Diffusion gases into soil

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. 

Volatilization causes contaminants to transfer from the dissolved phase to the gaseous phase. In general, factors affecting the volatilization of contaminants from ground water into soil gas include the contaminant concentration, the change in contaminant concentration with depth, the Henry s Law constant and diffusion coefficient of the compound, mass transport coefficients for the contaminant in both water and soil gas, sorption, and the temperature of the water. ... 

Figure 1. Schematic illustration of factors influencing the production and migration of radon in soils and into buildings. Geochemical processes affect the radium concentration in the soil. The emanating fraction is principally dependent upon soil moisture (1 0) and the size distribution of the soil grains (d). Diffusion of radon through the soil is affected primarily by soil porosity ( ) and moisture content, while convective flow of radon-bearing soil gas depends mainly upon the air permeability (k) of the soil and the pressure gradient (VP) established by the building.

Radon from the soil enters into buildings by convective flow of soil gas. Transport by diffusion is normally insignificant. In houses with very high radon concentration, diffusion need not be considered because it can only provide an insignificant fraction of the source strength. There are three conditions necessary for infiltration of soil gas containing radon into the building from the soil ... 

In the model, the internal structure of the root is described as three concentric cylinders corresponding to the central stele, the cortex and the wall layers. Diffu-sivities and respiration rates differ in the different tissues. The model allows for the axial diffusion of O2 through the cortical gas spaces, radial diffusion into the root tissues, and simultaneous consumption in respiration and loss to the soil. A steady state is assumed, in which the flux of O2 across the root base equals the net consumption in root respiration and loss to the soil. This is realistic because root elongation is in general slow compared with gas transport. The basic equation is... 

To collect a sample, the probe with a SPME fiber installed is inserted into the soil. Air is pumped through the probe, drawing subsurface soil vapors into the probe tip and across the SPME fiber. Pumping air across the fiber increases uptake of target analytes by the SPME fiber relative to what is collected by molecular diffusion alone. Once a sample is collected, the SPME fiber is removed from the probe for analysis. To analyze the sample, the SPME fiber is inserted into a modified inlet system attached to the Fido sensor. The modified inlet serves to heat the SPME fiber, causing rapid and quantitative desorption of trapped molecules of analyte. The vapor-phase analyte is then swept into the sensor for analysis by a flow of carrier gas. 

Radon (222Rn) is formed by the radioactive decay of uranium, BKU (Fig. 15.1a). As a result, the highest concentrations tend to be associated with soils derived from rocks with a high uranium content (Nazaroff and Nero, 1988 Boyle, 1988 Nero, 1989 Mose and Mushrush, 1997). Because radon is a gas that diffuses out of the soil, it can enter homes through cracks in the foundation, around loose-fitting pipes and wall joints, and through floor drains (e.g., Nero, 1989). The concentrations found in a home depend on the type of soil (including the moisture content) on which it sits and the extent of Rn penetration into the house. They also depend on the house ventilation rate and the particular location in the house in which the measurement is... 

Tecator Ltd. [16] have described a flow injection analysis method for the determination of 0.2 -1.4 mg/1 (as NH3N) of ammonia nitrogen in soil samples extractable by 2 M potassium chloride. The soil suspension in 2 M potassium chloride is centrifuged and filtered and introduced into the flow injection system for the analysis of ammonia (and nitrate) one parameter at a time. Ammonia is determined by the gas diffusion principle, in which a PTFE membrane is mounted in the gas diffusion cell. 

Several workers have demonstrated that diffusion of CO2 gas through the soil causes a fractionation in the CO2 that exits from the soil surface (Dorr and Milnnich, 1980 Ceding et al., 1991 Davidson, 1995 Amundson etal., 1998 see Chapter 5.01). These workers draw an important distinction between soil CO2 (present in the soil) and soil respired CO2 (gas that has passed across the soil surface and into the atmosphere). Simultaneous measurements of shallow soil CO2 (i.e., <0.5 m) and soil respired CO2 show that the 5 C of soil respired CO2 is often 4%c lower than soil CO2 (Cerling et al., 1991 Davidson, 1995). This magnitude of fractionation is similar to what would be expected by diffusion processes in air (4.4%c), given the theoretical diffusion coefficients of C02 and C02 (Cerling et al, 1991 Davidson, 1995). 

Experimental determination of He diffusion was attempted by Duddridge et al. (1991), who injected He-rich gas at a depth of 35 m into permeable limestones cut by a fault. They recorded a pulse of He in shallow soil gas 5-20 hours later within 10 m of the fault suboutcrop and up to 53 hours later 20 m from the fault suboutcrop. However, the concentration increase recorded (0.032 ppm) is well within the error of the analytical system (mass spectrometer with constant pressure inlet, as discussed below, and analytical sensitivity of 0.030 ppm), the data are patchy with many samples showing no pulse, and there is no estimate of background variation or the effect of changing environmental conditions. Conclusions about diffusion rates based on these data may not be reliable. 

In some cases the source of soil contamination is the soil itself. For example, soils rich in toxic elements such as arsenic, lead, mercury, and cadmium provide their own source of contamination. In addition, soils rich in uranium and its radioactive decay product radium provide continuous long-term sources of the radioactive gas radon in soil. The radon can diffuse from soil into the air of buildings or into groundwater, with resulting radiation exposures to human and animal populations. Other possible sources of contamination internal to soil itself are biological organisms, which are either themselves health threatening or which produce toxic chemicals. 

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