Vadose zone content

A U.S. EPA study (41) showed that soil vapor extraction (SVE) is an effective treatment for removing volatile contaminants from the vadose zone. Sandy soils are more effectively treated than clay or soils with higher organic content because higher air flows are possible in sand and clays—organic soils tend to adsorb or retain more contaminants. Removal of volatiles is rapid in the initial phase of treatment and thereafter decreases rapidly thereafter-an important consideration in the design of air emissions control over the life of the project. 

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

Vaughan PJ, Suarez DL. Constant capacitance model computation of boron speciation for varying soil water content. Vadose Zone J. 2003 2 253-258. 

Moisture Content/Field Capacity Within the vadose zone, moisture content is important since microbial growth is limited by excessively wet or dry soil. Moisture content, expressed as a percentage of the field (or holding) capacity, indicates the ratio of moisture to air in the soil. The recommended range for optimal growth is between 40 and 70%. 

Given these two main factors (i.e., oxygen availability and organic matter content), several generalizations can be made about solid phase surfaces, the vadose zone, and the groundwater region as follows ... 

Soils, typically, are not fully saturated by water the soil layer and the region reaching to the water table contain water contents below full saturation. These regions usually are referred to as the vadose zone and said to be unsaturated, but they are more correctly considered partially saturated. The degree of saturation is the ratio of the volume of water to the pore volume within the porous medium. Saturation levels usually are a few percent at land surface (or even zero in perpemaUy dry arid zones) and increase slowly with depth until the region of the capillary fringe (water table), where it increases rapidly to 100%. 

Holden, P.A. Hersman, L.E. Firestone, M.K. Water content mediated microaerophilic toluene biodegradation in arid vadose zone materials. Microb. Ecol. 2001, 42, 256-266. 

The duration of the remediation is dependent on the soil type, water content, and the nature of the contaminants. The HRUBOUT process cannot remove metals from soils. Polychlorinated biphenyls (PCBs) cannot be totally removed. The in situ HRUBOUT process is designed for removing contaminants from the vadose zone, (i.e., the zone between the surface aud the water table). Low permeability lowers system effectiveness and raises remediation costs. Soils with variable permeabilities may cause uneven delivery of air to contaminants. VOC removal rates may be reduced by high organic content in the soil because soil orgauics have a high VOC-sorption capacity. 

Beneath the soil profile, the vadose zone may consist of unconsolidated or consolidated materials in this text, only unconsolidated materials are considered. The movement of water in the vadose zone is more difficult to describe than flow in the saturated zone, because water transport occurs only via water-filled pores and the fraction of the pore spaces filled with water (the percent saturation) is highly variable. The water content 6 refers to the fraction of the bulk soil volume that contains water 6 can range between zero... 

Analysis of vadose zone transport is more complex than analysis of saturated zone transport, in part because changes in soil water content have a strong inbuence on both hydraulic conductivity and pore pressure. How does a decrease in soil water content affect ... 

Aeolianites that become submerged below the groundwater table may become cemented in the phreatic environment. In this zone, pore spaces between sand grains are completely filled with water and any cements derived from the interstitial waters are often isopachous in nature (Muller, 1971). Large solution volumes and longer residence times in the phreatic zone can result in coarser spar compared with the vadose zone. Increases in temperature, degree of supersaturation and NaCl content appear to result in larger cement crystal sizes (Badiozamani et al., 1977). 

WS = volumetric water content in vadose zone soils (cm3/cm3)... 

It has also been possible to identify two types of vadose seepage. The first, which may be termed low-Ca vadose seepage, is derived from low-Ca soils and its degree of calcite saturation and Ca content are controlled by the available soil Ca and soil Pco, Such seepage does not appear to dissolve significant amounts of Ca as it transits the vadose zone, probably because it is closed to further additions of CO2 and may thus arrive at the aquifer substantially undersaturated if its co, remains at the high levels acquired in the soil zone. Further, some of this low-Ca seepage (such as site 4) will remain undersaturated even if ventilated to the Pco, of a cave atmosphere (Fig. 2). 

Pore waters in oxic zones are characterized by a dissolved oxygen content greater than 0.5 ml/1. Oxic carbonates prevail in (i) subaerial environments, such as the vadose zone where the pores are periodically filled with gas, air and/or water (ii) immediately below the sediment-water interface in aquatic environments and (iii) in the phreatic zone below the water table where all the pores are regularly filled with water. The thickness of the oxic zone depends on the penetration, by diffusion or advection, of oxygen below the sediment surface. Oxygen diffusion into pore waters is largely controlled by the organic content and the rate of deposition. In marine and lacustrine sediments the... 

Table IV. Texture, Alkalinity and Moisture Contents of Soil Sanples (Oct. 27-28, 1981) from the Vadose Zone In Potato Fields and Aldlcarb Residue Concentrations (ppb) In the Soil and Soli Solution <"H20" In Last (blumn).

Soil samples collected from the vadose zone also show a variety of aldicarb residue concentration profiles (Table IV). In view of the low Kd value for aldicarb in sandy soils (22) it can be assumed that any aldicarb residues detected in soil samples are entirely dissolved in the soil moisture. If soil residue values are recalculated on a soil-moisture basis, the resulting peak concentrations are closer to those observed in the groundwater or leachate from the lysimeters. However, the pattern of residue concentrations in the vadose zone is also highly erratic and bears no apparent relation to soil texture, moisture content or alkalinity (Table IV). Equally erratic concentration distributions were observed for samples collected on other dates in the same year. Similarly erratic results were obtained for different soil profiles from Long Island potato fields (22). 

The migration of gas becomes an important consideration in certain geoenvironmental engineering applications when the gas content in a porous medium is sufficiently high such that the gas phase is continuous. Examples of such apphcations include the removal of VOCs and SVOCs from the imsaturated or vadose zone above the water table in the subsurface via the gas phase and the minimization of oxygen influx or radon efflux from engineered covers for tailings disposal applications. In the former case, removal efficiency is improved as gas permeability increases, whereas in the latter case, the objective is to minimize the gas permeability and therefore minimize the gas flow. 

The unsaturated (vadose) zone is contained in Hanford Formation sediments. This zone is approximately 9 m (30 ft) thick. These sediments typically are open-framework pebble- to boulder-sized gravels. Interstitial sand content is generally low and mud-sized sediment is limited to coatings on individual grains and rip-up clasts. Interstratified lenses of sand and mud may be encountered, but they are very localized. Drilling of one well at the 100-N Area near an active liquid waste disposal facility in 1984 led to the only known occurrence of a perched water table within the 100 Areas. 

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