Title: Introduction Text: In situ reactive zone (IRZ) treatment involves the active manipulation of subsurface conditions to promote the transformation and/or degradation of a target contaminant into a less mobile or less toxic form. This can be accomplished through geochemical and/or biological mechanisms. This Web Data Sheet provides a brief introduction to IRZ treatment including a discussion of the role of redox and site conditions, the selection of IRZ amendments and delivery mechanisms, and the overall advantages and limitations of this technology.

Title: In Situ Reactive Zone Remediation Text: IRZ technology is currently being used to treat a wide variety of groundwater contaminants including petroleum hydrocarbons, chlorinated solvents, heavy metals, nitrate, perchlorate, energetics, and radionuclides.

Title: In Situ Reactive Zone Remediation Text: IRZ amendments can include electron acceptors (e.g. oxygen) to promote aerobic biodegradation of petroleum hydrocarbons. An electron donor (e.g. ethanol) or a cometabolite (e.g. toluene) can be used to promote chlorinated solvent biodegradation. Various substrates (e.g. molasses) and chemical reagents (e.g. sodium sulfide) can also be used to promote heavy metal reduction and/or precipitation.

Title: In Situ Reactive Zone Remediation Text: IRZ treatment typically involves the delivery of an amendment into the subsurface to alter the geochemical conditions in groundwater and/or to promote the growth of target microorganisms. The primary concern at the field scale is often the ability to effectively deliver the selected amendment to the subsurface.

Title: In Situ Reactive Zone Remediation Text: It is important to monitor the remediation effort to ensure that contamination does not reach potential receptors, as well as to ensure that the proper amount and type of amendment is being supplied for efficient contaminant removal.

Title: Redox Conditions in Groundwater (1 of 2) Text: The oxidation-reduction potential (ORP) of the groundwater indicates the type of geochemical and biological transformation processes that are dominant at a given field site. The objective of any IRZ treatment is to drive the ORP in the subsurface toward the optimal conditions that will promote degradation of the target contaminant. ORP is defined as the electric potential required to transfer electrons from one compound (the electron donor) to another compound (the electron acceptor). Microorganisms such as bacteria obtain energy for growth by transferring electrons from an electron donor to an electron acceptor. It is important to gather sufficient geochemical data including the concentrations of available electron acceptors such as dissolved oxygen (DO), nitrate, and sulfate in groundwater to determine the prevailing redox condition at the site.

Title: Redox Conditions in Groundwater (2 of 2) Text: IRZ treatment can include the injection of an organic substrate into the subsurface to promote biodegradation near the contaminant source. The biodegradation of the organic substrate will rapidly deplete the dissolved oxygen (DO) levels near the contaminant source and the center of the plume will be driven toward a more anaerobic and reduced state. Once DO levels are depleted, anaerobic microorganisms will typically use available electron acceptors in the following order: nitrate, Mn(IV), Fe(III) hydroxide, sulfate, and carbon dioxide. Because the objective of IRZ treatment is to control the subsurface ORP levels, it is important to understand which ORP values correspond to different geochemical and biological conditions. Roll over the plume zones for more information.

Title: Oxidation-Reduction Potential Text: Aerobic biodegradation will occur when dissolved oxygen levels are higher than 1 mg/L (ORP ~ +820 mV).

Title: Oxidation-Reduction Potential Text: After oxygen has been depleted, the conditions will favor the growth of nitrate-reducing organisms (ORP ~ +740 mV) if sufficient nitrate is present in the groundwater. Nitrate levels >0.5 mg/L typically indicate nitrate-reducing conditions are possible.

Title: Oxidation-Reduction Potential Text: At a slightly lower ORP (~ +520 mV) than shown for denitrification, manganese reduction will occur and will result in the dissolution of reduced manganese in groundwater.

Title: Oxidation-Reduction Potential Text: At low ORP levels (~ -50 mV), iron reduction will predominate and dissolved iron levels will rise as Fe(III) hydroxide solid [Fe(OH)3] is transformed to the soluble Fe(II) form by microbial activity. Under these conditions, Fe(II) levels will tend to increase along the groundwater flowpath.

Title: Oxidation-Reduction Potential Text: ORP levels of ~ -220 mV will promote sulfate reduction to hydrogen sulfide and will result in a noticeable "rotten egg" odor to the groundwater. Under these conditions, sulfate will tend to decrease and hydrogen sulfide increase along the groundwater flowpath.

Title: Oxidation-Reduction Potential Text: At very low ORP values (~ -240 mV), methanogenesis will occur, which results in the production of methane from carbon dioxide. Under these conditions, methane levels will tend to increase along the groundwater flow path and dissolved hydrogen gas will be present at >5 nM. The production of excessive quantities of methane can lead to noxious odor and if produced in large quantities, it can even be explosive.

Title: Other Site Conditions Text: Other site conditions that may impact IRZ treatment effectiveness include: pH. Groundwater pH is an important parameter because values less than 5.0 or above 9.0 may impact the viability of native microbes. In addition, pH plays an important role in the precipitation and solubility of metals. Hydrogeology. Site-specific hydrogeology is important including depth to the water table, aquifer saturated thickness, hydraulic conductivity, hydraulic gradient, and the presence of vertical gradients. The hydraulic conductivity of the aquifer should be greater than 1 ft/day and groundwater velocity on the order of >30 ft/yr to successfully promote IRZ treatment within the subsurface. However, high groundwater flux can increase the cost of treatment by requiring more substrate and limiting substrate dispersion away from the groundwater flow path. Geology. Sites with low permeability or a high degree of aquifer heterogeneity may not be suitable for IRZ treatment due to the challenge of amendment distribution. Nature and extent of contamination. The area and depth of the plume to be treated also impacts the system footprint and cost-effectiveness of IRZ treatment. In addition, IRZ treatment is not appropriate for sites with impacted receptors, or with short travel time or distance to potential discharge and/or exposure points.

Title: IRZ Treatments for Selected Contaminants Text: IRZ treatment is applicable to a wide variety of contaminants in groundwater. Click on the table to review the types of amendments and treatment mechanisms applicable to each contaminant. Issues to consider in the selection of an IRZ amendment include chemical costs, adequate supply, presence of nuisance compounds or impurities, solubility in groundwater, capital and operation costs for injection equipment, and the ability to obtain permission for injection of the substance into the subsurface.

Title: Treatments for Selected Contaminants Text: Organic contaminants are treated through biodegradation. In this case, the IRZ treatment approach may be referred to as enhanced in situ biodegradation (EISB) or biostimulation.

Title: Treatments for Selected Contaminants Text: Includes benzene, toluene, ethylbenzene, and xylene (BTEX) and other petroleum hydrocarbons in groundwater resulting from the spill of gasoline, diesel, or fuel oil. The petroleum hydrocarbons may be present at the site in the form of light non-aqueous phase liquids (LNAPLs).

Title: Treatments for Selected Contaminants Text: Aerobic biodegradation is the most commonly used IRZ approach for petroleum hydrocarbons in groundwater. Aerobic processes typically involve more vigorous biomass growth and higher contaminant biodegradation rates than anaerobic processes. In order to establish an aerobic IRZ, oxygen should be delivered in some form to the subsurface. Oxygen can be delivered as a pure gas, air, hydrogen peroxide, or slow release compound such as magnesium oxide. In the presence of sufficient oxygen and nutrients, microorganisms will ultimately convert many petroleum hydrocarbon contaminants to carbon dioxide, water, and microbial cell mass.

Title: Treatments for Selected Contaminants Text: Includes perchloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), vinyl chloride (VC), and other chlorinated volatile organic compounds (CVOCs) in groundwater. These CVOCs may be present at the site in the form of dense non-aqueous phase liquids (DNAPLs).

Title: Treatments for Selected Contaminants Text: Aerobic biodegradation is suitable for a very limited number of CVOCs including VC, dichloromethane, chloromethane, and chloroethane. In order to establish an aerobic IRZ to degrade CVOCs, oxygen should be delivered in some form to the subsurface. Oxygen can be delivered as a pure gas, air, hydrogen peroxide, or a slow release compound such as magnesium oxide. The aerobic biodegradation of VC occurs faster than anaerobic reductive dechlorination due to vigorous biomass growth and higher biodegradation rates. In some cases, a combination of an anaerobic reaction zone near the contaminant source followed by an aerobic oxidation zone further downgradient may be highly effective for treating CVOC plumes.

Title: Treatments for Selected Contaminants Text: Most chlorinated solvents in groundwater are more readily degraded under anaerobic conditions. Anaerobic conditions can be promoted through the addition of an electron donor to the subsurface. The microbes use the organic or inorganic electron donor source (e.g. ethanol or hydrogen gas) for growth and then utilize the chlorinated solvent molecule as a terminal electron acceptor. This causes the reductive dechlorination of the chlorinated compound. These processes typically occur under nitrate- and iron-reducing conditions. However, the most rapid biodegradation rates and the widest range of CVOC biodegradation occurs under sulfate-reducing and methanogenic conditions.

Title: Treatments for Selected Contaminants Text: During cometabolism, microbes incidentally degrade the contaminant through the production of a non-specific enzyme. The chlorinated compound is not used as a carbon or energy source and the microbe receives no benefit from its degradation. Instead, a cometabolite is added to promote microbial growth and therefore enzyme production. Some cometabolic amendments include methane, ethane, propane, toluene, and phenol.

Title: Treatments for Selected Contaminants Text: Several inorganic contaminants in groundwater can be addressed through IRZ technology including heavy metals, nitrate, and perchlorate. Both biotic and abiotic processes are relied upon for IRZ treatment of inorganic constituents.

Title: Treatments for Selected Contaminants Text: Including chromium (e.g. Cr[VI] to Cr[III]), copper, silver, mercury, nickel, zinc, lead, and more.

Title: Treatments for Selected Contaminants Text: Nitrate is NO3- and perchlorate is ClO4-.

Title: Treatments for Selected Contaminants Text: The mobility of metals is typically reduced through sorption and/or precipitation. Many metals are most insoluble as sulfide precipitates. Sodium sulfide can be added to the subsurface to promote the precipitation of copper, silver, mercury, lead, and zinc. Other IRZ amendments which promote the precipitation of insoluble heavy metal solids include lime and ferrous sulfate.

Title: Treatments for Selected Contaminants Text: The toxicity of certain metals can be decreased by changing their valence state through the manipulation of redox conditions in the subsurface. For example, the injection of molasses into groundwater can promote the growth of microbes and the rapid depletion of oxygen within the subsurface. This depletion of oxygen lowers the ORP in the subsurface and causes the subsequent transformation of Cr(VI) to Cr(III), which has a lower toxicity to human and ecological receptors. The end product of this reaction is Cr(OH)3, which is insoluble and readily precipitates out of the groundwater.

Title: Treatments for Selected Contaminants Text: Groundwater contaminated with nitrate and/or perchlorate can be treated through establishing nitrate-reducing conditions in the aquifer. In the absence of oxygen, a variety of microorganisms will use nitrate or perchlorate as an electron acceptor. Nitrate is transformed to nitrite and then to nitrogen, while degradation of perchlorate produces the nontoxic byproducts of chloride and oxygen. Several electron donor amendments have been used for nitrate and perchlorate reduction including acetate, benzoate, ethanol, lactate, hydrogen, methanol, molasses, propane, sucrose, yeast extract, and vegetable oil.

Title: Treatments for Selected Contaminants Text: Energetics include organic compounds such as TNT (2,4,6-trinitrotoluene), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine).

Title: Treatments for Selected Contaminants Text: Although TNT can be degraded under aerobic conditions to intermediate byproducts, anaerobic pathways are the most prevalent for the complete degradation of other nitroaromatic compounds in groundwater. In the presence of a sufficient supply of electron donor, nitroaromatic compounds will biodegrade under iron-reducing, sulfate-reducing, and methanogenic conditions.

Title: Substrates Used for Biotic Zones (1 of 2) Text: The use of IRZ treatment to promote biodegradation can also be referred to as enhanced in situ biodegradation (EISB) or biostimulation. The selection of an appropriate IRZ substrate to promote biodegradation should be site-specific and is usually based on bench-scale tests. Microcosm studies are typically used to screen the selection of a substrate. This figure illustrates a common microcosm setup. The soil and water used in the microcosm are obtained from the site. More information on microcosm studies and IRZ treatment through biodegradation is available in a guidance document co-authored by NAVFAC titled: Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents.

Title: Substrates Used for Biotic Zones (2 of 2) Text: Examples are shown here of a variety of substrates that have been used for IRZ treatment at sites with CVOCs in groundwater. The injection of an organic substrate into the aquifer will promote microbial growth and create an anaerobic environment to enhance rates of CVOC biodegradation. These substrates listed here are classified as soluble substrates, viscous fluids, solid substrates, and experimental substrates. The physical nature of the substrate dictates the delivery technique and the frequency of injections.

Title: Delivery Mechanisms Text: Several methods can be used for the delivery of IRZ amendments into the subsurface including passive, semi-passive, and active injection scenarios. The design of the IRZ system should include specification of the following: Type of substrate to be used and the target dose rate Type of delivery mechanism (e.g. passive, semi-passive, or active injection) Radius of influence and proposed injection (and extraction) well spacing The following slides provide additional details about the various delivery mechanisms available.

Title: Permeable Reactive Barriers or Passive Wells Text: Passive delivery methods are most appropriate for viscous or solid IRZ amendment delivery. Passive delivery methods rely on the natural groundwater gradient and dissolution and dispersion to deliver the amendment into the subsurface. This type of configuration includes permeable reactive barriers or the placement of a slow-release compound into an array of unpumped or "passive" wells (shown here). The spacing between the passive wells is based on maintaining the IRZ amendment at a sufficient concentration within the reactive zone as groundwater migrates along the flow path between the points of injection.

Title: Direct Injection Text: This is a semi-passive delivery strategy which relies upon continuous or periodic forced injection of the IRZ amendment into one well or an array of wells. This direct injection technique is used with soluble or viscous IRZ amendments. Semi-passive systems are best suited to the reduction of contaminant levels in low-concentration plumes and/or to act as a biobarrier or "polishing step" for other remediation methods. Semi-passive systems do not provide hydraulic containment and may produce localized mounding depending on the injection strategy.

Title: Dual Vertical Well Recirculation Text: This configuration consists of a row of upgradient vertical injection wells paired with a row of downgradient vertical extraction wells. Groundwater is extracted from the downgradient wells, the soluble IRZ amendment is added, and the groundwater is reinjected in the upgradient wells. This approach is best suited for the treatment of high-concentration plumes or source areas and can be designed to provide hydraulic containment. It can also help to accelerate cleanup timeframes by promoting mixing within the aquifer.

Title: Vertical Well Recirculation Text: This active delivery strategy is similar to dual well recirculation, but the same well is used for both injection and extraction. The recirculation of groundwater helps to promote the mixing of the soluble IRZ amendment within the aquifer and allows for multiple passes of the contaminated groundwater through the treatment zone.

Title: Advantages Text: Advantages of using IRZ remediation include: IRZ technology can be used to treat a wide variety of groundwater contaminants including petroleum hydrocarbons, chlorinated solvents, heavy metals, nitrate, perchlorate, energetics, and radionuclides. Contamination can be addressed in both groundwater and saturated soils. Contaminants can be immobilized or transformed into less toxic forms. Remediation of the contaminants is performed in situ with minimal aboveground impact, minimal waste generation, and a small footprint at a given site. Relatively low capital costs compared to other ex situ remedial options because the primary capital expenditure is related to injection well installation or direct-push points. Flexible treatment options can be tailored to fit the site strategy such as use of vertical recirculation to expedite the cleanup time or use of a row of passive injections to serve as a barrier to off-site migration.

Title: Limitations Text: Limitations of using IRZ remediation include: Most in situ processes provide less control over system effectiveness and reliability compared to ex situ processes. This may result in longer cleanup timeframes. There may be site-specific conditions that limit treatment effectiveness such as low permeability, high degree of heterogeneity, or inhibitory geochemical conditions (e.g., pH). Some contaminants may not be completely or permanently transformed to innocuous products, and if transformation is incomplete, intermediates may be more toxic and/or mobile than the parent compound. Problems such as production of noxious gases, enhanced plume migration, and biofouling of injection wells could occur in certain cases. Significant regulatory coordination may be needed to obtain approval for amendment injection and/or groundwater reinjection. Some delivery mechanisms may provide only limited hydraulic control.

Title: References/Links Text: Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents. August 2004. Download Page. ESTCP Enhanced Anaerobic Bioremediation of Chlorinated Solvents Cost Estimating Tool. NAVFAC ERB Web Site. Technologies - Bioremediation (Enhanced In Situ). Interstate Technology Regulatory Council (ITRC). In Situ Bioremediation Documents. ITRC. Enhanced In Situ Biodenitrification Documents. EPA. CLU-IN Technology Focus - Bioremediation of Chlorinated Solvents.

Title: Contact Information Text: For more information about IRZ treatment, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil