In-situ air sparging (IAS) without inorganic nutrient amendment is an innovative strategy for enhancing the intrinsic (natural) biodegradation of petroleum hydrocarbons (PHCs) in groundwater systems. Published data show that IAS can significantly increase dissolved oxygen (DO) concentrations in PHC-contaminated groundwater systems, and that increased DO levels often stimulate an increase in the population densities of indigenous PHC-degrading microbes. Reported increases in microbial population densities stimulated by IAS are on the order of those stimulated by conventional in-situ bioremediation (ISB) strategies employing both molecular oxygen (O2) and inorganic nutrient amendment, suggesting that intrinsic biodegradation is often not inorganic nutrient limited.
Rationale for inorganic nutrient amendment during conventional ISB studies has typically been based on an assumption that groundwater is oligotrophic, and that inorganic nutrients limit intrinsic biodegradation within petroleum contaminant plumes. Previous bench-scale biotreatability studies have reported that inorganic nutrient amendment has stimulated an increase in microbial population densities; however, few studies have evaluated the effects of nutrient amendment on biodegradation kinetics. Moreover, many conventional ISB studies were performed without regard to the subsurface distribution of indigenous microbes at subject sites.
Microbial enumeration studies from the literature report that large population densities of PHC-degraders exist in petroleum contaminant plumes. These studies have shown an inverse relationship between DO and PHC concentration, and a positive correlation between population density and PHC concentration during intrinsic biodegradation. Pertinent conclusions which can be inferred from these studies with respect to intrinsic biodegradation are: (1) PHCs often stimulate microbial activity by providing an organic carbon/energy source to indigenous microbes; (2) DO depletion typically limits further biodegradation; and (3) ambient inorganic nutrient concentrations are usually not limiting.
Conventional ISB strategies were often limited by technical, logistical, and/or cost factors. The development of IAS in the mid to late 1980s potentially overcame many of the drawbacks associated with conventional ISB approaches. Since that time, IAS has been implemented to treat many petroleum-contaminated groundwater systems. IAS without inorganic nutrient amendment is an effective ISB strategy because petroleum contaminant plumes are typically oxygen and not inorganic nutrient limited. IAS can potentially supply significant amounts of O2 at relatively low cost. Additionally, inorganic nutrient amendment is typically not necessary and can be deleterious unless carefully controlled. The results of three case studies are presented to illustrate the success of IAS without inorganic nutrient amendment for stimulating aerobic biodegradation of petroleum constituents.
INTRODUCTION
For over 20 years in-situ bioremediation (ISB) has been an effective strategy for remediating many petroleum-contaminated overburden and some bedrock groundwater systems. ISB has typically involved injection of molecular oxygen (O2) and inorganic nutrients into contaminant plumes to enhance the aerobic biodegradation of petroleum constituents by indigenous microbes. Petroleum products targeted for ISB have included gasolines, turbine fuels, fuel oils and certain organic solvents.
The first reported use of ISB was by Suntech, Inc. in 1972 to remediate a gasoline release which contaminated a fractured bedrock aquifer in Ambler, Pennsylvania (United States Environmental Protection Agency [USEPA], 1985). A bench-scale biotreatability study was performed to assess whether microbes capable of metabolizing petroleum hydrocarbons (PHCs) were present in the contaminant plume. Based on biotreatability study findings, Suntech, Inc. concluded that lack of O2 and inorganic nutrients were limiting PHC biodegradation in-situ. An oxygen/nutrient-amendment strategy were developed. Ambient air was sparged continuously into the water column within wells to increase dissolved oxygen (DO) levels in the contaminant plume1. Inorganic nutrients were also supplied on a batch-feed schedule. Groundwater injection/extraction wells were operated to establish a treatment zone through which oxygen/nutrients were circulated. Results of this study indicated that gasoline constituents were not detected within on-site groundwater 10 months after oxygen and nutrient amendment ceased (USEPA, 1985).
An oxygen-amendment alternative to in-well sparging was utilized at a LaGrange, Oregon site in 1982 (USEPA, 1985). Groundwater was extracted and ambient air added using in-line static mixers to increase DO levels in groundwater recharged to the aquifer (hereafter, "in-line mixing"). Both in-well sparging and in-line mixing had limited oxygen loading rates due, in part, to the relatively low aqueous solubility limit of O2 (i.e., approximately 10 milligrams per liter [mg/l] at standard temperature and pressure). Consequently, these strategies were ineffective for significantly increasing DO levels. In fact, DO concentrations of 10 mg/l can only enhance biodegradation of 5 mg/l PHCs based on the stoichiometry of common petroleum constituents.
Based on the limited ability of these strategies to significantly increase DO levels in groundwater, alternative oxygen-amendment strategies involving O2, O2-enriched ambient air, ozone, and hydrogen peroxide (H2O2) were studied under field and laboratory conditions (USEPA, 1987). Based on several years of studies, H2O2 amendment became the strategy most frequently used to enhance PHC biodegradation (Baker and Herson, 1994). Due to its complete miscibility with water, the injection of H2O2 into groundwater systems can significantly increase DO concentrations (Norris, 1994). For example, the dissociation of 100 mg/l H2O2 yields 50 mg/l DO (Riser-Roberts, 1992).
According to USEPA (1985), one of the first reported uses of H2O2 amendment for ISB was in 1984 in Granger, Indiana. H2O2 was injected into a gasoline-contaminated aquifer at concentrations ranging from 100 to 500 mg/l. Inorganic nutrients were also injected with H2O2. Yaniga and Smith (1984) provided early evidence that H2O2 amendment enhanced biodegradation. They demonstrated that injection of 100 mg/l H2O2 increased DO concentrations from 4 to 10 mg/l resulting in a concomitant decrease in dissolved PHC concentrations from about 4 to 2.5 mg/l. Yaniga and Smith also reported that H2O2/nutrient amendment resulted in a substantial increase in microbial population densities.
- Although H2O2 amendment can supply significantly more O2 than the other oxygen-amendment strategies discussed above, the following drawbacks have restricted its widespread usage:
- Special precautions must be taken so as to not inhibit microbial activity during subsurface injection due to H2O2 cytotoxicity at high concentrations (Norris, 1994);
- Premature decomposition of H2O2 at concentrations exceeding 100 mg/l can cause oxygen to exsolve and form gas bubbles thereby reducing permeability (Baker and Herson, 1994);
- H2O2 amendment can cause metal oxides/hydroxides and magnesium/calcium phosphates to precipitate which can also reduce permeability (Riser-Roberts, 1992; Levin and Gealt, 1993);
- H2O2 is relatively expensive; and
- Transportation, storage, and handling issues make H2O2 unsuitable for smaller sites.
Irrespective of the oxygen-amendment strategy selected, inorganic nutrient amendment has been a fundamental element of ISB since its inception. Inorganic nutrients principally consist of nitrogen and phosphorus, but may include potassium, magnesium, calcium, sodium, and sulfur, as well as the trace elements copper, zinc, cobalt, iron, manganese, and molybdenum.
The injection of inorganic nutrients into contaminant plumes was generally based on the assumption that groundwater systems are oligotrophic and contain small population densities of indigenous petroleum-degrading microbes (Riser-Roberts, 1992). Levin and Gealt (1993) indicated that "as recently as the mid-1970s one could find in the literature statements about the extremely low numbers of microorganisms in groundwaters." Moreover, though microbial enumerations were sometimes performed on soil and groundwater samples collected from contaminant plumes, few studies included enumeration of samples from uncontaminated (background) locations such that population density distributions could be assessed. Furthermore, bench-scale biotreatability studies demonstrated that injection of O2/inorganic nutrients increased population densities substantially (USEPA, 1987; Canter and Knox, 1986). However, few studies compared the relative effect which oxygen and nutrient amendment induced on biodegradation kinetics, with that effect induced solely by oxygen amendment (Baker and Herson, 1994). Consequently, nutrient amendment has often been performed without regard to the population density and spatial distribution of indigenous microbes and the relationship between inorganic nutrient amendment and biodegradation rates.
In-Situ Air Sparging
In-situ air sparging (IAS) was first implemented in Germany in 1985 as a saturated zone remedial strategy (Brown, 1994), and was introduced to the United States in 1989 as a hybrid of soil vapor extraction (SVE). Ardito and Billings (1990) referred to this hybrid as the Subsurface Volatilization and Ventilation System. Because SVE treats the unsaturated zone whereas IAS treats the saturated zone, a discussion of SVE is beyond the scope of this paper. However, IAS is often performed with SVE as a source control measure.
IAS involves the injection of pressurized air into the saturated zone. The physics of air movement through the saturated zone in response to air sparging is not well understood. According to Brown (1994) and others, 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. This theory is hereafter referred to as the "transient flow" model in this paper. Alternatively, Hinchee (1994) and Johnson (1994) suggest that 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. Furthermore, Dahmani et al. (1994) suggest that air movement through the saturated zone typically does not occur as migrating air bubbles, with the exception of within homogeneous, highly permeable formations (e.g., unconsolidated course sand and gravel deposits). Hereafter, this theory is referred to as the "channel flow" model. To date, there is uncertainty with respect to the validity of each model and the conditions under which each may or may not apply. Many practitioners, however, have assumed the transient flow model to be the operative mechanism.
IAS enhances physical and/or biological attenuation processes within the sparge point radius of influence (Brown, 1994; Johnson, 1994)2. IAS enhances physical attenuation by volatilizing PHCs adsorbed to the formation matrix and stripping those dissolved in groundwater. IAS stimulates aerobic biodegradation of adsorbed and dissolved-phase PHCs amenable to metabolism. Physical processes are a more significant attenuation mechanism for volatile PHCs (VPHCs) of low aqueous solubility, whereas biological processes are a more significant attenuation mechanism for PHCs of low volatility and varying aqueous solubilities. Detailed discussion of enhanced physical attenuation due to IAS is beyond the scope of this paper.
Based on a literature review of 37 references in which enhanced biodegradation was a remedial objective of IAS,3 only one (i.e., Pijls et al., 1994) indicated that inorganic nutrient amendment was performed to enhance biodegradation. It is unclear why nutrient amendment has not been performed in these studies, considering nearly all pre-IAS approaches to ISB involved nutrient amendment.
PURPOSE AND OBJECTIVES
The purpose of this paper is to illustrate the effectiveness of IAS without inorganic nutrient amendment for aerobically enhancing petroleum biodegradation, especially in comparison to other aerobic ISB strategies. Specific objectives are to:
- Review the microbial ecology of petroleum-contaminated groundwater systems, and the principal factors which control intrinsic biodegradation4 in-situ;
- Discuss the effects of IAS on enhancing PHC biodegradation; and
- Summarize three case studies which illustrate the effectiveness of IAS for aerobically enhancing PHC biodegradation without inorganic nutrient amendment.
MICROBIAL ECOLOGY OF PETROLEUM CONTAMINANT PLUMES
The aerobic biodegradation of petroleum constituents in groundwater systems is effected by microorganisms which metabolize PHCs for organic carbon and energy.5 The microorganisms involved are primarily procaryotic soil bacteria such as Nocardia, Pseudomonads, Acinetobacter, Flavobacterium, Microcossus, Arthrobacter and Corynebacterium, though eucaryotic fungi may play a minor role (Riser-Roberts, 1992; Chapelle, 1993). Petroleum-degrading soil bacteria consist of two different groups distinguished by unique respiratory capabilities. Obligate aerobic heterotrophs consist of those soil bacteria which metabolize organic carbon only under oxic conditions, whereas facultative anaerobic heterotrophs consist of those bacteria which metabolize organic carbon under either oxic or anoxic conditions.6 The bulk of viable heterotrophs are attached to the formation matrix, and a proportionately smaller fraction are suspended in groundwater.
Intrinsic biodegradation is typically effected by a consortium of bacteria genera rather than a single genus. This is because ultimate bio-oxidation to carbon dioxide and water involves a series of biotransformations in which one genus converts one group of PHCs to intermediate compounds. The intermediate compounds are themselves metabolized by a different genus of bacteria.
In uncontaminated groundwater systems, indigenous heterotrophs obtain organic carbon and energy from dissolved organic carbon (DOC). The DOC leaches from soil organic matter in the unsaturated zone (Chapelle, 1993). In petroleum-contaminated groundwater systems, certain heterotrophic bacteria having the genetic capability to metabolize petroleum constituents are stimulated by the supplemental organic carbon supplied by PHCs. This occurs even though a portion of the microbial population may be inhibited by PHC toxicity. Heterotrophic bacteria metabolize DOC and PHCs by breaking carbon-carbon and carbon-hydrogen covalent bonds. Examples of 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, heterotrophs use O2 as a terminal electron acceptor7 to collect electrons released during metabolism, and ambient inorganic nutrients and organic carbon to maintain cell tissue and increase biomass. Although oxygen is consumed in this process, nutrients are generally conserved as they are recycled during production of waste materials and lysis of cellular tissue.
LIMITING FACTORS OF INTRINSIC BIODEGRADATION
The primary factors limiting intrinsic biodegradation of petroleum constituents in groundwater systems are biodegradability potential and microbial viability.
Biodegradability potential is a function of PHC type, size, structure, and concentration. For example, normal alkanes are more easily metabolized than isoprenoids or cycloalkanes, and single-ring aromatic PHCs are more easily metabolized than multi-ring compounds. Moreover, PHC concentrations must be within specific ranges. If concentrations are too low, indigenous heterotrophs 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. In general, in-situ biodegradability potential can readily be assessed via literature searches and simple bioassays (Schaffner et al., 1994).
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. The significance of these factors is discussed below.
Uncontaminated groundwater systems typically contain ambient DO concentrations of about 5 to 6 mg/l (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 occurring, indicating that heterotrophs deplete ambient DO during PHC biodegradation. Therefore, DO depletion is a significant factor limiting further biodegradation within most petroleum contaminant plumes.
Indigenous heterotrophs 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. Thus, 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 heterotroph population densities of about 102 to 103 colony forming units per milliliter (cfu/ml). However, densities of up to about 106 cfu/ml have been reported in some cases (Bitton and Gerba, 1984).
Microbial population densities within petroleum contaminant plumes typically increase in response to supplemental organic carbon supplied by dissolved/adsorbed-phase PHCs. Petroleum-contaminated groundwater typically contains significantly higher population densities than background groundwater, often by several-fold (Riser-Roberts, 1992). 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 osmotic/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. However, microbial inhibition due to pH can readily be assessed using simple bioassay technique.
Based on the inverse relationship 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.
IAS-ENHANCED BIODEGRADATION OF PHCs
IAS 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. Additionally, inorganic nutrient amendment is typically not necessary and can be deleterious unless carefully controlled. The benefits of IAS for stimulating aerobic biodegradation are discussed below.
Molecular Oxygen 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. Table 2 compares the potential effectiveness of IAS with that of H2O2 amendment for supplying O2 to groundwater systems. The table is a modification of one developed by Brown (1994) comparing potential O2 loading rates calculated for both approaches as a function of flow rate and utilization efficiency.8
Table 1. Molecular Oxygen Loading Rates, lbs/d.
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Notes:
1. "lbs/d" indicates pounds per day; "scfm" indicates standard cubic feet per minute; and "gpm" indicates gallons per minute.
2. The flow rates shown above are on the order of those utilized during full-scale implementation
of both technologies.
3. Utilization efficiency refers to that fraction of the oxygen load which is available to stimulate
biodegradation.
As shown in Table 1, IAS can potentially supply significantly more O2 than can be supplied by H2O2 amendment. For example, IAS can yield a higher oxygen loading rate at an air flow rate of 25 scfm (assuming 10 percent utilization) than even the injection of 100 mg/l H2O2 at 50 gpm (assuming 100 percent utilization). Though IAS can yield relatively high oxygen loads, utilization efficiency is controlled by the mechanics of air flow in the groundwater system (i.e., transient flow versus channel flow models). IAS is expected to be more efficient under transient flow than channel flow conditions due to the increased surface area of the air-water contact and the turbulence created by the migration of air bubbles through the formation matrix (Brown, 1984). However, careful engineering design of IAS systems can provide adequate O2 supplies even under channel flow conditions (Johnson, 1994).
Brown (1994), Billings et al. (1994), Kuiper et al. (1993), and many others have shown that IAS can significantly increase DO levels within petroleum contaminant plumes in a variety of hydrogeological settings.
Inorganic Nutrients
By increasing DO levels, IAS typically stimulates a significant increase in microbial population densities within petroleum contaminant plumes, often by as much as several orders of magnitude (Billings et al., 1994; Felten et al., 1992; Kuiper, et al., 1993). 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. For example, Canter and Knox (1986) report that combined oxygen/nutrient amendment stimulates an increase in microbial population densities of several orders of magnitude.
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;10
- 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).
In addition to being unnecessary, inorganic nutrient amendment may promote biofouling of the formation matrix and/or the injection system filter pack. For example, Schaffner et al. (1990) demonstrated that inorganic nutrient amendment induced significant reductions in the intrinsic permeability of simulated toluene-contaminated groundwater systems. The permeability reduction was attributed to biofouling of the porous media, which was caused by microbial activity stimulated by nutrient amendment. Problems associated with biofouling attributed to ISB have been reported by others (Wilson and Brown, 1989; Lee et al., 1988).
Enhanced Volatilization/Air Stripping of VPHCs
For contaminant plumes in which VPHC levels are high enough to become toxic to indigenous microbes, enhanced volatilization/air stripping is anticipated to significantly reduce VPHC concentrations enabling biodegradation to occur (Johnson, 1994). Hence, enhanced physical attenuation may reduce VPHC levels below toxicity thresholds thereby stimulating microbial degradation.
Increased pH
Uncontaminated groundwater systems often have pH values below neutral based on the authors experience.11 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. The following reactions illustrate the aerobic biodegradation of the aromatic hydrocarbon benzene and the subsequent production of H2CO3:
Acidic groundwater conditions can potentially limit microbial viability (Chapelle, 1993). IAS may strip CO2 from groundwater thereby increasing pH toward neutral.
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 (Hinchee, 1994; Brown, 1994; Ardito and Billings, 1990). In addition, reduced operation and maintenance costs resulting from shortened cleanup times could result in further cost savings.
IN-SITU AIR SPARGING CASE STUDIES
Three case studies are presented to illustrate the effectiveness of IAS without nutrient amendment for enhancing PHC biodegradation in groundwater systems. Case Study Nos. 1 and 2 were obtained by performing a literature search utilizing the National Ground Water Information Center's Ground Water Network. Case Study No. 3 presents previously unpublished data from an on-going project in New England in which an IAS system initially engineered to enhance physical attenuation was later found to also stimulate aerobic biodegradation.
CASE STUDY NO. 1
According to Felten et al. (1992), a gasoline release resulting in soil and groundwater contamination occurred at a retail facility. The facility operated from 1958 to 1989, at which time the underground storage tanks (USTs) were removed. Site geology consisted primarily of fine to medium sand with some silt and gravel overlying bedrock to a depth of about 29 to 35 feet. Overburden groundwater ranged in depth from about 19 to 25 feet. The source area was about 90 feet long and 75 feet wide, and contaminated soil extended to depths of about 14 to 26 feet. Estimates of subsurface PHC distributions indicated that about 31.5 percent of the total PHC mass (17,077 pounds) was residual in unsaturated zone soils, 68 percent was adsorbed to saturated zone soils, and about 0.5 percent was dissolved in groundwater.
A pilot test was performed to evaluate whether combined IAS/SVE could be used to remediate soil and groundwater contamination at the site. Following the pilot test, an IAS/SVE system was installed incorporating two combined IAS/SVE system couplets. The IAS sparge points consisted of two-foot stainless steel screens installed ten feet below the water table. The SVE wells were screened in the unsaturated zone from about 13 to 23 feet below ground surface. SVE was performed from November 1991 to March 1992, prior to start-up of the IAS system, and combined IAS/SVE was performed from March to August 1992. One of two air sparge points was not operated during July and August 1992. In addition, one of the two sparging wells had to be periodically surged with high pressure to control fouling. SVE was performed at a blower extraction rate of 90 scfm, with a vacuum of 44 inches of water. The IAS system was operated at a flow rate of 3.5 scfm, with a pressure of 10 pounds per square inch gauge (psig).
DO and microbial population density data were collected within the assumed radius of influence of the combined IAS/SVE system before and after start-up of the system. The data indicated that DO increased from about 1 to 5 mg/l and microbial population densities increased between 10- to 420-times in four of five monitoring wells in response to IAS. Microbial population densities decreased 20-times in the remaining monitoring well, which was attributed to very low DO (below 2 mg/l). Preliminary results indicate that about 1,400 pounds of PHCs were removed solely by SVE during the first four months of operation, and that an additional 600 pounds were removed by combined IAS/SVE from March to August 1992. The estimated PHC removal rate during operation of the SVE system was less than 7 pounds per day (lbs/d) for four months, compared to 12 lbs/d during operation of the combined IAS/SVE system during the first 30 days after start-up of the IAS system. Following the initial 30 days of operation, the combined IAS/SVE system removed less than 3 lbs/d of PHCs. In total, an estimated 2,000 pounds of PHCs were treated by IAS/SVE during the nine months of operation. However, the removal rate of hydrocarbons due to biodegradation was not discussed.
A pertinent finding of the paper is that the increase in DO concentrations due to IAS increased microbial population densities which enhanced PHC biodegradation in-situ.
CASE STUDY NO. 2
According to Kuiper, et al. (1993), a gasoline release occurred at a Wood Village, Oregon site. High BTEX levels including benzene at concentrations of up to 651 micrograms per liter (ug/l) were detected in groundwater at the site. Overburden soils consisted of clayey silt and sand underlain by more permeable gravel. Groundwater is generally between about 3 to 10 feet below ground surface.
A pilot test was performed within the source area to evaluate whether combined IAS/SVE could be used to treat PHCs in soils of relatively low permeability, and to provide remedial design information for a full-scale system. One SVE well was installed in a silty sand unit immediately below the asphalt pavement to a depth of 3 feet. An IAS well was placed near the SVE well in a hydraulically sidegradient location, and was screened within saturated silty clays. IAS was performed at an average air flow rate of 4 scfm for about 9 months, following a six week period where SVE was performed alone. Groundwater monitoring was performed before and after start-up of IAS for DO and hydrocarbon-degrader population densities.
Operated alone, the SVE system resulted in a gradual decrease in DO concentrations from 4.5 to 3.7 mg/l. Decreased DO levels were attributed to reduced pressure effected by SVE. Within 2-1/2-months of IAS start-up, DO levels increased between 41 to 154 percent in three monitoring wells hydraulically downgradient of the IAS system. DO remained elevated in these monitoring wells between 227 to 350 percent immediately after the system was shut down 9-1/2 months after start-up of the IAS system. DO concentrations were about 7 to 11 mg/l during treatment, on the order of the aqueous solubility limit of O2. Microbial enumeration data suggested an increase in the population density of Pseudomonads of two to four orders of magnitude in downgradient monitoring wells in response to IAS. The IAS radius of influence was estimated at about 10 to 20 feet.
During combined IAS/SVE, an 87 percent reduction in air flow was noted from the SVE wells. The reduced air flow was attributed to biofouling of the sparge points by iron-fixing bacteria. Dissolved PHC concentrations decreased within the IAS radius of influence from as high as 1,654 ug/l to less than 1 ug/l total BTEX during 9-1/2 months of system operation. During this time, benzene concentrations decreased from 651 ug/l to less than 1 ug/l. BTEX concentrations have remained at or below analytical detection limits within the radius of influence of the IAS system since shut-down. Biofouling of the IAS system and the resulting 87 percent decrease in air flow from the SVE wells resulted in a decrease in the rate of enhanced volatilization/air stripping. Nevertheless, the simultaneous increase in DO levels and petroleum-degrader population densities during the study suggested that biodegradation was enhanced. However, the removal rate of hydrocarbons due to biodegradation was not discussed. A pertinent finding of this study was that IAS was effective in enhancing PHC biodegradation due to the significantly increased DO levels and concomitant increase in microbial population densities.
CASE STUDY NO. 3
Gasoline contaminated soils and groundwater were identified at a retail gasoline facility located in New England in 1984. Two existing USTs were subsequently removed and replaced. Hydrogeological investigations were performed from 1984 to 1989 which included installation of overburden and bedrock groundwater monitoring wells at and adjacent to the site property.
Overburden site soils consist of glacial or glacio-fluvial/lacustrine deposits including 4 to 20 feet of sandy silt to silty sand beach deposits, underlain by two discrete glacial till horizons (upper and lower till). The upper till consists of 8 to 19 feet of silty sand with gravel; the less permeable lower till consists of a 6 to 31-feet thick silty clay with gravel horizon. An approximately 6- to 7-foot thick weathered bedrock unit underlies the lower till. Groundwater is at a depth of about 7 to 10 feet within the beach deposit horizon. The overburden contaminant plume is about 300 feet wide by 400 feet long. Concentrations of total BTEX in six groundwater monitoring wells ranged from about 163 to 624,000 ug/l in July 1993 prior to implementation of remedial activities at the site.
A combined IAS/SVE program was developed to remediate soil and groundwater contamination at the site. The objective of the program was to enhance volatilization/air stripping of VPHCs in-situ to mitigate source contamination. A pilot test was performed using two 1-1/4-inch sparge points with 12-inch screens positioned within the upper and lower glacial till units. SVE was accomplished using a 4-inch well screen placed within a 10-foot long horizontal extraction trench within the unsaturated zone. The effects of IAS/SVE were monitored using four 1-1/2-inch PVC SVE monitoring probes, four 1-1/2-inch diameter PVC well points, and several existing groundwater monitoring wells. Based on the pilot study results, a full-scale IAS/SVE system was installed. The SVE system consisted of approximately 165 linear feet of SVE trench in which horizontal 4-inch diameter well screens were placed beneath an asphalt pavement surface in two separate trench networks. The SVE wells were connected to a 1,000-scfm capacity blower. The IAS system consisted of seventeen 1-1/4-inch diameter sparge points with 12-inch length screens placed below the water table within the upper till unit with four series of sparge points positioned between and centered on the SVE trenches. The sparge points were connected to a 500-scfm capacity compressor for air injection. The vacuum within each of the two SVE wells was maintained between 10 to 36 inches of water, with a total air discharge rate of about 195 scfm. The IAS system was operated with air pressures of 8 to 10 psig, with a total air flow of about 114 scfm. A site plan depicting the layout of the IAS/SVE system is provided as Figure 1.
Figure 1. Site Plan
The SVE system has been operated since September 1993. The IAS system was operated from November 1993 to March 1994, and from August 1994 to the present. The system was shut down during spring and summer 1994 due to an elevated groundwater table related to unusually high groundwater recharge.
Concentrations of total volatile organic compounds (VOCs) in exhaust gas were measured using a photoionization detector at 351 parts per million by volume (ppmvol) during the start-up of the SVE system in September 1993. These concentrations decreased to about 45 to 84 ppmvol by the end of November 1993. Following start-up of the IAS system, total VOC concentrations in the SVE exhaust gas increased to about 56 to 104 ppmvol indicating that IAS was enhancing volatilization/air stripping of VPHCs. By the end of December 1993, total VOC concentrations decreased further to about 24 to 26 ppmvol.
A biodegradation screening study was performed in January 1995 to evaluate whether IAS was also enhancing intrinsic biodegradation. The screening study included groundwater quality monitoring for the following biodegradation indicator parameters: DO, oxidation-reduction potential (ORP), and pH. Groundwater samples were also enumerated for total heterotrophs using the serial dilution plate count method (Chapelle, 1993). The results of this study are summarized in Table 2.
Table 2. Biodegradation Screening Study Results.
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Notes:
1. "IAS On" refers
to samples collected with all sparge points in operation. (The
IAS/SVE system was turned off for 1 hour prior to sampling to
minimize the effect of possible channel flow short-circuiting
through the monitoring well filter pack).
2. "IAS Off" refers
to samples collected with all sparge points shut down. With respect
to water quality data, all the sparge points had been in operation
prior to shut down. With respect to microbial enumeration data,
one half of the sparge points had been in operation prior to shut
down.
3. "cfu/ml" indicates
colony forming units per milliliter.
4. "mv" indicates millivolts.
As shown on Table 2, IAS effected a 21 to 282 percent increase in DO levels, a 4 to 2,000 percent increase in ORP, and a 1 to 7 percent increase in pH.12 Based on the increased DO and ORP data, groundwater conditions have shifted from relatively anoxic (reducing) to oxic (oxidizing), suggesting that IAS was effective in increasing DO levels within the contaminant plume. Based on the small increase in pH, IAS also stripped CO2 from the groundwater, most notably in the monitoring wells of historically high PHC concentrations (i.e., wells RW-2 and RW-3). In addition to perturbing water chemistry, doubling the number of operating sparge points stimulated an increase in total heterotroph population densities in most monitoring wells (an overall increase of about 170% based on geometric mean). Moreover, microbial population densities were relatively high for both sampling rounds, and population densities in the more contaminated monitoring wells (MW-8, RW-1, MW-7, RW-2, and RW-3) are significantly larger than the population density in the less contaminated monitoring well (MW-11). The data suggests that intrinsic biodegradation is occurring and that inorganic nutrients are generally not limiting. Furthermore, overburden groundwater samples collected from the contaminant plume prior to start-up of the combined IAS/SVE system, and monitored on a triannual basis thereafter, indicated that geometric mean total BTEX concentrations decreased from about 11,400 ug/l in July 1993 to about 800 ug/l in November 1994 in response to IAS.
The pertinent finding of the biodegradation screening study was that, in addition to enhancing physical attenuation, IAS was effective in stimulating PHC biodegradation due to significantly increased DO concentrations and concomitant increases in microbial population densities.
CONCLUSION
In addition to enhancing the volatilization and air stripping of VPHCs, IAS without inorganic nutrient amendment enhances aerobic biodegradation in many petroleum-contaminated groundwater systems. IAS is an effective ISB strategy because it potentially (1) supplies more O2 than other oxygen-amendment strategies; (2) increases groundwater pH toward neutral; and (3) is relatively low cost. However, problems associated with biofouling have been reported. Furthermore, because ambient inorganic nutrient concentrations are typically not limiting, nutrient amendment is usually not necessary, and may actually be deleterious by promoting biofouling of the formation/injection system unless carefully controlled.
During the course of our study, we identified the following issues as needing resolution relative to more effective application of IAS:
- A better understanding of the physics of air flow in the saturated zone, particularly as it relates to transient/channel flow conditions; and
- Development of standardized equipment and procedures for monitoring the physical and biological attenuation mechanisms potentially enhanced by IAS.
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BIOGRAPHICAL SKETCHES
I. Richard Schaffner, Jr. conducted undergraduate studies in geology and graduate studies in contaminant hydrogeology at Brigham Young University. He is a member of the National Ground Water Association, the New England Water Environment Association, and the Northeast Branch of the American Society for Microbiology, and works as a consulting hydrogeologist. His interests include the fate and transport of synthetic organic solvents in groundwater systems, and the enhanced microbial degradation of aromatic and aliphatic petroleum hydrocarbons in the saturated and unsaturated zones. He can be contacted at GZA GeoEnvironmental, Inc., 380 Harvey Road, Manchester, New Hampshire 03103.
Armand A. Juneau, Jr. completed undergraduate studies in geology at the University of New Hampshire, where he focused on igneous and metamorphic geology and inorganic geochemistry. He has completed over 175 Phase I and Phase II Environmental Site Assessments, and over 70 hydrogeological investigations involving municipal landfills, leaking underground storage tanks, manufacturing facilities, and "Superfund" sites. His interests include igneous and metamorphic geology, and investigation and remediation of groundwater contamination within overburden soils and fractured bedrock. He can be contacted at GZA GeoEnvironmental, Inc., 380 Harvey Road, Manchester, New Hampshire 03103.