Soil Gas Analysis Research Articles

Characterization of hydrocarbon contaminated areas by multivariate statistical analysis: Case studies

Analysis of soil gases is a relatively rapid and inexpensive method to delineate and measure hydrocarbon contamination in the subsurface caused by diesel or gasoline. Techniques originally developed for petroleum exploration have been adapted to tracking hydrocarbons which have leaked or spilled at or below the earth's surface.Discriminant analysis (a multivariate statistical technique) is used to classify soil gas samples of C1 to C7 hydrocarbons as biogenic (natural soil gases) or thermogenic (contaminant hydrocarbons). Map plots of C1 to C7 total interstitial hydrocarbons, C2 to C7 interstitial hydrocarbons, and C1/ΣC n rations are used to further delineate and document the extent and migration of contamination.Three case studies of the technique are presented: each involves leakage of hydrocarbons from underground storage tanks. Soil gas analysis clearly defines the spread of contamination and can serve as the basis for the correct placement of monitoring wells. The method proved to be accurate, rapid, and cost-effective; it therefore has potential for widespread application to the identification of soil and groundwater contaminated by hydrocarbons.

Dispersion of soil gas around micron gold deposits

Kesler, S.E., Gerdenich, M,J., Steininger, R.C. and Smith, C., 1990. Dispersion of soil gas around micron gold deposits. In: S.E. Kesler (Editor), Soil and Rock Gas Geochemistry. J. Geochem. Explot., 38:117-132. Experimental weathering of sediment-hosted micron gold (SHMG) ore from the Alligator Ridge deposit indicates that ore and adjacent Pilot Shale wallrock release gas with larger concentrations of COS, CS2 and CH4, and lower CH4/(CH4 + C2H4 ) and CH4 / (CH4 + C2H 6 ) ratios than does barren Pilot Shale. The COS and CS2 are derived from decomposition of disseminated pyrite, which is probably more abundant in mineralized zones, and variations in the CH-gas ratios are thought to reflect increased thermal maturity of organic material in the mineralized zones. Surveys designed to determine whether these gases are present in detectable haloes around SHMG mineralization were carried out over the Yankee prospect, a relatively similar deposit several km south of Alligator Ridge. The surveys included analysis of interstitial soil gas, gases desorbed from soils, and gases desorbed from two sets of Carbotrap collectors, one left on the bedrock interface (at depths of about 30-70 cm below the surface) and another placed in holes drilled about 3 m into bedrock. All of the gases observed in the experiments were detected, as well as organo-sulfur and higher alkane and alkene gases. The surveys indicated systematic variations in gas dispersal related to faults and/or mineralization, with the most consistent patterns being elevated COS values and depressed CH4/(CH4 + C2H4) ratios over mineralization. Similar highs in COS, as well as other gases including SO2 and ethyl mercaptan, were associated with faults, and it was not possible to distinguish these two sources of anomalous gas patterns in all cases. In general, best results were obtained with gases desorbed from bulk soil, and no significant advantage was observed with the use of collectors placed in deep holes. Although all of the survey methods yielded anomalous patterns, none of these patterns are sufficiently strong to merit the use of gas geochemistry as a primary guide in exploration for SHMG deposits.

Application of soil-gas geochemistry to mineral exploration in Africa

Abstract Field methods of soil-gas analysis for CO2, O2, Rn and Tn are described and applied to the exploration for copper sulphide and gold/sulphide mineralisation in terrains ranging from tropical rain forest to sand desert in Africa. CO2 concentrations are high in soil gases overlying sulphide-bearing mineralisation and the patterns can be followed under increasing cover of sand to a depth of several tens of metres. The radioactive gases show promise in the mapping of basic rocks which are potential hosts to gold mineralisation. Strong seasonal effects on the CO2 distribution are noted in tropical rain forests, the absolute levels being much higher during or following the rainy season.

Basic Study on TCEs Behavior in Subsurface Environment

In order to clarify the aspects of initial stage of groundwater pollution due to organochlorines in the subsurface, some experiments on the movement of contaminants near the groundwater surface and solution process into groundwater were conducted. As one result, the oscillation of water level is assumed to support the solubility and the durability of contamination of groundwater. As the second subject of this work, the monitoring in a model region was pursued for the purpose of explaining the distribution of contamination. The method of soil gas analysis was then used to investigate the state of groundwater pollution.

Soil gas survey techniques for the investigation of underground storage tanks

AbstractThe sampling and analysis of near surface soil gas has shown great promise for the investigations of underground storage tanks. Trace to elevated concentrations of volatile organic components in soil gas samples have been used to identify source areas near buried tanks and map the migration of releases in groundwater and in the vadose zone. Soil gas surveys are also an effective tool to position soil borings, excavations, and monitoring wells, while support equipment (drilling rigs, backhoe, etc.) is still on site. Two examples are presented where soil gas surveys were used to direct the investigation of underground storage tanks.

Soil-gas surveying techniques

Delineation and remediation of subsurface contamination have become a major focus of environmental science during the past five years. Conventional technologies available for subsurface investigations always will be required to confirm and monitor subsurface contamination; however, quicker and less expensive techniques are useful for preliminary site evaluations. Soil-gas surveying is a technique that is applicable to a wide range of volatile organic compounds (VOCs) under a variety of geologic and hydrologic settings. The most common uses of soil-gas data include planning monitoring well networks and defining plume boundaries for remedial action. Preliminary screening techniques are effective in selecting locations for detailed sampling and analysis. Site investigators can use results from a preliminary soil-gas survey to drill monitoring wells at locations within the boundaries of a VOC volume. Soil-gas investigations also can be used to identify sources of VOCs and to distinguish between soil and groundwater contamination. Chemical analysis of soil gases has recently been used to monitor solvent and fuel leaks from underground storage tanks. In order to effectively design soil-gas surveys and interpret their results, the subsurface transport and fate of VOCs must be understood. These phenomena can have a profound impact on the presence and concentrations of VOCs more » in the soil atmosphere. Physical, chemical, and microbiological processes can be important in determining VOC concentrations in soil gas. « less

Shallow-Probe Soil-Gas Sampling for Indication of Ground-Water Contamination by Chloroform

Abstract Soil-gas sampling and analysis for indication of ground-water contamination by chloroform was field tested. The field testing included evaluation of: The repeatability of results, the correlation of results of soil-gas analyses with ground-water concentrations, the differences in results among closely spaced samples and the depth profile of chloroform concentrations in soil-gas samples. The sampling probe gave good repeatability, although leakage of sample from syringes between sampling and analysis is postulated as a reason for variability in results. Soil-gas concentrations correlated with ground-water data at a level above 95% significance. Short-range variability of results ranged from 12% to 43% over 2 meters with leakage of sample from syringes postulated as a reason for the higher variability. A depth-dependence of the chloroform concentration consistent with a vertical transport mechanism of gasphase diffusion was observed.

Use of Pressurized Vials for Loading Sample Loops in Soil Gas Analyses

AbstractA technique was developed using small (3 mL) evacuated glass vials for soil gas sample storage and loading of gas chromatographic sample loops. Vial volume was reduced 2 mL by inclusion of glass beads. The vials were pressurized with 3.2 mL sample and used directly for loop loading via a specially contructed vial guide. Time course determinations on stored samples of C2H2 and Kr indicated leak losses of 0 to 0.2% d−1 when compared to the ideal technique of exhaustive loop purging from a standard gas tank. The low cost, reusability, and known vacuity of the vials, as well as the elimination of a transfer stop from storage container to a syringe prior to injection into a gas chromatograph, should favor their use in agronomic field and laboratory studies where numerous samples of the smallest possible volume must be taken.

The Sampling and Gas Chromatographic Analysis of Gases from Landfill Sites

Abstract The safe development of landfill sites is daily increasing in importance. In this context the analysis of soil gases. especially for methane. is vital. Techniques have been developed for the sampling of such gases. and their subsequent analysis at the v.p.m. level by gas chromatography. Factors affecting the gas concentrations are considered.

Gases of mud volcanoes in the Copper River Basin, Alaska

The gases emitted from mud volcanoes in the Copper River Basin of Alaska fall into two distinct types which are not mixed during vertical migration. The gases in the eastern volcanoes are nearly pure carbon dioxide, whereas the western ones contain methane and nitrogen and almost no carbon dioxide. Chemical and carbon isotopic compositions suggest the carbon dioxide rich gases originated by solution of limestones and that methane rich gases probably formed by thermal decomposition of coals. Permafrost may be a strong factor in separating the Copper River Basin gases. Extending downward for several hundred feet, the permafrost would prevent shallow lateral migration and focus the energy of the gas into occasional mud volcano vents. Soil gas analyses show rapidly decreasing amounts of the methane to about 150 m and of carbon dioxide to about 20–40 m away from the mud volcano pools. Isotopic variations of these natural methane and carbon dioxide gases, which are not intermixed, indicate that calculations of formation temperatures based on δ 13C ratios cannot be used generally.

Geochemical Prospecting--A Critical Review: ABSTRACT

A recent visit to the Soviet Union confirms impressions from the literature that Soviet geochemists continue to be the most active in geochemical prospecting technology. The All Union Scientific Institute of Nuclear Geophysics and Geochemistry in Moscow has the prime responsibility for research and development of near-surface geochemical prospecting for practical application as a rapid reconnaissance tool, especially in new areas and for stratigraphic traps, in support of geophysics and geology exploration. The major topics of investigation include: vertical migration mechanisms, radiometry, geomicrobiology, soil gas analysis, and gas logging. In the United States, geochemical prospecting has had an erratic history of research and application, and although credited in whole or in part with some discoveries, it is not generally accepted as a commercially useful tool. Successful application of near-surface geochemical prospecting requires (1) migration of hydrocarbons from the accumulation to the near-surface zone, (2) detection and identification of these migrated hydrocarbons in micro quantities, and (3) correlation of the observed near-surface distribution of these hydrocarbons with their subsurface source. Modern analytical methods for detecting micro quantities of hydrocarbons in rocks, soil, or soil gas samples are accurate and definitive. Factors complicating the use of geochemical prospecting involve the vertical migration process with the associated complex environmental productive zones, non-commercial occurrences, and intervening source rocks. Field tests indicate that where hydrocarbons succeed in reaching the near-surface zones, they u ually do so in the form of seeps of restricted areal extent even at the micro concentration level. From the initial work of United States and Soviet geochemical investigators 25 years ago, there has developed a sophisticated geochemical technology now making important contributions to petroleum exploration through source bed evaluation, crude oil correlation, formation water analyses, and origin and migration investigations. End_of_Article - Last_Page 549------------

THE RELIABILITY OF GEOPHYSICAL ANOMALIES ON THE BASIS OF PROBABILITY CONSIDERATIONS

When data are used which contain random errors, there is always some question as to whether a supposed anomaly may not be real but due only to errors in the data. This paper describes a method of testing this point, which is worked out in detail assuming the Gaussian error law for the case of independent values. Possible applications of the method to the problems of contouring data, and evaluating and ranking anomalies are suggested. Examples of the use of the method on gravity and soil gas analysis data are also given.

ON GEOCHEMICAL PROSPECTING—I

The first type of geochemical prospecting, that of searching for and recognizing visible oil and gas seeps, is one of the oldest methods of prospecting for petroleum deposits. A second type of geochemical prospecting, that of soil gas analysis, has been of record in the literature for some ten years, and depends upon the microanalysis, for hydrocarbons, of “underground” or interstitial soft air. A third type of geochemical prospecting is described, that of soil analysis, which has been recently developed and depends upon the separation, identification and quantitative determination of significant constituents which are entrained, occluded and adsorbed by the soil particles. These significant constituents exhibit a wide range of molecular weights, from hydrogen through gaseous, liquid and solid hydrocarbons and their derivatives. All these constituents tend to show the same significant patterns, which in general fall into two classes, the narrow elongated pattern characteristic of faulting, and the localized “halo” pattern which tends to follow and define the edges of the underlying production. This “halo” pattern has been found over both structural and stratigraphic traps.