Fractured Chalk Research Articles

Finite element modelling of pipe piles driven in low-to-medium density chalk under monotonic axial loading

Percussive driving in low-to-medium density chalk creates a thin annulus of fully de-structured ‘putty’ chalk around pile shafts and an outer annular zone where fracturing is more intense than in the natural chalk. This damage and the related generation, and subsequent equalisation, of excess pore pressures impacts the piles’ time-dependent axial loading behaviour, as do other ageing processes. This paper presents finite element analyses of open steel tubular piles driven for the recent ALPACA and ALPACA Plus research projects under monotonic axial loading to failure after extended ageing periods. A nonlinear elastic stiffness model with a nonlocal deviatoric strain-based Mohr-Coulomb failure criterion was employed, with different sets of properties to represent the de-structured, fractured chalk and intact chalk. It is shown that the piles’ axial responses are mainly controlled by the puttified chalk annuli. A simplified but efficient means is adopted to impose pre-loading chalk effective stress conditions that explicitly capture the effects of installation damage and subsequent ageing. The potential for strain-softening in the brittle chalk is examined and the effective stress paths developed in representative chalk elements throughout the loading are considered in conjunction with the mobilisation of accumulated deviatoric strains. The simulations indicate 264% higher shaft capacity in compression than in tension, which is mainly attributed to the internal chalk plug.

Natural attenuation of dissolved petroleum fuel constituents in a fractured Chalk aquifer: contaminant mass balance with probabilistic analysis

A plume-scale mass balance is developed to assess the natural attenuation (NA) of dissolved organic contaminants in fractured, dual-porosity aquifers. This methodology can be used to evaluate contaminant distribution within the aquifer, plume source term, contaminant biodegradation and plume status. The approach is illustrated for a site on the UK Upper Chalk aquifer impacted by petroleum fuel containing methyl tert -butyl ether (MTBE) and tert -amyl methyl ether (TAME). Variability in site investigation data and uncertainty in the mass balance were assessed using probabilistic analysis. The analysis shows that benzene, toluene, m/p-xylene and o-xylene (BTEX) compounds are biodegraded primarily by denitrification and sulfate reduction in the aquifer, with an equivalent plume-scale first-order biodegradation rate of 0.49 a −1 . Other biodegradation processes are less important. Sorption contributes to hydrocarbon attenuation in the aquifer but is less important for MTBE and TAME. Uncertainty in the plume source term and site hydrogeological parameters had the greatest effect on the mass balance. The probabilistic analysis enabled the most likely long-term composition of the plume source term to be deduced and provided a site-specific estimate of contaminant mass flux for the prediction of plume development. The mass balance methodology provides a novel approach to improve NA assessments for petroleum hydrocarbons and other organic contaminants in these aquifer settings. Supplementary material: Plume organic chemistry, aquifer hydraulic conductivity and probability distribution functions used for mass balance inputs are available at https://doi.org/10.6084/m9.figshare.c.7016429 Thematic collection: This article is part of the Monitoring the aquifers collection available at: https://www.lyellcollection.org/topic/collections/monitoring-the-aquifers

Radionuclide transport in fractured chalk under abrupt changes in salinity

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.

Numerical investigation of a district scale groundwater heat pump system: A case study from Colchester, UK

Nearly 40% of Europe’s total energy consumption is dedicated to buildings and heating/cooling make a significant part of this consumption. Groundwater heat pumps (GWHP) are highly efficient, and low-carbon technology that can supply heating/cooling to buildings on small or large scales. Thus, they contribute to achieving European targets of net-zero greenhouse gas emissions by 2050. In the literature, studies on the utilisation of GWHP at a district scale, particularly in chalk aquifers, are relatively rare. The implementation of district-scale geothermal heat pump (GWHP) systems poses several challenges, including dealing with the scale and complexity of the systems, addressing geological variability, managing high initial investments, balancing energy demand and supply, ensuring proper maintenance and monitoring, and mitigating potential environmental impacts. These challenges require careful consideration and strategic planning to ensure the successful deployment and sustainable operation of these systems., This study numerically investigates a district-scale GWHP system and analyses the thermal plume development created due to the heating operation, offering insights into system performance. A good match was found between field results and simulation results for water level increase and drawdown. However, there is a difference of approximately 11% in system efficiency between field tests and simulations due to the lower abstraction temperature detected in the simulation. The simulation results show that cooler water injection into the fractured chalk aquifer creates a thermal plume radially spanning out to 50 m. The thermal plume has no effect on the abstraction temperature and system performance. This result can be attributed to the large distance between injection and abstraction wells and the low hydraulic gradient.

Can effective porosity be used to estimate near-well protection zones in fractured chalk?

Protection of fractured carbonate aquifers is often based on a single-porosity description of a dual-porosity system. However, it is difficult to assess a trustworthy value of the effective porosity based on scientific principles; thus, a range of estimates is often suggested. The complexity of the problem is compounded by the fact that the effective porosity may be scale-dependent. This paper investigates whether it is possible to describe solute transport in fractured carbonate rocks with an equivalent porous medium model using a constant value of effective porosity. It is assumed that the dual-porosity model provides an acceptable description of transport mechanisms in fractured porous rock and that it is possible to estimate the parameters needed in the single-porosity models from results generated by the dual-porosity model. The effective porosity is estimated from the dual-porosity results that are used as targets. For Danish chalk, an effective porosity of 13% (11–17%) is estimated. However, it is demonstrated that the estimated effective porosity is only valid at the specific transport time (1 year) from which simulation results of the dual-porosity model were extracted. The effective porosity is shown to increase with travel time until equilibrium conditions are realised between the fractures and matrix, following which, the effective porosity equals the matrix porosity and will maintain this value at larger transport times. Assuming that the dual-porosity model provides a trustworthy description of solute transport in fractured chalk and limestone, a method to estimate the effective porosity of an equivalent porous medium model is presented.

Water Weakening of Artificially Fractured Chalk, Fracture Modification and Mineral Precipitation during Water Injection—An Experimental Study

This experiment was designed to study the water-weakening effect of artificially fractured chalk caused by the injection of different compositions of brines under reservoir conditions replicating giant hydrocarbon reservoirs at the Norwegian Continental Shelf (NCS). NaCl, synthetic seawater (SSW), and MgCl2, with same ionic strength, were used to flood triaxial cell tests for approximately two months. The chalk cores used in this experiment originate from the Mons basin, close to Obourg, Belgium (Saint Vast Formation, Upper Cretaceous). Three artificially fractured chalk cores had a drilled central hole parallel to the flooding direction to imitate fractured chalk with an aperture of 2.25 (±0.05) mm. Two additional unfractured cores from the same sample set were tested for comparison. The unfractured samples exposed a more rapid onset of the water-weakening effect than the artificially fractured samples, when surface active ions such as Ca2+, Mg2+ and SO42− were introduced. This instant increase was more prominent for SSW-flooded samples compared to MgCl2-flooded samples. The unfractured samples experienced axial strains of 1.12% and 1.49% caused by MgCl2 and SSW, respectively. The artificially fractured cores injected by MgCl2 and SSW exhibited a strain of 1.35% and 1.50%, while NaCl showed the least compaction, at 0.27%, as expected. Extrapolation of the creep curves suggested, however, that artificially fractured cores may show a weaker mechanical resilience than unfractured cores over time. The fracture aperture diameters were reduced by 84%, 76%, and 44% for the SSW, MgCl2, and NaCl tests, respectively. Permeable fractures are important for an effective oil production; however, constant modification through compaction, dissolution, and precipitation will complicate reservoir simulation models. An increased understanding of these processes can contribute to the smarter planning of fluid injection, which is a key factor for successful improved oil recovery. This is an approach to deciphering dynamic fracture behaviours.

Analytical Solution for Fractional Well Flow in a Double-Porosity Aquifer with Fractional Transient Exchange between Matrix and Fractures

An analytical solution is presented for groundwater flow to a well in an aquifer with double-porosity behavior and transient transfer between fractures and matrix. The solution is valid for fractional flow dimensions including linear, cylindrical or spherical flow to the well and for fractional inter-porosity diffusive transfer including release from storage in infinite slabs, infinite cylinders or spherical matrix blocks. Approximations are also presented for small and large times that are easy to evaluate in practice. The solution can be used to analyze pumping tests via coupling with a parameter estimation code. The utility of the method is demonstrated by a practical example using data from a pumping test performed in a fractured chalk aquifer. The analytical solution allows the accurate modeling of pumping tests and the estimation of aquifer parameters that are statistically significant and physically relevant.

Assessing the Impact of Fractured Zones Imaged by ERT on Groundwater Model Prediction: A Case Study in a Chalk Aquifer in Voort (Belgium)

Fractured and karst aquifers are important groundwater reservoirs and are widely used to provide drinking water to the population. Because of the presence of the fractures with varying geometry and properties providing preferential flow paths, fractured aquifers are highly heterogeneous and difficult to characterize and model. In this context, geophysical methods can provide relevant spatially distributed data about the presence of fractures, that can be further integrated in hydrological and groundwater models. In this contribution, we present a case study of a groundwater extraction site in a fractured chalk aquifer in Voort (Belgium), used for the production of drinking water. First, the presence of fractures in the vicinity of the extraction site and their orientation is imaged using electrical resistivity tomography. Based on the available data and the objectives of the study, it is chosen to model only the groundwater component and to simplify the unsaturated zone processes through an average recharge rate. Then, the detected fractures are included in the groundwater model to improve the calibration and the predictive capacity of the model. The results show that a set of parallel fractures crosses the modeled area, whose orientation is in accordance with the tectonic setting. Including these fractures in the model, a more satisfactory calibration was achieved, helping to better understand the hydrogeological behavior of the aquifer. Finally, the acquired knowledge is used to propose new management scenarios for the extraction site minimizing its impact.

Long-Term Storage of Carbon Dioxide in Fractured Chalk by Accelerated Dissolution

Long-Term Storage of Carbon Dioxide in Fractured Chalk by Accelerated Dissolution

Chalk: all we need is a fracture log!

Chalk fracture logging is reviewed in the context of the broader geology needed to build conceptual ground models. Examples of drilling damaged core illustrate the many issues faced by core loggers including identification of marker beds (marl seams, hardgrounds, flint bands, fossil shell beds) and the ‘interpretations’ necessary to complete a fracture log. Stratabound fractures impart a special style of fracturing to each Chalk formation. Lithology is a key factor in development of fracture style where marl seams control inclined conjugate fracture sets, development of listric growth faults and interbed slides. Lateral changes in lithology and thickness and consequent controls on fracture evolution are related to intra-Chalk tectonic episodes and tectonic setting with onshore interpretations supported by offshore seismic profiles. Strike-slip faults are illustrated in the Chalk cliffs of the Sussex coast. Fracture log reports should highlight special features such as shear zones and use annotated core photographs to illustrate issues requiring discussion.

Colloid-facilitated transport of 238Pu, 233U and 137Cs through fractured chalk: Laboratory experiments, modelling, and implications for nuclear waste disposal

The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1–5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.

Continuous Dissolved Gas Tracing of Fracture‐Matrix Exchanges

AbstractTransport in fractured media plays an important role in a range of processes, from rock weathering and microbial processes to contaminant transport, and energy extraction and storage. Diffusive transfer between the fracture fluid and the rock matrix is often a key element in these applications. But the multiscale heterogeneity of fractures renders the field assessment of these processes extremely challenging. This study explores the use of dissolved gases as tracers of fracture‐matrix interactions, which can be measured continuously and highly accurately using mobile mass spectrometers. Since their diffusion coefficients vary significantly, multiple gases are used to probe different scales of fracture‐matrix exchanges. Tracer tests with helium, xenon, and argon were performed in a fractured chalk aquifer, and resulting tracer breakthrough curves are modeled. Results show that continuous dissolved gas tracing with multiple tracers provides key constrains on fracture‐matrix interactions and reveal unexpected scale effects in fracture‐matrix exchange rates.

Mobility of Radionuclides in Fractured Carbonate Rocks: Lessons from a Field-Scale Transport Experiment.

Current research on radionuclide disposal is mostly conducted in granite, clay, saltstone, or volcanic tuff formations. These rock types are not always available to host a geological repository in every nuclear waste-generating country, but carbonate rocks may serve as a potential alternative. To assess their feasibility, a forced gradient cross-borehole tracer experiment was conducted in a saturated fractured chalk formation. The mobility of stable Sr and Cs (as analogs for their radioactive counterparts), Ce (an actinide analog), Re (a Tc analog), bentonite particles, and fluorescent dye tracers through the flow path was analyzed. The migration of each of these radionuclide analogs (RAs) was shown to be dependent upon their chemical speciation in solution, their interactions with bentonite, and their sorption potential to the chalk rock matrix. The brackish groundwater resulted in flocculation and immobilization of most particulate RAs. Nevertheless, the high permeability of the fracture system allowed for fast overall transport times of all aqueous RAs investigated. This study suggests that the geochemical properties of carbonate rocks may provide suitable conditions for certain types of radionuclide storage (in particular, brackish, high-porosity, and low-permeability chalks). Nevertheless, careful consideration should be given to high-permeability fracture networks that may result in high radionuclide mobility.

Modeling of Permeability and Strain Evolution in Chemical Creep Compaction Experiments with Fractured and Unfractured Chalk Cores Conducted at Reservoir Conditions

Summary Understanding the effect of typical water-related improved oil recovery techniques is fundamental to the development of chalk reservoirs on the Norwegian Continental Shelf (NCS). We investigate the contribution and interplay of key parameters influencing the reservoir's flow and storativity properties, such as effective stresses, injecting fluid chemistry, and geomechanical deformation. This is done by developing a mathematical model that is applied to systematically interpret experimental data. The gained understanding is useful for improved prediction of permeability development during field life. The model we present is for a fractured chalk core whereby fluids can flow through the matrix and fracture domains in parallel. The core is subject to a constant effective stress above the yield, resulting in time-dependent compaction (creep) of the matrix, while the fracture does not compact. Reactive brine injection causes enhanced compaction but also permeability alteration. This again causes a redistribution of injected flow between the two domains. A previous version of the model parameterizing the relation between chemistry and compaction is here extended to quantify the effect on permeability and see the effect of flow in a fracture-matrix geometry. A vast set of experimental data were used to quantify the relations in the model and demonstrate its usefulness to interpret experimental data. Two outcrop chalk types (Aalborg and Liège) being tested at 130°C and various concentrations of Ca-Mg-Na-Cl brines are considered. However, assumptions were required, especially regarding the fracture behavior because directly representative data were not available. The tests with inert injecting brine were used to quantify the effect of matrix and fracture mechanical compaction on permeability trends. To be able to explain the tests with reactive brine, an important finding is that permeability not only decreased because of enhanced porosity reduction but also because of a quantifiable chemistry-related process (dissolution/precipitation). Sensitivity analyses were performed regarding varying fracture width, injection rate, and chemistry concentration to evaluate the effect on chemical creep compaction and permeability evolution in fractured cores. The model can be used to highlight parameters with great influence on the experimental results. An accurate quantification of such parameters will contribute to refining laboratory experiments and will provide valuable data for upscaling and field application.

The Mungo Field, Blocks 22/20a and 23/16a, UK North Sea

Abstract The Mungo Field is a mature producing asset located in the UK Central North Sea. Discovered in 1989 and brought on production in 1998, it is the largest field within the Eastern Trough Area Project (ETAP). Production occurs via a normally unattended installation and is tied back to the ETAP Central Processing Facility. It is a pierced, four-way dip closure against a salt diapir. Light oil is present within steeply dipping Late Paleocene sandstone and Early Paleocene–Late Cretaceous chalk intervals. The vertical relief of the salt stock is around 1500 m TVDSS and top of the salt canopy lies at about 1350 m TVDSS. The Paleocene sandstones (Forties Sandstone Member of the Sele Formation, Lista Formation and Maureen Formation) make up the primary reservoir and have been extensively developed in three phases since 1998 under water injection and natural depletion. The sandstones were deposited as deep-water turbidite complexes (submarine fans with local channels) on and around the flanks of the rising salt diapir. More recently, successful stimulation of the Chalk Group, coupled with re-evaluation of core and well-log data, has indicated that economic production rates could also be achieved from the underlying fractured chalk reservoir.

Dimethyl Ether Enhanced Oil Recovery in Fractured Reservoirs and Aspects of Phase Behavior

The efficiency of the dimethyl ether (DME) enhanced oil recovery (EOR) technique in a fractured chalk reservoir core plug was investigated. The coreflood experiment showed that DME EOR could lead t...

The Spatial Distribution of the Microbial Community in a Contaminated Aquitard below an Industrial Zone

The industrial complex Neot Hovav, in Israel, is situated above an anaerobic fractured chalk aquitard, which is polluted by a wide variety of hazardous organic compounds. These include volatile and non-volatile, halogenated, organic compounds. In this study, we characterized the indigenous bacterial population in 17 boreholes of the groundwater environment, while observing the spatial variations in the population and structure as a function of distance from the polluting source. In addition, the de-halogenating potential of the microbial groundwater population was tested through a series of lab microcosm experiments, thus exemplifying the potential and limitations for bioremediation of the site. In all samples, the dominant phylum was Proteobacteria. In the production plant area, the non-obligatory organo-halide respiring bacteria (OHRB) Firmicutes Phylum was also detected in the polluted water, in abundancies of up to 16 %. Non-metric multidimensional scaling (NMDS) analysis of the microbial community structure in the groundwater exhibited clusters of distinct populations following the location in the industrial complex and distance from the polluting source. Dehalogenation of halogenated ethylene was demonstrated in contrast to the persistence of brominated alcohols. Persistence is likely due to the chemical characteristics of brominated alcohols, and not because of the absence of active de-halogenating bacteria.

The Role of Stabilizer Concentration in the Mobility of Carbon‐Supported Nanozerovalent Iron (nZVI) in Fractured Media

AbstractThe mobility of nanozerovalent iron (nZVI) particles is of major importance when assessing their effectiveness for use in contaminated sites. Here, the influence of the stabilizer loading on nZVI mobility was investigated in fractured media. nZVI in the form of Carbo‐Iron Colloids (CIC), a composite material of activated carbon as a carrier for nZVI, was used for the tests. CIC was supplemented with carboxymethyl cellulose stabilizer, at three different loadings: 5% wt., 20% wt., and 80% wt., in a moderately saline solution of 250 mM. Transport experiments were conducted in a fractured chalk core, and the characteristics of the solutions and particles were analyzed. The effect of ionic strength was investigated by application of the 80% loading, also in a lower salinity solution of ~3 mM. Transport experiments were modeled, and all results were compared with especially high 1,600% wt. stabilizer loading, from a previous study. Among the different carboxymethyl cellulose loadings, significant differences in mobility and a correlation between mobility and stability were observed. Though, differences in stability were found insignificant. Higher stabilizer concentration and, to a greater extent, a higher number of free stabilizer molecules in solution enhance particle stability and, hence, mobility. A good fit was obtained between the transport experiment results and a model of solute and colloid transport in a single fracture. The model attachment parameters showed a good correlation with the decay rate constants of the stability tests, indicating yet again the significant role played by stability in CIC mobility and the possibility for mobility prediction.

Radionuclide transport in brackish water through chalk fractures

Mobility of radionuclides originating from geological repositories in the subsurface has been shown to be facilitated by clay colloids. In brackish water, however, colloids may flocculate and act to immobilize radionuclides associated with them. Furthermore, little research has been conducted on radionuclide interactions with carbonate rocks. Here, the impact of bentonite colloid presence on the transport of a cocktail of U(VI), Cs, Ce and Re through fractured chalk was investigated. Flow-through experiments were conducted with and without bentonite colloids, present as a mixture of bentonite and Ni-altered montmorillonite colloids. Ce was used as an analogue for reactive actinides in the (III) and (VI) redox states, and Re was considered an analogue for Tc. Filtered brackish groundwater (ionic strength = 170 mM) pumped from a fractured chalk aquitard in the northern Negev Desert of Israel, was used as a solution matrix.Rhenium transport was identical to that of the conservative tracer, uranine. The sorption coefficient (Kd) of U(VI), Cs and Re, calculated from batch experiments with crushed chalk, proved to be a good predictor of mass recovery in transport experiments conducted without bentonite colloids. A meaningful Kd value for Ce could not be calculated due to its precipitation as a Ce-carbonate colloids. Transport of both U(VI) and Cs was indifferent to the presence of bentonite colloids. However, the addition of bentonite in the injection solution effectively immobilized Ce, decreasing its recovery from 17-41% to 0.8–1.4%. This indicates that radionuclides which interact with clay colloids that undergo flocculation and deposition may effectively be immobilized in brackish aquifers. The results of this study have implications for the prediction of potential mobility of radionuclides in safety assessments for future geological repositories to be located in fractured carbonate rocks in general and in brackish groundwater in particular.

Field Scale Mobility and Transport Manipulation of Carbon-Supported Nanoscale Zerovalent Iron in Fractured Media.

In field applications, mostly in porous media, transport of stabilized nano zerovalent iron particles (nZVI) has never exceeded a few meters in range. In the present study, the transport of Carbo-Iron Colloids (CIC), a composite material of activated carbon as a carrier for nZVI stabilized by carboxymethyl cellulose (CMC), was tested under field conditions. The field site lies within a fractured chalk aquitard characterized by moderately saline (∼13 mS) groundwater. A forced gradient tracer test was conducted where one borehole was pumped at a rate of 8 L/min and CMC-stabilized CIC was introduced at an injection borehole 47 m up-gradient. Two CIC-CMC field applications were conducted: one used high 100% wt CMC (40 g/L) and a second used lower 9% wt loading (∼2.7 g/L). Iodide was injected as a conservative tracer with the CIC-CMC in both cases. The ratio between the CIC-CMC and iodide recovery was 76% and 45% in the high and low CMC loading experiments, respectively. During the low CMC loading experiment, the pumping rate was increased, leading to an additional CIC recovery of 2.5%. The results demonstrate the potentially high mobility of nZVI in fractured environments and the possibility for transport manipulation through the adjustment of stabilizer concentration and transport velocity.