Unsaturated zone movement

Water Movement in Unsaturated Zone of Soil Formation. 701... 

The movement of free product is dependent on soil permeability and moisture. The released gasoline first infiltrates downward vertically, mainly governed by the gravity force, into and through the unsaturated zone, then reaches the water table. If there is an impermeable layer above the water table, the free product will be purged and may not reach the water table directly. In the... 

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. 

In the upper unsaturated zone (above the capillary fringe), multiphase movement and transformation are typical. Vapor-phase gasoline becomes more important gasoline adsorption by soil, dissolution in pore water, and free product in the pore space can also be significant. 

Smith, D. B., Wearn, P. L., Richards, H. J., Rowe, P. C., Water movement in the unsaturated zone of high and low permeability strata by measuring natural lithium, In Isotope Hydrology 1970, Vienna, Interna. Atomic Energy Assoc., p. 73-87, 1970. 

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. 

Water movement in the unsaturated zone is dependent on the factors discussed previously — driving force, hydraulic conductivity, viscosity, and density — but is... 

Jones, R.E., Hansen, J.L., Romine, R.R., and Marquardt, T.E. Unsaturated zone studies of the degradation and movement of aldicarb and aldoxycarb residues, Environ. Toxicol Chem., 5 361-372, 1986. 

DBCP has been found In ground water In Hawaii, California, Arizona, South Carolina, and Maryland (5-7,19,77-79). Typical positives are 0.02-20 ppb. Areas with the highest frequency of positives and the highest well concentrations are the San Joaquin Valley In California and the region southwest of Phoenix, Arizona. The Hawaii contamination has occurred despite several hundred feet of overburden between the basal aquifer and the surface. One set of California soil core results show that ppb amounts of DBCP has leached about 15 m through the unsaturated zone ( ), whereas DBCP was not detected In another set of California soil cores sampled as deep as 10 m and five years after the last application (80). The latter results can possibly be explained by rapid movement of DBCP down the soil profile to depths greater than 10 m. 

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. 

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. 

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. 

Cook P. G., Jolly ID., Leaney F. W., Walker G. R., Allan G. L., Fifield L. K., and Allison G. B. (1994) Unsaturated zone tritium and chlorine-36 profiles from southern Austraha their use as tracers of soil water movement. Water Resour. Res. 30, 1709-1719. 

Gvirtzman H. and Magaritz M. (1986) Investigation of water movement in the unsaturated zone under an irrigated area using environmental tritium. Water Resour. Res. 22, 635-642. 

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. 

The previous sections have described the movement of groundwater in both the saturated and the unsaturated zones. The equations presented are sufficient to predict the transport of conservative chemicals that neither decay with time nor sorb onto particles. 

Smart, P.L. Friederich, H. (1986) Water movement and storage in the unsaturated zone of a maturely karstified aquifer, Mendip Hills, England. Proceedings of the Conference on Environmental Problems in Karst Terrains and their Solutions, 28-30 October, Bowling Green, Kentucky, National Water Wells Association, pp. 57-87. 

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. 

PRZM-2 Unsaturated zone/groundwater PRZM-2 is a combination of two models developed to simulate the one-dimensional movement of pesticides. It has been used predominantly for evaluation of pesticides in the root zone. 

CHEMFLO Unsaturated zone CHEMFLO is a one-dimensional flow and transport model designed to simulate the movement of water and chemicals into and through soils. 

Unsaturated soil zone below root depth (vadose zone) Movement Upward A Lateral Downward Transport % Degradation (chemical/biological) Soil Retention Transport. 

The driving force for movement of aldicarb residues is the movement of water. Therefore, the movement of water must be defined before the movement of aldicarb residues can be quantified. Factors influencing the movement of water include rainfall, irrigation, evapotranspiration and soil field capacity in the unsaturated zone. The amount of organic matter determines the rate of aldicarb movement relative to water movement in both the unsaturated and saturated zones. 

Unsaturated Zone Models. Various models exist for estimating the movement and degradation of pesticide residues in the unsaturated zone (20). Perhaps the best model available is PRZM developed by the U.S. EPA (21). The applicability of PRZM to the modeling of aldicarb residues has been demonstrated using data from Long Island (22), Florida (20), Wisconsin (23.23.24), and North Carolina (24). 

Saturated Zone Models. Results from unsaturated zone simulations can be used as inputs to saturated zone models to predict concentrations of aldicarb residues in groundwater. The saturated zone model used by the author takes the pesticide inputs into groundwater, as predicted by PRZM, and calculates the concentration and movement of aldicarb residues in the upper portion of the saturated zone. The core of the saturated zone model is a finite element solute transport calculation procedure developed at the University of Wisconsin (25). The accuracy of this model in estimating pesticide movement in groundwater is (as with other... 

The movement of aldicarb residues is quite complex, depending on a number of interacting factors. In most aldicarb use areas, residues degrade completely before moving through the unsaturated zone and into the saturated zone. In the few areas Where aldicarb residues have entered the saturated zone, residues are usually located in shallow groundwater near treated fields. 

Jones, R. L., J. L. Hansen, R. R. Romine, T. E. Marquardt, "Unsaturated Zone Studies on the Degradation and Movement of Aldicarb and Aldoxycarb Residues." Submitted for publication, 1985. 

Principles of Modeling Pesticide Movement in the Unsaturated Zone... 

Sorption. The interaction of a pesticide with the soil solids or organic matter, termed sorption, retards the pesticide movement through the unsaturated zone. This interaction is being increasingly associated with the soil organic matter (18), with the extent of the interaction dependent upon the type of organic materials and the molecular characteristics of the pesticide (19). Linear and Freundlich isotherms have been used to describe sorption, where... 

Green, R.E. Liu, C.C.K. Tamraker, N. "Modeling Pesticide Movement in the Unsaturated Zone in Hawaii Soils Under Agricultural Use", 189th American Chemical Society National Meeting, PEST 54, Florida, 1985. 

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