The results of a laboratory treatability study are
presented which indicate that diesel fuel oil-contaminated soils
(up to C20) may be successfully bioremediated by insitu
forced aeration. Oxygen was determined to be the limiting factor
for the biodegradation of these petroleum hydrocarboncontaminated
soils. Nutrient addition and associated costly nutrient amendment
delivery to the subsurface was determined to be unnecessary.
Oxygen can be provided to subsurface soils cost effectively by
using a blower, and air extraction and air inlet wells.
Bioremediation of petroleum hydrocarboncontaminated soils is accomplished with insitu forced aeration by providing oxygen to the indigenous subsurface microbes, thus accelerating aerobic biodegradation of the contaminants into carbon dioxide and water. Average densities of heterotrophic bacteria were observed to be one to two orders of magnitude higher in oilcontaminated soils than in background soils. These elevated microbial densities suggest that native soil bacteria are already degrading oil contamination and that the greatest impediment to more rapid biodegradation may be the supply of oxygen, which can be quickly used up by active biological populations and not replenished in the subsurface without a means of constant air supply.
The biotreatability study indicated that approximately four to eight percent of the petroleum hydrocarbon mass was biodegraded over the course of the 2month testing period. The amount of hydrocarbon degraded was evaluated by measuring the amount of carbon dioxide which was produced as samples of fuel oilcontaminated soil were aerated with carbon dioxidefree air. Not surprisingly, direct chemical analysis of portions of the treated soils before and after the treatability study proved less informative due to the inherent heterogeneity of soil contamination. The costs projected to remediate the soils by forced aeration are estimated to be about onehalf of those projected to remediate the soils by excavation and treatment or disposal.
BACKGROUND
A multiphase hydrogeologic and environmental evaluation of a site located at a major retail shopping mall in the northeastern United States. It was concluded that petroleum hydrocarbon compound (PHC)contaminated soils existed over an area of approximately 5,000 square feet (in plan view) in the vicinity of a former underground storage tank (UST), once located northwest of a department store. The tank served a former concrete company facility which was razed during mall construction. Based on the results of petroleum hydrocarbon fingerprinting analysis, these PHCs appeared to be an unweathered to slightly weathered No. 2/diesel fuel oil. Figure 1 shows the location of the former UST and the extent of oilcontaminated soils.
Soil conditions in the area of PHC contamination consist of approximately 5 to 10 feet of sandy fill material, overlying 30 to 40 feet of sand and gravel with about 10 to 20 percent silt. Relatively impermeable crystalline bedrock underlies the sand and gravel. The observed groundwater table was approximately 10 feet above the bedrock surface, resulting in a vadose zone which extends to a depth of approximately 30 to 40 feet below the ground surface. Observed oil contamination was generally restricted to the sand and gravel, and commonly extended throughout much of the thickness of the vadose zone and into the upper portion of the saturated zone. A typical subsurface profile across the area of oilcontaminated soils is presented as Figure 2.
DESCRIPTION, AND BASIS FOR SELECTION, OF AERATION-ENHANCED IN-SITU BIOREMEDIATION
State clean-up goals for fuel oil-contaminated soils are 100 parts per million (ppm) total petroleum hydrocarbons, and 1.0 ppm total benzene, toluene, ethylbenzene and xylenes (BTEX). These goals are similar to those established by many other State agencies (Bell, 1990). Based on the observed site hydrogeologic conditions, the type and distribution of contaminant, and site operational constraints, aeration-enhanced in-situ bioremediation remediation was selected as the preferred remedial option. The depth to the bottom of contamination (up to about 40 feet) and the location of the area of contamination directly adjacent to active mall buildings, precluded cost-effective excavation and treatment/disposal of the soils.
The availability of oxygen is commonly the limiting factor in biodegradation of petroleum hydrocarbons and other contaminants in the subsurface. Active contaminant degrading microbial populations are commonly present in the subsurface, as are sufficient quantities of nutrients (e.g., nitrogen, phosphorous) (Calabrese and Kostrecki, 1991). However, the absence of sufficient oxygen supply causes the subsurface environment to become anoxic, and causes microbes to function in a less efficient anaerobic manner. When oxygen is added to the subsurface establishing oxic conditions, facultative heterotrophs (facultative microbes capable of both aerobic and anaerobic metabolism, and heterotrophs derive their energy from oxidation of organic carbon compounds) will convert from anaerobic to aerobic metabolism, and more efficiently and quickly degrade the petroleum hydrocarbons.
It was proposed to implement insitu bioremediation by the process of forced aeration using a series of air extraction and air inlet supply wells, driven by a blower system similar to that used for the venting of volatiles from soils. Unlike simple venting of volatiles, however, the objective of the forced aeration was to remove the petroleum hydrocarbon contamination by providing oxygen to the microbes in the subsurface (accelerating aerobic bioremediation), and removing the carbon dioxide and water which are the end products of the hydrocarbon degradation. The first phase of the proposed remedial program was performance of a laboratory treatability study which is the focus of this paper.
OBJECTIVES OF LABORATORY TREATABILITY STUDY
Information in the literature indicates the PHCs identified at the site can biodegrade if site conditions are appropriate (Overcash and Pal, 1979). To confirm whether soil conditions at the site are conducive to biodegradation, a laboratory scale treatability study was conducted. The purpose of this study was twofold:
- To assess whether toxic conditions exist in the oilcontaminated soils that might inhibit bioremediation of the organic contaminants; and
- To develop a rough estimate of the overall rate of biodegradation that would occur during fullscale insitu operation.
Note that the purpose of this study was not to optimize the rate of biodegradation, but rather to assess whether bioremediation via forced aeration could be used to remediate the contaminated soils in-situ.
Bacterial enumerations were also performed on several soil samples collected from the site. The purpose of these bacterial counts was to assess the population density of indigenous, heterotrophic bacteria in the contaminated soils and in background soils. The results of these bacterial enumerations would augment the laboratory study in providing a general indication as to the likelihood of success for bioremediation.
COLLECTION AND ANALYSIS OF SOIL SAMPLES
Soil samples were collected from four test borings drilled in the vicinity of the location of the former fuel oil UST. Soil samples were also collected from another test boring located approximately 100 feet southwest of the location of the former UST to characterize background conditions. Test boring locations are indicated on Figure 1.
Soil samples for bacterial enumeration were collected "aseptically" so as to limit the potential for contamination of the samples by non-indigenous bacteria. Splitspoon samplers were prepared for sampling as follows: 1) sampling implements were washed and scrubbed using Extran, a nonphosphate, laboratorygrade detergent, 2) implements were rinsed with tap wafer followed by distilled water, and 3) implements were assembled and then rinsed with isopropyl alcohol, a 10-percent bleach solution, and sterilized distilled water. Samples were transferred from the split-spoon to presterilized plastic bags, placed immediately in an icepacked cooler, and transported to a laboratory for microbial enumeration.
Table 1 includes the results of two types of analyses on these soil samples: 1) EPA Method 418.1, a spectrophotometric/infrared method that measures total PHC content, and 2) Modified ASTM Method D3328 (similar to EPA Method 8100), a gas chromatographic method that also measures total PHC content.
At the time of the analyses, Method 418.1 was required by the NHDES for analysis of soil contaminated with fuel oiltype PHCs. Modified ASTM Method D3328 was performed because it is generally considered to be less weathering prone to interference, and it provides a more accurate representation of actual hydrocarbon concentrations. This method also provides an indication as to the type of PHC and degree of weathering.
| TABLE 1 HYDROCARBON CONTENT OF SOIL SAMPLES | |||
| Sample ID | Sample Depth (feet below ground surface) |
Total Petroleum Hydrocarbon Content (ppm) | Analytical Method |
| TSS1-S1 | 5-11 | 6,200 | EPA 418.1 |
| 3,800 | Modified ASTM D3328 | ||
| TSS1-S2 | 11-13 | 11,000 | EPA 418.1 |
| 14,000 (duplicate) | EPA 418.1 | ||
| 4,500 | Modified ASTM D3328 | ||
| TSS2-S1 | 5-7 | 3,400 | Modified ASTM D3328 |
| 6,500 | EPA 418.1 | ||
| 6,300 (duplicate) | EPA 418.1 | ||
| TSS4-S1 | 5-7 | 1,400 | EPA 418.1 |
| 520 | Modified ASTM D3328 | ||
| TSS4-S2 | 7-12 | 1,600 | EPA 418.1 |
| 850 | Modified ASTM D3328 | ||
| TSS5-S1 | 5-7 | 190 | EPA 418.1 |
| 67 (duplicate) | EPA 418.1 | ||
| 26 | Modified ASTM D3328 | ||
Observed PHC concentrations within soil samples were typically in the range of approximately 1,000 to 10,000 ppm. The characteristics of the chromatograms produced by the Modified ASTM Method D3328 indicate the presence of a petroleum product with a chromatogram similar to No. 2/diesel fuel oil; the phytane/nC 18 ratios range from 0.36 to 0.76 (average value 0.52) and indicate that this petroleum product is generally slightly weathered. These results are consistent with those previously obtained in this area. Selected soil samples were analyzed for volatile organic compounds (VOCs) using EPA Method 8240. Additionally, three samples were screened for VOCs using a gas chromatograph headspace technique. The results of these analyses indicate the presence of acetone in the 0.1 to 200 ppm range. Previous analysis of soil samples from this area also indicated the occasional presence of low levels (up to 0.7 ppm) of toluene, ethylbenzene, xylenes, 1,1,1trichloroethane, and 1,2dichloroethene. The presence of the aromatic hydrocarbons toluene, ethylbenzene and xylenes is consistent with the presence of fuel oil contamination. The source of the other VOCs is unclear.
BACTERIAL ENUMERATIONS
A laboratory was engaged to conduct enumerations of heterotrophic bacteria on four soil samples collected at the site. The bacterial enumerations were conducted using the serial dilution plate method technique. The results of these enumerations are summarized on Table 2. Average bacterial density in soil samples contaminated with petroleum hydrocarbons (TSS2S2A, TSS4S2A) is one to two orders of magnitude higher than that of background samples (TSS5S1, TSS5S2A). The elevated population densities of bacteria in the contaminated soil samples suggest that native soil bacteria are currently biodegrading the PHC contamination and that, therefore, the contaminated soils are amenable to bioremediation. The higher bacterial counts indicate that soil conditions are not inhibitory to microbial growth, and that contaminant levels are sufficient to support a viable population of PHCdegrading bacteria. It is likely, therefore, that the greatest impediment to more rapid biodegradation is the supply of oxygen, which is quickly used up in the subsurface without a means of constant air supply.
| TABLE 2 - BACTERIA ENUMERATION | ||
| Sample Designation | Sample Depth (feet below ground surface) |
Bacterial Population Density (Mean CFU+/-S.D. per g DW soil) |
| TSS2-S2A | 7-9 | 2.9+/-1.5x105 |
| TSS4-S2A | 7-9 | 3.4+/-1.3x106 |
| TSS5-S1 | 5-7 | 5.9+/-0.4x103 |
| TSS5-S2A | 7-9 | 2.3+/-1.0x105 |
BIOTREATABILITY STUDY
EXPERIMENT PROCEDURE
The extent of aerobic biodegradation was assessed by monitoring the evolution of carbon dioxide, a primary product of hydrocarbon degradation. The evolved carbon dioxide was collected onto an adsorbent medium for subsequent gravimetric measurement. The amount of hydrocarbon biodegraded was estimated using the relative molecular weights of carbon dioxide and the PHCs.
Figure 3 shows the experimental apparatus used for the study. Laboratory air enters the apparatus through a pressure regulator and passes through the following for conditioning and flow control:
- Calcium sulfate scrubber (Drierite) to remove water (the presence of humidity can quickly spend the caustic scrubber);
- Caustic scrubber (Ascarite) to remove carbon dioxide (to limit background interferences);
- Flow meter to measure and control air flow through the system;
- Humidifier to re-humidify the air to help prevent drying the soil; and
- Manifold to divide the flow amongst the test flasks.
In each test flask, the air enters near the soil surface (at the bottom of the flask) and exits close to the top of the flask. The air exiting each test flask passes through a calcium sulfate scrubber to remove water followed by two caustic scrubbers in series to adsorb the biologically evolved carbon dioxide.
Four test flasks were run for this study. These included:
- System blank (empty flask);
- Soil control/blank consisting of 400 grams (g) background soil; and
- Two test flasks, Sample Nos. TSS 1S2 and TSS2S 1, each containing 400 g of contaminated soil.
The amount of carbon dioxide evolved from each flask was measured periodically (daily to semiweekly) for a period of nine weeks. Following completion of the study, the amount of hydrocarbons remaining in the two test flasks was measured on samples from each flask.
EXPERIMENT RESULTS
The amount of hydrocarbons biodegraded from each test flask was calculated from the amount of carbon dioxide which was evolved. For simplification, these calculations assume that all biodegraded hydrocarbons had the same molecular weight to carbon atom ratio. Deviations from this assumption are likely minor because the ratio of molecular weight to carbon atoms does not vary substantially for the compounds of interest.
Specifically, it was assumed that hydrocarbons present in the soil exist in the molecular form HC, which represents are relatively conservative estimate regarding the amount of carbon dioxide evolved in relation to the mass of hydrocarbon degraded. Under proper environmental conditions, hydrocarbons will biodegrade as follows:
