewater (FPW) content (< 20%), fluorite precipitation at FPW contents < 60% and gibbsite precipitation at FPW contents above 10%. The colloids generated in the mixing zone aggregate when the synthetic FPW content exceeds 10%. LIBD analysis of the time-dependent colloid generation/aggregation revealed a low concentration of colloids to be stable with an estimated plateau value around 100–200 ppt and an average colloid diameter around 30 nm after 140 days reaction time at FPW admixture > 10%. SEM/EDX mostly identifies Al/Si containing colloidal phases and some sulfates could be found under certain admixture ratios. TRLFS studies show that the Cm speciation is strongly influenced by colloid formation in all solutions. In the Febex pore water/GGW mixing zone with high groundwater contents (> 80%) colloids are newly formed and Cm is almost quantitatively associated with most likely polysilicilic acid colloids.     

Modelling geochemical and microbial consumption of dissolved oxygen after backfilling a high level radiactive waste repository

Dissolved oxygen (DO) left in the voids of buffer and backfill materials of a deep geological high level radioactive waste (HLW) repository could cause canister corrosion. Available data from laboratory and in situ experiments indicate that microbes play a substantial role in controlling redox conditions near a HLW repository. This paper presents the application of a coupled hydro-bio-geochemical model to evaluate geochemical and microbial consumption of DO in bentonite porewater after backfilling of a HLW repository designed according to the Swedish reference concept. In addition to geochemical reactions, the model accounts for dissolved organic carbon (DOC) respiration and methane oxidation. Parameters for microbial processes were derived from calibration of the REX in situ experiment carried out at the Asp? underground laboratory. The role of geochemical and microbial processes in consuming DO is evaluated for several scenarios. Numerical results show that both geochemical and microbial processes are relevant for DO consumption. However, the time needed to consume the DO trapped in the bentonite buffer decreases dramatically from several hundreds of years when only geochemical processes are considered to a few weeks when both geochemical reactions and microbially-mediated DOC respiration and methane oxidation are taken into account simultaneously.     

Present state and future directions of modeling of geochemistry in hydrogeological systems

A first step towards understanding and controlling the fate and dissemination of radioactive waste is to create a concise and comprehensive theoretical framework for the rather non-linear processes involved--hence, the need for geochemical models. Two classes of geochemical models are commonly used, i.e., static and hydrodynamic models. In contrast to static models, hydrodynamic models combine geochemical reactions with hydrogeological processes such as ground-water flow, diffusion and dispersion. In this review, we examine the present state of geochemical models in terms of included processes, thermodynamic databases, missing phenomena, numerical behavior and performance. It is shown that over the past decade, significant progress has been made with respect to modeling of geochemistry in hydrodynamic systems: this is illustrated by describing several applications. Finally, we focus on the perspectives of geochemical modeling in the assessment of the safety of nuclear waste disposal.     

Inverse modeling of multicomponent reactive transport through single and dual porosity media

Compacted bentonite is foreseen as buffer material for high-level radioactive waste in deep geological repositories because it provides hydraulic isolation, chemical stability, and radionuclide sorption. A wide range of laboratory tests were performed within the framework of FEBEX (Full-scale Engineered Barrier EXperiment) project to characterize buffer properties and develop numerical models for FEBEX bentonite. Here we present inverse single and dual-continuum multicomponent reactive transport models of a long-term permeation test performed on a 2.5 cm long sample of FEBEX bentonite. Initial saline bentonite porewater was flushed with 5.5 pore volumes of fresh granitic water. Water flux and chemical composition of effluent waters were monitored during almost 4 years. The model accounts for solute advection and diffusion and geochemical reactions such as aqueous complexation, acid-base, cation exchange, protonation/deprotonation by surface complexation and dissolution/precipitation of calcite, chalcedony and gypsum. All of these processes are assumed at local equilibrium. Similar to previous studies of bentonite porewater chemistry on batch systems which attest the relevance of protonation/deprotonation on buffering pH, our results confirm that protonation/deprotonation is a key process in maintaining a stable pH under dynamic transport conditions. Breakthrough curves of reactive species are more sensitive to initial porewater concentration than to effective diffusion coefficient. Optimum estimates of initial porewater chemistry of saturated compacted FEBEX bentonite are obtained by solving the inverse problem of multicomponent reactive transport. While the single-continuum model reproduces the trends of measured data for most chemical species, it fails to match properly the long tails of most breakthrough curves. Such limitation is overcome by resorting to a dual-continuum reactive transport model.     

Migration of acidic groundwater seepage from uranium-tailings impoundments, 2. Geochemical behavior of radionuclides in groundwater

In this second paper of a series on groundwater seepage from uranium tailings, the general geochemical behavior of radionuclides is described and then applied to data from the field site, Seepage Area A of the Nordic Main impoundment near Elliot Lake, Ontario. The delineation of radionuclide behavior requires (1) the calculation of total element concentration by the summation of concentrations of each element's isotopes (the isotopic concentrations are calculated from the isotopes' radioactivities), (2) the evaluation of solid-liquid interactions using total element concentrations, and (3), for particular isotopes, the evaluation of the extent to which parental geochemical behavior causes a deviation in the isotope's behavior from that of its total element. A computerized speciation program, WATRAD, is used to evaluate aqueous complexation and mineral saturation indices of radium, actinium, thorium, and uranium. Data from Seepage Area A on isotopes of these four elements plus ²¹⁰Pb show that the geochemical behavior of radionuclides can be best defined on an individual isotopic basis rather than on an elemental basis.     

Microbially mediated redox processes in natural analogues for radioactive waste

Natural analogues allow scientists to investigate biogeochemical processes relevant to radioactive waste disposal that occur on time scales longer than those that may be studied by time-limited laboratory experiments. The Palmottu U-Th deposit in Finland and the Bangombé natural nuclear reactor in Gabon involve the study of natural uranium, and are both considered natural analogues for subsurface radioactive waste disposal. The microbial population naturally present in groundwater may affect the redox conditions, and hence, the radionuclide solubility and migration. Therefore, groundwater samples from the two sites were investigated for microbial populations. The total numbers of cells ranged from 10(4) to 10(6) cells ml(-1). Iron-reducing bacteria (IRB) were the largest culturable microbial population in the Palmottu groundwater and were present at up to 1.3 x 10(5) cells ml(-1). Sulfate-reducing bacteria (SRB) and acetogens could also be cultured from the Palmottu groundwater. The numbers of IRB and SRB were largest in groundwater with the lowest uranium concentrations. Removal of dissolved U(VI) from solution was concomitant with the growth of IRB enrichment cultures and the reduction of iron. The redox buffer in the Palmottu groundwater consists of iron and uranium species, both of which are affected by IRB. IRB and aerobic heterotrophs were cultured from the Bangombé groundwater, where redox potentials are buffered by iron and organic carbon species. Microbial populations similar to those found at Palmottu and Bangombé are found throughout the Fennoscandian Shield, a potential host rock for subsurface radioactive waste disposal. These results confirm that microorganisms can be expected to play a role in stabilizing radioactive waste disposed of in the subsurface by lowering redox potential and immobilizing radionuclides.     

Modelling reactions between cement pore fluids and rock: implications for porosity change

The migration of groundwater equilibrated with cement from a deep geological disposal facility for radioactive wastes will perturb the chemical, minerological and physical properties of the geosphere in advance of the migration of radionuclides. Preliminary modelling of a simplified scenario has been conducted to assess these changes using appropriate data for mineral dissolution kinetics, the chemical composition of cement pore fluids, and the hydrogeological characteristics of fractured crystalline rock. Chemical exchanges between rock immediately adjacent to the engineered barriers of a waste disposal facility and pore fluids were evaluated using the speciation-reaction path code. which revealed rapid loss of Ca, and gains in Na and Si of the evolved fluids, with little change in pH. Secondary minerals show a sequence of calcium silicate hydrates, and zeolites. Precise definition of the overall mass balance is uncertain due to the absence of both thermodynamic data for many zeolites and kinetic data for the precipitation of feldspars. Modelling has demonstrated that reaction kinetics will be important in governing chemical exchanges for length scales up to 20 m. Radionuclide retardation will be enhanced by the growth of zeolites and calcium silicate hydrates.     

Identification and Characterization of Potential Discharge Areas for Radionuclide Transport by Groundwater from a Nuclear Waste Repository in Sweden

This paper describes solute transport modeling carried out as a part of an assessment of the long-term radiological safety of a planned deep rock repository for spent nuclear fuel in Forsmark, Sweden. Specifically, it presents transport modeling performed to locate and describe discharge areas for groundwater potentially carrying radionuclides from the repository to the surface where man and the environment could be affected by the contamination. The modeling results show that topography to large extent determines the discharge locations. Present and future lake and wetland objects are central for the radionuclide transport and dose calculations in the safety assessment. Results of detailed transport modeling focusing on the regolith and the upper part of the rock indicate that the identification of discharge areas and objects considered in the safety assessment is robust in the sense that it does not change when a more detailed model representation is used.     

Analysis of water flow paths: methodology and example calculations for a potential geological repository in Sweden

Safety assessment related to the siting of a geological repository for spent nuclear fuel deep in the bedrock requires identification of potential flow paths and the associated travel times for radionuclides originating at repository depth. Using the Laxemar candidate site in Sweden as a case study, this paper describes modeling methodology, data integration, and the resulting water flow models, focusing on the Quaternary deposits and the upper 150 m of the bedrock. Example simulations identify flow paths to groundwater discharge areas and flow paths in the surface system. The majority of the simulated groundwater flow paths end up in the main surface waters and along the coastline, even though the particles used to trace the flow paths are introduced with a uniform spatial distribution at a relatively shallow depth. The calculated groundwater travel time, determining the time available for decay and retention of radionuclides, is on average longer to the coastal bays than to other biosphere objects at the site. Further, it is demonstrated how GIS-based modeling can be used to limit the number of surface flow paths that need to be characterized for safety assessment. Based on the results, the paper discusses an approach for coupling the present models to a model for groundwater flow in the deep bedrock.     

A coupled THC model of the FEBEX in situ test with bentonite swelling and chemical and thermal osmosis

The performance assessment of a geological repository for radioactive waste requires quantifying the geochemical evolution of the bentonite engineered barrier. This barrier will be exposed to coupled thermal (T), hydrodynamic (H), mechanical (M) and chemical (C) processes. This paper presents a coupled THC model of the FEBEX (Full-scale Engineered Barrier EXperiment) in situ test which accounts for bentonite swelling and chemical and thermal osmosis. Model results attest the relevance of thermal osmosis and bentonite swelling for the geochemical evolution of the bentonite barrier while chemical osmosis is found to be almost irrelevant. The model has been tested with data collected after the dismantling of heater 1 of the in situ test. The model reproduces reasonably well the measured temperature, relative humidity, water content and inferred geochemical data. However, it fails to mimic the solute concentrations at the heater-bentonite and bentonite-granite interfaces because the model does not account for the volume change of bentonite, the CO(2)(g) degassing and the transport of vapor from the bentonite into the granite. The inferred HCO(3)(-) and pH data cannot be explained solely by solute transport, calcite dissolution and protonation/deprotonation by surface complexation, suggesting that such data may be affected also by other reactions.     

Groundwater quality evolution based on geochemical modeling and aptness testing for ingestion using entropy water quality and total hazard indexes in an urban-industrial area (Tiruppur) of Southern India

Environmental Science and Pollution Research - This study used geochemical modeling to understand the chemical evolution of groundwater, entropy water quality index to assess the aptness of...     

Description of hydrogeologic heterogeneity and evaluation of radionuclide transport at an underground nuclear test

Realistic models of contaminant transport in groundwater demand detailed characterization of the spatial distribution of subsurface hydraulic properties, while at the same time programmatic constraints may limit collection of pertinent hydraulic data. Fortunately, alternate forms of data can be used to improve characterization of spatial variability. We utilize a methodology that augments sparse hydraulic information (hard data) with more widely available hydrogeologic information to generate equiprobable maps of hydrogeologic properties that incorporate patterns of connected permeable zones. Geophysical and lithologic logs are used to identify hydrogeologic categories and to condition stochastic simulations using Sequential Indicator Simulation (SIS). The resulting maps are populated with hydraulic conductivity values using field data and Sequential Gaussian Simulation (SGS). Maps of subsurface hydrogeologic heterogeneity are generated for the purpose of examining groundwater flow and transport processes at the Faultless underground nuclear test, Central Nevada Test Area (CNTA), through large-scale, three-dimensional numerical modeling. The maps provide the basis for simulation of groundwater flow, while transport of radionuclides from the nuclear cavity is modeled using particle tracking methods. Sensitivity analyses focus on model parameters that are most likely to reduce the long travel times observed in the base case. The methods employed in this study have improved our understanding of the spatial distribution of preferential flowpaths at this site and provided the critical foundation on which to build models of groundwater flow and transport. The results emphasize that the impacts of uncertainty in hydraulic and chemical parameters are dependent on the radioactive decay of specific species, with rapid decay magnifying the effects of parameters that change travel time.     

Sensitivity analysis of radionuclide migration in compacted bentonite: a mechanistic model approach

Mechanistic model calculations for the migration of Cs, Ra, Am and Pb in compacted bentonite have been carried out to evaluate sensitivities with respect to different parameter variations. A surface chemical speciation/electric double layer model is used to calculate: (i) porewater composition and radionuclide speciation in solution and at the bentonite surface, yielding the distribution of mobile and sorbed species and (ii) interaction of diffusing species with negatively charged pore walls to obtain diffusion parameters. The basic scenario considers the interaction of compacted bentonite with a fresh-type groundwater; variations include the presence of bentonite impurities and saline groundwater. It is shown that these scenarios result in significant variations of porewater composition that affect migration via three mechanisms that can partly compensate each other: (1) effects on sorption through radionuclide complexation in solution, and competition of major cations for surface sites; (2) changes in radionuclide solution speciation leading to different diffusing species under different conditions; (3) effects on diffusion through changes in the electric double layer properties of the clay pores as a function of ionic strength.     

Long-term oxygen depletion from infiltrating groundwaters: model development and application to intra-glaciation and glaciation conditions

Processes that control the redox conditions in deep groundwaters have been studied. The understanding of such processes in a long-term perspective is important for the safety assessment of a deep geological repository for high-level nuclear waste. An oxidising environment at the depth of the repository would increase the solubility and mobility of many radionuclides, and increase the potential risk for radioactive contamination at the ground surface. Proposed repository concepts also include engineered barriers such as copper canisters, the corrosion of which increases considerably in an oxidising environment compared to prevailing reducing conditions. Swedish granitic rocks are typically relatively sparsely fractured and are best treated as a dual-porosity medium with fast flowing channels through fractures in the rock with a surrounding porous matrix, the pores of which are accessible from the fracture by diffusive transport. Highly simplified problems have been explored with the aim to gain understanding of the underlying transport processes, thermodynamics and chemical reaction kinetics. The degree of complexity is increased successively, and mechanisms and processes identified as of key importance are included in a model framework. For highly complex models, analytical expressions are not fully capable of describing the processes involved, and in such cases the solutions are obtained by numerical calculations. Deep in the rock the main source for reducing capacity is identified as reducing minerals. Such minerals are found inside the porous rock matrix and as infill particles or coatings in fractures in the rock. The model formulation also allows for different flow modes such as flow along discrete fractures in sparsely fractured rocks and along flowpaths in a fracture network. The scavenging of oxygen is exemplified for these cases as well as for more comprehensive applications, including glaciation considerations. Results show that chemical reaction kinetics control the scavenging of oxygen during a relatively short time with respect to the lifetime of the repository. For longer times the scavenging of oxygen is controlled by transport processes in the porous rock matrix. The penetration depth of oxygen along the flowpath depends largely on the hydraulic properties, which may vary significantly between different locations and situations. The results indicate that oxygen, in the absence of easily degradable organic matter, may reach long distances along a flow path during the life-time of the repository (hundreds to thousands of metres in a million years depending on e.g. hydraulic properties of the flow path and the availability of reducing capacity). However, large uncertainties regarding key input parameters exist leading to the conclusion that the results from the model must be treated with caution pending more accurate and validated data. Ongoing and planned experiments are expected to reduce these uncertainties, which are required in order to make more reliable predictions for a safety assessment of a nuclear waste repository.     

Investigating the role of gas bubble formation and entrapment in contaminated aquifers: Reactive transport modelling

In many natural and contaminated aquifers, geochemical processes result in the production or consumption of dissolved gases. In cases where methanogenesis or denitrification occurs, the production of gases may result in the formation and growth of gas bubbles below the water table. Near the water table, entrapment of atmospheric gases during water table rise may provide a significant source of O(2) to waters otherwise depleted in O(2). Furthermore, the presence of bubbles will affect the hydraulic conductivity of an aquifer, resulting in changes to the groundwater flow regime. The interactions between physical transport, biogeochemical processes, and gas bubble formation, entrapment and release is complex and requires suitable analysis tools. The objective of the present work is the development of a numerical model capable of quantitatively assessing these processes. The multicomponent reactive transport code MIN3P has been enhanced to simulate bubble growth and contraction due to in-situ gas production or consumption, bubble entrapment due to water table rise and subsequent re-equilibration of the bubble with ambient groundwater, and permeability changes due to trapped gas phase saturation. The resulting formulation allows for the investigation of complex geochemical systems where microbially mediated redox reactions both produce and consume gases as well as affect solution chemistry, alkalinity, and pH. The enhanced model has been used to simulate processes in a petroleum hydrocarbon contaminated aquifer where methanogenesis is an important redox process. The simulations are constrained by data from a crude oil spill site near Bemidji, MN. Our results suggest that permeability reduction in the methanogenic zone due to in-situ formation of gas bubbles, and dissolution of entrapped atmospheric bubbles near the water table, both work to attenuate the dissolved gas plume emanating from the source zone. Furthermore, the simulations demonstrate that under the given conditions more than 50% of all produced CH(4) partitions to the gas phase or is aerobically oxidised near the water table, suggesting that these processes should be accounted for when assessing the rate and extent of methanogenic degradation of hydrocarbons.     

Specific association of 36Cl with low molecular weight humic substances in soils

Soils initially contaminated with ³⁶Cl in the chloride form were subjected to solid–liquid extractions using a variety of reagents including deionised water and 1 M sodium hydroxide (NaOH). 1 M NaOH was found to result in the greatest recovery of ³⁶Cl from the soils, a result which provided initial evidence that radioactive chlorine became attached to humic substances present naturally within the soils. Deionised water and 1 M NaOH extracts were subjected to analysis involving separation by gel filtration chromatography (GFC). It was found that ³⁶Cl in 1 M NaOH extracts associated preferentially with low molecular weight (LMW) fractions of humic substances whereas, in deionised water extracts, ³⁶Cl appeared to be present exclusively in the chloride form. Previous literature evidence, mainly from highly organic forest soils, suggests that conversion of stable chlorine from chloride to organic forms can occur as a result of biological action. The present paper also presents good evidence for the specific attachment of stable chlorine (³⁷Cl) to a LMW humic fraction, again demonstrated using GFC separation. Current risk assessments of the deep geological disposal of solid radioactive wastes containing ³⁶Cl typically assume a very low degree of sorption based on the notion that the predominant environmental species of radiochlorine is chloride. This paper concludes with a brief discussion on the implications of organochlorine formation in the biosphere for assessment of the radiological impact of deep geological disposal of solid radioactive wastes.     

Porosity investigation of compacted bentonite using XRD profile modeling

Many countries intend to use compacted bentonite as a barrier in their deep geological repositories for nuclear waste. In order to describe and predict hydraulic conductivity or radionuclide transport through the bentonite barrier, fundamental understanding of the microstructure of compacted bentonite is needed. This study examined the interlayer swelling and overall microstructure of Wyoming Bentonite MX-80 and the corresponding homo-ionic Na(+) and Ca(2+) forms, using XRD with samples saturated under confined swelling conditions and free swelling conditions. For the samples saturated under confined conditions, the interparticle, or so-called free or external porosity was estimated by comparing the experimental interlayer distances obtained from one-dimensional XRD profile fitting against the maximum interlayer distances possible for the corresponding water content. The results showed that interlayer porosity dominated total porosity, irrespective of water content, and that the interparticle porosity was lower than previously reported in the literature. At compactions relevant for the saturated bentonite barrier (1.4-1.8 g/cm(3)), the interparticle porosity was estimated to ≤3%.     

Large-scale modeling of reactive solute transport in fracture zones of granitic bedrocks

Final disposal of high-level radioactive waste in deep repositories located in fractured granite formations is being considered by several countries. The assessment of the safety of such repositories requires using numerical models of groundwater flow, solute transport and chemical processes. These models are being developed from data and knowledge gained from in situ experiments such as the Redox Zone Experiment carried out at the underground laboratory of Äspö in Sweden. This experiment aimed at evaluating the effects of the construction of the access tunnel on the hydrogeological and hydrochemical conditions of a fracture zone intersected by the tunnel. Most chemical species showed dilution trends except for bicarbonate and sulphate which unexpectedly increased with time. Molinero and Samper [Molinero, J. and Samper, J. Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden). J. Hydraul. Res., 42, Extra Issue, 157–172] presented a two-dimensional water flow and solute transport finite element model which reproduced measured drawdowns and dilution curves of conservative species. Here we extend their model by using a reactive transport which accounts for aqueous complexation, acid–base, redox processes, dissolution–precipitation of calcite, quartz, hematite and pyrite, and cation exchange between Na⁺ and Ca²⁺. The model provides field-scale estimates of cation exchange capacity of the fracture zone and redox potential of groundwater recharge. It serves also to identify the mineral phases controlling the solubility of iron. In addition, the model is useful to test the relevance of several geochemical processes. Model results rule out calcite dissolution as the process causing the increase in bicarbonate concentration and reject the following possible sources of sulphate: (1) pyrite dissolution, (2) leaching of alkaline sulphate-rich waters from a nearby rock landfill and (3) dissolution of iron monosulphides contained in Baltic seafloor sediments. Based on these results, microbially mediated processes are postulated as the most likely hypothesis to explain the measured increase of dissolved bicarbonates and sulphates after tunnel construction.