Transport unsaturated zone

Models for transport distinguish between the unsaturated zone and the saturated zone, that below the water table. There the underground water moves slowly through the sod or rock according to porosity and gradient, or the extent of fractures. A retardation effect slows the motion of contaminant by large factors in the case of heavy metals. For low level waste, a variety of dose calculations are made for direct and indirect human body uptake of water. Performance assessment methodology is described in Reference 22. 

Suction lysimeters are required for some field-scale groundwater monitoring studies to monitor the transport of compounds of interest through the unsaturated zone. Unlike monitoring wells or water supply wells that sample water from the saturated zone, suction lysimeters sample water from the unsaturated zone. This section provides a summary of the installation and sampling procedures for pressure-vacuum suction lysimeters. A detailed discussion of unsaturated zone sampling devices is available elsewhere. 

There are many components of gasoline that readily dissolve in water and are transported as solutes in the groundwater. Most gasoline products are volatile and can release gas into the soil void in gaseous form, particularly in the unsaturated zone. Besides these three forms, gasoline components can be adsorbed by the soil matrix and exist in the soil as adsorbates. 

For vapor to move in the unsaturated zone, the soil formations must be sufficiently dry to permit the interconnection of air passages among the soil pores. Vapor concentration and vapor flow govern its movement. Vapor can move by diffusion from areas of higher concentration to areas of lower concentration and ultimately to the atmosphere. Therefore, the transportation of the vapor phase of gasoline components in the unsaturated zone can pose a significant health and safety threat because of inhalation and explosion potential. 

The level of vapor movement in the unsaturated zone is much less important than transport in liquid form. However, this might not be true if the water content of the soil is very low or if there is a strong temperature gradient. The movement of vapor through the unsaturated zone is a function of temperature, humidity gradients, and molecular diffusion coefficients for water vapor in the soil. 

Groundwater can be found in the traditional sense at the water table below which the soil pore spaces are essentially saturated and the water is free to move, and in the unsaturated zone (or vadose zone) above the water table. It is possible for water to migrate through both of these zones, transporting dissolved components (or contaminants). The interaction of the various forces involved will determine the direction and rate of migration. 

Calvet R (1984) Behavior of pesticides in unsaturated zone. Adsorption and transport phenomenon. In Yaron B, Dagan G, Goldschmid J (eds) Pollutants in porous media. Springer, Heidelberg, pp 143-151... 

A continuous circulation of groundwater is generated in the area surrounding the remediation well, as aquifer waters replace the annulus water. The circulation thus delivers new contaminants to the stripping zone. Volatile contaminants dissolved in the groundwater are transferred from the liquid to the gas phase and are extracted from the groundwater surface via a double-cased screen. Soil air from the unsaturated zone is also extracted and transported to the off-gas treatment system. 

Illustrative Example 22.4 Transport of a Volatile Compound From the Groundwater Through the Unsaturated Zone Into the Atmosphere— Illustrative Example 19.2 Reconsidered Concluding Remarks... 

In Illustrative Example 19.2 we discussed the flux of trichloroethene (TCE) from a contaminated aquifer through the unsaturated zone into the atmosphere. The example was based on a real case of a polluted aquifer in New Jersey (Smith et al., 1996). These authors compared the diffusive fluxes, calculated from measured TCE vapor concentration gradients, with total fluxes measured with a vertical flux chamber. They found that the measured fluxes were often several orders of magnitude larger than the fluxes calculated from Fick s first law. In these situations the vapor profiles across the unsaturated zone were not always linear. The authors attributed this to the influence of advective transport through the unsaturated zone. In order to test this hypothesis you are asked to make the following checks ... 

There are always at least two phases involved, that is, water and solids, that are of comparable importance for the fate of organic compounds in porous media. A third phase (air) becomes important in the so-called unsaturated zone recent studies show that a fourth phase, colloids, are important for the transport of chemicals in porous media. Furthermore, if extreme situations are analyzed, such as chemical dump-sites, phases like nonaqueous phase liquids (NAPLs) also have to be considered. 

Unsaturated-Zone Airflow Implications for Natural Remediation of Groundwater by Contaminant Transport through the Subsurface... 

Figure 8. Measured and modeled results for 02 volume % within the unsaturated zone, showing oxygen transport in response to barometric pressure changes. The simulated 4% vol 02 is shown as a solid band. Reprinted with permission from Elberling et al. (1998). Copyright 1998, American Geophysical Union.

Modeling results of subsurface pressure gradients were used to simulate subsurface soil gas velocity throughout the unsaturated zone profile. Figure 15 shows vertical profiles of unsaturated-zone air velocities for 12-hr time periods for August and October 1996. Results show that subsurface airflow is almost never zero, as is assumed in a diffusion-only transport model. Air-phase solute transport models based solely on diffusion would therefore not be able to accurately predict contaminant flux from the subsurface. 

Elberling, B., Larsen, F., Christensen, S., and Postma, D. (1998). Gas transport in a confined unsaturated zone during atmospheric pressure cycles. Water Resources Research, 34(11), 2855-2862. 

Wietersen, R.C., T.C. Daniel, KJ. Fermanich, B. Lowery, and K. McSweeney (1993a). Irrigation and polymer effects on herbicide transport through the unsaturated zone of a Sparta sand. J. Environ. Qual., 22 819-824. 

Current multimedia models are inadequate in many respects. Description of intermedia transport across the soil-air and unsaturated soil-saturated soil zones suffers from the absence of a suitable theory for multiphase transport through the multiphase soil matrix. These phenomena are crucial in describing pollutant migration associated with hazardous chemical waste sites. Existing unsaturated-zone soil transport models fail to include mass transfer limitations associated with adsorption and desorption and with absorption and volatilization processes. Rather, most models assume equilibrium among the soil-air, soil-solid, solid-water, and soil-contaminant phases. 

Contaminants in the soil compartment are associated with the soil, water, air, and biota phases present. Transport of the contaminant, therefore, can occur within the water and air phases by advection, diffusion, or dispersion, as previously described. In addition to these processes, chemicals dissolved in soil water are transported by wicking and percolation in the unsaturated zone.26 Chemicals can be transported in soil air by a process known as barometric pumping that is caused by sporadic changes in atmospheric pressure and soil-water displacement. Relevant physical properties of the soil matrix that are useful in modeling transport of a chemical include its hydraulic conductivity and tortuosity. The dif-fusivities of the chemicals in air and water are also used for this purpose. 

Table 16 Description of homogenous, laminar transport processes of a mass C in the saturated and unsaturated zone (without dispersion and diffusion)...

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