Soil Gas CO2 Research Articles

Geochemical modeling of near-surface CO2 interactions: The critical element in cost-effective long-term monitoring

Effective long-term monitoring of commercial carbon dioxide (CO2) geosequestration sites must involve multiple and integrated measurement, monitoring, and sampling techniques in order to identify and quantify CO2 leakage. Measuring changes in CO2 soil gas concentrations in the shallow sub-surface can be problematic due to naturally variable background levels, water table elevation changes, and interference from surface operations. This project focuses on modeling CO2 physical and chemical fate and transport in saturated near-surface zones (<100 m) under varying geological conditions to identify key geochemical variables that can be used in long-term monitoring programs to detect and track CO2 leakage. Numerical simulations were performed using the TOUGHREACT modeling suite to approximate the reactive transport of CO2 introduced at various depths below the water table. Results indicate that CO2 reaction products in groundwater can be used as surrogate measures of CO2 transport. Site-specific characterizations of soil, rock, and groundwater compositions are critical to quantifying interactions that will affect chemical reactions within the rock matrix and groundwater. Sensitivity analyses have been performed to investigate the impacts of soil and water chemistry on geochemical markers such as pH, alkalinity, and major cation concentrations. Results from this study indicate that modeling of the near-surface reactive transport of CO2 can be used to guide sampling efforts, thereby improving the efficiency of monitoring programs aimed at detecting CO2 leakage from geosequestration.

Subsurface characterization and geological monitoring of the CO 2 injection operation at Weyburn, Saskatchewan, Canada

Abstract The IEA Weyburn Carbon Dioxide (CO 2 ) Monitoring and Storage Project analysed the effects of a miscible CO 2 flood into a Lower Carboniferous carbonate reservoir rock at an onshore Canadian oilfield. Anthropogenic CO 2 is being injected as part of a commercial enhanced oil recovery operation. Much of the research performed in Europe as part of an international monitoring project was aimed at analysing the long-term migration pathways of CO 2 and the effects of CO 2 on the hydrochemical and mineralogical properties of the reservoir rock. The pre-CO 2 injection hydrochemical, hydrogeological and petrographical conditions in the reservoir were investigated in order to recognize changes caused by the CO 2 flood and to assess the long-term fate of the injected CO 2 . The Lower Carboniferous (Mississippian) aquifer has a salinity gradient in the Weyburn area, where flows are oriented SW–NE. Hydrogeological modelling indicates that dissolved CO 2 would migrate from Weyburn in an ENE direction at a rate of about 0.2 m/annum under the influence of regional groundwater flow. Baseline gas fluxes and CO 2 concentrations in groundwater were also investigated. The gas dissolved in the reservoir waters allowed potential transport pathways to be identified. Analysis of reservoir fluids proved that dissolved CO 2 and methane (CH 4 ) increased significantly in the injection area between 2002 and 2003. Most of the injected CO 2 exists in a supercritical state, lesser amounts are trapped in solution and there is little apparent mineral trapping. The CO 2 has already reacted with the reservoir rock sufficiently to mask some of the strontium isotope signature caused by 40 years of water flooding. Experimental studies of CO 2 –porewater–rock interactions in the Midale Marly Unit indicated slight dissolution of carbonate and silicate minerals, followed by relatively rapid saturation with respect to carbonate minerals. Carbon dioxide flooding experiments on similar rock samples demonstrated that porosity and gas permeability increased significantly through dissolution of calcite and dolomite. Several microseismic events were recorded over a six-month period and these are provisionally interpreted as being related to small fractures formed by injection-driven fluid migration within the reservoir, as well as other oilfield operations. Experimental studies on the overlying and underlying units show similar reaction processes; however secondary gypsum precipitation was also observed. Reaction experiments were conducted with CO 2 and borehole cements. The size and tensile strength of the cement blocks were unaffected, however their densities increased. Pre- and post-injection soil gas survey data are consistent with a shallow biological origin for the measured CO 2 in soil gases. Isotopic (δ 13 C) data values are higher than in the injected CO 2 , and confirm this interpretation. No evidence for leakage of the injected CO 2 to ground level has been detected. The long-term safety and performance of CO 2 storage was assessed by the construction of a features, events and processes (FEP) database that provides a comprehensive knowledge base for the geological storage of CO 2 .

The impact of a naturally occurring CO 2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)

Recent research into CO 2 geological storage has shown that it has potential to be a safe and effective way to rapidly decrease short-term anthropogenic CO 2 emissions. Despite this progress, stakeholders must be convinced that the scientific community has studied all possible scenarios, including a potential leak into the biosphere. To better understand the potential impact of such an event, a detailed geochemical and biological study was conducted during two different seasons on a naturally occurring gas vent located within a Mediterranean pasture ecosystem (Latera geothermal field, central Italy). Results from botanical, soil gas, and gas flux surveys, and from chemical and biological analyses of shallow soil samples (0–20 cm depth), show that a significant impact is only observed in the 6 m wide centre of the vent, where CO 2 flux rates exceed 2000–3000 g m −2 d −1. In this “vent core” there is no vegetation, pH is low (minimum 3.5), and small changes are observed in mineralogy and bulk chemistry. In addition, microbial activities and populations are regulated in this interval by near-anoxic conditions, and by elevated soil gas CO 2 (>95%) and trace reduced gases (CH 4, H 2S, and H 2). An approximately 20 m wide halo surrounding the core forms a transition zone, over which there is a gradual decrease in CO 2 concentrations, a rapid decrease in CO 2 fluxes, and the absence of reactive gas species. In this transition zone grasses dominate near the vent core, but these are progressively replaced by clover and a greater plant diversity moving away from the vent centre. Physical parameters (e.g. pH, bulk chemistry, mineralogy) and microbial systems also gradually return to background values across this transition zone. Results indicate that, even at this anomalous high-flux site, the effects of the gas vent are spatially limited and that the ecosystem appears to have adapted to the different conditions through species substitution or adaptation.

FLUID GEOCHEMISTRY INVESTIGATIONS ON THE VOLCANIC SYSTEM OF METHANA

An extensive geochemical survey on the fluids released by the volcanic/geothermal system of Methana was undertaken. Characterization of the gases was made on the basis of the chemical and isotopie (He and C) analysis of 14 samples. CO2 soil gas concentration and fluxes were measured on the whole peninsula at more than 100 sampling sites. 31 samples of thermal and cold groundwaters were also sampled and analysed to characterize the geochemistry of aquifers. Anomalies referable to the geothermal system, besides at known thermal manifestations, were also recognized at some anomalous degassing soil site and in some cold groundwater. These anomalies were always spatially correlated to the main active tectonic system of the area. The total CO2 output of the volcanic system has been preliminary estimated in about 0.2 kg s~ . Although this value is low compared to other volcanic systems, anomalous C02 degassing at Methana may pose gas hazard problems. Such volcanic risk, although restricted to limited areas, cannot be neglected and further studies have to be undertaken for its better assessment.

Evolution of groundwater chemical composition by plagioclase hydrolysis in Norwegian anorthosites

The Precambrian Egersund anorthosites exhibit a wide range of groundwater chemical composition (pH 5.40–9.93, Ca 2+ 1.5–41 mg/L, Na + 12.3–103 mg/L). They also exhibit an evolutionary trend, culminating in high pH, Na-rich, low-Ca groundwaters, that is broadly representative of Norwegian crystalline bedrock aquifers in general. Simple PHREEQC modelling of monomineralic plagioclase–CO 2–H 2O systems demonstrates that the evolution of such waters can be explained solely by plagioclase weathering, coupled with calcite precipitation, without invoking cation exchange. Some degree of reaction in open CO 2 systems seems necessary to generate the observed maximum solute concentrations, while subsequent system closure can be invoked to explain high observed pH values. Empirical data provide observations required or predicted by such a model: (i) the presence of secondary calcite in silicate aquifer systems, (ii) the buffering of pH at around 8.0–8.3 by calcite precipitation, (iii) significant soil gas CO 2 concentrations ( PCO 2 > 10 −2 atm) even in poorly vegetated sub-arctic catchments, and (iv) the eventual re-accumulation of calcium in highly evolved, high pH waters.

Baseline studies of surface gas exchange and soil-gas composition in preparation for CO2 sequestration research: Teapot Dome, Wyoming

A baseline determination of CO2 and CH4 fluxes and soil-gas concentrations of CO2 and CH4 was made over the Teapot Dome oil field in the Naval Petroleum Reserve 3 in Natrona County, Wyoming, United States. This was done in anticipation of the experimentation with CO2 sequestration in the Pennsylvanian Tensleep Sandstone underlying the field at a depth of 5500 ft (1680 m). The measurements were made in January 2004 to capture the system with minimum biological activity in the soils, resulting in a minimum CO2 flux and a maximum CH4 flux. The CO2 fluxes were measured in the field with an infrared spectroscopic method. The CH4 fluxes were determined from gas-chromatographic measurements on discrete samples from under the flux chambers. The CO2 and CH4 were determined at 30-, 60-, and 100-cm (11-, 23-, and 39-in.) depths in soil gas by gas chromatography. A total of 40 locations had triplicate flux measurements using 1.00-m2 (10.763-ft2) chambers, and soil gas was sampled at single points at each of the 40 locations. Carbon dioxide fluxes averaged 227.1 mg CO2 m2 day1, a standard deviation of 186.9 mg m2 day1, and a range of 281.7 to 732.9 mg m2 day1, not including one location with subsurface infrastructure contamination. Methane fluxes averaged 0.137 mg CH4 m2 day1, standard deviation of 0.326 mg m2 day1, and a range of 0.481 to 1.14 mg m2 day1, not including the same contaminated location. Soil-gas CO2 concentrations increased with depth, averaging 618, 645, and 1010 ppmv at 30, 60, and 100 cm (11, 23, and 39 in.), respectively. Soil-gas CH4 concentrations averaged 0.128, 0.114, and 0.093 ppmv at 30, 60, and 100 cm (11, 23, and 39 in.), respectively. The decrease in CH4 with depth reflects a slow rate of methanotrophic oxidation, even during winter conditions. The 13C of the soil gas CO2 was also determined in the soil-gas samples and in the atmosphere. These data demonstrated that the increased CO2 with depth was derived from the biological oxidation of soil organic matter.

A geochemical perspective and assessment of leakage potential for a mature carbon dioxide–enhanced oil recovery project and as a prototype for carbon dioxide sequestration; Rangely field, Colorado

Measurements of CO2 and CH4 soil gas concentrations and gas exchange with the atmosphere at a large-scale CO2-enhanced oil recovery (EOR) operation at Rangely, Colorado, United States allowed assessment of the microseepage potential. Shallow and deep soil gas concentrations and direct transport of CO2 and CH4, complemented by carbon isotopes, have demonstrated an estimated microseepage rate to the atmosphere of approximately 400 t CH4/yr from the 78-km2 area of the Rangely field. Preliminary estimates of deep-sourced CO2 losses are in the range of 170 to less than 3800 t/yr. Several holes as much as 9 m deep for nested sampling of gas composition, stable carbon isotopic ratios for CO2 and CH4, and carbon-14 measurements on CO2 indicate that deep-sourced CO2 microseepage loss was detected. Methanotrophic oxidation of microseeping CH4 to CO2 in soils demonstrates significant contribution to soil gas CO2 in high CH4 flux areas, necessitating a revision of the estimated direct CO2 microseepage rate to less than 170 t/yr over the Rangely field.An evaluation of produced water quality from pre-CO2-EOR to 1999 demonstrates an increase in some parameters, particularly of Ca2+ and HCO3, indicating dissolution of ferroan calcite and ferroan dolomite in the Weber Formation. Inverse computer modeling suggested carbonate mineral sequestration was possible within the constraints of evolution of produced water quality. X-ray analyses of well scales, however, do not support the presence of significant mineral sequestration. Instead, modeling indicates that the bulk of the CO2 that has been injected since 1986 is sequestered as dissolved CO2.

Carbon dioxide and radon gas hazard in the Alban Hills area (central Italy)

The sudden and catastrophic, or slow and continuous, release at surface of naturally occurring toxic gases like CO 2, H 2S and Rn poses a serious health risk to people living in geologically active regions. In general this problem receives little attention from local governments, although public concern is raised periodically when anomalous toxic-gas concentrations suddenly kill humans or livestock. For example, elevated CO 2 concentrations have been linked to the death of at least 10 people in the central Italian region of Lazio over the last 20 years, while it was the CO 2 asphyxiation of 30 cows in a heavily populated area near Rome in 1999 which prompted the present soil-gas study into the distribution of the local health risk. A detailed geochemical survey was carried out in an area of about 4 km 2 in the Ciampino and Marino districts, whereby a total of 274 soil-gas samples were collected and analysed for more than 10 major and trace gas species. Data were then processed using both statistical and geostatistical methods, and the resulting maps were examined in order to highlight areas of elevated risk. General trends of elevated CO 2 and Rn concentrations imply the presence of preferential pathways (i.e. faults and fractures) along which deep gases are able to migrate towards the surface. The CO 2 and Rn anomalous trends often correspond to and are usually elongated parallel to the Apennine mountain range, the controlling structural feature in central Italy. Because of this fundamental anisotropy in the factors controlling the soil-gas distribution, it was found that a geostatistical approach using variogram analysis allowed for a better interpretation of the data. With regard to the health risk to local inhabitants, it was found that although some high risk areas had been zoned as parkland, others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil-gas CO 2 concentrations of more than 70% and radon values of more than 250 kBq m −3. It is recommended that land-use planners incorporate soil-gas and/or gas flux measurements in environmental assessments in areas of possible risk (i.e. volcanic or structurally active areas).

Experimental evidence against diffusion control of Hg evasion from soils

Elemental Hg (Hg 0) evolution from soils can be an important process and needs to be measured in more ecosystems. The diffusion model for soil gaseous efflux has been applied to modeling the fluxes of several gases in soils and deserves testing with regard to Hg 0. As an initial test of this model, we examined soil gaseous Hg 0 and CO 2 concentrations at two depths (20 and 40 cm) over the course of a controlled environment study conducted in the EcoCELLs at the Desert Research Institute in Reno, Nevada. We also compared small, spatially distributed gas wells against the more commonly used large gas wells. In this study, two EcoCELLs were first watered (June 2000) and then planted (July 2000) with trembling aspen ( Populus tremuloides). Following that, trees were harvested (October 2000) and one EcoCELL (EcoCELL 2) was replanted with aspen (25 April 2001). During most of the experiment, there was a strong vertical gradient of CO 2 (increasing with depth, as is typical of a diffusion-driven process), but no vertical gradient of soil gaseous Hg 0. Strong diel variations in soil gas Hg 0 concentration were noted, whereas diel variations in CO 2 were small and not statistically significant. Initial watering and planting caused increases in both soil gas CO 2 and Hg 0. Replanting in EcoCELL 2 caused a statistically significant increase in soil gas CO 2 but not Hg 0. Calculated Hg 0 effluxes using the diffusion model produced values two orders of magnitude lower than those measured using field chambers placed directly on the soil or whole-cell fluxes. Neither soil gas Hg 0 concentrations nor calculated fluxes were correlated with measured Hg 0 efflux from soil or from whole EcoCELLs. We conclude that (1) soil gas Hg 0 flux is not diffusion-driven and thus soil gas Hg 0 concentrations cannot be used to calculated soil Hg 0 efflux; (2) soil gas Hg 0 concentrations are increased by watering dry soil, probably because of displacement/desorption processes; (3) soil gas Hg 0 concentrations were unaffected by plants, suggesting that roots and rhizosphere processes are unimportant in controlling Hg 0 evasion from the soil surface. We recommend the use of the small wells in all future studies because they are much easier to install and provide more resolution of spatial and temporal patterns in soil gaseous Hg 0.

Short- and long-term gas hazard: the release of toxic gases in the Alban Hills volcanic area (central Italy)

In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur (Tivoli, Cava dei Selci, Solforata, Tor Caldara). The gas comprises a large proportion of CO 2, with minor CH 4, H 2S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). His release is triggered when the total pressure of the fluid phase exceeds the hydrostatic pressure, thus forming a free gas phase. Furthermore, both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO 2, H 2S and Rn) poses a serious health risk to people living in this geologically active area. This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH 4 and H 2S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ. In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO 2 concentrations of over 70% and a CO 2 flux of about 0.7 kg m −2 day −1, as well as radon values of more than 250 kBq/m 3. In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m 3) and very high values in the cellars (up to 250.000 Bq/m 3). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas).

Evaluation of Air Injection and Extraction Tests at a Petroleum Contaminated Site, Korea

Air injection and extraction tests were conducted at a site where soils and groundwater were contaminated by petroleum hydrocarbons, mainly composed of TEX (toluene, ethylbenzene,and xylene). Storage tanks of petroleum hydrocarbons located less than 20 m away from the center of the test site were suspected to be the contamination source. Six injection/extraction wells and 21 monitoring wells were installed to evaluate performance and radius of influence with respect tosoil vapor extraction and air injection. Effective radius ofinfluence of the air injection tests based on pressure changes ranged from 3.3 to 10.5 m. Soil gas pressures, concentrationsof O2, CH4 and CO2, and temperatures were measured during the tests. Air permeability and radii of influence with respect to gas pressure and oxygen supply by air injection were estimated. Dynamic changes in the concentrations of O2, CH4 and CO2 gases and temperature around the extent of their ranges were detected asair was injected. We evaluated extensively the variations ofthe geochemical parameters that were indicative of active microbial degradation in this study.

An assessment of a mesocosm approach to the study of microbial respiration in a sandy unsaturated zone.

Microbial respiration rates were determined through a 3.2 m thick, sandy unsaturated zone in a 2.4 m diameter x 4.6 m high mesocosm. The mesocosm was maintained under near constant temperature (18 degrees to 23 degrees C) and reached steady moisture content conditions after several hundred days. Soil-gas CO2 concentrations in the mesocosm ranged from 0.09% to 3.31% and increased with depth. Respiration rates within the mesocosm were quantified over a 342-day period using measured CO2 concentrations and a transient, one-dimensional finite-element model. Microbial respiration rates were 2 x 10(-1) micrograms C.g-1.d-1 throughout most of the system, but decreased to 10(-4) to 10(-3) micrograms C.g-1.d-1 within the capillary fringe. Microbial respiration rates were also determined in minicosms (500 g sample mass) over a range in temperatures (4 degrees to 30 degrees C) and volumetric moisture contents (0.044 to 0.37). The functional dependence of CO2 production on temperature and soil-moisture content was similar for the two scales of laboratory observation. Respiration rates in the minicosms, for temperatures and moisture contents in the mesocosm, were up to an order of magnitude greater than those determined for the mesocosm. The higher respiration rates in the minicosms, compared to the mesocosm, were attributed to greater disturbance of the samples and to shorter acclimation time in the minicosms. Extrapolating the laboratory respiration rates to field conditions yielded rates that were two to three orders of magnitude greater than rates previously determined in situ for C-horizon material. Results show that in situ microbial reaction rates determined using disturbed samples in minicosms and mesocosms yielded respiration rates that greatly exceeded field conditions. Mesocosms can, however, provide a useful environment for conducting process-related research in unsaturated environments.

Variations in Phytoremediation Performance with Diesel-Contaminated Soil

The potential for phytoremediation of high concentrations of petroleum hydrocarbons is poorly understood. This study examines variations in phytoremediation performance for a soil contaminated with diesel at 6400 mg TPH kg−1 dry mixture. Experiments on diesel-contaminated soil were conducted in cups using 200 g of soil, and in columns using 4,000 g. Root development and TPH levels were measured in both experiments. In addition, CO2 soil gas concentrations were measured in the column experiments. The results show that ryegrass enhanced the loss of TPH over controls, and that this benefit only became evident after full root establishment. A comparison of the two experiments shows that rooting intensity (mg root kg−1 soil) is the key factor leading to higher TPH loss rates in the smaller containers. No clear difference in TPH loss occurred at 100 and 260 mm depths. Soil gas CO2 did not correlate well with TPH loss rates. The research concludes that an understanding of root development is crucial to evaluating the potential for ryegrass phytoremediation.

Soil gas CO2, CH4, and H2 distribution in and around Las Cañadas caldera, Tenerife, Canary Islands, Spain

Diffuse degassing of CO2, CH4 and H2 was investigated at the surface environment of Cañadas caldera, Canary Islands, during the gas survey carried out in the summer of 1995. Soil CO2 concentration varied significantly from atmospheric levels to 30%, while soil CH4 and H2 contents ranged from 5 to 851ppm and from 0.5 to 620ppm, respectively. Soil CO2, CH4 and H2 distribution suggests that high diffuse degassing at Cañadas caldera is volcanic-structurally controlled. Anomalous soil H2 concentrations were identified at the summit of Teide and outside caldera boundaries, where the most recent eruption of Tenerife Island occurred. δ13C–CO2 data showed a magmatic, mixed magmatic–biogenic, and biogenic origin while a biogenic origin is suggested for soil CH4 at Cañadas caldera and its surroundings. By coupling the CO2/3He ratio with the 3He/4He ratio of fumarolic gas samples from the summit of Teide, we propose three possible sources for carbon: MORB-type, organic carbon and carbonate.

Monitoring Anaerobic Natural Attenuation of Petroleum Using a Novel In Situ Respiration Method in Low-Permeability Sediment

Assessing petroleum biodegradation rates is an important part of predicting natural attenuation in subsurface sediments. Monitoring carbon dioxide (CO2) and methane (CH4) produced in situ, and their radiocarbon 14C), stable carbon (13C) and deuterium (D). signature provide a novel method to assess anaerobic microbial processes. Our objectives were to: (1) estimate the rate of anaerobic petroleum hydrocarbon (PH) mineralization by monitoring the production of soil gas CH4 and CO2 in the vadose zone of low-permeability sediment, (2) evaluate the dominant microbial processes using δ13C and δD, and (3) determine the proportion of CH4 and CO2 attributable to anaerobic mineralization of PH using 14C analysis. Argon was sparged into the subsurface to dilute existing CO2 and CH4 concentrations. Vadose zone CO2, CH4, oxygen, total combustible hydrocarbons, and argon concentrations were measured for 75 days. CO2 and CH4 samples were collected on day 86 and analyzed for 14C, δ13C, and δD. Based on CH4 soil gas production, the anaerobic biodegradation rate was estimated between 0.017 to 0.055 mg/kg soil-d. CH4 14C (2.6 pMC), δ13C (-45.64‰), and δD (-316‰) values indicated that fermentation of PH was the sale source of CH4 in the vadose zone. CO2 14C (62 pMC) indicated that approximately 47% of the total CO2 was from PH mineralization and 53% from plant root respiration. Although low-permeability sediment increases the difficulty of completely replacing in situ soil gas and assuring anaerobic conditions, this novel respiration method distinguished between anaerobic processes responsible for PH degradation.

Physical versus Biological Hydrocarbon Removal during Air Sparging and Soil Vapor Extraction

Although physical removal of contaminants may account for the majority of mass of petroleum hydrocarbons (PH) removed using air sparging and soil vapor extraction (AS/SVE), biological processes contribute to mass removal both during system operation and during system shutdown. Biodegradation rates are difficult to measure during active AS/SVE due to the forced entry of air. This study examined the relative rates of physical and biological removal of PH during AS/SVE. Before system startup, soil gas CO2 and O2 were measured to estimate biodegradation rates based stoichiometrically on the mass of hexane. Biodegrada tion rates calculated from average CO2 production (0.08% d-1) were 1.3 times less than those based on average O2 utilization (0.26% d-1). A simulation model (Stella II) incorporating air injection and microbial respiration measured during AS/SVE shutdown predicted initial O2 concentrations during AS/SVE well but overestimated steady-state O2 by up to 2.5% potentially because respiration may be underestimated if more O2 depletion occurred during AS/SVE than was measured when the system was off. Although biodegradation accounted for 77 times less contaminant mass removal than physical removal, biological contaminant removal may be underestimated using standard respiration tests.

CO2 degassing along the San Andreas Fault, Parkfield, California

The fluxes, concentrations, and carbon isotopic compositions of soil CO2 were measured along the Parkfield segment of the San Andreas fault (SAF). Single or double‐peak CO2 flux anomalies (&gt; 18 g m−2 d−1) were observed along 12 of 16 fault‐crossing transects at five sites. Flux anomalies did not coincide with concentration anomalies. Flux anomalies paralleled the fault trace, suggesting zones of high diffusivity/permeability. One flux anomaly may have been accentuated by a 0.3 mm creep event. However, values of δ13C (‐23.7 to ‐21.6 ‰) and 14C (110.5 to 111.9 pmC) for soil gas CO2 are characteristic of CO2 of biogenic origin. The CO2 flux anomalies are therefore consistent with fault‐related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2.

Isotopic evidence for biological controls on migration of petroleum hydrocarbons

The isotopic compositions of potential metabolic byproducts of petroleum hydrocarbon biodegradation in soil gas and groundwater samples from a shallow plume of aviation gas (AVGAS) were analyzed to assess levels and pathways of intrinsic bioremediation occurring at the site. Gasoline range organic compounds (GROs) in soil gas samples from the original source area were low (<1000 ppm), but GRO levels under an adjacent, asphalt-covered parking lot exceeded 100,000 ppm. Soil gas CH 4 was >20% in the central part of the plume and correlated well with GRO concentrations. The 14C contents of the CH 4, associated soil gas CO 2 and dissolved inorganic carbon compounds (DIC) in the groundwater were all less than 0.1 times modern, indicating they were primarily formed from degradation of AVGAS and not other potential carbon sources such as degradation of natural organic matter or dissolution of carbonate shells. The δD and δ 13C values of the CH 4 indicate that it was produced via acetate fermentation. The δ 13C values of CO 2 and DIC in the central part of the plume were high, suggesting that a large fraction of the CO 2 in that area was also produced by acetate fermentation. At the down-gradient edges of the plume, CH 4 levels dropped to zero and the δ 13C values of CO 2 were much lower (−26‰), indicating that aerobic degradation of the AVGAS was dominant in this area. Beyond the down-gradient edges of the plume, the 14C contents of both groundwater DIC and soil gas CO 2 were significantly below modern levels, implying that most of the carbon there was derived from hydrocarbons and had migrated beyond the edge of the plume. These data show that methanogenic activity in the central part of the plume was slowly degrading the AVGAS, but aerobic activity at the edges of the plume was effectively limiting migration of the hydrocarbons.

Structural pattern and CO 2–CH 4 degassing of Ustica Island, Southern Tyrrhenian basin

Brittle tectonics and ground degassing, including fracture-field, soil–gas and exhalation flux analyses of CO 2 and CH 4, were studied at Ustica Island, a Pleistocene volcanic complex in the Southern Tyrrhenian Sea. The mesoscopic fracture pattern perfectly fits an E–W-trending left-lateral strike–slip master fault, in agreement with the main morpho-structural submarine alignment including Ustica Island and Anchise Seamount. Along the SW–NE high-angle normal Arso Fault, geological evidence of reactivation with different kinematics (left- to right-lateral displacements) was recognised. Major CO 2 and CH 4 degassing (with fluxes up to 93,750 and 20 t km −2 a −1, respectively, and soil–gas concentrations of 11.3% and 5.7 ppm) occur over the Arso Fault. Although this fault is mapped just in the SW sector of the island, soil–gas CO 2 anomalies point out its clear continuation up to the NE margin of the island. These data, together with those of previous geophysical and geochemical results from off-shore Ustica, suggest that the Arso Fault is the local evidence of a more important active, gas-bearing structure. This tectonic feature is interpreted as a reactivation of a preexistent SW–NE trend, inherited as a second-order structure of the E–W deep shear zone. The reactivation is related to the interplay among different structures of the Southern Tyrrhenian basin.

Combined 14C and δ13C Monitoring of in Situ Biodegradation of Petroleum Hydrocarbons

Measurements of the stable carbon isotope ratios (δ13C) of microbial metabolic end products presents a promising method for monitoring in situ bioremediation of petroleum hydrocarbons. Differences between the δ13C values of hydrocarbons and indigenous carbon sources (e.g., plant matter, soil carbonates) can be exploited to trace the origins of metabolic end products. However, in zones of methanogenesis and/or where the δ13C values of endogenous plant matter overlap those of hydrocarbons, δ13C measurements can produce ambiguous results. In such cases, simultaneous measurement of the radiocarbon (14C) contents of metabolic end products can be used to determine their sources. This method was applied at a gasoline station spill site where hydrocarbons were the only source of 14C-free carbon. Combined δ13C and 14C measurements of soil gas CO2 and dissolved inorganic carbon in groundwater enabled quantification of carbon inputs. Furthermore, low 14C contents of high δ13C CO2 were crucial in establishing that the soil gas CO2 was derived from methanogenesis of hydrocarbons and not shell dissolution. In addition, low 14C content coupled with a 16‰ drop in the δ13C values of CO2 across a semipermeable layer beneath the gas station building confirmed that microbial oxidation of methane was occurring within this zone.