Relative contribution of DNAPL dissolution and matrix diffusion to the long-term persistence of chlorinated solvent source zones

Sustainable application of surfactants in soil remediation: Selective pollutants adsorption and hydrogen peroxide-driven adsorbent regeneration

The uncontrolled disposal of the liquid lindane wastes have led to the formation of dense non-aqueous phase liquids (DNAPL), consisting of 28 chlorinated organic compounds (COCs), contaminating soil and groundwater. Surfactant-enhanced aquifer remediation is proposed as technology to treat these sites. However, the polluted emulsion generated must be manged on-site. In this work a two-step process is applied to treat emulsion composed of E-Mulse® 3 (4 g·L-1) as surfactant and a DNAPL (2 gCOCs·L-1). In the first, the COCs were selectively adsorbed in a granular activated carbon (GAC) column with Fe (II) previously adsorbed (10-20mg·g-1) onto the carbon surface, recovering an aqueous phase with surfactant for their reuse. In the second step, the spent GAC was regenerated with a 40 g·L-1 solution of hydrogen peroxide fed to the column at 2 mL·min-1 to promote the oxidation of the COCs adsorbed in the GAC. The kinetic and adsorption model in a multisolute (surfactant and DNAPL) system has been proposed. Five successive cycles of regeneration/adsorption have been successfully applied in the column process. About 50 % of the COCs were retained from the emulsion, and more than 70 % of the surfactant was recovered. The consumption of unproductive oxidants decreased with the number of regeneration cycles. The water effluent obtained after regeneration of GAC did not present chlorinated compounds desorbed and nontoxic by-products generated, such as short-chain acids.

Quantification of contaminant mass discharge from point sources in aquitard/aquifer systems based on vertical concentration profiles and 3D modeling

Point sources with contaminants, such as chlorinated solvents, per- and polyfluoroalkyl substances (PFAS), or pesticides, are often located in low-permeability aquitards, where they can act as long-term sources and threaten underlying groundwater resources. We demonstrate the use of a 3D numerical model integrating comprehensive hydrogeological and contamination data to determine the contaminant mass discharge (CMD) from an aquitard into the underlying aquifer. A mature point source with a dissolved chlorinated solvent in a clayey till is used as an example. The quantitative determination is facilitated by model calibration to high-resolution vertical concentration profiles obtained by direct-push sampling techniques in the aquifer downgradient of the contaminant source zone. The concentration profiles showed a plume sinking with distance from the source characteristic for such aquitard/aquifer settings. The sinking is caused by the interplay between infiltrating water and horizontal groundwater flow. The application of 3D solute transport modeling on high-resolution profiles allowed for determining the infiltration rate, the hydraulic conductivity in the aquitard, and, ultimately, the CMD. Different source zone conceptualizations demonstrate the potential effects of fractures and sorption in source zones in aquitards on CMD development. Fractures in the aquitard had a minor influence on the current CMD determined with the presented approach. Still, fractures with hydraulic apertures larger than 10 μm were crucial for the temporal development of the CMD and plume. A thorough characterization of the source zone conditions combined with high-resolution concentration profiles and detailed modeling is valuable for shedding light on the probable future development of groundwater contamination arising from sources in aquitard/aquifer settings and evaluating remedial actions.

Contaminant back-diffusion from layered aquitards subjected to barrier-controlled source zones

Low-permeability aquitards may serve as secondary sources of slow-releasing contaminants into the adjacent aquifer system, creating considerable obstacles to groundwater cleanup. Accurately capturing the exchange of contaminant mass between aquitards and aquifers can facilitate site management and remediation. Previous simulation studies were mainly limited to one-dimensional (1D) back diffusion from aquitards during the remediation of the source zone. In this study, a novel two-dimensional (2D) back-diffusion model is developed to investigate the storage and release of contaminants in aquitards after source isolation. This model coupled the dynamical decay of isolated sources and the diffusion-sorption process of contaminants in the layered aquitards. Exact analytical solutions for the present 2D multilayer model were derived using the finite cosine transform, Duhamel Theorem, separation of variables, and transfer matrix method. Results indicated that the previous 1D model would overestimate the contaminant concentration in the aquitard and the back-diffusion risk when the source zone was isolated. The proposed 2D back-diffusion model enables quantitative prediction of how source zone width, source concentration, and aquitard heterogeneity impact plume trailing time, thus aiding in understanding the mechanisms of aquifer contamination beyond barrier-controlled source zones.

The era of low-permeability sites remediation and corresponding technologies: A review

Rational utilization of soil resources and remediation of contaminated soils are imperative due to the rapidly growing demand for clean soils. Currently, many in-situ remediation technologies are less suitable at low-permeability sites due to the limitations of soil permeability. This work defines a low-permeability site as a site with hydraulic conductivity less than 10^-4 cm/s, and summarizes the migration characteristics of representative contaminants at low-permeability sites, and discusses the principles and practical applications of different technologies suitable for the remediation of low-permeability sites, including electrokinetic remediation technology, polymer flushing technology, fracturing technology, and in-situ thermal remediation technology. Enhanced and combined remediation technologies are further described because one remediation technology cannot remediate all contaminants. The prospects for the application of remediation technologies to low-permeability sites are also proposed. This work highlights the necessity of low-permeability sites remediation and the urgent need for new remediation technologies, with the hope to inspire future research on low-permeability sites.

An approach for quantification of the heterogeneity of DNAPL source zone geometries

Many studies have investigated the migration and entrapment processes of source zones from dense non-aqueous phase liquid (DNAPL) contamination under different conditions. However, the characterization of occupying area by source zone (or source shape) in water-saturated aquifers is still rudimentarily considered. In this study, we demonstrated this issue (1) by providing a brief review of existing approaches for source shape consideration, (2) by proposing an approach with simple shape parameters based on the non-uniformity of source widths, and (3) by providing exemplary applications of our proposed approach on shapes already published in previous research works. Our literature review suggested that the source zone in mathematical approaches is generally characterized as simple geometrical shapes (arbitrary lines or rectangles) or system-defined parameters that contrast to complex and discontinuous shapes observed in the real world. But the characterization of such complex shapes is still not possible with acceptable efforts. Therefore, we proposed an approach to parameterize the source shape by considering the variation of width and midpoints over the depth of the entire source zone and formulate four parameters based on population statistics (mean, standard deviation). To illustrate the suitability of our approach, we applied it to the results of lab experiments, and by analyzing these complex shapes, we highlighted the potential for improving the characterization techniques of non-uniformity of the source zones.

Enhanced semi-volatile DNAPL accessibility at sub-boiling temperature during electrical resistance heating in heterogeneous porous media

Successful remediation of semi-volatile contaminants using electrical resistance heating (ERH) coupled technologies requires a deep understanding of contaminant migration and accessibility, especially with stratigraphic heterogeneity and dense nonaqueous phase liquid (DNAPL) occurrence. Here, we chose nitrobenzene (NB) as a model contaminant of semi-volatile DNAPL and uniquely demonstrated that temperature variation during ERH could induce NB DNAPL migration out of the low permeability zone (LPZ) even below water boiling temperature. When heating the system using alternating current (AC) of 140 V to a temperature range of 50-79 °C, obvious DNAPL migration was visually observed. The upward migration of DNAPL would considerably increase the mass of accessible contaminant by other remediation measures. The downstream cumulative NB mass of 1092 mg in 140 V system raised 56-folds compared to that of 19 mg in the control experiment with only groundwater flow. This migration was mainly attributed to a complex natural convection caused by temperature gradient. Comparing with traditional AC heating, ERH powered by pulsed direct current (PDC-ERH) showed a higher and more uneven heating pattern, resulting in a stronger convection at the same voltage that enhanced the DNAPL migration out of LPZ. These results revealed the importance of natural convection in the ERH process, which could be further optimized to improve the energy efficiency of remediation.

Effects of co-occurrence of PFASs and chlorinated aliphatic hydrocarbons on microbial communities in groundwater: A field study

The effects of per- and polyfluoroalkyl substances (PFASs) and chlorinated aliphatic hydrocarbons (CAHs) co-contamination on the microbial community in the field have not been studied. In this study, we evaluated the presence of PFASs and CAHs in groundwater collected from a fluorochemical plant (FCP), and carried out Illumina MiSeq sequencing to understand the impact of mixed PFASs and CAHs on the indigenous microbial community. The sum concentrations of 20 PFASs in FCP groundwater ranged from 2.05 to 317.40 μg/L, and the highest PFOA concentration was observed in the deep aquifer (60 m below ground surface), co-contaminated by dense non-aqueous-phase liquid (DNAPL). The existence of PFASs and CAHs co-contamination in groundwater resulted in a considerable decrease in the diversity of microbial communities, while the abundance of metabolisms associated with contaminants biodegradation has increased significantly compared to the background wells. Furthermore, Acinetobacter, Pseudomonas and Arthrobacter were the dominant genera in PFASs and CAHs co-contaminated groundwater. The presence of high concentrations of PFASs and CAHs has been positively associated with the genus of Citreitalea. Finally, geochemical parameters, such as ORP, sulfate and nitrate were the key factors to shape up the structure of the microbial community and sources to rich the abundance of the potential functional bacteria.

Biotic and abiotic reductive dechlorination of chloroethenes in aquitards

Chlorinated solvents occur as dense nonaqueous phase liquid (DNAPL) or as solutes when dissolved in water. They are present in many pollution sites in urban and industrial areas. They are toxic, carcinogenic, and highly recalcitrant in aquifers and aquitards. In the latter case, they migrate by molecular diffusion into the matrix. When aquitards are fractured, chlorinated solvents also penetrate as a free phase through the fractures. The main objective of this study was to analyze the biogeochemical processes occurring inside the matrix surrounding fractures and in the joint-points zones. The broader implications of this objective derive from the fact that, incomplete natural degradation of contaminants in aquitards generates accumulation of daughter products. This causes steep concentration gradients and back-diffusion fluxes between aquitards and high hydraulic conductivity layers. This offers opportunities to develop remediation strategies based, for example, on the coupling of biotic and reactive abiotic processes. The main results showed: 1) Degradation occurred especially in the matrix adjacent to the orthogonal network of fractures and textural heterogeneities, where texture contrasts favored microbial development because these zones constituted ecotones. 2) A dechlorinating bacterium not belonging to the Dehalococcoides genus, namely Propionibacterium acnes, survived under the high concentrations of dissolved perchloroethene (PCE) in contact with the PCE-DNAPL and was able to degrade it to trichloroethene (TCE). Dehalococcoides genus was able to conduct PCE reductive dechlorination at least up to cis-1,2-dichloroethene (cDCE), which shows again the potential of the medium to degrade chloroethenes in aquitards. 3) Degradation of PCE in the matrix resulted from the coupling of reactive abiotic and biotic processes-in the first case, promoted by Fe²⁺ sorbed to iron oxides, and in the latter case, related to dechlorinating microorganisms. The dechlorination resulting from these coupling processes is slow and limited by the need for an adequate supply of electron donors.

Improved assessment and performance monitoring of a biowall at a chlorinated solvent site using high-resolution passive sampling

This study contrasts the use of high-resolution passive sampling and traditional groundwater monitoring wells (GWMW) to characterize a chlorinated solvent site and assess the effectiveness of a biowall (mulch, compost and sand) that was installed to remediate trichloroethene (TCE), the primary contaminant of concern. High-resolution passive profilers (HRPPs) were direct driven hydraulically upgradient, within, and hydraulically downgradient of the biowall and in close proximity to existing GWMWs. Compared with hydraulically upgradient locations, the biowall was highly reducing, there were higher densities of bacteria/genes capable of reductive dechlorination, and TCE was being reductively transformed, but not completely, as cis-1,2-dichloroethene (cis-DCE) was detected within and hydraulically downgradient of the biowall. However, based on the high-resolution data, there were a number of important findings which were not discoverable using data from GWMWs alone. Data from the HRPPs indicate that the biowall was completely transforming TCE to ethene (C₂H₄) except within a high velocity interval, where the concentrations were reduced, but breakthrough of cis-DCE was apparent. Hydraulically upgradient of the biowall, concentrations of TCE increased with depth where a very low permeability zone exists that will likely remain as a long-term source. In addition, although low concentrations of cis-DCE were present downgradient of the biowall, surfacing into a downgradient stream was not detected. This study demonstrates the advantages of high-resolution passive sampling of aquifers to assess the performance of remediation techniques compared to traditional methods such as GWMWs.

Using MODFLOW and RT3D to simulate diffusion and reaction without discretizing low permeability zones

Low permeability zones (LPZs) are major sources of groundwater contamination after active remediation to remove pollutants in adjacent high permeability zones (HPZs). Slow back diffusion from LPZs to HPZs can extend management of polluted sites by decades. Numerical models are often used to simulate back diffusion, estimate cleanup times, and develop site management strategies. Sharp concentration gradients of pollutants are present at the interface between HPZs and LPZs, and hence accurate simulation requires fine grid sizes resulting in high computational burden. Since the MODFLOW family of codes is widely used in practice, we develop a new approach for modeling pollutant back diffusion using MODFLOW/RT3D that eliminates the need for fine discretization of the LPZ. Instead, the LPZ is treated as an impermeable region in MODFLOW, while in RT3D the LPZ is conceptualized as a series of immobile zones coupled with a mobile zone at the HPZ/LPZ interface. Finite volume discretization of diffusion and reaction within the LPZ is then modeled as mass transfer and reaction among several immobile species. This results in a simulation domain with significantly fewer grid cells compared to that required if all LPZs are discretized, providing potential for improved computational efficiency. Cases, including a layer of HPZ over an LPZ, a thin/thick lens of LPZ embedded in HPZ, and multiple lens of LPZs embedded in HPZ are tested by the new approach for tracer and reactive scenarios.

Fate and contaminant transport model-driven probabilistic human health risk assessment of DNAPL-contaminated site

In this study, fate and contaminant transport model-driven human health risk indexes were calculated due to the presence of dense non-aqueous phase liquids (DNAPLs) in the subsurface environment of air force base area in Florida, USA. Source concentration data of DNAPLs was used for the calculation of transport model-driven health risk indexes for the children and adult sub-population via direct oral ingestion and skin dermal contact exposure scenario using 10,000 Monte Carlo type simulations. The highest variation in the probability distribution of transformed DNAPL compound (cis-dichloroethene (cis-DCE) > vinyl chloride (VC)) was observed as compared to parent DNAPL (tetrachloroethene (PCE)) based on the 50-year simulation timespan. Transformed DNAPL compounds (VC, cis-DCE) posed the highest risk to human health for a longer duration (up to 15 years) in comparison to parent DNAPL (PCE), as non-carcinogenic hazard quotient varied from 400 to 1100. Carcinogenic health risks were observed as 3-order of magnitude higher than safe limit (HQ_Safe < 10^-6) from 2nd to 5th year timespan and fall in the high-risk zone, indicating the need for a remediation plan for a contaminated site. Variance attribution analysis revealed that concentration, body weight, and exposure duration (contribution percentage - 70 to 95%) were the most important parameters, highlighting the impact of dispersivity and exposure model in the estimation of risk indexes. This approach can help decision-makers when a contaminated site with partial data on hydrogeological properties and with higher uncertainty in model parameters is to be assessed for the formulation of remediation measures.

Impact of diffuse layer processes on contaminant forward and back diffusion in heterogeneous sandy-clayey domains

Low-permeability aquitards can significantly affect the transport, distribution, and persistence of contaminant plumes in subsurface systems. Although such low-permeability materials are often charged, the key role of charge-induced electrostatic processes during contaminant transport has not been extensively studied. This work presents a detailed investigation exploring the coupled effects of heterogeneous distribution of physical, chemical and electrostatic properties on reactive contaminant transport in field-scale groundwater systems including spatially distributed clay zones. We performed an extensive series of numerical experiments in three distinct heterogeneous sandy-clayey domains with different levels of complexity. The flow and reactive transport simulations were performed by explicitly resolving the complex velocity fields, the small-scale electrostatic processes, the compound-specific diffusive/dispersive fluxes and the chemical processes utilizing a multi-continua based reactive transport code (MMIT-Clay). In each particular domain, numerical experiments were performed focusing on both the forward and back diffusion through the sandy-clayey interfaces. The results illuminate the control of microscopic electrostatic mechanisms on macroscopic mass transfer. Coulombic interactions in the clay's diffuse layer can significantly accelerate or retard a particular species depending on its charge. Furthermore, the chemical heterogeneity plays a major role in mass storage and release during reactive transport. Neglecting such processes can lead to substantial over- or underestimation of the overall transport behavior, which underlines the need for integrated physical, chemical and electrostatic approaches to accurately describe mass transfer processes in systems including low-permeability inclusions.

Contaminant occurrence and migration between high- and low-permeability zones in groundwater systems: A review

In recent decades, water quality problems that impact human health, especially groundwater pollution, have been intensely studied, and this has contributed to new ideas and policies around the world such as Low Impact Development (LID) and Superfund legislation. The fundamental to many of these problems is pollutant occurrence and migration in saturated porous media, especially in groundwater. Such environments often contain contrasting zones of high and low permeability with significant differences in hydraulic conductivity (~10^-4 and 10^-8 m/s, respectively). High-permeability zones (HPZs) represent the primary pathways for pollutant transport in groundwater, while low-permeability zones (LPZs) are often diffusion dominated and serve as both sinks and sources (i.e., via back-diffusion) of pollutants over many decades. In this review, concepts and mechanisms of solute source depletion, contaminant accumulation, and back-diffusion in high- and low-permeability systems are presented, and new insights gained from both experimental and numerical studies are analyzed and summarized. We find that effluent monitoring and novel image analysis techniques have been adroitly used to investigate temporal and spatial evolutions of contaminant concentration; simultaneously, mathematical models are constantly upscaled to verify, optimize and extend the experimental data. However, the spatial concentration data during back-diffusion lacks diversity due to the limitations of pollutant species in studies, the microscopic mechanisms controlling pollutant transformation are poorly understood, and the impacts of these reactions on contaminant back-diffusion are rarely considered. Hence, most simulation models have not been adequately validated and are not capable of accurately predicting pollutant fate and cleanup in realistic heterogeneous aquifers. Based on these, some hypotheses and perspectives are mentioned to promote the investigation of contaminant migration in high- and low-permeability systems in groundwater.

Analytical and Numerical Methods for a Preliminary Assessment of the Remediation Time of Pump and Treat Systems

Several remediation technologies are currently used to address groundwater pollution. “Pump and treat” (P&T) is probably one of the most widely applied, being a process where contaminated groundwater is extracted from the subsurface by pumping and then treated before it is discharged or reinjected into the aquifer. Despite being a very adaptable technology, groundwater remediation is often achieved in long and unsustainable times because of limitations due to the hydrogeological setting and contaminant properties. Therefore, the cost–benefit analysis over time results in an inefficient system and a preliminary evaluation of the clean-up time is crucial. The aim of the paper is to compare, in an integrated manner, the application of some models to estimate the time to compliance of a P&T system in relation to the specific hydrogeological condition. Analytical solutions are analyzed and applied to an industrial site and to a synthetic case. For both cases, batch flushing and the advection-dispersion-retardation (ADR) model underestimate remediation times comparing the results to real or simulated monitoring data, whereas the Square Root model provided more reliable remediation times. Finally, for the synthetic case, the reliability of analytical approaches and the effects of matrix diffusion are tested on the basis of a numerical groundwater transport model specifically implemented, which confirm the results of the analytical methods and the strong influence of the matrix diffusion on the results.

Low permeability zone remediation of trichloroethene via coupling electrokinetic migration with in situ electrochemical hydrodechlorination

To address the challenge of trichloroethene (TCE) remediation in low permeability zone, an inexpensive Cu-Ni bimetallic cathode was proposed in electrokinetic (EK) remediation system to couple electrokinetic migration with in situ electrochemical hydrodechlorination. Aqueous phase TCE was originally added into the anolyte so that breakthrough curves through the low permeability porous soil compartment could be obtained to better understand TCE migration driven by electroosmosis flow using different cathodes. The Cu-Ni cathode resulted in more TCE migration of 7.64 mg compared to that of 5.99 mg with Ni and 4.22 mg with mixed metal oxide (MMO) cathode, suggesting that the Cu-Ni cathode was capable of driving more TCE flux out of the contaminated soil. With the Cu-Ni cathode, 98.4% of TCE flux that reached the cathode was electrochemically reduced on the cathode, which was much higher than that with MMO cathode (77.9%) or Ni cathode (59.6%). TCE mass that was transported by electroosmosis flow increased from 2.04 to 6.68 mg when the voltage gradient increased from 1 to 4 V cm^-1, with the normalized energy consumption increasing from 0.06 to 0.16 kWh kg^-1 per unit water movement, and from 0.54 to 2.55 kWh g^-1 per unit TCE transport. For TCE that did reach the cathode compartment, > 98% degradation maintained at the Cu-Ni cathode with various voltage gradients. The coupled electrokinetic and electrochemical hydrodechlorination technology appears to be a promising strategy for the remediation of low permeability porous media.

Controls on the persistence of aqueous-phase groundwater contaminants in the presence of reactive back-diffusion

The persistence of groundwater contaminants is influenced by several interacting processes. Physical, physico-chemical, and (bio-)chemical processes all influence the transport of contaminants in the subsurface. However, for a given hydrogeological system, it is generally unclear to which degree each of these phenomena acts as a control on plume behaviour. Here, we present a comprehensive investigation of these processes and their influences on plume behaviour and persistence in layered sedimentary systems. We investigate different scenarios that represent fundamental configurations of common contaminant situations. A confined aquifer over- and underlain by aquitard layers is investigated in a source-removal scenario and a constant-source equilibrium scenario. Additionally, an aquitard overlain and underlain by high permeability units is investigated in a source-removal scenario. In these investigations, we vary layer thickness, as well as parameters governing advection, (back-)diffusion, sorption, and degradation. Extensive analysis of these results enables quantification of the influence of these parameters on maximum down-gradient concentration, plume persistence duration, and plume spatial extent. Finally, parameter space dimensionality reduction is used to establish trends and regimes in which certain processes dominate as controls. A lower limit to plume extent as a function of a novel constructed parameter is also determined. These results provide valuable quantitative information for the assessment of the fate of groundwater contaminants and are applicable to a wide range of aqueous-phase solutes.

Do CSIA data from aquifers inform on natural degradation of chlorinated ethenes in aquitards?

Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δ¹³C-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δ¹³C increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.

Revealing chlorinated ethene transformation hotspots in a nitrate-impacted hyporheic zone

Hyporheic zones are increasingly thought of as natural bioreactors, capable of transforming and attenuating groundwater pollutants present in diffuse baseflow. An underappreciated scenario in the understanding of contaminant fate in hyporheic zones is the interaction between point-source trichloroethene (TCE) plumes and ubiquitous, non-point source pollutants such as nitrate. This study aims to conceptualise critical biogeochemical gradients in the hyporheic zone which govern the export potential of these redox-sensitive pollutants from carbon-poor, oxic aquifers. Within the TCE plume discharge zone, discrete vertical profiling of the upper 100 cm of sediment pore water chemistry revealed an 80% increase in dissolved organic carbon (DOC) concentrations and 20-60 cm thick hypoxic zones (<2 mg O₂ L^-1) within which most reactive transport was observed. A 33% reduction of nitrate concentrations coincided with elevated pore water nitrous oxide concentrations as well as the appearance of manganese and the TCE metabolite cis-1,2-dichloroethene (cDCE). Elevated groundwater nitrate concentrations (>50 mg L^-1) create a large stoichiometric demand for bioavailable DOC in discharging groundwater. With the benefit of a high-resolution grid of pore water samplers investigating the shallowest 30 cm of hypoxic groundwater flow paths, we identified DOC-rich hotspots associated with submerged vegetation (Ranunculus spp.), where low-energy metabolic processes such as mineral dissolution/reduction, methanogenesis and ammonification dominate. Using a chlorine index metric, we show that enhanced TCE to cDCE transformation takes place within these biogeochemical hotspots, highlighting their relevance for natural plume attenuation.

A robust optimization technique for analysis of multi-tracer experiments

Fate and transport of solutes in heterogeneous porous media is largely affected by diffusive mass exchange between mobile and immobile water zones. Since it is difficult to directly measure and determine the effect in the aquifers, multi-tracer experiments in combination with mathematical modeling are used to obtain quantitative information about unknown system parameters such as the effective mobile and immobile porosity, and the diffusive mass exchange between mobile and immobile water zones. The Single Fissure Dispersion Model (SFDM) describing nonreactive transport of solutes in saturated dual-porosity media, has been employed as a modeling approach to explain dual-porosity experiments in the field and laboratory (column experiments). SFDM optimization with conventional methods of minimization was immensely difficult due to its complex analytical form. Thus, previous studies used a trial and error procedure to fit it to the experimental observations. In this study, a rigorous optimization technique based on the newly developed scatter search method is presented that automatically minimizes the SFDM to find the optimal values of the hydrogeologically related parameters. The new program (OptSFDM) is accompanied with an easy-to-use graphical user interface (GUI) that is flexible and fully integrated. The program usability is showcased by a few, previously presented experimental case studies, and compared against the currently available, trial-and-error based, command-line executable, SFDM code.

Modeling groundwater contaminant transport in the presence of large heterogeneity: A case study comparing MT3D and RWhet

A case study is presented that implements two numerical models for simulating a 30-year PAT operation conducted at a large contaminated site for which high-resolution data sets are available. A Markov chain based stochastic method is used to conditionally generate the realizations with random distribution of heterogeneity for the Tucson International Airport Area (TIAA) federal Superfund site. The fields were conditioned to data collected for 245 boreholes drilled at the site. Both MT3DMS and the advanced random walk particle method (RWhet) were used to simulate the PAT-based mass removal process. The results show that both MT3DMS and RWhet represent the measured data reasonably, with Root Mean Square Error (RMSE) less than 0.03. The use of fine grids and the total-variation-diminishing method (TVD) limited the effects of numerical dispersion for MT3DMS. However, the effects of numerical dispersion were observed when compared to the simulations produced with RWhet using a larger number of particles, which provided more accurate results with RMSE diminishing from 0.027 to 0.024 to 0.020 for simulations with 1, 20, and 50 particles. The computational time increased with more particles used in the model, but was still much less than the time required for MT3DMS, which is an advantage of RWhet. By showing the results using both methods, this study provides guidance for simulating long-term PAT systems. This work will lead to improve understanding of contaminant transport and plume persistence, and in turn will enhance site characterization and site management for contaminated sites with large plumes.

Experimental and numerical evaluation of Groundwater Circulation Wells as a remediation technology for persistent, low permeability contaminant source zones

Contaminants removal stoked inside low permeability zones of aquifers is one of the most important challenge of groundwater remediation process today. Low permeability layers can be considered persistent secondary sources of contamination because they release pollutants by molecular diffusion after primary source of contamination is reduced, causing long plum tails (Back-Diffusion). In this study, the Groundwater Circulation Well (GCW) system was investigated as an alternative remediation technology to the low efficient traditional pumping technologies to restore contaminated low permeability layers of aquifers. The GCW system creates vertical groundwater circulation cells by drawing groundwater through a screen of a multi-screen well and discharging it through another screen. The suitability of this technology to remediate contaminated low permeability zones was investigated by laboratory test and numerical simulations. The collected data were used to calibrate a model created to simulate the Back-Diffusion process and to evaluate the effect of different pumping technologies on the depletion time of that process. Results show that the efficiency of the GCW is dependent on the position and on the geometry of the low permeability zones, however the GCW system appears more suitable to restore contaminated low permeability layers of aquifers than the traditional pumping technology.

Determining the long-term operational performance of pump and treat and the possibility of closure for a large TCE plume

The purpose of this study is to evaluate the impact of heterogeneity on the long-term performance of a large pump-and-treat (PAT) system that has been in operation for 30 years at a site located in Tucson, AZ. A 3D numerical model was developed. Three different concentrations were examined: composite concentration in the influent to the treatment plant, resident concentration in the aquifer, and concentration for downgradient boundary discharge. The time scales needed for concentrations measured in these ways to reach the Maximum Contaminant Levels (MCLs) are significantly different, with ∼125 years required for treatment-plant influent compared to ∼225 years for downgradient boundary discharge and >>227 years (total simulated time) for the resident concentration in the aquifer. These large time scales, compared to 36 years for a hypothetical homogeneous system, demonstrate the significant impacts of permeability heterogeneity on remediation at this site. The possibility of closure of the site was investigated by examining the mass discharge from the site boundary and the concentration rebound after simulating shutdown of the PAT system. The results of this study provide insight on evaluation of closure potential for large, complex contamination sites and a reference on selecting performance metrics for site management.

Diffusion of solutes from depleting sources into and out of finite low-permeability zones

Two important factors that affect groundwater contaminant persistence are the temporal pattern of contaminant source depletion and solute diffusion into and out of aquitards. This study provides a framework to evaluate the relative importance of these effects on contaminant persistence, with emphasis on the importance of thin aquitards. We developed one-dimensional (1D) analytical solutions for forward and back diffusion in a finite domain with a no flux boundary using the method of images and demonstrated their applicability to measured data from three well-controlled laboratory diffusion experiments with exponentially depleting sources. We used both in situ aquitard solute concentrations and aquifer breakthrough curves for sorbing and non-sorbing solutes. The finite-domain no flux boundary solutions showed better agreement with measured data than was available with semi-infinite approaches, with increasing discrepancy for dimensionless relative diffusion length scale beyond a critical threshold value (Z_d > 0.7). We also used a mass balance to demonstrate that the temporal pattern of contaminant source depletion controls the duration of solute mass accumulation in the aquitard, as well as the total solute mass release back into the aquifer. Lower rates of source depletion result in a longer period of mass accumulation in the aquitard and later back diffusion initiation time. The amount of solute mass stored in the aquitard increases with longer loading duration, thereby contributing to overall longer contaminant persistence in aquifers. This study entails widespread implications for anthropogenic waste and contamination sites, which are all dependent on efficient and cost-effective contaminant management strategies.

Spatial variability of microbial communities in a fractured sedimentary rock matrix impacted by a mixed organics plume

Dissolved phase contaminants, transported by diffusion into the low permeability matrix of fractured sedimentary rock, pose a challenge to groundwater cleanup efforts because this stored mass may persist even when the upgradient source zone is removed. In this context, if contaminant biodegradation takes place within the low permeability matrix, plume persistence may be substantially reduced. Therefore, it is important to characterize microbial communities within the low permeability, rock matrix pores, instead of only from groundwater samples, which represent biomass from fast flowing fractures. This research relies on depth-discrete data from both core and groundwater samples collected from two locations representing a mid-plume and plume front condition within an aged, mixed organic contaminant plume in a sedimentary rock aquifer. Results from multiple analyte measurements on rock and groundwater indicate that biodegradation in the lower permeability matrix of fractured sedimentary rocks and the microbial consortia is spatially variable due to differences in hydrochemistry, redox conditions, and contaminant concentrations. Dechlorinating microorganisms were detected in the sandstone matrix at both locations, but the detected microbial diversity calculated with PCR-DGGE was significantly higher in samples collected from the core located closer to the source zone, where contaminant concentrations are higher and contaminant compositions more diverse, compared to samples from the plume front location.

Semi-analytical method for matrix diffusion in heterogeneous and fractured systems with parent-daughter reactions

A semi-analytical/numerical method for modeling matrix diffusion in heterogeneous and fractured groundwater systems is developed. This is a significant extension of the Falta and Wang (2017) method that only applied to diffusion in an aquitard of infinite thickness. The current solution allows for the low permeability matrix to be embedded within a numerical gridblock, having finite average thickness, a specified volume fraction and a specified interfacial area with the high permeability domain. The new formulation also allows for coupled parent-daughter decay reactions with multiple species that each have independent retardation factors, decay rates, and yield coefficients in both the high and low permeability parts of the system. The method uses a fitting function to approximate the transient concentration profile in the low permeability part of each gridblock so that the matrix diffusion flux into the high permeability part of the gridblock can be computed as a concentration dependent source-sink term. This approach is efficient because the only unknowns in each gridblock are the concentrations in the high permeability domain, so there is practically no increase in computational effort compared to a conventional transport simulation. The method is shown to compare favorably with an analytical solution for matrix diffusion in fractured media with parallel fractures, with an analytical solution for matrix diffusion with parent-daughter decay reactions, with laboratory experiments of matrix diffusion in a layered system, with a laboratory experiment involving lens shaped inclusions, and with fine grid numerical simulations of transport in highly heterogeneous systems.

Assessing the effect of chlorinated hydrocarbon degradation in aquitards on plume persistence due to back-diffusion

This modeling study aims to investigate how reactive processes in aquitards impact plume persistence in adjacent aquifers. For that purpose the migration of a trichloroethene (TCE) plume in an aquifer originating from dense non-aqueous phase liquid (DNAPL) source dissolution and back-diffusion from an underlying reactive aquitard was simulated in a 2D-numerical model. Two aquitard degradation scenarios were modeled considering one-step degradation from TCE to cis-dichloroethene (cDCE): a uniform (constant degradation with aquitard depth) and a non-uniform scenario (decreasing degradation with aquitard depth) and were compared with a no-degradation scenario. In the no-degradation scenario, a long-term TCE tailing above the Maximum Contaminant Level (MCL) caused by back-diffusion after source removal was observed. In contrast, in the aquitard degradation scenarios, TCE back-diffusion periods were shorter, whereby the extent of back-diffusion reduction depended on the aquitard degradation depth and the rate. For high degradation rates (half-life: 30-80days), an aquitard degradation depth greater than 65cm prevented TCE plume persistence after source removal but generated a long-term tailing above the MCL for the produced cDCE. For slow degradation rates (half-life: <200days), TCE was only partially degraded after source removal, independent of the aquitard degradation depth, leading to a long-term dual contamination of the aquifer by cDCE and TCE. A sudden enrichment of ¹³C in TCE and cDCE was observed after source removal in the uniform and non-uniform degradation scenarios that was distinct from δ¹³C patterns observed when aquifer degradation occurs (continuous enrichment of ¹³C along the plume axis) and for when there is absence of degradation (no change of isotope ratios). This demonstrates that δ¹³C measurements in the aquifer can be used as a diagnostic tool to demonstrate aquitard degradation, which simplifies the identification of reactive processes in aquitards, as aquifers are usually easier to monitor than aquitards.

Identification of Degradation Pathways of Chlorohydrocarbons in Saturated Low-Permeability Sediments Using Compound-Specific Isotope Analysis

This study aims to investigate whether compound-specific carbon isotope analysis (CSIA) can be used to differentiate the degradation pathways of chlorohydrocarbons in saturated low-permeability sediments. For that purpose, a site was selected, where a complex mixture of chlorohydrocarbons contaminated an aquifer-aquitard system. Almost 50 years after contaminant releases, high-resolution concentration, CSIA, and microbial profiles were determined. The CSIA profiles showed that in the aquitard cis-dichloroethene (cDCE), first considered as a degradation product of trichloroethene (TCE), is produced by the dichloroelimination of 1,1,2,2-tetrachloroethane (TeCA). In contrast, TeCA degrades to TCE via dehydrohalogenation in the aquifer, indicating that the aquifer-aquitard interface separates two different degradation pathways for TeCA. Moreover, the CSIA profiles showed that chloroform (CF) is degraded to dichloromethane (DCM) via hydrogenolysis in the aquitard and, to a minor degree, produced by the degradation of carbon tetrachloride (CT). Several microorganisms capable of degrading chlorohydrocarbons were detected in the aquitard, suggesting that aquitard degradation is microbially mediated. Furthermore, numerical simulations reproduced the aquitard concentration and CSIA profiles well, which allowed the determination of degradation rates for each transformation pathway. This improves the prediction of contaminant fate in the aquitard and potential magnitude of impacts on the adjacent aquifer due to back-diffusion.

Contaminant back-diffusion from low-permeability layers as affected by groundwater velocity: A laboratory investigation by box model and image analysis

Low-permeability lenses represent potential sources of long-term release when filled from contaminant solute through direct contact with dissolved plumes. The redistribution of contaminant from low to high permeability aquifer zones (Back-Diffusion) was studied. Redistribution causes a long plume tail, commonly regarded as one of the main obstacles to effective groundwater remediation. Laboratory tests were performed to reproduce the redistribution process and to investigate the effect of pumping water on the remediation time of these contaminated low-permeability lenses. The test section used is representative of clay/silt lenses (k≈1∗10^-10m/s/k≈1∗10^-7m/s) in a sand aquifer (k≈1∗10^-3m/s). Hence, an image analysis procedure was used to estimate the diffusive flux of contaminant released by these low-permeability zones. The proposed technique was validated performing a mass balance of a lens saturated by a known quantity of tracer. For each test, performed using a different groundwater velocity, the diffusive fluxes of contaminant released by lenses were compared and the remediation times of the low-permeability zones calculated. For each lens, the obtained remediation timeframes were used to define an analytical relation vs groundwater velocity and the coefficients of these relations were matched to grain size of the low-permeability lenses. Results show that an increase of the velocity field is not useful to diminish the total depletion times as the process mainly diffusive. This is significant when the remediation approach relies on pumping technology.

Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Low Permeability Zone Remediation via Oxidant Delivered by Electrokinetics and Activated by Electrical Resistance Heating: Proof of Concept

This study proposes and proves (in concept) a novel approach of combining electrokinetic (EK)-assisted delivery of an oxidant, persulfate (PS), and low temperature electrical resistivity heating (ERH), to activate PS, to achieve remediation of contaminated, low permeability soil. This unique combination is able to overcome existing challenges in remediating low permeability materials, particularly associated with delivering remediants. A further benefit of the approach is the use of the same electrodes for both EK and ERH phases. Experiments were conducted in a laboratory-scale sand tank packed with silt and aqueous tetrachloroethene (PCE) and bracketed on each side by an electrode. EK first delivered unactivated PS throughout the silt. ERH then generated and sustained the target temperature to activate the PS. As a result, PCE concentrations decreased to below detection limit in the silt in a few weeks. Moreover, it was found that activating PS at ∼36 °C eliminated more PCE than activating it at >41 °C. It is expected this results from the reactive SO₄^•- radical being generated more slowly, which ensures more complete reaction with the contaminant. The novel application of EK-assisted PS delivery followed by low temperature ERH appears to be a viable strategy for low permeability contaminated soil remediation.

The impact of well-field configuration and permeability heterogeneity on contaminant mass removal and plume persistence

The purpose of this study is to investigate the effects of well-field hydraulics and permeability heterogeneity on mass-removal efficiency for systems comprising large groundwater contaminant plumes. A three-dimensional (3D) numerical model was used to simulate the impact of different well-field configurations on pump-and-treat mass removal for heterogeneous domains. The relationship between reduction in contaminant mass discharge (CMDR) and mass removal (MR) was used as the metric to examine remediation efficiency. The impacts of well-field configuration on mass removal behavior are attributed to mass-transfer constraints related to regions of low flow associated with the well field, which can be muted by the influence of permeability heterogeneity. These impacts are reflected in the associated CMDR-MR profiles. Systems whose CDMR-MR profiles are below the 1:1 relationship line are associated with more efficient well-field configurations. The impact of domain heterogeneity on mass-removal effectiveness was investigated in terms of both variance and correlation scale of the random permeability distributions and indexed by the CMDR-MR relationship. Data collected from pump-and-treat operations conducted in a section of the Tucson International Airport Area (TIAA) federal Superfund site were used as a case study. The comparison between simulated and measured site data supports the general validity of the numerical model, and results from the case study are consistent with the conclusions of the theoretical study. These results illustrate that the CMDR-MR relationship can be an effective way to quantify the impacts of different factors on mass-removal efficiency.

Electrokinetic-enhanced permanganate delivery and remediation of contaminated low permeability porous media

Back diffusion of contaminants from low permeability strata has inhibited site remediation and closure due to an inability to deliver remediants into these strata. This study demonstrates the potential of electrokinetics (EK) to significantly reduce back diffusion of chlorinated compounds from low permeability porous media. Experiments were conducted in a two-dimensional sandbox packed with vertical layers of coarse sand and silt contaminated with aqueous trichloroethene (TCE). Three experiments, each approximately 41 days in duration, compared EK-enhanced in situ chemical oxidation (EK-ISCO) to EK or ISCO alone. EK-ISCO successfully delivered the oxidant (permanganate, PM) throughout the silt cross-section while ISCO without EK resulted only in PM delivery to the edges of the silt layer fringes. EK-ISCO resulted in a 4.4-fold reduction in TCE concentrations in the coarse sand compared to a 3.5-fold reduction from ISCO alone. EK-ISCO with a 25 mA current was found to be more effective than with 300 mA current. Overall, this study suggests that electrokinetics coupled with an appropriate in situ remediation technique, such as ISCO, can enhance remediation of lower permeability strata and limit the extent of contaminant back diffusion.

Molecular and carbon isotopic characterization of an anaerobic stable enrichment culture containing Dehalobacterium sp. during dichloromethane fermentation

Biodegradation of dichloromethane (DCM) under reducing conditions is of major concern due to its widespread detection in contaminated groundwaters. Here, we report an anaerobic enrichment culture derived from a membrane bioreactor operating in an industrial wastewater treatment plant, capable of fermenting DCM and the brominated analogue dibromomethane (DBM). Comparative analysis of bacterial 16S rDNA-DGGE profiles from fresh liquid medium inoculated with single colonies picked from serial dilution-to-extinction agar vials showed that cultures degrading DCM contained a predominant band belonging to Dehalobacterium, however this band was absent in cultures unable to degrade DCM. Analysis of the microbial composition of the enrichment by bacterial 16S rRNA gene amplicon paired-end sequencing confirmed the presence of Dehalobacterium together with three additional phylotypes belonging to Acetobacterium, Desulfovibrio, and Wolinella, representing all four operational taxonomic units >99.9% of the retrieved sequences. The carbon isotopic fractionation (ε) determined for DCM degradation in this culture was -27±2‰. This value differs from the ε previously reported for the DCM-fermentative bacteria Dehalobacter (-15.5±1.5‰) but they are both significantly different from those reported for facultative methylotrophic organisms (ranging from -45 to -61‰). This significant difference in the ε allows differentiating between hydrolytic transformation of DCM via glutathione-dependent dehalogenases and fermentation pathway.

CAPSULE

The carbon isotopic fractionation of dichloromethane by an enriched Dehalobacterium-containing culture has significant potential to monitor biodegradation of DCM in groundwaters.

A semi-analytical method for simulating matrix diffusion in numerical transport models

A semi-analytical approximation for transient matrix diffusion is developed for use in numerical contaminant transport simulators. This method is an adaptation and extension of the heat conduction method of Vinsome and Westerveld (1980) used to simulate heat losses during thermally enhanced oil recovery. The semi-analytical method is used in place of discretization of the low permeability materials, and it represents the concentration profile in the low permeability materials with a fitting function that is adjusted in each element at each time-step. The resulting matrix diffusion fluxes are added to the numerical model as linear concentration-dependent source/sink terms. Since only the high permeability zones need to be discretized, the numerical formulation is extremely efficient compared to traditional approaches that require discretization of both the high and low permeability zones. The semi-analytical method compares favorably with the analytical solution for transient one-dimensional diffusion with first order decay, with a two-layer aquifer/aquitard solution, with the solution for transport in a fracture with matrix diffusion and decay, and with a fully numerical solution for transport in a thin sand zone bounded by clay with variable decay rates.

Solute source depletion control of forward and back diffusion through low-permeability zones

Solute diffusive exchange between low-permeability aquitards and high-permeability aquifers acts as a significant mediator of long-term contaminant fate. Aquifer contaminants diffuse into aquitards, but as contaminant sources are depleted, aquifer concentrations decline, triggering back diffusion from aquitards. The dynamics of the contaminant source depletion, or the source strength function, controls the timing of the transition of aquitards from sinks to sources. Here, we experimentally evaluate three archetypical transient source depletion models (step-change, linear, and exponential), and we use novel analytical solutions to accurately account for dynamic aquitard-aquifer diffusive transfer. Laboratory diffusion experiments were conducted using a well-controlled flow chamber to assess solute exchange between sand aquifer and kaolinite aquitard layers. Solute concentration profiles in the aquitard were measured in situ using electrical conductivity. Back diffusion was shown to begin earlier and produce larger mass flux for rapidly depleting sources. The analytical models showed very good correspondence with measured aquifer breakthrough curves and aquitard concentration profiles. The modeling approach links source dissolution and back diffusion, enabling assessment of human exposure risk and calculation of the back diffusion initiation time, as well as the resulting plume persistence.

Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites

Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach. The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released. The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated to be <3years due to its high effective solubility. While this was approximately an order-of-magnitude shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability zone at the end of the simulation period (26kg) was larger than that predicted for 1,1,1-TCA (17kg). Even 80years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-dioxane within transmissive portions of the source zone is quickly depleted due to characteristics that favor both diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means. Furthermore, the results highlight the differences between 1,4-dioxane and chlorinated solvent source zones, suggesting that back diffusion of 1,4-dioxane mass may be serving as the dominant long-term "secondary source" at many contaminated sites that must be managed using alternative approaches.

Image analysis procedure for studying Back-Diffusion phenomena from low-permeability layers in laboratory tests

In this study, the long-term tailing derived from the storage process of contaminants in low-permeability zones is investigated. The release from these areas in the groundwater can be considered a long-term source that often undermines remediation efforts. An Image Analysis technique is used to analyze the process and evaluate the concentrations of a tracer at different points of the test section. Furthermore, the diffusive flux from the low-permeability lenses is determined. To validate the proposed technique, the results are compared with samples, and the diffusive fluxes resulting from the low-permeability zones of the reconstructed aquifer are compared with a theoretical approach.

Estimating Tortuosity Coefficients Based on Hydraulic Conductivity

While the tortuosity coefficient is commonly estimated using an expression based on total porosity, this relationship is demonstrated to not be applicable (and thus is often misapplied) over a broad range of soil textures. The fundamental basis for a correlation between the apparent diffusion tortuosity coefficient and hydraulic conductivity is demonstrated, although such a relationship is not typically considered. An empirical regression for estimating the tortuosity coefficient based on hydraulic conductivity for saturated, unconsolidated soil is derived based on results from 14 previously reported diffusion experiments performed with a broad range of soil textures. Analyses of these experimental results confirm that total porosity is a poor predictor for the tortuosity coefficient over a large range of soil textures. The apparent diffusion tortuosity coefficient is more reliably estimated based on hydraulic conductivity.

Quantification of Degradation of Chlorinated Hydrocarbons in Saturated Low Permeability Sediments Using Compound-Specific Isotope Analysis

This field and modeling study aims to reveal if degradation of chlorinated hydrocarbons in low permeability sediments can be quantified using compound-specific isotope analysis (CSIA). For that purpose, the well-characterized Borden research site was selected, where an aquifer-aquitard system was artificially contaminated by a three component chlorinated solvent mixture (tetrachloroethene (PCE) 45 vol %, trichloroethene (TCE) 45 vol %, and chloroform (TCM) 10 vol %). Nearly 15 years after the contaminant release, several high-resolution concentration and CSIA profiles were determined for the chlorinated hydrocarbons that had diffused into the clayey aquitard. The CSIA profiles showed large shifts of carbon isotope ratios with depth (up to 24‰) suggesting that degradation occurs in the aquitard despite the small pore sizes. Simulated scenarios without or with uniform degradation failed to reproduce the isotope data, while a scenario with decreasing degradation with depth fit the data well. This suggests that nutrients had diffused into the aquitard favoring stronger degradation close to the aquifer-aquitard interface than with increasing depth. Moreover, the different simulation scenarios showed that CSIA profiles are more sensitive to different degradation conditions compared to concentration profiles highlighting the power of CSIA to constrain degradation activities in aquitards.

Colloid Mobilization in a Fractured Soil: Effect of Pore-Water Exchange between Preferential Flow Paths and Soil Matrix

Exchange of water and solutes between contaminated soil matrix and bulk solution in preferential flow paths has been shown to contribute to the long-term release of dissolved contaminants in the subsurface, but whether and how this exchange can affect the release of colloids in a soil are unclear. To examine this, we applied rainfall solutions of different ionic strength on an intact soil core and compared the resulting changes in effluent colloid concentration through multiple sampling ports. The exchange of water between soil matrix and the preferential flow paths leading to each port was characterized on the basis of the bromide (conservative tracer) breakthrough time at the port. At individual ports, two rainfalls of a certain ionic strength mobilized different amounts of colloids when the soil was pre-exposed to a solution of lower or higher ionic strength. This result indicates that colloid mobilization depended on rainfall solution history, which is referred as colloid mobilization hysteresis. The extent of hysteresis was increased with increases in exchange of pore water and solutes between preferential flow paths and matrix. The results indicate that the soil matrix exchanged the old water from the previous infiltration with new infiltrating water during successive infiltration and changed the pore water chemistry in the preferential flow paths, which in turn affected the release of soil colloids. Therefore, rainfall solution history and soil heterogeneity must be considered to assess colloid mobilization in the subsurface. These findings have implications for the release of colloids, colloid-associated contaminants, and pathogens from soils.

An upscaled approach for transport in media with extended tailing due to back-diffusion using analytical and numerical solutions of the advection dispersion equation

The mono-continuum advection-dispersion equation (mADE) is commonly regarded as unsuitable for application to media that exhibit rapid breakthrough and extended tailing associated with diffusion between high and low permeability regions. This paper demonstrates that the mADE can be successfully used to model such conditions if certain issues are addressed. First, since hydrodynamic dispersion, unlike molecular diffusion, cannot occur upstream of the contaminant source, models must be formulated to prevent "back-dispersion." Second, large variations in aquifer permeability will result in differences between volume-weighted average concentration (resident concentration) and flow-weighted average concentration (flux concentration). Water samples taken from wells may be regarded as flux concentrations, while soil samples may be analyzed to determine resident concentrations. While the mADE is usually derived in terms of resident concentration, it is known that a mADE of the same mathematical form may be written in terms of flux concentration. However, when solving the latter, the mathematical transformation of a flux boundary condition applied to the resident mADE becomes a concentration type boundary condition for the flux mADE. Initial conditions must also be consistent with the form of the mADE that is to be solved. Thus, careful attention must be given to the type of concentration data that is available, whether resident or flux concentrations are to be simulated, and to boundary and initial conditions. We present 3-D analytical solutions for resident and flux concentrations, discuss methods of solving numerical models to obtain resident and flux concentrations, and compare results for hypothetical problems. We also present an upscaling method for computing "effective" dispersivities and other mADE model parameters in terms of physically meaningful parameters in a diffusion-limited mobile-immobile model. Application of the latter to previously published studies of systems that exhibit early breakthrough and extended tailing shows that the upscaled mADE model is able to describe the observed behavior with reasonable accuracy given only known physical parameters for the systems without any model calibration.

Implications of soil mixing for NAPL source zone remediation: Column studies and modeling of field-scale systems

Soil remediation is often inhibited by subsurface heterogeneity, which constrains contaminant/reagent contact. Use of soil mixing techniques for reagent delivery provides a means to overcome contaminant/reagent contact limitations. Furthermore, soil mixing reduces the permeability of treated soils, thus extending the time for reactions to proceed. This paper describes research conducted to evaluate implications of soil mixing on remediation of non-aqueous phase liquid (NAPL) source zones. The research consisted of column studies and subsequent modeling of field-scale systems. For column studies, clean influent water was flushed through columns containing homogenized soils, granular zero valent iron (ZVI), and trichloroethene (TCE) NAPL. Within the columns, NAPL depletion occurred due to dissolution, followed by either column-effluent discharge or ZVI-mediated degradation. Complete removal of TCE NAPL from the columns occurred in 6-8 pore volumes of flow. However, most of the TCE (>96%) was discharged in the column effluent; less than 4% of TCE was degraded. The low fraction of TCE degraded is attributed to the short hydraulic residence time (<4 days) in the columns. Subsequently, modeling was conducted to scale up column results. By scaling up to field-relevant system sizes (>10 m) and reducing permeability by one-or-more orders of magnitude, the residence time could be greatly extended, potentially for periods of years to decades. Model output indicates that the fraction of TCE degraded can be increased to >99.9%, given typical post-mixing soil permeability values. These results suggest that remediation performance can be greatly enhanced by combining contaminant degradation with an extended residence time.

Back diffusion from thin low permeability zones

Aquitards can serve as long-term contaminant sources to aquifers when contaminant mass diffuses from the aquitard following aquifer source mass depletion. This study describes analytical and experimental approaches to understand reactive and nonreactive solute transport in a thin aquitard bounded by an adjacent aquifer. A series of well-controlled laboratory experiments were conducted in a two-dimensional flow chamber to quantify solute diffusion from a high-permeability sand into and subsequently out of kaolinite clay layers of vertical thickness 15 mm, 20 mm, and 60 mm. One-dimensional analytical solutions were developed for diffusion in a finite aquitard with mass exchange with an adjacent aquifer using the method of images. The analytical solutions showed very good agreement with measured breakthrough curves and aquitard concentration distributions measured in situ by light reflection visualization. Solutes with low retardation accumulated more stored mass with greater penetration distance in the aquitard compared to high-retardation solutes. However, because the duration of aquitard mass release was much longer, high-retardation solutes have a greater long-term back diffusion risk. The error associated with applying a semi-infinite domain analytical solution to a finite diffusion domain increases as a function of the system relative diffusion length scale, suggesting that the solutions using image sources should be applied in cases with rapid solute diffusion and/or thin clay layers. The solutions presented here can be extended to multilayer aquifer/low-permeability systems to assess the significance of back diffusion from thin layers.

Abiotic dechlorination in rock matrices impacted by long-term exposure to TCE

Field and laboratory tests were performed to evaluate the abiotic reaction of trichloroethene (TCE) in sedimentary rock matrices. Hydraulically conductive fractures, and the rock directly adjacent to the hydraulically conductive fractures, within a historically contaminated TCE bedrock aquifer were used as the basis for this study. These results were compared to previous work using rock that had not been exposed to TCE (Schaefer et al., 2013) to assess the impact of long-term TCE exposure on the abiotic dechlorination reaction, as the longevity of these reactions after long-term exposure to TCE was hitherto unknown. Results showed that potential abiotic TCE degradation products, including ethane, ethene, and acetylene, were present in the conductive fractures. Using minimally disturbed slices of rock core at and near the fracture faces, laboratory testing on the rocks confirmed that abiotic dechlorination reactions between the rock matrix and TCE were occurring. Abiotic daughter products measured in the laboratory under controlled conditions were consistent with those measured in the conductive fractures, except that propane also was observed as a daughter product. TCE degradation measured in the laboratory was well described by a first order rate constant through the 118-d study. Observed bulk first-order TCE degradation rate constants within the rock matrix were 1.3×10(-8) s(-1). These results clearly show that abiotic dechlorination of TCE is occurring within the rock matrix, despite decades of exposure to TCE. Furthermore, these observed rates of TCE dechlorination are expected to have a substantial impact on TCE migration and uptake/release from rock matrices.

Reduction of diffusive contaminant emissions from a dissolved source in a lower permeability layer by sodium persulfate treatment

Residual contamination contained in lower permeability zones is difficult to remediate and can, through diffusive emissions to adjacent higher permeability zones, result in long-term impacts to groundwater. This work investigated the effectiveness of oxidant delivery for reducing diffusive emissions from lower permeability zones. The experiment was conducted in a 1.2 m tall × 1.2 m wide × 6 cm thick tank containing two soil layers having 3 orders of magnitude contrast in hydraulic conductivity. The lower permeability layer initially contained dissolved methyl tert-butyl ether (MTBE) and benzene, toluene, ethylbenzene, and p-xylenes (BTEX). The treatment involved delivery of 10% w/w nonactivated sodium persulfate (Na2S2O8) solution to the high permeability layer for 14 days. The subsequent diffusion into the lower permeability layer and contaminant emission response were monitored for about 240 days. The S2O8(2-) diffused about 14 cm at 1% w/w into the lower permeability layer during the 14 day delivery and continued diffusing deeper into the layer as well as back toward the higher-lower permeability interface after delivery ceased. Over 209 days, the S2O8(2-) diffused 60 cm into the lower permeability layer, the BTEX mass and emission rate were reduced by 95-99%, and the MTBE emission rate was reduced by 63%. The overall treatment efficiency was about 60-110 g-S2O8(2-)delivered/g-hydrocarbon oxidized, with a significant fraction of the oxidant delivered likely lost by back-diffusion and not involved in hydrocarbon destruction.

Sustainable in-well vapor stripping: A design, analytical model, and pilot study for groundwater remediation

A sustainable in-well vapor stripping system is designed as a cost-effective alternative for remediation of shallow chlorinated solvent groundwater plumes. A solar-powered air compressor is used to inject air bubbles into a monitoring well to strip volatile organic compounds from a liquid to vapor phase while simultaneously inducing groundwater circulation around the well screen. An analytical model of the remediation process is developed to estimate contaminant mass flow and removal rates. The model was calibrated based on a one-day pilot study conducted in an existing monitoring well at a former dry cleaning site. According to the model, induced groundwater circulation at the study site increased the contaminant mass flow rate into the well by approximately two orders of magnitude relative to ambient conditions. Modeled estimates for 5h of pulsed air injection per day at the pilot study site indicated that the average effluent concentrations of dissolved tetrachloroethylene and trichloroethylene can be reduced by over 90% relative to the ambient concentrations. The results indicate that the system could be used cost-effectively as either a single- or multi-well point technology to substantially reduce the mass of dissolved chlorinated solvents in groundwater.

Architecture, persistence and dissolution of a 20 to 45 year old trichloroethene DNAPL source zone

A detailed field-scale investigation of processes controlling the architecture, persistence and dissolution of a 20 to 45year old trichloroethene (TCE) dense non-aqueous phase liquid (DNAPL) source zone located within a heterogeneous sand/gravel aquifer at a UK industrial site is presented. The source zone was partially enclosed by a 3-sided cell that allowed detailed longitudinal/fence transect monitoring along/across a controlled streamtube of flow induced by an extraction well positioned at the cell closed end. Integrated analysis of high-resolution DNAPL saturation (Sn) (from cores), dissolved-phase plume concentration (from multilevel samplers), tracer test and permeability datasets was undertaken. DNAPL architecture was determined from soil concentration data using partitioning calculations. DNAPL threshold soil concentrations and low Sn values calculated were sensitive to sorption assumptions. An outcome of this was the uncertainty in demarcation of secondary source zone diffused and sorbed mass that is distinct from trace amounts of low Sn DNAPL mass. The majority of source mass occurred within discrete lenses or pools of DNAPL associated with low permeability geological units. High residual saturation (Sn>10-20%) and pools (Sn>20%) together accounted for almost 40% of the DNAPL mass, but only 3% of the sampled source volume. High-saturation DNAPL lenses/pools were supported by lower permeability layers, but with DNAPL still primarily present within slightly more permeable overlying units. These lenses/pools exhibited approximately linearly declining Sn profiles with increasing elevation ascribed to preferential dissolution of the uppermost DNAPL. Bi-component partitioning calculations on soil samples confirmed that the dechlorination product cDCE (cis-dichloroethene) was accumulating in the TCE DNAPL. Estimated cDCE mole fractions in the DNAPL increased towards the DNAPL interface with the uppermost mole fraction of 0.04 comparable to literature laboratory data. DNAPL dissolution yielded heterogeneous dissolved-phase plumes of TCE and its dechlorination products that exhibited orders of magnitude local concentration variation. TCE solubility concentrations were relatively localised, but coincident with high saturation DNAPL lens source areas. Biotic dechlorination in the source zone area, however, caused cDCE to be the dominant dissolved-phase plume. The conservative tracer test usefully confirmed the continuity of a permeable gravel unit at depth through the source zone. Although this unit offered significant opportunity for DNAPL bypassing and decreased timeframes for dechlorination, it still transmitted a significant proportion of the contaminant flux. This was attributed to dissolution of DNAPL-mudstone aquitard associated sources at the base of the continuous gravel as well as contaminated groundwater from surrounding less permeable sand and gravel horizons draining into this permeable conduit. The cell extraction well provided an integrated metric of source zone dissolution yielding a mean concentration of around 45% TCE solubility (taking into account dechlorination) that was equivalent to a DNAPL mass removal rate of 0.4tonnes per annum over a 16m(2) cell cross sectional area of flow. This is a significant flux considering the source age and observed occurrence of much of the source mass within discrete lenses/pools. We advocate the need for further detailed field-scale studies on old DNAPL source zones that better resolve persistent pool/lens features and are of prolonged duration to assess the ageing of source zones. Such studies would further underpin the application of more surgical remediation technologies.

Mass discharge assessment at a brominated DNAPL site: Effects of known DNAPL source mass removal

Management and closure of contaminated sites is increasingly being proposed on the basis of mass flux of dissolved contaminants in groundwater. Better understanding of the links between source mass removal and contaminant mass fluxes in groundwater would allow greater acceptance of this metric in dealing with contaminated sites. Our objectives here were to show how measurements of the distribution of contaminant mass flux and the overall mass discharge emanating from the source under undisturbed groundwater conditions could be related to the processes and extent of source mass depletion. In addition, these estimates of mass discharge were sought in the application of agreed remediation targets set in terms of pumped groundwater quality from offsite wells. Results are reported from field studies conducted over a 5-year period at a brominated DNAPL (tetrabromoethane, TBA; and tribromoethene, TriBE) site located in suburban Perth, Western Australia. Groundwater fluxes (qw; L(3)/L(2)/T) and mass fluxes (Jc; M/L(2)/T) of dissolved brominated compounds were simultaneously estimated by deploying Passive Flux Meters (PFMs) in wells in a heterogeneous layered aquifer. PFMs were deployed in control plane (CP) wells immediately down-gradient of the source zone, before (2006) and after (2011) 69-85% of the source mass was removed, mainly by groundwater pumping from the source zone. The high-resolution (26-cm depth interval) measures of qw and Jc along the source CP allowed investigation of the DNAPL source-zone architecture and impacts of source mass removal. Comparable estimates of total mass discharge (MD; M/T) across the source zone CP reduced from 104gday(-1) to 24-31gday(-1) (70-77% reductions). Importantly, this mass discharge reduction was consistent with the estimated proportion of source mass remaining at the site (15-31%). That is, a linear relationship between mass discharge and source mass is suggested. The spatial detail of groundwater and mass flux distributions also provided further evidence of the source zone architecture and DNAPL mass depletion processes. This was especially apparent in different mass-depletion rates from distinct parts of the CP. High mass fluxes and groundwater fluxes located near the base of the aquifer dominated in terms of the dissolved mass flux in the profile, although not in terms of concentrations. Reductions observed in Jc and MD were used to better target future remedial efforts. Integration of the observations from the PFM deployments and the source mass depletion provided a basis for establishing flux-based management criteria for the site.