Source Zone Remediation Research Articles

Long‐term evaluation of hydrocarbon degradation rates within the source zone under natural and enhanced attenuation for site closure purposes

AbstractDespite addressing the cleanup of petroleum hydrocarbon contamination plumes in groundwater, challenges with site closure persist due to the ongoing long‐term contaminant release from source zones. Thus, source zone monitoring even in the presence of residual light nonaqueous‐phase liquid (LNAPL) is mandated by many countries' regulatory agencies before site closure. In this study, four independent controlled‐release sites, each containing a particular fraction of ethanol in gasoline, that is, 10%, 24%, 25%, and 85% (v/v), were subjected to monitored natural attenuation (MNA) alone (E24 and E85), and with nitrate (E25) and sulfate (E10) biostimulation at the vicinity of the source zone. To the best of the authors' knowledge, this is the longest controlled gasohol release monitoring study performed to date at the source zone for site closure (nearly 18 years of monitoring). Both natural source zone depletion and biostimulation were found to reduce benzene concentrations to acceptable remediation target levels according to environmental regulatory councils. Benzene biodegradation rates (λs) were one order of magnitude higher than those reported in the literature for diluted concentrations in groundwater plumes. The decay rate of benzene varied between 0.45 and 1.75/year and was strongly influenced by the mass of ethanol. Compared to biostimulation with nitrate (half‐life of 1.52 years), MNA alone resulted in faster benzene removal rates (half‐life of 0.96 years). Anaerobic biostimulation with sulfate had negligible effects on benzene, toluene, ethylbenzene, and xylene (o‐, m‐, and p‐xylene [BTEX]) biodegradation compared to natural attenuation alone. The highest ethanol concentration (E85) led to faster benzene removal (with a half‐life of 0.40 years), which was even higher than MNA under lower ethanol concentrations (E24). Benzene half‐life was 2.2‐fold slower in the site with the lowest ethanol (E10) compared to E85, indicating that the negative effects of ethanol on BTEX biodegradation appeared to be short‐lived and may need to be re‐evaluated over the long term. Benzene decay rates were approximately six times slower in the source zone than the rates obtained for groundwater plumes, emphasizing the importance of targeting the LNAPL after plume retreat. The study's findings can assist practitioners in better predicting the lines of evidence for BTEX bioremediation and remediation cleanup times at a lower cost in the presence of ethanol, as well as providing guidance for determining whether biostimulation is necessary to expedite source zone remediation for site closure.

Managing the remediation strategy of contaminated megasites using field-scale calibration of geo-electrical imaging with chemical monitoring

Groundwater contamination is a threat to drinking water resources and ecosystems. Remediation by injection of chemical reagents into the aquifer may be preferred to excavation to reduce cost and environmental footprint. Yet, successful remediation requires complete contact between contamination and reagents. Subsurface heterogeneities are often responsible for diffusion into low-permeable zones, which may inhibit this contact. Monitoring the spatial distribution of injected reagents over time is crucial to achieve complete interaction. Source zone contamination at megasites is particularly challenging to remediate and monitor due to the massive scale and mixture of contaminants. Source zone remediation at Kærgård Plantation megasite (Denmark) is monitored here, with a new methodology, using high-resolution cross-borehole electrical resistivity tomography (XB-ERT) imaging calibrated by chemical analyses on groundwater samples. At this site, high levels of toxic non-aqueous phase liquids (NAPL) are targeted by in-situ chemical oxidation using activated persulfate. It may take numerous injection points with extensive injection campaigns to distribute reagents, which requires an understanding of how reagent may transport within the aquifer. A geophysical (XB-ERT) monitoring network of unprecedented size was installed to identify untreated zones and help manage the remediation strategy. The combination of spatially continuous geophysical information with discrete but precise chemical information, allowed detailed monitoring of sulfate distribution, produced during persulfate activation. Untreated zones identified in the first remediation campaign were resolved in the second campaign. The monitoring allowed adjusting the number of injection screens and the injection strategy from one campaign to the next, which resulted in better persulfate distribution and contaminant degradation in the second campaign. Furthermore, geophysical transects repeated over the timespan of a remediation campaign allowed high-resolution time-lapse imaging of reagent transport, which could in the future improve the predictability of transport models, compared to only using on a-priori assumptions of the hydraulic conductivity field.

Remediation of chlorinated solvents with Electrical Resistance Heating (ERH) at an active industrial site in Italy

Italian legislation defines stringent groundwater chemical quality criteria, to be applied at a site’s downgradient property boundary, irrespective of whether the underlying aquifer is, or could be, used for water resource purposes. In some scenarios, the regulatory authorities may identify less stringent standards, but this rarely occurs. This means that many sites with groundwater contamination are managed using hydraulic barriers, as source zone remediation may not achieve the stringent groundwater standards required due to technology limits or time constraints; therefore, the parties responsible for contamination often decide to continue to operate these hydraulic barriers indefinitely. This article describes the first application in Italy of source treatment using Electrical Resistance Heating (ERH), a remediation technology capable of removing a large percentage of contaminant mass, at a site where a hydraulic barrier is operating within a low yielding aquifer that is not used for water supply. The implementation of this technology was possible since the source zone was far from the downgradient site boundary, thus making achievement of the stringent quality standards at the boundary possible within a reasonable timeframe. The ERH system recovered of about 600 kg of contaminants within a timeframe of 8 months and achieved a reduction of contaminant concentrations in the most impacted areas greater than 90%. This article also emphasizes that, in similar low yielding aquifers, setting less stringent groundwater standards at the site boundary whilst still protecting downgradient receptors may promote more widespread implementation of source remediation activities in Italy.

A probabilistic assessment of surface water-groundwater exchange flux at a PCE contaminated site using groundwater modelling

Polluted groundwater discharge at a chlorinated solvent contaminated site in Hagfors, Sweden, is affecting a nearby stream flowing through a sparsely populated area. Because of difficulties related to source zone remediation, decision makers recently changed the short-term site management objective to mitigating discharge of polluted groundwater to the stream. To help formulating targeted remediation strategies pertaining to the new objective, we developed a groundwater numerical decision-support model. To facilitate reproducibility, the modelling workflow was scripted. The model was designed to quantify and reduce the uncertainty of surface water-groundwater (SW-GW) exchange fluxes for the studied period (2016–2020) through the use of history-matching. In addition to classical observations, thermal anomalies detected in fiber optic distributed temperature sensing (FO-DTS) measurements were used to inform the model of groundwater discharge. After assessing SW-GW exchange fluxes, we used measurements of surface water chemistry to provide a probabilistic estimation of mass influx and spatio-temporal distributions of contaminated groundwater discharge. Results show 1) SW-GW exchange fluxes are likely to be significantly larger than previously estimated, and 2) prior estimations of mass influx are located near the center of the posterior probability distribution. Based on this, we recommend decision makers to focus remediation action on specific segments of the stream.

Mass-flux Reduction and Mass Removal Relationships (MFR-MR) of Pool-dominated Multicomponent DNAPLs in Heterogeneous Geological System

<p>Contamination of groundwater by chlorinated solvents such as trichloroethylene (TCE, tetrachloroethylene (PCE), and 1,1-dichloroethylene (1,1DCE) is a widespread problem that can seriously threaten human health and the environment. These compounds are often present as dense nonaqueous phase liquid (DNAPL) mixtures in the subsurface that serve as a long-term groundwater contamination sources. The objective of this study is to test the effectiveness of different in-situ flushing agents (SDS, Tween 80, MCD, and water) for the recovery of DNAPL mixtures (TCE, PCE, 1,1DCE) entrapped in heterogeneous aquifer media. A series of batch tests were first performed to assess the multiphase behavior of water-DNAPL mixture-reagent system. Intermediate scale tank experiments were then performed to compare the performance of the reagent solutions. The flushing breakthrough curves were also used to investigate the impact of solubilization agents on mass-flux reduction/mass-removal behavior relationships. Performance of agents on DNAPL mixture source zone remediation was found to be in the following order: Tween 80>SDS>MCD>>Water. Individual DNAPL constituents exhibited transient effective solubilities that depend on the reagent contents and DNAPL compositions. Although high fractions of DNAPL source zones in heterogeneous porous media were removed through chemical flushing (SDS, Tween 80, MCD, and water), the heterogeneous system exhibited an extended multi-step concentration behavior. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher-saturation zones. Our results also showed that the distribution and the emplacement of organic liquid, flow-field heterogeneity, sorption and desorption, and flushing solution all influenced the DNAPL mixture removal. The results from this study underline the need for understanding the multiphase behaviour of a system, particularly when DNAPL mixtures are present, prior to field-scale remediation implementation.</p>

Conductive-polymer-supported palladium-iron bimetallic nanocatalyst for simultaneous 4-chlorophenol and Cr(VI) removal: Enhanced interfacial electron transfer and mechanism

Nanoscale zerovalent iron (nZVI) reduction offers a wide range of applications in source-zone remediation, but the reactivity of nZVI is largely hampered due to its low electron-transfer ability and tendency to aggregate. Based on the dual function of conductive polymers (CPs) as support and electron transfer carrier, we combined CPs with nZVI and prepared a series of Pd/Fe bimetallic materials that successfully address the challenges of nZVI reduction. These Pd/Fe@CPs particles showed strong catalytic ability for the simultaneous removal of 4-chlorophenol (4-CP) and Cr(VI). The removal rate of 4-CP was significantly enhanced by 1.5–6.2 times after supporting Pd/Fe nanoparticles (NPs) with CPs. The enhanced reactivity of supported Pd/Fe NPs was attributed to their highly stabilized and dispersed state and the promoted electron transfer due to the synergistic effect between CPs and nZVI bimetallic particles. The various catalytic activity over Pd/Fe@CPs was attributed to the distinctive properties of CPs and their different interfacial electron transfer ability. Importantly, this study provides insights into distinguishing both mechanisms of direct electron transfer and atomic-hydrogen-mediated indirect electron transfer, and their quantitative relationship to the dehalogenation performance over Pd/Fe@CPs materials. This work provides better understanding of the remediation process and mechanisms of nZVI reduction.

Stochastic cost-optimization and risk assessment of in situ chemical oxidation for dense non-aqueous phase liquid (DNAPL) source remediation

This study involved development of a computer program to determine optimal design variables for in situ chemical oxidation (ISCO) of dense nonaqueous phase liquid (DNAPL) sites to meet site-wide remediation objectives with minimum life-cycle remediation cost while taking uncertainty in site characterization data and model predictions into consideration. A physically-based ISCO performance model computes field-scale DNAPL dissolution, instantaneous reaction of oxidant with contaminant and with readily oxidizable natural oxidant demand (NOD), second-order kinetic reactions for slowly oxidizable NOD, and time to reach ISCO termination criteria. Remediation cost is computed by coupling the performance model with a cost module. ISCO termination protocols are implemented that allow different treatment subregions (e.g., zones with different estimated contaminant concentrations) to be terminated independently based on statistical criteria related to confidence limits of contaminant concentrations estimated from soil and/or groundwater sampling data. The ISCO model was implemented in the program called Stochastic Cost Optimization Toolkit, which includes modules for additional remediation technologies that can be implemented serially or in parallel coupled with a dissolved plume model to enable design optimization to meet plume-scale cleanup objectives. This study focuses on optimization of ISCO design to meet specified source zone remediation objectives. ISCO design parameters considered for optimization include oxidant concentration and injection rate, frequency and number of soil or groundwater samples, and cleanup criteria for termination of subregion injection. Sensitivity studies and example applications are presented to demonstrate the benefits of proposed stochastic optimization methodology.

Surfactant-Enhanced Permanganate Oxidation on Mass-Flux Reduction and Mass Removal (MFR-MR) Relationship for Pool-Dominated TCE Source Zones in Heterogeneous Porous Media

Chlorinated solvents generally enter the subsurface as dense nonaqueous phase liquids (DNAPL) and accumulation mostly occurs in aquifers as pool-dominated zones that can cause long-term aqueous phase groundwater contamination. In situ remediation of DNAPL source zones in such systems is crucial for protecting and/or restoring groundwater quality in these aquifer systems having significant groundwater potential. The objective of the project was to investigate the surfactant-enhanced permanganate oxidation efficacy for pool-dominated DNAPL source zones in heterogeneous aquifer media. A complementary objective of this study was to investigate the impact of nonuniform distribution of DNAPL source zones, surfactant-enhanced dissolution and surfactant-enhanced permanganate oxidation conditions on mass-flux reduction/mass-removal behavior relationships. A series of 2-D flow-cell tank experiments using various grain sizes silica sand and natural soil were conducted as part of this study. DNAPL trichloroethene (TCE) was used as a chlorinated solvent, and SDS (sodium dodecyl sulfate) and KMnO4 (potassium permanganate) were used as anionic surfactant and oxidant (remediation agents), respectively. The results were compared with a water-flooding experiment to test the remediation effort. Although, high fractions of TCE source zones in the heterogeneous porous media were removed by sodium dodecyl sulfate (SDS)-enhanced flushing, TCE removal in this system exhibited an extended multi-step concentration elution behavior. This nonideal behavior was observed for both the water-flood and SDS-flushing experiments. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher saturation zones. Our results also showed that the distribution and the emplacement of organic liquid and flow-field heterogeneity had a significant influence on remediation and removal for both flushing solution (SDS and water). It was postulated that when SDS/MnO4 was applied with sufficient dosage and provided enough contact time, pool-dominated source zones could be remediated more efficiently compared to surfactant flushing alone. As a result, the performance of technology depends on the site characteristics which are critical to characterize effective DNAPL remediation strategies for contaminated sites.

A numerical model to simulate the NAPL source zone remediation by injecting zero-valent iron nanoparticles

A continuum-scale numerical model is developed to simulate the flow, mass-transfer, and reactive processes taking place during the injection of zero-valent iron nanoparticles (nZVI) in a water-saturated porous medium, containing a chlorinated non-aqueous phase liquid (NAPL) at its residual saturation (source zone). Firstly, the transport of the nanoparticles (NP) is modelled by coupling the nanoparticles transport with their deposition in a porous medium. A systematic sensitivity analysis is done to examine the individual effect of each parameter on the spatial and temporal evolution of nanoparticle concentration along the column length. The parameters quantifying the kinetics of attachment/detachment between the nanoparticles and the porous medium (sand grains) are estimated via inverse modeling. Secondly, the kinetics of the reaction of nZVI with dissolved tetrachloroethylene (PCE) is based on the numerical predictions of the statistical shrinking-core model that couples the mass-transfer with the reactive processes taking place at a nanoparticle scale. The continuum NP transport model is then extended to include the reactive flows by combining the dynamics of PCE ganglia dissolution and nZVI reactions with mass balances for the residual PCE saturation, the dissolved PCE concentration, and the nZVI concentration in the aqueous phase. Sensitivity analyses are performed on the NP transport-NAPL remediation model with a view to clarify the effects of dimensionless parameters (dimensionless flow velocity, Damköhler numbers) on the fate of nZVI and trapped PCE ganglia. Finally, the residual PCE remediation efficiency is numerically predicted as a function of the injected NP mass. This efficiency is compared with the respective experimental results of a PCE source zone remediation test performed in a sand column, by using as adjustable parameters the kinetic coefficients of the nanoparticles deposition on sand grains. The remediation efficiency of the PCE source zone is maximized when the injected nZVI maintains its reactivity throughout its flow in the porous medium. To design an efficient NAPL source zone remediation strategy, the parameters must be set to values which will ensure that the nZVI is kept to a higher than zero concentration throughout the length of the porous medium.

Dechlorination of Excess Trichloroethene by Bimetallic and Sulfidated Nanoscale Zero-Valent Iron.

Nanoscale zerovalent iron (nZVI) likely finds its application in source zone remediation. Two approaches to modify nZVI have been reported: bimetal (Fe-Me) and sulfidated nZVI (S-nZVI). However, previous research has primarily focused on enhancing particle reactivity with these two modifications under more plume-like conditions. In this study, we systematically compared the trichloroethene (TCE) dechlorination pathway, rate, and electron selectivity of Fe-Me (Me: Pd, Ni, Cu, and Ag), S-nZVI, and nZVI with excess TCE simulating source zone conditions. TCE dechlorination on Fe-Me was primarily via hydrogenolysis while that on S-nZVI and nZVI was mainly via β-elimination. The surface-area normalized TCE reduction rate ( k'SA) of Fe-Pd, S-nZVI, Fe-Ni, Fe-Cu, and Fe-Ag were ∼6800-, 190-, 130-, 20-, and 8-fold greater than nZVI. All bimetallic modification enhanced the competing hydrogen evolution reaction (HER) while sulfidation inhibited HER. Fe-Cu and Fe-Ag negligibly enhanced electron utilization efficiency (εe) while Fe-Pd, Fe-Ni, and S-nZVI dramatically increased εe from 2% to ∼100%, 69%, and 72%, respectively. Adsorbed atomic hydrogen was identified to be responsible for the TCE dechlorination on Fe-Me but not on S-nZVI. The enhanced dechlorination rate along with the reduced HER of S-nZVI can be explained by that FeS conducting major electrons mediated TCE dechlorination while Fe oxides conducting minor electrons mediated HER.

Rotary drum soil blending for source zone remediation: Various application scenarios

AbstractMechanical blending of contaminated soil with amendments has recently reemerged as an important treatment technology. From its original application using large‐diameter augers in the early 1990s to the current use of rotary drum blenders, soil blending is being used as an alternative to other remediation technologies like amendment injection and soil vapor and groundwater extraction. Shallow (approximately 10 m below ground surface [bgs] or less) soil blending also offers an alternative to excavation and disposal. Soil blending has been used to remediate a site with various contaminants including, but not limited to, chlorinated solvents, petroleum, and metals. The types of soils susceptible to soil blending vary from sands and gravels to silts and clays to fractured rock and combinations of all of these. The types of amendments blended include oxidants, reducing agents, biological enhancements, and stabilizing amendments. Soil blending systems deliver the power to the mixing head to adequately mix the soil and amendment to enhance remediation effectiveness. Since long‐term contamination is often a result of heterogeneously distributed residual contaminant in localized source zones that are difficult to access, the typical aim of soil blending is to homogenize the soil while effectively distributing amendment to these zones made accessible by blending. By effectively homogenizing the soil, however, soil blending will increase the void ratio and disrupt the shear strength and bearing capacity of the soil so an important component of a soil blending technology is proper recovery of these geotechnical parameters. This can be achieved by using well‐known soil improvement techniques such as amending all or a portion of the blended area with Portland cement or lime. Several case studies of soil blending treatments of different contaminants and amendments in various soil types are provided.

Unintentional contaminant transfer from groundwater to the vadose zone during source zone remediation of volatile organic compounds

Historical heavy use of chlorinated solvents in conjunction with improper disposal practices and accidental releases has resulted in widespread contamination of soils and groundwater in North America and worldwide. As a result, remediation of chlorinated solvents is required at many sites. For source zone treatment, common remediation strategies include in-situ chemical oxidation (ISCO) using potassium or sodium permanganate, and the enhancement of biodegradation by primary substrate addition. It is well known that these remediation methods tend to generate gas (carbon dioxide (CO2) in the case of ISCO using permanganate, CO2 and methane (CH4) in the case of bioremediation). Vigorous gas generation in the presence of chlorinated solvents, which are categorized as volatile organic contaminants (VOCs), may cause gas exsolution, ebullition and stripping of the contaminants from the treatment zone. This process may lead to unintentional ‘compartment transfer’, whereby VOCs are transported away from the contaminated zone into overlying clean sediments and into the vadose zone. To this extent, benchtop column experiments were conducted to quantify the effect of gas generation during remediation of the common chlorinated solvent trichloroethylene (TCE/C2Cl3H). Both ISCO and enhanced bioremediation were considered as treatment methods. Results show that gas exsolution and ebullition occurs for both remediation technologies. Facilitated by ebullition, TCE was transported from the source zone into overlying clean groundwater and was subsequently released into the column headspace. For the case of enhanced bioremediation, the intermediate degradation product vinyl chloride (VC) was also stripped from the treatment zone. The concentrations measured in the headspace of the columns (TCE ∼300ppm in the ISCO column, TCE ∼500ppm and VC ∼1380ppm in the bioremediation column) indicate that substantial transfer of VOCs to the vadose zone is possible. These findings provide direct evidence for the unintended spreading of contaminants as a result of remediation efforts, which can, under some circumstances, result in enhanced risks for soil vapour intrusion.

2D sandbox experiments of surfactant foams for mobility control and enhanced LNAPL recovery in layered soils

The ability of surfactant foam to enhance mobility control and LNAPL recovery in a heterogeneous porous media was investigated through sandbox experiments with p-xylene used as LNAPL. A layered heterogeneous porous media was represented in a 2D sandbox filled with two layers of coarse and medium silica sand. Based on previous tests, the surfactant solution used was Ammonyx Lo at a concentration of 0.1% (w/w). The sandbox experimental program included tracer tests done under both uncontaminated and contaminated conditions, foam injection under uncontaminated conditions and surfactant solution and foam injection under contaminated conditions. Tracer tests indicated that the permeability contrast between sand layers was increased by LNAPL contamination. Foam injection under uncontaminated conditions presented a S-shaped front that indicated a better mobility control than the piston-shaped front obtained during tracer tests. During foam injection, complete sweep of the sandbox was achieved with 1.8pore volume (PV) compared to 2.8PV during tracer injection, thus indicating better mobility control with foam. Pre-flush of the contaminated sandbox with surfactant solution initiated p-xylene mobilization but no free phase was recovered at the effluent. A negligible p-xylene residual saturation was reached following foam injection in the contaminated sandbox. However, mass balance indicated a total recovery of only 48% of the initial p-xylene, thus indicating an underestimation of the recovery by volatilization. Recovery by volatilization was corrected, which gave the following proportions of LNAPL recovery mechanisms: 19% by mobilization, 16% by dissolution and 65% by volatilization. Results show the potential efficiency of foam remediation of LNAPL source zones in heterogeneous porous media. Still, further developments are needed prior to field scale application, which could benefit from in situ foam production that would simplify on-site operations.

Smoldering Combustion (STAR) for the Treatment of Contaminated Soils: Examining Limitations and Defining Success

Smoldering combustion, commercially available as the Self-sustaining Treatment for Active Remediation (STAR) technology, is an innovative technique that has shown promise for the remediation of contaminant source zones. Smoldering combustion is an exothermic reaction (net energy producing) converting carbon compounds and an oxidant (e.g., oxygen in air) to carbon dioxide, water, and energy. Thus, following ignition, the smoldering combustion reaction can continue in a self-sustaining manner (i.e., no external energy or added fuel input following ignition) as the heat generated by the reacting contaminants is used to preheat and initiate combustion of contaminants in adjacent areas, propagating a combustion front through the contaminated zone provided a sufficient flux of air is supplied. The STAR technology has applicability across a wide-range of hydrocarbons in a variety of hydrogeologic settings; however, there are limitations to its use. Impacted soils must be permeable enough to allow a sufficient flux of air to the combustion front and there exists a minimum required concentration of contaminants such that the soils contain sufficient fuel for the reaction to proceed in a self-sustaining manner. Further limitations, as well as lessons learned and methods to mitigate these limitations, are presented through a series of case studies. In summary, the successful implementation of STAR will result in >99 percent reduction in contaminant concentrations in treated areas, limited residual contaminant mass, reduced groundwater contaminant mass flux which can be addressed through monitored natural attenuation; and an enhanced site exit strategy, reduced lifecycle costs, and reduced risk. ©2016 Wiley Periodicals, Inc.

First Pilot Test on the Integration of GCW (Groundwater Circulation Well) with ENA (Enhanced Natural Attenuation) for Chlorinated Solvents Source Remediation.

The remediation of aged source zone affected by residual chlorinated aliphatic hydrocarbons (CAHs) represents one of the main challenges in contaminated aquifers. Groundwater Circulation Wells (GCWs) could be considered a strategy for the progressive source zone remediation; this in situ remediation technology is designed to create an in situ vertical groundwater circulation cells by drawing groundwater from an aquifer through one screened section of a multi-screened well and discharging it through another screened section. The pressure gradient between the two hydraulically separated screen sections in the well induces a circulation flow in the aquifer forcing water through less permeable layer where usually CAHs residual source are located. The groundwater moves through the treatment zone both horizontally and vertically and as a consequence the low permeable layer is constantly penetrated by the vertical flow of the GCWs. We tested the possibility to use GCW to enhance in-situ bioremediation (ISB) in an operative industrial site heavily contaminated by different chlorinated solvents (at concentration up to 100 mg/L) in a complex hydrogeological saturated zone. A 30 mts deep GCW, with three screen sections, was designed and installed at the site for a pilot testing. Groundwater is pumped towards two screen sections of the GCW and is reinjected into the aquifer by another screen section after passing through an external unit treatment. External treatment unit is composed of a sand filter tank and two reactors: one reactor was filled with a biodegradable polymer (poly- hydroxy-butyrrate, PHB) and the other one with a mixture of zero-valent iron (ZVI) and PHB. Results from the first eight months of operation clearly demonstrated how groundwater recirculation through the PHB reactor allowed delivering continuously electron donors in the contaminated aquifer enhancing the mobilization of CAHs and stimulating Natural Attenuation biological processes.

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 (<4days) in the columns. Subsequently, modeling was conducted to scale up column results. By scaling up to field-relevant system sizes (>10m) 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.

The influence of humic acid and clay content on the transport of polymer-coated iron nanoparticles through sand

The introduction of nanoscale zero valent iron (nZVI) into the subsurface has recently received significant attention as a potentially effective method for remediation of source zones of chlorinated solvents present as dense nonaqueous phase liquids (DNAPL). One of the challenges in the deployment of nZVI is to achieve good subsurface nZVI mobility to permit delivery of the nZVI to the target treatment zone. Stabilization of nZVI with various polymers has shown promise for enhancing nZVI subsurface mobility, but the impact of subsurface conditions on nZVI mobility has not been fully explored. In this study, the effect of humic acid and kaolinite on the transport of polymer-stabilized nZVI (carboxylmethyl cellulose-surface modified nZVI, CMC90K-RNIP) in sand was investigated using column experiments. In addition, effects of electrolytes on the stability of CMC90K-RNIP in the presence of humic acid, and the stability of humic acid-coated reactive nanoscale iron particles (HA-RNIP) at various humic acid concentrations were investigated. Humic acid enhanced the mobility of bare RNIP, whereas the transport of CMC90K-RNIP was not significantly affected by humic acid injected as a background solution, except at the highest concentration of 500mg/L. At lower pore water velocity, the effect of humic acid on the transport of CMC90K-RNIP was greater than that at high water velocity. Adding kaolinite up to 2% by weight to the sand column reduced the retention of CMC90K-RNIP, but further increases in kaolinite content (to 5%) did not significantly affect nZVI retention. The impact of kaolinite on nZVI retention was more pronounced at lower pore water velocities.

Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation

Practical, non-invasive tools do not currently exist for mapping the remediation of dense non-aqueous phase liquids (DNAPLs). Electrical resistivity tomography (ERT) exhibits significant potential but has not yet become a practitioner's tool due to challenges in interpreting the survey results at real sites. This study explores the effectiveness of recently developed four-dimensional (4D, i.e., 3D space plus time) time-lapse surface ERT to monitor DNAPL source zone remediation. A laboratory experiment demonstrated the approach for mapping a changing NAPL distribution over time. A recently developed DNAPL-ERT numerical model was then employed to independently simulate the experiment, providing confidence that the DNAPL-ERT model is a reliable tool for simulating real systems. The numerical model was then used to evaluate the potential for this approach at the field scale. Four DNAPL source zones, exhibiting a range of complexity, were initially simulated, followed by modeled time-lapse ERT monitoring of complete DNAPL remediation by enhanced dissolution. 4D ERT inversion provided estimates of the regions of the source zone experiencing mass reduction with time. Results show that 4D time-lapse ERT has significant potential to map both the outline and the center of mass of the evolving treated portion of the source zone to within a few meters in each direction. In addition, the technique can provide a reasonable, albeit conservative, estimate of the DNAPL volume remediated with time: 25% underestimation in the upper 2m and up to 50% underestimation at late time between 2 and 4m depth. The technique is less reliable for identifying cleanup of DNAPL stringers outside the main DNAPL body. Overall, this study demonstrates that 4D time-lapse ERT has potential for mapping where and how quickly DNAPL mass changes in real time during site remediation.

Nanoscale zero-valent iron flakes for groundwater treatment

Even today the remediation of organic contaminant source zones poses significant technical and economic challenges. Nanoscale zero-valent iron (NZVI) injections have proved to be a promising approach especially for source zone treatment. We present the development and the characterization of a new kind of NZVI with several advantages on the basis of laboratory experiments, model simulations and a field test. The developed NZVI particles are manufactured by milling, consist of 85 % Fe(0) and exhibit a flake-like shape with a thickness of <100 nm. The mass normalized perchloroethylene (PCE) dechlorination rate constant was 4.1 × 10−3 L/g h compared to 4.0 × 10−4 L/g h for a commercially available reference product. A transport distance of at least 190 cm in quartz sand with a grain size of 0.2–0.8 mm and Fe(0) concentrations between 6 and 160 g/kg (sand) were achieved without significant indications of clogging. The particles showed only a low acute toxicity and had no longterm inhibitory effects on dechlorinating microorganisms. During a field test 280 kg of the iron flakes was injected to a depth of 10–12 m into quaternary sand layers with hydraulic conductivities ranging between 10−4 and 10−5 m/s. Fe(0) concentrations of 1 g/kg (sand) or more [up to 100 g/kg (sand)] were achieved in 80 % of the targeted area. The iron flakes have so far remained reactive for more than 1 year and caused a PCE concentration decrease from 20.000–30.000 to 100–200 µg/L. Integration of particle transport processes into the OpenGeoSys model code proved suitable for site-specific 3D prediction and optimization of iron flake injections.

Risk-Based Approaches for the Remediation of a UST Site: A Case Study

Risk-based corrective action (RBCA) is a scientifically-accepted approach to remediate polluted sites. Under the RBCA approach, the risks to human health associated with polluted sites are assessed and appropriate remedial measures are taken to reduce risks. RBCA evaluations involve the use of risk models to assess health risks to different receptors. In Taiwan, the Soil and Groundwater Remediation Act was enforced in 2000. The government of Taiwan also use risk assessment protocols to obtain acceptable remediation goals for polluted sites with low risks. In this study, the application of risk assessment to derive remediation goals and develop remedial strategies at a fuel-oil spill site were performed. After the risk evaluation, the soil and groundwater remediation goals for total petroleum hydrocarbon (TPH), benzene, toluene, ethylbenzene, and xylenes (BTEX) were determined. The remediation levels meet with the requirements for minimum target risk levels (cancer risk = 1×10-6 and hazard quotient = 1). The developed risk-based cleanup goals are calculated based on actual land use and exposure pathways rather than on assumed maximum exposure. After the completion of RBCA process, the remediation cost can be significantly reduced. Based on the risk assessment results, source zone remediation, natural attenuation, and long term monitoring were recommended as the remediation strategies of the studied site to reduce risks to human health. Results from this study provide a streamlined process for future risk assessment work at petroleum-hydrocarbon polluted sites in Taiwan. RBCA is a sound and defensible basis for site closure, and it offers a more logical framework for making site closure decisions.