SUMMARY OF THE REMEDIATION TECHNOLOGIES DEVELOPMENT FORUM
PERMEABLE REACTIVE BARRIERS ACTION TEAM MEETING

Capital Hilton
Washington, D.C.
November 6–7, 2002

WELCOME AND INTRODUCTIONS
Bob Puls, U.S. Environmental Protection Agency (EPA)
John Vidumsky, DuPont

Bob Puls and John Vidumsky, co-chairs of the Remediation Technologies Development Forum's (RTDF's) Permeable Reactive Barriers (PRB) Action Team, opened the meeting by welcoming attendees (see Attachment A [PDF, 145kb]).

Puls said that the Action Team has been active for 7 years; many of its original objectives, such as initiating dialogue, disseminating information about PRB case studies, and supporting research and training, have been accomplished. More can still be done to promote PRB technologies, Puls said, and he asked attendees for their input on issues that the Action Team should address in the future. Toward this end, a questionnaire was passed out for attendees to fill out.

Puls provided a quick overview of the meeting's agenda. He said that the talks to be presented would cover five broad categories: (1) PRB activity in Europe, (2) PRBs' ability to meet site compliance goals, (3) site design advances in PRB implementations, (3) advances in monitoring PRBs, and (4) long-term performance of PRBs. He also said that a poster session would be held. The poster abstracts are included as Attachment B.

PRB ACTIVITY IN EUROPE

PRBs in Germany and Austria—Overview of 10 PRB Sites and Upcoming Projects
Volker Birke, University of Applied Sciences–North East Lower Saxony

Volker Birke, whose presentation is included as Attachment C Part 1 (PDF, 2128kb) Part 2 (PDF, 2345kb), discussed the status of PRB technology in Germany and Austria. While enthusiasm for PRB technology is lukewarm in Austria, he said, the technology is strongly embraced in Germany, as is evidenced by the fact that the German government has already spent $14 million on PRBs to date. In addition, two research and development networks—SAFIRA and RUBIN—have been created to study PRBs in Germany. Between these two organizations, basic research is performed on reactive materials, PRB design, and PRB performance. Studies are also performed to compare the cost of PRBs against other cleanup technologies.

Birke presented information about PRBs that have been installed at nine German sites (Bernau, Bitterfeld, Denkendorf, Edenkoben, Karlsruhe, Oberursel, Reichenbach, Rheine, and Tübingen) and one Austrian site (Brunn am Gebirge). Detailed information about the 10 sites can be found in Attachment C Part 1 (PDF, 2128kb) Part 2 (PDF, 2345kb), and at the following two Web sites: (1) http://safira.ufz.de and (2) http://www.rubin-online.de . (The latter Web site offers information about all German PRBs in English.) In summary, the PRBs in Germany and Austria are being investigated for their potential to address a wide range of contaminants, including chlorinated volatile organic compounds (e.g., trichloroethene [TCE], tetrachloroethene [PCE], cis-dichloroethene [cis-DCE], vinyl chloride (VC), and trichloroethane [TCA]); benzene, toluene, ethylbenzene, and xylene (BTEX); polycyclic aromatic hydrocarbons (PAHs); chlorobenzenes; and phenols. Most of the PRBs, Birke said, are of the funnel-and-gate design, but some efforts have been made to develop continuous reactive barriers and drain-and-gate designs. At several of the 10 sites, Birke said, the reactors/gates have been installed near the ground surface. As a result, researchers are able to open the reactors, investigate them, and control them in a relatively easy fashion. Various materials are being used in the reactors. Zero-valent iron (ZVI) and granulated activated carbon (GAC) have received the most attention, but efforts are underway to investigate other materials, such as "iron sponge," microbes, palladium, Oxygen Release Compound, and nutrients. Birke said that there is strong interest in exploring even more reactive materials, noting that novel reactive media are needed to address the complex contaminant mixtures present at real-world sites. PRB performance data are not available for all 10 sites, but data have been collected at some of them. These data suggest, Birke said, that some of the PRBs are not meeting remediation goals yet and some are experiencing hydraulic problems.

Birke also noted that PRBs will be installed at two new sites (Offenbach and Wiesbaden) in the near future. At the Offenbach site, a funnel-and-gate design will be installed to mitigate a BTEX/PAH plume. GAC will be used as the reactive material and microbial activity will be encouraged in an effort to augment the PRB's cleanup potential. At the Wiesbaden site, researchers are trying to identify a PRB design that can be used to extract arsenic from ground water.

PRBs in the United Kingdom: New Agency Guidance, Monkstown ZVI System, and New Sequential Reactors
Bob Kalin, Queen's University Belfast

Bob Kalin presented information about PRB-related activities that are ongoing in the United Kingdom. He presented information about a new guidance document, provided an update on the Monkstown PRB site, and discussed activities that have been performed at a complex gasworks site. Detailed information about all three topics is provided in Kalin's presentation, which is included as Attachment D Part 1 (PDF, 2190kb) Part 2 (PDF, 2076kb). The highlights were:

• Guidance document. In September 2002, the Environmental Agency's National Ground Water and Contaminated Land Centre published Guidance on the Design, Construction, Operation and Monitoring of Permeable Reactive Barriers. This guidance was written to provide Agency representatives, consultants, and remediation contractors good practices guidance; to encourage the effective use of sustainable remediation techniques, such as PRBs; and to provide a legal framework to support PRB emplacement. In the United Kingdom, Kalin said, it is illegal to install a PRB without first obtaining a Waste Management Licence. With the guidance document in place, however, the Environmental Agency can issue statements that free PRB installers from the threat of prosecution. The guidance document provides the framework for the development and justification of PRB design, monitoring regime, and decommissioning. The latter has received significant attention in the United Kingdom, but is rarely discussed in the United States. Regulatory officials in the United Kingdom, Kalin said, are concerned that PRBs will become sources of contaminants if left in the ground indefinitely. This is one reason why the United Kingdom favors funnel-and-gate designs over continuous barriers. Not only does the funnel-and-gate design allow engineers to exert more control over water flow, Kalin explained, but it is easier to clean out and decommission. Kalin agreed to send the guidance document to anyone who wanted a copy.



• Monkstown site. Kalin said that a ZVI funnel-and-gate system was installed at the Monkstown site several years ago. In the intervening years, extensive monitoring has been performed to determine how TCE concentrations are changing and whether iron reactivity is diminishing over time. As part of this analysis, the site's reactor was opened and core materials were removed and analyzed for isotope signatures. Results indicate that the reaction rates at the front end of the reactor have decreased by half, but that the rates in other parts of the reactor have doubled. Kalin said that the PRB has performed well. At first, TCE concentrations detected in the reactor's monitoring wells fluctuated significantly, but this problem was eliminated when site managers started recirculating a portion of the plume in an effort to make flow more consistent. More information about the PRB can be found in the Contaminated Land: Applications in Real Environments Technology Demonstration Project Report 4, and in a recent Queen's University Belfast report.



• Complex gas works site. More than 1,500 gasworks sites in the United Kingdom require remediation. At one of them, a site located in Portadown, a biological PRB has been installed to address a variety of contaminants, including multiple PAHs and complex cyanides. Kalin described the desk studies, site investigations, and laboratory feasibility studies that were performed at the site. He also described the conceptual geological framework, the hydrogeologic framework, and the slurry-wall-and-reactor system that was developed for the site. The reactor, he said, supports aerobic degradation. Once the system became operational, researchers found that the site's high ammonia concentrations diminished and aerobic microbial degradation was spurred in the process. According to monitoring results, after passing through the reactor, most of the site's contaminants fell below detection limits. Kalin said that the system was taken apart in 2002 to collect microbiological samples and to determine how easy it would be to take the system apart and put it back together again. Kalin said that this proved difficult, in part because the decommissioning activity coincided with a heavy rainfall event. The reactor system was damaged in the process. It will be fixed and upgraded soon.

Kalin acknowledged that several more PRB-related activities are ongoing in the United Kingdom, and he encouraged attendees to contact him for additional information. He concluded by saying that he thinks it is important for the PRB Action Team to start talking about decommissioning issues.

SESSION I: MEETING SITE COMPLIANCE GOALS
Moderator: Bob Puls, EPA

Assessment of Reactive Iron Barrier Performance at a Complex Australian Site
James Fairweather, Orica Engineering Pty Ltd.

James Fairweather provided information about a site in Australia that is contaminated with a variety of chlorinated hydrocarbons, some of which have been detected at concentrations as high as 220 milligrams per liter (mg/L). (Contaminants include carbon tetrachloride, PCE, TCE, VC, ethylene dichloride, and 1,1,2,2-tetrachloroethane.) The site is complex, Fairweather said, noting that the subsurface consists of sands, silty sands, sandy clays, clayey sands, peaty sands, and peat lenses; the total organic carbon (TOC) concentration is naturally high (i.e., greater than 500 mg/L); sulphide concentrations exceed 30 mg/L; and the pH is low (i.e., less than 5). When trying to identify a remediation strategy for the site, researchers considered PRB technology. But they were not sure how well a PRB would perform in such a geochemically complex setting. To find out, they performed a laboratory column study and a pilot-scale PRB demonstration project. The latter effort, which was completed in February 1999, involved installing a 1.5 meter thick, 5 meter wide, 3 meter tall PRB into the subsurface. The PRB, which was filled with ZVI, was constructed using a sheet piling installation technique. A network of monitoring wells and piezometers were also installed as part of the pilot-scale demonstration project.

The results from the laboratory column tests and the pilot-scale project are summarized in Fairweather's presentation, which is included as Attachment E (PDF, 673kb). In summary, the pilot-scale test showed that ZVI did indeed degrade a broad range of dissolved chlorinated hydrocarbons. (Researchers drew this conclusion after examining ground-water sampling data that were collected at the 1-month, 3-month, 6-month, 9-month, 12-month, 19-month, and 39-month time periods.) Mass reduction rates varied with time, depth, influent concentration, and TOC concentrations. (Month 39 data showed anomalously high mass removal of PCE in the first 400 millimeters of the wall, Fairweather said, noting that this may have been due to biodegradation processes.⁽¹⁾) Also, for some of the contaminants, the reaction half-lives were not as favorable as those that have been observed at sites that exhibit less complicated geochemistry. Nevertheless, Fairweather said, the overall results suggest that a full-scale PRB will perform well enough to meet the qualitative goal that has been set for this site, which is to prevent contaminated ground water from impacting a nearby surface-water body. Fairweather said that the design for a full-scale PRB has not been finalized yet, but that the PRB will most likely be a continuous ZVI barrier that is about 270 meters long, 0.4 meters thick, and 7 to 10 meters deep. The biopolymer slurry trench technique will be used to install the PRB. Installation will be completed before the end of 2003.

Monitoring of a ZVI PRB at the Somersworth Superfund Site Two Years After Installation
Tom Krug, GeoSyntec

Tom Krug, whose presentation is included as Attachment F Part 1 (PDF, 2525kb) Part 2 (PDF, 2477kb), provided information about a PRB that was installed at the Somersworth Sanitary Landfill Superfund site in 2000. The wall treats contaminated overburden ground water before it reaches a nearby wetland. The goal, Krug said, is to reduce chlorinated ethene (e.g., TCE) concentrations below interim cleanup levels. Krug provided a brief description of the PRB's design: It is a 915-foot-long continuous wall that is split into eight sections, each 100 to 150 feet in length. Zero valent granular iron is the reactive medium. Some of the wall's eight sections are filled with pure iron; others have a significant amount of sand mixed in with the iron.

The wall was installed using a biopolymer slurry. With this approach, a trench is dug, guar is pumped into the trench to keep it open, and then reactive materials are poured in, displacing the slurry. Any slurry that remains in the trench degrades via microbial activity. Two problems can arise with the bioslurry method: iron's reactivity can be reduced if it contacts the guar, and the site's hydraulics can be altered as residual guar degrades. At the Somersworth site, the following steps were taken to avoid such problems: (1) the iron was pre-wetted and excess water was added to the backfill, and (2) the reactive medium was added to the trench through a tremie pipe that was positioned near the bottom (rather than the top) of the trench. Krug said that the wall's 23 panels were installed in an alternating pattern: one panel was installed, the next was skipped, and then another was installed. This first string of panels that were installed are referred to as primary panels. Each primary panel was dug out, filled with guar, left to sit overnight, and then backfilled with reactive materials the next day. After the primary panels were constructed, construction workers installed the secondary panels. The installation team learned an important lesson after installing two of the secondary panels: guar degraded quickly and it became difficult to maintain trench stability overnight. (This may have been because the guar that had been added to the primary panels had already attracted a large amount of microbes to the area.) After this realization was made, the construction crew started building the secondary panels in one day rather than allowing the trench to sit overnight before backfilling. Krug said that efforts were made to ensure that the grain size of the iron and the sand met certain specifications. (If the wall had been filled with fines, site hydraulics could have been impacted.) For example, the construction crew double-washed sand to remove fines and monitored the iron/sand backfill frequently during the construction process to ensure that grain size did indeed meet specifications. After construction, they collected core samples from the wall to determine whether the sand/iron mixture stratified in the subsurface. Settlement was only detected in the two secondary panels that had experienced trench stability problems during installation.

Krug said that 2 years worth of post-construction monitoring data have been collected. Water level and water chemistry data indicate that the PRB is performing well. No significant mounding has been observed upgradient of the wall, and the PRB does not appear to be pushing ground water into the underlying bedrock or exerting significant impacts on the site's overall ground-water flow patterns. Furthermore, the data show that contaminants are being treated as they move through the PRB and that no significant amounts of contamination are moving around or beneath the wall. These results hold true, Krug said, even in the two secondary panels that exhibit settlement. Hydraulic tests are also being performed at the site.

One attendee asked Krug for information on installation costs. The iron cost $1.2 million, Krug said, and construction was $800,000. Although this might seem expensive, he said, the installation method chosen was cheaper than other methods that were being considered. Another attendee asked whether the installation crew encountered problems when trying to excavate panel corners. Krug said that a special extension was added to the trenching equipment to ensure that the corners were properly excavated.

Results of the Kinston Jetted PRB and Source Treatment
Steve Shoemaker, DuPont

Steve Shoemaker provided information about DuPont's Kinston Plant, a polyester manufacturing site that has been operational since the 1940s. Ground water underneath the site is contaminated, Shoemaker said: a 1,000-foot-long TCE plume emanates from the site, traveling at a rate of about 0.1 feet per day toward a nearby creek. TCE concentrations in the source area are 50 to 60 mg/L. Concentrations in the distal part of the plume average 100 to 150 micrograms per liter (µg/L). In the early 1990s, DuPont installed a pump-and-treat system in an effort to contain the plume. This system was not cost effective: operation and maintenance costs were about $100,000 per year and only 2 pounds of TCE were removed over a 5-year period. In the mid-1990s, DuPont started looking for alternative remedial approaches.

Shoemaker described the remedial objectives established for the site. The principal objectives were to contain the plume, prevent contaminants from migrating into the creek, and reduce TCE concentrations in the plume to 5 µg/L. In addition, DuPont had a secondary objective: shut down the expensive pump-and-treat system. In an effort to meet these objectives, DuPont installed a 350-foot-long ZVI PRB using high-pressure jetting technology. (See Attachment G [PDF, 1615kb] for details on the installation process and the challenges that existed at this site.) Three years of post-construction monitoring data have been collected. In one well, the PRB caused TCE concentrations to drop from about 450 µg/L to about 10 µg/L. The regulatory agency involved with the site expressed satisfaction with the results; as a result, DuPont was allowed to shut off its pump-and-treat system in 2001. Thus, this site's PRB has met remedial objectives.

Shoemaker also said that ZVI is being used to treat the site's source zone. As part of this effort, a treatment slurry (95 percent kaolinite clay and 5 percent Peerless ZVI) has been injected into the subsurface, and a low-permeability cofferdam (also containing 95 percent clay mixed with 5 percent ZVI) has been installed. The goal is twofold: (1) treat the subsurface by exposing TCE to ZVI, and (2) render the source area stagnant to divert ground water around the source region. The remedial effort appears to be working, Shoemaker said, noting that TCE concentrations are dropping in the source zone soils. (Soil samples were collected from 16 locations before and after the ZVI-clay slurry was injected. Before injection, average TCE concentrations were 25 to 50 parts per million. A few months after the injection, TCE was at or near nondetect levels in 14 of the 16 locations.) During the injection process, Shoemaker said, some contaminated soil was displaced. These spoils (which had some ZVI-clay slurry mixed in) were placed in boxes and left to "cook" ex situ until TCE levels dropped to nondetect. TCE has not, however, dropped significantly in the source area's ground water.

Performance Monitoring of the Spill Site 7 ZVI PRB
Stephen Hart, URS Corporation

In 1999, Stephen Hart said, trench-box construction techniques were used to install a continuous 568-foot-long iron-filled PRB at Warren Air Force Base's Spill Site 7. The wall was installed to address Zone D, one of the contaminated ground-water zones that have been identified at Warren Air Force Base. Spill Site 7 is contaminated with chlorinated organic compounds (COCs), such as TCE, which has been detected at concentrations as high as 21,000 µg/L. The PRB, Hart said, was installed as an interim remedial action. The primary remedial objectives for this site are to: (1) reduce the concentration of COCs in the upper 15 feet of ground water to maximum contaminant levels (MCLs), and (2) minimize COC loading to nearby Diamond Creek. Expanding on the latter, Hart noted that the creek was contaminated with TCE and VC.

Hart described the performance monitoring network that has been established for the PRB, and summarized the results of post-construction monitoring data. (See Attachment H [PDF, 1351kb] for details.) The results reveal the following: (1) COC concentrations have fallen below detection limits in wells within and immediately downgradient of the PRB, (2) COC concentrations are showing decreasing trends in wells that are located 30 to 50 feet downgradient of the PRB, (3) TCE and VC concentrations are decreasing in Diamond Creek, (4) the PRB is not impacting the site's overall ground-water flow pattern or affecting the site's vertical or horizontal ground-water gradients and no mounding has been observed upgradient of the PRB, (5) water-quality parameters and inorganics are performing as expected, and (6) no contaminant degradation byproducts are impacting Diamond Creek. Hart provided some detail on monitoring well 186 (MW-186). In this well, which is located about 40 feet downgradient of the PRB, VC concentrations rose after the PRB was installed. Investigations are being performed to determine why this occurred. For example, a bioevaluation (involving phospholipid fatty acid analyses and biomass evaluations) has been conducted. The results, which are shown in Attachment H (PDF, 1351kb), do suggest that microbial activity in the vicinity of MW-186 differs from that which is observed in other parts of the site. Carbon isotope analyses are also being performed. These analyses, which are being conducted by the University of Toronto, should be completed by next year.

Before closing, Hart discussed future activities that are planned for Site 7. For starters, he said, PRB performance monitoring will continue, and efforts will be made to determine how the PRB can be incorporated into a final remedy for the site. Also, four treatability studies are currently being performed. One of the studies involves drilling to depths of more than 50 feet below ground surface (bgs) and using jetting/jet grouting techniques to create an iron-filled PRB.

Six Years of Intensive Monitoring of the First PRB To Treat a Mixed Waste Plume: U.S. Coast Guard Site in Elizabeth City, North Carolina
Bob Puls, EPA

In 1996, Puls said, an iron-filled PRB was installed at the Elizabeth City site. The PRB—the first full-scale continuous wall ever installed—is about 150 feet long, 24 feet deep, and 2 feet thick. The PRB's primary goal is to remediate chromium VI and contain a contaminated ground-water plume that emanates from a chrome plating shop. The PRB also has a secondary goal: to treat chlorinated solvents, such as TCE, cis-DCE and VC, that are present in the area. While some of these chlorinated solvents can be attributed to the chrome plating shop, Puls said, others come from a different source. Investigations are currently underway to better characterize the source. In hindsight, Puls said, he wished that a more thorough characterization had been performed before the wall was designed and installed. Puls said that 10 compliance monitoring wells have been installed at the site and multilevel samplers have been established along three transects. In total, the site has about 150 sampling points. Puls presented detailed information (see Attachment I [PDF, 551kb]) about the PRB performance data that have been collected over the last 6 years. In summary, the major findings were:

• The PRB has succeeded in meeting its primary objective—containing and treating the chrome plating shop plume. The PRB has successfully removed chromium VI from the plume, Puls said, noting that this contaminant has not been detected above MCLs in samples collected downgradient of the PRB. As a result, the chrome plating shop is moving toward site closure.



• Two distinct chlorinated solvent plumes enter the PRB; both are being treated by the PRB. Puls said that chlorinated solvents enter the PRB via (1) a shallow plume that emanates from the chrome plating shop plume and (2) a deeper plume that has an unknown source. The deeper plume has higher contaminant concentrations and does not foster natural degradation processes. Regardless of which plume the chlorinated solvents come from, Puls said, most of them appear to be remediated by the PRB, as is evidenced by the fact that contaminants have been detected below MCLs in most of the downgradient locations that have been sampled. There is some concern, however, that cis-DCE and VC concentrations are increasing at the back of the wall. Additional investigations will be performed to determine whether this is the case.

Puls also provided data on geochemical parameters. These results are presented in Attachment I (PDF, 551kb).

SESSION II: SITE DESIGN ADVANCES IN PRB IMPLEMENTATIONS
Moderator: John Vidumsky, DuPont

Electrically Induced Redox Barriers (E-barriers)
Tom Sale, Colorado State University

Tom Sale, whose presentation is included as Attachment J (PDF, 2352kb), provided information about e-barriers, a PRB technology that is still in its infancy. Sale said that e-barriers, developed by Dave Gilbert and himself, operate as follows: electrodes are installed in the subsurface, they are powered by an electrical source, they alter the in situ oxidation and reduction conditions, and contaminants are degraded in the process. Sale said that several laboratory studies were performed to prove the concept. For starters, simple beaker experiments were performed to evaluate how contaminated water responds when exposed to e-barriers that consist of one positive electrode and one negative electrode. The studies showed that some contaminants degrade readily under such a scenario. The experiments also showed that concentration reductions occur in three areas: at the e-barrier, upstream of the e-barrier, and downstream of the e-barrier. These findings suggest that the barrier affects oxidation and reduction conditions in areas distal to the electrodes. According to the laboratory results, Sale said, e-barriers appear to be a highly effective remedial option for chlorinated ethanes (e.g., 1,1,1-TCA) and energetics (e.g., RDX and TNT). Experiments performed with the latter were conducted as part of the "Sequential Electrolytic Degradation of Energetics in Compounds in Ground Water" project.

Sale said that he and Gilbert have started experimenting with e-barriers in the field. At CFB Borden—the first site chosen for field testing—Sale and Gilbert learned that e-barriers can indeed be scaled up for field application and that they can be installed in the subsurface. They also learned that e-barriers are capable of sustaining their electrical properties for at least 5 months; that pH, electron activity, and reference potentials shift as expected; and that large quantities of PCE can be removed at a cost as low as $0.01 per day. Sale said that PCE degradation rates at CFB Borden still need to be investigated more thoroughly, pointing out that it is unclear how much PCE was actually degraded and how much was adsorbed. He also said that the e-barrier at the CFB Borden site promoted the generation of cis-DCE. Research still needs to be performed to examine the processes that caused cis-DCE to accumulate. Sale also noted that an e-barrier has been installed at Warren Air Force base, but that current is not running through the system yet. More details about this e-barrier, which cost about $300 per square foot to install, is provided in Attachment J (PDF, 2352kb).

Sale concluded by saying that he was not sure e-barriers could compete with ZVI barriers in treating contaminants like PCE or TCE. E-barriers might prove to be very valuable, however, in treating chlorinated ethanes, energetics, and any other compound that does not respond well to ZVI treatment. One attendee asked Sale about the sequencing of positive and negative electrodes. Sale said that contaminant degradation is almost always more pronounced when contaminated water is passed through the positive electrode before going through the negative electrode.

Biopolymer Slurry Wall Installation of a ZVI PRB at the Former Carswell Air Force Base, Texas
Cynthia Crane, HydroGeoLogic, Inc.

Cynthia Crane, whose presentation is included as Attachment K (PDF, 1721kb), focused her talk on innovative construction techniques that were used to install a 1,126-foot-long PRB at the former Carswell Air Force Base. The wall was installed in March 2002. It extends from the bedrock layer (about 40 feet bgs) to a height that is 2-3 feet above the highest historical ground-water level. The wall, which is 2 feet thick, is filled with a reactive material that consists of 50 percent ZVI and 50 percent sand by dry weight. Fine sand has been poured on top of the reactive media and fills the space between the PRB's ceiling and the PRB's cap. Crane said that several factors came into play when deciding where to place the wall. For example, because the site is still operational, the PRB could not be installed in an area that would interfere with flight paths, existing buildings, or powerlines. Also, aquifer characteristics and contaminant distribution played a role in choosing the PRB's location. (At this site, TCE is the major contaminant, but cis-DCE, trans-DCE, PCE, and VC are also present.)

The biopolymer slurry construction method was used to install the PRB. Crane described the steps involved: (1) excavate a trench down to the bedrock layer, (2) pump in the biopolymer slurry to keep the trench open, (3) backfill the trench with reactive media up to the desired depth, (4) backfill from the top of the reactive media to 3–4 ft below grade with fine sand, (5) inject enzymes into the wall to degrade the guar, and (6) cap the trench with native material. The PRB was installed in two sections: one was 811 feet long, and the other was 315 feet long. Because the excavation and backfilling of the PRB occurred simultaneously, only a relatively short length of the PRB was open trench at any given time. Steps were taken to ensure that the biopolymer slurry would not break down before the construction crew had a chance to backfill the trench. For example, a biostat was added to the slurry and efforts were made to keep the slurry's pH above 9, because basic conditions inhibit microbial activity. The latter effort was compromised somewhat by the fact that the fine sand used to backfill above the reactive media reduced the biopolymer slurry pH to 8. Crane said that several lessons were learned during the installation process. First, she said, the installation team learned that it took a long time to create a 50/50 mix of ZVI and sand. Adding water and using a cement mixer, however, did help speed up the process. Second, Crane said, the installation team encountered problems with tremie pipe plugging when they first tried to emplace reactive materials (stored temporarily in a storage box) into the trench. They fixed the problem by cutting slots into the base of the tremie pipe and discharging the reactive media directly from the cement mixer at a constant rate. Although the installation was a success, Crane said, if she had to do it again she would make some changes. That is, she would select a different backfill material to be placed above the reactive media, perhaps a sand that had some limestone mixed in (to buffer its impact on the slurry's pH) and a coarser gradation (to cut down on how easily it could be pulled along the open portion of the trench via wave action). In addition, she would make greater efforts to limit the amount of sediment that mixes with the slurry. She would also use shorter overlapping trenches. Before closing, Crane provided some performance and cost data. (See Attachment K [PDF, 1721kb] for details.)

Probabilistic Design and Construction Verification of a PRB at an Industrial Site in Virginia
Grant Hocking, GeoSierra LLC

Grant Hocking provided information about a 1,200-foot-long iron PRB that was constructed at the Arrowhead Superfund Site. This wall, which extends between 5 feet bgs and 44 feet bgs, was constructed with the Azimuth Controlled Vertical Hydraulic Fracturing Technology. (Hocking reminded attendees that PRBs are no longer limited to shallow depths; in fact, the Azimuth technology can install walls 300 feet bgs and can be employed across a wide range of complex geological environments, such as those characterized by flowing sands, gravel and cobbles, and clay tills. The technology is limited, however, by the fact that it cannot be used to install walls that are more than 9 inches thick.) Hocking focused his talk on two phases of the Arrowhead Superfund Site PRB project:

• Design. Hocking recommended using probabilistic design methodologies to design PRBs. Using this approach, designers use distributions for input data, forecast performance, and predict the likelihood of achieving cleanup objectives. For example, a PRB designer would be able to say that there is an 85 percent probability of achieving cleanup objectives using PRB design A, but only a 75 percent probability using design B. Parameters are ranked by their sensitivity, Hocking said. As a result, probabilistic design methodologies provide insight on which parameters play the most important role in determining performance. Hocking described some of the steps that are involved in a probabilistic design analysis. For example, he said, PRB designers must obtain information about contaminant half-lives and percent molar conversions, solve simultaneous differential equations, and use Monte Carlo statistical analysis methods to generate probabilistic outcomes. Additional details are provided in Attachment L (PDF, 1578kb).



• Construction verification. Hocking stressed the importance of performing construction verification tests to ensure that PRB construction is successful. Hocking recommended conducting: (1) real-time PRB geometry imaging, (2) inclined PRB thickness profiling, and (3) hydraulic pulse interference tests. The latter, which Hocking described as an excellent and highly accurate test, can be used to determine whether PRBs are impacting a site's overall ground-water flow patterns. Additional details are provided in Attachment L (PDF, 1578kb).

PRB Installation Using Edible Oil Substrate
Bob Borden, Solutions-IES

Bob Borden talked about Edible Oil Substrate (EOS™), an innovative technology that degrades chlorinated solvents by enhancing reductive dechlorination processes. Borden said that EOS™ is an oil-in-water emulsion that is prepared using food-grade edible oils. Once injected into the subsurface, the emulsion serves as a carbon source for in situ microorganisms that drive reductive dechlorination. Borden said that EOS™ can be injected into source areas, distributed throughout a plume to enhance natural attenuation, or used as a permeable barrier to cut off a migrating plume. Borden focused his talk on the latter application. If EOS™ is to be used as a permeable barrier, he said, it is important to make sure that the emulsion does not plug up subsurface soils. To ensure that this does not happen, the emulsion is broken up into very tiny droplets before it is injected. EOS™ is not the first slow-release organic substrate to be investigated as a remedial tool, Borden said, but it differs from its counterparts in that it can be distributed over a large area, through a variety of soil types, and is a longer-lasting carbon source that does not require frequent reinjections. Borden presented data from column studies and a 3-dimensional "sand box" trial to back up his statements. (See Attachment M [PDF, 767kb] for details.)

Borden said that EOS™ has been tested in the field: pilot sites have been performed at three air force bases and full-scale applications have been deployed at three industrial facilities. He provided data on the Altus Air Force Base pilot study. In December 2001, Borden said, remediators established an EOS™ barrier at this site by injecting EOS™ into six injection wells. Samples have been collected three times since the injections were performed. The results indicated that: (1) EOS™ moved at least 25 feet in low hydraulic conductivity weathered/fractured shale, (2) high sulfate concentrations (i.e., 500 to 2,000 mg/L) did not impede the performance of the EOS™ barrier, and (3) EOS™ stimulated dechlorination. Expanding on the latter, Borden noted that there was a 90 percent reduction in TCE and that ethene, ethane, and VC were produced. He said that he is concerned about the rise in VC concentrations but is hopeful that this contaminant will degrade (via microbial activity) over time. Borden concluded his presentation with some cost information (see Attachment M [PDF, 767kb]), noting that EOS™ barriers are significantly cheaper than iron PRBs but a bit more expensive than monitored natural attenuation approaches.

One attendee asked Borden whether he would consider injecting microbes into the subsurface to enhance VC degradation at the Altus Air Force Base site. Borden said this idea is being considered, but that he wants to wait to see if the VC degrades on its own first. Another attendee asked Borden to comment further on the applications he envisions as appropriate for EOS™ barriers. Borden said that EOS™ barriers might be an excellent option at sites where monitored natural attenuation is being considered as a remedial approach. These barriers are not an ideal solution, however, for sites where receptors are located nearby and regulators demand a high level of certainty regarding contaminant capture and remediation.

Performance of a Hydraulically Controlled PRB for the Removal of Strontium-90
David R. Lee, Atomic Energy of Canada, Ltd.

David Lee presented information about a wall-and-curtain PRB that has been installed in Chalk River, Ontario, Canada. This system, which has been operational since December 1998, is being used to address a strontium-90 plume that is migrating toward a wetland area. Lee described the components of the Chalk River site's wall-and-curtain design. Remediators constructed the PRB, Lee said, by driving sheet piling into the ground and then installing a curtain of reactive materials upgradient of the sheet piling. The curtain is filled with granular clinoptilolite, a zeolite that has a high affinity for strontium-90. A complex drainage system has also been incorporated into the design; it allows the system's operators to exert hydraulic control and to perform effluent monitoring. Lee said that the system is designed to funnel large quantities of ground water through a small zone of clinoptilolite. Efforts are made, however, to separate the clean from the contaminated water and to prevent the former from moving through the curtain. Toward this end, a drainage system has been installed upgradient of the wall-and-current treatment system. It captures and shuttles shallow, uncontaminated ground water to the nearby wetland, bypassing the entire treatment system. By reducing the volume of flow through the curtain, the drainage system extends the lifespan of reactive materials. Lee also said that the system is designed to be as easy to monitor and operate as a pump-and-treat system. (If it had not met these criteria, he said, the wall-and-curtain system would probably not have received approval as a remedial strategy for the site.) The PRB is designed so that operators can easily measure flow, contaminant concentrations, hydraulic conductivity, and a variety of other parameters whenever they wish. These data can be obtained for any interval that investigators want to evaluate and at a precision up to five significant figures. In addition, the system has been designed to give operators control over the PRB's capture zone.

The wall-and-curtain PRB has performed exceptionally well over the last 4 years, Lee said. One problem was recently identified, however: a portion of the contaminant plume appears to be bypassing the clinoptilolite. In an effort to fix this situation, he said, operators have increased ground-water flow rates through the PRB. Lee said that additional monitoring wells will be installed near the bypass area and that tracer studies will be used to determine whether modifying the flow rate has successfully prevented bypass. He concluded by noting that operators should never assume that all of the ground water at a site is being treated. Instead, they must go out and perform studies to determine the extent of their capture zone.

PRB as Part of an Integrated Containment Remedy at the DuPont Newport Site
Steve Shoemaker, DuPont

Steve Shoemaker provided information about the Newport South Landfill, a site surrounded by a river and wetlands. The landfill, which served as a repository for spent ores and other wastes for about 50 years, was declared a Superfund site in 1990 after elevated concentrations of barium, zinc, copper, cadmium, lead, cobalt, nickel, and manganese were detected. In situ stabilization, a remedy that would have cost about $17 million, was identified as the preferred remedial approach in the Record of Decision. Given the expense, DuPont started searching for alternative remedial strategies that EPA would find acceptable. In 2001, EPA approved an integrated containment remedy which involves three components: (1) a landfill cap, (2) an impermeable slurry wall that is positioned to prevent ground water from flowing into the river, and (3) a PRB that is positioned to protect the wetlands. This alternative remedy, Shoemaker said, costs about $4 million.

EPA Region 3 indicated that the PRB would have to be designed to reduce barium concentrations to 7.6 mg/L, zinc to 0.12 mg/L, and manganese to 1.0 mg/L. To meet these goals, a mixture of sand, calcium sulfate (to remove barium), ZVI (to sorb zinc), and magnesium carbonate (to remove manganese) was chosen as the reactive material for the PRB. Then a field demonstration was performed to evaluate the PRB's performance. In situ reactive wells were used to emplace the reactive materials, Shoemaker said, noting that this is a fairly innovative installation approach. The field demonstration PRB performed well: it reduced all three of the contaminants below treatment standards. Given these results, the decision was made to move forth with a full-scale PRB. Installation activities were just recently completed.

The capping component of the remedial strategy played a crucial role in convincing EPA to accept DuPont's proposal to use a PRB at this site. This is because the cap, which is designed to last for hundreds of years, has the ability to extend the life of the PRB. Shoemaker said that the landfill is isolated hydrogeologically, noting that rainfall that infiltrates landfill wastes is the only source of water for the site's underlying ground water. Thus, capping the landfill, which will reduce rainfall infiltration to a rate less than 0.003 inches per year, will decrease ground-water flow rates. Reduced ground-water flow velocity, he said, equates to increased PRB residence time and wall life. In fact, DuPont was able to show that the PRB will function for about 600 years. Additional details about the capping designs considered and the techniques that were used to examine their performance are provided in Shoemaker's presentation, which is included as Attachment N (PDF, 1896kb).

SESSION III: ADVANCES IN MONITORING PRBS
Moderator: Tim Sivavec, General Electric

Performance Monitoring Using Passive Diffusion Bag Samplers at the Somersworth Site
Karen Berry-Spark, GeoSyntec Consultants

Karen Berry-Spark, whose presentation is included as Attachment O (PDF, 587kb), provided information about the passive diffusion bag (PDB) samplers that are being used to sample volatile organic compounds (VOCs) at the Somersworth Superfund site. (This is the same site that Tom Krug discussed earlier in the meeting.) Berry-Spark opened her presentation by describing how PDB samplers work. She said that a PDB sampler is about 1 to 2 feet long and 1.2 inches wide; it consists of a low-density polyethylene (LDPE) sleeve that is filled with laboratory-grade deionized water. VOCs pass through the sleeve readily. PDB samplers are lowered into wells, left in place for 2 weeks to allow water in the sleeve to equilibrate with water in the well, and then removed and sampled to determine how much VOC was present in the well. Berry-Spark said that PDB samplers offer some advantages over conventional purge-water sampling techniques. For example, because PDB samplers do not generate purge water and are not reused between wells, there is no need to measure purging parameters, to treat purge water, or to perform decontamination procedures between wells. Also, sediment cannot pass through the LDPE. Additionally, PDB samplers are inexpensive to use and are easy to deploy and recover. Berry-Spark did note some limitations of PDB samplers. For example, they cannot be used to collect data on inorganic contaminants and are not appropriate to use for all VOCs. Additionally, if a site has stagnant water, chemical stratification, or vertical flow, PDB samplers might not yield representative data.

Berry-Spark described a proof of concept study that was performed at the Somersworth Superfund Site to determine whether PDB samplers could be used. At this site, she said, more than 45 monitoring wells have been installed to measure the performance of the site's multi-component remedial strategy, which consists of a PRB (to treat overburden ground water) an extraction system (to address bedrock ground water) and natural attenuation (to address the wetland). Thirty of these wells are monitored solely for VOCs. Interest was expressed in using PDB samplers to monitor these wells rather than purge-water sampling techniques, since an annual cost savings of $25,000 could be realized by using the former over the latter. A comparison study was performed to determine whether PDB samplers provide acceptable data. This involved placing PDB samplers in the center of the well screen in 23 wells, leaving the bags in place for 14 days, retrieving the bags, sampling their contents, and then purging and sampling the well. The results collected from the PDB samplers were then compared to the results collected using the purge-water sampling technique. Strong correlation was revealed in 17 wells. EPA agreed that it would be acceptable, therefore, to use PDB samplers in these wells rather than the more costly purge-water sampling techniques. Strong correlation was not observed in six bedrock wells. Upon additional investigation, however, researchers were able to achieve strong correlations in four of the six wells when they moved the PDB samplers to depths other than the center of the well screen. For these four wells, EPA agreed, PDB samplers could be used if they were indeed placed at the screening level that had been identified as being the most appropriate for each well. Purge-water sampling techniques must be used, however, on the two wells where strong correlation could not be achieved. Berry-Spark concluded by saying that PDB samplers produce results that compare very well to those collected using conventional purging techniques. She said that an additional study will be performed to determine whether these correlations hold when PDB samplers are left in wells for time periods longer than 14 days.

Berry-Spark fielded several questions from the audience. One attendee asked about the length of the well screens at the Somersworth site. Berry-Spark said that well screens in the bedrock wells range from 10 to 40 feet, and that the well screens in the overburden wells range from 10 to 30 feet. Another attendee complimented Berry-Spark on the comparison study that was performed. He reminded attendees, however, that one cannot automatically assume that PDB samplers will perform satisfactorily at all sites. Another attendee asked whether strong correlations between the two sampling techniques are expected to persist across different seasons. Berry-Spark said that the comparison test at the Somersworth site actually spanned several seasons; no seasonality impacts were observed on the correlations. She reminded attendees, however, that they must make sure that the PDB samplers remain submersed in water if they leave the bags in place over multiple seasons. Another attendee asked Berry-Spark to explain why a comparison test is needed to determine whether PDB samplers will perform adequately when left in wells for more than 2 weeks. Berry-Spark responded by saying that biofouling and precipitation are concerns. Sivavec said that General Electric has used PDB samplers at eight sites. At some of the sites, the PDB samplers were left in the wells for several months; no fouling was observed. Another attendee said that the U.S. Air Force has also experimented with long-term PDB sampler deployments and has experienced no biofouling problems. Berry-Spark commented that orange material on some of the PDBs has been observed at the Somersworth site after 14 day deployment and reminded the audience that this is a landfill site with some landfill leachate in groundwater.

The Use of PDB Samplers for Construction Verification and Performance Monitoring of a Deep PRB
Michelle Thomson, URS Corporation

Michelle Thomson provided information about a former manufacturing site that plans to use a deep ZVI PRB to address trichlorofluoromethane and carbon tetrachloride. Contaminants at this site are located deep in the aquifer, Thomson said; thus the PRB must be installed deep in the ground. This depth requirement, coupled with the fact that the site has flowing sands and a wetland located nearby, presents several construction challenges. Thus, the decision was made to perform a technology demonstration project at this site before proceeding with a full-scale PRB. The project's goals, she said, were to determine whether the PRB could indeed by emplaced 120 feet bgs, to determine whether a 6-inch-thick ZVI treatment zone would reduce contaminants to acceptable treatment objectives, and to study downgradient fate and transport of daughter products and co-contaminants. In 2001, she said, a 110-foot-long pilot PRB was installed at the site using the Azimuth Controlled Vertical Hydraulic Fracturing Technology. Construction verification studies (e.g., qualitative mass balance analyses, electrical resistivity mapping, directional drilling to core wall) were performed to determine whether construction was successful. Thomson said that the results are encouraging, but that investigators must wait to obtain performance data to fully evaluate the PRB's potential.

Thomson said that deep wells have been established 10, 40, and 70 feet downgradient of the PRB to evaluate its performance. Samples are collected at three discrete depth intervals using low-flow sampling techniques. PDB samplers are also being used. Thomson provided detailed information about the data that have been collected thus far. In summary, Thomson concluded, the data collected at this site suggest that: (1) in situ PRB emplacement down to 120 feet bgs can be achieved, (2) contaminants are being treated across the entire vertical interval, and (3) PDB samplers are effective tools for PRB construction and performance verification. She also pointed out that an important lesson was learned at this site: installing wells too close to a PRB wall can be problematic. At this site, she said, evidence suggests that the well that is supposed to be 10 feet downgradient of the PRB was installed at an angle and passes through the PRB. As a result, the top of the well is located downgradient of the PRB, but the bottom of the well is located upgradient. (Investigators first became aware that there was a problem when they found dramatic variations in contaminant concentrations across the well's depth. Results collected during a directional logging survey and pictures obtained with a camera provided insight as to the nature of the problem.) Thomson said that field monitoring will continue at this site and that efforts will be made to monitor the degradation of daughter products and co-contaminants downgradient of the PRB as the system moves toward steady state. If the regulatory community is satisfied with the pilot study's results, she said, another 400-foot-long section of PRB will be added to the site.

One attendee asked whether vertical flow has been measured at this site. Thomson said that site investigators did try to collect vertical flow data, but abandoned the effort after experiencing technical problems with the vertical flow meter. Efforts will resume soon. Another attendee commented on the well that passed through the PRB, noting that it is very difficult to drill a deep straight hole.

Recent Field Experience With Velocity and Directional Sensors
Bruce Sass, Battelle

Bruce Sass opened his presentation by noting that there is significant variability associated with hydraulic characterizations. For example, at any given site, hydraulic conductivity, ground-water gradients, flow rates, and flow directions might differ dramatically across different parts of the site. This variability can make it difficult to obtain a clear conceptual site model. Work has been performed, Sass said, to determine whether obtaining specific point and space data, such as that which is generated using flow sensors, will contribute anything to a researcher's knowledge about a site's overall hydraulic characteristics. If so, he said, this may help PRB designers build walls that are long enough to prevent bypass and thick enough to perform adequately. Sass focused his talk on field tests that have been performed to assess HydroTechnics™ sensors and colloidal borescopes at Dover Air Force Base and Lowry Air Force Base—two sites that have PRBs. Attachment P Part 1 (PDF, 2257kb) Part 2 (PDF, 2306kb) provides detailed information about the two assessment tools, the field studies performed at the air force bases, and the results obtained. In summary, Sass offered the following evaluations:

• HydroTechnics™ sensors. These sensors provide continuous data recordings over many months. They are able to record effects of rainfall events and seasonal and annual ground-water fluctuations. They measure very localized ground-water flow, and they provide velocity and direction for single points in space. They also provide temporal data. Additional work needs to be performed to determine how the sensors will perform inside a ZVI barrier.



• Colloidal borescopes. These devices measure particle movement along preferential flow paths. Results are biased toward high conductivity zones. The device only works when flow is stable, and it is not advisable to collect data immediately following atypical weather conditions. It is unclear whether borescope measurements are representative of overall flow conditions.

Sass concluded by saying that water-level measurements remain the best indicator of bulk ground-water flow. Selective use of HydroTechnics™ sensors (which measure very localized flow) and colloidal borescopes (which measure preferential flow), however, may be very useful at some highly heterogeneous sites.

Commenting on one of the slides that Sass presented, one attendee asked why some of the sensors showed ground water moving in the opposite direction of the PRB's gate. Sass said that these flow reversals coincided with high precipitation. During high-precipitation events, mounding may occur in front of the PRB and force ground water to flow away from the wall. Another attendee asked whether it would have been useful to use bromide tracer tests during the field studies. Sass said that he would have liked to do so, but such studies were not budgeted into the project. Vidumsky said that Sass' presentation makes it clear that the PRB Action Team still has a long way to go before it fully understands ground-water flow issues.

In Situ Monitoring of the Ten-Year Old PRB at CFB Borden, Ontario
Stephanie O'Hannesin

Stephanie O'Hannesin talked about the PRB that was installed at the CFB Borden site in 1991. This PRB—the first that was ever constructed—was evaluated in 1996 and in 2001 to determine whether it was still performing adequately after being in the ground for 5 and 10 years, respectively. Attachment Q (PDF, 2220kb) provides information about the CFB Borden site, the PRB design, the reactive material used, performance data collected after 5 years of operation, and the approach used to perform the 10-year evaluation. O'Hannesin focused on the latter, describing the study design and summarizing the results obtained. She said that much of the data was generated by Tamara Reynolds, a graduate student at the University of Waterloo. The main goal of the 10-year evaluation study, O'Hannesin said, was to examine the PRB to determine whether there have been any changes in hydraulic conductivity, accumulation of precipitates, increases in microbial activity, or changes in iron reactivity. To obtain this information, studies were performed both in the laboratory and in the field. In the laboratory, column tests were performed using materials that had been extracted from the 10-year old PRB via coring samples. The columns were spiked with 10 mg/L of TCE and evaluated to determine how well the PRB materials degraded the contaminant. In addition the core materials were examined for their iron content, microbial activity, and precipitates. In the field, an In Situ Tester (IST) was used to assess the PRB's reactivity. In effect, O'Hannesin said, the IST allows for in situ solute loading and sampling to characterize chemical reactions. The device is installed into the subsurface, ground water is extracted, and a Teflon bag—filled with the site's ground water and spiked with a contaminant—is returned to the ground. The spiked water is then sampled over time to determine how it is affected by the PRB. O'Hannesin provided a detailed description of laboratory column data and the IST data (see Attachment Q [PDF, 2220kb]). In summary, she made the following major points:

• IST device. The IST device, O'Hannesin said, is a promising tool for evaluating the reactivity of PRBs. It can be used at a variety of location and across a variety of depths, and might provide more accurate results than laboratory column tests. When it is used to test low-level contaminant concentrations, she said, there may be a deviation from 1^st order kinetics. This could be due to mass transfer limitations with low iron content, but may be less dominant in PRBs that consist of 100 percent iron.



• PRB performance at the 10-year mark. The studies indicated that the PRB was still reactive and that it could go on performing for several more years. No indications of biofouling were observed. There was evidence of precipitates (mostly calcium carbonate and iron[III]hydroxide) in the first 0 to 30 centimeters of the wall. The wall's hydraulic conductivity is equal to or one order of magnitude greater that the aquifer. Microbial activity is only slightly elevated.

SESSION IV: LONG-TERM PERFORMANCE OF PRBS
Moderator: Bruce Sass, Battelle

Long-Term Monitoring at the Sunnyvale PRB: Lessons Learned/Future Expectations
Scott Warner, Geomatrix

Scott Warner provided information about a control-and-gate PRB that was installed at a site in Sunnyvale, California, in 1994. This PRB, he said, was installed to address chlorinated solvent contamination and to restore the site's economic viability. (Before the PRB was installed, a pump-and-treat and an above-ground air stripper were being used to address the site; new tenants could not be invited to the site until the above-ground treatment system was removed.) Warner stressed that this site is a commercial site, not a research site; some of the activities that he would like to perform at this site, such as flow sensor testing, are not within the budget. Warner's presentation, which is included as Attachment R (PDF, 614kb), provides detailed information about the PRB's design and the installation methods that were used to emplace the wall. He noted that the wall is probably thicker than it needs to be and that it is filled with more iron than necessary. This is due to the fact that the PRB was designed at a time when very little was known about how much iron was needed to address contaminated sites.

Warner focused his talk on performance monitoring. He said that water level measurements have provided interesting information. For example, these data suggest that some mounding occurs upgradient of the PRB during the winter when California gets high rainfall. Also, in May 1995, abnormal water level measurement data was recorded when excavation activities at a neighboring property damaged the PRB's slurry wall. The damage has since been fixed, Warner said, and measures have been taken to prevent this kind of problem from arising again. That is, alarms have been installed to alert site managers if someone digs near the PRB. Warner said that PDB samplers are being used to evaluate changes in contaminant concentrations. To convince regulators that the PDB samplers produce acceptable data, these data were compared to data generated using more conventional low-flow purging techniques and traditional bailer methods. Warner also provided information (see Attachment R [PDF, 614kb]) on data that have been collected on pH trends, water quality parameters, redox conditions, inorganic chemistry, and dissolved hydrogen and methane gas. The latter, he said, are producing very interesting results. Warner concluded by making the following points: (1) the ZVI PRB is meeting site remediation goals, (2) the PDB samplers provide representative results, (3) the hydrogen gas data suggest continued reactions, (4) the hydrogen data may provide useful information, (5) the methane gas data suggest that biological activity is ongoing, (6) the inorganic data suggest mineralization is occurring in the peagravel zone, (7) the corrosion reaction appears to be long lasting, (8) hydraulics are key for design/performance, (9) hydraulic conditions at the site are seasonal, (10) the PRB is an economical remedy, and (11) the PRB is expected to perform adequately for several more years.

Effect of Co-Solutes on the Long-Term Reactivity of Granular Iron Toward TCE and Nitroaromatic Compounds
A. Lynn Roberts, Johns Hopkins University

Lynn Roberts provided information about laboratory column studies that have been conducted at Johns Hopkins University and Tyndall Air Force Base. These studies, she said, have been performed to assess the longevity of iron-filled PRBs by evaluating (1) how hydraulic residence times and hydraulic residence time distributions (HRTDs) change over time, (2) how solutes (bicarbonate, chloride, silica, natural organic matter [NOM], and organic contaminants) affect the reactivity of iron in the interfacial region, and (3) changes in interfacial composition over time. Roberts described the experimental design that was used and the analytical techniques (e.g., AES, XPS, micro Raman, SEM, TEM, and XRD) that were employed to generate data. Some of the column tests were performed over a time period as long as 1,100 days. Ground-water constituents, such as sodium bicarbonate, calcium carbonate, chloride, silica, and NOM, were added to some of the columns. Other columns were spiked with organic contaminants, such as chlorinated hydrocarbons (e.g., TCE, 1,2,3-trichloropropane, and 1-1-dichloroethane) or nitroaromatic compounds (e.g., 2-nitrotoluene, 4-nitroacetophenone, 4-nitroanisole). Exposure was continuous in some of the columns, intermittent in others. Detailed data from the column studies will be published soon, Roberts said. In summary, the results suggest the following: (1) alterations in PRB performance toward TCE removal stem more from changes in specific reactivity than from HRTD changes; (2) transport property changes (HRTDs, immobile water content) exert relatively minor effects; (3) carbonate, silica, and NOM all diminish the reactivity of iron toward TCE; (4) continuous versus intermittent TCE experiments suggest that PRB longevity may be a function of contaminant concentration; and (5) surface spectroscopic techniques furnish a better understanding of the species present at the iron surface.

Long-Term Permeability and Reactivity of Granular Iron: Effects of Gas Formation and Mineral Precipitation
Lai Gui, University of Waterloo

Lai Gui discussed column studies that have been performed at the University of Waterloo to examine the effects that gas formation and mineral precipitation have on PRB permeability and the reactivity of granular iron. Gui explained why these studies are relevant: in order for an iron PRB to be more cost-effective than a pump-and-treat system, the PRB must remain reactive and permeable for long periods. Concern has been expressed, she said, about the potential for entrapped gas and precipitates to accumulate in iron walls, reduce their reactivity, clog them, and turn them into impermeable barriers. In response to these concerns, the University of Waterloo performed experiments to (1) study the effects of entrapped gas and calcium carbonate on porosity and hydraulic conductivity in granular iron columns and (2) examine the effect of precipitates on iron reactivity using TCE as an indicator. Attachment S (PDF, 478kb) provides detailed information about the experimental setup, the techniques used to generate data, and the results obtained. The following conclusions emerged from the study:

• Effects on hydraulic conductivity. Gui said that entrapped gas initially caused rapid declines in hydraulic conductivity but that hydraulic conductivity did rebound somewhat over time. (The gas caused about a 10 percent porosity loss, which translated to a hydraulic conductivity loss of about 50 to 80 percent.) Calcium carbonate precipitate formation caused a porosity loss of 7 percent, she said, but had negligible effects on hydraulic conductivity.



• Effects on iron reactivity. Gui said that calcium carbonate precipitates do reduce TCE degradation rates. She also noted that these precipitates form as a progressing front, and that the front progresses slowly under field conditions.

Gui ended her presentation by saying that the rate of precipitate formation may be predictable. If so, she said, PRBs can be designed to take the effects of precipitate formation into account.

Evaluation of Corrosion and Mineral Precipitation on the Performance of ZVI Barriers
Bruce Sass, Battelle

Bruce Sass discussed studies that examined PRB longevity at Lowry Air Force Base and Moffett Federal Air Field. These studies, he said, were performed in the laboratory and in the field. For example, in the laboratory, column experiments were performed to simulate long-term field performance, and changes in degradation rates and corrosion compounds were examined as the iron in the column aged. The field investigation included ground-water analyses, geochemical modeling, iron core analysis, and hydraulic monitoring. Sass described the results obtained from these studies. In summary, the following was learned: (1) iron reactivity declines with prolonged exposure to ground water, (2) the rate of decline is influenced by the amount of dissolved solid that is present in the ground water, (3) precipitate migration rates increase as ground-water flow rates increase, (4) precipitation generally follows the predictions of geochemical modeling, (5) Oxidation Reduction Potential (ORP) and pH are not good early-warning indicators of iron reactivity loss, and (6) iron's reactivity rate can decline by several factors before any evidence of clogging is observed. The studies also suggest that a researcher's ability to transfer laboratory column test results to the field depends, in part, on how well that researcher understands the actual hydraulic conditions that are operating in the field.

Sass concluded with some predictions about the longevity of the PRBs at Lowry Air Force Base and Moffett Federal Air Field. The data suggest, he said, that both of these PRBs will perform adequately for 30 years. (This is the amount of time that it will take for the reactivity of each of these PRBs to decline by a factor of two.) Given this finding, using a PRB at these sites will be more economical than using a pump-and-treat system. (Sass provided some information that suggests that PRBs are less expensive than pump-and-treat systems in cases where the PRB is able to last for 10 or more years.

Spatial and Temporal Trends in Ground-Water Chemistry and Precipitate Formation at the Elizabeth City PRB
Richard Wilkin, EPA

Richard Wilkin described investigations that have been performed to assess the long-term performance of the ZVI PRB at the Elizabeth City site. (This is the same site that Bob Puls discussed earlier in the meeting.) To perform a thorough assessment of a PRB's performance, Wilkin said, one must examine contaminant behavior, ground-water chemistry, mineral precipitates, microbial communities, and hydraulic performance. Wilkin focused his talk on the last four items on this list, saying that the first item—contaminant behavior—had already been covered during Puls' presentation. Wilkin said that 6 years worth of PRB performance data have been collected at the Elizabeth City site. Attachment T (PDF, 1153kb) provides detailed information about the methodologies that were used to collect these data and the results obtained. In summary, Wilkin made the following major points:

• Ground-water flow rate and total dissolved solid (TDS) concentrations influence PRB longevity. The more ground water and TDS that passes through a PRB, Wilkin said, the more material precipitates out. Thus, lower ground-water flows and lower TDS concentrations translate to less precipitate buildup and longer PRB performance. Wilkin said that TDS concentrations are low at the Elizabeth City site.



• Minerals and biomass do build up on PRBs, but rates vary across depths. Wilkin said that ZVI is a long-term sink for carbon, sulfur, calcium, silica, nitrogen, magnesium, and manganese. Biomass can also accumulate on PRBs. Buildup does not occur at the same rates across an entire wall's depth—certain areas are more vulnerable than other areas. Wilkin said that more work needs to be performed to evaluate vertical differences in hydrologic and geochemical data.



• Porosity loss can be estimated. Wilkin said that one needs the following information to calculate porosity loss: (1) ground-water flow rates, (2) solute flux rates, (3) influent sulfate and bicarbonate concentrations, (4) sulfate and bicarbonate removal efficiencies, (5) the PRB's initial porosity, (6) iron corrosion rates, and (7) mineral molar volumes. At the Elizabeth City site, Wilkin said, porosity losses are low. This is due, in part, to the low TDS level in the site's ground water. He said that higher porosity losses have been recorded at the Denver Federal Center, a site with higher TDS values.

Wilkin ended by saying that strong correlations between declining performance and changing geochemical parameters have not yet been identified at the Elizabeth City site.

Heterogeneity Development and Its Influence on Long-Term PRB Performance: Column Study
Wiwat Kamolpornwijit, Oak Ridge National Laboratory

Wiwat Kamolpornwijit presented data from some field column studies that Oak Ridge National Laboratory performed at the Y-12 PRB site. He said that these data have been incorporated into a document that summarizes a three-agency (EPA, the Department of Defense, and the Department of Energy) effort to assess the long-term performance of PRBs. During his presentation, which is included as Attachment U (PDF, 826kb), Kamolpornwijit gave detailed information on the column study's experimental design, the analytical techniques used, and the results obtained. In summary, he said that the studies revealed the following:

• Heterogeneity in flow developed in the column studies. Kamolpornwijit said that mineral precipitation was the main factor that caused heterogeneity in flow. Gas production and microbial action also played a role, but their contribution was minor.



• pH is a good indicator of hydraulic flow distribution if ZVI maintains its reactivity.



• Aqueous chemical sampling is a good technique to use to assess the extent of precipitation.



• Thermal gravimetric analysis is a useful tool for quantifying major phases in precipitants and calculating the corrosion rates of iron.

Kamolpornwijit also said that efforts are underway to scale up the column study data to the field scale. This will allow researchers to predict how long the in situ PRB at the Y-12 site will perform. Preliminary results suggest that the PRB, subjected to similar ground water, will experience flow heterogeneity development (in addition to natural variation) after it has been operational for 1 year. He encouraged attendees to contact him if they wanted to discuss the data in greater detail.

CLOSING REMARKS

Puls thanked attendees for their participation and expressed appreciation to those who played a role in planning and implementing the meeting. Before closing, he noted that several attendees had approached him during the meeting asking for information on the cost of PRBs. He advised reading Economic Analysis of the Implementation of Permeable Reactive Barriers for Remediation of Contaminated Ground Water, a report that EPA released in June 2002. This report can be found at http://www.epa.gov/ada/pubs/reports.html.

Attachments A through U

Attachments A through U are available on the Internet. To view these attachments, visit the RTDF home page at http://www.rtdf.org , click on the "Permeable Reactive Barriers Action Team" button, then click on the "Team Meetings" button. The attachments will be available as part of the November 2002 meeting summary.

Attachment A: Final Attendee List (PDF, 145kb)

Attachment B: Abstracts From the Poster Session

• Permeable Reactive Barrier Installation by the Bio-Polymer Slurry Method at the MDL Site in Needham, MA (Kenneth Andromalos) (PDF, 105kb)



• Use of Biogenic Apatite for Stabilization of Subsurface Metal Contaminants (Bill Bostick) (PDF, 60kb)



• Remediation of Groundwater Contaminated with Zn, Pb, and Cd Using a Permeable Reactive Barrier With Apatite II (James Conca) (PDF, 133kb)



• Natural Groundwater Constituents as Tracers for the Reactivity Assessment of Fe(0) PRBs (Markus Ebert) (PDF, 73kb)



• Treatment of Vapor-Phase Organohalide Contaminants Using Fe(0) and Bimetallic Reductants (Adam Grenier) (PDF, 66kb)



• Design, Construction and Installation Verification of a 1200' Long Iron Permeable Reactive Barrier (Grant Hocking) (PDF, 92kb)



• Heterogeneity Development and Its Influence on Long-Term PRB Performance (Wiwat Kamolpornwijit) (PDF, 83kb)



• Investigation of the Long-Term Performance of Zero Valent Iron for Reductive Dechlorination of Trichloroethylene (Tie Li) (PDF, 20kb)



• Rantec G150 Biopolymer, Biodegradable Liquid Shoring, General Characteristics and Use Procedures (Lloyd Marsten) (PDF, 57kb)



• Performance of the Monticello Millsite Permeable Reactive Barrier (Paul Mushovic) (PDF, 66kb)



• Mineralogical Characteristics and Transformations During Long-Term Operation of a Zero Valent Iron Reactive Barrier (Debra Phillips) (PDF, 58kb)



• Arsenic Remediation in Simulated Groundwater Using Zero Valent Iron: Laboratory Column Tests (Chunming Su) (PDF, 49kb)



• In Situ Remediation With Nanoscale Iron Particles (Wei-xian Zhang) (PDF, 63kb)

Attachment C: PRBs in Germany and Austria—Overview of 10 PRB Sites and Upcoming Projects (Volker Birke): Part 1 (PDF, 2128kb) Part 2 (PDF, 2345kb)

Attachment D: PRBs in the United Kingdom: New Agency Guidance, Monkstown ZVI System, and New Sequential Reactors (Bob Kalin): Part 1 (PDF, 2190kb) Part 2 (PDF, 2076kb)

Attachment E: Assessment of Reactive Iron Barrier Performance at a Complex Australian Site (James Fairweather) (PDF, 673kb)

Attachment F: Monitoring of a ZVI PRB at the Somersworth Superfund Site Two Years After Installation (Tom Krug): Part 1 (PDF, 2525kb) Part 2 (PDF, 2477kb)

Attachment G: Results of the Kinston Jetted PRB and Source Treatment (Steve Shoemaker) (PDF, 1615kb)

Attachment H: Performance Monitoring of the Spill Site 7 ZVI PRB (Stephen Hart) (PDF, 1351kb)

Attachment I: Six Years of Intensive Monitoring of the First PRB To Treat a Mixed Waste Plume: U.S. Coast Guard Site in Elizabeth City, North Carolina (Bob Puls) (PDF, 551kb)

Attachment J: Electrically Induced Redox Barriers (E-barriers) (Tom Sale) (PDF, 2352kb)

Attachment K: Biopolymer Slurry Wall Installation of a ZVI PRB at the Former Carswell Air Force Base, Texas (Cynthia Crane) (PDF, 1721kb)

Attachment L: Probabilistic Design and Construction Verification of a PRB at an Industrial Site in Virginia (Grant Hocking) (PDF, 1578kb)

Attachment M: PRB Installation Using Edible Oil Substrate (Bob Borden) (PDF, 767kb)

Attachment N: PRB as Part of an Integrated Containment Remedy at the DuPont Newport Site (Steve Shoemaker) (PDF, 1896kb)

Attachment O: Performance Monitoring Using Passive Diffusion Bag Samplers at the Somersworth Site (Karen Berry-Spark) (PDF, 587kb)

Attachment P: Recent Field Experience With Velocity and Directional Sensors (Bruce Sass): Part 1 (PDF, 2257kb) Part 2 (PDF, 2306kb)

Attachment Q: In Situ Monitoring of the Ten-Year Old PRB at CFB Borden, Ontario (Stephanie O'Hannesin) (PDF, 2220kb)

Attachment R: Long-Term Monitoring at the Sunnyvale PRB: Lessons Learned/Future Expectations (Scott Warner) (PDF, 614kb)

Attachment S: Long-Term Permeability and Reactivity of Granular Iron: Effects of Gas Formation and Mineral Precipitation (Lai Gui) (PDF, 478kb)

Attachment T: Spatial and Temporal Trends in Ground-Water Chemistry and Precipitate Formation at the Elizabeth City PRB (Richard Wilkin) (PDF, 1153kb)

Attachment U: Heterogeneity Development and Its Influence on Long-Term PRB Performance: Column Study (Wiwat Kamolpornwijit) (PDF, 826kb)