In situ analysis of soil atmospheres can reveal patterns of reactions in soil according to their products. Carbon dioxide and methane, for example, represent aerobic and anaerobic reactions, respectively. An investigation of soil gas composition on a petroleum hydrocarbon contamination site suggests that aerobic biodegradation of petroleum in soil accounts for the majority of CO2 produced.56 Also, methane produced in the anoxic region of wetlands is transported to the atmosphere via diffusion, ebullition, or through vascular plants.66-68 A significant proportion (50 to 90%) of the methane produced is oxidized to carbon dioxide by methanotropic bacteria before reaching the soil surface. Depth profiles of the gas concentrations in the peatlands, with measurements from chromatographic methods, will help reveal the kinetics and mechanisms of how methane is produced and reacts.9 This is important because our understanding of the global budget of methane and carbon dioxide is still far from complete. Unknown terrestrial sinks of 1.3 Tg C/year (equivalent to 30% of the contribution from fossil fuel combustion and cement production) are required to balance the CO2 budget, and considerable uncertainties still remain regarding many components of the CH4 budget.27'31'42'43'69'70 Understanding the links of soil and atmospheric CH4 and CO2 will help resolve the puzzles of their global budgets as well as global environmental changes.
Gas, such as carbon dioxide, discharges over seismically active faults often links to a long-term, permanent phenomenon which indicates that active faults are characterized by a high permeability and act as preferential conduits in the crust. This permeability in fault gouges and intensely sheared zones can generate complex geochemical patterns in soil atmosphere. This characteristic is employed to search for active faults as well as for monitoring, in seismic and volcanic areas, distribution in soils, using carbon dioxide, along with other gases.14,71 Two regional faults72 and an underground hydrothermal aquifer28 in Italy are, respectively, located by searching for abnormal CO2 flux of soil origin. A soil gas emission monitoring program at Furnas volcano, Azores, shows that a potential health hazard of high concentrations of CO2 from the hydrothermal system beneath that area could occur without warning and reach a disastrous level.73 Determination made directly in the field becomes particularly important during periods of increased volcanic activity and movement of magma beneath the volcanic edifice. Chromatographic analyses of these gases thus become a useful tool.19
From the standpoint of environmental hazard and liability, identifying the source and origin of soil CH4 accumulations, for example, near petroleum spill sites, has become very important. On-site GC measurement of gas composition in the soil and carbon isotopic composition analysis in the laboratory, along with geological, geochemical, and land use data, help distinguish biogenic and anthropogenic sources of CH4. Appropriate methods for site remediation can also be chosen according to these results.56,74 Similar evaluations have been conducted by using concentrations of dissolved H2 to determine the distribution of redox processes in anoxic groundwater, which are heavily contaminated with organic compounds such as petroleum products or landfill leachate.51
Also, there is a need to identify soil emission hotspots such as accurate locations of organic soil pollution, and the effects of local soil composition and fertilizer distribution. Repeated sampling and analysis over long periods can be used to integrate cumulative emission rates of soil gases, such as methane and carbon dioxide. For example, Rahn et al.59 conducted a series of measurements of the CO2 emission that is concluded to be responsible for tree mortality at the site on the flanks of Mammoth Mountain. Dai and co-workers75 found a strong correlation between cumulative mass of CO2 from five incubated Arctic soils and the relative percentages of polysaccharides in these soil samples. Chromatographic methods are appropriate in terms of high spatial resolution, short analysis time, small sample volume, and little sample manipulation. The demand to improve the ability of equipment mobility has also been risen to make real-time analysis and monitoring in the field a routine job.
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