Unsaturated zone differences

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. 

There are some important differences between the behavior and flow of water in the unsaturated zone (vadose zone) above the water table and the saturated zone below the water table. The surface tension of water or other fluids becomes important when there is a gas phase in contact with the fluid phase and the solid phase. If the total volume of a porous medium (VT) is divided into the volume of the solid portion (Vs,),... 

The unsaturated zone, where most of the low-level waste is buried, could probably be used for high-level waste after cooling but it does not lend itself to the regional approach to defining permeability. Additionally, the laboratory methods of evaluating permeability of unsaturated material are extremely difficult to perform consistently. However, an air permeability technique that may prove useful in evaluating the zone between the surface and the water table was developed by Weeks W. The technique uses normal barometric fluctuations measured at different depths in the formation to determine permeability to air which can be used to determine permeability to water. 

Simple Bottleneck Boundaries A Simple Noninterface Bottleneck Boundary Illustrative Example 19.1 Vertical Exchange of Water in a Lake Two- and Multilayer Bottleneck Boundaries Bottleneck Boundary Between Different Media Illustrative Example 19.2 Diffusion of a Volatile Compound from the Groundwater Through the Unsaturated Zone into the Atmosphere... 

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. 

A study was designed to define the relative rates of dealkylation of selected triazine herbicides and two monodealkylated triazine degradation products in the unsaturated zone and in surface runoff. Atrazine and propazine degrade to DEA by deethylation and deisopropylation, respectively. Similarly, atrazine and simazine can both dealkylate to DIA by removal of an isopropyl and ethyl side chain, respectively (Figure 30.12). Differences in the concentration of the dealkylated degradation product from the two different sources should indicate any preferential removal of ethyl versus isopropyl side chain. Furthermore, because monodealkylated DEA and DIA have different side chains remaining, their relative rate of removal should provide additional information on the liability of the ethyl side chain versus an isopropyl side chain. 

The advantage of using fugacity to calculate the equilibrium distribution coefficients becomes apparent when one compares the fugacity capacities of a HOP for several different phases. For example, consider a region of the unsaturated zone just below the ground surface where naphthalene is distributed between air, water, pure phase octanol, and soil at equilibrium. The fugacity capacities for these phases are repeated below in Eqs. (46)-(49) ... 

In the unsaturated zone, water movement is caused by both gravity and by pore water pressure differences arising from variations in the water content from one location to another water may even move vertically upward through the soil profile if evaporation or plant roots remove it from the near-surface soil. Water flow is impeded, however, by the fact that water can only move via the relatively thin film of water coating the particles. Such flow contrasts with water flow in the saturated zone, where water can move through the entire pore volume and occupy the full cross-sectional area of the pore spaces. 

SESOIL Unsaturated zone SESOIL is a one-dimensional, finite difference flow and transport model. The model estimates the rate of vertical solute transport and transformation from the land surface to the water table. 

VLEACH Unsaturated zone VLEACH is a one-dimensional, finite difference model developed to simulate the transport of contaminants displaying linear partitioning behavior through the vadose zone to the water table by aqeuous advection and diffusion. 

Figure 8.1 defines the different types of subsurface-waters. The zone above the water table is called the zone of aeration, vadose zone, or unsaturated zone. Moving downward in this zone we encounter first soil water or soil moisture, then intermediate vadose water, and then capillary water. Capillary water rises into rock and sediment pores above the water table because of capillary forces. The capillary zone may be as much as 30 ft (9 m) thick in clayey sediments. To the extent the unsaturated zone can truly be called the zone of aeration, then air and atmospheric oxygen are present and conditions are oxidizing. 

TABL 13.9 Assumed solubility-limiting solids and concentrations of radioelements in iow-Eh reference ground-waters, in geological formation types proposed for repository development by different repository programs (McKinley and Savage 1994), and in oxidized (high-Eh> unsaturated zone waters similar to those in the proposed repository horizon at Yucca Mountain, NV... 

The first accounted only for C02 retention caused by dilution to dissolved inorganic carbon. The second accounted for additional dilution to an adsorbed C phase predicted from C02 -loss experiments. The geochemical models were separately coupled with a two-dimensional, finite-difference model for gas diffusion to simulate the distribution of P C02 in the unsaturated zone near a disposal trench at a low-level radioactive waste-disposal site near Sheffield, Illinois. Comparison of simulated P C02 distribution with onsite data supported the presence of the adsorbed C phase. 

The simulations were performed using a finite-difference model for solution of the diffusion equation in porous media (. For modeling purposes, the cross section (Figure 1) was divided into a 3-layered, 36 by 53 block-centered grid. The layers represented the Toulon Member, the Radnor Till Member, and eolian silts (Roxana Silt and Peoria Loess). Physical properties of the unsaturated zone that were used for modeling were identical to those listed in Table I. 

Local hydro-geological conditions were considered for each zone where the radionuclides transport occurs, namely for the contaminated zone, for the unsaturated zone and for the saturated zone, using different densities, porosities, hydraulic conductivities, radionuclide distribution coefficients and thicknesses. The well is located at the down-gradient edge of the contamination source. 

Comparable in many respects to the H- He method, the tracer pair H- Kr removes many of the difficulties inherent to the use of alone. However, in contrast to the H- He method, a knowledge of the H-input function is always needed, because Kr is not linked to the H-decay. Another difference between He and Kr lies in their transport behavior through the unsaturated zone. Weise et al. (1992a) showed that while the fast diffusing He was at the atmospheric level in the soil air down to 25 m depth, the Kr activity in the soil gas decreased significantly at depths larger than about 10 m. A time lag between the atmospheric input function and the concentrations in the soil air at the water table in deep unsaturated zones affects not only Kr but also the dating methods based on CFCs (Cook and Solomon 1995) and SFe (Zoellmann et al. 2001). 

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