Soil gas surveys

Karably, L. and K. Babcock. Effects of Environmental Variables on Soil Gas Surveys In Superfund 87, The 8th National Conference... 

Soil Gas The minmum 02 concentration that can support aerobic metabolism in unsaturated soil is approximately 1%. 02 diffuses into soil because of pressure gradients, and CO 2 moves out of soil because of diffusivity gradients. Excess water restricts the movement of 02 into and through the soil. A minimum air-filled pore volume of 10% is considered adequate for aeration. Soil gas surveys using a mobile geoprobe unit have become a valuable tool to demonstrate a zone of enhanced microbial metabolism in the subsurface. 

Due to the tendency of hydrocarbons in the soil to undergo subsurface oxidation, measuring COj levels in the soil gas could be used as a cost-effective field screening tool. In one soil-gas survey, COj levels in soil gas correlated well with petroleum hydrocarbons in the soil (Diem et al. 1988). 

Kerfoot FIB. 1990. Soil-gas surveys for detection and delineation of groundwater contamination. Trends in Analytical Chemistry 9 157-163. 

Soil gas surveys are widely used for delineating sites with VOC contamination. Even though soil gas data may be generated with the most definitive and accurate analytical method available, such as gas chromatography/mass spectrometry (GC/MS), they will always remain screening data due to uncertainty associated with the sample matrix. 

The percent-methane compositions from the auger hole surveys conducted over the Sacramento and San Joaquin Basins are plotted in Fig. 5-21. There is a decrease from 98% methane in the north of the Sacramento basin to 90% in the south part, whilst the soil gas over the San Joaquin Basin has 82% methane. These data imply that a soil-gas grid would have defined local differences regionally. Furthermore these geochemical data are repeatable (Table 5-Xl) the percent-methane values on Fig. 5-21 were all determined at least two or three times over a three-year period and found to be repeatable. Compositional data have remained repeatable throughout our experience with soil-gas surveys. 

Comparison of results of free soil-gas and disaggregated soil-gas surveys, Rosehill, Virginia (Richers, 1984)... 

Subsequently the scientific research well was drilled as planned. When it penetrated Lower Ordovician carbonates it produced 16.3 x lO" m per day of natural gas. The scientific research well thus became the discovery well in this region. Since the well was drilled directly on the surface hydrocarbon gas anomaly, the soil gas survey traverse was thought to cross an apical or linear anomaly. Further work, however, was to reveal that the gas field has an annular anomaly. 

In contrast to the very restricted size of the soil-gas anomaly at Jabiluka, Zonghua and Yangfen (1981) describe broad anomalies associated with a buried skam Cu deposit near Shanghai, China. The ore body is confined to the contact zone of granodiorite and Palaeozoic and Mesozoic sedimentary rocks. The major sulphide minerals in the ore body are chalcopyrite, pyrite and molybdenite. Part of the deposit has been oxidised to limonite. The area is covered by 140-180 m of alluvium. A soil-gas survey of the area was complemented by a multielement study of soils including the determination of Hg. Significant anomalies of Hg in soil and soil gas occurred above the buried sulphide... 

Dadashev et al. (1971) describe a soil-gas survey over the Filizchai pyrite deposit in Azerbaijan. They report a definite relation between the position of the ore body and the composition of the soil air. As the survey line enters the ore field in the southwest, there... 

Corazza, E., Magro, G., Pieri, S. and Rossi, U., 1993. Soil gas survey in the geothermal area of Bolscna Lake (Vulsini Mountains, central Italy). Gcothermics, 22 201-214. 

Reimer, G.M., 1986. Helium soil-gas survey of the Aurora uranium deposit, McDcrmitt caldera complex, Oregon. J. Geophys. Res. 91 12,355-12,358. 

Robbins GA, Deyo BG, Temple MR, et al. 1990. Soil-gas surveying for subsurface gasoline contamination using total organic vapor detection instruments Part II. Field experimentation. 

Ciotoli, G., Guerra, M., Lombardi, S., and Vittori, E., Soil gas survey for tracing seismogenic faults a case study in the Fucino basin. Central Italy, J. Geophys. Res., 103(B10), 23781-23794, 1998. 

Soil gas surveys can be done rapidly and easily and can cover a wide area. One new type uses a Cortex shoelace with absorbent pellets inside (called the Goresorber). The shoelace is inserted into the ground with a two-foot push tube. (Clearance with a metal detector is recommended.) The lace is withdrawn in a day or so and the pellets analyzed for CWM or explosive breakdown products. A geographic information systems (CIS) computer printout is made extrapolating areas of increased concentrations. This method can be used to locate all manner of landfills or contaminated soils. Similarly, soil samples can hint at burial sites. Where most soil samples are clean but some reveal CWM or explosive breakdown products, this may present evidence of a nearby burial site. 

The first generation of sophistication of radon and heiium soil-gas surveys—that of collection of a soil-gas sample in the field at an instant and analysis of its radon or heiium content— has had highly variable usefulness.As in ail soil-gas exploration surveys, commonly meteoroiogic variations and diurnal thermal and plant respiratory effects cause greater variations in trace gas content of the soil gas than does the ore deposit induced anomaly. Therefore, the utility of radon and heiium soil-gas surveys has been greatly enhanced and will be more so in the future through the development and use of accumulator measurements, whereby the amount of radon or heiium that reaches the soil or soil-gas over a period of time (a month or two) is measured. 

Radon and helium soil-gas surveys may be useful in detailed or semi-regional exploration programmes by virtue of the fact that both are inert gases derived from the radioactive decay of uranium and its daughters. As such, the gases may migrate away from a buried uranium deposit to yield anomalous concentrations of radon and/or helium in soil gas. Thus, a buried uranium deposit may be remotely detectable indirectly by detection of anomalous radon and/or helium concentrations in soil gas. 

A soil-gas survey is conducted to determine if the subsurface is oxygen limited. Soil gas is analyzed for concentrations of oxygen and carbon dioxide. 

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