Title: Introduction Text: This video explains the process behind the reductive dechlorination of perchloroethene (PCE) and trichloroethene (TCE) to dichloroethene (DCE), vinyl chloride (VC), and ethene. Click on the buttons at left to view the video. When the reductive dechlorination process is incomplete, the levels of DCE and VC in groundwater can build up over time. This is referred to as DCE stall and it can limit the ability to meet cleanup goals and obtain site closure. This training tool discusses the suspected causes of DCE stall, along with potential solutions for this problem. Visual Direction: Video of chemical structure diagrams showing how perchloroethene and trichloroethene are converted into dichloroethene, vinyl chloride, and ethene by removal of chlorine and replacement with hydrogen.

Title: Causes of DCE Stall Text: Reductive dechlorination of PCE and TCE to DCE appears to be universal at sites where conditions are at least sulfate-reducing. At some sites, all of the necessary conditions for efficient, complete dechlorination of PCE or TCE to ethene are not present and degradation stalls at DCE. There are two basic requirements for a complete reductive dechlorination pathway including: 1) sufficient electron donor (usually derived from a fermentable carbon source) to achieve strongly reducing conditions and 2) bacteria capable of efficient dechlorination of DCE to ethene. Play the graphic animation to see this process in action. DCE stall will typically occur when there are either electron donor or biological limitations. It should also be noted that biological activity can be hindered at some sites by extreme conditions that are not related to the above requirements, including extreme pH, presence of biotoxins, micronutrient limitations, and other factors. In addition, the lack of VC, ethene, or ethane at a site may be attributed instead to the direct transformation of DCE to carbon dioxide via alternate pathways rather than reductive dechlorination. These alternate pathways include: anaerobic or aerobic oxidation (DCE to carbon dioxide) and abiotic degradation of DCE to carbon dioxide via mechanisms such as iron monosulfides. Visual Direction: Animated graphic showing effects of increasing electron donor. With low electron donor, aerobic microbes deplete oxygen. With increases, anaerobic organisms utilize nitrate, reduce iron and sulfate, and PCE and TCE are consumed, then finally DCE and VC.

Title: Biostimulation Text: Biostimulation refers to electron donor addition for the purpose of creating strongly reducing conditions and facilitating complete reductive dechlorination by indigenous microorganisms. Visual Direction: No graphic.

Title: Electron Donor Limitations Text: The first potential reason for DCE stall is a lack of sufficient electron donor (usually a fermentable carbon source) to achieve the necessary strongly reducing conditions. This occurs when either natural or introduced carbon sources are sufficient to achieve iron- or sulfate-reducing conditions, but are exhausted before the natural sulfate. The following examples present this phenomenon pre- and post-biostimulation. As shown in these examples, a lack of electron donor prevented conditions from being reducing enough to facilitate complete dechlorination. The solutions devised for these sites will be further discussed later on in the tool. Pre-biostimulation examples Post-biostimulation examples Visual Direction: Graphic of organisms only having enough electron donor to reduce iron. PCE, TCE, DCE and VC are not degraged.

Title: Pre-Biostimulation Examples Text: Test Area North: A former waste injection well (TSF-05) received a variety of organic and inorganic wastes for about 15 years until 1972. By the late 1990s, organic carbon in the groundwater was essentially zero with generally anoxic conditions. No dissolved iron was observed and sulfate levels were consistent with background conditions of 30 to 40 mg/L. Thus, several decades after waste disposal, the site was only under nitrate-reducing conditions. Naval Weapons Station Seal Beach Site 40: This site is a locomotive maintenance shop where solvents and some petroleum hydrocarbons were released over a period of several years. A natural attenuation evaluation previously concluded that reductive dechlorination was occurring in an isolated area, but primarily to cis-DCE, indicating a stall in the reductive dechlorination process. In July 2001, very little organic carbon was observed in monitoring wells at the site prior to a biostimulation pilot test. Sulfate levels ranged from 160 to 480 mg/L and the oxidation reduction potential (ORP) was approximately +150 mV. This is well above the ORP of -210 mV that signifies sulfate-reducing conditions. Donor Limitations Home Visual Direction: 1) Diagram of a site plan showing TCE concentration isopleths and a table of pre-biostimulation conditions for Test Area North. 2) Diagram of NWS Seal Beach Site 40 and table of contaminant concentrations.

Title: Post-Biostimulation Examples Text: Ft. Lewis, Washington East Gate Disposal Yard A Reductive Anaerobic Biological In Situ Treatment Technology (RABBIT) demonstration was conducted using butyric acid as an electron donor at this TCE-contaminated site. Initial TCE concentrations were about 5,200 ug/L, cis-DCE concentrations were about 5,800 ug/L, and vinyl chloride (VC) concentrations were about 12 ug/L. Butyric acid was injected continuously at 550 mg/L at a target rate of about 0.5 L/min into three injection wells. The downgradient monitoring well results indicated that cis-DCE accumulated, VC was produced at low levels, and ethene was not observed in significant quantities. During the demonstration, maintenance issues with the pump led to an accidental injection of a high concentration butyric acid slug. This increase in electron donor at Week 17 was coincident with a methane spike, a temporarily low ORP level, and VC production. Therefore, DCE stall was only mitigated when the electron donor was increased and recurred when electron donor was limiting following pump repair. The final report indicated that the redox potential was never depressed enough to achieve significant levels of cis-DCE dechlorination. The fact that methane and VC production followed the injection of a butyric acid slug support the conclusion that increased electron donor levels would have helped to address the DCE stall (Battelle et al. 2002). Visual Direction: Time lapse diagrams of the changes of TCE, cis-DCE, and VC from week 6 to week 24.

Title: Post-Biostimulation/Bioaugmentation (continued) Text: New Jersey Superfund Site At a site in New Jersey, one lesson learned was that bioaugmentation is typically not needed as long as D. ethenogenes is present at the site. Instead, biostimulation should always be performed first to ensure that a site is not strictly carbon limited. Although D. ethenogenes was detected at the New Jersey site, it was decided to perform bioaugmentation combined with the injection of lactate, acetate, and methanol as electron donors. Electron donor injection was first initiated in February 2001. After more than 1 year of electron donor injection, the primary terminal electron accepting process appeared to be sulfate reduction. Reductive dechlorination of TCE stalled at cis-DCE at most wells with insignificant production of VC or ethene. In one exception, monitoring well B-23, significant VC was produced, but negligible ethene. To address the DCE Stall, the amount of electron donor was increased in February 2002 and again in December 2002. The use of acetate was stopped in February 2002 and lactate, ethanol, and methanol were applied at a higher rate. With the increased concentration of electron donor, impacted wells saw significant conversion of DCE to VC to ethene for the first time. This observation confirmed the fact that the site was strictly carbon limited, not biologically limited. Donor Limitations Home Visual Direction: Graph showing changes in PCE, TCE, c-DCE, VC, and ethene concentrations from February 2001 to June 2002.

Title: Biological Limitation Text: The second possible reason for DCE stall is that no bacteria are present at the site that are capable of efficiently dechlorinating DCE to ethene. Dehalococcoides ethenogenes was the first microbe isolated in a laboratory that was demonstrated to be capable of complete dechlorination of PCE or TCE to ethene. There are likely other microbial strains that are capable of this transformation, but they have not been cultured yet in a laboratory setting. Characterization of microbial communities at sites all over the world has revealed that D. ethenogenes is present in a wide variety of environments, but is not ubiquitous. For example, in a survey of dechlorinating sites in North America and Europe, it was observed that D. ethenogenes was detected at all 21 sites with complete dechlorination, and none of the 3 sites with DCE stall. Click here to see examples Dover AFB Kelly AFB Visual Direction: Map of the U.S. and Europe with different cities identified as having complete dechlorination, partial dechlorination, or a bioaugmentation plot.

Title: Biological Limitation Examples Text: Dover AFB (EPA 2000) A biostimulation pilot test was conducted from September 1996 to May 1997 with lactate as an electron donor, resulting in conversion of TCE to cis-DCE. In spite of complete removal of sulfate and production of methane, no degradation to VC or ethene was observed. Complete dechlorination to ethene was observed after a 3-month lag following addition of 350 L of culture containing D. ethenogenes - after 9 months, TCE and DCE were absent. Back to Biological Limitations Home Visual Direction: Animated cross-section diagram of contaminated groundwater at Dover AFB that was injected with D. ethenogenes culture to degrade the contaminants. Diagram shows injection solution, injection wells, contaminated groundwater, and recovery wells.

Title: Kelly AFB (Major et al. 2002) Text: Kelly AFB (Major et al. 2002) Acetate and methanol addition at this site began on Day 89. After 87 days of this addition, dechlorination of TCE had only proceeded to cis-DCE as shown in this graph of sampling results from a nearby monitoring well. On Day 176, a D. ethenogenes-containing culture was added and complete dechlorination to ethene was observed about 75 days later. Back to Biological Limitations Home Visual Direction: Graph showing the change in concentrations of PCE, cis-DCE, TCE, VC, and ethene over 300 days at Kelly AFB.

Title: NWS Seal Beach (BEI 2002) Text: NWS Seal Beach (BEI 2002) A 6-month biostimulation pilot study was performed at this site beginning in August 2001 to treat PCE using lactate as an electron donor. At the end of the study, complete conversion of PCE to cis-DCE was observed along with methano-genesis, but no VC or ethene. DNA analysis revealed that D. ethenogenes could not be detected at the site. Bioaugmentation was later implemented at this site and the results are discussed under the solution strategies section of this tool. Back to Biological Limitations Home Visual Direction: Figure of compound concentrations vs. date for ethene, VC, DCE, TCE, and PCE at NWS Seal Beach.

Title: Solution Strategies Text: A variety of potential strategies for solving the DCE stall problem exist as follows: No action Monitored natural attenuation (MNA) MNA - Wait for complete dechlorination to ethene Biostimulation for reductive dechlorination (addition of electron donor) Bioaugmentation for reductive dechlorination (addition of D. ethenogenes) Biostimulation for enhanced biological oxidation The best strategy will be dependent on site-specific issues including: Initial contaminant concentrations Acceptable cleanup time frame Travel time to receptors Course of events that led to DCE stall Acceptance of innovative technologies (risk tolerance) Budget For each of the above solution strategies, the site-specific parameters that should be evaluated will be discussed. Visual Direction: No graphic.

Title: Solution Strategy: No Action Text: It may be appropriate to take no additional steps to remedy DCE stall at sites with low parent compound concentrations. In this case, DCE stall may not prevent meeting cleanup standards at the site because DCE is less toxic than PCE or TCE. As shown in the Table, the cleanup standard for cis-DCE is 14 times higher than PCE or TCE. As long as the conditions at the site are at least sulfate-reducing, the conversion of PCE and TCE to cis-DCE should occur in a matter of months. This approach will only work at sites with sufficient carbon to result in the conversion of PCE and TCE to cis-DCE. This approach is considered a "no action" strategy because once the degradation of PCE and TCE to cis-DCE is accomplished no additional steps will be taken to remedy the DCE stall. With low initial PCE and TCE levels, the resulting cis-DCE levels will likely be well below the 70 µg/L cleanup standard. Visual Direction: Table of chemicals of concern and their corresponding maximum contaminant levels.

Title: No Action Advantages and Limitations Text: Advantages: At some sites, very fast conversion from PCE or TCE can be naturally stimulated in a few months. In this case, no further action would be taken to remedy the DCE stall because of the less stringent cleanup standards for DCE compared to PCE, TCE, or VC. Travel time to receptors is not important because cleanup standards can be met quickly. Biological limitations are not a concern and may even be an advantage because DCE stall is preferable to the presence of elevated levels of the more toxic VC compound. Conversion of PCE and TCE to cis-DCE is almost universal in the presence of sufficient carbon. Limitations: Only applicable for low concentrations PCE or TCE sites. Requires good electron donor distribution. Visual Direction: No graphic.

Title: Solution Strategy: MNA Text: Many more natural attenuation pathways exist for DCE compared to PCE or TCE. For PCE, the only reported biological pathway is reductive dechlorination. For TCE, in addition to reductive dechlorination, intrinsic cometabolic oxidation has been observed, but only at slow rates. For DCE, in addition to those pathways for PCE and TCE, direct oxidation has been reported. In addition, the lower toxicity of DCE relative to PCE and TCE may bring natural attenuation pathways into play that would not have existed for the parent compound. Natural attenuation pathways for DCE are listed below: Dilution/Dispersion Cometabolic Oxidation Direct Oxidation Abiotic Degradation In general, natural attenuation rates for DCE are much higher under mildly reducing or aerobic conditions. Visual Direction: No graphic.

Title: Natural Attenuation Pathways for DCE Text: Dilution/Dispersion Pathway: Given sufficient distances from receptors, the DCE levels will be attenuated by dilution and dispersion as groundwater flows downgradient. Because DCE is less toxic and has a higher MCL than PCE and TCE, this mechanism may be more acceptable for higher concentrations of DCE compared to PCE and TCE. Click here to return to MNA Home Visual Direction: Graphic of dilution/dispersion showing plumes of TCE and DCE concentrations decreasing with distance as they spread and approach the receptor. TCE is still 20X the MCL but DCE is near the MCL.

Title: Natural Attenuation Pathways for DCE Text: Cometabolic Oxidation Pathway: PCE is not subject to any biological oxidation either through direct or cometabolic pathways. However, TCE, DCE, and VC can all undergo cometabolic oxidation. This process is a chance oxidation of the contaminants that occurs while bacteria are producing enzymes for other purposes such as methane oxidation. Click here for more information. Click here to return to MNA Home Visual Direction: Animated graphic showing cell enzymes binding to methane and more enzymes being released to oxidize DCE and TCE.

Title: Cometabolic Oxidation Information Text: Although cometabolic oxidation is not often considered to be significant for MNA, it has been observed in at least one case to be important (Sorenson et al. 2000). Cometabolic oxidation may be especially important following generation of methane during biostimulation, though this has not been demonstrated in the field. This approach was included in a pilot test work plan for Naval Weapons Station (NWS) Seal Beach, Site 40. Visual Direction: No graphic.

Title: Natural Attenuation Pathways for DCE Text: Direct Oxidation Pathway: Neither PCE nor TCE can be used directly as a carbon source by any known biological process. DCE, however, has been shown in a few studies to undergo direct oxidation as a sole carbon source. Klier et al. (1999) and Bradley and Chapelle (2000) showed DCE mineralization (conversion to carbon dioxide) by mixed laboratory cultures. *Coleman et al. (2002) were able to isolate a bacterium that used DCE as a sole carbon source. *It was noted that this activity appeared to be uncommon, even at sites contaminated with DCE (only 2 of 18 samples were positive) Click here to return to MNA Home Visual Direction: Animated graphic of organism utilizing DCE as the electron donor with electron acceptors such as oxygen or nitrate present.

Title: Natural Attenuation Pathways for DCE Text: Abiotic Degradation Pathways: Abiotic degradation of chlorinated ethenes by iron sulfides and other reactive inorganics may occur naturally under some site conditions. Iron sulfides are very common at sites with reducing conditions due to iron and sulfate reduction. The ferrous iron acts as an abiotic electron donor like zero-valent iron, thereby reducing chlorinated ethenes. Abiotic degradation in the presence of iron sulfides may also occur at some sites following biostimulation. In the post-biostimulation case, DCE stall may be reached through an active remedy, but then may be mitigated through natural abiotic processes. An example of how this natural process can enhance an active remedy is shown here. Reductive dechlorination appeared to stop at DCE downgradient of a mulch biowall at Altus AFB, but DCE levels did not accumulate and migrate downgradient (Henry et al. 2003). A 90% reduction in total moles of VOCs without the accumulation of DCE, and without generation of VC or ethene was cited as evidence that DCE stall was being managed through natural processes (Figure A). A combination of mechanisms including reactions with iron sulfide, which is abundant at the site, may be responsible for the observed contaminant behavior (Figure B). Click here to return to MNA Home Visual Direction: Figure A is a graph of total molar concentration of VOCs vs. distance from the biowall (upgradient and downgradient) in July 2002, Sept. 2002 and March 2003. Figure B is a table with concentrations of TCE, cDCE, and VC at different well locations.

Title: MNA Advantages and Limitations Text: Advantages: DCE can be degraded under a wide range of redox conditions including mildly reducing and even aerobic conditions. May be very useful in combination with biostimulation. Relatively low cost as monitoring and reporting are the only major activities. Limitations: Generally not applicable for high concentrations. Travel time to receptors still a consideration. Lack of dissolved oxygen (DO) may limit degradation rates for direct oxidation pathways. Some DCE degradation mechanisms have only been demonstrated recently and regulatory acceptance may require additional sampling and some education. Visual Direction: No graphic.

Title: Solution Strategy: MNA Wait Text: A less conventional approach to MNA for DCE that might be applicable to low risk sites is to wait an extended period of time, while monitoring, to see if complete dechlorination eventually occurs. This will only be possible if a site is not electron donor limited, and redox conditions are appropriate for recruitment and growth of D. ethenogenes. In the case of biological limitations, it is possible that establishment of a viable D. ethenogenes population may just require sufficient time with the appropriate conditions for their growth. This idea is currently based only on ecological principles, deductive reasoning, and some preliminary data. A potential example of this phenomenon is illustrated by a case study from a dry cleaner site. Visual Direction: No graphic.

Title: Sages Cleaners Site Text: The Sages Cleaners Site in Florida initially had PCE contamination with only minor amounts of dechlorination degradation products. An ethanol flood was performed to remove DNAPL, leaving high concentrations of ethanol in the ground with residual PCE. Conversion of PCE to cis-DCE was significant after about 13.5 months, but no significant dechlorination to VC and ethene occurred until about 40 months post-flush. Thus, it appears that biology rather than electron donor was limiting for a period of about 3.5 years, but D. ethenogenes eventually seems to have grown to sufficient numbers to facilitate dechlorination to ethene. Click on the buttons to view constituent concentrations over time. (Data used courtesy of S. Mravik, EPA, Ada, OK) Visual Direction: Animated time lapse graphics of contaminant concentrations at the Sages Cleaners site for ethanol, PCE, cis-DCE, VC, and ethene.

Title: MNA Wait Advantages and Limitations Text: Advantages: May be technically sound and cost-effective at low risk sites, allowing resources to be focused on higher risk sites. Applicable for relatively new natural attenuation sites (co-contaminant with non-chlorinated organics and chloroethenes) or biostimulated sites that are not carbon limited. Limitations Probably only applicable at low-risk sites because may require 3 to 4 years of waiting or more. Site must not be carbon limited. Has not been rigorously demonstrated at multiple sites. May be perceived as a stall tactic or an attempt to avoid taking cleanup action. Visual Direction: No graphic.

Title: Solution Strategy: Biostimulation Text: Biostimulation may be appropriate for sites that appear to exhibit DCE stall under "natural" conditions and are electron donor-limited. Biostimulation alone may be sufficient to mitigate DCE stall as illustrated in Example 1. It may also be used at sites that are both electron donor- and biologically-limited to prepare the site for bioaugmentation as shown in Example 2. It is often difficult to determine whether or not a site has biological limitations based solely on DNA testing or laboratory microcosms. For this reason, it is recommended that biostimulation be tested first for a few months at any site where bioaugmentation is under consideration. Visual Direction: Animated cartoon of electron donor increasing and organisms utilizing sulfate and then CO2 and DCE/VC simultaneously.

Title: Biostimulation Example 1 Text: DOE's Test Area North: As discussed previously, the site was carbon limited and no dechlorination beyond DCE was apparent. Biostimulation using sodium lactate as an electron donor was begun in January 1999 resulting in complete dechlorination of TCE to ethene. Biostimulation Home Visual Direction: Graph of TCE, cis-DCE, trans-DCE, VC and ethene concentrations vs. date for Test Area North showing a decrease in chlorinated compounds and an increase in ethene.

Title: Biostimulation Example 2 Text: NWS Seal Beach: A previous natural attenuation evaluation concluded reductive dechlorination was occurring in isolated areas, but primarily to cis-DCE. In July 2001, very little organic carbon was observed prior to the biostimulation pilot test; sulfate ranged from 160 to 480 mg/L, and ORP was approximately +150 mV (BEI, 2002). Biostimulation via sodium lactate addition began in August 2001. The treatment area was initially dominated by PCE and dechlorination proceeded to DCE within about 2 months, but DCE stall was encountered even after 6 months, and no D. ethenogenes could be detected at the site. After confirmation that biostimulation alone was not sufficient, a bioaugmentation demonstration was performed as discussed in the next section. Biostimulation Home Visual Direction: (1) Schematic of a system used to inject sodium lactate solution into a well. (2) Graph of COD concentration over time at different wells. (3) Graph showing the results of reductive dechlorination at a well. PCE and TCE decreased but DCE increased.

Title: Biostimulation Advantages and Limitations Text: Advantages: Can be effective from low concentrations up to residual DNAPL conditions. Much faster than MNA (assuming carbon is limited). Treatment generally occurs within electron donor distribution area, so the proximity of receptors is less important than for MNA. For sites that are already at least mildly reducing, biostimulation works with tendencies of natural system. Biostimulation is among the least expensive active treatment technologies. Limitations: Only applicable at electron donor-limited sites. Will require long cleanup times if large volumes of DNAPL are present. Lag times are likely to be longer at aerobic sites both for the onset of dechlorination and for complete dechlorination. Extremely high bioavailable iron, or high sulfate may present challenges. Other environmental factors such as extreme pH, high sulfide, other chemical toxicity issues, and trace nutrient limitations may limit biostimulation, but are uncommon. Visual Direction: No graphic.

Title: Solution Strategy: Bioaugmentation Text: Bioaugmentation may be a viable option at sites without the appropriate native microbes to complete DCE biodegradation. This application involves the addition of a D. ethenogenes containing microbial culture to site groundwater to facilitate complete dechlorination to ethene. Examples of this approach are provided below: Dover AFB NWS Seal Beach Bachman Road Site Visual Direction: Animated cartoon of electron donor and native microbes that can not degrade DCE/VC. Bioaugmentation is applied to add a microbial culture of D. ethenogenes, which then degrades DCE/VC.

Title: Bioaugmentation Example 1 Text: Dover AFB Biostimulation was performed for 269 days by injecting lactate into a TCE-contaminated aquifer that was initially aerobic. DCE stall was encountered, as observed in previous microcosm and column studies using site media. Bioaugmentation was performed by injecting 351 L of D. ethenogenes-containing culture enriched from DOE's Pinallas site. Following a 90-day lag period, dechlorination of DCE to ethene began. After 509 days, only ethene was detected in the test area (Ellis et al. 2000). Bioaugmentation Home Visual Direction: Four graphs of TCE, cDCE, VC, and ethene levels plotted over time as a result of biostimulation and bioaugmentation at Dover AFB.

Title: Bioaugmentation Example 2 Text: NWS Seal Beach A 6-month biostimulation pilot conducted at a PCE-contaminated site using lactate resulted in DCE stall. DNA analysis revealed an absence of D. ethenogenes. Bioaugmentation was begun in April 2003 through addition of 20 L of D. ethenogenes-containing culture (KB-1) in each of two inoculation wells. Within one month of inoculation, monitoring of the inoculation wells and two downgradient wells revealed some conversion of DCE to VC, although DCE persisted (Figure A). After several more months, all contaminants were near MCLs in the treatment area although DCE loss was not fully accounted for by the VC and ethene generated. D. ethenogenes DNA was detected in wells about 8 ft downgradient from the inoculation wells after 3 months, and about 16 ft downgradient in 4 months. Real-time polymerase chain reaction (PCR) was performed at the University of California Berkeley to monitor the survival and proliferation of D. ethenogenes (Figure B). Bioaugmentation Home Visual Direction: Figure A is a graph of contaminant levels at NWS Seal Beach before and after bioaugmentation. Figure B is a bar graph of quantitative PCR results for Dehalococcoides from April through August 2003 at different wells on site.

Title: Bioaugmentation Example 3 Text: Bachman Road Site A test to compare bioaugmentation and biostimulation was performed side-by side at a shallow PCE- and DCE-contaminated site in Michigan. The bioaugmentation culture was enriched from the contaminated site and 200 L was injected in an area that initially exhibited DCE stall. As the area was carbon limited, a biostimulation test was performed nearby using lactate as an electron donor. The bioaugmentation plot showed nearly complete conversion of DCE to ethene in 43 days. The biostimulation plot experienced a 3-month lag, but showed nearly complete conversion to ethene after 121 days (Lendvay et al. 2003). Bioaugmentation Home Visual Direction: Figure shows layout of test plots for comparing bioaugmentation and biostimulation at the Bachman Rd site. Three additional figures show resulting aqueous concentrations of contaminants for control plot, bioaugmentation plot and biostimulation plot.

Title: Implementing Bioaugmentation Text: Is biostimulation sufficient to meet site needs (including schedule)? Are redox conditions suitable for bioaugmentation? Is the site electron donor limited? Are environmental conditions suitable for bioaugmentation (e.g., pH, competing electron acceptors, biotoxins, etc.)? Can bacteria be disturbed cost-effectively in a relevant time frame for site cleanup goals? Dover AFB NWS Seal Beach Bachman Road Site Bioaugmentation Home Visual Direction:

Title: Bioaugmentation Advantages/Limitations Text: Advantages: May help to overcome biological limitations at a site, provided that electron donor limitations or other factors have been ruled out as the cause of the DCE stall. Potentially reduces lag times. Limitations: Effective distribution of the bacteria is not trivial; may require large volumes of culture in a few locations, small volumes in many locations, construction of a recirculation system, or a lot of time to achieve. Cost of effective distribution may be a significant challenge and a method for effective delivery of cultures into the subsurface must be developed such as direct push points, injection wells, etc. Distribution or mode of transport throughout the aquifer should be considered (e.g. recirculation wells, multiple direct push points coupled with chemotaxis, etc.). Will the microbes be transported from the point of introduction passively or through active engineered methods? Organism morphology probably will play a large role in their transport. These cultures do not all have the same physiological characteristics and some will likely transport better than others. A case study of results from one culture in one location may not be directly transferable to a different site with a different hydrogeologic and geochemical environment. There may be competition with native cultures. Competitive exclusion of the introduced culture with existing microbes may create conditions in which there is a low survivability. The augmented cultures are usually grown in a bioreactor under ideal conditions. Once introduced to a different geochemical environment they may not perform as expected. In other words there may be some survival, however, their ability to produce the functional enzymes may be compromised. Business models vary considerably for use of this approach and costs can be high compared to performance benefits. (Cultures sold by the liter vs. unlimited supply for site based on license). The most appropriate way to monitor these sites has not been fully developed. Recent work in molecular biology promises better techniques, but these are not fully developed at present. In other words, the mere presence of the organism may not be as helpful as determining whether it is generating the necessary enzymes. Visual Direction: No graphic.

Title: Solution Strategy: Enhanced Biological Oxidation Text: An alternative approach to dealing with biological limitations is to switch from anaerobic biodegradation processes to aerobic degradation processes. DCE and VC are both susceptible to cometabolic and catabolic (for growth) degradation under aerobic conditions. The injection of oxygen into a zone already containing DCE and/or VC, along with methane (or another substrate that facilitates cometabolism), will result in the growth of methanotrophs and ultimately cometabolic oxidation will occur. Pt. Mugu Example Visual Direction: Cartoon figure showing addition of oxygen allowing microbes to use methane as a substrate and resulting in release of oxygenase to break down DCE and VC.

Title: Enhanced Biological Oxidation Example Text: Naval Base Ventura County - Pt. Mugu Biostimulation was performed beginning in December 1998 using lactic acid to treat TCE and DCE. Complete and rapid conversion to VC was accomplished, with much slower conversion to ethene. In other words, VC stall was encountered rather than DCE stall. Microcosm experiments suggested that the absence of TCE may have stalled further dechlorination of VC. This implies that VC dechlorination was cometabolic and was induced or accelerated by the presence of TCE (L. Semprini, personal communication). A pulsed flow strategy of oxygen and methane injection was undertaken for 3 months to induce cometabolic oxidation of VC. In principle, this approach should also work for the cometabolic oxidation of DCE, but it has not been demonstrated at the field-scale. Enhanced Biological Oxidation Home Visual Direction: 1) Map of site layout, 2) schematic diagram of system, 3) Diagram including all components between the extraction well and injection well, 4) Graph showing VC and dissolved methane concentrations over time for three sampling locations.

Title: Enhanced Bio Oxidation Advantages/Limitations Text: Advantages: The bacteria involved in this process are ubiquitous. Bacterial growth rates and potentially degradation rates are higher under aerobic conditions. Much faster than MNA (assuming electron donor is limited). Treatment generally occurs within oxygen and methane distribution area, so proximity of receptors less important than for MNA options. Cometabolic oxidation is a fairly well established and accepted process. Limitations: Aboveground equipment requirements are greater than for other options. Concentrations higher than a few mg/L can inhibit the reaction. Reduced aquifers need to be "peroxidized" with oxygen. Design effort may be more intensive than other options. Biofouling can be a problem because of higher growth rates. Probably more expensive than anaerobic options. Visual Direction: No graphic.

Title: Site Specific Assessment Text: The following decision flow diagram has been developed to guide site management decisions. Please use the flowchart to help determine the best course of action at your site. Question: Are you experiencing DCE stall at your site? Yes, No, Unsure, Not Applicable Visual Direction: Summary flowchart to guide site management decisions. Flowchart can be enlarged, printed, or used interactively as a decision tree.

Title: Summary Text: The purpose of this tool was to provide Remedial Project Managers (RPMs) with the information necessary to identify underlying causes of DCE stall and to present solution strategies applicable to site specific conditions. The phenomenon of DCE accumulation in the reductive dechlorination process is commonly referred to as "DCE stall." DCE stall is typically caused by insufficient electron donor to achieve strongly reducing conditions. Under less reducing conditions, the DCE concentrations in groundwater may accumulate without the apparent accumulation of VC, ethene, or ethane. At these sites, biological activity may be hindered by lack of sufficient electron donor. It can also be impacted extreme conditions such as pH, presence of biotoxins, micronutrient limitations, and other factors. It is also possible that the expected products of VC and ethene are not formed because microbial oxidation or abiotic pathways are dominant (e.g. DCE transformation directly to carbon dioxide). All of these factors should be carefully considered before exploring a biostimulation or bioaugmentation approach at a given site. As discussed above, there are several practical limitations to the effectiveness of bioaugmentation including the survivability and distribution of introduced microbes within the subsurface. Although the use of bioaugmentation has been demonstrated at the lab-scale and with small-scale field projects, it has not been demonstrated to be effective at the full-scale and use of this technology is not currently encouraged Visual Direction: No graphic.

Title: Contact Information Text: For more information about strategies for addressing DCE Stall, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil Visual Direction: No graphic.

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