Soil Gas Samples Research Articles

Vapor Intrusion of VOCs and Indoor Exposure Risk: A Field Study in an Urban Neighborhood

Background: Characterizing vapor intrusion (VI) exposure risk often involves the collection of indoor air samples from affected buildings. Interpretation of field data and the assessment of health risks are widely recognized as challenging. Aims: To conduct field measurements at a well characterized site with the goal of improving VI models of transport mechanisms and risk characterization. Methods: Owners of three different properties (each with different foundation-types) were recruited to participate in our assessment in a Boston area neighborhood (USA) where VI of chlorinated solvents (VOCs) was known to be occurring and regulatory action underway. Ten (10) exterior soil gas sampling locations (nested at multiple depths) and three subslab sampling locations were installed. We conducted five quarterly sampling events. At each event, soil gas, indoor air, and groundwater samples were collected. Soil moisture content was also monitored and the soil geology was qualitatively described and evaluated. We communicated results with property owners and regulatory agencies. Results: Saturated soil zones appear to act as impervious barriers to vapor transport. Geologic features may also affect vapor transport. In several locations geologic stratigraphy correlated with soil gas concentrations. We identified the infiltration of sewer gas at one property resulting in PCE concentrations in indoor air that were nearly two-orders of magnitude higher compared to when infiltration of sewer gas was not known to be occurring. We are identifying implications of how our research can inform and improve VI characterization methods. Conclusion: Site models for VI should consider sewer lines as potential sources of volatile organic chemicals to indoor air, as well as geologic features that may restrict vapor transport. The integration of modeling results and field data, can be used to better understand site conditions and the potential for vapor intrusion exposure and health risk.

Poly-Use Multi-Level Sampling System for Soil-Gas Transport Analysis in the Vadose Zone

Soil-gas turnover is important in the global cycling of greenhouse gases. The analysis of soil-gas profiles provides quantitative information on below-ground turnover and fluxes. We developed a poly-use multi-level sampling system (PMLS) for soil-gas sampling, water-content and temperature measurement with high depth resolution and minimal soil disturbance. It is based on perforated access tubes (ATs) permanently installed in the soil. A multi-level sampler allows extraction of soil-gas samples from 20 locations within 1 m depth, while a capacitance probe is used to measure volumetric water contents. During idle times, the ATs are sealed and can be equipped with temperature sensors. Proof-of-concept experiments in a field lysimeter showed good agreement of soil-gas samples and water-content measurements compared with conventional techniques, while a successfully performed gas-tracer test demonstrated the feasibility of the PMLS to determine soil-gas diffusion coefficients in situ. A field application of the PMLS to quantify oxidation of atmospheric CH4 in a field lysimeter and in the forefield of a receding glacier yielded activity coefficients and soil-atmosphere fluxes well in agreement with previous studies. With numerous options for customization, the presented tool extends the methodological choices to investigate soil-gas transport in the vadose zone.

Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting, Tapti graben, Deccan Syneclise, India

The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of petroleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons (C1–C4) and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane (C1 = 518 ppb) and ethane plus higher hydrocarbons (ΣC2+ = 977 ppb) were observed. Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from −22.5‰ to −30.2‰ PDB, −18.0‰ to 27.1‰ PDB and 16.9‰–32.1‰ PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentrations were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons.

Distinguishing contributions to diffuse CO2 emissions in volcanic areas from magmatic degassing and thermal decarbonation using soil gas 222Rn–δ13C systematics: Application to Santorini volcano, Greece

Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ a new approach using 222Rn–δ13C systematics to identify and quantify the source of diffuse degassing at Santorini during the period of renewed activity. Soil CO2 flux measurements were made across a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were collected in April and September 2011 for isotopic analysis of CO2 (δ13C), and radon detectors were deployed during September 2011 to measure (222Rn). Our results reveal a change in the pattern of degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO2 emissions between September 2010 and January 2012. High-CO2-flux soil gas samples have δ13C∼0‰. Using this value and other evidence from the literature we conclude that these CO2 emissions from Santorini were a mixture between CO2 sourced from magma, and CO2 released by the thermal or metamorphic breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources may account more broadly for the typical range of δ13C values of CO2 (from ∼−4‰ to ∼+1‰) in diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke unusual mantle source compositions. At Santorini a mixing model involving magmatic CO2 (with δ13C of −3±2‰ and elevated (222Rn)/CO2 ratios ∼105–106 Bqkg−1) and CO2 released from decarbonation of crustal limestone (with (222Rn)/CO2 ∼ 30–300 Bq kg−1, and δ13C of +5‰) can account for the δ13C and (222Rn)/CO2 characteristics of the ‘high flux’ gas source. This model suggests ∼60% of the carbon in the high flux deep CO2 end member is of magmatic origin. This combination of δ13C and (222Rn) measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing of CO2 in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute significantly to the CO2 flux.

Vapor Intrusion Investigation at a Site in New Jersey: A Field Study

AbstractA vapor intrusion investigation was conducted at a New Jersey site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). Several residential and commercial buildings are nearby and may be impacted, including a middle school, a learning center, and a church. Groundwater water samples were collected from the site and analyzed for volatile organic compounds (VOCs). Based on the results of the groundwater samples, subslab soil gas and indoor samples were collected from the buildings located in the area surrounding the site and analyzed for VOCs by using the U.S. Environmental Protection Agency Method TO-15. The analytical data were compared against background ambient air data and the soil gas and indoor air data were compared against New Jersey state screening levels. Both PCE and TCE were found above screening levels in groundwater and subslab soil gas samples. It is recommended that further groundwater and vapor intrusion investigations be performed at the nearby buildings and an appr...

Distribution of methane and carbon dioxide concentrations in the near-surface zone and their genetic characterization at the abandoned “Nowa Ruda” coal mine (Lower Silesian Coal Basin, SW Poland)

The closure of hard coal mines in the Lower Silesian Coal Basin (SW Poland) resulted in migration of coal-bed gases (mostly methane and carbon dioxide) towards the surface. The gases were driven out by rising groundwater table in the Pennsylvanian aquifer (so-called “water piston effect”). Because such migration may be a hazard for inhabitants living in the post-mining lands a surface geochemical survey was carried out in the area of the closed “Nowa Ruda” hard coal mine. The research aimed at: (i) evaluating the distribution of methane and carbon dioxide concentrations in the near-surface zone, (ii) determining the genetic characteristics of soil gases and (iii) comparing their characteristics with that of coal-bed gases. The surface geochemical survey included sampling of soil gases and chromatographic analyses of samples for methane and carbon dioxide concentrations and for stable carbon isotope composition in samples of increased methane and carbon dioxide concentrations. Samples were collected along 8 sampling lines located in the mining fields “Piast”, “Słupiec” and “Wacław”. In total, 390 soil gas samples were collected from 1.2m depth. Maximum methane and carbon dioxide concentrations were 7.8 and 5.8vol.%, respectively. Almost all methane encountered in the analyzed samples of soil gases are of microbial origin and were generated during recent anaerobic processes in the near-surface zone. High concentration of carbon dioxide was generated by aerobic biodegradation of organic matter in abandoned mines. Moreover, carbon dioxide was originated during the near-surface oxidation of methane. Only sometimes insignificant thermogenic component of coal-bed methane and carbon dioxide which migrated to near-surface zone from Pennsylvanian coal-bed strata occurs. Moreover an insignificant component of endogenic carbon dioxide also migrated though the faults from deep-seated volcanic chambers and/or even from upper mantle in the “Piast” mining field. The results of our research revealed that the changes of soils gas concentrations in the “Słupiec” field do not indicate “water piston effect”. The highest state of gas emergency by inflow of deep-gases to near-surface zone could take place in the years 2008–2010 therefore much later than the near-surface measurement sessions in 2004. In the “Piast” field a relatively high number of anomalous concentrations of methane and carbon dioxide were measured. Such values results from degassing of groundwaters, the table of which stabilized within the Pennsylvanian aquifer in 2000. Gases migrated towards the surface along the fracture systems and faults, particularly along the Main and the Great faults. A great number of anomalously high carbon dioxide concentrations were measured in the “Wacław” field, which is an effect of groundwater table stabilization within the Pennsylvanian rocks. Carbon dioxide migrates towards the surface along the bedding planes of Pennsylvanian coal-bearing strata and along the linings of remediated “Wacław” and “Wanda” shafts. The highest state of gas emergency by inflow of deep-gases to near-surface zone caused by the uprising of Pennsylvanian water table (“piston effect”) could take place in “Piast” mining field in the years 1998–2000, and in “Wacław” mining field in the years 1978–1980, therefore much earlier than the near-surface measurement sessions in 2004.

Pollution of near-surface zone in the vicinity of gas wells

The results of a surface geochemical and geoelectrical survey carried on around gas wells drilled in the vicinity of Sucha Beskidzka town (Western Carpathians, southern Poland) are presented. The study area includes three wells located 8–10m apart, from which one has already been plugged and abandoned (P&A). In this area 46 soil gas samples were collected from a depth of 1.2m below the surface. Samples were analyzed for concentrations of light hydrocarbons and carbon dioxide. Mean methane concentration in soil gas reached almost 2.7vol.%, with a maximum value exceeded 26vol.%. Mean concentration of total alkanes C2–C5 was 0.02vol.%, with a maximum concentration close to 0.75vol.%. Mean concentration of total alkenes C2–C4 was 0.00013vol.%, with a maximum concentration close to 0.0056vol.%. Mean concentration of carbon dioxide reached 2.3vol.%, with a maximum concentration of 7.7vol.%. Relationships between these measured hydrocarbons and isotopic composition of ethane indicate deep source gases. A detailed geochemical survey was completed concurrently with a geoelectrical survey using electric resistivity tomography (ERT) and DC resistivity, pole–dipole sounding. Results showed that the distribution of hydrocarbon concentrations in soil gas correlates well with the electrical resistivity of near-surface rocks. Zones of lowered resistivity (caused by increased amounts of clays) correspond to the zones of lower hydrocarbon concentrations, whereas high hydrocarbon concentrations coincide with the zones of increased resistivity. Simultaneously, zones of much lower resistivity were identified, suggesting the presence of groundwater possibly containing increased TDS.Results of this study show that soil gas surveys conducted in the vicinity of oil wells can be used to determine the presence of anomalous concentrations of light hydrocarbons and carbon dioxide. Measured concentrations of methane can exceed the explosive limit (5%), which could create hazardous conditions. In addition, groundwater can be polluted with brine seeping from leaking well casings. These results suggest that pollution of the near-surface soil/aquatic environment associated with leakage from improperly closed oil and gas wells can cause a disturbance of near-surface geological formations associated with oil field operations.

A simple and sensitive gas chromatography–electron capture detection method for analyzing perfluorocarbon tracers in soil gas samples for storage of carbon dioxide

Co-injection of a conservative tracer during the geological sequestration of CO2 can imprint a marker to the injected gas that can be easily recognized during soil gas surveys in case of CO2 leakage from the reservoir toward the surface.In this work, an ultra-trace detection method, based on gas chromatography with electron capture detection for analyzing perfluorocarbon tracers (PFTs) in soil gas samples was optimized.Three totally fluorinated cycloalcane compounds consisting of five and six atom carbon rings were selected for this purpose. We evaluated the feasibility of collecting PFTs on adsorbent tube packed with a commercial graphitized carbon black (Carbotrap™ 100) sampling 2L of soil gas. The sorbent tubes were then analyzed by using a two-stage thermal desorption process.The developed method allows to quickly determine these compounds at very low fL/L level, method identification limits ranged from 1.3 to 5.8fL/L. Moreover, it shows good precision, evaluated by within-day and between-day studies.A preliminary survey of the PFT soil gas background concentrations, conducted by analyzing some soil gas samples collected in two different areas in Central Italy and in the Po Plain, ascertained the PFT background concentration lower than MIL.

Monitoring of a Simulated CO2 Leakage in a Shallow Aquifer Using Stable Carbon Isotopes

Artificial carbon dioxide leakage into a shallow aquifer was monitored using stable carbon isotope measurements at a field site near the town of Wittstock, Brandenburg, Germany. Approximately 400 000 L of CO(2) were injected into a shallow aquifer at 18 m depth over 10 days. The (13)C/ (12)C ratios of the CO(2) were measured in both groundwater and soil gas samples to monitor the distribution of the injected CO(2) plume and to evaluate the feasibility and reliability of this approach to detect potential CO(2) leakage, for example from carbon capture and storage (CCS) sites. The isotopic composition of the injected CO(2) (δ(13)C -30.5 ‰) was differentiable from the background CO(2) (δ(13)C -21.9 ‰) and the artificial CO(2) plume was monitored over a period spanning more than 204 days. The results demonstrate that this stable isotope monitoring approach can be used to identify CO(2) sources and detect potential CO(2) migration from CCS sites into overlying shallow aquifers or even into the upper subsurface. A significant difference between the isotope ratios of the natural background and the injected CO(2) is required for this monitoring approach to be effective.

A numerical investigation of vapor intrusion — The dynamic response of contaminant vapors to rainfall events

The U.S. government and various agencies have published guidelines for field investigation of vapor intrusion, most of which suggest soil gas sampling as an integral part of the investigation. Contaminant soil gas data are often relatively more stable than indoor air vapor concentration measurements, but meteorological conditions might influence soil gas values. Although a few field and numerical studies have considered some temporal effects on soil gas vapor transport, a full explanation of the contaminant vapor concentration response to rainfall events is not available. This manuscript seeks to demonstrate the effects on soil vapor transport during and after different rainfall events, by applying a coupled numerical model of fluid flow and vapor transport. Both a single rainfall event and seasonal rainfall events were modeled. For the single rainfall event models, the vapor response process could be divided into three steps: namely, infiltration, water redistribution, and establishment of a water lens atop the groundwater source. In the infiltration step, rainfall intensity was found to determine the speed of the wetting front and wash-out effect on the vapor. The passage of the wetting front led to an increase of the vapor concentration in both the infiltration and water redistribution steps and this effect is noted at soil probes located 1m below the ground surface. When the mixing of groundwater with infiltrated water was not allowed, a clean water lens accumulated above the groundwater source and led to a capping effect which can reduce diffusion rates of contaminant from the source. Seasonal rainfall with short time intervals involved superposition of the individual rainfall events. This modeling results indicated that for relatively deeper soil that the infiltration wetting front could not flood, the effects were damped out in less than a month after rain; while in the long term (years), possible formation of a water lens played a larger role in determining the vapor intrusion risk. In addition, soil organic carbon retarded the transport process, and damped the contaminant concentration fluctuations.

Soil gas 222Rn concentration in northern Germany and its relationship with geological subsurface structures

222Rn in soil gas activity was measured across the margins of two active salt diapirs in Schleswig-Holstein, northern Germany, in order to reveal the impact of halokinetic processes on the soil gas signal. Soil gas and soil sampling were carried out in springtime and summer 2011. The occurrence of elevated 222Rn in soil gas concentrations in Schleswig-Holstein has been ascribed to radionuclide rich moraine boulder material deposits, but the contribution of subsurface structures has not been investigated so far. Reference samples were taken from a region known for its granitic moraine boulder deposits, resulting in 222Rn in soil gas activity of 40 kBq/m3. The values resulting from profile sampling across salt dome margins are of the order of twice the moraine boulder material reference values and exceed 100 kBq/m3. The zones of elevated concentrations are consistent throughout time despite variations in magnitude. One soil gas profile recorded in this work expands parallel to a seismic profile and reveals multiple zones of elevated 222Rn activities above a rising salt intrusion. The physical and chemical properties of salt have an impact on the processes influencing gas migration and surface near radionuclide accumulations. The rise of salt supports the breakup of rock components thus leading to enhanced emanation. This work provides a first approach regarding the halokinetic contribution to the 222Rn in soil gas occurrence and a possible theoretical model which summarizes the relevant processes was developed.

Volatile Organic Compounds in the Unsaturated Zone from Radioactive Wastes

Volatile organic compounds (VOCs) are often comingled with low-level radioactive wastes (LLRW), but little is known about subsurface VOC emanations from LLRW landfills. The current study systematically quantified VOCs associated with LLRW over an 11-yr period at the USGS Amargosa Desert Research Site (ADRS) in southwestern Nevada. Unsaturated-zone gas samples of VOCs were collected by adsorption on resin cartridges and analyzed by thermal desorption and GC/MS. Sixty of 87 VOC method analytes were detected in the 110-m-thick unsaturated zone surrounding a LLRW disposal facility. Chlorofluorocarbons (CFCs) were detected in 100% of samples collected. Chlorofluorocarbons are powerful greenhouse gases, deplete stratospheric ozone, and are likely released from LLRW facilities worldwide. Soil-gas samples collected from a depth of 24 m and a horizontal distance 100 m south of the nearest waste-disposal trench contained >60,000 ppbv total VOCs, including >37,000 ppbv CFCs. Extensive sampling in the shallow unsaturated zone (0-2 m deep) identified areas where total VOC concentrations exceeded 5000 ppbv at the 1.5-m depth. Volatile organic compound concentrations exceeded background levels up to 300 m from the facility. Maximum vertical diffusive fluxes of total VOCs were estimated to be 1 g m yr. Volatile organic compound distributions were similar but not identical to those previously determined for tritium and elemental mercury. To our knowledge, this study is the first to characterize the unsaturated zone distribution of VOCs emanating from a LLRW landfill. Our results may help explain anomalous transport of radionuclides at the ADRS and elsewhere.

Detailed compositional analysis of hydrocarbons in soil gases above multi-horizon petroleum deposits – A case study from western Poland

A surface geochemical survey was carried on in an area with a cluster of petroleum deposits in western Poland. The largest of these accumulations is the Paproć-Cicha Góra dry gas deposit reservoired in the Rotliegend sediments. Other, similar gas deposits are: the Paproć gas field discovered in the top part of the Carboniferous formation and the Paproć W, Paproć N, and Nowy Tomyśl gas fields localized in the Zechstein Limestone reef structures. Moreover, the Paproć gas-condensate and the Jastrzębsko oil deposits were found at shallower depths, in the Main Dolomite formation. This paper aims to analyze the variability of composition and distribution of near-surface gaseous hydrocarbons in relation to hydrocarbon accumulations reservoired in various stratigraphic units at depths. The occurrence of CH4, its heavier homologues and unsaturated hydrocarbons was measured in 267 soil gas samples. The sampling sites were located along six survey lines (A–A, B–B, C–C, D–D, E–E and F–F) at a 200 m spacing. Concentrations of CH4, total alkanes (C2–C4) and total alkenes (C2–C3) reached maxima of 0.26 vol.%, 5.32 ppm and 5.88 ppm, respectively. Soil–gas alkanes heavier than CH4 were interpreted as being derived from subsurface hydrocarbon accumulations. These compounds penetrated to the near-surface zone by means of active and passive microseepage. Source strength of alkanes microseepage from subsurface accumulations to the land surface (determined by the C2H6/C2H4 ratio) is not a factor that substantially changes the patterns of surface anomalies. Analysis of the values of CH4/C2H6 and C2H6/C3H8 coefficients confirmed the presence of compositionally diversified, deep hydrocarbon accumulations. The study revealed that even small accumulations of gas condensate produce very strong surface geochemical signatures, contrasting with dry gas deposits. The reason may be a specific effect of hydrocarbon migration within the existing petroleum system. The Main Dolomite, which is both the source and the reservoir rock in the Fore-sudetic monocline, plays the role of a “filter”, which modifies the flux of gases ascending from deep-seated reservoirs. As a result of such “filtering”, the geochemical signature of hydrocarbons recorded at the surface indicates the dominance of gas condensate and oil–gas accumulations at depths, giving rise to obviously incorrect interpretations. These interpretations were evidenced by compositional analysis of soil gases, which did not explicitly confirm the existence of a dry gas deposit in the Rotliegend reservoir (Paproć-Cicha Góra gas field). On the other hand, the identified anomalies generally reflect the outline of this accumulation. It supports the applicability of surface geochemical surveys to hydrocarbon exploration in the Fore-sudetic Monocline. The presence of the Main Dolomite at shallower depth must be taken into account in the interpretation of data used for positioning of seismically identified Rotliegend structures. The results of this project indicate that the Main Dolomite may obscure the geochemical signature of near-surface gases derived from dry gas accumulations in both the deeper Rotliegend and Carboniferous reservoirs.

Integrated geophysics and soil gas profiles as a tool to characterize active faults: the Amer fault example (Pyrenees, NE Spain)

The combined use of geophysical and soil gas composition exploration methods allows to rapidly obtain at relative low cost information that might be related to seismic activity conditions. In this study, we carried out geochemical soil gas sampling (222Rn, 220Rn and CO2), electrical resistivity tomography and seismic refraction profiles in two selected zones near the town of Amer in the Spanish Pyrenees, where the presence of recent fractures is evident in the field. Data analysis clearly reveals anomalous values for each gas at specific positions along the electrical imaging transects. Geomorphologic and hydrogeologic data and the integration of geophysical data and soil gas measurements indicate that: (1) endogene gases radon (222Rn) and carbon dioxide (CO2) are released from the meta-sedimentary basement rocks across the main fractured zones with higher permeability values, while lower Cenozoic detrital sedimentary formations act as an impervious boundary; (2) sites with highest radon concentrations (52 kBq m−3) coincide with the zones in the Amer fault showing more recent geomorphic evidence of activity, and more specifically with those areas covered by thinner surficial formations; (3) the lowest 222Rn values (0.2–0.4 kBq m−3) were recorded just on the master active fault plane. This pattern could be explained by a dilution effect resulting from high rates of soil CO2 efflux (267 g m−2 day−1); (4) soil thoron (220Rn) activity is maximum (143 kBq m−3) in areas with high surficial fracturing; (5) groundwater pumping may cause important distortions in the natural flow dynamics and in the measured concentrations of gases. The agreement between the different data (geochemical, geophysical, and hydrogeological) and field observations (geology and geomorphology) leads us to propose a preliminary tectonic-gravitational model for the study area.

Soil gas composition above gas deposits and perspective structures of the Carpathian Foredeep, SE Poland

Abstract In 2001 a surface geochemical survey was carried out in the Carpathian Foredeep, in the area between Jaroslaw and Radymno (SE Poland) where multihorizon gas deposits were discovered. These deposits accumulate microbial CH 4 with small amounts of N 2 and higher molecular weight gaseous hydrocarbons. Soil–gas composition in the hydrocarbon fields in the study area is relatively different from the original composition of natural gas occurring in the subsurface reservoir. In 449 analyzed soil gas samples collected from 1.2 m depth relatively low concentrations were found for CH 4 (median value 2.2 ppm) and its homologues (median value of total alkanes C 2 –C 4 – 0.02 ppm). Alkenes were encountered in 36.3% of the analyzed samples (mean value of total alkenes C 2 –C 4 – 0.015 ppm) together with distinctly higher concentrations of H 2 (maximum value – 544 ppm, mean value – 42 ppm) and CO 2 (maximum value – 10.26 vol.%, mean value – 2.27 vol.%). Individual, very high concentrations of CH 4 (up to about 35 vol.%) resulted from sub-surface biochemical reactions whereas higher alkanes detected in soil gases (up to about 68 ppm) originated from deep gas accumulations. Both the H 2 and alkenes may be indirect indicators of deep hydrocarbon accumulations. Carbon dioxide may also be useful for hydrocarbon exploration, revealing increased concentrations in those sampling sites where CH 4 concentrations are strongly depleted, presumably due to bacterial oxidation. These relationships are valid only for the study area and should not be extended as an universal principle. The interpreted zones of cumulative anomalies of CH 4 , higher molecular weight gaseous hydrocarbons, H 2 and CO 2 related to the results of geological and geophysical studies directly or indirectly point to the presence of undiscovered, deep gas accumulations and suggest larger sizes for known deposits. The character of the anomalies along with the relationships between the components of soil gases support the opinion that gas accumulations occur at various depths. Moreover, the results indicate that the surface geochemical pattern is significantly influenced by tectonic discontinuities.

LIGHT HYDROCARBONS IN SOIL GAS ABOVE PROSPECTIVE OIL‐ AND GAS‐BEARING STRUCTURES: POMERANIAN SYNCLINORIUM, NW POLAND

The occurrence of methane and heavier homologues and unsaturated hydrocarbons was recorded in 226 soil gas samples recovered from a study area in the Pomeranian Synclinorium, NW Poland. Samples were collected at stations located along three survey lines (I–I', II–II’ and III–III') at a 200 m spacing. Concentrations of methane, total alkanes (C2‐C4) and total alkenes (C2‐C4) reached 13.7 vol. %, 18.4 ppm and 0.56 ppm, respectively. Soil gas alkanes heavier than methane were interpreted to be derived from subsurface hydrocarbon accumulations. These hydrocarbons migrated up into the near‐surface zone along structural discontinuities and fractures which were observed on seismic profiles.The migration rate of hydrocarbons from subsurface accumulations towards the surface was determined by the ethane/ethylene (C2/C2=) ratio. The statistical distribution of the ethane:propane (C2/C3) ratio and plots of the C2/C3+C4 and C1/C2+C3 ratios indicated that accumulations of condensate, gas or oil with a gas cap probably occur in the study area. Variations in normalized values of total alkane C2‐C4 concentrations allowed surface geochemical anomaly zones to be identified. These anomaly zones were evidence for the occurrence of subsurface hydrocarbon accumulations.Hydrocarbon accumulations are most likely to occur beneath the central part of profile II–II' and may also occur in the SW and NE parts of profile III–III' where both the Carboniferous and the Zechstein Main Dolomite are prospective. In these areas, hydrocarbon accumulations may occur in fault‐bounded anticlinal highs.Surface geochemical anomalies also confirm the presence of a non‐commercial hydrocarbon accumulation in the Main Dolomite which was discovered by the D‐1 well, and the possibility of another subsurface accumulation in an adjacent tectonic block. Soil gas analyses combined with seismic data provide evidence for the hydrocarbon prospectivity of the study area.

The example of background determination and mathematical processing of data from surface geochemical survey for the purposes of petroleum exploration

The paper reports on the methods of mathematical processing of the results of surface geochemical survey applied to petroleum exploration. The results include concentrations of methane analyzed in 1251 soil gas samples collected from the area over the Wierzchowice gas field (SW Poland). The methods were presented of determining the geochemical background, the anomalous threshold as well as of data normalization and filtration. The advantages were presented of iterative method for determination of background values proposed by the authors. In the authors' opinion, selection of the background should be preceded by statistical analysis of each set received during the iteration process. The method presented relies on the assumption of normal distribution of this dataset. The spatial distributions were compared of raw (measured) concentrations and mathematically process values. The applied mathematical data processing enables the researcher to eliminate the “noise” and to select objectively the anomalous values. Thus, such an attempt facilitates the comprehensive interpretation of geochemical data referred to geological model of a field. Results of the studies have demonstrated that the distribution of surface anomalies of methane over the Wierzchowice gas field has been disturbed by long-lasting exploitation. The anomalies appear mainly over dislocation zones, and in the SE part of the area they take the “halo” form around exploited portions of the field due to unexploited areas off the field.

Soil gas radon measurements in a region of the Bohemian Massif: investigations in the framework of an Austrian pilot study

Soil gas radon measurements are carried out in a pilot study in three municipalities in Upper Austria. The selected municipalities are characterised by a high radon potential. Sixty measuring sites--well distributed over the region and over the different geological areas--were selected. Additionally, the permeability of the soil was determined where the soil gas samples were taken and at various sites where soil samples were analysed by gamma spectrometry. Long-term soil-gas radon concentration measurements are carried out at several sites to study the long-term behaviour of radon activity concentration in soil, the influence of meteorological parameters and seasonal variations. The final goal of the project is to correlate the collected data with geological data and indoor radon concentration. First results of this ongoing study are presented and discussed.

Soil-gas helium and surface-waves detection of fault zones in granitic bedrock

Fracture and fault networks are conduits that facilitate groundwater movement in hard-rock terrains. Soil-gas helium emanometry has been utilized in Wailapally watershed, near Hyderabad in southern India, for the detection of fracture and fault zones in a granite basement terrain having a thin regolith. Based on satellite imagery and geologic mapping, three sites were selected for detailed investigation. High spatial resolution soil-gas samples were collected at every one meter at a depth of >1.5m along 100m long profiles (3 in number). In addition, deep shear-wave images were also obtained using the multichannel analysis of surface waves. The study clearly indicates several soil-gas helium anomalies (above 200 ppb) along the profiles, where the shear-wave velocity images also show many near-surface vertical low velocity zones. We thus interpret that the soil-gas helium anomalous zones and the vertical low-velocity zones are probable traces of fault/fracture zones that could be efficient natural recharge zones and potential groundwater conduits. The result obtained from this study demonstrates the efficacy of an integrated approach of soil-gas helium and the seismic methods for mapping groundwater resource zones in granite/gneiss provinces.

Beneficial effects of oxygen addition and hybrid poplar phytoremediation of petroleum-contaminated soils

Laboratory scale experiments were conducted to investigate the effect of a phytoremediation system using poplar trees on the smear zone-soil gases for petroleum hydrocarbons contaminated soil. Eight clear polyvinyl chloride (PVC) columns (1.5 m × 0.1 m) were packed with the contaminated soil covered with clean top soil. Five ports were installed on the front side of each column for soil gas sampling. Hybrid poplar trees (Populus deltoides X nigra DN34) were grown in 4 columns. The remaining four columns were used as controls (no trees). Three of the tree columns and 3 of the controls had induced air flow. An experimental apparatus was established to allow for the induced air to flow through the air flow columns. Average air flow rates of 0.43 L/d, 0.71 L/d and 1.50 L/d were tested. The columns were operated for 78 days. During the experiments, the effect of oxygen addition, through the induced air, on the development of poplar roots in the hydrocarbons contaminated soil was investigated. In addition, the compositions and concentrations of the soil gases at different soil depths were evaluated during the course of the experiments.