Soil Gas CO2 Concentrations Research Articles

Application of portable multi-component gas analyzer to mineral exploration in semi-arid steppes of northern China: A case study from the Qinjiaying Ag–Pb–Zn prospect

Soil gases, particularly sulfur gases, have not been widely used in mineral exploration due to their high mobility and reactivity. However, our team developed a portable multi-component gas analyzer (PMGA) capable of on-site and real-time measurement of H2S, SO2, CH4, and CO2 concentrations in soil gas. This study showcases the applicability and effectiveness of PMGA in the geochemical exploration of concealed ore deposits in the Qinjiaying Ag–Pb prospect in semi-arid steppes. Comparison tests between different analyzers and periods were conducted in the Qinjiaying Ag–Pb prospect. The results showed that PMGA has high stability, consistency, and reproducibility. Then, soil gas geochemical surveys were carried out by applying PMGA along eight prospecting lines, with three prospecting lines passing through the known mineralized zone and the remaining five prospecting lines focusing on the covered areas. Three composite anomalies of H2S, SO2, CH4, and CO2 were identified. The No. 1 soil gas anomaly corresponds well to the known No. I mineralized zone. The No. 3 gas anomaly is characterized by large-scale and strong multi-gas anomalies, indicating the presence of great prospecting potentials. Finally, drilling verification revealed a medium-sized Ag–Pb–Zn deposit in the No. 3 gas anomaly. The results demonstrate that PMGA is a powerful tool for mineral exploration in the study area. This successful practice provides a paradigm for applying PMGA in mineral exploration in semi-arid steppes.

Shear-wave splitting properties of the upper crust, during the 2013–2014 seismic crisis, in the CO2-rich field of Florina Basin, Greece

The Florina basin (NW Greece) embodies sources of natural CO2 emissions, which are aligned parallel to the NE-SW striking regional major normal faults. The study area is characterized by low to moderate seismicity, as historical information and instrumental data reveal. During February 2013 to January 2014 an outburst of seismic activity emerged, consisting of more than 2000 located earthquakes, with a mainshock of Mw = 4.1 on 17 February 2013 (16:48 UTC). A local temporary network was installed in July 2013, whose recordings provided the necessary data for performing a Shear-wave Splitting (SwS) study of the upper crust. After the application of strict selection criteria, 532 SwS measurements were obtained for two stations, using a mixture of manual and fully automatic methods. Two polarization directions of the fast shear-wave were identified at one of the stations, with the main one striking NE-SW and the secondary in a NW-SE direction. The first direction is in agreement with the local stress field, as it is identified by focal mechanisms, and with the strike of faults along which soil gas CO2 concentrations are distributed and can be explained by the Anisotropic Poro-Elasticity (APE) model. The second direction is consistent with the strike of mapped local faults, inherited since the Late Miocene extension, which are responsible for the formation of the Florina basin. The SwS results at the second station revealed an almost N-S dominant polarization direction, which is not consistent with the local stress field. This direction, obtained from events belonging to the southern group of the sequence, that was probably triggered by fluid intrusion, might be interpreted by spatiotemporal patterns of seismicity and CO2 upwelling.

Spatial and temporal variations of CO2 emissions from the active fault zones in the capital area of China

The chemical and isotopic composition of 26 soil gas wells and 11 springs along the active fault zones in the capital area of China was determined to investigate the origin of CO2, the spatial and temporal variations of CO2 emission, and their mechanisms. The chemical and isotopic data indicated that biogenic CO2 was the primary source for CO2 from the soil gas wells in both the basin and orogen regions, the crust-derived and mantle-derived CO2 could have ascended through the deep-cut faults and got into the springs, with a few diffusing into the soil gas wells. Minor air could have intruded into the soil gas wells in the orogen region through the faults cutting the surface, owing to the barometric pressure fluctuation. Relatively high He concentration, CO2 concentration, and flux of soil gas from wells were observed in the basin region than those in the orogen region. A significant amount of CO2 is produced during the process of oxidation of organic matter in the basin, where the organic matter accumulates in thick clayey strata. Further amounts of CO2 are produced by interactions between groundwaters and carbonates, which are widely represented in the basin region and by intruded air into the soil gas wells in the orogen region due to barometric pressure fluctuations. Temporal variations of CO2 concentration and CO2 flux were observed in soil gas wells in the basin region, where seismic activity is more frequent compared to those in the orogen, indicating that seismic activity could be responsible for the jumpily temporal variations of CO2 concentration and flux in wells. Monitoring activities of soil gas CO2 concentrations and CO2 fluxes in active fault zones should be carried out.

UK: Plasgran – plastic compounds

Soil gas baseline and assurance monitoring was performed over the CO2CRC Otway Project demonstration site for six years (2007–2012). The main objective of the monitoring was assurance monitoring, to demonstrate to regulators that there were no adverse environmental impacts on soils. The use of the method for leakage detection was also of research interest. Soil gas sampling was carried out annually over a defined grid, in preparation for and during the experimental storage of 65,000 tonnes of carbon dioxide (CO2) into the Waarre C Formation at a depth of about 2 km. CO2, methane, oxygen and nitrogen were the main gases analysed. Advanced analytics included helium and δCCO213 measurements.Baseline monitoring allowed us to develop an understanding of the local variability in soil gas CO2 concentrations in anticipation of the CO2 injection. Interpretation of the concentrations of CO2 in terms of possible leakage rates (fluxes) is not possible without more detailed models and measurements of the possible transport mechanisms of CO2 to and through the vadose zone. However, the comparison of post-injection with baseline data, the examination of fixed gas relationships and C/13CCO212 isotopes showed that most CO2 in the soil was of biogenic origin. A deep subsurface source, which would indicate deep gas migration to the surface, was not apparent.This shows that there is little impact of the injected CO2 on the local ecosystem and, in conjunction with other monitoring data, helps to satisfy our regulatory requirements. Our work is a case study of a practical soil gas monitoring programme and illustrates both the utility and the limitations of the technique.

Structural recovery in three selected forest soils after compaction by forest machines in Lower Saxony, Germany

Harvesting and logging with heavy forest machines cause soil damage that may restrict forest soil functions. Although the recovery ability of compacted forest soils depends on the soil properties, little is known about the long-term structure recovery of different soils following forest operations. The aim of our study was to evaluate the soil structure recovery of three different soil types. Therefore, we applied a space-for-time substitution approach (10, 20, 30 and 40years after the last machine impact) to study selected sites in Lower Saxony, Germany, using the following as proxies: bulk density, carbon dioxide (CO2) concentration in soil gas, and the relative apparent gas diffusion coefficient (DS/D0). At sites with high biological activity and high clay content (Cambisols on limestone), recovery occurred 10–20years after last traffic impact. At these sites, 10years after the last traffic impact, gas diffusivity at the wheel track was half of the gas diffusivity of the undisturbed soil, and soil gas CO2 concentrations were significantly higher at the wheel tracks. At the 20-, 30-, and 40-year-old skid trails, there were no significant differences between the untrafficked reference and the soil frequented by vehicles. Regardless of the kind of traffic impact (wheel track, mid line, side strip or undisturbed reference soil), all investigated parameters indicated that soil structure becomes more favourable with increasing time since the last forest interference. In contrast, loamy sandy soils (Podzols on glacial drift and sand) showed low recovery ability. Forty years after the last machine impact, gas diffusivity was still significantly reduced at the wheel track. Cambisols at loess-covered sandstone showed neither strong impact of forest traffic on soil structure nor changes in soil structure 20–40years after last traffic impact. In general, bulk density turned out not to be a sufficient proxy for soil structure recovery.

Identifying blind geothermal systems with soil CO2 surveys

Diffuse soil CO2 flux surveys are a widely applied approach for delineating zones of elevated heat and mass transfer in areas with geothermal surface features including hot-springs, mud pools, and geysers. However, many geothermal systems are capped by relatively impermeable layers that diminish the surface expression of potential resources present at depth. Here, we report diffuse soil CO2 flux survey results with complementary δ13CO2 values and shallow soil temperatures for the Ngatamariki geothermal system (Taupo Volcanic Zone, New Zealand), a largely surface blind system, in an effort to explore the utility of such data as indicators of magmatic carbon emission in geothermal systems with low CO2 flux. Our results include that: (1) the majority (54%) of the soil CO2 flux measurements are below background levels (15gCO2m−2day−1); (2) only 5.6% of the shallow (10cm) soil temperatures exceed 30°C; and (3) no correlation is present between soil temperature and CO2 flux at Ngatamariki. These results belie the fact that a recently developed geothermal resource is present beneath Ngatamariki. Yet, δ13CO2 values when interpreted in the context of soil gas CO2 concentrations, demonstrate that the magmatic-hydrothermal system beneath Ngatamariki can be distinguished from soil-zone carbon sources using diffuse soil gas geochemical tracers. Modeling the Ngatamariki CO2 system as a simple mixture of geothermal biogenic and atmospheric end-members allows for the apportionment of CO2 flux at different locations across the field. Based on these findings, we suggest that delineating potential geothermal sites and assessing the relative contributions of mixed fluid end-members is possible even with low diffuse CO2 flux and limited surface expression of geothermal systems at depth.

Soil gas monitoring of the Otway Project demonstration site in SE Victoria, Australia

Soil gas baseline and assurance monitoring was performed over the CO2CRC Otway Project demonstration site for six years (2007–2012). The main objective of the monitoring was assurance monitoring, to demonstrate to regulators that there were no adverse environmental impacts on soils. The use of the method for leakage detection was also of research interest. Soil gas sampling was carried out annually over a defined grid, in preparation for and during the experimental storage of 65,000tonnes of carbon dioxide (CO2) into the Waarre C Formation at a depth of about 2km. CO2, methane, oxygen and nitrogen were the main gases analysed. Advanced analytics included helium and δCCO213 measurements.Baseline monitoring allowed us to develop an understanding of the local variability in soil gas CO2 concentrations in anticipation of the CO2 injection. Interpretation of the concentrations of CO2 in terms of possible leakage rates (fluxes) is not possible without more detailed models and measurements of the possible transport mechanisms of CO2 to and through the vadose zone. However, the comparison of post-injection with baseline data, the examination of fixed gas relationships and C/13CCO212 isotopes showed that most CO2 in the soil was of biogenic origin. A deep subsurface source, which would indicate deep gas migration to the surface, was not apparent.This shows that there is little impact of the injected CO2 on the local ecosystem and, in conjunction with other monitoring data, helps to satisfy our regulatory requirements. Our work is a case study of a practical soil gas monitoring programme and illustrates both the utility and the limitations of the technique.

Soil CO2 flux baseline in an urban monogenetic volcanic field: the Auckland Volcanic Field, New Zealand

The Auckland Volcanic Field (AVF) is a dormant monogenetic basaltic field located in Auckland, New Zealand. Though soil gas CO2 fluxes are routinely used to monitor volcanic regions, there have been no published studies of soil CO2 flux or soil gas CO2 concentrations in the AVF to date or many other monogenetic fields worldwide. We measured soil gas CO2 fluxes and soil gas CO2 concentrations in 2010 and 2012 in varying settings, seasons, and times of day to establish a baseline soil CO2 flux and to determine the major sources of and controlling influences on Auckland's soil CO2 flux. Soil CO2 flux measurements varied from 0 to 203 g m−2 day−1, with an average of 27.1 g m−2 day−1. Higher fluxes were attributed to varying land use properties (e.g., landfill). Using a graphical statistical approach, two populations of CO2 fluxes were identified. Isotope analyses of δ13CO2 confirmed that the source of CO2 in the AVF is biogenic with no volcanic component. These data may be used to assist with eruption forecasting in the event of precursory activity in the AVF, and highlight the importance of knowing land use history when assessing soil gas CO2 fluxes in urban environments.

Multiple lines of evidence to demonstrate vinyl chloride aerobic biodegradation in the vadose zone, and factors controlling rates

A field-based investigation was conducted at a contaminated site where the vadose zone was contaminated with a range of chlorinated hydrocarbons. The investigation consisted of groundwater and multilevel soil-gas monitoring of a range of contaminants and gases, along with isotope measurements and microbiology studies. The investigation provided multiple lines of evidence that demonstrated aerobic biodegradation of vinyl chloride (VC) was occurring in the vadose zone (i) above the on-site source zone, and (ii) above the downgradient off-site groundwater plume location. Data from both the on-site and off-site locations were consistent in showing substantially greater (an order of magnitude greater) rates of VC removal from the aerobic vadose zone compared to more recalcitrant contaminants trichloroethene (TCE) and tetrachloroethene (PCE). Soil gas VC isotope analysis showed substantial isotopic enrichment of VC (δ¹³C -5.2 to -10.9‰) compared to groundwater (δ¹³C -39.5‰) at the on-site location. Soil gas CO₂ isotope analysis at both locations showed that CO₂ was highly isotopically depleted (δ¹³C -28.8 to -33.3‰), compared to soil gas CO₂ data originating from natural sediment organic matter (δ¹³C= -14.7 to -21.3‰). The soil gas CO2 δ¹³C values were consistent with near-water table VC groundwater δ¹³C values (-36.8 to -39.5‰), suggesting CO₂ originating from aerobic biodegradation of VC. Bacteria that had functional genes (ethene monooxygenase (etnC) and epoxyalkane transferase (etnE)) involved in ethene metabolism and VC oxidation were more abundant at the source zone where oxygen co-existed with VC. The distribution of VC and oxygen vadose zone vapour plumes, together with long-term changes in soil gas CO₂ concentrations and temperature, provided information to elucidate the factors controlling aerobic biodegradation of VC in the vadose zone. Based on the overlapping VC and oxygen vadose zone vapour plumes, aerobic vapour biodegradation rates were independent of substrate (VC and/or oxygen) concentration. The high correlation (R=0.962 to 0.975) between CO₂ concentrations and temperature suggested that aerobic biodegradation of VC was controlled by bacterial activity that was regulated by the temperature within the vadose zone. When assessing a contaminated site for possible vapour intrusion into buildings, accounting for environmental conditions for aerobic biodegradation of VC in the vadose zone should improve the assessment of environmental risk of VC intrusion into buildings, enabling better identification and prioritisation of contaminated sites to be remediated.

Continuous gas monitoring in the West Bohemian earthquake area, Czech Republic: First results

Two stations monitoring concentrations of carbon dioxide and radon in soil gas (Oldřisska and Nový Kostel) and one station monitoring flow of carbon dioxide at a mofette (Soos) have been operated in the area of the West Bohemian earthquake swarms. We present preliminary results obtained on the base of four-year observations. We found that data are not influenced considerably by barometric pressure. Although the CO2 concentration varies greatly, the long-term trends at stations Oldřisska and Nový Kostel are similar, which indicates that the CO2 flow is controlled by common geogenic processes. Also temporal trends of CO2 and Rn concentrations in soil gas at individual stations are analogous. We found diurnal variations of both CO2 concentration in soil gas and the CO2 flow at mofettes due to the earth tides. A response to tides of semi-diurnal period is insignificant in CO2 concentration and only weak in the CO2 flow. We also examined possible pre-seismic, co-seismic and post-seismic effects of the intensive 2008 earthquake swarm on the CO2 concentration at Oldřisska and Nový Kostel, and on the CO2 flow at Soos. However, all potential indications were insignificant and there has not been proven any influence of the swarm on the CO2 concentration as well as on the CO2 flow. Nevertheless, a gradual decrease of amplitudes of diurnal variations before the swarm and the lowest amplitudes during the swarm is a noteworthy phenomenon, which might indicate the strain changes of the rock associated with earthquake swarm.

ＣＯ２地中貯留のナチュラルアナログ―松代での地化学調査―

Environmental assessments and safety control during and after CO2 injection are essential for CO2 geological storage, and we are required to evaluate long-term environmental changes and safety. However, long-term changes are difficult to detect directly because the leakage of CO2 is expected to be small and the evaluation is, sometimes, requested to cover more than 1,000 years. To solve this problem, a natural-analogue study, which inquires into environmental changes at present through a comparison with past geological phenomena, is one possible approach. When the Matsushiro earthquake swarm began in 1965, a large volume of subsurface water accompanied by CO2 gas was discharged along fracture zones. A natural-analogue study on the CO2 discharge during the earthquake swarm should be helpful to create a scenario of leakage and a guideline for the safety of CO2 geological storage. Surveys of the CO2 content in soil gas and CO2 flux emissions from the surface were carried out with carbon isotope ratio measurements to understand the current state at Matsushiro, and to make a conceptual model for environmental assessments and safety control. From geological and geophysical points of view, it is said that deep water gushing out from the surface caused the swarm of earthquakes. As this deep water is still gushing out, we planned to measure CO2 concentrations in soil gas and CO2 flux to examine present CO2 activities at Matsushiro. Because CO2 in the soil is also produced by activities of microbes, however, we decided to measure the isotope ratio of the carbon to distinguish CO2 in deep groundwater origin from that produced by microbes. We selected five survey lines and three survey areas based on previous geochemical measurements and fissure distribution during the earthquake swarm, and measured CO2 concentration in soil, CO2 flux, and isotope ratio. Although there were survey points on the thick fan deposit where CO2 concentration in the soil and CO2 flux were high, the isotope ratio indicated that the carbon is produced by the activity of microbes. On the other hand, the isotope ratio of the samples collected from the thin fan deposit area shows deep subsurface water as the origin. An investigation well was drilled into the basement. Subsurface water samples were collected near the bottom of the well in the igneous rock formation. Geochemical analyses and carbon isotope ratio measurements show higher concentrations of chloride and abiogenic CO2, indicating that groundwater of a deep origin with CO2 is still rising. We are now making a conceptual model of hydrogeological history at the next step. This natural analogue study of CO2 seepage could indicate the importance of understanding shallow hydrogeological characteristics in a CO2 storage field.

Computer modeling of methanotrophic oxidation of hydrocarbons in the unsaturated zone from an enhanced oil recovery/sequestration project, Rangely, Colorado, USA

Measurements of CH4 and CO2 concentrations in soil gas, and as exchange rates (fluxes) with the atmosphere were made over the Rangely oil field, Colorado, USA. The Rangely field has been operated as a large CO2 enhanced oil recovery (EOR) project since 1986, and may serve as a prototype for CO2 sequestration in depleted oil/gas fields. The first of this pair of papers reported on the details of the methodology and results of field measurements of these gases, including light alkanes. The interpretation was enhanced by the extensive use of 13C and some 14C measurements. The fluxes of both CH4 and CO2 to the atmosphere were estimated. This research has demonstrated and quantified the methanotrophic oxidation of microseeping CH4 in the unsaturated zone. Strong spatial variations were found and studied further by nested soil gas sampling in holes up to 9 m deep. Areas of microseepage were found, as well as areas where atmospheric CH4 was the only source for methanotrophic oxidation. Computer modeling was used to determine rate constants for the oxidation process. Strong seasonal differences exist in the rates of CH4 oxidation because of the semiarid and severe climate of Rangely. Rate constants of <1 to >1000 day−1 were determined. During the summer, the highest rates were near the surface, but shifted downward to about 2 m depth in the winter. Methanotrophic oxidation of CH4 in the unsaturated zone will be the most important component of amelioration of CH4 microseepage induced by overpressuring of the reservoir undergoing CO2 injection. These types of measurements and interpretation will be necessary in order to fully understand the environmental effects of CO2 sequestration in oil/gas fields.

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.

Winter CO2 fluxes in a boreal forest

We measured soil respiration during two winters in three different ecotypes of the BOREAS northern study area. The production of CO2 was continuous throughout the winter and, when totaled for the winter of 1994–1995, was equivalent to the release of ∼40–55 g C/m2 from the soil surface. As soils cooled in the early winter, the CO2 production rate decreased in a manner that appeared to be exponentially related to shallow soil temperatures. This exponential relationship was not observed when soils began to warm, possibly indicating that there may be additional or different processes responsible for increased CO2 production during winter warming events. We also measured CO2 concentrations in soil gas and the Δ14C of the soil CO2. These measurements show that the CO2 produced in winter is not simply the return to the atmosphere of the carbon fixed during the previous growing season. We suggest that the wintertime production of CO2 originates, at least in part, from the decomposition of old organic carbon stored at depth in the soil.