AFCEE PROTOCOL FOR ENHANCED ANAEROBIC BIOREMEDIATION USING EDIBLE OILS

Transcription

1 Paper 3D-03, in: A.R. Gavaskar and A.S.C. Chen (Eds.), Remediation of Chlorinated and Recalcitrant Compounds Proceedings of the Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2004). ISBN , published by Battelle Press, Columbus, OH, AFCEE PROTOCOL FOR ENHANCED ANAEROBIC BIOREMEDIATION USING EDIBLE OILS Robert C. Borden and Christie Zawtocki (Solutions Industrial & Environmental Services, Inc., Raleigh, NC) Michael D. Lee (Terra Systems, Inc., Wilmington, DE) Bruce M. Henry (Parsons. Denver, CO) Erica S. Becvar (Air Force Center for Environmental Excellence/ERS, Brooks City-Base, TX) Patrick E. Haas (Mitretek Systems, San Antonio, TX) ABSTRACT: Distribution of edible oils in the subsurface is a very promising approach for stimulating in situ biodegradation of chlorinated solvents and other anaerobically biodegradable contaminants (e.g., nitrate, perchlorate, RDX, etc.). The oil provides a slowrelease carbon source and electron donor to support long-term anaerobic biodegradation. In addition, chlorinated solvents will partition into the oil, reducing the mass flux of dissolved contaminants and the risk to downgradient receptors. Capital costs for this approach are expected to be much lower than competing technologies since most of the injection equipment can be reused at multiple sites. Long-term operation and maintenance costs should also be lower since much less frequent substrate addition would be required. To aid users of this process, the Air Force Center for Environmental Excellence (AFCEE) has developed a detailed technical protocol for design, field implementation and monitoring of edible oil bioremediation projects. This protocol provides detailed guidance on: (1) the effects of edible oils on the fate and transport of chlorinated solvents in the subsurface; (2) approaches for source area treatment and barrier installation; (3) methods for distribution of edible oils as a NAPL or as an oil-in-water emulsion; and (4) pilot test planning, execution and data analysis. Design issues addressed in the protocol include: (a) determining the amount of oil to inject and area to be treated (barrier length, width, and depth); (b) identifying the best approach for distributing the oil including appropriate injection point spacing, volume of chase water, injection pressure and flow rate; and (c) the effect of oil injection on formation permeability, soil gas emissions, and downgradient water quality. BACKGROUND Enhanced anaerobic bioremediation can be a cost-effective approach for treating a variety of groundwater contaminants including certain heavy metals, nitrate, perchlorate, acid mine drainage and chlorinated organics. Chlorinated solvents amenable to in situ anaerobic bioremediation include tetrachloroethene (PCE), trichloroethene (TCE), cis- 1,2-dichloroethene (cis-dce), vinyl chloride (VC), 1,1,1-trichloroethane (1,1,1-TCA), 1,1,2-trichloroethane (1,1,2-TCA), 1,2-dichloroethane (1,2-DCA), carbon tetrachloride (CT), and chloroform (CF). Anaerobic bioremediation processes can be stimulated through addition of soluble substrates (lactate, butyrate, propionate, acetate, molasses, and refined sugars), solid

2 substrates (bark mulch, compost, chitin and peat), and slowly-soluble substrates including edible oil. Soluble, rapidly biodegradable substrates have been demonstrated to be very effective for enhancing reductive dechlorination. However, these materials are quickly depleted due to their rapid biodegradation and by downgradient migration with the flowing groundwater. Consequently, soluble substrates must be frequently added to the aquifer, increasing capital and operation and maintenance (O&M) costs. While solid substrates are more long lasting, there are significant cost and technical limitations associated with introducing these materials into the subsurface. In contrast, edible oils can be more easily injected into the subsurface reducing both capital and O&M costs. Two general approaches have been used to distribute edible oils in the subsurface: (1) injection of pure liquid oil as a nonaqueous-phase liquid (NAPL); and (2) injection of an oil-in-water emulsion followed by a water flush to distribute emulsion throughout the treatment zone. Edible oils can be injected directly into the soil as a NAPL using conventional wells or through temporary direct push points (Boulicault et al., 2000). However, it can be very difficult to move NAPL oil a significant distance away from the injection point. Lee et al. (2001) found that injection of NAPL soybean oil followed by water flushing was not effective in moving NAPL oil out away from the injection point. This observation is supported by laboratory studies (Coulibaly and Borden, 2004) which show that NAPL soybean oil can be distributed short distances in sands without excessive permeability loss only if the oil can be displaced to residual saturation. However, multiple pore volumes of water must be injected to displace the oil to residual saturation. If the oil is not displaced to residual saturation, permeability losses will be high and there is potential for upward migration of the oil due to buoyancy effects. Distribution of edible oils in the subsurface can be enhanced by injecting the oil as an oil-in-water emulsion followed by water flushing to distribute and immobilize the oil droplets. The emulsion is prepared to: (1) be stable for extended time periods (e.g., non-coalescing); (2) have small, uniform droplets to allow transport in most aquifers; and (3) have a negative surface charge to reduce droplet capture by the solid surfaces. Laboratory permeameter studies demonstrated that these emulsions can be effectively distributed with a low residual saturation in sands and clayey sands with only modest reductions in aquifer permeability (Coulibaly and Borden, 2004). Field pilot studies have demonstrated that this approach can effectively distribute emulsified oils over twenty feet away from the injection point and provide a long-lasting carbon source to support reductive dechlorination (Lee et al., 2001). Figure 1 shows the emulsion being prepared from soybean oil, food grade surfactants and water prior to injection. The white appearance is from the very small, uniform droplets. Edible oils have most commonly been used FIGURE 1. Emulsion preparation prior to injection. to treat contaminated groundwater in a permeable reactive barrier (PRB) configuration by injecting an emulsion through a series of temporary or permanent wells installed

3 perpendicular to groundwater flow. As groundwater moves through the emulsion treated zone under the natural hydraulic gradient, a portion of the trapped oil dissolves providing a carbon and energy source to accelerate anaerobic biodegradation processes. Most barriers are installed using emulsions to minimize permeability loss by entrapped oil. If permeability loss were excessive, contaminated groundwater could flow around the barrier and not be treated. Edible oils are also commonly used to treat source areas by stimulating anaerobic biodegradation of dissolved and sorbed contaminants.oil addition can reduce the effective aqueous concentration of chlorinated solvents by acting as a sponge to adsorb a portion of the dissolved contaminant. If NAPL oil is injected into a source area, the oil will block soil pores, reducing the aquifer permeability and reducing the dissolved mass flux out of a source area. EDIBLE OIL PROTOCOL While field and laboratory results have shown that edible oil injection can be very effective in stimulating anaerobic biotransformation processes, there are a number of factors that must be considered. To aid users, AFCEE has developed a detailed technical protocolfor the site selection, design, field implementation, and monitoring of enhanced anaerobic bioremediation projects using edible oils. The protocol provides background information on anaerobic biodegradation of chlorinated solvents, use of edible oils as an electron donor to drive reductive dechlorination, sources and properties of common vegetable oils, and transport of edible oils as NAPL and emulsions in the subsurface. The protocol also provides guidance on the following issues. How Much Oil Do You Need to Add? The volume of oil injected into the aquifer should be calculated to create a reaction zone sufficient to meet remedial objectives. Typically, the oil is injected in either a barrier configuration to intersect a groundwater plume, preventing downgradient migration of contaminants or a grid configuration to treat a source area. Figures 2a and 2b show general schematics for a barrier and source area treatment, respectively. For a permeable reactive barrier design, the amount of oil required for a specified design life is calculated based on the length and depth of the barrier, groundwater flow velocity, concentration of contaminants and competing electron acceptors entering the Groundwater Flow Injection Point Groundwater Flow Injection Point Source Area Clean Groundwater Source Area Zone of Influence FIGURE 2a. Barrier treatment. FIGURE 2b. Source area treatment.

4 barrier, and losses due to methane production and release of dissolved organic carbon. Laboratory studies indicate that this mass balance approach can provide a reasonably accurate prediction of barrier life. Typically, barriers are designed to provide sufficient carbon to support anaerobic biodegradation processes for 5 to 10 years. When conducting a source area treatment, the amount of oil added should be selected to match the time required to completely degrade the contaminants. Source area treatments are designed in a similar manner. After determining the length, width, and thickness of the area targeted for treatment, the amount of substrate can be determined based on the treatment volume, contaminant concentrations, and competing electron acceptor concentrations. The potential mass flux of contaminants and competing electron acceptors into the treatment area can also be calculated using the upgradient concentrations, width and depth of the treatment area, and the groundwater flow velocity. The goal of the treatment is to provide sufficient substrate to destroy the contaminant mass within the treatment area and any contaminants or competing electron acceptors that enter the area during the treatment period. How Do You Inject the Oil? Several oil injection scenarios can be considered for application. In general, oil is either injected as a NAPL or as an oil-in-water emulsion. Edible oils can be injected directly through a direct-push probe, can be added through injection wells or direct-push injection points, or can be emplaced by conventional construction techniques such as trenching or in an excavation. Advantages and disadvantages of each approach are discussed in the protocol. Figure 2 shows a typical schematic of the injection process. What is the Injection Point Spacing? When injecting NAPL oil, injection points must be closely spaced. However when injecting emulsions, a much wider injection point spacing can be used. Determination of the optimum injection point spacing is a trade-off between drilling costs and labor costs. Wider spacing of the injection points reduces injection well installation costs, but increases the time/labor required for injection. Drilling costs are affected by the geology and the depth to groundwater. Labor costs are determined by the time required to inject the emulsion which may be limited by the aquifer permeability. When well installation costs are high (e.g. deep groundwater, difficult drilling), a large injection well spacing may be used. However when drilling costs are low (e.g. wells installed by direct push), a closer injection point spacing is used to reduce injection times. Often multiple wells can be injected simultaneously to reduce the time required for injection. Figure 3 shows several points manifolded together during the injection process. Each point has a separate flow meter and control valve (Figure 4). When the amount of emulsion and/or chase water reaches a predetermined value, the valve is closed and flow is diverted to the remaining wells. Pre-injection permeability tests can be conducted to estimate the time required to complete an injection project. How Much Overlap is Needed between Injection Points? When injecting emulsions, the amount of emulsified oil and chase water required is calculated to provide complete coverage of the treatment zone with a small factor of safety to allow for subsurface heterogeneities. Oil-in-water emulsions are completely miscible with water and so they will migrate through the subsurface similar to a dissolved solute that can sorb to the

5 FIGURE 3. Typical emulsion injection process. FIGURE 4. Injection system manifold. aquifer material. Similar to any other solute, emulsions will move more rapidly through high permeability zones and more slowly through low permeability zones. As a consequence, high permeability zones may be over-treated and some lower permeability zones may remain untreated. Dispersion will provide some spreading of aqueous organic carbon increasing the reactive zone. However, injections are commonly designed to provide 5 to 10% overlap (factor of safety) between injection points. However in more heterogeneous environments, a greater factor of safety (overlap) may be desired. What Injection Pressure/Flow Rate Can Be Used? Regardless of the injection scenario selected, injection pressures should be selected based on site-specific conditions. For NAPL injections, the minimum injection pressures will depend on the hydrostatic pressure and pore space entry pressure. Maximum injection pressures will be limited by fracturing of the aquifer and/or surface breakout at the injection point. Injection pressures in excess of the overburden pressure will induce hydraulic fracturing, which is generally not desirable due to the resulting non-uniform distribution of oil throughout the injection zone. To inject NAPL oil into the aquifer matrix, the injection pressure must overcome the pore space entry pressure. This entry pressure may be highly variable, depending on the size of the aquifer pores and the properties of the NAPL oil. In fine-grained aquifers (e.g., clayey sands or silts, carbonates), the pore space entry pressure can exceed the overburden pressure and it may not be possible to effectively distribute NAPL oil. What is the Effect of Oil Injection on Permeability? Injection of NAPL oil into the subsurface reduces permeability by blocking aquifer pore space. For source area treatments, a reduction in permeability may be desirable. However for permeable reactive barriers, reductions in permeability may reduce flow through the barrier thereby decreasing barrier effectiveness. Injection of NAPL oil will typically result in high oil residual saturations and high permeability losses. In contrast, emulsions can be distributed into the subsurface with very low to moderate permeability losses. To minimize potential impacts on the aquifer permeability, the emulsion must be stable with droplet sizes significantly smaller than the mean pore size of the sediment. Slug tests or other in-situ permeability

6 tests are typically conducted before and after oil injection to evaluate any changes in aquifer permeability. What is the Effect of Oil Injection on Contaminant Biodegradation? Oil injection will stimulate contaminant biodegradation by providing a long-term carbon source to promote microbial activity. Immediately after oil injection, contaminant concentrations may initially decrease due to sorption into the oil. However, over time, the adsorbed contaminant mass will partition back into the reaction zone at a controlled rate dependent on the rate of biodegradation in the aqueous phase and the dissolution of the vegetable oil. Once in the subsurface, the oil slowly biodegrades over time providing a slow continuous source of dissolved organic carbon (i.e., fermentation products). Degradation of the oil results in removal of oxygen and production of hydrogen (H 2 ). The hydrogen itself then drives the desired anaerobic biological metabolism. What Size Barrier is Needed for Effective Treatment? Barriers are typically installed across a plume, perpendicular to groundwater flow. Normally, the barrier will be wider than the plume to allow for uncertainties in contaminant distribution, changes in aquifer permeability due to oil injection and/or biological activity, and variations in groundwater flow direction with time. The depth of the barrier is dependent on the depth of the contaminated groundwater. Typically, barriers are designed to treat the entire saturated zone to minimize potential for contaminants to flow under the barrier without being treated. Depending on the thickness of the saturated zone and the vertical distribution of the contaminants, multiple injection wells screened at different intervals may be needed. The overall goal of the design should be to treat the entire impacted zone and minimize the potential for contaminants to migrate around the barrier without being treated. What is the Effect of Oil Injection on Downgradient Water Quality? Secondary groundwater quality (color, odor, dissolved iron, manganese, turbidity, etc.) may be degraded within the reactive zone. Elevated levels of DOC, lower redox geochemistry compared to background, and elevated levels of metabolic byproducts (e.g., carbon dioxide, ferrous iron, methane) may also be observed immediately downgradient of the oil treated zone. However, available groundwater monitoring data indicate that changes in secondary groundwater quality parameters are not detectable more than feet from oil injection zones. Where downgradient impacts are a concern, injection points should be located a substantial distance upgradient from critical receptors. Are Soil Gas Emissions an Issue? Methane may be produced from injected oil and could potentially pose a problem in nearby buildings. Whenever edible oils are injected in close proximity to buildings or other confined areas, soil gas should be monitored for methane and other gases that might be produced. SUMMARY The Edible Oil Protocol provides detailed guidance on: (1) site applicability screening, (2) the effects of edible oils on the fate and transport of chlorinated solvents in the subsurface; (3) treatment approaches including source area treatment and barriers; (4) methods for injection and distribution of edible oils including injection as a NAPL or

7 as an oil-in-water emulsion; (5) pilot test planning, execution and data analysis, and longterm performance monitoring. Design issues addressed in the protocol include: (a) determining the configuration of the treatment zone (grid vs. barrier) (b) calculating the amount of oil to inject and area to be treated (barrier length, width, and depth); (c) identifying the best approach for distributing the oil including appropriate injection point spacing, volume of chase water, injection pressure and flow rate; and (d) the effect of oil injection on formation permeability, soil gas emissions, and downgradient water quality. The protocol and related field efforts are part of the on-going AFCEE Enhanced In Situ Bioremediation Initiative, where enhanced in situ bioremediation has been demonstrated at nearly 30 sites. Other protocols offered by AFCEE include those on monitored natural attenuation of fuel hydrocarbons (Wiedemeier et al., 1995), chlorinated solvents (Wiedemeier et al., 1998), and Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents (in press). ACKNOWLEDGMENTS The research presented in this manuscript was supported in part by the Air Force Center for Environmental Excellence Environmental Restoration Directorate. REFERENCES Boulicault, K.J., R.E. Hinchee, R.H. Wiedemeier, S.W. Hoxworth, T.P. Swingle, E. Carver, P.E. Haas, Vegoil: A Novel Approach for Stimulating Reductive Dechlorination. In: Wickramanayake, G.B., Gavaskar, A.R., Alleman, B.C., and Magar, V.S. (Eds.), Bioremediation and Phytoremediation of Chlorinated and Recalcitrant Compounds, Proceedings of the Second International Conference on Remediation of Chlorinated and Recalcitrant Compounds. Battelle Press, Columbus, OH. Vol. C2-4, pp Coulibaly, K.M., and R.C. Borden, Impact of edible oil injection on the permeability of aquifer sands, J. Cont. Hydrol., in press. Lee, D.M., T.M. Lieberman, R.C. Borden, W. Beckwith, T. Crotwell and P.E. Haas, Effective distribution of edible oils results from five field applications. In: Magar, V.S., Fennell, D., Morse, J., Alleman, B.C., and Leeson, A. (Eds.), Anaerobic Degradation of Chlorinated Solvents, Proceedings of the Sixth International In Situ and On-Site Bioremediation Symposium. Battelle Press, Columbus, OH. Vol. 6(7). Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, R.N. Miller, and J.E. Hansen, Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater, Air Force Center for Environmental Excellence, San Antonio, TX. Wiedemeier, T.H., M.A. Swanson, D.E. Moutoux, E.K. Gordon, J.T. Wilson,, B.H. Wilson, D.H. Kampbell, P.E. Haas, R.N. Miller, J.E. Hansen, and F.H. Chapelle, Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, EPA/600/R-98/128 (ftp://ftp.epa.gov.pub/ada/reports/ protocol.pdf).