Subsurface Gaseous Phase

Groundwater may contain dissolved gases as a result of exposure to the surface environment prior water infiltration, contact with the subsurface gaseous phase, and gas produced biologically below the water table. The most important dissolved gas in groundwater is CO. ... 

Reversible and irreversible retention of contaminants on the subsurface solid phase is a major process in determining pollutant concentrations and controlling their redistribution from the land surface to groundwater. After being retained in the solid, contaminants may be released into the subsurface liquid phase, displaced as water-immiscible liquids, or transported into the subsurface gaseous phase or from the near surface into the atmosphere. The form and the rate of release are governed by the properties of both contaminant and solid phase, as well as by the subsurface environmental conditions. We consider here contaminants adsorbed on the solid phase. 

Many geochemical processes occur in which a fluid remains in contact with a gaseous phase. The gas, which could be the Earth s atmosphere or a subsurface gas reservoir, acts to buffer the system s chemistry. By dissolving gas species from the buffer or exsolving gas into it, the fluid will, if reaction proceeds slowly enough, maintain equilibrium with the buffer. 

The aqueous solution here refers to free water in the subsurface having a composition affected by the interaction between the incoming water and the solid and gaseous phases. This composition is achieved under a dynamic equilibrium with natural processes and may be disturbed by anthropogenic activities. The chemical composition of the snbsnrface aqneous solution at a given time is the end product of all the reactions to which the liqnid water has been exposed. 

Adsorption is the net accumulation of matter on the sohd phase at the interface with an aqueous solution or gaseous phase. In this process, the solid surface is the adsorbent and the matter that accumulates is the adsorbate. Adsorption also may be defined as the excess concentration of a chemical at the subsurface solid interface compared to that in the bulk solution, or the gaseous phase, regardless of the nature of the interface region or the interaction between the adsorbate and the sohd surface that causes the excess. Surface adsorption is due to interactions between electrical charges, or nonionized functional groups, on mineral and organic constituents. 

Adsorption removes a compound from the bulk phase and thus affects its behavior in the subsurface environment. Due to some hysteresis effects, sometimes reflected in formation of bound residues, the release of compounds from the solid phase to the liquid or gaseous phase does not always reach the amount of adsorbate retained on solid surfaces. 

Contaminants may reach the subsurface in a gaseous phase, dissolved in water, as an immiscible hquid, or as suspended particles. Contaminant partitioning in the subsurface is controlled by the physicochemical properties and the porosity of the earth materials, the composition of the subsurface water, as well as the properties of the contaminants themselves. While the physicochemical and mineralogical characteristics of the subsurface sohd phase define the retention capacity of contaminants, the porosity and aggregation stams determine the potential volume of liquid and air that are accessible for contaminant redistribution among the subsurface phases. Enviromnental factors, such as temperature and water content in the subsurface prior to contamination, also affect the pollution pattern. 

One way that contaminants are retained in the subsurface is in the form of a dissolved fraction in the subsurface aqueous solution. As described in Chapter 1, the subsurface aqueous phase includes retained water, near the solid surface, and free water. If the retained water has an apparently static character, the subsurface free water is in a continuous feedback system with any incoming source of water. The amount and composition of incoming water are controlled by natural or human-induced factors. Contaminants may reach the subsurface liquid phase directly from a polluted gaseous phase, from point and nonpoint contamination sources on the land surface, from already polluted groundwater, or from the release of toxic compounds adsorbed on suspended particles. Moreover, disposal of an aqueous liquid that contains an amount of contaminant greater than its solubility in water may lead to the formation of a type of emulsion containing very small droplets. Under such conditions, one must deal with apparent solubility, which is greater than handbook contaminant solubility values. 

Water evaporation and contaminant volatilization from subsurface aqueous solutions are two companion processes that affect contaminant partitioning between the liquid and gaseous phases. Temperature-induced evaporation may affect the concentration of the natural constituents of the subsurface water and thus affect contaminant dissolution in this water. 

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. 

After reaching the subsurface, contaminants are partitioned among the solid, liquid, and gaseous phases. A fraction of the contaminated gaseous phase is transported into the atmosphere, while the remaining part may be adsorbed on the subsurface solid phase or dissolved into the subsurface water. Contaminants dissolved in the subsurface aqueous phase or retained on the subsurface solid phase are subjected, over the course of time, to chemical, biochemical, and surface-induced degradation, which also lead to formation of metabolites. 

Contaminants in soil can partition between the soil and air, soil and water, and soil and solids however, the physical structure and chemical composition of surface and subsurface soil are highly variable. Compared to aqueous systems, soil is a complex heterogeneous media composed of solid, liquid, and gaseous phases. The four major components of soil are the inorganic (mineral) fraction, organic matter, water, and air. Soil consists of 50% pore space, which is occupied by air and water 45% minerals and 5% organic matter. Figure 2.5 illustrates a typical soil structure that is important to consider for remediation. 

For k>kf the adspecies mass transfer process is described by the diffusion Eq. (63). If the species migration in the subsurface region and the exchange with the gaseous phase occur fast, then k — l, therefore the boundary condition comprises the 3rd kind condition. Otherwise, it would be necessary to take into account the temporal evolution of the species in subsurface layers k , and the kinetic equations for these layers can contain the time derivatives. Most works devoted to mass transfer problems and also to the surface segregation of the alloy components [155,173]. The boundary conditions in the non-ideal systems are discussed in Ref. [174]. They require the use of equations for the pair functions of the type d(6,Jkq)/dx — 0. When describing the interphase boundary motion, the 3rd kind boundary conditions are also possible, although the 1st and the 2nd kind conditions are used more often. The latter are mainly applied to the description of many problems with species redistribution in the closed volume [175],... 

Under natural conditions, contaminants often reach the earth s surface as a mixture of (potentially) toxic chemicals, having a range of physicochemical properties that affect their partitioning among the gaseous, liquid, and solid phases. As a consequence, contaminant retention properties in the subsurface are highly diverse. Contaminants may reach the subsurface from the air, water, or land surface. 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

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