Difference between revisions of "Bioremediation - Anaerobic Secondary Water Quality Impacts"

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Bioremediation is the process by which contaminants in soil and/or groundwater are treated biologically, primarily by microorganisms or biomolecules generated and released by the cells. This article overviews key design considerations when planning and implementing a bioremediation remedy.  This article focuses on enhanced in situ bioremediation (EISB) for the anaerobic biodegradation of organic contaminants, particularly [[Chlorinated Solvents | chlorinated solvents]], in soil and groundwater. However, much of the information provided is applicable to other contaminant types. There are numerous resources for design and implementation of bioremediation applications as well as detailed case studies and performance evaluations. This article provides a summary of key design considerations and parameters for anaerobic biodegradation for treatment of common organic contaminants.
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Secondary water quality impacts (SWQIs) from [[Bioremediation - Anaerobic | anaerobic bioremediation]] are changes in water chemistry that result from adding an organic substrate to the subsurface to enhance contaminant biodegradation. Common SWQIs that occur at most sites include increases in dissolved [[wikipedia: Manganese | manganese]], [[wikipedia: Iron | iron]], and [[wikipedia: Methane | methane]]. Fortunately, SWQIs are usually restricted to areas that are already contaminated and attenuate with distance downgradient in well-understood ways.
  
 
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'''Related Article(s):'''
 
'''Related Article(s):'''
 
*[[Bioremediation - Anaerobic]]
 
*[[Bioremediation - Anaerobic]]
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*[[Injection Techniques for Liquid Amendments]]
  
  
'''CONTRIBUTOR(S):''' [[Michaye McMaster, M.Sc.]] and [[Leah MacKinnon, M.A.Sc., P. Eng.]]
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'''Contributor(s):''' [[Dr. Robert Borden, P.E.]]
  
  
'''Key Resource(s)''':
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'''Key Resource(s):'''  
*[[Media:EPA_542_R_13_018.pdf|Introduction to In Situ Bioremediation of Groundwater]]<ref>USEPA,  2013. Introduction to In Situ Bioremediation of Groundwater. [[Media:EPA_542_R_13_018.pdf|Report pdf]]</ref>
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*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131/ER-2131/(language)/eng- Extent and Persistence of Secondary Water Quality Impacts after Enhanced Reductive Bioremediation] <ref name="Bordenetal2015">Borden, R. C., Tillotson, J. M., Ng, G.-H. C., Bekins, B. A., Kent, D. B., Curtis, G. P., 2015. Extent and persistence of secondary water quality impacts after enhanced reductive bioremediation. ER-2131. Strategic Environmental Research and Development Program, Arlington, VA. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131/ER-2131/(language)/eng- ER-2131]</ref>  
 
 
*[http://www.navfac.navy.mil/content/dam/navfac/Specialty%20Centers/Engineering%20and%20Expeditionary%20Warfare%20Center/Environmental/Restoration/er_pdfs/d/navfacexwc-ev-tm-1501-erd-design-201503f.pdf Design Considerations for Enhanced Reductive Dechlorination. TM-NAVFAC-EXWC-EV-1501.]<ref>NAVFAC, 2015. Design considerations for Enhanced Reductive Dechlorination. TM-NAVFAC-EXWC-EV-1501. [http://www.navfac.navy.mil/content/dam/navfac/Specialty%20Centers/Engineering%20and%20Expeditionary%20Warfare%20Center/Environmental/Restoration/er_pdfs/d/navfacexwc-ev-tm-1501-erd-design-201503f.pdf Report pdf]</ref>
 
  
 
==Introduction==
 
==Introduction==
Key considerations for designing [[Bioremediation - Anaerobic | in situ anaerobic bioremediation]] applications include the conceptual site model (CSM) and remedial goals, which determine the remedial configuration, amendment types and dosage, longevity of amendments, and modes of delivery. Often, additional site characterization, laboratory microcosm studies, or small-scale field tests are necessary to evaluate the technology and support of the full-scale remedy.
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Electron donor addition can be very effective in stimulating enhanced [[Bioremediation - Anaerobic | anaerobic bioremediation]] (EAB) of a wide variety of groundwater contaminants including [[Chlorinated Solvents | chlorinated solvents]], high explosives, [[Perchlorate |perchlorate]], and certain [[Metal and Metalloid Contaminants | metals]] and radionuclides<ref>AFCEE (Air Force Center for Engineering and the Environment), NFESC (Naval Facilities Engineering Service Center), ESTCP (Environmental Security Technology Certification Program), 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. Prepared by Parsons Infrastructure & Technology Group, Inc., Denver, CO, USA. [[Media:AFCEE_Principles_and_Practices.pdf|Report pdf]]</ref><ref>Stroo, H.F., M.R. West, B.H. Kueper, R.C. Borden, C.H. Ward, 2014. In Situ Bioremediation of Chlorinated Ethene Source Zones, Chlorinated Solvent Source Zone Remediation, Ed. B.H. Kueper, H.F. Stroo and C.H. Ward, Springer, New York, NY. [http://link.springer.com/chapter/10.1007/978-1-4614-6922-3_12 doi:10.1007/978-1-4614-6922-3_12]</ref><ref>Borden, R.C., 2007, Anaerobic Bioremediation of Perchlorate and 1,1,1-Trichloroethane in an Emulsified Oil Barrier, Journal of Contaminant Hydrology, 94, 13-33. [http://www.sciencedirect.com/science/article/pii/S016977220700071X doi:10.1016/j.jconhyd.2007.06.002]</ref><ref>Yurovsky, M., D. Cacciatore, L. Hudson, D.P. Leigh., C. Bhullar, and W. Shaheen, 2009. Operation of in situ biological system for treatment of hexavalent chromium at the Selma Pressure Treating Superfund Site. In: Proceedings of the Tenth International In Situ and On-Site Bioremediation Symposium, Baltimore, MD. Battelle Press. [https://clu-in.org/products/tins/tinsone.cfm?id=9474771&query=groundwater%20OR%20ground%20water&numresults=25&startrow=1801 ISBN:9780981973012]</ref>. However, this can result in secondary water quality impacts (SWQIs) including decreased levels of dissolved [[wikipedia: Oxygen | oxygen]] (O<sub>2</sub>), [[wikipedia: Nitrate | nitrate]] (NO<sub>3</sub><sup>-</sup>), and [[wikipedia: Sulfate | sulfate]] (SO<sub>4</sub><sup>2-</sup>), and elevated levels of dissolved [[wikipedia: Manganese | manganese]] (Mn<sup>2+</sup>), dissolved [[wikipedia: Iron | iron]] (Fe<sup>2+</sup>), [[wikipedia: Methane | methane]] (CH<sub>4</sub>), [[wikipedia: Sulfide | sulfide]] (S<sup>2-</sup>), organic carbon, and naturally occurring hazardous compounds (e.g., [[wikipedia: Arsenic | arsenic]])<ref name="Bordenetal2015"/>. Fortunately, the impacted groundwater is usually confined within the original contaminant plume and is unlikely to adversely impact potable water supplies<ref name="TB2015">Tillotson, J. M. and R. C. Borden, 2015. Statistical Analysis of Secondary Water Quality Impacts from Enhanced Reductive Bioremediation: Groundwater Monitoring and Remediation [http://onlinelibrary.wiley.com/doi/10.1111/gwmr.12132/abstract doi:10.1111/gwmr.12132]</ref>.
 
 
==Conceptual Site Model Interpretation for Design==
 
Successful implementation of anaerobic bioremediation must consider the CSM to develop a robust site-specific design. A CSM is developed and refined during site investigation activities to describe the site conditions, contaminant sources and extent, and the risk they pose to receptors. The following components of a CSM are evaluated to develop the optimal remedial design:
 
*Site geology and hydrogeology
 
*Contaminant type, distribution, and concentrations, including source zones
 
*Groundwater biogeochemical conditions
 
*Site infrastructure (i.e., buildings, below-ground utilities and conduits)
 
*Human health and ecological risks
 
*Remedial goals
 
 
 
[[File:Mackinnon-Article 2- figure 1.PNG|300px|thumbnail|right|Figure 1. Amendment addition for biobarrier.]]
 
The remedial goals and contaminant distribution are key to defining the target treatment area and remedial configuration. The contaminant type, biogeochemistry, and hydrogeology (aquifer permeability) determines the amendment type and optimal dosage. The site infrastructure and hydrogeology also determine the delivery method, including spacing between injection points, volumes of injectate, and injection frequency.
 
 
 
==Remedial Configurations==
 
In-situ anaerobic bioremediation of contaminated soil and groundwater involves introducing amendments and microbial cultures, if bioaugmentation is used, into the saturated treatment zone. The two most common treatment configurations include:  
 
*A grid of injection and/or extraction points for targeted treatment of a source zone or plume, and
 
*A linear treatment zone, referred to as a biobarrier (Fig. 1), treats contaminants as they flow through the biologically active zone to control plume migration. The biobarrier consists of a row(s) of injection wells/points or a trench filled with solid substrate.  
 
 
 
The configuration is selected based on the remedial objectives, remedial timeframe, and site conditions and restrictions.
 
 
 
==Delivery Modes==
 
Amendments can be delivered into the subsurface using injection wells (e.g. batch injection, recirculation, push-pull), direct injections (e.g., [[Direct Push (DP) Technology | direct push technology]], hydraulic and pneumatic fracturing) or excavation and backfill. Only liquid amendments (quick release or slow release) can be emplaced through well screens. Direct injections can be used for delivery of all liquid amendments as wells as microscale-particulates (e.g., EHC<sup>®</sup>, ABC+<sup>®</sup>, which contain zero valent iron and solid phase carbon), while biobarriers containing mulch or compost are installed using trenches. The design spacing of direct injection points or wells are based on the estimated radius of influence, which is dependent on the site-specific geology and amendment characteristics.
 
 
 
Amendments can be applied in a passive or active manner to target the contaminant zones.  
 
*In a passive treatment approach, amendments are typically delivered through injection wells or direct injection points in one injection event. Alternatively, solid amendments may be installed via excavation and backfill, or direct injection. Natural flow of groundwater is then relied upon to deliver contaminated groundwater to biologically active areas where treatment occurs.  
 
 
 
*In a semi-passive approach, liquid or solid amendments are injected periodically, with intermittent periods of passive treatment between injection events. Recirculation may be employed during the active treatment periods, while during the passive treatment periods native flow of groundwater is relied upon for delivery.  
 
 
 
*Active bioremediation approaches for groundwater involve recirculation of dissolved amendments within the targeted saturated zone (Fig. 2). Recirculation may improve substrate distribution in the discrete targeted depth and area, provide hydraulic containment, enhance contaminant/substrate mixing, and accelerate treatment time.
 
[[File:Mackinnon-Article 2- figure 2.PNG|400px|thumbnail|right|Figure 2. Groundwater recirculation system.]]
 
 
 
==Amendment Types==
 
Amendments for anaerobic bioremediation include carbon-based electron donors, electron acceptors, nutrients, pH buffers, and microbial cultures. For each type of amendment that is used the critical design considerations in selection include the contaminant type, site geochemical/hydrogeological conditions, and delivery configuration/mode, as well as the amendment solubility, anticipated rate of consumption (or growth in the case of bioaugmentation), longevity, cost, and ability to be distributed in the subsurface. Bench scale testing may be used to confirm the most applicable amendment type and dose.
 
 
 
===Electron Donors===
 
Many materials have been used as electron donors for anaerobic reductive bioremediation applications for anaerobic reduction of contaminants such as chlorinated solvents, energetics and perchlorate. These materials are typically classified as quick release compounds (lactate, sodium benzoate, molasses, whey) or slow release compounds (emulsified vegetable oils, Hydrogen Release Compound [HRC<sup>®</sup>], EHC<sup>®</sup>, ABC+<sup>®</sup>, mulch, compost). Quick release compounds are often selected for active approaches to bioremediation where the electron donor may be continually replenished in the target treatment area. Slow release compounds are often selected for passive approaches where the greater longevity is desirable. In some cases, a mixture of amendments will be used; for example, a quick release compound may be used to provide initial rapid bacterial growth and a slow release compound may be used to provide a long-term source of electron donor.
 
  
===Electron Acceptors===
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==SWQI Production and Attenuation Conceptual Model==
Electron acceptors such as nitrate, iron(III), or sulfate can be used as amendments for the anaerobic oxidative bioremediation of contaminants such as aromatic hydrocarbons, fuels, and some chloroethenes<ref>Edwards, E. A., Wills, L.E., Reinhard, M., Grbic-Galic, D., 1991. Anaerobic Degradation of Toluene and Xylene by Aquifer Microorganisms under Sulfate-Reducing Conditions. Applied and Environmental Microbiology 58:2663-2666.</ref><ref>Lovley, D.R., 1997. Potential for anaerobic bioremediation of BTEX in petroleum-contaminated aquifers. Journal of Industrial Microbiology and Biotechnology, 18(2-3), 75-81. [https://doi.org/10.1038/sj.jim.2900246 doi 10.1038/sj.jim.2900246]</ref><ref>Suflita, J.M. and Sewell, G.W., 1991. Anaerobic biotransformation of contaminants in the subsurface. Environmental Research Brief (USA).</ref><ref>Bradley, P.M. and Chapelle, F.H., 1997. Kinetics of DCE and VC mineralization under methanogenic and Fe (III)-reducing conditions. Environmental Science & Technology, 31(9), 2692-2696. [https://doi.org/10.1021/es970110e doi 10.1021/es970110e]</ref>. While these electron acceptors may promote slower kinetics compared to using oxygen, they may be chosen due to higher solubility, ease of delivery, and/or compatibility with existing geochemical conditions. Other considerations when using these electron acceptors include:
 
*Nitrate is highly soluble in water and after oxygen, provides the most energy for the microbial reaction. However, the EPA’s maximum contaminant level (MCL) for nitrate is 10 mg/L in groundwater. Therefore, the concentration and migration of nitrate needs to be carefully managed.
 
*Iron(III) is only slightly soluble in water, but gets reduced to iron(II) which is soluble in water. Water quality thresholds for iron include a secondary MCL guideline for iron of 0.3 mg/L for color, taste and staining effects.
 
*Sulfate is very soluble in water and reduces to sulfide, which typically precipitates with the naturally occurring iron in the subsurface. However, under acidic conditions, the sulfide can become hydrogen sulfide gas, which is toxic to breathe. The secondary MCL of sulfate is 250 mg/L due to taste, but does not present a risk to human health. Some common sulfate amendments include magnesium sulfate (Epsom salts), calcium sulfate (gypsum), and commercially-available products such as Nutrisulfate<sup>®</sup>.
 
  
===Nutrients===
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During EAB, large amounts of easily fermented organic substrates are added to the target treatment area to degrade or immobilize the contaminants of concern (CoC). These substrates are fermented to hydrogen (H<sub>2</sub>), acetate, and other volatile fatty acids that are then used as electron donors by microbes to mediate oxidation-reduction (redox) reactions that reduce dissolved oxygen, nitrate, and sulfate as well as Fe<sup>3+</sup> and Mn<sup>3/4+</sup> containing minerals and the CoC.  Figure 1 shows a typical pattern of SWQI parameters with time in the: 1) injection area; 2) near plume (25 m downgradient); 3) medium-distance plume (50 m downgradient); and 4) far plume (100 m downgradient). These curves were generated using a geochemical model developed to simulate SWQI production and attenuation<ref name="Ng2015">Crystal Ng, G.H., Bekins, B.A., Cozzarelli, I.M., Baedecker, M.J., Bennett, P.C., Amos, R.T. and Herkelrath, W.N., 2015. Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN. Water Resources Research. [https://pubs.er.usgs.gov/publication/70159670 doi:10.1002/2015WR016964]</ref>, and calibrated to [[ Long-Term Monitoring (LTM) - Data Analysis | long-term monitoring data]] from field sites where organic-rich materials have entered the subsurface<ref name=Bordenetal2015 />. The time frame for production and attenuation of SWQIs can vary from 10 to 100+ years, depending on the amount and duration of substrate addition, [[ Advection and Groundwater Flow | groundwater flow]] velocity, and background electron acceptor concentrations.
Under natural conditions, typical aquifers typically contain suitable amounts of trace nutrients for microbial growth, however substrate amendments may be used to provide additional nutrients such as nitrogen, phosphorous, sulfur, vitamin B12 and yeast extracts. While these nutrients may be valuable at some sites, there can be disadvantages to using nutrients, particularly at high levels, as the nutrients may precipitate with natural minerals and cause aquifer plugging. Nutrients may also compete for electron donors, when used. Additionally, nutrients such as vitamin B12 can be expensive to apply, and the cost-benefit of these additional amendments must be considered in the design process.
 
  
===pH Buffers===
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===Total Organic Carbon (TOC)===
The activity of the microorganisms that degrade the target compounds, and in particular the chlorinated volatile organic compound (cVOC)-dechlorinating microorganisms, can be inhibited at low pH (less than 6.0)<ref name="Robinson2009">Robinson, C., Barry, D.A., McCarty, P.L., Gerhard, J.I. and Kouznetsova, I., 2009. pH control for enhanced reductive bioremediation of chlorinated solvent source zones. Science of the Total Environment, 407(16), 4560-4573. [http://dx.doi.org/10.1016/j.scitotenv.2009.03.029 doi:10.1016/j.scitotenv.2009.03.029]</ref>. Low groundwater pH can be a result of the geologic materials, contaminant impacts (i.e. acids), or can occur during EISB due to the fermentation of electron donors and/or dehalogenation of cVOCs. The addition of a [[pH Buffering in Aquifers | 'buffer']] (i.e., sodium bicarbonate, calcium carbonate) or base (i.e., magnesium hydroxide) may be required for some EISB applications, to neutralize pre-existing acidic groundwater conditions or to maintain pH > 6.0 during EISB. When buffering is required for an anaerobic bioremediation application it is critical to complete bench scale tests with both groundwater and aquifer solids to confirm the buffer capacity of the site materials.
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[[File:Fig1_SWQI.png|thumbnail|left|650px|Figure 1. Typical variation in SWQI Parameters over time with distance from injection. Graphs compare concentrations in injection area, 25 m, 50 m and 100 m downgradient for 40 years post-injection<ref name=Bordenetal2015 />.]]
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Organic substrate addition causes a rapid increase in total organic carbon (TOC) within injection area monitoring wells with maximum concentrations typically ranging from 50 to 500 mg/L (Fig. 1).  However, much higher TOC concentrations were observed at some sites<ref name="TB2015" />. TOC concentrations often remain high for several years in the injection area due to the use of slow release electron donors (e.g., emulsified vegetable oil or ((EVO)) and/or repeated substrate injections, and then decline once substrate addition ends. However, low levels of TOC may continue to be released from endogenous decay of accumulated biomass<ref>Sleep, B.E., A.J. Brown, and B.S. Lollar. 2005. Long-term tetrachlorethene degradation sustained by endogenous cell decay. Journal of Environmental Engineering Science, 4(1), 11–17. [http://www.nrcresearchpress.com/doi/abs/10.1139/s04-038#.VoqpE_krLcs doi:10.1139/s04-038]</ref><ref>Adamson, D.T., and C.J Newell. 2009. Support of source zone bioremediation through endogenous biomass decay and electron donor recycling. Bioremediation Journal, 13(1), 29-40. [http://www.tandfonline.com/doi/abs/10.1080/10889860802690539 doi:10.1080/10889860802690539]</ref>. Increases in carbon loading are expected to result in greater SWQI formation. However, these SWQIs will attenuate with time and distance downgradient. Reducing the carbon loading to decrease SWQIs production may reduce treatment efficiency, possibly resulting in greater exposure to chlorinated solvents and other contaminants.  
  
===Bioaugmentation===
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Maximum TOC concentrations in downgradient wells are generally much lower than in the injection area, indicating TOC in the aqueous phase is rapidly consumed and does not migrate long distances downgradient. Rapid consumption of TOC in the injection area is due to reactions with background electron acceptors (e.g., O<sub>2</sub>, NO<sub>3</sub><sup>-</sup>, Mn<sup>4+</sup>, Fe<sup>3+</sup>, SO<sub>4</sub><sup>2-</sup>), the target contaminants, and fermentation to CH<sub>4</sub>. Since TOC is largely restricted to the injection area, these redox reactions are also largely restricted to the same area. Thermodynamic calculations indicate that reduction reactions should proceed in the order of O<sub>2</sub>, NO<sub>3</sub><sup>-</sup>, Mn<sup>4+</sup>, Fe<sup>3+</sup>, SO<sub>4</sub><sup>2-</sup>, and CO<sub>2</sub>. However, these processes often overlap (e.g., methane production occurring before complete sulfate reduction) due to spatial variability, energy limitations from low reactant concentrations, slow reaction kinetics, and addition of excess electron donor<ref>Cozzarelli, I.M., J.M. Sulfita, G.A. Ulrich, S.H. Harris, M.A. Scholl, J.L. Schlottmann, and S.C. Christenson, 2000. Geochemical and microbiological methods for evaluating anaerobic processes in an aquifer contaminated by landfill leachate. Environmental Science and Technology, 34(18), 4025-4033. [http://pubs.acs.org/doi/abs/10.1021/es991342b doi: 10.1021/es991342b]</ref><ref>Jakobsen, R., and Postma, D., 1999. Redox zoning, rates of sulfate reduction and interactions with Fe-reduction and methanogenesis in a shallow sandy aquifer, Rømø, Denmark. Geochimica et Cosmochimica Acta, 63(1), 137-151. [http://www.sciencedirect.com/science/article/pii/S0016703798002725 doi:10.1016/S0016-7037(98)00272-5]</ref>.
Adding microbial cultures for bioaugmentation may be considered at a site when an appropriate population of anaerobic microorganisms is not present or sufficiently active to stimulate complete anaerobic degradation of the existing contaminants. For example, the presence of Dehalococcoides-related microorganisms has been linked to complete dechlorination of PCE and TCE to ethene in the field<ref>Parsons, 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. AFCEE, NFEC, ESTCP. [[Media:AFCEE_Principles_and_Practices.pdf|Report pdf]]</ref><ref>Major, D.W., McMaster, M.L., Cox, E.E., Edwards, E.A., Dworatzek, S.M., Hendrickson, E.R., Starr, M.G., Payne, J.A. and Buonamici, L.W., 2002. Field demonstration of successful bioaugmentation to achieve dechlorination of tetrachloroethene to ethene. Environmental Science & Technology, 36(23), 5106-5116. [https://doi.org/10.1021/es0255711 doi 10.1021/es0255711]</ref><ref>Hendrickson, E.R., Payne, J.A., Young, R.M., Starr, M.G., Perry, M.P., Fahnestock, S., Ellis, D.E. and Ebersole, R.C., 2002. Molecular analysis of Dehalococcoides 16S ribosomal DNA from chloroethene-contaminated sites throughout North America and Europe. Applied and Environmental Microbiology, 68(2), 485-495. [https://doi.org/10.1128/aem.68.2.485-495.2002 doi 10.1128/aem.68.2.485-495.2002]</ref><ref>Lendvay, J.M., Löffler, F.E., Dollhopf, M., Aiello, M.R., Daniels, G., Fathepure, B.Z., Gebhard, M., Heine, R., Helton, R., Shi, J., Krajmalnik-Brown, R., Major Jr., C.L., Barcelona, M.J., Petrovskis, E., Hickey, R., Tiedje, J.M., Adriaens, P., 2003. Bioreactive barriers: a comparison of bioaugmentation and biostimulation for chlorinated solvent remediation. Environmental Science & Technology, 37(7), 1422-1431. [https://doi.org/10.1021/es025985u doi 10.1021/es025985u]</ref>. Commercially available bioaugmentation products that contain these microorganisms include KB-1<sup>®</sup>, SDC-9™, and Bio-Dechlor Inoculum<sup>®</sup> Plus.  
 
  
==Amendment Dose==
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===Oxygen (O<sub>2</sub>) and Nitrate (NO<sub>3</sub><sup>-</sup>)===
Once the amendments for an application are selected, the quantity, concentration and frequency of amendment addition can be evaluated. To define the amendment dose, the site conditions are reviewed in terms of remedial objectives (i.e., treatment targets, longevity) to obtain the basis of dosing calculations including dimensions of the targeted subsurface zone and associated pore volume, groundwater velocity, concentrations of terminal electron acceptors (for electron donors), the type of contaminants and geochemical characteristics (e.g., initial redox state, pH). Although conservative designs are typically applied, the potential for producing methane gas or other gases (i.e. hydrogen sulfide) must also be considered, especially in shallow aquifers. As described above, the results from bench scale testing may be used to support this evaluation.
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In the injection area, O<sub>2</sub> and NO<sub>3</sub><sup>-</sup> decline rapidly following substrate addition and often remain low for years after TOC declines due to reduction of O<sub>2</sub> and NO<sub>3</sub><sup>-</sup> by sediment organic carbon and/or reduced minerals. Concentrations of O<sub>2</sub> and NO<sub>3</sub><sup>-</sup> in downgradient wells decline with the arrival of anaerobic, oxygen- and nitrate-depleted water. In most cases, there is little or no increase in O<sub>2</sub> or NO<sub>3</sub><sup>-</sup> with distance downgradient due to the limited mixing between the anaerobic plume and background aerobic groundwater<ref name="BB1986">Borden, R.C. and P.B. Bedient, 1986. Transport of dissolved hydrocarbons influenced by reaeration and oxygen limited biodegradation: 1. Theoretical development. Water Resources Research, 22(13), 1973-1982. [http://onlinelibrary.wiley.com/doi/10.1029/WR022i013p01983/full doi:10.1029/WR022i013p01983]</ref><ref>Schirmer, M., G.C. Durrant, J.W. Molson, and E.O. Frind, 2001. Influence of transient flow on contaminant biodegradation. Groundwater, 39(2), 276-282. [http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2001.tb02309.x/abstract doi:10.1111/j.1745-6584.2001.tb02309.x]</ref>.
  
In the case of electron donors, dosing is based on “electron donor demand” which accounts for consumption of the amendment by treating the target constituent(s) as well as competing electron acceptors. An on-line [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200626 Planning Aqueous Amendment Injection Systems Soluble Substrate Design Tool] is available from ESTCP for estimating electron donor demand<ref name= "Borden2012">Borden, R.C., Cha, K.Y., Simpkin, T. and Lieberman, M.T., 2012. Development of a Design Tool for Planning Aqueous Amendment Injection Systems Soluble Substrate Design Tool (No. ER-200626). North Carolina State Univ. at Raleigh, NC. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200626/ER-200626 Report pdf]</ref>, and electron donor vendors may also have design tools to support EISB applications. These calculations can also be used to evaluate the need for and frequency of repeat injection events, which will then be confirmed based on performance monitoring. However, it is important to note that previous applications have shown that the treatment period may be extended beyond the longevity of the amended electron donor as a result of endogenic cell decay of biomass. Thus, the initial biomass growth stimulated by electron donor addition may serve as a secondary source of electron donor<ref>Adamson, D.T. and Newell, C.J., 2009. Support of source zone bioremediation through endogenous biomass decay and electron donor recycling. Bioremediation Journal, 13(1), 29-40. [http://dx.doi.org/10.1080/10889860802690539 doi: 10.1080/10889860802690539]</ref>.
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===Sulfate (SO<sub>4</sub><sup>2-</sup>)===
 +
SO<sub>4</sub><sup>2-</sup> concentrations follow the same general pattern as O<sub>2</sub> and NO<sup>3-</sup>, with an initial decline in the injection area following substrate addition. However, biodegradation coupled to sulfate reduction is less energetically favorable than biodegradation coupled to O<sub>2</sub>, NO<sub>3</sub><sup>-</sup>, Mn<sup>4+</sup>, and Fe<sup>3+</sup> reduction. As a result, SO<sub>4</sub><sup>2-</sup> is depleted more slowly than these other terminal electron acceptors (TEAs) and substantial amounts of SO<sub>4</sub><sup>2-</sup> may persist in the injection area if background SO<sub>4</sub><sup>2-</sup> levels are high. In downgradient wells, SO<sub>4</sub><sup>2-</sup> concentrations decline with the arrival of anaerobic, low SO<sub>4</sub><sup>2-</sup> groundwater. In most cases, there is little increase in SO<sub>4</sub><sup>2-</sup> with distance downgradient due to the very limited mixing between the anaerobic low SO<sub>4</sub><sup>2-</sup> plume and background higher SO<sub>4</sub><sup>2-</sup> groundwater. The amount of dissolved S<sup>2-</sup> produced from SO<sub>4</sub><sup>2-</sup> reduction depends on the SO<sub>4</sub><sup>2-</sup> concentration, extent of SO<sub>4</sub><sup>2-</sup> reduction, and the amount of Fe<sup>2+</sup> and Mn<sup>2+</sup> in groundwater. If S<sup>2-</sup> is in excess (Saturation Index of FeS>1), then S<sup>2-</sup> will persist and limit the extent of Fe<sup>2+</sup> released to solution. In practice, at most sites sufficient sediment-bound Mn and Fe are present to react with S<sup>2-</sup> and precipitate as low solubility sulfide minerals<ref>Cozzarelli, I.M., J.S. Herman, M.J. Baedecker, and J.M. Fischer, 1999. Geochemical heterogeneity of a gasoline-contaminated aquifer. Journal of Contaminant Hydrology, 40(3), 261-284. [http://www.sciencedirect.com/science/article/pii/S0169772299000509 doi:10.1016/S0169-7722(99)00050-9]</ref>. As a result, aqueous sulfide concentrations are low in the injection area and downgradient aquifer<ref name="TB2015" />. In uncommon cases where SO<sub>4</sub><sup>2-</sup> concentrations are high and solid phase Fe is low, some dissolved sulfide may migrate a short distance downgradient before reacting or precipitating. Once injection area TOC declines, SO<sub>4</sub><sup>2-</sup> is expected to recover somewhat more rapidly than O<sub>2</sub> or NO<sub>3</sub><sup>-</sup>.
  
==Milestones, Metrics, and Endpoints==
+
===Manganese, Iron, and Arsenic===
Common parameters to evaluate the success of bioremediation applications include operational monitoring during implementation (e.g., amendment concentrations, injection rates, injection pressures, achieved volumes) and treatment performance monitoring. The performance monitoring program typically includes the following parameters to evaluate:
+
Excess TOC in the injection area will stimulate reduction of solid phase Mn<sup>4+</sup> and Fe<sup>3+</sup>, causing a gradual increase in Mn<sup>2+</sup> and Fe<sup>2+</sup>. Much of the reduced Mn<sup>2+</sup> and Fe<sup>2+</sup> produced in these reactions will be retained on the aquifer material within the injection area through a variety of processes including ion exchange and surface complexation reactions<ref>Davis, J.A., and D.B. Kent, 1990. Surface complexation modeling in aqueous geochemistry. In:  M.F. Hochella and A.F. White (eds.), Mineral-Water Interface Geochemistry. Washington, DC. Mineralogical Society of America, 23, 177-260. [http://rimg.geoscienceworld.org/content/23/1/177.citation Article]</ref>. However, a portion of the reduced Mn<sup>2+</sup> and Fe<sup>2+</sup> will be in the aqueous phase and migrate downgradient with natural groundwater flow. Sorption reactions will diminish the rate of transport of Fe and Mn compared to the groundwater flow rate as well as the maximum concentrations observed in downgradient wells. However, significant increases in Mn<sup>2+</sup> and Fe<sup>2+</sup> have been observed in monitoring wells at significant distances downgradient<ref name="TB2015" />. In some aquifers, Mn<sup>2+</sup> and Fe<sup>2+</sup> concentrations have been observed to decline before TOC is depleted, presumably due to depletion of bioavailable Mn<sup>4+</sup> and Fe<sup>3+</sup> in the injection-area aquifer materials. Once TOC levels decline in the injection area, production of additional Mn<sup>2+</sup> and Fe<sup>2+</sup> will slow and dissolved Mn and Fe concentrations in monitoring wells should start to decline. 
*the distribution of amendments (e.g., conductivity, turbidity, total organic carbon, volatile fatty acids, methane)
 
*the resulting changes in the geochemistry (e.g., redox, pH, anions, cations)
 
*associated changes in desired microbiological populations (e.g., molecular and enzyme analyses) using [[Molecular Biological Tools - MBTs | molecular biological tools]]
 
*influence on the target contaminants (i.e., cVOCs and their daughter products)
 
*confirmation of degradation, and potentially degradation processes, through [[Compound Specific Isotope Analysis (CSIA) | compound specific isotope analysis]]
 
  
Potential for adverse effects from amendments and/or their by-products (i.e., methane in soil gas, hydrogen sulfide, metals mobilization) is also evaluated on a site-specific basis. The monitoring plan should be adaptive to accommodate observed changes in groundwater quality at the site. Performance thresholds or triggers should be used to evaluate the performance monitoring results and identify when additional remedial contingencies may be needed to achieve the remedial objectives and/or when the remedy is complete.
+
In some aquifers, arsenic (As) is naturally present as As<sup>4+</sup> sorbed or coprecipitated with Fe<sup>3+</sup> or other minerals<ref>Pierce, M.L., and C.B. Moore, 1982. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Research, 16(7), 1247-1253. [http://www.sciencedirect.com/science/article/pii/0043135482901439 doi:10.1016/0043-1354(82)90143-9]</ref><ref>Smedley, P.L., and D.G. Kinniburgh, 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17(5), 517-568. [http://www.sciencedirect.com/science/article/pii/S0883292702000185 doi:10.1016/S0883-2927(02)00018-5]</ref><ref>Kent, D.B., and P.M. Fox, 2004. The influence of groundwater chemistry on arsenic concentrations and speciation in a quartz sand and gravel aquifer. Geochemical Transactions, 5(1), 1-12. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1475781/ doi:10.1186/1467-4866-5-1]</ref>. The general term sorption is used to encompass all binding mechanisms of As to Fe<sup>3+</sup> phases. If arsenic is present in these forms, the TOC addition to remediate contaminants could release dissolved arsenic to groundwater by both reduction of Fe<sup>3+</sup> to Fe<sup>2+</sup> and As<sup>5+</sup> to As<sup>3+</sup>. As Fe-rich groundwater migrates downgradient, Fe<sup>2+</sup> sorbs to the sediment or precipitates as Fe-bearing minerals (FeS, carbonates, magnetite)<ref>Fredrickson J.K., J.M. Zachara, D.W. Kennedy, H. Dong, T.C. Onstott, N.W. Hinman, and S. Li. 1998. Biogenic iron mineralization accompanying dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium. Geochimica et Cosmochimica Acta, 62, 3239-3257. [http://www.sciencedirect.com/science/article/pii/S0016703798002439 doi:10.1016/S0016-7037(98)00243-9]</ref> and aqueous Fe concentrations decline<ref name="Ng2015"/>. Available monitoring data suggest that arsenic follows a similar pattern and aqueous arsenic concentrations decline as the anaerobic plume migrates downgradient and encounters Fe<sup>3+</sup>-rich sediments<ref>Cozzarelli, I.M., M.E. Schreiber, M.L. Erickson, and B.A. Ziegler, 2015. Arsenic cycling in hydrocarbon plumes: secondary effects of natural attenuation. Groundwater, 54, 35–45. [http://onlinelibrary.wiley.com/doi/10.1111/gwat.12316/full doi:10.1111/gwat.12316]</ref><ref name="TB2015" />. Once TOC concentrations in the injection area decline, Fe<sup>3+</sup> and As<sup>5+</sup> reduction is expected to decline with a concurrent decline in arsenic release.  At sites where concentrations of sediment-bound As<sup>5+</sup> are low, minimal arsenic will be released.  
  
==Field Demonstrations and Performance==
+
===Methane===
There have been numerous field demonstrations of anaerobic bioremediation documented in publicly available literature and reports, including:
+
TOC fermentation products (H<sub>2</sub> and acetate) in the injection area that are not consumed in microbially-mediated reactions with contaminants or background electron acceptors will be fermented to CH<sub>4</sub>. This can result in high CH<sub>4</sub> concentrations in the injection area and downgradient aquifer<ref>Jacob, C., E.F. Weber, J.N. Bet, and A.K. Macnair, 2005. Full-scale enhanced reductive dechlorination using sodium lactate and vegetable oil. In: Proceedings of the Eighth International In Situ and On-Site Bioremediation Symposium (Baltimore, MD; June 2005), Battelle Press, Columbus, OH.</ref>. If the sum of gas partial pressures (mainly N<sub>2</sub> + CO<sub>2</sub> + CH<sub>4</sub>) exceeds the hydrostatic pressure, these gases will come out of solution and form bubbles<ref>Amos, R.T. and K.U. Mayer, 2006. Investigating the roles of gas bubble formation and entrapment in contaminated aquifers: reactive transport modelling. Journal of Contaminant Hydrology, 87(1-2), 123-154. [http://www.sciencedirect.com/science/article/pii/S0169772206000891 doi:10.1016/j.jconhyd.2006.04.008]</ref>. This occurs primarily near the water table due to the lower hydrostatic pressure there compared to deeper levels. At greater depths, groundwater can appear to be supersaturated with CH<sub>4</sub> due to the higher pressure. In relatively homogeneous, coarse grained sediments, gas bubbles can migrate upward into the vadose zone, removing CH<sub>4</sub> from the aquifer. However, in finer grained sediments, upward migration of gas bubbles will be more limited. If the gas bubbles are not released to the vadose zone, they may eventually dissolve, releasing CH<sub>4</sub> back into groundwater. Dissolved CH<sub>4</sub> produced in the injection area will migrate downgradient with groundwater flow. Since mixing between the anaerobic plume and aerobic background groundwater is low, aerobic methane oxidation will be limited<ref name="BB1986" />. Recent research suggests that CH<sub>4</sub> plume migration may be limited by anaerobic oxidation of methane (AOM) using NO<sub>3</sub><sup>2-</sup><ref>Raghoebarsing, A.A., A. Pol, K.T. van de Pas-Schoonen, A.J.P. Smolders, K.F. Ettwig, W.I.C. Rijpstra, S. Schouten, J.S.S. Damste, H.J.M. Op den Camp, M.S.M. Jetten, and M. Strous. 2006.  A microbial consortium couples anaerobic methane oxidation to denitrification.  Nature 440, no. 7086, 918-921. [http://www.nature.com/nature/journal/v440/n7086/abs/nature04617.html doi:10.1038/nature04617]</ref>, Mn<sup>3/4+</sup> and Fe<sup>3+</sup><ref>Beal, E.J., C.H. House, and V.J. Orphan, 2009.  Manganese- and iron-dependent marine methane oxidation. Science 325, no. 5937: 184-187, [http://www.sciencemag.org/content/325/5937/184.abstract doi:10.1126/science.1169984]</ref><ref>Amos, R.T., B.A Bekins, I.M Cozzarelli, M.A. Voytek, J.D. Kirshtein, E.J.P. Jones, and D.W. Blowes, 2012. Evidence for iron-mediated anaerobic methane oxidation in a crude oil-contaminated aquifer. Geobiology, 10(6), 506-517. [http://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2012.00341.x/abstract doi:10.1111/j.1472-4669.2012.00341.x]</ref> or SO<sub>4</sub><sup>2-</sup><ref>Caldwell, S.L., Laidler, J.R., Brewer, E.A., Eberly, J.O., Sandborgh, S.C. and Colwell, F.S., 2008. Anaerobic oxidation of methane: mechanisms, bioenergetics, and the ecology of associated microorganisms. Environmental sScience & Technology, 42(18), 6791-6799. [http://dx.doi.org/10.1016/j.jconhyd.2007.06.002 doi:10.1021/es800120b]</ref><ref>Grossman, E.L., L.A. Cifuentes, and I.M. Cozzarelli, 2002. Anaerobic methane oxidation in a landfill-leachate plume. Environmental Science & Technology, 36(11), 2436-2442. [http://pubs.acs.org/doi/abs/10.1021/es015695y doi:10.1021/es015695y]</ref> as terminal electron acceptors. However, this reaction may be slow or may not occur at some sites. As a result, dissolved CH<sub>4</sub> can migrate long distances in some aquifers. Once TOC in the injection area declines, CH<sub>4</sub> production will stop and dissolved CH<sub>4</sub> should be transported downgradient by groundwater flow.
*U.S. EPA Contaminated Site Clean-Up Information (CLU-IN)<ref>USEPA 2016. Anaerobic Bioremediation (Direct) Application.</ref>
 
*In Situ Bioremediation of Chlorinated Ethene DNAPL Source Zones: Case Studies<ref name= "ITRC2007">ITRC, 2007. In Situ Bioremediation of Chlorinated Ethene DNAPL Source Zones: Case Studies, BioDNAPL-2, 173 pgs. [[Media:ITRC-2007-Bioremed_of_Chlorinated_Ethene.pdf|Report pdf]]</ref>  
 
*ESTCP Demonstrations: Cost and Performance Reports
 
**ER-0008: Biodegradation of Dense Non-Aqueous Phase Liquids (DNAPLs) through Bioaugmentation of Source Areas - Dover National Test Site<ref>ESTCP, 2008. Biodegradation of Dense Non-Aqueous Phase Liquids (DNAPLs) through Bioaugmentation of Source Areas - Dover National Test Site, Dover, Delaware: ESTCP Cost and Performance Report. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200008/ER-200008 ESTCP Project ER-0008]</ref>  
 
**ER-0221: Edible Oil Barriers for Treatment of Chlorinated Solvent Groundwater<ref>Lieberman, M.T. and Borden, R.C., 2009. Edible Oil Barriers for Treatment of Chlorinated Solvent Contaminated Groundwater. Solutions Industrial and Environmental Services, Inc. Raleigh, NC. [[Media:PRB-ER-0221-FR.pdf|Report pdf]]</ref>  
 
**ER-200219: Comparative Demonstration of Active and Semi-Passive In Situ Bioremediation Approaches for Perchlorate Impacted Groundwater: Active In Situ Bioremediation Demonstration (Aerojet Facility)<ref name= "Cox2012">Cox, E. and Krug, T., 2012. Comparative Demonstration of Active and Semi-Passive In Situ Bioremediation Approaches for Perchlorate Impacted Groundwater: Active In Situ Bioremediation Demonstration (Aerojet Facility) ESTCP Project ER-200219. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-200219 ER-200219]</ref>
 
**ER-200627: Loading Rate and Impacts of Substrate Delivery for Enhanced Anaerobic Bioremediation<ref name= "ESTCP2010LR">Henry, B., 2010. Loading Rate and Impacts of Substrate Delivery for Enhanced Anaerobic Bioremediation. ESTCP Project ER-200627, 90 pgs. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200627 ER-200627]</ref>
 
*Regulatory / Guidance Documents<ref name="Robinson2009"/><ref name= "Borden2012"/><ref name= "ITRC2007"/><ref name= "Cox2012"/><ref name= "ESTCP2010LR"/>
 
  
==Summary==
+
==Implications for Remediation==
The design of a successful anaerobic bioremediation application depends on a strong understanding of the CSM and remedial goals for the site. Bioremediation can be adapted to work in a wide range of site conditions, and the amendment selection and delivery methods are designed to be effective for the site-specific hydrogeologic conditions, contaminants, biogeochemistry, and infrastructure constraints to satisfy the remedial goals.
+
When implementing enhanced anaerobic bioremediation systems, designers should be aware that some level of SWQIs occur at every site. However, concentrations of all parameters decline with distance downgradient following reasonably well understood and predictable processes. Importantly, elevated levels of SWQI parameters are usually confined within the original contaminant plume and are unlikely to adversely impact potable water supplies<ref name="TB2015" />.
  
 
==References==
 
==References==
Line 102: Line 49:
  
 
==See Also==
 
==See Also==
*[https://clu-in.org/techfocus/default.focus/sec/Bioremediation/cat/Anaerobic_Bioremediation_(Direct)/p/2  Bioremediation]
+
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-2131 Numerical Modeling of Post-Remediation Impacts of Anaerobic Bioremediation on Groundwater Quality]
*[https://clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/epa_2006_engin_issue_bio.pdf In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated Sites]
 
*[https://clu-in.org/download/remed/Bioaug2005.pdf Bioaugmentation For Remediation of Chlorinated Solvents]
 
*[https://frtr.gov/costperformance/pdf/remediation/principles_and_practices_bioremediation.pdf Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents]
 
*[http://www.itrcweb.org/Team/Public?teamID=23 Bioremediation of DNAPLs]
 
*[http://www.itrcweb.org/Team/Public?teamID=33 In Situ Bioremediation]
 
*[http://navfac.navy.mil/content/dam/navfac/Specialty%20Centers/Engineering%20and%20Expeditionary%20Warfare%20Center/Environmental/Restoration/er_pdfs/b/navfac-ev-fs-biorem-dnapl-20120412.pdf Using Bioremediation in Dense Non-Aqueous Phase Liquid Source Zones]
 
*[http://navfac.navy.mil/content/dam/navfac/Specialty%20Centers/Engineering%20and%20Expeditionary%20Warfare%20Center/Environmental/Restoration/er_pdfs/d/navfacexwc-ev-tm-1501-erd-design-201503f.pdf Technical Memorandum - Design Considerations for Enhanced Reductive Dechlorination]
 
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200008  Biodegradation of Dense Non-Aqueous Phase Liquids (DNAPL) Through Bioaugmentation of Source Areas]
 

Latest revision as of 02:01, 28 April 2022

Secondary water quality impacts (SWQIs) from anaerobic bioremediation are changes in water chemistry that result from adding an organic substrate to the subsurface to enhance contaminant biodegradation. Common SWQIs that occur at most sites include increases in dissolved manganese, iron, and methane. Fortunately, SWQIs are usually restricted to areas that are already contaminated and attenuate with distance downgradient in well-understood ways.

Related Article(s):


Contributor(s): Dr. Robert Borden, P.E.


Key Resource(s):

Introduction

Electron donor addition can be very effective in stimulating enhanced anaerobic bioremediation (EAB) of a wide variety of groundwater contaminants including chlorinated solvents, high explosives, perchlorate, and certain metals and radionuclides[2][3][4][5]. However, this can result in secondary water quality impacts (SWQIs) including decreased levels of dissolved oxygen (O2), nitrate (NO3-), and sulfate (SO42-), and elevated levels of dissolved manganese (Mn2+), dissolved iron (Fe2+), methane (CH4), sulfide (S2-), organic carbon, and naturally occurring hazardous compounds (e.g., arsenic)[1]. Fortunately, the impacted groundwater is usually confined within the original contaminant plume and is unlikely to adversely impact potable water supplies[6].

SWQI Production and Attenuation Conceptual Model

During EAB, large amounts of easily fermented organic substrates are added to the target treatment area to degrade or immobilize the contaminants of concern (CoC). These substrates are fermented to hydrogen (H2), acetate, and other volatile fatty acids that are then used as electron donors by microbes to mediate oxidation-reduction (redox) reactions that reduce dissolved oxygen, nitrate, and sulfate as well as Fe3+ and Mn3/4+ containing minerals and the CoC. Figure 1 shows a typical pattern of SWQI parameters with time in the: 1) injection area; 2) near plume (25 m downgradient); 3) medium-distance plume (50 m downgradient); and 4) far plume (100 m downgradient). These curves were generated using a geochemical model developed to simulate SWQI production and attenuation[7], and calibrated to long-term monitoring data from field sites where organic-rich materials have entered the subsurface[1]. The time frame for production and attenuation of SWQIs can vary from 10 to 100+ years, depending on the amount and duration of substrate addition, groundwater flow velocity, and background electron acceptor concentrations.

Total Organic Carbon (TOC)

Figure 1. Typical variation in SWQI Parameters over time with distance from injection. Graphs compare concentrations in injection area, 25 m, 50 m and 100 m downgradient for 40 years post-injection[1].

Organic substrate addition causes a rapid increase in total organic carbon (TOC) within injection area monitoring wells with maximum concentrations typically ranging from 50 to 500 mg/L (Fig. 1). However, much higher TOC concentrations were observed at some sites[6]. TOC concentrations often remain high for several years in the injection area due to the use of slow release electron donors (e.g., emulsified vegetable oil or ((EVO)) and/or repeated substrate injections, and then decline once substrate addition ends. However, low levels of TOC may continue to be released from endogenous decay of accumulated biomass[8][9]. Increases in carbon loading are expected to result in greater SWQI formation. However, these SWQIs will attenuate with time and distance downgradient. Reducing the carbon loading to decrease SWQIs production may reduce treatment efficiency, possibly resulting in greater exposure to chlorinated solvents and other contaminants.

Maximum TOC concentrations in downgradient wells are generally much lower than in the injection area, indicating TOC in the aqueous phase is rapidly consumed and does not migrate long distances downgradient. Rapid consumption of TOC in the injection area is due to reactions with background electron acceptors (e.g., O2, NO3-, Mn4+, Fe3+, SO42-), the target contaminants, and fermentation to CH4. Since TOC is largely restricted to the injection area, these redox reactions are also largely restricted to the same area. Thermodynamic calculations indicate that reduction reactions should proceed in the order of O2, NO3-, Mn4+, Fe3+, SO42-, and CO2. However, these processes often overlap (e.g., methane production occurring before complete sulfate reduction) due to spatial variability, energy limitations from low reactant concentrations, slow reaction kinetics, and addition of excess electron donor[10][11].

Oxygen (O2) and Nitrate (NO3-)

In the injection area, O2 and NO3- decline rapidly following substrate addition and often remain low for years after TOC declines due to reduction of O2 and NO3- by sediment organic carbon and/or reduced minerals. Concentrations of O2 and NO3- in downgradient wells decline with the arrival of anaerobic, oxygen- and nitrate-depleted water. In most cases, there is little or no increase in O2 or NO3- with distance downgradient due to the limited mixing between the anaerobic plume and background aerobic groundwater[12][13].

Sulfate (SO42-)

SO42- concentrations follow the same general pattern as O2 and NO3-, with an initial decline in the injection area following substrate addition. However, biodegradation coupled to sulfate reduction is less energetically favorable than biodegradation coupled to O2, NO3-, Mn4+, and Fe3+ reduction. As a result, SO42- is depleted more slowly than these other terminal electron acceptors (TEAs) and substantial amounts of SO42- may persist in the injection area if background SO42- levels are high. In downgradient wells, SO42- concentrations decline with the arrival of anaerobic, low SO42- groundwater. In most cases, there is little increase in SO42- with distance downgradient due to the very limited mixing between the anaerobic low SO42- plume and background higher SO42- groundwater. The amount of dissolved S2- produced from SO42- reduction depends on the SO42- concentration, extent of SO42- reduction, and the amount of Fe2+ and Mn2+ in groundwater. If S2- is in excess (Saturation Index of FeS>1), then S2- will persist and limit the extent of Fe2+ released to solution. In practice, at most sites sufficient sediment-bound Mn and Fe are present to react with S2- and precipitate as low solubility sulfide minerals[14]. As a result, aqueous sulfide concentrations are low in the injection area and downgradient aquifer[6]. In uncommon cases where SO42- concentrations are high and solid phase Fe is low, some dissolved sulfide may migrate a short distance downgradient before reacting or precipitating. Once injection area TOC declines, SO42- is expected to recover somewhat more rapidly than O2 or NO3-.

Manganese, Iron, and Arsenic

Excess TOC in the injection area will stimulate reduction of solid phase Mn4+ and Fe3+, causing a gradual increase in Mn2+ and Fe2+. Much of the reduced Mn2+ and Fe2+ produced in these reactions will be retained on the aquifer material within the injection area through a variety of processes including ion exchange and surface complexation reactions[15]. However, a portion of the reduced Mn2+ and Fe2+ will be in the aqueous phase and migrate downgradient with natural groundwater flow. Sorption reactions will diminish the rate of transport of Fe and Mn compared to the groundwater flow rate as well as the maximum concentrations observed in downgradient wells. However, significant increases in Mn2+ and Fe2+ have been observed in monitoring wells at significant distances downgradient[6]. In some aquifers, Mn2+ and Fe2+ concentrations have been observed to decline before TOC is depleted, presumably due to depletion of bioavailable Mn4+ and Fe3+ in the injection-area aquifer materials. Once TOC levels decline in the injection area, production of additional Mn2+ and Fe2+ will slow and dissolved Mn and Fe concentrations in monitoring wells should start to decline.

In some aquifers, arsenic (As) is naturally present as As4+ sorbed or coprecipitated with Fe3+ or other minerals[16][17][18]. The general term sorption is used to encompass all binding mechanisms of As to Fe3+ phases. If arsenic is present in these forms, the TOC addition to remediate contaminants could release dissolved arsenic to groundwater by both reduction of Fe3+ to Fe2+ and As5+ to As3+. As Fe-rich groundwater migrates downgradient, Fe2+ sorbs to the sediment or precipitates as Fe-bearing minerals (FeS, carbonates, magnetite)[19] and aqueous Fe concentrations decline[7]. Available monitoring data suggest that arsenic follows a similar pattern and aqueous arsenic concentrations decline as the anaerobic plume migrates downgradient and encounters Fe3+-rich sediments[20][6]. Once TOC concentrations in the injection area decline, Fe3+ and As5+ reduction is expected to decline with a concurrent decline in arsenic release. At sites where concentrations of sediment-bound As5+ are low, minimal arsenic will be released.

Methane

TOC fermentation products (H2 and acetate) in the injection area that are not consumed in microbially-mediated reactions with contaminants or background electron acceptors will be fermented to CH4. This can result in high CH4 concentrations in the injection area and downgradient aquifer[21]. If the sum of gas partial pressures (mainly N2 + CO2 + CH4) exceeds the hydrostatic pressure, these gases will come out of solution and form bubbles[22]. This occurs primarily near the water table due to the lower hydrostatic pressure there compared to deeper levels. At greater depths, groundwater can appear to be supersaturated with CH4 due to the higher pressure. In relatively homogeneous, coarse grained sediments, gas bubbles can migrate upward into the vadose zone, removing CH4 from the aquifer. However, in finer grained sediments, upward migration of gas bubbles will be more limited. If the gas bubbles are not released to the vadose zone, they may eventually dissolve, releasing CH4 back into groundwater. Dissolved CH4 produced in the injection area will migrate downgradient with groundwater flow. Since mixing between the anaerobic plume and aerobic background groundwater is low, aerobic methane oxidation will be limited[12]. Recent research suggests that CH4 plume migration may be limited by anaerobic oxidation of methane (AOM) using NO32-[23], Mn3/4+ and Fe3+[24][25] or SO42-[26][27] as terminal electron acceptors. However, this reaction may be slow or may not occur at some sites. As a result, dissolved CH4 can migrate long distances in some aquifers. Once TOC in the injection area declines, CH4 production will stop and dissolved CH4 should be transported downgradient by groundwater flow.

Implications for Remediation

When implementing enhanced anaerobic bioremediation systems, designers should be aware that some level of SWQIs occur at every site. However, concentrations of all parameters decline with distance downgradient following reasonably well understood and predictable processes. Importantly, elevated levels of SWQI parameters are usually confined within the original contaminant plume and are unlikely to adversely impact potable water supplies[6].

References

  1. ^ 1.0 1.1 1.2 1.3 Borden, R. C., Tillotson, J. M., Ng, G.-H. C., Bekins, B. A., Kent, D. B., Curtis, G. P., 2015. Extent and persistence of secondary water quality impacts after enhanced reductive bioremediation. ER-2131. Strategic Environmental Research and Development Program, Arlington, VA. ER-2131
  2. ^ AFCEE (Air Force Center for Engineering and the Environment), NFESC (Naval Facilities Engineering Service Center), ESTCP (Environmental Security Technology Certification Program), 2004. Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. Prepared by Parsons Infrastructure & Technology Group, Inc., Denver, CO, USA. Report pdf
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See Also