Unsaturated zone models

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. 

Soil models tend to be based on first-order kinetics thus, they employ only first-order rate constants with no ability to correct these constants for environmental conditions in the simulated environment which differ from the experimental conditions. This limitation is both for reasons of expediency and due to a lack of the data required for alternative approaches. In evaluating and choosing appropriate unsaturated zone models, the type, flexibility, and suitability of methods used to specify needed parameters should be considered. 

Many models are available in the literature, and some of these models can be applied only to specific environmental situations and only for chemicals for which they were developed. Obviously, all models do not provide the same numerical results when employed to provide answers to a particular problem, so care must be taken in choosing an appropriate unsaturated zone model, or when specifying a volatilization rate. For modeling algorithms, and numerical examples the reader is referred to the work of Lyman et al. (6), Bonazountas Wagner (5) and others listed in these references. 

Kerfoot [6] examined the performance of a grab sampling technique for soil-gas measurement analyses, at a site with groundwater known to be contaminated with chloroform. The study assessed the correlation between soil-gas and groundwater analyses with chloroform as a model volatile organic compound. Chloroform concentration in soil gas increased linearly with depth in the unsaturated zone. 

Flutson, J.L. and Wagner, R.J. (1992) Leaching Estimation and Chemistry Model. A Process Based Model of Water and Solute Movement, transformation, Plant Uptake and Chemical Reactions in the Unsaturated Zone. Version 3. Dept, of Soil, Crop and Atmospheric Sciences, Series No. 92-3, Cornell University, Ithica, New York. 

Carsel, R.F., Nixon, W.B., and Ballantine, L.G. Comparison of pesticide root zone model predictions with observed concentrations for the tobacco pesticide metalaxyl in unsaturated zone soils. Environ. Toxicol Chem., 5(4) 345-353,1986. 

The above results will be useful for the two-film model of air-water exchange (Chapter 20). A very different bottleneck boundary, that is, the unsaturated zone of a soil, is discussed in Illustrative Example 19.2. 

The unsaturated zone can be modeled as a bottleneck boundary of thickness 8 = 4 m. The TCE concentration at the lower end of the boundary layer is given by the equilibrium with the aquifer and at the upper end by the atmospheric concentration of TCE, which is approximately zero. Thus, you need to calculate the nondimensional Henry coefficient of TCE at 10°C, KTCB a/w(10°C). 

Advective air and water currents are much smaller in soil systems but still influence the movement of chemicals that reside in soil. Advection of water in the saturated zone is usually solved numerically from hydrodynamic models. Advection of air and water in the unsaturated zone is complicated by the heterogeneity of these soil systems. Models are usually developed for specific soil property classes, and measurements of these soil properties are made at a specific site to determine which soil-model layers to link together. 

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. 

Wagenet, R.J. and J.L. Hutson (1989). LEACHM Leaching estimation model - a process based model for water and soute movement, transformation, plant uptake and chemical reactions in the unsaturated zone. Continuum Vol. 2. Water Resources Institute, Cornell University Ithaca, NY. 

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. 

All chemical reactions comprise at least two species. For models of transport processes in groundwater or in the unsaturated zone reactions are frequently simplified by a basic sorption or desorption concept. Hereby, only one species is considered and its increase or decrease is calculated using a Ks or Kd value. The Kd value allows a transformation into a retardation factor that is introduced as a correction term into the general mass transport equation (chapter 1.1.4.2.3). 

If the unsaturated zone is composed of relatively fine sediment (silt and fine sands) a quasi-uniform seepage flow can be assumed for the unsaturated zone in humid climate zones over long time spans. Therefore, the transport of infiltration water can be simulated in PHREEQC as a monotonous movement in accordance with the "piston flow model. A constant flow of infiltration water of 0.5 m per year is assumed for the following simulation. Furthermore, it is considered simplistically that the infiltrating precipitation has a tritium activity of 2000 TU (tritium units) over a period of 10 years. Then, it is assumed that the tritium activity decreases to zero again. 

Fig. 69 shows the modeled tritium concentrations in the unsaturated zone after 5, 10, 15, 20, 25, 30, and 35 years. Contrary to the modeling with an impulse-like tritium input (Fig. 42) the concentrations in the uppermost meters of the soil do not immediately drop back to zero because some tritium-containing water keeps infiltrating, even though with lower tritium concentrations. Thus, the tritium peaks do not show a symmetrical curve as with the impulse-like input, but a slightly left-inclined distribution. 

Padilla F., LaFrance P., Robert C., and ViUeneuve J. (1988) Modeling the transport and the fate of pesticides in the unsaturated zone considering temperature effects. Ecol. Model. 44, 73-88. 

Reible DD, Malhiet ME, lllangasekare TH. 1989. Modeling gasoline fate and transport in the unsaturated zone. J Hazard Mater 22 359-376. 

Flow is more difficult to model in the unsaturated zone because hydraulic... 

Hutson JL, Wagenet RJ. LEACHM Leaching estimation and chemistry model. A process-based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone. Version 3. Ithaca, NY Department of Soil, Crop and Atmospheric Sciences. Research series no. 92-3, Cornell University, 1992. 

---