U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

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PRB Update

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Remaining Simulcast Questions and Answers from October 11, 2012

Question 1: (question was noted on call for further follow up) Are there any good examples of PRB technologies being implemented in cold climates? - Environmental Consultant; Toronto, Canada

  • (trainer name): (response to be added)

Question 2: Other than the Elizabeth PRB closure mentioned by the last speaker, are there other sites where PRBs have been closed? Any thoughts on the prospects of unrestricted future land use where a PRB was used to attain groundwater standards? Realizing different regulatory requirements, as well as type of PRB, may determine whether the PRB could be left in place, it sounds like keeping a PRB in place would not necessarily exclude an unrestricted future land use. Your thoughts appreciated. - Environmental Consultant; Syracuse, NY, United States

  • (trainer name): (response to be added)

Question 3: Are you aware of any media that could address selenium? U.S. EPA participant; Helena, MT

  • (trainer name): (response to be added)

Remaining Simulcast Questions and Answers from April 10, 2012

Question 1: The Speaker mentioned that Biowalls can be replenished by used oil. Can used oil from car maintenance and from frying food be reused for that purpose? If so, that will be a wonderful example for recycling used oils. Thank you. - State Regulator; cypress, CA, United States

  • John Doyon: The types of oils that may be used would typically be food grade oils. Theoretically, "recycled" food grade oils may be used, but I'm personally not aware of any commercially avialable products that market themselves as such.

Question 2: At Chalk River what were the Sr-90 (Strontium-90) influent activity/concnetration (pCi/l)?

  • John Doyon: The influent water has a gross beta measurement of 85 Bq/L which is reduced to .6 Bq/L. The standard is 5 Bq/L.

Question 3: How is the soil waste from trenching handled normally? Thanks! - Environmental Consultant; Orange, CA, United States

  • John Doyon: The way the excavated soil is handled will depend on the level and type of contamination and the regulatory restrictions depending on whether is classified as a RCRA waste, and what constraints state and local regulations/ordinances may play in directing the material. Typically, the unsaturated zone soil would not have been impacted and should be able to be placed back into the excavation over top of the reactive media.

Question 4: Is subsidence a consideration with mulch PRBs, as the material decomposes over time? - Environmental Consultant; Lakewood, CO, United States

  • Bruce Henry: Yes, compaction of the mulch mixture may occur depending on the ratio of mulch to sand in the mixture. I have only seen a biowall settle 12 to 18 inches at the most for a 50:50 mulch to sand ratio. In practice we often mound soil about a foot over the biowall, then go back and re-grade a year or so later if settling occurs. Active areas with roads, parking, sidewalks, etc. warrant special consideration for safety. For low loads (walkways) steel plating can be placed across the trench. A geotechnical engineer should be consulted for areas requiring support of higher loads (roadways).

Question 5: Can this be used as a preventative method for sites that are at high risk for releases (i.e. refineries and gas stations)? - State Regulator; Norristown, PA, United States

  • John Doyon: It could theoretically be used for such a purpose, but it would most likely be more cost effective to design these systems not to leak in the first place or to have some sort of a recovery system in place rather than put in this type of a system when it may not be needed.

Remaining Simulcast Questions and Answers from January 24, 2012

Question 1: How many of the PRBs systems have been installed at privately owned facilities? - Environmental Consultant; Virginia, MN, United States

  • Cannon Silver: The previous PRB-4 document published in 2005 included a table listing 84 full-scale PRB installations, of which two-thirds were installed at privately-owned facilities. Since 2004, numerous additional PRBs have been installed, including many at non-government facilities.

Question 2: Have you been involved in creating a bacterial and nutrient barrier to treat btex contamination in a groundwater plume? - Environmental Consultant; New Baltimore, MI, United States

  • Cannon Silver: No. I have been involved in several groundwater treatment systems that injected air or oxygen to stimulate biodegradation of BTEX compounds, which provided adequate treatment to meet remedial goals. Supplemental addition of bacteria or nutrients was not necessary at these sites, but could be engineered if deemed necessary.

Question 3: For mulch biowalls, could you please expand on the amounts of mulch per unit volume that are used, and what other amendments it is typically mixes with (for a silty-sand aquifer)? Is settling observed over time as the mulch is "consumed"? - Environmental Consultant; Chicago, IL, United States

  • Bruce Henry: Mulch is typically added at 40 to 60 percent by volume with the remainder primarily sand. Small gravel up to 10 percent by volume may be added as a weighting material. Other organics may also be added such as compost (up to 5 percent by volume) or vegetable oil (1-2 percent by volume) to supplement the mulch. The ratio of sand to mulch should be based on 1) the permeability of the surrounding formation, and 2) the amount of settling or compaction that is acceptable. The higher the permeability of the surrounding formation and the less settling and compaction that is acceptable, then the higher the ratio of sand that should be used. It is advisable to obtain samples of the mulch and sand and to mix and test various ratios with geotechnical compaction tests if the PRB will need to support a load (e.g., roadways). Also note that sand will fill void space within the mulch, so there is a reduction in overall volume when mixing volumes of sand and gravel, perhaps on the order of a 40 percent reduction in total volume. Again, mixing samples at different ratios will help determine what the reduction is, and what the final density of the mixture will be.

Question 4: How challenging it is to create a PRB wall with good integrity (good media distribution)? - Environmental Consultant; Shanghai, China

  • Cannon Silver: This can be challenging, particularly when placing material that differs in density (e.g., mulch insufficiently mixed with sand/gravel) or injecting material into a heterogeneous lithology. However, following best management practices, including those identified in the PRB-5 document, allow PRBs with good integrity to be routinely installed. Quality controls and monitoring, both during installation (e.g., coring, tilt meters) and afterwards (e.g., hyrdaulic, geochemical) help confirm the integrity of the installed PRB.

Question 5: Although not explicitly stated, it seems that PRBs are a management of migration technology that can be used to protect downgradient receptors, but is NOT a source remediation technology. So it really should be combined with a source control technology to be effective long term. Do you agree? - Environmental Consultant; Concord, NH, United States

  • Bruce Henry: Yes, the concept of a PRB as part of a treatment train is emphasized in the PRB update document. The duration for which the PRB must sustain its performance should relate to the time it will take for attenuation or remediation of the plume source.

Question 6: For treating a nitrate plume, is there an upper end concentration beyond which a PRB is not feasible? Most of the literature shows beginning nitrate concentrations of <100 ppm nitrate. Can you also touch on challenges to a nitrate PRB where significant groundwater ammonia concentrations are present? - Environmental Consultant; Saskatoon, Canada

  • Eric Nuttall: A) Most nitrate plumes are below 200 ppm. I have successfully used a PRB of molasses to treat a nitrate plume with about 160 ppm. Nitrate values above 200 ppm may require two PRBs in series. B) Ammonia plus nitrate is a problem/challenge. The ammonia is a long term source of nitrate and is not treated anaerobically. There are various strategies to address combined ammonia/nitrate plumes.

Question 7: In addition to arsenic, what other chemical compounds can iron and steel slag be used for PRB treatment? - Environmental Consultant; Charleston, WV, United States

  • Eric Nuttall: Iron will reduce and precipitate various metals such as uranium, hex chrome, selenium, etc. The ITRC BCR mining team is completing a document on this topic.

  • Peter Zawislanski: Slag has also shown potential for treating chlorinated ethenes and ethanes, as well as phosphate.

Question 8: With California likely lowering the Maximum Contaminant Level(MCL)for hex chrome, what are the lowest downgradient concentrations or detection limits observed with PRBs for hex chrome? - Environmental Consultant; Marina, CA, United States

  • Eric Nuttall: Recently Kleinfelder, Inc., treated a hex chrome plume with injections of an EOS type carbon source. The chrome level went to non-detect, i.e., well below the MCL.

Question 9: What happens when absorbent materials completely dissolve? - State Regulator; Arkansas, United States

  • Peter Zawislanski: The sorptive materials used in PRBs have very low solubility in groundwater. For instance, zeolites and apatites are minerals with solubility constants around 10^-20. This is much lower than, for example, that of limestone, which has a solubility constant of around 10^-9. Although some dissolution of these media will take place over time, it will be a very slow process. Similarly, the solubility of organoclays is effectively negligible.

Remaining Simulcast Questions and Answers from October 18, 2011

Question 1: Please speak to the role temperature plays in the effectiveness of PRBs and describe your experience with PRBs in cold climates. - Engineering Firm Participant; Chugiak, AK, United States

  • Bruce Henry: Most biological and chemical reactions that occur in the subsurface are influenced by temperature. While I do not have experience with chemical reactions such as with ZVI, I can share my experience with biological degradation of chlorinated solvents in Alaska. The impact of temperature on native dechlorinating species is often overlooked in bioremediation studies (Friis et al., 2007). Bradley et al. (2005) report that TCE was dechlorinated to cis-DCE and VC in microcosms constructed with soil and groundwater collected from two sites in Alaska. The microcosms were incubated at 4 degrees Celsius and spiked with radiolabeled (carbon 14) TCE, cis-DCE, and VC. However, dechlorination to ethene or ethane was not observed, and reductions in cis-DCE and VC (ranging from 25 to 70 percent) were attributed to anaerobic oxidation based on an accumulation of radio-labeled carbon dioxide. Bradley et al. (2005) concluded that assumptions regarding low to insignificant microbial activity at water temperatures below 5 degrees Celsius do not consider the presence of cold-adapted (psychrotolerant and psychrophilic) microorganisms. However, it is not clear from this study which microorganisms are facilitating the dechlorination reactions and whether oxidation of DCE and VC will occur at rates sufficient to limit accumulation. Data collected for enhanced bioremediation demonstrations at the DP98 Site and the Kenney Avenue Plume at Elmendorf AFB showed near molar conversion of TCE to cis-DCE, with limited dechlorination to VC or ethene. Microbial data indicate that the growth of native Dehalococcoides species was limited under conditions of 7 to 8 degees Celsius, with only trace detections. In 2002, the KB-1 bioaugmentation culture was used at a site contaminated with PCE at the River Terrace Site in Soldotna, Alaska (Oasis Environmental). Prior to bioaugmentation, PCE was converted through TCE and stalled at cis-DCE. VC concentrations peaked 1 to 2 years after bioaugmentation, and some ethene production was observed approximately 2.5 years after bioaugmentation. This suggests that while Dehalococcoides may be able to grow at low groundwater temperatures, the rate of growth and dechlorination activity will be slow.

Remaining Simulcast Questions and Answers from August 25, 2011

Question 1: Is there experience with injecting media (e.g. EZVI) into fractured bedrock (e.g. to remediate TCE), and where can examples/descriptions of this information be obtained? - Environmental Consultant; Ottawa, Canada

  • Cannon Silver: Yes, Section 6.2.3. of the ITRC PRB-5 document mentions pneumatic fracturing and injection applied to low permeability and bedrock sites, including highly weathered fractured bedrock at depths up to 160 feet (50 m). Project descriptions are available at the CLU-IN.org website: http://www.clu-in.org/products/fracrock/viewsites.cfm (search for "ZVI" or "EHC").

Question 2: How long after the construction of the barrier (and how often) should the permeability be measured to evaluate any possible problems with the flow of groundwater through the PRB? - University Participant; Rome, Italy

  • Scott Warner: Hydraulic conditions within the PRB can be evaluated using several methods. The most common include water level measurements through the monitoring well network and development of potentiometric flow maps. The general rule is to develop these quarterly (though some sites are now in semi-annual to annual monitoring). For direct assessment of permeability (or hydraulic conductivity) that may be developed using tracer tests (single well dilution tests, primarily), some sites use 5-year intervals (following a baseline assessment within the first year after construction) to make these assessments, providing there are no other indications of substantial permeability reductions (as determined through water level measurements).

Question 3: How would it be wise to install a 4-foot think ZVI wall WITHOUT intentionally mixing something granular? Seems like one would just be overlooking conductivity decreases as compared to the aquifer either at time of installation, or later as some oxidation and/or precipitation occurs. - Environmental Consultant; Nashville, TN, United States

  • Scott Warner: We assume this question is referring to the discussion and photograph of the first commercial installation back in 1994. At that time, without a long history of field evidence, the 4-ft ZVI system was intended primarily to provide enough capacity for the chemical treatment with the 2-ft pea gravel sections (before and after the ZVI core) to assist with hydraulics. During construction, it was observed that the gravel and ZVI mixed and formed a transitional zone. Later monitoring and assessment indicated the positive benefit of this mixed zone from both a hydraulic and geochemical perspective. That being said, even without the mixed zone, detailed monitoring by the USEPA for the Elizabeth City, NC Coast Guard Station suggested that the rate of mineralization is no more than a percent or two a year relative to potential pore-space filling. Therefore, permeability loss in the 100% ZVI core still is considered a slow, though chronic, condition that still should not interfere in the overall treatment for many years.

Question 4: If the ZVI walls can continue successfully for 15 years, why doesn't the patent holder guarantee that for something that is at least similar, rather than a year or 2? Do you have any thoughts on that? - Environmental Consultant; Nashville, TN, United States

  • Multiple instructors: Questions concerning patents should be addressed to the patent holder directly. While you can expect a ZVI wall to perform well for 15+ years if properly designed and site conditions are suitable, there are conditions (e.g., high nitrate or high carbonate sites) where it may not perform to expectations.

Question 5: How would you reconcile the failure of the Hill AFB PBR (slide 71) with the client? How would you deal with a private client using their own money to pay for remediation? - Environmental Consultant; Atlanta, GA, United States

  • Cannon Silver: This question highlights the challenge of how to communicate bad news when performance does not meet expectations. It also highlights the need to involve the client throughout the project including the design phase. All clients must balance costs with level of acceptable risk, answering the question how much of a safety factor can the project afford? In the case of Hill AFB, which was installed when iron prices were high, no safety factor was included in the design. Fortunately, other projects can learn from these lessons and best practices to calculate the appropriate amount of material, and decide on the right balance of costs and risks for that given project.

Question 6: For mineral precipitation in ZVI PRB, even if the divalent iron precipitate is still able to conduct electrons, will the contaminant degradation rate be reduced? - Environmental Consultant; Pittsburgh, PA, United States

  • Cannon Silver: As discussed in the PRB-5 Section 8.2.1, divalent iron conducts electrons. However, if precipitation is excessive and causes a thick layer of deposits, the sheer distance that the electrons have to travel (between ZVI and the bulk fluid) may inhibit the desired reactions. Moreover, the excessive quantity of precipitates may reduce the porosity and hydraulic conductivity within the barrier, thereby reducing the residence time for treatment and overall ZVI performance.

Question 7: About gas generation in PRBs, is there any case that gas generation had accumulated in PRBs and resulted in reduction of porosity and hydraulic performance? For example, hydrogen gas could be generated due to anaerobic corrosion of iron in groundwater. - Environmental Consultant; Pittsburgh, PA, United States

  • Scott Warner: Gas generation in ZVI PRBs is a known condition. Pilot testing has shown the actual production of separate phase bubbles, and early monitoring of in situ 100% ZVI PRB systems shows solubility levels of hydrogen, for example, being produced. While the potential porosity interference is considered to be possible, we are unaware of any specific cases where hydraulic conditions have been so adversely impacted by gas generation that the PRB system is compromised. Some sites measure hydrogen gas generation as part of their long-term performance monitoring program (on an approximately 5-year interval). This data is valuable for evaluating whether the rate of corrosion is continuing adequately. The 1994 PRB installation results show that substantial hydrogen continues to be generated (nearly 50% solubility levels of hydrogen) 15 years after installation.

Question 8: There often is not budget to conduct the extensive site characterizations emphasized in the ITRC document. What are the minimum groundwater geochemical test parameters needed to evaluate use of PRBs and the type of chemical to use? - Environmental Consultant; Plymouth, WI, United States

  • Scott Warner: Previous ITRC documents (see http://www.itrcweb.org/guidancedocument.asp?TID=5) provide guidance as to the geochemical characterization methods and parameters for PRB design and selection. However, we do suggest that comprehensive site characterization should always be conducted to assure the PRB design will be sustainable and appropriate for the site. The risks of having an inadequate design increases if the level of characterization is low.

Question 9: Other than increased cost, is there a down side to use of ZVI as a backfill enhancement below the water table as a residual treatment method? This would be a source treatment approach, or possibly just at the downgradient edge of the source excavation. - Environmental Consultant; Plymouth, WI, United States

  • Cannon Silver: This enhancement is a great idea! We successfully used this approach in an excavation at a Navy site in San Francisco. As long of the ZVI (or other selected reactive media) is appropriate for the site contaminants and meets other site objectives, this type of passive horizontal barrier can be very cost-effective and provide long-term reduction of contaminant mass.

Question 10: How do you determine whether backfill with ZVI or an organic carbon source (mulch, EOS, etc.) is a better option? It seems ZVI lasts a long time, carbon doesn't. - Environmental Consultant; Plymouth, WI, United States

  • Peter Zawislanski: I would contribute the following specifically with respect to metals: Treatment of divalent metals, such as copper and zinc, has been largely accomplished using mulch biowalls, often with the addition of limestone for pH buffering. ZVI has also been shown to be effective for divalent metals, but would likely not be selected due to costs relative to mulch. On the other hand, mulch biowalls are not well-suited for the treatment of arsenic, which has been successfully treated using ZVI. Therefore, the choice of PRB material to be used for the treatment of metals will depend on which metals are present and may require a mixture of mulch and ZVI.

  • Cannon Silver: The choice of PRB reactive material will depend on multiple site-specific factors including cost, desired longevity, and the contaminants needing treatment. For example, as Peter stated, mulch biowalls are capable of treating copper and zinc, while ZVI is better suited for the treatment of arsenic. As another example, if treating chlorinated ethenes and formation of daughter products (e.g., vinyl chloride) is of particular concern to nearby receptors, this may favor use of reactive media (e.g., ZVI) that generate abiotic degradation over sequential reductive dechlorination. Please refer to the various considerations detailed in the PRB-5 document.

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