The theory previously presented for the contaminated aquifer (Section 3.0) and pond/surface impoundment (Section 4.0) source zones is relatively straightforward because no synergistic or antagonistic interactions between loss processes occur, and because the source zone dimensions remain constant.  On the other hand, the theory presented here for the contaminated vadose zone source zone is relatively complex.  Firstly, auxiliary theory implemented in the source-term release module to calculate selected parameters used in the vadose zone mass loss equations is relatively extensive.  Because of this, this section begins with separate subsections giving detailed descriptions of theory related to water balance calculations, wind emissions calculations, and overland sediment loss calculations.  Secondly, synergistic and antagonistic interactions between contaminant loss processes do occur in the general vadose zone source zone scenario.  Therefore, for clarity, this section first presents derivations of mass loss expressions for individual processes (assuming they are acting alone), and then presents derivations of the mass loss expressions when interactions can occur.  Thirdly, three of the possible loss processes may cause the dimensions of the source zone to change.  This time variation of source zone dimensions complicates the mass loss theory, and requires a more involved numerical solution procedure.  Fourthly, this section also discusses two types of remediation methodologies for contaminated vadose zones for which special theory has been implemented.  And finally, the nature of volatilization in a vadose zone is more complex in and of itself.

There are many different possible types of vadose zone source zones; and the volatilization from different types of sources may be controlled by different mechanisms or combinations of mechanisms.  Furthermore, for certain specific types of contamination sources, the mechanisms controlling contaminant volatilization may change over time (as the source zone changes over time) and may be different for different contaminants or contaminant concentrations.  Therefore, a complete description of each type of contaminant source may encompass one or more completely different types of volatilization model.

Each type of volatilization scenario may be described by unique volatilization theory (i.e., each may have a different equation that is appropriate for estimating the mass loss from the source zone as a function of time due to volatilization).  For this reason, previous versions of MEPAS required the user to select one of a number of possible volatilization scenarios for a contaminated vadose zone.  MEPAS then computed the emission rate based on idealized volatilization theory that was appropriate to that particular volatilization scenario, under the assumption that volatilization was the only loss process occurring.  Each of these theories implicitly assumed that the contaminants were present either entirely in a NAPL phase, or that a NAPL phase never existed.  In addition, the fundamental conceptualization of the source zone for some of these volatilization models was different than that for models of other loss processes.  This was not a problem as long as loss routes were analyzed independently; but it is a problem for a source-term release module where mass-balanced, coupled loss theory is required.  Currently, only one hybrid-type of volatilization scenario has been implemented in the source-term release module for a vadose zone source zone.  The basis of this scenario was the so-called "Land Treatment Facilities" case of the previous configuration of MEPAS (see Droppo and Buck 1996).  However, the theory for this scenario has been greatly enhanced to accommodate source zones with or without a NAPL phase (or that contain a NAPL phase initially that disappears during the course of the simulation).  The theory is commensurate with multiple interactive loss process and complex NAPL phase partitioning.

Consider that the contaminated source zone is located somewhere in the vadose zone.  The source-term release module conceptualizes this zone to be a rectangular parallelopiped with horizontal cross-sectional area A.  There may be a layer of uncontaminated soil above the source zone initially.  The position of the soil surface (i.e., the upper boundary of the vadose zone) may recede over time because of wind suspension or water erosion of particles.  The bottom of the source zone always remains at the same location.  However, the top of the source zone may recede (to lower elevations) over time because of the aforementioned wind suspension or water erosion processes, or because of volatilization.  Therefore, the thickness of the source zone, h, and its volume, V, may vary over time.

The source zone contains an aqueous phase (the vadose zone pore water), a vapor phase (the air-filled pore space), a solid-sorbent phase (the soil matrix), and may contain a NAPL phase (if the masses of any of the contaminants that may partition into a NAPL phase are above the saturation limits of the other two phases).  The source zone is assumed to be a so-called "well-mixed reactor," which means that its properties are assumed to be spatially uniform throughout, for all mass loss theory except for the case of an ISS waste form.  (However, there is one vadose zone scenario where a simplified spatial-gradient expression for volatilization is used as a bounding value when the primary theory would predict unphysically high loss rates.)

Section 2.2.2 describes how the source-term release module tests to determine if a NAPL phase exists (based on the current masses of contaminants in the source zone) at the beginning of each time step.  Equation 2.1 in Section 2.2.2 is the test criterion in general form.  The test criterion that the module actually uses for the contaminated vadose zone source zone is obtained by expressing the source zone volume (that appears in Equation 2.1) in terms of other specific source zone parameters.  This expression may be a time-varying function, and may be given by different formulae, depending on the interaction of the loss processes.  Because this theory is developed only later in this section, the specific test criterion for NAPL phase existence will be reported at the end of Section 5.5.

Because all contaminant masses and loss fluxes are updated within a time step (before the numerical solution algorithm proceeds to the next time step) the module also has a complete record of contaminant mass produced in a time step based on chain decay of some parent species.  This new mass is included in the total mass for that contaminant for the next time step for all scenarios except for the scenario where the vadose zone has been subjected to an ISS remediation methodology.  For this case, the special theory needed requires that the accumulation and subsequent loss of daughter products of concern be handled in a different manner (discussed explicitly below).