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| Resource Library > Technology Transfer > Programs and Initiatives > Bioventing > Site Screening |
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| Bioventing: Site Screening |
Site characterization is an important step in determining the feasibility of bioventing and in providing information for a full-scale bioventing design. Volume 2 of the Bioventing Principles and Practice Manual discusses site characterization methods that are recommended for bioventing sites based on field experience and a statistical analysis of the Bioventing Initiative data. These parameters have proven to be the most useful in predicting the potential applicability of bioventing at a contaminated site. The sequence of site characterization activities that should be conducted at a potential bioventing include the following:
- Review existing site data
- Conduct soil gas survey
- Characterize soil
- Perform in situ respiration testing
- Perform soil gas permeability testing
In general, sites may be deemed amenable for bioventing if the following parameters are met:
- Low oxygen in soil gas (less than 2% vol/vol) compared to background (e.g., 15 - 21%)
- High carbon dioxide in soil gas (greater than 2%) compared to background (e.g., 0.5%)
- Elevated volatile hydrocarbons in soil gas
- Contaminants of concern above cleanup goals
Thus, one can use standard soil gas techniques to confirm the appropriateness of bioventing. At sites where the contamination is at sufficiently shallow depths (typically < 20 ft [6.1 m]), a soil-gas survey should be conducted initially to determine whether oxygen-limited conditions exist. Oxygen-limited conditions are a good indicator that bacteria capable of degrading the contaminants of concern are present, given that soil gas in uncontaminated vadose zone soils generally will exhibit oxygen concentrations equivalent to ambient air. The soil gas survey also assists in delineating the extent of contamination and locating suitable areas for vent well and monitoring point placement. Data on soil gas concentrations of oxygen, carbon dioxide, and total petroleum hydrocarbons (TPH) can provide valuable insight into the extent of subsurface contamination and the potential for in situ bioventing. Addendum One, Test Plan and Technical Protocol for A Field Treatability Test for Bioventing - Using Soil Gas Surveys to Determine Bioventing Feasibility and Natural Attenuation Protocol, provides a working knowledge of how soil gas can be used as an indicator of subsurface hydrocarbon contamination and how bioventing feasibility can be determined using soil gas monitoring techniques.
Technical Basis:
Soil microbial and geochemical process can alter soil gas composition. Geochemical changes in soil gas are likely to be similar in background uncontaminated versus petroleum hydrocarbon-impacted areas. However, microbial biodegradation of petroleum hydrocarbons produces a predictable fingerprint that will only be exhibited in the area of the contamination.
Following a hydrocarbon spill, soil microorganisms begin to use available soil gas oxygen. As the population of fuel-degrading microorganisms increases, the supply of soil gas oxygen is often depleted, creating an anaerobic volume of contaminated soil. Under anaerobic conditions, fuel biodegradation generally proceeds at significantly slower rates. In some cases, aerobic biodegradation will continue because the diffusion or advection of oxygen into soils from the atmosphere exceeds biological oxygen utilization rates. Under these circumstances the site is naturally aerated, and the hydrocarbons will be naturally attenuated over time.
Carbon dioxide is produced as a by-product of the complete biodegradation of natural or refined hydrocarbons, and can also be produced or buffered by the soil carbonate cycle. Carbon dioxide levels in soil gas are generally elevated in fuel-contaminated soils when compared to levels in clean background soils. However, due to the buffering capacity of alkaline soils, the relationship between contaminant biodegradation and carbon dioxide production is not always a reliable indicator. In acidic soils, such as exist at Tyndall Air Force Base (AFB), Florida, carbon dioxide production was directly proportional to oxygen utilization.
Volatile hydrocarbons found in soil gas can also provide valuable information on the extent and magnitude of subsurface contamination. Fuels such as gasoline, which contain a significant fraction of C6 and lighter compounds, are easily detected using soil gas monitoring techniques. Heavier fuels, such as diesel, contain fewer volatiles and are more difficult to locate based on volatile hydrocarbon monitoring. Methane is frequently produced as a by-product of anaerobic biodegradation and, like oxygen depletion, can also be used to locate the most contaminated soils at a site.
The use of soil gas to determine bioventing feasibility and bioventing progress has several economic and technical advantages over more traditional drilling and soil sampling techniques. In shallow (<20 feet), predominantly sand soils, the labor and equipment cost for a two-person soil gas survey team is approximately one-third the cost of a three-person conventional drilling and sampling team. Many new hydraulically driven, multi-purpose probes can be used for soil gas sampling, as well as for collecting soil and groundwater samples at depth. These probes can be advanced as quickly as conventional augers and do not produce drill cuttings which require expensive analysis and disposal.
An additional advantage of soil gas sampling is that a properly collected gas sample can represent the average chemistry of several cubic feet of soil as compared to a discrete soil sample, which can only describe a few cubic inches of the subsurface. This advantage is of particular importance in risk-based remediation projects where the degree of benzene removal can most accurately be determined by using multiple soil gas sampling locations.
Soil gas techniques have several limitations which must be acknowledged if this approach is to be properly applied. Soil gas monitoring is often impossible in very moist soils and particularly in fined-grained units. Attempts to gather soil gas samples from low-permeability soils often result in the leakage of atmospheric air into the sampling system and inaccurate sampling results.
Although hydraulically driven probes such as cone penetrometers are extending the depth of application, deep contamination and contamination in tight or cobble soils can best be assessed by using standard drilling techniques to install permanent soil gas monitoring points.
Examples:
Good Bioventing Candiates:
Discussion on FTA-2: This site displays the classical profile of low soil gas oxygen, high carbon dioxide and volatile organic compounds. Remediation is very likely to be enhanced via air injection bioventing. Additional information: Soils appeared to be permeable based on physical descriptions as sandy soils and relatively low vacuum levels on soil gas sampling pumps. A relatively high vacuum pump reading or and inability to obtain a soil gas sample would raise a red flag with respect to bioventing feasibility.
Discussion on Site S-4: Site S-4 also displays the classic low oxygen, high carbon dioxide and volatile hydrocarbon profile that is consistent with bioventing. Technical note: The relate
Poor Bioventing Candidates:
Discussion on Fire Training Area 1: Even though this fire training area has been subject to numerous releases of waste oils, fuel hydrocarbons, and spent solvents, the above profile suggests that volatile fuel hydrocarbons are not present in significant quantity, as per the Total Volatile Hydrocarbon (TVH) measurements. In addition, the site soil gas does not exhibit the presence of an oxygen demand or the corresponding carbon dioxide buildup. Both oxygen and carbon dioxide are present at atmospheric concentrations. This site has been naturally aerated as a result of sandy, permeable soils and a shallow water table which prevented petroleum hydrocarbons from infiltrating deep into on-site soils.
Discussion on Aquasystem Site: This site does not display a strong profile with low oxygen, high carbon dioxide, and high TVH. Carbon dioxide levels appear to be higher at greater depths which may suggest that there may be residual contamination at greater depths. These field data can be used to select the next sampling location to bound the contamination vertically and horizontally during a single field mobilization. This can result in a very significant cost savings since waiting weeks to months for reports based on off-site laboratory data is costly, time consuming, and with even the best effort could require multiple field mobilizations. Given the data above, this site does not appear to require further remediation and bioventing is not indicated.
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