Overview

In the presence of aerobic conditions and appropriate nutrients, microorganisms can convert many organic contaminants to carbon dioxide, water, and microbial cell mass. Aerobic bioremediation uses oxygen as the electron acceptor. Aerobic metabolism is more commonly exploited and can be effective for hydrocarbons and other organic compounds, such as petroleum hydrocarbons and some fuel oxygenates (e.g., methyl tertiary-butyl ether [MTBE]). Many organisms are capable of degrading hydrocarbons using oxygen as the electron acceptor and the hydrocarbons as carbon and energy sources. Aerobic technologies may also change the ionic form of metals. If a site contains mixed metal and organic wastes, it is necessary to consider whether the oxidized forms of the metal species (such as arsenic) will be environmentally acceptable (EPA 2006a).

Aerobic bioremediation is most often used at sites with mid-weight petroleum products (e.g., diesel fuel and jet fuel), because lighter products such as gasoline tend to volatilize readily and can be removed more rapidly using other technologies (e.g., air sparging or soil vapor extraction). Heavier petroleum products such as lubricating oils generally take longer to biodegrade than the lighter products, but enhanced aerobic bioremediation technologies may still be effective. It is generally not practical to use enhanced aerobic bioremediation technologies to address free mobile product or petroleum contamination in low permeability soil (e.g., clay) (EPA 2004). Environmental fracturing may enhance bioremediation applications in low permeability areas.

Figure 1 shows the redox zones established in response to a typical plume moving through an aquifer. A plume moving with groundwater flow typically will develop distinct redox zones - once an electron acceptor is depleted, a new redox reaction using a new electron acceptor occurs. The electron acceptor that would lead to the next largest generation of energy during the reaction will dominate (EPA 2000).

Figure 1. Redox zones of a typical contaminant plume (Source: Parsons 2004).

Aerobic oxidation can occur naturally under proper conditions, but oxygen, which is often considered to be the primary growth-limiting factor for hydrocarbon-degrading bacteria, is normally depleted in zones that have been contaminated with hydrocarbons. Enhanced aerobic bioremediation technologies focus in part on increasing oxygen levels and can potentially increase biodegradation by several orders of magnitude over naturally-occurring, non-stimulated rates. Enhancements can be used to address contaminants in the unsaturated zone, the saturated zone, or both. Enhancement technologies work by providing a supplemental supply of oxygen to the subsurface, which becomes available to aerobic, hydrocarbon-degrading bacteria. The stoichiometric ratio of oxygen per hydrocarbon is 3 M O₂ per 1 mole of hydrocarbons. The success of aerobic bioremediation highly depends on the ability to deliver oxygen to hydrocarbon-degrading microorganisms. The balance between oxygen sources, oxygen uptake, and the degree to which oxygen is transported through the subsurface largely dictate the effectiveness of a bioremediation system (EPA 2004).

The following sections provide a brief overview of different enhancement technologies.

In Situ

Technologies to accelerate naturally-occurring in situ bioremediation include biosparging, bioventing, and the use of oxygen-releasing substances such as pure oxygen, hydrogen peroxide, ozone, and commercial oxygen-releasing compounds. These technologies work by providing an additional supply of oxygen to the subsurface, which then becomes available to aerobic bacteria. Most enhanced aerobic bioremediation technologies primarily address contaminants that are dissolved in groundwater or are sorbed to soil particles in the saturated zone. Enhanced aerobic bioremediation technologies have typically been used outside source areas (EPA 2004).

Soil

Bioventing involves supplying air or oxygen, and nutrients if needed, into the unsaturated zone. Oxygen is delivered to the unsaturated zone by forced air movement either through extraction or injection of air to increase oxygen concentrations. Direct air injection is used more commonly to control air flow rates and provide only enough oxygen to sustain microbial activity. Bioventing is designed primarily to treat contaminants in the vadose zone or capillary fringe (EPA 2006a).

Bioventing has a strong record of treating aerobically degradable contaminants such as fuels, but also has been used to treat a variety of other contaminants, including nonhalogenated solvents (e.g., benzene, acetone, toluene, and phenol), lightly halogenated solvents (e.g., 1,2-dichloroethane, dichloromethane, and chlorobenzene), and some semi-volatile organic compounds (SVOCs) (e.g., lighter polycyclic aromatic hydrocarbons [PAHs]) (EPA 2006b). To be responsive to bioventing, a site must generally have: low oxygen in soil gas compared to background (<2% volume/volume compared to approximately 15-21%); high (>2%) CO₂ in soil gas compared to background; and elevated levels of volatile hydrocarbons in the soil gas (AFCEE 2012).

While bioventing is relatively inexpensive, this method can take a few years to clean up a site, depending on contaminant concentrations and site-specific removal rates (EPA 2006a, EPA 2006b).

For guidance on bioventing, see Chapter III of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: a Guide for Corrective Action Plan Reviewers. Volume I of Leeson and Hinchee (1996) Principles and Practices of Bioventing Manual contains principles of bioventing in relation to the physical, chemical, and microbial processes occurring in the field and Volume II focuses on bioventing design and process monitoring.

Groundwater or Saturated Soil

Biosparging involves the injection of air or oxygen, and nutrients if needed, into the saturated zone to stimulate microbial activity (although contaminants adsorbed to soils in the unsaturated zone can also be treated by biosparging, bioventing is typically more effective for this situation). The effectiveness of biosparging depends on two primary factors — permeability of the soil and the biodegradability of the constituents. In general, the type of soil will determine its permeability. Fine-grained soil such as clay and silt have lower permeabilities than coarse-grained soil like sand and gravel. Soil with higher permeability will allow more air to move through it to reach the microorganisms (EPA 1994b).

Biosparging can be used to reduce the concentrations of petroleum constituents that are dissolved in groundwater, adsorbed to soil below the water table, and within the capillary fringe. When volatile compounds are present, biosparging is often combined with other remedial technologies such as soil vapor extraction (SVE) or bioventing. When biosparging is combined with SVE, the vapor extraction system creates a negative pressure in the vadose zone through a series of extraction wells that control the migration of the vapor plume (EPA 1994b).

For guidance on biosparging, see Chapter VIII of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers.

Introduction of Oxygen-Releasing Substances is another approach to enhancing aerobic bioremediation in the saturated zone. Commonly used substances include hydrogen peroxide, ozone, commercial oxygen-releasing compounds, and pure oxygen. Oxygen-releasing substances can be used to stimulate bioremediation in the unsaturated zone, but are more commonly introduced into contaminated groundwater or saturated soil. This technique may be used to address source areas, entire plumes, or plume tails (EPA 2004).

Pure Oxygen Injection can be relatively efficient at increasing levels of dissolved oxygen to promote aerobic bioremediation. More oxygen can be transferred to the saturated zone when it is introduced in pure form than if the dissolved oxygen is derived from forced contact between groundwater and atmospheric air, as is done with biosparging. Pure oxygen is most commonly introduced into the subsurface via vapor-phase injection. Vapor-phase oxygen (approximately 95% oxygen) is injected into the saturated zone near the base of the contamination using a network of sparge wells. A series of vertical oxygen injection wells are often alternately sparged in order to increase dissolved oxygen levels more efficiently over larger areas (EPA 2004).

Chapter XII of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: a Guide for Corrective Action Plan Reviewers provides more information on how to apply this enhancement.

Oxygen-Releasing Compounds are introduced to the saturated zone in solid or slurry phases and include calcium and magnesium peroxides. These compounds release oxygen to the aquifer when hydrated by groundwater as the peroxides undergo conversion to their respective hydroxides. Oxygen-releasing compounds may be introduced into the saturated zone in several ways. The most common approaches include (EPA 2004):

• Placing the compounds into drilled boreholes or other excavations.
• Injecting a compound slurry into direct-push borings (e.g., Geoprobe).
• Mixing oxygen-releasing compounds directly with contaminated soil and then using the mixture as backfill or hauling it to a disposal site.
• Suspending oxygen-releasing compounds contained in "socks" in groundwater monitoring wells.
• A combination of the above.

Chapter XII of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers provides more information on how to apply this enhancement.

Oxygen-releasing compounds, including calcium peroxide and magnesium peroxide products, are available through a variety of commercial vendors, each of whom provides product-specific guidance to clients.

Hydrogen Peroxide Enhancement involves the circulation of a dilute solution of hydrogen peroxide through the contaminated groundwater zone to increase the oxygen levels. Hydrogen peroxide decomposes rapidly to oxygen, significantly increasing existing oxygen levels in the saturated zone where hydrogen peroxide is introduced. For each part of hydrogen peroxide introduced into groundwater, one-half part of oxygen can be produced (EPA 2004).

Though hydrogen peroxide has the potential of providing some of the highest levels of available oxygen to contaminated groundwater (theoretically 10% hydrogen peroxide could provide 50,000 ppm of available oxygen), it is cytotoxic to microbes at concentrations greater than 100-200 ppm. Hydrogen peroxide also decomposes quickly to oxygen, potentially within four hours (EPA 2004). This limits the extent to which hydrogen peroxide can be distributed in the subsurface before it is transformed. In addition, increased oxygen bubbles in the saturated zone can potentially reduce hydraulic permeability, which could prevent the distribution of oxygen and circulation of nutrients (ICSS 2006). The precipitation of chemical oxidants such as iron oxides in response to hydrogen peroxide introduction presents additional problems (EPA 2004, Hazen 2010). Significant fouling issues can arise, depending on the concentrations of naturally occurring levels of inorganic compounds, such as iron, in the subsurface (EPA 2004).

To introduce hydrogen peroxide into the saturated zone, a dilute solution is first mixed with extracted groundwater. The hydrogen peroxide-amended groundwater is pumped into infiltration galleries or injection wells located in or near suspected source areas. Generally, the infiltration/injection and groundwater extraction scheme is designed to promote the circulation and distribution of hydrogen peroxide and dissolved oxygen through the treatment area (EPA 2004).

Chapter XII of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: a Guide for Corrective Action Plan Reviewers provides more information on how to apply this enhancement.

Ozone Injection is a technique used both for chemical oxidation and enhanced aerobic bioremediation. Ozone is a strong oxidant and has an oxidation potential greater than that of hydrogen peroxide. Ozone is also 10 times more soluble in water than pure oxygen and about half of dissolved ozone introduced into the subsurface decomposes to oxygen within approximately 20 minutes. Because of its oxidizing potential, ozone injected into the subsurface can be toxic to microbes and can actually suppress subsurface biological activity, but this is generally temporary and a sufficient number of bacteria can survive and resume biodegradation after ozone has been applied (EPA 2004).

Ozone may be injected into the subsurface in a dissolved or gaseous phase. Injection or sparging of gaseous ozone, typically at a 5% concentration, into a contaminated area in the subsurface is more common. Ozone then dissolves in the subsurface water, reacts with subsurface organics, and decomposes to oxygen. Vapor control equipment, such as a soil vapor extraction and treatment system, may be used when ozone injection rates are high enough to emit excess ozone to the unsaturated zone. Less commonly, groundwater is extracted and treated and then used to transport (through re-injection or re-infiltration) the dissolved phase ozone and oxygen into the subsurface contaminated area (EPA 2004).

Chapter XII of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: a Guide for Corrective Action Plan Reviewers provides more information on how to apply this enhancement.

A Permeable Reactive Barrier (PRB) is a passive in situ method for remediating contaminated groundwater. Groundwater flows under a natural gradient through a treatment zone, which contains chemical (e.g., zero-valent iron) or biological (i.e., microorganisms) agents. The contaminants are concentrated and either degraded or retained in the barrier zone. The application of in situ bioremediation using a PRB design is commonly referred to as a biobarrier. Introduction of solid oxygen-releasing materials (e.g. calcium or magnesium peroxide) or direct delivery of oxygen gas into the barrier area is the most common method for promoting sustainable aerobic conditions for treatment of contaminants (ITRC 2005b). The CLU-IN web page on Permeable Reactive Barriers contains more information and guidance on how PRBs can be used to clean up contaminated groundwater.

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Ex Situ

Soil

Biopiles are used to reduce concentrations of contaminants in excavated soil. Bioremediation through use of biopiles involves placing contaminated, excavated soil into piles ("cells") and stimulating microbial activity through aeration and/or addition of amendments such as minerals, nutrients, and moisture. Biopiles are typically maintained at soil temperatures ranging from 10° to 45°C (EPA 1994a, EPA 2006a). The treatment area typically includes an impermeable liner, a leachate collection system, and an aeration system. An air distribution system is buried in the soil as the biopile is constructed. Oxygen exchange can be achieved utilizing vacuum, forced air, or natural draft air flow. Low air flow rates are desirable to minimize contaminant volatilization. If volatile constituents are present in significant concentrations, the biopile may require a cover and treatment of the offgas. Surface drainage and moisture from the leachate collection system may be treated and then recycled back to the contaminated soil. Nutrients such as nitrogen and phosphorus are often added to the recycled water. Alkaline or acidic substances may also be added to the recycled water to modify or stabilize pH (EPA 2006a).

Biopile treatment lasts from a few weeks to a few months, depending on the contaminants present and the design and operational parameters selected for the biopile. This technology is most often applied to readily degradable contaminants, such as petroleum hydrocarbons (EPA 2006a).

Guidance on biopiles, including information on necessary site and constituent characteristics, biopile design evaluation, and operation and monitoring evaluation, can be found in Chapter IV of How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: a Guide for Corrective Action Plan Reviewers and Battelle 1996a and 1996b.

Composting is a controlled biological process that treats organic contaminants using microorganisms under thermophilic conditions, generally at 40° to 50°C. Similar to biopiles, during composting, soils are excavated and mixed with amendments. Bulking agents and organic amendments, such as wood chips and vegetative wastes, may be added to enhance the porosity of the mixture to be decomposed (EPA 2006a) when porosity is low or recalcitrant compounds are being treated (NAVFACa). Degradation of the bulking agent heats up the compost, creating thermophilic conditions (EPA 2006a).

Oxygen content, moisture levels, and temperatures are monitored and manipulated to optimize degradation. Oxygen content usually is maintained by frequent mixing, such as daily or weekly turning of windrows, while moisture content is maintained by surface irrigation. Temperatures are controlled, to a degree, by mixing, irrigation, and air flow, but are also dependent on the degradability of the bulk material and ambient conditions (EPA 2006a).

There are three designs commonly applied for composting (EPA 2006a):

• Aerated static piles—Compost is formed into piles and aerated with blowers or vacuum pumps.
• Mechanically agitated in-vessel composting—Compost is placed in a reactor vessel, in which it is mixed and aerated.
• Windrow composting—Compost is placed in long, low, narrow piles (i.e., windrows) and periodically mixed with mobile equipment.

Composting has been successfully applied to soils and biosolids contaminated with petroleum hydrocarbons, solvents, chlorophenols, pesticides, herbicides, PAHs, and nitroaromatic explosives such as TNT and RDX (EPA 1997, EPA 2006a). Composting also has been used successfully at several sites to treat soils contaminated with perchlorate (ITRC 2005a, ITRC 2008). Composting is not likely to be successful for highly chlorinated substances, such as PCBs, or for substances that are difficult to degrade biologically (EPA 2006a).

For more information on composting, refer to the EPA (2006a) Engineering Issue: In Situ and Ex Situ Biodegradation Technologies for Remediation of Contaminated Sites and the NAVFAC focus page on Biopiles-Composting

Land Farming involves spreading excavated contaminated soils in a thin layer on the ground surface and stimulating aerobic microbial activity within the soils through aeration and/or the addition of minerals, nutrients, and moisture. It is useful in treating aerobically degradable contaminants (EPA 1994). Land farming is suitable for non-volatile contaminants at sites where large areas for treatment cells are available. Land treatment of contaminated soil at the site usually involves the tilling of an 8- to 12-inch layer of the soil to provide aeration; moisture is added when needed. In some cases, amendments may be added to improve the tilth of the soil, supply nutrients, or moderate pH. Typically, full-scale land treatment would be conducted in a prepared-bed land treatment unit, such as an open, shallow reactor with an impermeable lining on the bottom and sides to contain leachate, control runoff, and minimize erosion and with a leachate collection system under the soil layer (EPA 2006a).

The performance of land treatment varies, depending on the contaminants to be treated (EPA 2006a). Sample performance data from land farming systems are summarized below:

Contaminant/Site Type	Pre-Treatment Levels	Post-Treatment Levels	Time Frame
Pentachlorophenol (wood preservation site)	100 mg/kg (in soil)	<5 mg/kg	4 months
Pesticide storage facility	86 ppm	5 ppm	n/a
Fuel oil spill	6,000 ppm (total petroleum hydrocarbons)	100 ppm	120 days

Modified from Land Farming (UHI 2012)

Surface Water

Constructed Wetlands. The CLU-IN web page on Phytotechnologies contains more information on how constructed treatment wetlands can be used to clean up contamination in surface water.

Solid-Water Mixtures

Slurry Bioreactors are used for treatment of soil, sediments, sludge, and other solid or semi-solid wastes. Slurry biodegradation has been shown to be effective in treating highly contaminated soils and sludges that have contaminant concentrations from 2,400 mg/kg to 250,000 mg/kg (EPA 1990). The advantages of treating contaminants in a slurry include: increased mass transfer rate and increased contact between contaminants and microbes; higher rate of biodegradation than with in situ bioremediation; and the ability to control and optimize environmental parameters such as pH and temperature (Robles-González et al. 2008). Generally, wastes are first screened to remove debris and other large objects. They are then mixed with water in a tank or other vessel until solids are suspended in the liquid phase. Typical slurries are 10-30% solids by weight. Tanks are used when mechanical mixing is necessary. Aeration, with submerged aerators or spargers, is often used in lagoons and may be combined with mechanical mixing to achieve the desired results (EPA 2006a).

Microorganisms used for biodegradation in a slurry bioreactor may grow in suspension in the fluid or be attached on a solid growth support medium. The support matrix is often an adsorptive medium, such as activated carbon, that can adsorb contaminants and slowly release them to the microorganisms for degradation (NAVFACb). Indigenous microbes may be used or microorganisms may be added initially to seed the bioreactor or may be added continuously to maintain proper biomass levels (EPA 2006a).

Residence time in the bioreactor varies with the matrix as well as the type and concentration of contaminant (EPA 2006a).

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Performance Monitoring

Typically in aerobic degradation involving a source zone, the area around the source quickly becomes anaerobic and a series of different redox areas is established downgradient from the source, with aerobic degradation occurring at the perimeter of the dissolved contaminant plume (EPA 2000). If the contaminant is not very amenable to anaerobic degradation and bioremediation is chosen as the remedial technology, the addition of an oxidant (e.g., air, oxygen) generally will be needed to keep the aquifer/source zone aerobic, and dispersion of the oxidant should be monitored.

Remedial progress or success with source zone mass removal also can be estimated using flux techniques and tracers. For nonspecific performance monitoring techniques that might be useful, see Remediation Measurement Tools.

The following table lists parameters that can be used to understand the type of biodegradation that is taking place under a certain set of geochemical conditions, as well as whether degradation is occurring. The type of information needed and the associated parameters would be determined in the site-specific data quality objectives process.

Performance Parameter	Method	Data Use	Performance Expectation	Recommended Frequency of Analysis
Contaminants of Concern (COCs)	VOCs, EPA SW-846: 8260B SVOCs, SW-846: 8270D. Other methods might be more appropriate depending upon the COC (e.g., SW-846: 8310 for PAHs and SW-846: 8330A for Nitroaromatics). Field GC or GC/MS might also be appropriate.	Used to determine background and source/plume concentrations of target analytes for subsequent comparison with target analyte concentrations following treatment.	Contaminants are typically expected to decline to less than regulatory compliance levels within the contaminant area or in the case of monitored natural attenuation the plume should stabilize or shrink.	Baseline and recommended for each sampling round.
Degradation products, if appropriate (e.g., nitrobenzene produces aniline)	VOCs, EPA SW-846: 8260B SVOCs, SW-846: 8270D. Field GC or GC/MS can also be appropriate.	Used to determine if degradation is occurring and what products are being produced.	Hazardous degradation products are typically expected to decline to less than regulatory compliance levels within the contaminant area or in the case of monitored natural attenuation the plume should stabilize or shrink.	Baseline and recommended for each sampling round.
Appropriate degrading microorganism	Quantified by quantitative polymerase chain reaction - specialty laboratory	Used to determine the presence and quantity of appropriate microorganism at baseline period or after bioaugmentation.	Appropriate microorganisms will be detected and increase as a consequence of adding electron acceptor.	Baseline prior to remedy initiation and quarterly based on the numbers achieved. Once a high titer is measured and growth is ensured, the test can be continued but is not critical
Compound Specific Isotope Analysis (CSIA),	Specialty laboratory	Used to measure the amounts of stable isotopes in contaminants to determine the extent of specific chemical and biochemical reactions impacting the contaminant.	Can be used to determine a chemical's source, degradation mechanism, and rate of degradation	Baseline if the occurrence of biodegradation is not obvious. Periodically to determine if the rate of degradation is occurring as expected.
Soluble Iron (Suggested by San Mateo County 1996)	Preferred method is to field filter (0.45 �m filter) and ICP 200.7 (measures total of ferric and ferrous iron); alternate field method measuring ferrous iron: Colorimetric Hach Method 8146 (ITRC 2008)	Used to determine the presence and quantity of ferrous iron. Elevated levels of ferrous iron indicate that the groundwater environment is sufficiently reducing to sustain iron reduction and for anaerobic degradation to occur—hence conditions are probably not conducive for aerobic degradation.	Indicator of extent of anaerobic degradation. Useful at sites where MNA is chosen (San Mateo County 1996). Not necessary at sites where active aeration is taking place.	Recommended for each sampling round. Typically measured at the wellhead to protect samples from exposure to oxygen. (ITRC 2008)
Chloride	EPA Method E300.1 or SW-846: 9056A (laboratory methods) Hach chloride test kit Model 8-P, or ISE for field measurements	Used as an indication that dechlorination is occurring if observed concentrations are greater than 3 times background and consistent with CH molar concentrations.	Indicator parameter only.	Baseline and every subsequent sampling event.
Nitrate	EPA Method 300.1 or SW-846: 9056A  (laboratory- based ion chromatography methods)  ISE may be used in field	Used to determine if the nutrient nitrate needs to be injected into the subsurface and whether it is being consumed or accumulated. (EPA 2004)	May indicate need for phosphate amendment.	Baseline and quarterly thereafter (EPA 2004)
Dissolved Phosphate	EPA Method 300. EPA SW-846: 9056A (laboratory)	Used to determine if the nutrient phosphate needs to be injected into the subsurface and whether it is being consumed or accumulated. (EPA 2004)	May indicate need for phosphate amendment.	Baseline and quarterly thereafter (EPA 2004)
Dissolved Oxygen (DO)	DO meter calibrated in the field according to the manufacturer's specifications (APHA et al. 1992: 4500-O G) (field). Should be done with downhole probe or flow-through cell to avoid atmospheric contact.	If MNA, used to determine where aerobic degradation is possible. If active, used to determine the success of the oxygen amendment application.	Active injection of an oxygen amendment (air, oxygen, ozone, hydrogen peroxide, Oxygen Release Compound®) into a source area and somewhat down gradient should re-establish aerobic degradation.	Baseline and quarterly for MNA (Pope 2004) For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
Carbon Dioxide	APHA et al. 1992: 4500-CO2  C (titrimetric) or 4500-CO2 D (calculation requiring known values for total alkalinity and  pH) ISE for field measurement	Carbon dioxide is a by-product of both aerobic and anaerobic degradation. Elevated levels of carbon dioxide indicate microbial activity has been stimulated.	Indicator parameter.	Optional for active system (ITRC 2008). For MNA baseline and quarterly thereafter (San Mateo County 1996)
Oxidation Reduction Potential (ORP)	Direct-reading meter, A2580B, or USGS A6.5 (field)	Used to provide data on whether or not aerobic conditions are present in the groundwater. Oxidizing conditions are required for aerobic degradation of contaminants. Used in conjunction with other geochemical parameters to determine whether or not groundwater conditions are optimal for aerobic biodegradation.	Positive ORP values (>0.0 mV) in conjunction with elevated levels of DO indicate aerobic conditions and the success of oxidant addition.	Optional for MNA remediation. For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004). Avoid aeration of sample during measurement.
Temperature	Field probe with direct-reading meter (APHA et al. 1992: 2550 B)	Used as a general water quality parameter and as a well purging stabilization indicator. Microbial activity is slower at lower temperatures.	Indicator parameter. Typically used as a well purge stabilization parameter.	Optional but generally done as part of well purge stabilization parameter suite.
pH	Field probe with direct-reading meter calibrated in the field according to the manufacturer's specifications (APHA et al. 1992: 4500-H+B)	Used to confirm pH conditions are stable and suitable for microbial bioremediation or to identify trends of concern. (EPA 2004)	The optimum pH for bacterial growth is approximately 7. Enhanced aerobic bioremediation can be effective over a pH range of 5 to 9 pH units (EPA 2004).	For MNA: baseline and quarterly thereafter (EPA 2004). For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
Alkalinity	APHA et al. 1992: 2320 B, or Hach alkalinity test kit model AL AP MG-L, or Hach Method #8203 (field or laboratory) ISE for field measurement	Used as an indicator of biodegradation and aquifer buffering capacity (neutralization of weak acids). Used in conjunction with pH. An increase in alkalinity/stable pH indicate the aquifer's buffering capacity is sufficient to neutralize metabolic acids produced by degradation process (San Mateo County 1996).	Concentrations of alkalinity that remain at or below background in conjunction with pH <5 indicates that a buffering agent may be required (ITRC 2008).	Baseline and all sampling events thereafter (San Mateo County 1996).
Water Table Elevations	Water/air interface meter	Used to estimate the direction of groundwater flow and flow rate.	For MNA groundwater flow direction and rate should fall within the expected range. Determines if hydraulic conditions (groundwater flow) are consistent with design intent or if enhanced aerobic bioremediation technology application has had an unanticipated effect on these conditions (EPA 2004).	For MNA baseline and all sampling events thereafter (San Mateo County 1996). For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
SOIL VAPOR
Carbon Dioxide	Draeger Tube Bacharach Fyrite® Gas Analyzer	Carbon dioxide is a by-product of both anaerobic and aerobic degradation. Elevated levels of carbon dioxide are used as an indication microbial activity has been stimulated.	Indicator parameter.	For MNA at least bi-annually (Pope 2004) For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
Methane	EPA Method 18 (GC) EPA SW-846: 5021A Robert S. Kerr Laboratory RSK-175 SOP followed by GC or GC/MS	Used as an indicator that biodegradation is occurring	Methane is usually the product of anaerobic biodegradation. High levels will give an indication that biodegradation is proceeding. High levels of methane will also indicate that efforts to aerate the target area may not be succeeding.	For MNA at least biannually (EPA 2004).
Oxygen	Bacharach Fyrite® Gas Analyzer	If MNA, used to determine where aerobic degradation is possible. If active, used to determine the success of the oxygen amendment application.	Reduced oxygen levels are expected during aerobic biodegradation in MNA systems. Steady state or enriched oxygen levels are one indication that soil conditions are appropriate for aerobic biodegradation.	For MNA at least bi-annually (EPA 2004). For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
Volatile COCs	EPA SW-846: 8260B Laboratory Field GC or GC/MS	Used to determine the presence of CoCs and to measure the success of the remedial effort.	Suggests residual sources in soil or fugitive emissions associated with the remedial effort.	For active systems daily for the startup phase (7-10 days) and weekly to monthly thereafter (EPA 2004).
SOIL
COCs	VOCs, EPA SW-846: 8260B    SVOCs, SW-846: 8270D	Used to determine the success of remediation.	Soil contaminant concentrations should be dropping over time.	For MNA a statistically significant number of continuous soil cores located throughout the area of contamination (Pope 2004). For active systems quarterly to annually (EPA 2004).

Sources: EPA 2004 and San Mateo County 1996.

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