Title: Introduction to Permeable Mulch Biowalls Text: Permeable mulch biowalls are types of biobarriers used to remediate chlorinated solvents and other contaminants in groundwater. Permeable mulch biowalls use natural organic substrates such as mulch, compost, and/or vegetable oil to create an anaerobic reaction zone to enhance bioremediation within the aquifer. Due to the low cost of these organic materials, permeable mulch biowalls can be installed for approximately 1/4 to 1/3 of the cost of zero valent iron permeable reactive barriers (PRBs). Permeable mulch biowalls also offer minimal maintenance, while cost-effectively preventing migration of chlorinated solvent plumes at a fraction of the cost of pump-and-treat systems. This Web tool reviews design and installation considerations for permeable mulch biowalls and highlights case study results at Navy and Air Force sites.

Title: Biobarrier Text: A biobarrier is a biologically active flow-through zone in an aquifer that is established downgradient of the source zone or on the leading edge of a contaminant plume. As contaminated groundwater passes through the biobarrier, the contaminant is converted by microorganisms into innocuous byproducts such as carbon dioxide and water. The microbial population is established with biostimulation and/or bioaugmentation. Biobarriers can be used to create aerobic or anaerobic conditions. Biobarrier configurations include biowalls, bioborings, or injection wells to add substrates to groundwater as it passively flows through the biologically active zone.

Title: Applicable Contaminants Text: Permeable mulch biowalls are used to enhance anaerobic bioremediation of chlorinated solvents and other contaminants found in groundwater in an oxidized state (e.g., perchlorate). These chemicals are generally recalcitrant in groundwater because they are not readily degradable under naturally aerobic or mildly anaerobic conditions. Permeable mulch biowalls can treat several types of contaminants (roll over contaminant label to view example sites): Chlorinated solvents and their dechlorination products: -PCE (tetrachloroethene) -TCE (trichloroethene) -TCA (trichloroethane) -CT (carbon tetrachloride) -DCE (dichloroethene) -VC (vinyl chloride) Oxidizers (perchlorate) Energetic compounds (RDX, TNT) Dissolved metals (hexavalent chromium) Nitrate and sulfate In addition, there is potential for biowall applications with the following compounds: Chlorinated pesticides (chlordane), Polychlorinated biphenyls (PCBs), and Chlorinated cyclic hydrocarbons

Title: Degradation Pathways (1 of 2) Text: Two main degradation pathways which break down chlorinated compounds under anaerobic conditions are sequential biological reductive dechlorination and non-sequential biogeochemical reduction. Sequential biological reductive dechlorination occurs under anaerobic conditions and involves the step-wise replacement of chlorine atoms with hydrogen atoms. The parent compound (e.g., PCE or TCE) is biodegraded through a succession of daughter products (e.g., DCE, VC, and ethene).

Title: Degradation Pathways (2 of 2) Text: A second type of degradation pathway is non-sequential biogeochemical reduction. For example, the degradation of TCE by non-sequential biogeochemical reduction involves: Ferric iron hydroxides and hydrogen sulfide reacting to form iron mono-sulfide The iron mono-sulfide then reacting with TCE to form iron hydroxide, sulfate, and acetylene This reductive process may require a shorter residence time than sequential biological reductive dechlorination because intermediate chlorinated degradation products are not produced.

Title: Biowall Design Text: Permeable mulch biowalls have the potential for widespread application, given their low-cost and ability to target a number of contaminants. The following slides outline the key parameters involved in designing effective biowalls, including site selection, dimensions, materials, and possible design variations.

Title: Site Selection Criteria Text: The criteria in the matrix shown here are used to determine the suitability of a site for biowall installation. Key parameters to evaluate include: existing infrastructure, contaminant concentrations, lithology, plume depth, groundwater flow, and pH. Of these, the required trenching depth is generally the most critical. The suitability of the site lithology is based on the fact that the biowall must be more permeable than the surrounding lithology; otherwise, the groundwater will flow around (circumvent) the biowall. This fact makes biowalls suitable to low permeability silts and clays or bedrock.

Title: Dimensions - Depth Text: Biowall depth primarily depends on the depth to the confining layer. When designing the biowall depth, the following guidelines should be considered: The maximum trench depth is typically 25 to 35 ft, but it may be possible to use a biopolymer slurry to install a deeper biowall. Make sure the trench is deep enough to effectively treat the contaminant and to avoid contamination bypass. Key the trench into a confining layer, if possible, to avoid "hanging biowalls." Key the trench into a confining layer to maintain a higher permeability in the trench versus the formation.

Title: Dimensions - Width Text: Biowalls are typically 1.5-ft to 3-ft wide using conventional trenching technology. The desired width depends primarily on the contaminant concentration and groundwater velocity. High groundwater flow may require a dual biowall configuration (shown here) Low groundwater flow rate will likely only require a single biowall The design should follow these steps: Estimate the maximum seepage velocity (remember that the rate of groundwater flow is proportional to the hydraulic gradient, the hydraulic conductivity, and the effective porosity) Evaluate potential degradation rates vs. the residence time required in the reactive zone Consider generation and rate of degradation of toxic intermediate products (e.g., DCE, VC, chloroethane) Compare residence time to rates of degradation necessary to meet remedial objectives Sorption will increase effective residence time on order of 2 to 4 times

Title: Materials Text: Permeable mulch biowall fill materials can include a variety of organic and other natural sources: Tree Mulch - provides bulk source of cellulose and lignin, but avoid low pH pine mulch (maximum 50% by volume) Compost and other organics - provide more easily degraded organic carbon with nutrients; include cotton seed and cotton burrs, blended corn cobs, silage, and agricultural waste Sand or pea gravel - maintains permeability and provides weighting material (minimum 50% by volume); crushed limestone can be used for buffering pH Materials to promote abiotic degradation by reactive iron monosulfides - include high iron sand backfill, iron ore, crushed gypsum or sulfate fertilizer pellets Tip: You need enough bioavailable organic carbon so the biowall material can last and maintain effective biodegradation (> 20 mg/L TOC)

Title: Design Variations Text: As shown here, liquid substrates such as vegetable oil can be added into mulch/compost biowalls to extend their useful life. The liquid can be injected using perforated piping installed in the trench to provide "recharge" over time. Alternatively, liquid substrates can be directly injected into borings or wells. Click here for a list of alternate substrates. Other variations in biowall design include: Multiple biowalls/trenches perpendicular to groundwater flow with continuous “reactive zones” between the biowalls Biowalls installed at various angles to groundwater flow to increase residence time within the biowall Additional biowall materials, such as vegetable oil, high iron sand and gypsum (sulfate), or crushed limestone (pH control) Bioaugmentation at a relatively low volume to inoculate with DCE/VC dechlorinators (e.g., Dehalococcoides ethenogenes) Bioreactors, bioborings, or horizontal injection wells below the biowalls

Title: Longevity Text: The longevity of mulch biowalls depends on two factors. First, the woody chips themselves are comprised of lignin and cellulose, with the lignin encasing most of the cellulose. Lignin is very slow to degrade, so the woody portion of the mulch is expected to last 10 to 15 years or more. Sampling at Offutt AFB in 2006 (7 years post installation) has confirmed this expectation. The second, and more critical, factor is how much bioavailable organic carbon is present and the threshold value at which it is no longer able to sustain effective degradation. These values are highly site-specific. For example, the threshold may be much higher at high sulfate sites. The readily bioavailable organic carbon may last from 2 to even 5 years. Because of this relatively short availability, cotton burrs or vegetable oil are added to the mulch mixture to boost the amount of bioavailable organic carbon. It is important to note that the use of dissolved organic carbon (DOC) may not always be a true indication of the ability to sustain anaerobic activity. The microbes grow on the mulch, and consequently, may utilize bioavailable organic carbon as soon as it is produced by degradation of the mulch. At some sites, sulfate reduction and methanogenisis continues to be observed even though DOC is only 10 to 20 mg/L. DOC is just one parameter used to determine when to recharge a biowall. As far as recharging biowalls without piping, there is no need to replace the mulch substrate because the woody structure is still present. Rather, it is much more cost effective to directly inject emulsified vegetable oil using a direct push method, which is readily accomplished in the soft biowall backfill material. Also it is important to note that to date very few biowalls have been recharged. The science behind biowall recharge is still being developed.

Title: Alternate Substrates Text: There are several types of alternate substrates that can be injected or added to create in situ biobarriers: Soluble Substrates (e.g., lactate, ethanol, sugars) - substrates migrate and may last up to 90 days, which creates a larger reaction zone, but requires frequent injections (days to months) Viscous Fluid Substrates (e.g., vegetable oil microemulsions or HRC®) - require adequate residence time in the reaction zone often with multiple staggered rows of injection points and must be replenished every 3 to 4 years

Title: Regulatory Concerns Text: To date, there have been few regulatory hurdles in the application of this innovative technology. The two most common concerns of regulatory agencies are: 1) how to dispose of the excavated trench soils and 2) potential impact to overall water quality.

Title: Soil Disposal Text: The primary regulatory concern is management of excavated soil. Ideally, excess soil can be used for trench backfill and cover material. However, the ability to reuse the trench spoils is affected by their contaminant concentrations. The likelihood for generating contaminated trench spoils is increased when contaminants have high sorption potential onto soil and elevated concentrations in groundwater. Because off-site disposal may render the technology cost-prohibitive, other soil management options should be evaluated. Land farming is one type of on-site management, which takes place inside of a lined cell, that is an effective option for degrading contaminants. Amendments are added to the excavated soil (e.g., citric acid, buffer, and nutrients) for the process to occur; then the cell is flooded and covered. Land farming at the NWIRP McGregor site (shown here) anaerobically degraded perchlorate from 500,000 μg/kg to less than 100 μg/kg in 6 to 8 weeks.

Title: Secondary Water Quality Text: Anaerobic conditions can sometimes impact overall water quality including the presence of iron and manganese in the water. These constituents are subject to secondary maximum contaminant levels (SMCLs) for taste, odor, and other water quality issues. Therefore, degradation of water quality is another potential regulatory concern. However, biowalls installed to date meet these secondary water quality criteria downgradient of the biowall. The water quality results from a biowall at Seneca Army Depot, NY are shown here. These results indicate that secondary water quality parameters in groundwater downgradient of the biowall returned to concentrations similar to those observed upgradient of the biowall.

Title: Installation Methods Icons Text:

Title: Installation Methods Text: Permeable mulch biowall installation methods can include: Conventional backhoe to 10 to 15 feet (installed with or without side-wall shoring/trench boxes) Continuous chain trencher to 25 to 35 feet (up to 200 linear feet per day) Biopolymer slurry to 50 feet Long-arm excavators with biopolymer solids, possibly as deep as 100 feet

Title: Conventional Backhoe Text: Conventional Backhoe The maximum depth for the conventional backhoe ranges from 10 to 15 feet. If only minor sloughing occurs, then side-wall shoring (trench boxes) will unlikely be needed.

Title: Continuous Chain Trencher Text: Continuous Chain Trencher The maximum depth for a continuous chain trencher ranges from 25 to 35 feet. This specialized technology has higher mobilization costs, but is very effective in simultaneously excavating and backfilling the trench and can install up to 200 linear feet per day of trench.

Title: Biopolymer Slurry Text: Biopolymer Slurry The maximum depth for the biopolymer slurry is 50 feet. This method has yet to be used for biowall installation.

Title: Long-arm Excavators Text: Long-Arm Excavators with Biopolymer Solids The maximum depth for the long-arm excavators with biopolymer solids is 100 feet. This method has yet to be used for biowall installation.

Title: Case Studies Text: Two permeable mulch biowall case studies are highlighted. Click on the map to view the installation name and to see details of the case study.

Title: Altus AFB Icons Text:

Title: Altus AFB (1 of 5) Text: In June 2002, a 455-ft long permeable mulch biowall was installed as part of a demonstration project at Landfill 3, Operable Unit 1 at the Altus Air Force Base (AFB) in Oklahoma. The objective was to treat a TCE plume with concentrations ranging from 1,000 to 8,000 µg/L. Based on the successful demonstration results, a full-scale 5,000-ft long and 35-ft deep biowall system was installed along the AFB property boundary during the spring of 2005.

Title: Altus AFB (2 of 5) Text: Demonstration Biowall: Biowall trench installed in June 2002 using continuous chain trencher 455-ft long by 24-ft deep by 1.5-ft wide Approximately 300 cubic yards of tree mulch, 60 cubic yards of cotton gin compost, and 265 cubic yards of sand Performance monitoring: 1, 3, 9, 17, and 34 months

Title: Altus AFB (3 of 5) Text: Demonstration Biowall Results: Within nine months of installation, TCE concentrations within and immediately downgradient of the biowall declined to less than 5 µg/L TCE continued to be reduced by 92%-99% in the biowall after 34 months Biogeochemical reduction initially appeared to be the predominant degradation pathway within the biowall, based on the very low accumulation of DCE or VC in the biowall However, over time, the biowall became less effective in TCE removal with an accumulation of cis-1,2-DCE This potential DCE stall corresponds to a depletion of soluble organic carbon and a rebound in sulfate levels within the biowall

Title: Altus AFB (4 of 5) Text: Full-Scale Biowall: Based on the demonstration results, a full-scale permeable mulch biowall was installed between March 2005 and April 2006 to treat chlorinated solvents within other groundwater plumes at Altus AFB. The potential annual savings for this mulch biowall was estimated to be $600,000-$700,000 compared with a pump-and-treat system. The TCE plume passes through three successive biowall sections installed in a fractured clay and shale aquifer. This biowall was designed as an in situ containment measure to prevent off-base migration of contaminants in groundwater. The biowall is approximately one mile along the base boundary, has a 2-ft wide and 35-ft deep trench, and 9,700 cubic yards of mulch. The monitoring wells consist of 15 transects with three wells per transect.

Title: Altus AFB (5 of 5) Text: Full-Scale Biowall Results: When upgradient wells are compared to wells in the full-scale biowall, the following average reductions have been achieved across the biowall: 85% TCE reduction across entire biowall system 73% toxicity reduction after final wall treatment of the TCE plume core* *Based on the “MCL equivalents” in upgradient and in-wall wells. This toxicity calculation accounts for DCE production and minor VC detections in the wall.

Title: NWIRP McGregor Icons Text:

Title: NWIRP McGregor (1 of 5) Text: Both pilot-scale biowalls (Area F) and a full-scale biowall network (Area S) have been installed at the Naval Weapons Industrial Reserve Plant (NWIRP) in McGregor, TX. The NWIRP McGregor site was used to manufacture and test rocket motor propulsion systems until 1995. Perchlorate concentrations as high as 91,000 µg/L were detected at the site during early site investigations. The perchlorate plume is located primarily in the upper portions of an unconfined 5-ft to 35-ft thick fractured limestone aquifer. The depth to groundwater ratio varies seasonally from depths of 2 to 10 ft below ground surface. The biowalls at NWIRP McGregor contain organic materials, including compost and soybean oil, which act as electron donor sources for microbes to reduce perchlorate as groundwater moves through the passive barriers. This site is well-suited to a biowall application because the impacted aquifer is relatively shallow.

Title: NWIRP McGregor (2 of 5) Text: Substrate Study (Area F): As shown here, several combinations of electron donors were tested including both solid and soluble forms of organic carbon. The amendments tested include compost, wood chips, acetic acid, and vegetable oil. Different combinations of these substrates were used at each of the five pilot-scale trenches.

Title: NWIRP McGregor (3 of 5) Text: Pilot-Scale Installation (Area F): At Area F, five pilot-scale biowalls (Trenches 1-5) were installed to assess biobarrier construction issues. The pilot-scale barriers ranged from 75-ft to 100-ft in length and were approximately 12-feet-deep. The primary objectives of the study were to investigate the longevity of the biobarrier media, the nature of the organic carbon distribution, and the optimal biobarrier media mix. Other site-specific issues of concern were the impact of the fractured limestone on barrier hydraulic performance and the ability to co-treat perchlorate and TCE in groundwater.

Title: NWIRP McGregor (4 of 6) Text: Pilot-Scale Perchlorate Treatment Results (Area F): Click through the figures to see sample results from the pilot-scale biowalls. The first two figures show the reduction of perchlorate downgradient of Trenches 1 and 2. The last two figures show the total organic carbon concentrations within the biowalls with time. Based on these and other data, it was determined that carbon regeneration (e.g., soybean oil application) would be required every one to two years.

Title: NWIRP McGregor (5 of 6) Text: Full Scale System for Perchlorate Treatment (Area S): Based on the success of the pilot study, a full-scale biowall system was constructed in late 2002 to address the off-site migration of the perchlorate plume. The full-scale biowall was constructed in the vicinity of Area S, the Explosives Classification and Disposal Area, which was the official burning ground for off-specification ammonium perchlorate. A total of 3,500 linear feet of trench was installed at Area S in seven trench segments. The segments were installed in the downgradient plume direction approximately 1,000 feet apart. Each trench was backfilled with a mixture of gravel (70%), mushroom compost (20%), and soybean oil-soaked woodchips (10%). Over 4,200 tons of material were used to backfill the trenches. A subsurface piping system was also installed to provide even distribution of chemicals. The video to the left shows the changes in the perchlorate plume from 1999 before the barrier installation through 2003, several months after barrier installation. The later portion of the video shows the dramatic impact of the barriers on the perchlorate plume at the leading edge. Project monitoring conducted to date indicates that the perchlorate concentrations continue to decrease to non-detectable concentrations in groundwater passing through the biowall system. NAVFAC has estimated an overall capital cost avoidance of $3 million as a result of using a biowall versus a conventional pump-and-treat system at this site.

Title: NWIRP McGregor (6 of 6) Text: Off-Site "Bioborings" (Area M): In addition to the biowalls, Area M was selected as the location for 200 bioborings. Bioborings consist of a deep hole through which substrate is placed to treat groundwater as it passively flows through the biologically active zone. This type of biobarrier was less intrusive and less expensive than the trench system in this area. A natural organic carbon source of 25 lbs of cottonseed meal was used as the substrate in each boring. A perchlorate-reducing microbe (PRM) and sodium acetate were also added. Results indicate that degradation is occurring, but it is difficult to distinguish this degradation from the effects of upgradient biowalls.

Title: Costs (1 of 2) Text: Permeable mulch biowalls are generally very cost-effective for shallow groundwater applications, compared to other remedial technologies. Some key cost observations include: Trenching is the single most expensive line item for construction of biowall systems Overall cost to install biowalls ranges from $100 to $1,000 per linear foot, depending on the length and depth Due to the large cost of mobilization ($20,000 to $60,000 alone), there is an economy of scale in trenching costs. The plot to the left shows this economy of scale; as the trench length increases, the cost per foot decreases Long-term operation costs consist primarily of annual groundwater monitoring Modifications and contingencies will increase costs

Title: Costs (2 of 2) Text: Example: To illustrate typical permeable mulch biowall costs, a spreadsheet is provided here for the biowall system at the BG05 Site, Ellsworth AFB, SD Biowall dimensions: 580-ft-long by 32-ft-deep by 2-ft wide mulch biowall installed in June 2005 Total construction cost: $293,000 Of this, $155,000 was for the trenching subcontractor and $30,000 for biowall materials Iron ore and gypsum pellets were added to one segment of the biowall to stimulate biogeochemical reduction

Title: References Text: Environmental Security Technology Certification Program (ESTCP). 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. August. NAVFAC ERB Web Site. Technologies - Bioremediation (Enhanced In Situ) Air Force Center for Environmental Excellence (AFCEE). Enhanced In Situ Anaerobic Bioremediation Interstate Technology Regulatory Council (ITRC). In Situ Bioremediation Documents Federal Remediation Technologies Roundtable. In Situ Biological Treatment Profile

Title: Contacts Text: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil AFCEE POC (210) 536-4314 erica.becvar@brooks.af.mil