2.2.1 Allowable Contaminants in Phases Present

In general, the source-term release module assumes that a contaminant may be present in the source zone in multiple phases (i.e., in aqueous solution, sorbed to solid particles, in vapor-filled pore space, or in a separate NAPL that is immiscible with water and air).  However, not every phase is present in each type of contaminant source zone.  For example, a contaminated aquifer or contaminated pond/surface impoundment source zone has no vapor phase, a contaminated pond/surface impoundment source zone may not have a solid-sorbent phase, and any type of source zone may not have a NAPL phase.  Furthermore, the source-term release module does not allow all contaminants to partition into every phase that is present.  This is reasonable because even though, in a strict thermodynamic sense, every contaminant should have a nonzero aqueous solubility, NAPL solubility, saturated vapor pressure, and solid surface partition coefficient, there are practical limits on the phase partitioning behavior of different contaminants in real-world systems.

 The source-term release module assumes that all contaminants can partition into an aqueous, solid-sorbent, or vapor phase (if a solid-sorbent or vapor phase is present in the source zone).  However, this does not mean that all contaminants will actually be present in the solid-sorbent or vapor phase.  Some of the contaminants in the chemical property database have values of zero for the sorption coefficient, or for some other chemical property that is used to estimate the sorption coefficient.  This means that solid-sorbent terms in the phase partitioning equations will be calculated to be identically equal to zero.  In addition, most radionuclides and inorganic contaminants have negligibly small saturated vapor pressures and Henry's Law constants.  To reflect this, the chemical property database contains values of zero for these properties for some of them.  There are also organic contaminants in the database with low values of saturated vapor pressure and Henry's Law constant.  Therefore, even though a vapor-phase term is always included in the partitioning theory, it can be identically zero or negligibly small (i.e., computationally equal to zero) if the contaminant's properties cause it to be.

On the other hand, the source-term release module does not assume that all contaminants can partition into a NAPL phase (if one is present in the source zone).  A reasonably good conceptualization of the NAPL phase would be to assume that it is composed of all organic contaminants in the source zone, and that it contains none of the radionuclides or inorganic contaminants in the source zone.  However, the source-term release module (associated with RAAS Version 1.1) is not currently tied into the RAAS contaminant classification scheme.  Therefore it cannot explicitly identify whether a contaminant is an organic compound.  However, organic contaminants in the database tend to have higher values for the Henry's Law constant than radionuclides or inorganic contaminants.  So, the source-term release module currently uses a contaminant's Henry's Law constant as a quantitative metric (i.e., contaminants with modified Henry's Law constants [dimensionless form] greater than or equal to 10^-7 are assumed to partition into the NAPL phase).