APPLICATION OF FORENSIC GEOCHEMICAL METHODS IN
|Application of Forensic Geochemical Methods in Assessment of Hydrocarbon Releases. (Slide Show)|
Soil gas geochemical methods have been recently utilized by the environmental industry Baehr (1984), Bond et al. (1982), Lappala et al. (1983), Eklund et al. (1985), Kerfoot et al. (1986), Marrin (1987a, 1987b), Mohsen et al. (1983), Spittler, et al. (1985), Robins et al. (1990a, 1990b, 1995). Surface geochemistry is the oldest and first method ever used for determining the presence of unrefined petroleum, both surface and subsurface, Teplitz and Rogers (1932), Link (1952), Jones (1979), Janezic (1979), Mousseau and Williams (1979), Weismann (1980), Drozd et al. (1981), Williams et al. (1981), Jones and Thune (1982), Jones and Drozd (1983), Jones (1983), Richers (1984), Price and Heatherington (1984), Matthews et al. (1984), Jones and Burtell (1996), Jones et al. (2000). A brief outline of the history of surface geochemical exploration is provided in Appendix A in a paper entitled "Predictions of Oil or Gas Potential by Near-Surface Geochemistry" published by Jones and Drozd (1983). This work, initially conducted by the Gulf Oil Company research laboratory proved that the light gases (methane, ethane, propane, and butanes) present in near surface seeps measured directly over various types of producing fields could be compositionally related directly to the type of hydrocarbons reservoired within the producing field. Even more impressive is the fact that this type of measurement could be extended to frontier basins in order to predict the oil versus gas potential in advance of drilling.
During the research and development phase of applying this technology to exploration problems, Gulf Oil company scientists recognized the potential usefulness of surface geochemical methods for conducting environmental assessments over all types of refined petroleum product transportation, processing, storage and distribution areas. This includes salt dome storage caverns, mined drifts, underground coal gasification reactors, leaky well casings, pipelines, refineries and underground petroleum storage tanks. Examples were first published in "Proceedings: Sampling and Analytical Methods for Determining Petroleum Hydrocarbons in Groundwater and Soil" (an API workshop held in November of 1984). A more recent paper by Jones and Burtell (1996) published as AAPG Memoir 66 is included in Appendix B for reference.
A. Light C1-C4 Gases
Although refined products, such as gasoline, are typically thought of as containing only C5 to C12 hydrocarbons, as reported by Johnson et al. (1988) (see Table in Appendix C) fresh gasoline contains significant levels of butanes along with trace levels of propane. As early as 1975 Gulf scientists were using the light gas composition of gasoline to recognize interference from shallow gasoline contamination when conducting soil gas surveys in exploration applications. In fact, as shown by actual analyses of both fresh and weathered samples, gasoline contains very significant levels of the very lightest alkanes: ethane, propane, isobutane and normal butane. In contrast to typical soil gas seepage measured over deep reservoirs, concentrations of these light gases are found to be in inverse order in gasoline; with normal butane > isobutane > propane > ethane.
Light hydrocarbon analyses (methane, ethane, propane and butanes) measure the lightest, most volatile constituents present in gasolines and other petroleum products. In addition to providing excellent trace gases, these light hydrocarbon compounds also tend to dissipate more rapidly with time and/or distance from the point(s) at which petroleum constituents are introduced into the subsurface environment. Light hydrocarbon analyses thus allow for the identification and differentiation of natural gas, biogenic gas, gasoline and many other refined petroleum product constituents.
Empirical evidence has also demonstrated that almost all refined products, such as gasoline, kerosene, diesel, fuel oils and lube oils have distinctive signatures, not only in their heavier fractions, but also within the traces of light gases contained within the products. Since the relative yield of light gases decreases significantly with heavier molecular weight products, such as kerosene or diesel, detection of, and recognition of these heavier products can be greatly improved by including the C5 plus, gasoline range volatiles in the vapor analysis.
For complex mixtures, such as gasoline, it is possible to identify and quantify each of the known gasoline components (see table, Johnson et al. 1988, 1990, Appendix C). Due to the large number of individual compounds present in gasoline and other petroleum products, the results of C5 plus analyses must be done using high resolution capillary gas chromatography to generate "fingerprints" of the specific products. These C5 plus signatures can be used in both the vapor, vapor headspace over water, and for the free product in determining the type of product present and the relative weathering of individual samples.
Light and gasoline range hydrocarbon analyses yield a quantitative measure of the actual concentration, by volume, of gasoline type vapors present in near surface soils. However, they do not represent, nor allow a prediction of the volume of either the residual, free phase, nor the dissolved phase hydrocarbon concentrations at depth. Although C5 plus, gasoline range hydrocarbons dissipate more slowly than lighter fraction (C1-C4) compounds, both tend to remain in higher concentrations in soils containing free products because of a buffering effect generated by the free product. The concentrations of all the volatiles establish equilibrium concentrations in both the vapor and dissolved state that is dependent not only on their vapor pressures and water solubility, but also on their relative solubility in the gasoline, or other free product mixtures. Thus the solubility of each gas in the free product mixture exerts significant control over the equilibrium saturation in the vapor or dissolved phase.
Published examples listing the composition of typical fresh and weathered gasolines are included in Appendix C along with numerous examples which illustrate the light (methane, ethane, propane and butane) and the pentane to xylene gasoline range hydrocarbons making up the vapors associated with typical gasolines. In cases where unusual or mixed products are involved, it is critical to view the C5 plus chromatograms, which generally provide the first initial fingerprints of the contamination.
Another very important consideration in conducting soil gas surveys is to take note of and to use the presence of biogenic gases as a positive aspect for mapping subsurface pollution, Jones and Agostino (1998). A paper outlining 12 case studies is given in Appendix D. Bacteria which generate carbon dioxide under aerobic conditions and methane under anaerobic conditions are ubiquitous and occur at almost every site, Marrin (1987b), Vance (1993), Chapelle et al. (1996). Not only do these bacterial actions occur, but the levels of carbon dioxide and methane generated generally range into the percent levels, often making biogenic gases the largest magnitude components in the entire soil gas mixture. In general, the longer that the pollution has existed in the subsurface, the higher are these biogenic gas levels. Both carbon dioxide and methane can be measured with fairly reasonable accuracy in the field using infrared detectors, and are highly recommended for field screening.
Although carbon dioxide is generated by the biodegradation of all types of organic materials and must be used with caution, the presence of a concentrated petroleum source such as gasoline, diesel, kerosene, etc. causes a concentrated buildup of carbon dioxide in the subsurface. The average concentration of carbon dioxide in ambient air is only 0.03 percent. Typical soil organic matter biodegration generally yields soil gas values between 0.2 to 3-5 percent. Higher concentrations of carbon dioxide measured in various soil vapor samples collected in the vicinity of subsurface petroleum products yields values as high as 5 to 30 percent, an indication that biodegration is significantly enhanced within the area surveyed.
Ambient air methane ranges from 1.5 to 2 ppm by volume. Soil gas values where there is no pollution or deep gas migration, generally range from 0.5 to 1 ppm, suggesting that normal soils act as a sink for atmospheric methane. Since biogenic methane is generated under anaerobic conditions, it is usually generated deeper in the subsurface sediments than carbon dioxide and appears to correlate very well with the location of free product. As with carbon dioxide, the longer that the pollution has existed, the higher are the soil gas levels. Petroleum contaminated sites often exhibit biogenic methane levels ranging from several thousand ppm to percent levels. For example, JP4 jet fuel under the Miami Airport has been found to have generated methane levels as high as 60 percent, Hayman et al. (1988).
Biogenic methane and carbon dioxide data, when measured in tandem with specific organic vapor components (C1-C4 and C5 plus), are very useful in defining the horizontal extent of hydrocarbon contaminants in the subsurface environment. The presence or absence of specific petroleum related hydrocarbons aids in interpretation and confirms the relationship of the biogenic gases to their appropriate sources. These two biogenic gases generally can be used for mapping the distribution of contaminated soils, even when the contamination is very old and the lighter hydrocarbon volatiles are nearly gone. In some cases only Total Petroleum Hydrocarbons (TPH) remain to confirm the previous presence of the degraded petroleum residues.
Given the demonstrable utility of soil gas methods, it is surprising that so many people have such a poor opinion of the technique. This problem has been addressed in considerable detail by Nyquist et al. (1990) and Robbins et al. (1990a, 1990b) who have found that a large number of the reported failures in the application of this technology are caused by the use of organic vapor detection instruments which do not yield a correct response under the environmental conditions in which they are applied. The factors influencing the response of these total organic measuring instruments are significantly increased by the presence of large concentrations of biogenic methane and carbon dioxide. The Nyquist and Robbins papers are included in Appendix E for reference.
Another major reason that soil gas applications fail is that the soil gas site is often treated as if it was a water well. No attempt should be made to remove three volumes of vapor before taking the sample. The objective is to measure the naturally occurring equilibrium established between the soil gas vapors and the subsurface contamination. This equilibrium is in delicate balance (particularly in low-permeability clays) and is easily disturbed.
Soil gas anomalies occur in "hydrocarbon spots" defined by geological depositional features, and as such, are naturally discontinuous, Jones et al. (2000). This limitation is easily overcome by high density sampling, coupled with a proper understanding of the geological environment of deposition. A 15 meter grid is generally recommended as a good compromise, although situations have occurred where a 3 meter grid was required. The soil gas vapors are derived from residual, free phase and dissolved phase hydrocarbons. Obviously much larger soil gas values are derived from residual phase contaminated soils than from dissolved phase. These differences can be accounted for during interpretation, provided that adequate sampling has occurred.
The importance of spacing cannot be overemphasized, both for soil gas and for monitor wells. The objectives of soil gas and monitor well investigations are to provide the true horizontal and vertical extent of the contaminated areas. Monitor wells cannot substitute for soil gas unless they are drilled on a very close spacing. In addition they must be sampled continuously from the surface down to the water table. Neither of these options are practical for monitor wells because of their high cost. Used together, soil gas, followed by selected borings (sampled continuously) and monitor wells can accomplish the desired objectives at a reasonable cost.
In the majority of situations, contamination has occurred near the surface and the pollution has migrated down to the water table by first creating a shallow residual phase with its attendant vapors. This is followed by continued lateral and downward migration of both free and dissolved phase hydrocarbons toward the water table. Geoprobe core and water samples very often find both phases trapped in small volumes above the water table, even when no obvious free phase exists on the water table. Published studies have shown that more product is trapped within the vadose zone than typically gets to the water table, Wilson and Brown (1989), Valkenburg (1994). Whenever free phase hydrocarbons form on the water table, the contamination is generally within a very advanced stage, Abdul et al. (1989).
Contrary to general opinion, surface geochemical sampling does not have to be confined only to soil gases collected entirely above the water table. Equivalent results can be obtained by measuring the gases dissolved in shallow perched or ground water aquifers which are always in dynamic equilibrium with their surroundings. If anomalous gases are contained in the soil gas pores, then they will generally be dissolved in the adjacent subsurface waters.
Admittedly, gas magnitudes in the free pore space and dissolved in the associated waters will be different from one another, However, they will both be very close to zero in background areas and generally show sufficient contrast from background whenever they are in close proximity to contamination. In areas of very shallow ground water the best sample is generally obtained from analyzing the dissolved gases in the subsurface water sample. If sufficient contrast to background is obtained, it is even possible to mix sampling types in a surface geochemical reconnaissance survey, Jones et al. (2000).
In addition to petroleum hydrocarbons, applications of surface geochemical methods have shown excellent results over areas contaminated by chlorinated solvents, even though these compounds are heavier than water. Of particular interest is the detection of ethane and ethylene anomalies found in close proximity to the chlorinated hydrocarbons. The presence of these two biogenically derived products greatly increases the possibility of detecting chlorinated solvents trapped in deep aquifers, since these two hydrocarbons are light volatiles that will rise to the surface, rather than sink to the bottom. Field observations have been verified by actual laboratory experiments, Belay et al. (1987), Barrio-Lage et al. (1990), DiStefano et al. (1991), Lesage et al. (1996). Pathways for the reduction of chloroethenes by vitamin B12 have been proposed by Burris et al. (1996). A recent publication by Lesage et al. (1996) has shown that reductive dechlorination of tetrachloroethylene mediated by vitamin B12 proceeds all the way to ethene without the accumulation of vinyl chloride. During one of their column flow experiments they stopped the flow for four days causing a significant amount of gas to form and accumulate at the top of the column. The gas mixture was found to contain predominantly ethene and ethane with a small quantity of vinyl chloride. This behavior under static conditions is exactly what would happen under natural ambient conditions within sediments contaminated by chlorinated hydrocarbons. Even when conditions are not optimized for biological reduction, these two hydrocarbon gases will rise and accumulate in the shallowest horizon possible. In areas where the soils and ground water have been contaminated for long term (40 plus years) the levels in four foot soil gas samples have been observed to reach as high as 500,000 ppm (50%) ethene. A chlorinated example is provided in Appendix F.
Soil gas sampling is an accurate surveying method that can quickly and inexpensively measure the presence and horizontal extent of contamination from a large variety of volatile organic chemicals such as gasoline, jet fuel, diesel and even other volatiles such as chlorinated solvents. The two most important advantages of soil gas surveys are cost and time. Soil gas surveys have the advantage of being non-disruptive to normal business operations. The technology can be used in congested areas within a refinery or gas plant where no other method can be as easily deployed.
Soil gases reveal the presence of volatile contaminants, but do not directly
image nor reflect the actual concentration of residual phase, the thickness
of free phase, or the concentration of dissolved phase hydrocarbons in
the ground water. Additional samples must always be gathered from geoprobe
borings and monitor wells to determine the true horizontal and vertical
extent of any subsurface contamination. Because the horizontal extent
of subsurface contamination is determined by geologic factors, a surface
geochemical map should never be considered to provide an exact picture
of any or all contaminated layers. Each layer can and will have its own
variability and identity. The most common mistake in the use of soil gas
data is to extend the interpretation beyond the survey design. Permeable
horizons in the vadose zone do not necessarily match the direction of
ground water flow. Multi-site and multi-depth samples must be taken and
analyzed to fully evaluate the horizontal and vertical contamination within
any given area. Proper geological models can help to reduce this complexity
to a manageable level.
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