IN SITU AIR SPARGING
In Situ Air Sparging

In situ air sparging (IAS) was first implemented in Germany in 1985 as a saturated zone remedial strategy. IAS involves the injection of pressurized air into the saturated zone. IAS induces a transient, air-filled porosity in which air temporarily displaces water as air bubbles migrate laterally from the sparge point and also vertically towards the water table. IAS induces a separate phase flux in which air travels in continuous, discrete air channels of relatively small diameter from the sparge point to the water table. Air movement through the saturated zone typically does not occur as migrating air bubbles, with the exception of within homogeneous, highly permeable formations of unconsolidated course sand and gravel deposits.

IAS enhances physical and/or biological attenuation processes and physical attenuation by volatilizing PHCs adsorbed to the formation matrix and stripping those dissolved in groundwater. IAS stimulates aerobic biodegradation of absorbed and dissolved-phase PHCs amenable to metabolism. Physical processes are a more significant attenuation mechanism for volatile PHCs, whereas biological processes are a more significant attenuation mechanism for PHCs of low volatility and varying aqueous solubilities.

Microbial Ecology of Petroleum Contaminant Plumes

The aerobic biodegradation of petroleum in groundwater systems is effected by microorganisms which metabolize PHCs for organic carbon and energy. The microorganisms involved are primarily procaryotic soil bacteria such as Nocardia, Pseudomonads, Acinetobacter, Flavobacterium, Microcossus, Anthrobacter, and Corynebacterium (Riser-Roberts, 1992; Chapelle, 1993). Petroleum-degrading soil bacteria consist of two different groups distinguished by unique respiratory capabilities. Obligate aerobic microbes consist of those soil bacteria which metabolize organic carbon only under oxic conditions, whereas facultative anaerobic microbes consist of those bacteria which metabolize organic carbon under either oxic or anoxic conditions.

Intrinsic biodegradation is typically effected by a bacteria group rather than a single bacteria. This is because ultimate bio-oxidation to carbon dioxide and water involves a series of biotransformations in which one bacteria converts one group of petroleum hydrocarbons to intermediate compounds. The intermediate compounds are themselves metabolized by a different bacteria.

In uncontaminated groundwater systems, indigenous microbes obtain organic carbon and energy from dissolved organic carbon (DOC). The DOC leaches from soil organic matter in the unsaturated zone. In petroleum-contaminated groundwater systems, certain bacteria having the genetic capability to metabolize petroleum constituents are stimulated by the supplemental organic carbon supplied by PHCs. Bacterial metabolize DOC and PHCs by breaking carbon-carbon and carbon-hydrogen covalent bonds. PHCs amenable to intrinsic biodegradation include the aliphatic hydrocarbons with carbon number ranges of C10 to C25 and the aromatic hydrocarbons benzene, toluene, ethyl benzene, and xylenes (BTEX).

During bio-oxidation of DOC/PHCs, microbes use O2 as a terminal electron acceptor to collect electrons released during metabolism, and ambient inorganic nutrients and organic carbon to maintain cell tissue and increase biomass. Although oxgyen is consumed in this process, nutrients are generally conserved as they are recycled during production of waste materials and cellular tissue. Alternate terminal electron acceptors include nitrate/nitrite, sulfate/sulfite, and carbon dioxide. However, O2 is generally the most energetic electron acceptor for stimulating biodegradation.

Limiting Factors of Intrinsic Biodegradation

The primary factors limiting intrinsic biodegradation of petroleum in groundwater systems are biodegradability potential and microbial viability.

Biodegradability potential is a function of PHC type, size, structure, and concentration. PHC concentrations must be within specific ranges. If concentrations are too low, indigenous microbes may not use PHCs as a primary source of organic carbon in preference to DOC; however, PHCs may be inhibitory if concentrations are too high.

Given the availability of biodegradable PHCs, microbial viability is controlled by a variety of factors including O2, inorganic nutrients, osmotic/hydrostatic pressure, temperature, and pH.

Uncontaminated groundwater systems typically contain ambient DO concentrations of about 5 to 6 mg/1 (Brown, et al., 1994). DO levels are depressed below the aqueous solubility limit of O2 due to the presence of DOC which exerts a biochemical oxygen demand (BOD) on the groundwater. Supplemental organic carbon supplied by PHCs typically exerts an even larger BOD than does naturally-occurring DOC, resulting in greater DO depletion. Petroleum-contaminated groundwater typically contains significantly lower DO concentrations than background groundwater, and is often entirely depleted of DO (Levin and Gealt, 1993; Riser-Roberts, 1992). Hence, there is an inverse relationship between DO and PHC concentrations under conditions in which intrinsic biodegradation is occuring, indicating that microbes deplete ambient DO during PHC biodegradation. Therefore, DO depletion is a significant factor limiting further biodegradation within most petroleum contaminant plumes.

Indigenous microbes use ambient inorganic nutrients and organic carbon to maintain cell tissue and increase biomass. Consequently, inorganic nutrient availability is reflected in microbial population densities within contaminant plumes in which intrinsic biodegradation is occurring. Although other factors influence microbial viability, none are as directly related to population density as inorganic nutrient and organic carbon availability. Population density is an indicator of ambient organic carbon and inorganic nutrient availability. According to USEPA (1987), groundwater samples collected from background locations hydraulically upgradient/sidegradient of petroleum contaminant plumes typically contain total population densities of about 102 to 103 colony forming units per milliliter (cfu/ml).

Microbial population densities within petroleum contaminant plumes typically increase in response to supplemental organic carbon supplied by dissolved/adsorbed-phase PHCs. Hence, there is a positive correlation between population densities and PHC concentrations within contaminant plumes under conditions in which intrinsic biodegradation is occurring. This correlation indicates that indigenous heterotrophs are stimulated to metabolize PHCs, and that ambient inorganic nutrient levels are not limiting biodegradation in-situ.

Other potential limiting factors include hydrostatic pressure, temperature, and pH, however, these factors are frequently within the range of microbial viability (Schaffner et al., 1990), and typically do not limit intrinsic biodegradation, with the possible exception of pH.

Based on the inverse relationships between DO and PHC concentration and the positive correlation between heterotroph population density and PHC concentration observed within many petroleum contaminant plumes, three generalizations are offered with respect to intrinsic biodegradation in most settings:

PHCs stimulate microbial activity by providing organic carbon to indigenous microbes;

DO depletion limits further biodegradation; and

Ambient inorganic nutrient concentrations are not limiting.

ISA-enhanced Biodegradation of PHCs

ISA without inorganic nutrient amendment is an effective ISB strategy for enhancing intrinsic biodegradation because most petroleum-contaminated groundwater systems are oxygen limited and not inorganic nutrient limited. IAS can potentially supply more O2 than other oxygen- amendment strategies at relatively low cost.

Molecular Oxgyen Supply

Efficient O2 supply to the contaminant plume is critical for ISB because DO depletion is the primary factor limiting aerobic biodegradation. Both IAS and H2O2 amendment have been demonstrated to be effective in significantly increasing DO concentrations in groundwater.

Inorganic Nutrients

By increasing DO levels, IAS typically stimulates a significant increase in microbial population densities within petroleum contaminant plumes. This increase suggests that ambient inorganic nutrients are not limiting PHC biodegradation. Moreover, increased population densities stimulated by IAS are often on the order of those stimulated by both O2 and inorganic nutrient amendment.

Inorganic nutrient amendment will not be necessary for most IAS projects in which enhanced biodegradation is a remedial objective; however, it may be for some. Examples of situations in which nutrient amendment may be necessary are listed below:

Site-specific bench-scale biotreatability testing demonstrates that biodegradation is both O2 and inorganic nutrient limited;

Microbial population densities remain constant during pilot-scale testing/full-scale IAS operation, indicating that inorganic nutrients are limiting;

Microbial population densities decrease without concomitant reductions in PHC concentrations during pilot-scale testing/full-scale operation, indicating that inorganic nutrients are becoming limiting (e.g., nutrients are flushed from the formation rather than recycled).

Increased pH

Uncontaminated groundwater systems often have pH values below neutral. This is primarily related to the hydrolysis of carbon dioxide (CO2) to carbonic acid (H2CO3). According to Freeze and Cherry (1979), CO2 is delivered to groundwater as a result of (1) exposure of precipitation to earth's atmosphere prior to groundwater recharge; (2) contact with soil gas during recharge through the unsaturated zone; and/or (3) gas production below the water table due to chemical and biological reactions involving groundwater, mineral species, naturally-occurring organic carbon, and soil bacterial activity.

Contaminated groundwater systems typically have depressed pH relative to background due to increased microbial activity related to intrinsic biodegradation of PHCs:

2C6H6 + 15O2 -- 6H2O + 12CO2 (bio-oxidation of benzene to water and CO2)

12CO2 + 12H2O -- 12H2CO3 (hydrolysis of CO2 to H2CO3)

Low Cost

Design, capital equipment, and installation costs for IAS systems are typically lower than conventional "pump and treat" remediation systems as well as other ISB strategies. In addition, reduced operation and maintenance costs resulting from shortened cleanup times could result in further cost savings.