Combined MODFLOW‐FRACTRAN Application to Assess Chlorinated Solvent Transport and Remediation in Fractured Sedimentary Rock

Detailed field investigations and numerical modeling were conducted to evaluate transport and fate of chlorinated solvent contamination in a fractured sedimentary bedrock aquifer (sandstone/siltstone/mudstone) at a Superfund site in central New Jersey. Field investigations provided information on the fractured rock system hydrogeology, including hydraulic gradients, bulk hydraulic conductivity, fracture network, and rock matrix, and on depth discrete contaminant distribution in fractures (via groundwater sampling) and matrix (via detailed subsampling of continuous cores). The numerical modeling endeavor involved application of both an equivalent porous media (EPM) model for flow and a discrete fracture network (DFN) model for transport. This combination of complementary models, informed by appropriate field data, allowed a quantitative representation of the conceptual site model (CSM) to assess relative importance of various processes, and to examine efficacy of remedial alternatives. Modeling progressed in two stages: first a large‐scale (20 km x 25 km domain) 3‐D EPM flow model (MODFLOW) was used to evaluate the bulk groundwater flow system and contaminant transport pathways under historic and current aquifer stress conditions and current stresses. Then, results of the flow model informed a 2‐D DFN transport model (FRACTRAN) to evaluate transport along a 1,000‐m flowpath from the source represented as a 2‐D vertical cross‐section. The combined model results were used to interpret and estimate the current and potential future extent of rock matrix and aqueous‐phase contaminant conditions and evaluate remedial strategies. Results of this study show strong effects of matrix diffusion and other processes on attenuating the plume such that future impacts on downgradient well fields under the hydraulic stresses modeled should be negligible. Results also showed futility of source remediation efforts in the fractured rock, and supported a technical impracticability (TI) waiver for the site. © 2013 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.     

Stratigraphic flux—A method for determining preferential pathways for complex sites

Experience with groundwater remediation over several decades has demonstrated that successful outcomes depend on quantitative conceptual site models (CSMs). Over the last 30 years, we have progressed from groundwater pump‐and‐treat remedies, which were largely designed based on a water supply perspective, to in situ and combined remedy strategies, which are only beginning to benefit from understanding the aquifer architecture and distribution of contaminant mass to assess plume maturity, mass flux, and more reliable means of fate and transport assessment. The U.S. Air Force funded the development of the Stratigraphic Flux approach to provide a framework for understanding contaminant transport pathways at its complex sites and enable more reliable and cost‐effective remediation. Stratigraphic Flux enables the development of quantitative, flux‐based CSMs that are founded in sequence stratigraphy, and high‐resolution hydraulic conductivity and contaminant distribution measurements. The result is a three‐dimensional graphical mapping of relative contaminant flux and classification of transport potential that is easy for all stakeholders to understand. The Stratigraphic Flux graphical model is based on a hydrofacies classification system that describes transport potential in three segments of the aquifer: transport zones—where the majority of groundwater flow occurs and transport rates are measured in feet per day; slow advection zones—where transport rates are measured in feet per year; and storage zones—where typically less than 1% of flow occurs, and diffusion dominates contaminant transport. The hydrofacies architectures are based on stratigraphy and transport potential is defined by grouping facies by orders of magnitude classes in hydraulic conductivity. By combining the hydrofacies architecture with contaminant concentration distributions, one can map relative contaminant flux to define and target the complex pathways that control contaminant transport and cleanup behavior. In this article, we describe the Stratigraphic Flux framework, focusing on the key information needed and the methods of analysis. We illustrate the results of its application to evaluate migration pathways for trichlorethylene and chromium at a former chrome pit at Air Force Plant 4 in Fort Worth, Texas. A comprehensive guidance document that describes the approach with a broad spectrum of tools and several site examples can be requested from the authors.     

A deterministic approach to evaluate and implement monitored natural attenuation for chlorinated solvents

A US EPA directive and related technical protocol outline the information needed to determine if monitored natural attenuation (MNA) for chlorinated solvents is a suitable remedy for a site. For some sites, conditions such as complex hydrology or perturbation of the contaminant plume caused by an existing remediation technology (e.g., pump‐and‐treat) make evaluation of MNA using only field data difficult. In these cases, a deterministic approach using reactive transport modeling can provide a technical basis to estimate how the plume will change and whether it can be expected to stabilize in the future and meet remediation goals. This type of approach was applied at the Petro‐Processors Inc. Brooklawn site near Baton Rouge, Louisiana, to evaluate and implement MNA. This site consists of a multicomponent nonaqueous‐phase source area creating a dissolved groundwater contamination plume in alluvial material near the Mississippi River. The hydraulic gradient of the groundwater varies seasonally with changes in the river stage. Due to the transient nature of the hydraulic gradient and the impact of a hydraulic containment system operated at the site for six years, direct field measurements could not be used to estimate natural attenuation processes. Reactive transport of contaminants were modeled using the RT3D code to estimate whether MNA has the potential to meet the site‐specific remediation goals and the requirements of the US EPA Office of Solid Waste and Emergency Response Directive 9200.4‐17P. Modeling results were incorporated into the long‐term monitoring plan as a basis for evaluating the effectiveness of the MNA remedy. As part of the long‐term monitoring plan, monitoring data will be compared to predictive simulation results to evaluate whether the plume is changing over time as predicted and can be expected to stabilize and meet remediation goals. This deterministic approach was used to support acceptance of MNA as a remedy. © 2007 Wiley Periodicals, Inc.     

Migration of VOC Plume in the Subsurface Domain at the Y‐12 National Security Site

During the production of thermonuclear fusion weapons at the Y‐12 National Security Complex (Y‐12 NSC) in Oak Ridge, Tennessee, between 1950 and 1963, the regional environment was extensively contaminated by volatile organic compounds (VOCs). Old Salvage Yard (OSY) on the western side of the site has been characterized as the major source of VOCs. In order to analyze the long‐term fate and transport of chlorinated VOC sources, an integrated surface and subsurface flow and transport model was developed for the Y‐12 NSC using the hydrodynamic and transport numerical package MIKE‐SHE. The model was developed considering the recent hydrogeological investigations on preferential flow and transport pathways at the site. The model was calibrated using the recorded groundwater flow and water‐quality data. The modeling simulated migration of the VOC plume for the next 100 years. Considering a range of hydrogeological and transport parameters, uncertainty of the results is discussed. The modeling predicted that tetrachloroethene, trichloroethene, and 1,2‐dichloroethene may exceed human health–related risk levels for the next 10 to 20 years. However, the contamination is unlikely to migrate to surface water under the current hydrogeological conditions and will decay below acceptable risk levels within approximately 20 years. © 2013 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.     

Life cycle based risk assessment of recycled materials in roadway construction

This paper uses a life-cycle assessment (LCA) framework to characterize comparative environmental impacts from the use of virgin aggregate and recycled materials in roadway construction. To evaluate site-specific human toxicity potential (HTP) in a more robust manner, metals release data from a demonstration site were combined with an unsaturated contaminant transport model to predict long-term impacts to groundwater. The LCA determined that there were reduced energy and water consumption, air emissions, Pb, Hg and hazardous waste generation and non-cancer HTP when bottom ash was used in lieu of virgin crushed rock. Conversely, using bottom ash instead of virgin crushed rock increased the cancer HTP risk due to potential leachate generation by the bottom ash. At this scale of analysis, the trade-offs are clearly between the cancer HTP (higher for bottom ash) and all of the other impacts listed above (lower for bottom ash). The site-specific analysis predicted that the contaminants (Cd, Cr, Se and Ag for this study) transported from the bottom ash to the groundwater resulted in very low unsaturated zone contaminant concentrations over a 200 year period due to retardation in the vadose zone. The level of contaminants predicted to reach the groundwater after 200 years was significantly less than groundwater maximum contaminant levels (MCL) set by the US Environmental Protection Agency for drinking water. Results of the site-specific contaminant release estimates vary depending on numerous site and material specific factors. However, the combination of the LCA and the site specific analysis can provide an appropriate context for decision making. Trade-offs are inherent in making decisions about recycled versus virgin material use, and regulatory frameworks should recognize and explicitly acknowledge these trade-offs in decision processes.     

In-situ bioremediation of hanford groundwater

The U.S. Department of Energy has generated liquid wastes containing radioactive and hazardous chemicals throughout the more than forty years of operation at its Hanford site in Washington State. Many of the waste components, including nitrate and carbon tetrachloride (CCl₄), have been detected in the Hanford groundwater. In-situ bioremediation of CCl₄ and nitrate is being considered to clean the aquifer. Preliminary estimates indicate that this technology should cost significantly less than ex-situ bioremediation and about the same as air stripping/granular activated carbon. In-situ bioremediation has the advantage of providing ultimate destruction of the contaminant and requires significantly less remediation time. Currently, a test site is under development. A computer-aided design tool is being used to design optimal remediation conditions by linking subsurface transport predictions, site characterization data, and microbial growth and contaminant destruction kinetics.     

Effective contaminant detection networks in uncertain groundwater flow fields

A mass transport simulation model tested seven contaminant detection-monitoring networks under a 40 degrees range of groundwater flow directions. Each monitoring network contained five wells located 40 m from a rectangular landfill. The 40-m distance (lag) was measured in different directions, depending upon the strategy used to design a particular monitoring network. Lagging the wells parallel to the central flow path was more effective than alternative design strategies. Other strategies allowed higher percentages of leaks to migrate between monitoring wells. Results of this study suggest that centrally lagged groundwater monitoring networks perform most effectively in uncertain groundwater-flow fields.     

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.     

Operating windows to assess whether monitored natural attenuation is a technically feasible remediation option for BTEX‐contaminated groundwater

When used in combination with source management strategies, monitored natural attenuation (MNA) is likely to be a technically feasible remediation option if the contaminant persistence time along the flow path is less than (a) the transport time to the compliance point and (b) the time available for groundwater remediation objectives to be achieved. Biodegradation is often the most significant natural attenuation process for benzene, toluene, ethylbenzene, and xylenes (BTEX) in groundwater. While BTEX transport rates increase with groundwater velocity, examination of data obtained from the published literature for seven sites undergoing MNA revealed significant positive correlations between groundwater velocity and first‐order biodegradation rates for toluene (r = 0.83, P < 0.05), ethylbenzene (r = 0.93, P < 0.01), m‐ and p‐xylene (r = 0.96, P < 0.01), and o‐xylene (r = 0.78, P < 0.05). This is attributed to increased dispersion at higher velocities leading to more mixing of electron acceptors with the contaminant plume. There was no positive correlation between groundwater velocity and first‐order biodegradation rates for benzene due to noise in the relationship caused by variations in (a) the concentrations of electron acceptors in the uncontaminated groundwater and (b) the proportions of benzene in the total BTEX concentration in the source area. A regression model of the relationship between groundwater velocity and the first‐order biodegradation rate can be used to delineate operating windows for groundwater velocity within which the contaminant persistence time is less than the transport and remediation times for a given source concentration, target concentration, distance to compliance point, retardation factor, and remediation time. The operating windows can provide decision makers with a rapid indication of whether MNA is likely to be a technically feasible remediation option at a given site. © 2005 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.     

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.     

Model-based prediction of long-term leaching of contaminants from secondary materials in road constructions and noise protection dams

In this study, contaminant leaching from three different secondary materials (demolition waste, municipal solid waste incineration ash, and blast furnace slag) to groundwater is assessed by numerical modeling. Reactive transport simulations for a noise protection dam and a road dam (a typical German autobahn), in which secondary materials are reused as base layers, were performed to predict the breakthrough of a conservative tracer (i.e., a salt) and sorbing contaminants (e.g., PAHs like naphthalene and phenanthrene or heavy metals) at the groundwater table. The dam constructions have a composite architecture with soil covers in inclined layers and distinct contrasts in the unsaturated hydraulic properties of the used materials. Capillary barrier effects result in strong spatial variabilities of flow and transport velocities. Contaminant breakthrough curves at the groundwater table show significant tailing due to slow sorption kinetics and a wide distribution of travel times. While conservative tracer breakthrough depends primarily on subsoil hydraulic properties, equilibrium distribution coefficients and sorption kinetics represent additional controlling factors for contaminant spreading. Hence, the three secondary materials show pronounced differences in the temporal development of leached contaminant concentrations with consequences for breakthrough times and peak concentrations at the groundwater table. Significant concentration reductions due to dispersion occur only if the source concentrations decrease significantly prior to the arrival of the contaminant at the groundwater table. Biodegradation causes significant reduction of breakthrough concentrations only if flow velocities are low.     

Application of an Adapted Version of MT3DMS for Modeling Back‐Diffusion Remediation Timeframes

Simulation of back‐diffusion remediation timeframe for thin silt/clay layers, or when contaminant degradation is occurring, typically requires the use of a numerical model. Given the centimeter‐scale vertical grid spacing required to represent diffusion‐dominated transport, simulation of back‐diffusion in a 3‐D model may be computationally prohibitive. Use of a local 1‐D model domain approach for simulating back‐diffusion is demonstrated to have advantages but is limited to only some applications. Incorporation of a local domain approach for simulating back‐diffusion in a new model, In Situ Remediation‐MT3DMS (ISR‐MT3DMS) is validated based on a benchmark with MT3DMS and comparisons with a highly discretized finite difference numerical model. The approach used to estimate the vertical hydrodynamic dispersion coefficient is shown to have a significant influence on the simulated flux into and out of silt/clay layers in early time periods. Previously documented back‐diffusion at a Florida site is modeled for the purpose of evaluating the sensitivity of the back‐diffusion controlled remediation timeframe to various site characteristics. A base case simulation with a clay lens having a thickness of 0.2 m and a length of 100 m indicates that even after 99.96 percent aqueous TCE removal from the clay lens, the down‐gradient concentrations still exceed the MCL in groundwater monitoring wells. This shows that partial mass reduction from a NAPL source zone via in situ treatment may have little benefit for the long‐term management of contaminated sites, given that back‐diffusion will sustain a groundwater plume for a long period of time. Back‐diffusion model input parameters that have the greatest influence on remediation timeframe and thus may warrant more attention during field investigations, include the thickness of silt/clay lenses, retardation coefficient representing sorbed mass in silt/clay, and the groundwater velocity in adjacent higher permeability zones. Therefore, pump‐and‐treat systems implemented for the purpose of providing containment may have an additional benefit of reducing back‐diffusion remediation timeframe due to enhanced transverse advective fluxes at the sand/clay interface. Remediation timeframes are also moderately sensitive to the length of the silt/clay layers and transverse vertical dispersivity, but are less sensitive to degradation rates within silt/clay, contaminant solubility, contact time, tortuosity coefficient, and monitoring well‐screen length for the scenarios examined. ©2015 Wiley Periodicals, Inc.     

Application of a contaminant mass balance method at an old landfill to assess the impact on water resources

Old and unlined landfill sites pose a risk to groundwater and surface water resources. While landfill leachate plumes in sandy aquifers have been studied, landfills in clay till settings and their impact on receiving water bodies are not well understood. In addition, methods for quantitatively linking soil and groundwater contamination to surface water pollution are required. This paper presents a method which provides an estimate of the contaminant mass discharge, using a combination of a historical investigation and contaminant mass balance approach. The method works at the screening level and could be part of a risk assessment. The study site was Risby Landfill, an old unlined landfill located in a clay till setting on central Zealand, Denmark. The contaminant mass discharge was determined for three common leachate indicators: chloride, dissolved organic carbon and ammonium. For instance, the mass discharge of chloride from the landfill was 9.4 ton/year and the mass discharge of chloride to the deep limestone aquifer was 1.4 ton/year. This resulted in elevated concentrations of leachate indicators (chloride, dissolved organic carbon and ammonium) in the groundwater. The mass discharge of chloride to the small Risby Stream down gradient of the landfill was approximately 31 kg/year. The contaminant mass balance method worked well for chloride and dissolved organic carbon, but the uncertainties were elevated for ammonium due to substantial spatial variability in the source composition and attenuation processes in the underlying clay till.     

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.     

Detecting and quantifying reductive dechlorination under monitored natural attenuation conditions

Field sampling and testing were used to investigate the relationship between baseline geochemical and microbial community data and in situ reductive dechlorination rates at a site contaminated with trichloroethene (TCE) and carbon tetrachloride (CTET). Ten monitoring wells were selected to represent conditions along groundwater flow paths from the contaminant source zone to a wetlands groundwater discharge zone. Groundwater samples were analyzed for a suite of geochemical and microbial parameters; then push‐pull tests with fluorinated reactive tracers were conducted in each well to measure in situ reductive dechlorination rates. No exogenous electron donors were added in these tests, as the goal was to assess in situ reductive dechlorination rates under natural attenuation conditions. Geochemical data provided preliminary evidence that reductive dechlorination of TCE and CTET was occurring at the site, and microbial data confirmed the presence of known dechlorinating organisms in groundwater. Push‐pull tests were conducted using trichlorofluoroethene (TCFE) as a reactive tracer for TCE and, in one well, trichlorofluoromethane (TCFM) as a reactive tracer for CTET. Injected TCFE was transformed to cis‐ and trans‐dichlorofluoroethene and chlorofluoroethene, and, in one test, injected TCFE was completely dechlorinated to fluoroethene (FE). In situ TCFE transformation rates ranged from less than 0.005 to 0.004/day. In the single well tested, injected TCFM was transformed in situ to dichlorofluoromethane and chlorofluoromethane; the TCFM transformation rate was estimated as 0.001/day. The results indicate that it is possible to use push‐pull tests with reactive tracers to directly detect and quantify reductive dechlorination of chlorinated ethenes and ethanes under monitored natural attenuation conditions, which has not previously been demonstrated. Transformation rate estimates obtained with these techniques should improve the accuracy of contaminant transport modeling. © 2012 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.     

Phytoremediation of dissolved phase organic compounds: Optimal site considerations relative to field case studies

The United States Environmental Protection Agency/Environmental Response Team (EPA/ERT) has been supporting and implementing the use of phytoremediation to remediate dissolved phase organic compounds at Superfund sites since March 1996. Since then, ERT has applied phytoremediation field plots, both pilot and full scale, to a variety of field conditions. These active sites vary considerably as to depth to groundwater, groundwater transmissivity, contaminant concentrations, contaminant hydrophobicity, climate, planting design, plant selection, planting technique, and type of monitoring. Here we compare the literature and discuss lessons learned at five Superfund sites. Current maintenance and monitoring techniques at these sites are also discussed with recommendations for the maintenance and monitoring of similar sites in the future. © 2003 Wiley Periodicals, Inc.