3.0 Contaminated Aquifer
Consider that the contaminated source zone is located below the water
table in a groundwater aquifer. The source-term release module conceptualizes
this zone to be a rectangular parallelopiped with volume V that has length
Lx in the horizontal direction parallel to the average groundwater
flow, width Ly in the horizontal direction perpendicular to
the average groundwater flow, and height Lz in the vertical
direction.
The source zone contains an aqueous phase (the groundwater), a
solid-sorbent phase (the porous aquifer 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).
Because the source zone is below the water table there is no vapor phase.
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.
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 volume
of the contaminated aquifer source zone is given by
where
Lx is the dimension of the contaminated
aquifer source zone parallel to flow (cm)
Ly is the horizontal dimension of the contaminated
aquifer source zone perpendicular to flow (cm)
Lz is the vertical dimension of the contaminated
aquifer source zone perpendicular to flow (cm).
Substituting Equation 3.1 into Equation 2.1 the test criterion can be rewritten
as
Note that because the source zone is in an aquifer (where the volumetric
air content is zero, the volumetric water content is equal to the total
porosity, and the bulk density of the solid sorbent is more appropriately
defined as the soil bulk density), Equation 3.2 is equivalent to
where
q t is the total
porosity of the soil (unitless)
ßs is the soil bulk density for an aquifer
or vadose zone (g cm-3).
Equation 3.3 is the test criterion that the module actually uses for contaminated
aquifer simulations.
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.