Thermal conductive heating in fractured bedrock: Screening calculations to assess the effect of groundwater influx

A two-dimensional semi-analytical heat transfer solution is developed and a parameter sensitivity analysis performed to determine the relative importance of rock material properties (density, thermal conductivity and heat capacity) and hydrogeological properties (hydraulic gradient, fracture aperture, fracture spacing) on the ability to heat fractured rock using thermal conductive heating (TCH). The solution is developed using a Green’s function approach in which an integral equation is constructed for the temperature in the fracture. Subsurface temperature distributions are far more sensitive to hydrogeological properties than material properties. The bulk ground water influx (q) can provide a good estimate of the extent of influx cooling when influx is low to moderate, allowing the prediction of temperatures during heating without specific knowledge of the aperture and spacing of fractures. Target temperatures may not be reached or may be significantly delayed when the groundwater influx is large.     

Numerical Analysis of Thermal Remediation in 3D Field‐Scale Fractured Geologic Media

Thermal methods are promising for remediating fractured geologic media contaminated with volatile organic compounds, and the success of this process depends on the coupled heat transfer, multiphase flow, and thermodynamics. This study analyzed field‐scale removal of trichloroethylene (TCE) and heat transfer behavior in boiling fractured geologic media using the multiple interacting continua method. This method can resolve local gradients in the matrix and is less computationally demanding than alternative methods like discrete fracture‐matrix models. A 2D axisymmetric model was used to simulate a single element of symmetry in a repeated pattern of extraction wells inside a large heated zone and evaluate effects of parameter sensitivity on contaminant recovery. The results showed that the removal of TCE increased with matrix permeability, and the removal rate was more sensitive to matrix permeability than any other parameter. Increasing fracture density promoted TCE removal, especially when the matrix permeability was low (e.g., <10^?17 m²). A 3D model was used to simulate an entire treatment zone and the surrounding groundwater in fractured material, with the interaction between them being considered. Boiling was initiated in the center of the upper part of the heated region and expanded toward the boundaries. This boiling process resulted in a large increase in the TCE removal rate and spread of TCE to the vadose zone and the peripheries of the heated zone. The incorporation of extraction wells helped control the contaminant from migrating to far regions. After 22 d, more than 99.3% of TCE mass was recovered in the simulation.     

Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water

Heat as a tracer in fractured porous aquifers is more sensitive to fracture-matrix processes than a solute tracer. Temperature evolution as a function of time can be used to differentiate fracture and matrix characteristics. Experimental hot (50 °C) and cold (10 °C) water injections were performed in a weathered and fractured granite aquifer where the natural background temperature is 30 °C. The tailing of the hot and cold breakthrough curves, observed under different hydraulic conditions, was characterized in a log–log plot of time vs. normalized temperature difference, also converted to a residence time distribution (normalized). Dimensionless tail slopes close to 1.5 were observed for hot and cold breakthrough curves, compared to solute tracer tests showing slopes between 2 and 3. This stronger thermal diffusive behavior is explained by heat conduction. Using a process-based numerical model, the impact of heat conduction toward and from the porous rock matrix on groundwater heat transport was explored. Fracture aperture was adjusted depending on the actual hydraulic conditions. Water density and viscosity were considered temperature dependent. The model simulated the increase or reduction of the energy level in the fracture-matrix system and satisfactorily reproduced breakthrough curves tail slopes. This study shows the feasibility and utility of cold water tracer tests in hot fractured aquifers to boost and characterize the thermal matrix diffusion from the matrix toward the flowing groundwater in the fractures. This can be used as complementary information to solute tracer tests that are largely influenced by strong advection in the fractures.     

Bioremediation in Fractured Rock: 1. Modeling to Inform Design,Monitoring, and Expectations

Field characterization of a trichloroethene (TCE) source area in fractured mudstones produced a detailed understanding of the geology, contaminant distribution in fractures and the rock matrix, and hydraulic and transport properties. Groundwater flow and chemical transport modeling that synthesized the field characterization information proved critical for designing bioremediation of the source area. The planned bioremediation involved injecting emulsified vegetable oil and bacteria to enhance the naturally occurring biodegradation of TCE. The flow and transport modeling showed that injection will spread amendments widely over a zone of lower‐permeability fractures, with long residence times expected because of small velocities after injection and sorption of emulsified vegetable oil onto solids. Amendments transported out of this zone will be diluted by groundwater flux from other areas, limiting bioremediation effectiveness downgradient. At nearby pumping wells, further dilution is expected to make bioremediation effects undetectable in the pumped water. The results emphasize that in fracture‐dominated flow regimes, the extent of injected amendments cannot be conceptualized using simple homogeneous models of groundwater flow commonly adopted to design injections in unconsolidated porous media (e.g., radial diverging or dipole flow regimes). Instead, it is important to synthesize site characterization information using a groundwater flow model that includes discrete features representing high‐ and low‐permeability fractures. This type of model accounts for the highly heterogeneous hydraulic conductivity and groundwater fluxes in fractured‐rock aquifers, and facilitates designing injection strategies that target specific volumes of the aquifer and maximize the distribution of amendments over these volumes.     

Induced Temperature Gradients to Examine Groundwater Flowpaths in Open Boreholes

Techniques for characterizing the hydraulic properties and groundwater flow processes of aquifers are essential to design hydrogeologic conceptual models. In this study, rapid time series temperature profiles within open‐groundwater wells in fractured rock were measured using fiber optic distributed temperature sensing (FO‐DTS). To identify zones of active groundwater flow, two continuous electrical heating cables were installed alongside a FO‐DTS cable to heat the column of water within the well and to create a temperature difference between the ambient temperature of the groundwater in the aquifer and that within the well. Additional tests were performed to examine the effects of pumping on hydraulic fracture interconnectivity around the well and to identify zones of increased groundwater flow. High‐ and low‐resolution FO‐DTS cable configurations were examined to test the sensitivities of the technique and compared with downhole video footage and geophysical logging to confirm the zones of active groundwater flow. Two examples are presented to demonstrate the usefulness of this new technique for rapid characterization of fracture zones in open boreholes. The combination of the FO‐DTS and heating cable has excellent scope as a rapid appraisal tool for borehole construction design and improving hydrogeologic conceptual models.     

Specification of Matrix Cleanup Goals in Fractured Porous Media

Semianalytical transient solutions have been developed to evaluate what level of fractured porous media (e.g., bedrock or clay) matrix cleanup must be achieved in order to achieve compliance of fracture pore water concentrations within a specified time at specified locations of interest. The developed mathematical solutions account for forward and backward diffusion in a fractured porous medium where the initial condition comprises a spatially uniform, nonzero matrix concentration throughout the domain. Illustrative simulations incorporating the properties of mudstone fractured bedrock demonstrate that the time required to reach a desired fracture pore water concentration is a function of the distance between the point of compliance and the upgradient face of the domain where clean groundwater is inflowing. Shorter distances correspond to reduced times required to reach compliance, implying that shorter treatment zones will respond more favorably to remediation than longer treatment zones in which back‐diffusion dominates the fracture pore water response. For a specified matrix cleanup goal, compliance of fracture pore water concentrations will be reached sooner for decreased fracture spacing, increased fracture aperture, higher matrix fraction organic carbon, lower matrix porosity, shorter aqueous phase decay half‐life, and a higher hydraulic gradient. The parameters dominating the response of the system can be measured using standard field and laboratory techniques.     

Analytical and Numerical Modeling of Solute Intrusion,Recovery, and Rebound in Fractured Bedrock

Contaminated groundwater in fractured bedrock can expose ecosystems to undesired levels of risk for extended periods due to prolonged back-diffusion from rock matrix to permeable fractures. Therefore, it is key to characterize the diffusive mass loading (intrusion) of contaminants into the rock matrix for successful management of contaminated bedrock sites. Even the most detailed site characterization techniques often fail to delineate contamination in rock matrix. This study presents a set of analytical solutions to estimate diffusive mass intrusion into matrix blocks, it is recovered by pumping and concentration rebound when pumping ceases. The analytical models were validated by comparing the results with (1) numerical model results using the same model parameters and (2) observed chloride mass recovery, rebound concentration, and concentration in pumped groundwater at a highly fractured bedrock site in Alberta, Canada. It is also demonstrated that the analytical solutions can be used to estimate the total mass stored in the fractured bedrock prior to any remediation thereby providing insights into site contamination history. The predictive results of the analytical models clearly show that successful remediation by pumping depends largely on diffusive intrusion period. The results of initial mass from the analytical model was used to successfully calibrate a three-dimensional discrete fracture network numerical model further highlighting the utility of the simple analytical solutions in supplementing the more detailed site numerical modeling. Overall, the study shows the utility of simple analytical methods to support long-term management of a contaminated fractured bedrock site including site investigations and complex numerical modeling.     

利用含可控裂缝人工岩样研究裂缝密度对各向异性的影响

利用新方法制作出含可控裂缝的双孔隙人工砂岩物理模型,具有与天然岩石更为接近的矿物成分、孔隙结构和胶结方式,其中裂缝密度、裂缝尺寸和裂缝张开度等裂缝参数可以控制以得到实验所需要的裂缝参数,岩样具有真实的孔隙和裂缝空间并可以在不同饱和流体状态下研究流体性质对于裂缝介质性质的影响.本次实验制作出一组具有不同裂缝密度的含裂缝人工岩样,对岩样利用SEM扫描电镜分析可以看到真实的孔隙结构和符合我们要求的裂缝参数,岩样被加工成八面棱柱以测量不同方向上弹性波传播的速度,用0.5 MHz的换能器使用透射法测量在饱和空气和饱和水条件下各个样品不同方向上的纵横波速度,并得出纵横波速度、横波分裂系数和纵横波各向异性强度受裂缝密度和饱和流体的影响.研究发现流体对于纵波速度和纵波各向异性强度的影响较强,而横波速度、横波分裂系数和横波各向异性强度受饱和流体的影响不大,但是对裂缝密度的变化更敏感.     

基于数字岩心的含裂隙储层砂岩介电性质研究

了解储层岩石的介电特性在石油工业的各个方面都有重要的应用.小尺度裂隙是影响岩石介电性质的地质因素之一,获得裂隙对含裂隙岩石介电性质影响的定量关系具有重要的理论和实践意义.以含裂隙人造砂岩的三维微观数字结构为基础,通过基于三维有限差分算法计算的岩石介电性质与实验数据的对比验证数值计算方法的有效性.在此基础上,通过理论模型获得不同孔隙度基质的介电性质,并在不含裂隙人造砂岩的三维微观数字结构中人为添加以裂隙密度和纵横比为定量表征参数的裂隙,应用验证后的数值算法模拟随频率变化的含裂隙砂岩的介电性质,分析和研究不同孔隙度基质中定向排列裂隙对砂岩介电性质的影响.结果表明,当裂隙孔隙度随裂隙纵横比或裂隙密度发生改变时,含裂隙砂岩的介电性质与裂隙密度以及裂隙纵横比呈正相关关系,而当裂隙孔隙度保持不变时,含裂隙砂岩的介电性质随裂隙纵横比的减小而增大;裂隙参数的改变对不同基质孔隙度的含裂隙砂岩的介电性质的影响趋势较为一致,但随着基质孔隙度的减小,裂隙对砂岩介电性质的影响逐渐增大.裂隙参数和基质孔隙度对含裂隙砂岩介电性质影响的研究结果为基于介电特性的裂缝性油气储层的定量表征提供了依据,在油气勘探开发中具有重要的应用前景.     

Long-term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan

Abstract Long-term monitoring of temperature distribution in an active fault zone was carried out using the optical fiber temperature-sensing technique. An optical fiber cable was installed in a borehole drilled into the Nojima Fault in Awaji Island, south-west Japan, and the temperature profile to a depth of 1460 m had been measured for 2.5 years (July 1997–January 2000). Although the obtained temperature records showed small temporal variations due to drifts of the measurement system all along the cable, local temperature anomalies were detected at two depths. One at around 80 m seems to correspond to a fracture zone and may be attributed to groundwater flow in the fracture zone. This anomaly had been stable throughout the monitoring period, whereas the other anomaly at around 500 m was a transient one. The water level in the borehole could be estimated from the diurnal temperature variations in the uppermost part of the borehole and may provide information on the hydrological characteristics of the fault zone, which is connected to the borehole through perforations on the casing pipe. Except for these minor variations, the temperature profile had been very stable for 2.5 years. The conductive heat flow calculated from this profile and the thermal conductivity measured on core samples increases with depth, probably resulting from errors in thermal conductivity due to sampling problems and/or from advective heat transfer by regional groundwater flow. Assuming that the middle part of the borehole (less fractured granite layer) is least affected by these factors, heat flow at this site is estimated to be approximately 70 mW/m².     

Matrix Diffusion Effects on Contaminant Migration from an Injection Well in Fractured Sandstone

Deep-well injection into fractured sandstone is an option for the disposal of contaminated mine dewatering discharge from an open pit uranium mine. As part of the assessment of potential contaminant migration from deep-well injection, the effect of matrix diffusion was evaluated. An analytical mathematical model was developed for the simulation of the radial movement of a contaminant front away from an injection point under steady flow conditions in a planar fracture with uniform properties. The model includes the effects of advection in the fracture, diffusion of contaminants from the fracture into the rock matrix, and equilibrium adsorption on the fracture surface as well as in the rock matrix. Effective diffusion coefficients obtained from laboratory experiments on 11 intact core samples varied from 3.4 × 10⁻⁸ to 3.2 × 10⁻⁷ cm²/s. Model simulations were made with diffusion coefficient values in this range and with single-fracture injection rates estimated from fracture frequencies in boreholes, and from bulk hydraulic conductivity values obtained from field tests. Because of matrix diffusion, the rate of outward movement of the front of the nonreactive contaminants from the injection well is much slower than the rate of water flow in the fractures. Simulations of the movement of contaminants that undergo adsorption indicate that even a small distribution coefficient for the rock matrix causes the contaminants to remain very close to the injection well during the one-year period. The results of the simplified model demonstrate that matrix diffusion is an important process that cannot be neglected in the assessment of a waste disposal scheme located in fractured porous rock. However, in order to make a definitive assessment of the capability of matrix diffusion and associated matrix adsorption to significantly limit the extent of contaminant migration around injection wells, it would be necessary to conduct field tests such as a preliminary or experimental injection.     

Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt,Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl- concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault     

Plume Detachment and Recession Times in Fractured Rock

The influence of source zone concentration reduction on solute plume detachment and recession times in fractured rock was investigated using new semianalytical solutions to transient solute transport in the presence of advection, dispersion, sorption, matrix diffusion, and first-order decay. Novel aspects of these solutions are: (1) the source zone concentration behavior is simulated using a constant concentration with the option for either an instantaneous reduction to zero concentration or an exponentially decaying source zone concentration initiated at some time (t^*) after the source is introduced, and (2) different biodegradation rates in the fracture and rock matrix. These solutions were applied for sandstone bedrock and revealed that biodegradation in the matrix, not the fracture, may be the most significant attenuation mechanism and therefore may dictate remediation time scales. Also, instantaneous and complete source concentration reduction in aged plumes may not be beneficial with respect to plume response because back-diffusion can sustain plume migration for long periods of time. Moderate source zone concentration reduction has a similar impact on the rate of advance of the leading edge of the plume as aggressive concentration reduction. If the source zone concentration reduction half-life is less than the plume decay half-life, then volatile organic compound (VOC) mass sequestered in the rock matrix will ultimately dictate plume persistence and not the presence of the source zone.     

The stress dependent elastic properties of thermally induced microfractures in aeolian Rotliegend sandstone

The impact of thermally induced microfractures on the stress‐sensitive elastic wave properties of aeolian Rotliegend sandstone samples is analysed. It is found that to identify the effects of the microfracture contribution accurately, a correction must first be made to account for water loss (representing a mass loss of 4–6%) from the pore throats and clays due to the heating process, despite care being taken to ensure that the thermally fractured samples re‐adsorb room moisture. Both the original and thermally fractured rocks are stress‐sensitive at the ultrasonic wave frequencies of the laboratory. However, a distinct shift in the estimated distribution of internal rock compliance indicates that the population of thermal microfractures differs in nature from that caused solely by core‐plug extraction damage. In particular, the ratio of normal to tangential compliance is observed to be higher for the thermally generated microfractures than for the broken grain‐grain contacts created by extraction unloading. This can be explained by the intragranular thermal‐fracture surfaces being smoother when compared to the intergranular boundaries. Mechanical hysteresis is observed between the up‐ and downgoing test cycles for both the original and, to a greater extent, the thermally fractured rock. This indicates that there is compaction‐induced movement of the fractures in the samples during application of stress in the laboratory.     

Impact of fracture network geometry on free convective flow patterns

The effect of fracture network geometry on free convection in fractured rock is studied using numerical simulations. We examine the structural properties of fracture networks that control the onset and strength of free convection and the patterns of density-dependent flow. Applicability of the equivalent porous medium approach (EPM) is also tested, and recommendations are given, for which situations the EPM approach is valid. To date, the structural properties of fracture networks that determine free convective flow are examined only in few, predominantly simplified regular fracture networks. We consider fracture networks containing continuous, discontinuous, orthogonal and/or inclined discrete fractures embedded in a low-permeability rock matrix. The results indicate that bulk permeability is not adequate to infer the occurrence and magnitude of free convection in fractured rock. Fracture networks can inhibit or promote convection depending on the fracture network geometry. Continuous fracture circuits are the crucial geometrical feature of fracture networks, because large continuous fracture circuits with a large vertical extent promote convection. The likelihood of continuous fracture circuits and thus of free convection increases with increasing fracture density and fracture length, but individual fracture locations may result in great deviances in strength of convection between statistically equivalent fracture networks such that prediction remains subject to large uncertainty.     

A mathematical model for 2D heat transfer dynamics in fluid systems with localized sink of magmatic fluid into local fractured zones above the top of crystallizing intrusions

A model describing two-dimensional (2D) dynamics of heat transfer in the fluid systems with a localized sink of a magmatic fluid into local fractured zones above the roof of crystallizing crustal intrusions is suggested. Numerical modeling of the migration of the phase boundaries in 2D intrusive chambers under retrograde boiling of magma with relatively high initial water content in the melt shows that, depending on the character of heat dissipation from a magmatic fluid into the host rock, two types of fluid magmatic systems can arise. (1) At high heat losses, the zoning of fluidogenic ore formation is determined by the changes in temperature of the rocks within the contact aureole of the intrusive bodies. These temperature variations are controlled by the migration of the phase boundaries in the cooling melt towards the center of the magmatic bodies from their contacts. (2) In the case of a localized sink of the magmatic fluid in different parts of the top of the intrusive chambers, a specific characteristic scenario of cooling of the magmatic bodies is probably implemented. In 2D systems with a heat transfer coefficient ?? _k < 5 × 10⁴ W/m² K, an area with quasi-stationary phase boundaries develops close to the region of fluid drainage through the fractured zone in the intrusion. Therefore, as the phase boundaries contract to the sink zone of a fluid, specific thermal tubes arise, whose characteristics depend on the width of the fluid-conductive zone and the heat losses into the side rocks. (3) The time required for the intrusion to solidify varies depending on the particular position of the fluid conductor above the top of the magmatic body.     

Seismic measurements of the internal properties of fault zones

The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites.     

Anisotropic azimuthal surface-wave inversion for a vertically fractured and transverse isotropy medium: Theory and examples from a controlled field experiment

Fractures in elastic media add compliance to a rock in the direction normal to the fracture strike. Therefore, elastic wave velocities in a fractured rock will vary as a function of the energy propagation direction relative to the orientation of the aligned fracture set. Anisotropic Thomson–Haskell matrix Rayleigh-wave equations for a vertically transverse isotropic media can be used to model surface-wave dispersion along the principal axes of a vertically fractured and transversely isotropic medium. Furthermore, a workflow combining first-break analysis and azimuthal anisotropic Rayleigh-wave inversion can be used to estimate P-wave and S-wave velocities, Thomsen's ε, and Thomsen's δ along the principal axes of the orthorhombic symmetry. In this work, linear slip theory is used to map our inversion results to the equivalent vertically fractured and transversely isotropic medium coefficients. We carried out this inversion on a synthetic example and a field example. The synthetic data example results show that joint estimation of S-wave velocities with Thomsen's parameters ε and δ along normal and parallel to the vertical fracture set is reliable and, when mapped to the corresponding vertically fractured and transversely isotropic medium, provides insight into the fracture compliances. When the inversion was carried out on the field data, results indicated that the fractured rock is more compliant in the azimuth normal to the visible fracture set orientation and that the in situ normal fracture compliance to tangential fracture compliance ratio is less than half, which implies some cementation may have occurred along the fractures. Such an observation has significant implications when modelling the transport properties of the rock and its strength. Both synthetic and field examples show the potential of azimuthal anisotropic Rayleigh-wave inversion as the method can be further expanded to a more general case where the vertical fracture set orientation is not known a priori.     

Understanding of hydraulic properties from configurations of stochastically distributed fracture networks

A systematic investigation of the effect of configurations of stochastically distributed fracture networks on hydraulic behaviour for fractured rock masses could provide either quantitative or qualitative correlation between the structural configuration of the fracture network and its corresponding hydraulic behaviour, and enhance our understanding of appropriate application of groundwater flow and contaminant transport modelling in fractured rock masses. In this study, the effect of block sizes, intersection angles of fracture sets, standard deviations of fracture orientation, and fracture densities on directional block hydraulic conductivity and representative elementary volume is systematically investigated in two dimensions by implementing a numerical discrete fracture fluid flow model and incorporating stochastically distributed fracture configurations. It is shown from this investigation that the configuration of a stochastically distributed fracture network has a significant quantitative or qualitative effect on the hydraulic behaviour of fractured rock masses. Compared with the deterministic fracture configurations that have been extensively dealt with in a previous study, this investigation is expected to be more practical and adequate, since fracture geometry parameters are inherently stochastically distributed in the field. Moreover, the methodology and approach presented in this study may be generally applied to any fracture system in investigating the hydraulic behaviours from configurations of the fracture system while establishing a ‘bridge’ from the discrete fracture network flow modelling to equivalent continuum modelling in fractured rock masses. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessment of Groundwater Quality Improvements and Mass Discharge Reductions at Five In Situ Electrical Resistance Heating Remediation Sites

A recent study assessing the state‐of‐the‐practice of in situ thermal remediation technologies (e.g., electrical resistive heating [ERH], conductive heating, steam‐based heating, in situ large‐diameter auger soil mixing with steam/hot air injection, and radio‐frequency heating) identified 182 applications in the 1988 to 2007 period and summarized the geologic settings in which these technologies were applied, chemicals treated, design parameters, and operating conditions. That study concluded that documentation for less than 8% of those applications contained sufficient data to assess the effect remediation had on groundwater quality. Consequently, post‐treatment data were collected at five ERH sites, with emphasis on assessing reductions in dissolved groundwater concentrations and mass discharge (mass flux) to the aquifer. For each site, dissolved groundwater concentrations and hydraulic conductivities were determined across a vertical transect oriented perpendicular to groundwater flow and at the downgradient edge of the treatment zone. Dissolved concentration and mass discharge reductions ranged from about less than 10× to 100×, with post‐treatment groundwater concentrations ranging from about 10¹ to 10⁴μg/L and mass discharges ranging from about 10^–1 to 10² kg/y. The primary factors differentiating sites with greater and lesser dissolved concentration and mass discharge reductions were the adequacy of pre‐treatment source zone delineation, the extent to which the treatment zone encompassed the source zone, and the duration of treatment at the design operating temperature. The results suggest that ERH systems are capable of reducing groundwater concentrations to 10 to 100 μg/L levels and lower in some settings, but only if the source zone is adequately delineated and fully encompassed by the treatment system, and the treatment system is operated for a sufficiently long period of time.