Interior versus exterior configurations of passive wells with filter cartridges for cleaning contaminated groundwater

This study considered alternative configurations of passive wells equipped with filter cartridges for removing contaminated groundwater. The wells fully penetrated a simulated unconfined aquifer. Both homogeneous and heterogeneous hydraulic conductivity distributions were considered. An initial configuration comprised wells along the downgradient perimeter of a contaminant plume, spaced 0.5 m in the direction transverse to regional groundwater flow. Additional wells near the downgradient tip of the plume prevented off‐site contamination. Alternative configurations had the same number of wells, but some included wells along higher (interior) concentration contours to facilitate quicker removal of the contaminant plume. Results suggest that downgradient configurations generally outperform alternatives, although repositioning a few outer wells near the contaminant source may be effective in some cases. © 2009 Wiley Periodicals, Inc.     

Low‐Capacity Well Pairs for Treating Contaminated Groundwater in Homogeneous and Heterogeneous Aquifers

Groundwater remediation alternatives were simulated for homogeneous and heterogeneous aquifers with a numerical mass transport model. Low‐energy alternatives involved an injection–extraction well pair positioned along a downgradient linear transect. This transect was located 5 m from the contaminant plume and oriented perpendicular to the regional hydraulic gradient. Through numerous trials, for one homogeneous and three heterogeneous settings, the model identified an optimal spacing and minimum pumping rate for a well pair: (1) centered on the downgradient tip of the plume (best centered), and (2) anywhere along the downgradient transect (best overall). Results suggest that low‐energy well pairs are an effective means for containing and removing some contaminant plumes, and best‐performing configurations are generally not centered on the downgradient tip of the initial contaminant plume. ©2015 Wiley Periodicals, Inc.     

Bioremediation of a former dry cleaner using potassium lactate

Chlorinated solvents were released to the surficial groundwater underneath a former dry cleaning building, resulting in a groundwater plume consisting of high concentrations of trichloroethene (TCE) and cis‐1,2‐dichloroethene (cis‐1,2‐DCE) and low concentrations of tetrachloroethene (PCE) and vinyl chloride. The initial remedial action included chemical oxidation via injection of 14,400 gallons of Fenton's Reagent in March 2002, and an additional 14,760 gallons in April 2002. A sharp reduction of contaminant concentrations in groundwater was observed the following month; however, rebound of contaminant concentrations was evident as early as October 2002. A source area of PCE‐impacted soils was excavated in June 2004. Following the excavation, Golder Associates Inc. (2007) implemented a biostimulation plan by injecting 55 gallons of potassium lactate (PURASAL® HiPure P) in September 2005, and again in February 2006. Comparing the preinjection and postinjection site conditions, the potassium lactate treatments were successful in accomplishing a 40 to 70 percent reduction in mass within four months following the second injection. Elevated vinyl chloride concentrations have persisted through both injection events; however, significant vinyl chloride reduction has been observed in one well with the highest total organic carbon (TOC) concentrations following each injection. © 2008 Wiley Periodicals, Inc.     

Field‐scale evaluation of a biobarrier for the treatment of a trichloroethene plume

In the 1960s, trichloroethene (TCE) was used at what is now designated as Installation Restoration Program Site 32 Cluster at Vandenberg Air Force Base to flush missile engines prior to launch and perhaps for other degreasing activities, resulting in releases of TCE to groundwater. The TCE plume extends approximately 1 kilometer from the previous launch facilities beyond the southwestern end of the site. To limit further migration of TCE and chlorinated degradation by‐products, an in situ, permeable, reactive bioremediation barrier (biobarrier) was designed as a cost‐effective treatment technology to address the TCE plume emanating from the source area. The biobarrier treatment would involve injecting carbon‐based substrate and microbes to achieve reductive dechlorination of volatile organic compounds, such as TCE. Under reducing conditions and in the presence of certain dechlorinating microorganisms, TCE degrades to nontoxic ethene in groundwater. To support the design of the full‐scale biobarrier, a pilot test was conducted to evaluate site conditions and collect pertinent design data. The pilot test results indicated possible substrate delivery difficulties and a smaller radius of influence than had been estimated, which would be used to determine the final biobarrier well spacing. Based on these results, the full‐scale biobarrier design was modified. In January 2010, the biobarrier was implemented at the toe of the source area by adding a fermentable substrate and a dechlorinating microbial culture to the subsurface via an injection well array that spanned the width of the TCE plume. After the injections, the groundwater pH in the injection wells continued to decrease to a level that could be detrimental to the population of Dehalococcoides in the SDC‐9^TM culture. In addition, 7 months postinjection, the injection wells could not be sampled due to fouling. Cleaning was required to restore their functions. Bioassay and polymerase chain reaction analyses were conducted, as well as titration tests, to assess the need for biobarrier amendments in response to the fouling issues and low pH. Additionally, slug tests were performed on three wells to evaluate possible localized differences in hydraulic conductivity within the biobarrier. Based on the test results, the biobarrier was amended with sodium carbonate and inoculated a second time with SDC‐9^TM. The aquifer pH was restored, and reductive dechlorination resumed in the treatment zone, evidenced by the reduction in TCE and the increase in degradation products, including ethene. © 2011 Wiley Periodicals, Inc.     

Re‐evaluation of treatment approach for a site contaminated with Freon‐113 and 1,1,1‐trichloroethane

This study demonstrates a remedial approach for completing the remediation of an aquifer contaminated with 1,1,2‐trichlorotrifluoroethane (Freon‐113) and 1,1,1‐trichloroethane (TCA). In 1987, approximately 13,000 pounds of Freon‐113 were spilled from a tank at an industrial facility located in the state of New York. The groundwater remediation program consisted of an extraction system coupled with airstripping followed by natural attenuation of residual contaminants. In the first phase, five recovery wells and an airstripping tower were operational from April 1993 to August 1999. During this time period over 10,000 pounds of CFC‐13 and 200 pounds of TCA were removed from the groundwater and the contaminant concentrations decreased by several orders of magnitude. However, the efficiency of the remediation system to recover residual Freon and/or TCA reduced significantly. This was evidenced by: (1) low levels (< 10 ppb) of Freon and TCA captured in the extraction wells and (2) a slight increase of Freon and/or TCA in off‐site monitoring wells. A detailed study was conducted to evaluate the alternative for the second‐phase remediation. Results of a two‐year groundwater monitoring program indicated the contaminant plume to be stable with no significant increase or decrease in contaminant concentrations. Monitored geochemical parameters suggest that biodegradation does not influence the fate and transport of these contaminants, but other mechanisms of natural attenuation (primarily sorption and dilution) appear to control the fate and transport of these contaminants. The contaminants appear to be bound to the soil matrix (silty and clay units) with limited desorption as indicated by the solid phase analyses of contaminant concentrations. Results of fate and transport modeling indicated that contaminant concentrations would not exceed the action levels in the wells that showed a slight increase in contaminant concentrations and in the downgradient wells (sentinel) during the modeled timeframe of 30 years. This feasibility study for natural attenuation led to the termination of the extraction system and a transaction of the property, resulting in a significant financial benefit for the original site owner. © 2003 Wiley Periodicals, Inc.     

Matrix Diffusion Modeling Applied to Long‐Term Pump‐and‐Treat Data: 1. Method Development

Despite the installation in the 1980s and 1990s of hydraulic containment systems around known source zones (four slurry walls and ten pump‐and‐treat systems), trichloroethene (TCE) plumes persist in the three uppermost groundwater‐bearing units at the Middlefield‐Ellis‐Whisman (MEW) Superfund Study Area in Mountain View, California. In analyzing TCE data from 15 recovery wells, the observed TCE mass discharge decreased less than an order of magnitude over a 10‐year period despite the removal of an average of 11 pore volumes of affected groundwater. Two groundwater models were applied to long‐term groundwater pump‐and‐treat data from 15 recovery wells to determine if matrix diffusion could explain the long‐term persistence of a TCE plume. The first model assumed that TCE concentrations in the plume are controlled only by advection, dispersion, and retardation (ADR model). The second model used a one‐dimensional diffusion equation in contact with two low‐permeability zones (i.e., upper and lower aquitard) to estimate the potential effects of matrix diffusion of TCE into and out of low‐permeability media in the plume. In all 15 wells, the matrix diffusion model fit the data much better than the ADR model (normalized root mean square error of 0.17 vs. 0.29; r² of 0.99 vs. 0.19), indicating that matrix diffusion is a likely contributing factor to the persistence of the TCE plume in the non‐source‐capture zones of the MEW Study Area's groundwater‐extraction wells. © 2013 Wiley Periodicals, Inc.     

Performance assessment of hanging funnel‐and‐gate structures designed by reverse particle tracking for capturing polluted groundwater

The objective of this study was to evaluate the capability of partially penetrating (hanging) funnel‐and‐gate structures, designed using reverse flow trajectories, for capturing plumes of contaminated groundwater. Linear capture structures, comprised of two slurry cutoff walls on either side of a permeable gate, were positioned perpendicular to regional groundwater flow in a hypothetical unconfined aquifer. A four‐step approach was used for each of two simulated settings: (1) a numerical mass transport model generated a contaminant plume originating from a source area; (2) a particle‐tracking model projected groundwater flow paths upstream from a treatment gate; (3) the structure was widened and deepened until bounding path lines contained the plume; and (4) mass transport simulation tested the ability of the structure to capture the plume. Results of this study suggest that designing funnel‐and‐gate structures using reverse particle tracking may result in too small a structure to capture a contaminant plume. This practice generally ignores effects of hydrodynamic dispersion, which may enlarge plumes such that contaminants move beneath or around a capture structure. This bypassing effect may be considerable even for low values of dispersivity. Particle‐tracking approaches may also underestimate the amount of time required to reduce contaminant concentrations to acceptable levels. © 2007 Wiley Periodicals, Inc.     

Chloroethene dechlorination in acidic groundwater: Implications for combining fenton's treatment with natural attenuation

A sulfuric acid leak in 1988 at a chloroethene‐contaminated groundwater site at the Naval Air Station Pensacola has resulted in a long‐term record of the behavior of chloroethene contaminants at low pH and a unique opportunity to assess the potential impact of source area treatment technologies, which involve acidification of the groundwater environment (e.g., Fenton's‐based in situ chemical oxidation), on downgradient natural attenuation processes. The greater than 75 percent decrease in trichloroethene (TCE) concentrations and the shift in contaminant composition toward predominantly reduced daughter products (dichloroethene [DCE] and vinyl chloride [VC]) that were observed along a 30‐m groundwater flow path characterized by highly acidic conditions (pH = 3.5 ± 0.4) demonstrated that chloroethene reductive dechlorination can continue to be efficient under persistent acidic conditions. The detection of Dehalococcoides‐type bacteria within the sulfuric acid/chloroethene co‐contaminant plume was consistent with biotic chloroethene reductive dechlorination. Microcosm studies conducted with ¹⁴C‐TCE and ¹⁴C‐VC confirmed biotic reductive dechlorination in sediment collected from within the sulfuric acid/chloroethene co‐contaminant plume. Microcosms prepared with sediment from two other locations within the acid plume, however, demonstrated only a limited mineralization to ¹⁴CO₂ and ¹⁴CO, which was attributed to abiotic degradation because no significant differences were observed between experimental and autoclaved control treatments. These results indicated that biotic and abiotic mechanisms contributed to chloroethene attenuation in the acid plume at NAS Pensacola and that remediation techniques involving acidification of the groundwater environment (e.g., Fenton's‐based source area treatment) do not necessarily preclude efficient chloroethene degradation. © 2007 Wiley Periodicals, Inc.     

The in situ treatment of BTEX,MTBE, and TBA in saline groundwater

A pilot‐scale test was conducted in a saline aquifer to determine if a petroleum hydrocarbon (PHC) plume containing benzene (B), toluene (T), ethylbenzene (E), xylenes (X), methyl tert‐butyl ether (MTBE), and tert‐butyl alcohol (TBA) could be treated effectively using a sequential treatment approach that employed in situ chemical oxidation (ISCO) and enhanced bioremediation (EBR). Chemical oxidants, such as persulfate, have been shown to be effective in reducing dissolved concentrations of BTEX (B + T + E + X) and additives such as MTBE and TBA in a variety of geochemical environments including saline aquifers. However, the lifespan of the oxidants in saline environments tends to be short‐lived (i.e., hours to days) with their effectiveness being limited by poor delivery, inefficient consumption by nontargeted species, and back‐diffusion processes. Similarly, the addition of electron acceptors has also been shown to be effective at reducing BTEX and associated additives in saline groundwater through EBR, however EBR can be limited by various factors similar to ISCO. To minimize the limitations of both approaches, a pilot test was carried out in a saline unconfined PHC‐impacted aquifer to evaluate the performance of an engineered, combined remedy that employed both approaches in a sequence. The PHC plume had total BTEX, MTBE, and TBA concentrations of up to 4,584; 55,182; and 1,880 μg/L, respectively. The pilot test involved injecting 13,826 L of unactivated persulfate solution (19.4 weight percent (wt.%) sodium persulfate (Na₂S₂O₈) solution into a series of injection wells installed within the PHC plume. Parameters monitored over a 700‐day period included BTEX, MTBE, TBA, sulfate, and sulfate isotope concentrations in the groundwater, and carbon and hydrogen isotopes in benzene and MTBE in the groundwater. The pilot test data indicated that the BTEX, MTBE, and TBA within the PHC plume were treated over time by both chemical oxidation and sulfate reduction. The injection of the unactivated persulfate resulted in short‐term decreases in the concentrations of the BTEX compounds, MTBE, and TBA. The mean total BTEX concentration from the three monitoring wells within the pilot‐test area decreased by up to 91%, whereas MTBE and TBA mean concentrations decreased by up to 39 and 58%, respectively, over the first 50 days postinjection in which detectable concentrations of persulfate remained in groundwater. Concentrations of the BTEX compounds, MTBE, and TBA rebounded at the Day 61 marker, which corresponded to no persulfate being detected in the groundwater. Subsequent monitoring of the groundwater revealed that the concentrations of BTEX continued to decrease with time suggesting that EBR was occurring within the plume. Between Days 51 and 487, BTEX concentrations decreased an additional 84% from the concentration measured on Day 61. Mean concentrations of MTBE showed a reduction during the EBR phase of remediation of 33% while the TBA concentration appeared to decrease initially but then increased as the sulfate concentration decreased as a result of MTBE degradation. Isotope analyses of dissolved sulfate (³⁴S and ¹⁸O), and compound‐specific isotope analysis (CSIA) of benzene and MTBE (¹³C and ²H) supported the conclusions that ISCO and EBR processes were occurring at different stages and locations within the plume over time.     

Enhanced anaerobic bioremediation of a TCE source at the Tarheel Army Missile Plant using EOS

EOS, or emulsified oil substrate, was used to stimulate anaerobic biodegradation of trichloroethene (TCE) and tetrachloroethene (PCE) at a former Army‐owned manufacturing facility located in the Piedmont area of North Carolina. Previous use of chlorinated solvents at the facility resulted in soil and groundwater impacts. Ten years of active remediation utilizing soil vacuum extraction and air sparging (SVE/AS) were largely ineffective in reducing the TCE/PCE plume. In 2002, the Army authorized preparation of an amended Remedial Action Plan (RAP) to evaluate in situ bioremediation methods to remediate TCE in groundwater. The RAP evaluated eight groundwater remediation technologies and recommended EOS as the preferred bioremediation alternative for the site. Eight wells were drilled within the 100 × 100 feet area believed to be the primary source area for the TCE plume. In a first injection phase, dilute EOS emulsion was injected into half of the wells. Distribution of the carbon substrate through the treatment zone was enhanced by pumping the four wells that were not injected and recirculating the extracted water through the injection wells. The process was repeated in a second phase that reversed the injection/extraction well pairs. Overall, 18,480 pounds of EOS were injected and 163,000 gallons of water were recirculated through the source area. Anaerobic groundwater conditions were observed shortly after injection with a corresponding decrease in both PCE and TCE concentrations. Dissolved oxygen, oxidation‐reduction potential, and sulfate concentrations also decreased after injection, while TCE‐degradation products, ferrous iron, and methane concentrations increased. The reduction in TCE allowed the Army to meet the groundwater remediation goals for the site. Approximately 18 months after injection, eight wells were innoculated with a commercially prepared dechlorinating culture (KB‐1) in an attempt to address lingering cis‐1,2‐dichloroethene (cis‐DCE) and vinyl chloride (VC) that continued to be observed in some wells. Dehalococcoides populations increased slightly post‐bioaugmentation. Both cis‐DCE and VC continue to slowly decrease. © 2007 Wiley Periodicals, Inc.     

Remediation of chlorinated ethenes,ethanes, and methanes in groundwater using carbon‐ and iron‐based electron donor

Field‐scale pilot tests were performed to evaluate enhanced reductive dechlorination (ERD) of dissolved chlorinated solvents at a former manufacturing facility located in western North Carolina (the site). Results of the site assessment indicated the presence of two separate chlorinated solvent–contaminated groundwater plumes, located in the northern and southern portions of the site. The key chlorinated solvents found at the site include 1,1,2,2‐tetrachloroethane, trichloroethene, and chloroform. A special form of EHC^® manufactured by Adventus Americas was used as an electron donor at this site. In this case, EHC is a pH‐buffering electron donor containing controlled release carbon and ZV Iron MicroSphere 200, a micronscale zero‐valent iron (ZVI) manufactured by BASF. Approximately 3,000 pounds of EHC were injected in two Geoprobe^® boreholes in the saprolite zone (southern plume), and 3,500 pounds of EHC were injected at two locations in the partially weathered rock (PWR) zone (northern plume) using hydraulic fracturing techniques. Strong reducing conditions were established immediately after the EHC injection in nearby monitoring wells likely due to the reducing effects of ZV Microsphere 200. After approximately 26 months, the key chlorinated VOCs were reduced over 98 percent in one PWR well. Similarly, the key chlorinated solvent concentrations in the saprolite monitoring wells decreased 86 to 99 percent after initial increases in concentrations of the parent chlorinated solvents. The total organic carbon and metabolic acid concentrations indicated that the electron donor lasted over 26 months after injection in the saprolite aquifer. © 2009 Wiley Periodicals, Inc.     

Innovative applications of subgrade biogeochemical reactors: Three case studies

Subgrade biogeochemical reactors (SBGRs) are an in situ remediation technology shown to be effective in treating contaminant source areas and groundwater hot spots, while being sustainable and economical. This technology has been applied for over a decade to treat chlorinated volatile organic compound source areas where groundwater is shallow (e.g., less than approximately 30 feet below ground surface [ft bgs]). However, this article provides three case studies describing innovative SBGR configurations recently developed and tested that are outside of this norm, which enable use of this technology under more challenging site conditions or for treatment of alternative contaminant classes. The first SBGR case study addresses a site with groundwater deeper than 30 ft bgs and limited space for construction, where an SBGR column configuration reduced the maximum trichloroethene (TCE) groundwater concentration from 9,900 micrograms per liter (μg/L) to <1 μg/L (nondetect) within approximately 15 months. The second SBGR is a recirculating trench configuration that is supporting remediation of a 5.7‐acre TCE plume, which has significant surface footprint constraints due to the presence of endangered species habitat. The third SBGR was constructed with a new amendment mixture and reduced groundwater contaminant concentrations in a petroleum hydrocarbon source area by over 97% within approximately 1 year. Additionally, a summary is provided for new SBGR configurations that are planned for treatment of additional classes of contaminants (e.g., hexavalent chromium, 1,4‐dioxane, dissolved explosives constituents, etc.). A discussion is also provided describing research being conducted to further understand and optimize treatment mechanisms within SBGRs, including a recently developed sampling approach called the aquifer matrix probe.     

In situ soil stabilization for source control of a lithium and bromate groundwater plume

Historic mineral ore processing operations at the former Cyprus Foote Mineral Site located in East Whiteland Township, Pennsylvania, have resulted in the creation of an approximately 10,000‐foot‐long off‐site groundwater plume impacted with lithium and bromate. The plume emanating from the site is impacting the groundwater quality of downgradient private residences. As an early part of the remedial implementation, the private residences were provided with public water connections while the source control efforts were being designed and implemented. Bromate and lithium have recently emerged as groundwater contaminants subjected to increased regulatory scrutiny. This is evidenced in a recently lowered Federal Maximum Contaminant Level (MCL) for bromate of 0.010 milligrams per liter and a Medium‐Specific Concentration (MSC) of 0.005 mg/L for lithium recently proposed by the Pennsylvania Department of Environmental Protection (PADEP) for all groundwater within the Commonwealth. Elevated concentrations for bromate and lithium were detected above the Proposed Remediation Goals (PRGs) for the site, MCLs, and MSCs at a distance of 7,300 feet and 9,200 feet from the source area, respectively. To reduce the contaminant concentrations within the groundwater plume, which will ultimately result in a regressing plume, and to enable the Brownfield redevelopment of this Superfund site, auger‐based, in situ soil stabilization (ISS) with depths of up to 75 feet below ground surface (bgs) was selected as the remedy. The remedial implementation required the temporary removal and relocation of over 100,000 cubic yards of overburden to expose the lithium‐bearing tailings prior to treatment. Using customized 90‐foot‐long, 9‐foot‐diameter augers attached to cranes and drilling platforms, ancillary support excavators, and approximately 21,000 tons of reagent; 2,019 ISS columns were advanced to depths ranging from 10 to 74 feet bgs. This resulted in the creation of an in situ low‐permeablity 117,045‐yd³ “quasi‐monolith,” which encompasses a lateral extent of approximately three acres. The integration of a comprehensive ISS design with a comprehensive long‐term groundwater‐monitoring plan ensured the success of the ISS implementation and will enable a continued evaluation of the off‐site groundwater quality. © 2009 Wiley Periodicals, Inc.     

Seven‐year performance evaluation of a permeable reactive barrier

In June and July 2001, the Massachusetts Department of Environmental Protection (MassDEP) installed a permeable reactive barrier (PRB) to treat a groundwater plume of chlorinated solvents migrating from an electronics manufacturer in Needham, Massachusetts, toward the Town of Wellesley's Rosemary Valley wellfield. The primary contaminant of concern at the site is trichloroethene (TCE), which at the time had a maximum average concentration of approximately 300 micrograms per liter directly upgradient of the PRB. The PRB is composed of a mix of granular zero‐valent iron (ZVI) filings and sand with a pure‐iron thickness design along its length between 0.5 and 1.7 feet. The PRB was designed to intercept the entire overburden plume; a previous study had indicated that the contaminant flux in the bedrock was negligible. Groundwater samples have been collected from monitoring wells upgradient and downgradient of the PRB on a quarterly basis since installation of the PRB. Inorganic parameters, such as oxidation/reduction potential, dissolved oxygen, and pH, are also measured to determine stabilization during the sampling process. Review of the analytical data indicates that the PRB is significantly reducing TCE concentrations along its length. However, in two discrete locations, TCE concentrations show little decrease in the downgradient monitoring wells, particularly in the deep overburden. Data available for review include the organic and inorganic analytical data, slug test results from nearby bedrock and overburden wells, and upgradient and downgradient groundwater‐level information. These data aid in refining the conceptual site model for the PRB, evaluating its performance, and provide clues as to the reasons for the PRB's underperformance in certain locations. © 2008 Wiley Periodicals, Inc.     

Pilot‐scale evaluation of in situ cometabolic bioremediation of TCE in groundwater using PHOSter® technology

A study was conducted to evaluate the efficacy of PHOSter® technology for treating groundwater contaminated with trichloroethene (TCE) at Edwards Air Force Base, California. The technology consists of injecting a gaseous mixture of air, methane, and nutrients into groundwater with the objective of stimulating the growth of methanotrophs, a naturally occurring microbial group that is capable of catalyzing the aerobic degradation of chlorinated solvents into nontoxic products. Injection operations were performed at one well for a period of three months. Six monitoring wells were utilized for groundwater and wellhead vapor monitoring and for groundwater and microbial sampling. In the five monitoring wells located within 44 feet of the injection well, the following results were observed: dissolved oxygen concentrations increased to a range between 6 and 8 milligrams per liter (μg/L); the biomass of target microbial groups increased by one to five orders of magnitude; and TCE concentrations decreased by an average of 92 percent, and to below the California primary maximum contaminant level (MCL; 5 micrograms per liter [µg/L]) in the well closest to the injection well. © 2008 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

     

Insufficient source area remediation results in the rebound of TCE breakdown products in groundwater

In the early 1990s, a soil removal action was completed at a former disposal pit site located in southern Michigan. This action removed waste oil, cutting oil, and chlorinated solvents from the unsaturated zone. To contain groundwater contaminant migration at the site, a groundwater pump‐and‐treat system comprised of two extraction wells operating at a combined flow of 50 gallons per minute, carbon treatment, and a permitted effluent discharge was designed, installed, and operated for over 10 years. Groundwater monitoring for natural attenuation parameters and contaminant attenuation modeling demonstrated natural attenuation of the contaminant plume was adequate to attain site closure. As a result of incomplete contaminant source removal, a rebound of contaminants above the levels established in the remedial action plan (RAP) has occurred in the years following system shutdown and site closure. Groundwater concentrations have raised concerns regarding potential indoor air quality at adjacent residential properties constructed in the past 9 to 10 years. The only remedial option available in the original RAP is to resume groundwater pump‐and‐treat. To remediate the source area, an alternate remediation strategy using an ozone sparge system was developed. The ozone sparge remediation strategy addresses the residual saturated zone contaminants beneath the former disposal pit and reestablishes site closure requirements without resumption of the pump‐and‐treat system. A pilot study was completed successfully; and the final system design was subsequently approved by the Michigan Department of Environmental Quality. The system was installed and began operations in July 2010. As of the January 2011 monitoring event, the system has shown dramatic improvement in site contaminant concentrations. The system will continue to operate until monitoring results indicate that complete treatment has been obtained. The site will have achieved the RAP objectives when the system has been shut down and meets groundwater residential criteria for four consecutive quarters. © 2011 Wiley Periodicals, Inc.     

Air Sparging Pilot Testing Including In-Well Temperature Measurement

Air sparging was pilot tested at a site where a groundwater plume containing cis-1,2-dichloroethene (cis-DCE), vinyl chloride (VC) and arsenic resulted from landfill operations. In addition to the commonly used methods for estimating air sparging zone of influence (ZOI), in-well temperature was monitored using sensitive thermocouples and data loggers at several monitoring wells of various screened intervals during the test. Following 42 days of pilot testing, the downgradient monitoring well samples were below maximum contaminant levels (MCLs)for all contaminants of concern, VC and dissolved arsenic were below detection limits (0.5 and 10 milligrams per liter [μg/L], respectively) in all of the downgradient monitoring wells. The ZOI monitoring results indicated that at some locations use of mounding data may overestimate the ZOI when the temperature data suggest that no sparged air was entering the well screen. Therefore, monitoring in-well temperature may provide additional useful information for estimating air sparging ZOI and is more indicative of air pathways than other monitoring methods. In addition, the temperature data were valuable for selecting a pulse frequency and duration to optimize groundwater mixing.     

The use of zero‐valent iron injection to remediate groundwater: Results of a pilot test at the Marshall Space Flight Center

In situ treatability studies are being conducted to evaluate various in situ technologies to manage groundwater contamination at the NASA Marshall Space Flight Center in Huntsville, Alabama. The focus of these studies is to evaluate remediation options for contaminated (mostly aerobic) groundwater occurring within the basal portion of a clayey residuum called the rubble zone. The tension‐saturated media and unsaturated media lying above the rubble zone are also being treated where they make up a significant component of the contaminant mass. An in situ chemical reduction field pilot test was implemented (following bench‐scale tests) during July and August 2000. The test involved the injection of zero‐valent iron powder in slurry form, using the Ferox^SM process patented by ARS Technologies, Inc. The pilot test focused on trichloroethene (TCE)‐contaminated groundwater within the rubble zone. Maximum pre‐injection concentrations of about 72,800 micrograms per liter (μg/l) were observed and no secondary sources are believed to exist beneath the area. The potential presence of unexploded ordnance forced an implementation strategy where source area injections were completed, as feasible, followed by overlapping injections in a down gradient alignment to create a permeable reactive zone for groundwater migration. Eight post‐injection rounds of groundwater performance monitoring were completed. The results are encouraging, in terms of predicted responses and decreasing trends in contaminant levels. © 2003 Wiley Periodicals, Inc.     

Natural attenuation with varying degrees of aquifer heterogeneity: Implications for groundwater monitoring

This study evaluated the effect of heterogeneity in hydraulic conductivity on the tendency for contaminant plumes to attenuate via dilution, hydrodynamic dispersion, and molecular diffusion in simulated aquifers. Simulations included one homogeneous and four increasingly heterogeneous hydraulic conductivity fields. A numerical mass transport model generated an initial contaminant plume for each case; all initial plumes had the same mass. Next, the model simulated plume migrations through the simulated aquifers. Results suggest that highly heterogeneous settings are potentially effective at plume attenuation. Low‐velocity zones in heterogeneous settings delay plume travel, enabling more time for natural processes to lower contaminant concentrations in groundwater. © 2012 Wiley Periodicals, Inc.     

Use of a low‐cost substrate in a continuous recirculation process to stimulate plumewide anaerobic dechlorination of chlorinated solvents

Over the past 20 years, significant time and money have been spent on better understanding and successfully applying bioremediation in the field. The results of these efforts provide a deeper un‐derstanding of aerobic and anaerobic microbial processes, the microbial species and environ‐mental conditions that are desirable for specific degradation pathways, and the limitations that may prevent full‐scale bioremediation from being successfully applied in heterogeneous subsur‐face environments. Numerous substrates have been identified as effective electron donors to stimulate anaerobic dechlorination of chlorinated ethenes, but methods of delivering these sub‐strates for in situ bioremediation (direct‐push injections, slug injections, high‐pressure injections, fracture wells, etc.) have yet to overcome the main limitation of achieving contact between these substrates and the contaminants. Therefore, although it is important (from a full‐scale remedia‐tion standpoint) to select an appropriate, low‐cost substrate that can be supplied in sufficient quantity to promote remediation of a large source area and its associated plume, it is equally im‐portant to ensure that the substrate can be delivered throughout the impacted plume zone. Failure to achieve substrate delivery and contact within the chlorinated solvent plume usually re‐sults in wasted money and limited remediation benefit. Bioremediation is a contact technology that cannot be effectively implemented on a large scale unless a method for rapidly delivering the low‐cost substrate across the entire source and plume areas is utilized. Unfortunately, many cur‐rent substrate delivery methods are not achieving sitewide distribution or treatment of the sorbed contaminant mass that exists in the organic fraction of a soil matrix. The following discussion sum‐marizes substrate delivery using an aggressive groundwater recirculation approach that can achieve plumewide contact between the contaminants and substrate, thus accelerating dechlori‐nation rates and shortening the overall remediation time frame. © 2006 Wiley Periodicals, Inc.