Steven Goldberg, IT Corporation
Investigation and Remediation of a Karst Groundwater System

A multi-phased investigation was completed at a Karst bedrock site in north central New York State to characterize surface and subsurface hydrologic/hydrogeologic characteristics, and to define the nature and extent of contamination. Field investigations included surficial geologic mapping, fracture trace analysis, surface and borehole geophysical techniques, dye tracing, conventional and angle drilling of boreholes and monitoring wells, aquifer testing, and sampling of environmental media. The investigation identified a network of interconnected, solution-enlarged fractures and incised drainage lines. The bulk of the groundwater flow from the site was found to be accommodated by discrete pathways that were not individually discernable, but were integrated into the regional flow system. Remedial measures entailed the blasting of interceptor trenches in the bedrock downgradient of the contamination sources to create common collection points to remove contaminated groundwater.

Florin Gheorghiu, Golder Associates
Enhanced Western Groundwater Control System Hydrogeologic Design, and Field Testing, Modern Landfill, York, Pennsylvania

The poster presents a project recently completed at Modern Landfill in York, PA. The Modern Landfill project is unique because of its magnitude (i.e., installation of a 2,800-foot long blast trench), design and testing challenges, and more importantly because of its demonstrated effectiveness. Controlled rock blasting was designed to increase hydraulic conductivity of the bedrock materials to 10^-2cm/s to allow conveyance and collection of leachate-impacted groundwater. Accumulated groundwater at the downgradient end of the 2,800-foot long blast trench (BT) is extracted by a well field and treated at the on-site wastewater treatment plant. The hydrogeologic design was completed using detailed numerical modeling extensively calibrated to site data. The effectiveness of the BT was demonstrated in the field by detailed hydrogeologic testing, numerical groundwater flow model verification, and groundwater sampling. Hydrogeologic field testing of blast-shattered bedrock required use of hydraulic head derivative for the identification of the test flow model given to the heterogeneities and complicated boundary conditions. The success story of this project is based on detailed understanding of bedrock geology (based on more than 500 investigation boreholes/coreholes), fracture flow hydrogeology (based on more than 250 hydrogeologic tests), and groundwater chemistry (based on chemistry data collected at the site since 1985.)

Nathan Hagelin, Harding Lawson Assoc.
Case Studies in the Evaluation of Remediation Technologies at Fractured Bedrock Sites

Since 1989, Harding ESE has been investigating fractured bedrock sites contaminated with chlorinated solvents using state of the art borehole geophysical methods. Only recently, these techniques have come into broader use. These methods include Borehole Image Processing Systems (BIPs), Optical Televiewer (OPTV), Acoustic Televiewer (ATV), heat pulse flow meter (HPFM) in combination with caliper, and temperature and electrical logging techniques. These methods greatly aid interpretation of the geologic structure(s) affecting groundwater flow and the migration of free phase and dissolved contaminants within the subsurface, but only provide part of a conceptual understanding of a site. To complete the site conceptual model and be in a position to properly assess the feasibility of actual remediation of fractured bedrock environments, bedrock matrix properties and the effect of matrix diffusion on contaminant accessibility must also be evaluated. Evaluation of remedial approaches must address the degree and accessibility of the contamination that resides both within and outside of the fractures. Most emerging technologies currently in vogue for remediation of chlorinated and recalcitrant compounds are not likely to be capable of overcoming these matrix effects. This poster session will present and discuss innovative investigation and modeling methods used at two sites to support two successful technical impracticability (TI) wavers at Loring AFB. The basis of the TI waivers included one site with potentially inaccessible DNAPL in fractured bedrock contrasted by the other site that had no DNAPL but a large matrix diffusion component. Data from a third Superfund site in Maine will be presented where ongoing work suggests matrix diffusion may not be a limiting factor to remediation of DNAPL in deep fractured bedrock.

Rob Blair, Golder Associates
Geological/Hydrogeological Correlation Between the Smithville CWML Site and DNAPL Sites in Niagara Falls, New York

The poster demonstrates the geological/hydrogeological correlation between the Smithville CWML site and DNAPL sites in Niagara Falls, New York, including the Bell Aerospace site. Stratigraphic horizons directly effected by DNAPL will be shown and general hydraulic parameters of the bedrock structure will be identified to provide a comparison between the various sites. The poster will be coordinated with similar poster efforts of other Golder personnel that will be attending the conference.

Jeff Orient, Tetra Tech NUS
Kathy Davies, U.S. EPA
Ron Sloto, USGS
Investigation and Remediation of a Fractured Bedrock Aquifer Contaminated with DNAPL

A site underlain by the Stockton Formation, which consists of fractured sandstone and mudstone units, was investigated to determine the nature and extent of groundwater contamination (primarily trichloroethene in both dissolved and DNAPL form). The Stockton Formation is the major source of groundwater in the area, with groundwater primarily transmitted through fractures, joints, and bedding plane partings. Most groundwater flow is associated with the coarser grained units, with the finer grained shale and siltstone units acting as aquitards. Complicating the investigation of the site was the presence of a 250 gpm municipal water supply well located approximately 1,900 feet downgradient from the site. Preliminary drilling and sampling work within and downgradient of the source area revealed the potential for TCE to be present in DNAPL form. Due to concerns that source area groundwater investigation/ remediation work could potentially mobilize the DNAPL and threaten the municipal well, investigation activities were temporarily suspended while a pre-existing treatment system for the municipal well was upgraded. Interceptor wells were drilled between the site and the municipal well for use in cutting off a high-concentration slug of contaminated groundwater that may be released through source area drilling activities. Strict procedures were developed for field work at the site, including continuous monitoring of drilling discharge air, water, and cuttings, screening for the presence of DNAPL using multiple methods, depth-specific sampling, and frequent sampling/field GC analysis of surrounding monitoring wells. Groundwater extraction wells were drilled in the source area to maximize contaminant removal. Inherent uncertainties regarding the prediction of well yields in fractured rock aquifers and the presence of DNAPL were addressed by hydraulic testing of each well as it was drilled, along with quick-turnaround sampling and real-time data evaluation. This data was used to site subsequent extraction wells and to optimize the design and performance of the source area remediation system. The presence of DNAPL was confirmed in an area approximately 80 feet in diameter. After approximately one year of extraction system operation, substantial decreases in TCE concentrations in the downgradient area have been achieved, while source area concentrations have remained consistent.

Bruce Hunter, Maine DEP
Maine Case Studies

The poster presents several sites in Maine where we have acquired a lot of data and have used it for making remedial decisions. At this time we have 8-10 sites that we are considering using for this purpose.

Nancy Kinner, University of New Hampshire
University of New Hampshire Bedrock Bioremediation Center

The Bedrock Bioremediation Center (BBC), situated within the Environmental Research Group (ERG) at the University of New Hampshire (UNH), specializes in multi-disciplinary research on bioremediation of organically contaminated aquifers. The Center is comprised of a consortium of faculty from ERG and the University's Departments of Microbiology, Earth Sciences, and Natural Resources. The BBC has received funding from the USEPA's Robert S. Kerr Laboratory (Ada, OK) to develop, test, and evaluate innovative technologies for enhanced bioremediation of organically contaminated bedrock aquifers. The long-term objectives of the BBC are to conduct research in order to: more efficiently and economically characterize the direction of groundwater flow and fracture patterns in contaminated bedrock aquifers; improve and develop new field technologies to characterize and control hydraulic flow conditions in the contaminant zone; develop laboratory and field methods to estimate and accelerate in situ rates of bioremediation of organic contaminants in bedrock aquifers; develop and apply innovative microbiological and molecular biology techniques to identify and characterize the specific components of the microbial communities in the fractures or competent bedrock responsible for contaminant biodegradation; and enhance in situ bioremediation and assess the effectiveness of bioremediation strategies. In order to be able to achieve its goals, the BBC is creating a field test site at the Pease International Tradeport (Portsmouth, NH) where new technologies can be evaluated in situ. One problem with evaluating the effectiveness of methods developed for use in bedrock is that it is difficult to track a parcel of water as it moves through an aquifer. To address this problem, the BBC's field facility will consist of three sets of paired boreholes. Each set of paired boreholes will be connected by at least one common fracture, so that it will be possible to monitor water as it moves through the fracture from the upgradient to the downgradient borehole. Such monitoring will help determine the ability of a new technology to reduce contamination. The paired boreholes will be located in an uncontaminated area of the aquifer (a control zone) and in zones of high and low contamination. This will enable testing of new methods under a variety of conditions to determine the full range of their effectiveness.

Vincent B. Dick, Haley & Aldrich, Inc.
The Bedrock Blast-Fracturing Process and its Use for Permeable Reactive Barrier Emplacement

Permeable reactive barriers are rapidly assuming a prominent role as a partial or complete remedy for contaminated site remediation. While most conceptual designs and actual applications to date have focused on remediation in unconsolidated overburden deposits there is a distinct need for an effective remedy in fractured bedrock settings. Very few applications have attempted emplacement of barrier materials in consolidated bedrock.

Bedrock blast fracturing provides a method to create a zone of favorable hydraulic conductivity across contaminant plumes, which is necessary for PRB emplacement in bedrock. These controlled permeable zones provide a means of intersecting and treating plumes, or redirecting plumes to treatment cells, creating a passive alternative for site remediation.

Controlled bedrock blast fracturing requires conventional characterization of bedrock strength and hydrogeologic properties, development of an engineered blasting plan, and construction with oversight to verify the blast plan is carried out and the Blasted Bedrock Zone (BBZ) is created as desired. The overall blast design is intended to balance hole spacing, charge weights, and timing of detonation within and between holes (millisecond-delays between charge detonation) to reduce air overpressure (sound) and ground/structure vibrations. The bedrock "rubble" zone created by the blasting becomes the BBZ (i.e. typically no bedrock is excavated from the subsurface for BBZ construction). Materials needed for the PRB treatment can be emplaced within the BBZ depending on whether the BBZ will be operated passively, or actively pumped to further control plume flow across or through the barrier.

For the poster, typical construction methods used for BBZ construction will be shown, along with a summary of data from a recently-published compilation of BBZ projects by the Groundwater Remediation Technologies Analysis Center (GWRTAC). Example designs will also be shown for bioremediation, and other treatment methods.

Paul M. Tornatore, Haley & Aldrich, Inc.
Migration Control Options in Bedrock Using Blast-Fracturing, Conventional, and Vacuum-Enhanced Pumping: Case Studies at a Single Site

Groundwater migration control in fractured bedrock is a challenging task due to vertical and lateral heterogeneity, unpredictable fracture networks, and transport issues caused by the contaminant characteristics. Despite these challenges, a variety of methods exist that can effectively manage migration in difficult fractured-bedrock aquifers.

The poster presents case studies developed at a large manufacturing facility in upstate New York where a variety of migration control methods have been employed to manage groundwater flow in a fractured bedrock setting. Original methods attempted the use of conventional groundwater wells, with later enhancements including bedrock blast fracturing/active pumping, and vacuum-enhanced pumping. A variety of methods were ultimately employed for chlorinated VOC contaminant plume reduction and migration control in the fractured sedimentary bedrock underlying the site. Data will be presented to show the effectiveness of these different methods under the different bedrock hydrogeologic conditions prevailing at each plume/source area. Overall, the degree of effectiveness at each plume location depends on the degree of bedrock fracture interconnection, whether native or created, and the degree with which the particular removal or treatment method accesses those fractures. This poster will present data compiled over a 1,200± acre facility, for a period of up to 13 years, on site bedrock geology, bedrock plume extent, locations and effectiveness of conventional pumping for plume recovery, high-vacuum extraction from bedrock, and recovery using blasted bedrock zones (BBZs).

Ken Goldstein, Malcom Pirnie, Inc.
Innovative Investigative Technologies to Determine the Extent of Groundwater Contamination at a Fractured Bedrock Site

Watervliet Arsenal, located in Watervliet, New York, is the United States' oldest continuously operating cannon manufacturing facility. The Arsenal is near the eastern boundary of Albany County, New York, on the west bank of the Hudson River, approximately four miles south-southwest of the confluence of the Mohawk and Hudson rivers. The Arsenal consists of two contiguous areas that comprise a total of 140 acres. The Main Manufacturing Area of the Arsenal is a 125-acre tract on which manufacturing and administrative operations occur. The second area, known as the Siberia Area, is used as a shipping yard and for the interim storage of raw and hazardous materials, finished goods and supplies for the Arsenal. The local groundwater recharge area at the Arsenal is a topographic high, located at the western property line. The discharge area for the Arsenal is located along the eastern property boundary, adjacent to the Hudson River, the regional discharge area. Chlorinated organic compounds have been detected in the fractured bedrock at the Main manufacturing Area. Dense, Non-Aqueous Phase Liquids consisting primarily of tetrachloroethene were detected at depth in the fractured bedrock.

The extent of groundwater contamination at the Arsenal was investigated through the installation of monitoring wells along with innovative investigative techniques. During the first two phases of the investigation, groundwater monitoring wells were installed at the water table and in the intermediate bedrock flow zones, ranging in depth from approximately 20 to approximately 80 feet below the groundsurface. The results of the first phase of the investigation indicated that the distribution of groundwater contamination was limited to three primary areas within the Main Manufacturing Area, with the highest concentrations of DNAPLs, being detected at the discharge boundary at approximately 70 feet below the ground surface.

Strategic placement of subsequent monitoring wells was through the use of borehole geophysical techniques such as temperature and resistivity logging, traditional video logging and the relatively new technique of enhanced digital Borehole Imaging Processing system (BIPs). The use of the BIPs allowed for the determination of fracture orientation and fracture aperture width and frequency, which focused the placement of the third-phase wells. The results of the third phase of the investigation indicated that the dissolved phase chlorinated organic contamination extended to a depth in excess of 160 feet below ground surface adjacent to the discharge boundary with DNAPL occurring in a zone of approximately 55 to 70 feet below grade.

Additional borehole geophysical logging techniques were conducted to further define the "high" flow zones in the contaminated bedrock section. These techniques included acoustic televiewer, gamma logging (long/short, normal, and fluid column combination), and electro-magnetic (EM) flowmeter under pumping conditions. This last method, EM flowmeter is very accurate and is capable of defining the active fractures within the system.

Through the use of the techniques, Malcom Pirnie intends to calculate the contaminant mass flux through the fractured bedrock from the site to the regional discharge area, the Hudson River. Potential impacts of the contaminants discharging to the ecologically sensitive receptor (Hudson River) will be evaluated. In addition, the use of these techniques will enable Malcom Pirnie to define the active flow/transport zone in the fractured media, which will facilitate a focused remedial approach for a limited zone of fractured bedrock.

Boris Faybishenko, Lawrence Berkeley National Laboratory
Field Characterization of Flow Phenomena in Unsaturated Fractured Rocks: Review of Innovative Methods and Unresolved Problems

Field observations used to characterize flow and transport in unsaturated fractured rocks have shown complex patterns of spatial and temporal variations in seepage behavior on a variety of scales. This behavior has led to severe contamination of the vadose zone at many sites. Episodic infiltration enhances rapid seepage and contaminant transport along localized preferential pathways in unsaturated fractured rocks. Localized preferential pathways mat be associated with different types of fracturing. Observed instabilities of the flow systems may be caused by film flow along fracture surfaces, the interplay between gravity, capillary, and viscous forces, which creates a periodic opening and closing of pore throats and intrafracture dripping behavior, as well as a variable surface area of the fracture-matrix interaction. Conventional methods of vadose zone investigations based on using a variety of single probes may not be applicable in the complex environment. To characterize seepage in the fractured rocks, we need to conduct controlled infiltration and air injection/extraction experiments with tracers on a hierarchy of scales to determine the geometry of flow and seepage behavior. These tests should be conducted using a combination of noninvasive (seismic, radar, electrical resistivity, etc.) and invasive (boreholes, trenching, etc.) characterization techniques. Field studies should be supplemented by laboratory investigations on cores and appropriate numerical modeling. Examples or results from field investigations in fractured basalt near the Idaho National Engineering and Environmental Laboratory will be presented.

Kent S. Sorenson, Jr., Idaho National Engineering and Environmental Laboratory
Enhanced Biodegradation of Trichloroethene through Lactate Addition in Fractured Rock

A 1-year field evaluation of enhanced biodegradation of chloroethenes was recently performed at Test Area North of the Idaho National Engineering and Environmental Laboratory. A residual source of chloroethenes, primarily trichloroethene (TCE) with some tetrachloroethene (PCE) and dichloroethene (DCE), is present in the fractured basalt aquifer at the site, about 60 to 120 m below land surface. The TCE plume emanating from this source is approximately 3 km long. The objective of the field evaluation was to determine whether the addition of an electron donor to the residual source area could enhance reductive dechlorination of TCE sufficiently to stop its downgradient flux from the source area. Based on results from the laboratory experiments, sodium lactate was chosen as the electron donor and was injected in concentrations ranging from 3% to 60%. TCE concentrations during the evaluation were decreased from concentrations as high as 3,800 µg/L to less than 10 µg/L in the most dramatic case. It was found in several cases that daughter product concentrations exceeded the initial aqueous concentrations of the parent compounds, suggesting that biodegradation either transformed sorbed contaminants along with aqueous contaminants, or that it increased the flux of parent compounds into the aqueous phase.

After 8 months of lactate addition, concentrations within 15 m or the injection were consistently less than 10 µg/L except in the midst of the residual source where kinetics appeared to limit the dechlorination. Significant dechlorination was observed as far as 40 m from the injection point. The residual electron donor (acetate, propionate, and butyrate) present in the treatment cell after 8 months was then monitored for an additional 4 months without lactate addition. It was found that the efficiency of reductive dechlorination actually increased during this period, potentially due to a shift in the microbial population in response to the absence of regular lactate additions. Thus, electron donor utilization patterns as well as electron donor distribution significantly affected dechlorination.

Doug Yeskis, U.S. EPA - Region 5
Field Investigations at Multiple Sites within a Fractured Carbonate Aquifer in Illinois and Wisconsin

The U.S. Environmental Protection Agency - Region 5 and the Water Resources Division of the U.S. Geological Survey are completing a multi-year project on aquifer properties within the Galena-Platteville aquifer, which is a major aquifer in northern Illinois and southern Wisconsin. Data collected at seven Superfund sites within the carbonate aquifer have been evaluated at several scales. Several of the field studies have been completed just for this project to ensure similar types and collection methods were used. The data collected included cross-hole aquifer testing, single-well and multiple-well aquifer tests, two tracer tests, detailed geophysical logging, including downhole-ground-penetrating radar, and rock cores. Evaluations of different data collection methods, which included hydrophysical logging and different downhole-geophysical-logging techniques, were completed at two sites within the study area. Time-sequential sampling of an open borehole was completed to determine the possible bias introduced during vertical aquifer sampling by straddle packer systems. At two locations within the study area, multiple-scale evaluations of aquifer properties are under way with the evaluation of field data and ground-water-flow models.

Rick Colwell, Idaho National Engineering and Environmental Laboratory
Longitudinal and Vertical Variations in the Microbial Ecology of a Fractured Basalt Aquifer with Respect to a Contaminant Plume

The microbial ecology of a basalt aquifer at Test Area north (TAN), Idaho National Engineering and Environmental Laboratory (INEEL) was characterized along two transects with respect to a contaminant plume - longitudinally along the plume axis and vertically within three coreholes. The course of the plume was an injection well that originally contained sewage, chlorinated hydrocarbons, and radionuclides. The aquifer consists of multiple basalt flows where dense fracture networks conduct the groundwater. Cores were collected for microbiological analyses from the saturated zone in the coreholes from ca. 220' to 440' below land surface. One corehole (TAN-37) was located proximal (ca. 30 m) to the injection well and is influenced heavily by the waste; the second hole (TAN-33) was located more distally (300 m) where the major contaminant is thought to be dissolved trichloroethylene (TCE) (ca. 500 ppb); and, the third corehole (TAN-48) was located yet further (ca. 1 km) from the source with lower concentrations of TCE (< 200 ppb). Microbial analyses of cores included biomass and community structure by phospholipid fatty acid (PLFA) analyses; cultural enumerations of methanotrophs, propanotrophs, phenol-oxidizers, ammonia oxidizers, iron reducers, sulfate reducers, methanogens and fermentors; sequencing of polymerase chain reaction (PCR) products derived from 16s rDNA extracted from the samples and acetate mineralization. Polymerase chain reaction-most probable number (PCR-MPN) analyses for methanotrophs were performed on selected groundwater samples to corroborate culture-based determinations of these microorganisms. Data from intentionally introduced tracers (microspheres and perfluorocarbons) and indigenous microbial tracers indicated several orders of magnitude reduction in potential contaminants was achieved by paring the core. Microbial biomass in the basalt from the distal two coreholes (TAN-33 & 48) was at the limit of detection for most assays; minimal acetate mineralization was detected for select samples. On the other hand, basalt from near the injection well (TAN-37) showed low but measurable biomass by PLFA (ca. 3 pmol/g), much higher levels of acetate mineralization, and positive enrichments for nearly all physiological types of microorganisms in all samples. These data were also corroborated by molecular determinations of the microorganisms in the cores, which indicated that TAN-37 had much higher levels of amplifiable DNA than TAN-33. Cloning and sequencing of TAN-37 PCR bands indicated a diversity of microorganisms in the samples from the top of the aquifer in TAN-37, including types that are common in soil environments (e.g., Acinetobacter, Pseudomonas, and actinomycetes). Estimates of numbers of free-living methanotrophs as derived independently from PCR-MPN and culture-based methods were comparable within a given well and highest in TAN-37. In TAN-37, considerable vertical variability existed in the microbiology of the cores. Correlations between vertical distributions of hydraulic conductivity and biological and geochemical characteristics were performed. Vertically averaged data from the longitudinal transect indicate that bacteria associated with basalt may be present in low numbers in pristine areas of the aquifer while a substantial stimulation of similar organisms may result from organic contamination. The variability of microbiological parameters along a vertical axis without a single corehole can rival that seen on a longitudinal gradient, although the predictability of this vertical variation is more elusive. The inability to predict vertical variation of microbial characteristics may be related to the lack of a significant hydraulic gradient at the TAN site, which may be a more significant factor in determining microbial distribution than differences in permeability.

Richard C. Dorrler, Arcadis Geraghty & Miller, Inc.
Characterization, Remediation, Performance Evaluation in Fractured Rock - A Case Study

A site located in the Piedmont of South Carolina has been undergoing remediation of PCE contamination since 1985. Initial remediation efforts included groundwater pumping for plume containment and soil vapor extraction for remediation of residual PCE contamination within the shallow saprolite soils. Starting in 1995, remediation efforts focused on the DNAPL source areas at the upgradient end of the site.

First, the DNAPL source areas were carefully delineated, both horizontally and vertically. Residual contamination was found along relic foliation fractures within the saprolite soil, however the bulk of the free phase PCE/DNAPL was found within a transition zone (weathered zone between the soils and rock) and within the fractures in the shallow bedrock. The primary zone targeted for remediation was determined to be at a depth of between 45 to 75 feet (13.7 to 22.9m) below ground surface. Also, an intensive investigation was undertaken to determine the hydraulic properties and to map the geologic structures and subsurface characteristics of the soil and rock. The investigative techniques included oriented split-spoon sampling, rock coring, downhole video logging, isolated straddle packer sampling, and pressure response testing.

Following a successful 6-month pilot test, and after obtaining the necessary State approvals, a full-scale source area remediation system was implemented to address the DNAPL contamination. The remediation approach utilized multiple wells, with applied high pressure and vacuum to open pathways between wells and establish a Crosshole flow condition throughout the fractured rock zone targeted for remediation. Once these pathways were developed and all the wells were interconnected, a variety of innovative cleaning processes were applied, starting with high vacuum dewatering. The high vacuum was needed to rapidly remove the groundwater and any dissolved phase contamination, as well as any liquid or free phase DNAPL. The high vacuum also helped to create steep water and air flow gradients, which contained contaminants within the zone targeted for remediation. Once this was accomplished, the use of forced hot air injection was initiated. The source of the hot air was from in-well heaters that operate at 1,000°F and at flow rates of 100 cubic feet per minute (cfm) and 60 psi. The high temperatures and flow rates were needed to change the remaining residual phase DNAPL from a liquid phase into a vapor phase, which was more readily removed by standard vapor extraction techniques.

As explained in the Semi-Annual and Annual Effectiveness Reports provided to the State, the overall performance of the DNAPL source area remediation system was based on the amount of PCE contamination removed during the 3 year period of operation (1995 - 1998); and on the subsequent changes in PCE concentrations as measured quarterly and semi-annually in a network of up to 87 monitoring wells installed at the site. PCE concentrations within the remaining hot spots have dropped below the 1 mg/l target level and are now at levels that are equivalent to those within the surrounding areas of the dissolved plume (i.e., hundreds of µg/L).

Finally, since some portion of the PCE could be absorbed into the matrix of the soil or rock, there is always some concern that when remediation pumping stops and water levels recover, that concentrations will rebound due to dilution of the PCE retained in the matrix. To evaluate the potential for rebound, a one year shut-down test was conducted at the site. Despite a full recovery of water levels and complete flooding of the target remediation zone, PCE concentrations have remained below 200 µg/L and are similar to those in the dissolved plume as measured in the surrounding monitoring wells.

Susan S. Suthersan, Frank C. Lenzo, and Jeffrey S. Burdick
Enhanced Reductive Dechlorination of PCE in Bedrock: Pilot to Full-Scale

The results are presented for a pilot test demonstrating the enhanced reductive dechlorination of tetrachloroethene (PCE) in bedrock groundwater using an in-situ reactive zone (IRZ). The pilot test was implemented in the mid-portion of an approximately 3,000-foot long plume. Initial PCE groundwater concentrations were approximately 120 micrograms per liter (µg/L).

Historical groundwater monitoring data included volatile organic concentrations (VOC). Concentrations were in equilibrium throughout the plume, with little evidence of the degradation of PCE. Baseline biogeochemical sampling was performed, and the results indicated the environment in each area was aerobic to transitional. A review of the electron acceptor and donor analytical data indicated the rate of attenuation was also limited due to a lack of organic carbon (electron donor/substrate).

A pilot test was initiated that consisted of an injection well and two downgradient monitoring wells. A reagent of molasses and water was injected on either a weekly or bi-weekly interval. The results of the pilot tests indicate anaerobic and reducing conditions were established and organic carbon concentrations increased in and downgradient of the injection wells. The increased amount of organic carbon coupled with the anaerobic and reducing conditions promoted the reductive dechlorination of PCE to lesser chlorinated compounds (TCE, DCE, VC, and ethene). The increased rate of attenuation decreased the ratio of PCE to daughter products, and reduced overall VOC mass within the pilot test areas. The results of the 15-month long pilot test were used to implement a full-scale in-site treatment system for the site.