Infiltration into the Subsurface

Viscosity Effect The infiltration and redistribution of two hydrocarbons in moist silt loam and loamy sand soils, with viscosities of 4.7 (soltrol) and 77 (mineral oil) times greater than that of water, was reported by Cary et al. (1989). The spatial distribution of the two hydrocarbons and water is presented Fig. 12.12 the infiltration rate of 

The kerosene conductivity of the soil was affected strongly by the soil texture and initial moisture content. In sand, the kerosene conductivity was affected slightly by the initial moisture content, as high as 70% field capacity, but decreased thereafter. The kerosene conductivity of the loam soil was similar in oven-dried and air-dried soils and increased significantly in soils at 70% and full field capacity. No kerosene flow was observed in the clay soils from oven-dried to field-capacity moisture contents. 

These behaviors can be explained as follows. First, the decrease in kerosene conductivity in sand is caused by a mechanically induced change in the original 

Retention in Porous Media As NAPL migrates through the subsurface, some of it becomes entrapped according to its retention capacity. In addition, NAPL constituents may become redistributed in the gaseous phase. 

The extent of kerosene trapping was determined quantitatively in a series of laboratory and outdoor experiments with Swedish soils (Jarsjo et al. 1994), yielding an empirical equation for the kerosene residual content as a function of soil composition  

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Arsenic in precipitation from unpolluted ocean air averages about 0.019 pg L 1 (Hering and Kneebone, 2002), 157 and terrestrial rainwater concentrations (at least over the USA) also have similar averages of around 0.013-0.032 pgL-1 ((Smedley and Kinniburgh, 2002), 522 Table 3.17). As the precipitation infiltrates into the subsurface, its chemistry changes as it reacts with sediments, soils, and rocks. Therefore, the arsenic chemistry of the groundwater of an area may be very different than its precipitation chemistry. 

Subsurface runoff. When precipitation hits the land surface, the vast majority does not go directly into the network of streams and rivers in fact, it may be cycled several times before ever reaching a river and the ocean. Instead, most precipitation that is not intercepted by the vegetation canopy and re-evaporated infiltrates into the soil, where it may reside as soil moisture, percolate down to ground-water, or be transpired by plants. 

The landfill liner, cover, and hydraulic barrier all belong to the subsurface pollutant engineered containment system. The liner is designed at the bottom of a landfill to contain downward leachate. The cover is designed at the top of a landfill to prevent precipitation from infiltrating into the landfill. The hydraulic barrier, or cutoff walls, is a vertical compacted earthen system to contain horizontal flow of plume. The ultimate purpose of these barriers is to isolate contaminants from the environment and, therefore, to protect the soil and groundwater from pollution originating in the landfill or polluted site. 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

In the previous sections of this book, we focused on the nature of contaminants and the geochemical reactions that can occur in the subsurface environment. Chemical compounds introduced into infiltrating water or in contact with soil or rock surfaces are subject to chemically and biologically induced transformations. Other compounds are retained by the soil constituents as sorbed or bound residues. Thus, in terms of geochemical interactions and reactions among dissolved chemical species, interphase transfer occurs in the form of dissolution, precipitation, volatilization, and various forms of physicochemical retention on the solid surfaces. 

In this chapter, we examine the various mechanisms that influence chemical redistribution in the subsurface and the means to quantify these mechanisms. The same basic principles can be applied to both saturated and partially saturated porous media in the latter case, the volumetric water content (and, if relevant, volatilization of NAPL constiments into the air phase) must be taken into account. Also, such treatments must assume that the partially saturated zone is subject to an equilibrium (steady-state) flow pattern otherwise, for example, under periods of heavy infiltration, the volumetric water content is both highly space and time dependent. When dealing with contaminant transport associated with unstable water infiltration processes, other quantification methods (e.g., using network... 

Saturated zones are common in porous material, and provide significant pathways for the subsurface transport of water and solutes. The term "ground water" commonly refers to continuously saturated zones of appreciable thickness. Saturated conditions also occur on a small scale or short term basis in association with the infiltration and drainage of precipitation or surface runoff. As indicated previously, the pressure and elevation components are the primary contributors to the total moisture potential in the saturated zone. These two are commonly combined into a "piezometric head", representing the addition of the water pressure head to the elevation at which the pressure head is measured. The hydraulic conductivity does not change significantly with... 

Natural drainage is defined as the infiltration of water from the surface into the fill mass (subsurface drainage) in combination with (uncontrolled) surface runoff (surface drainage) (see Figure 10.1). If the intensity of the precipitation (and/or overtopping) exceeds the infiltration capacity of the fill mass, ponding and, subsequently, natural surface runoff will occur. 

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