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 L_x in the horizontal direction parallel to the average groundwater flow, width L_y in the horizontal direction perpendicular to the average groundwater flow, and height L_z 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

L_x  is the dimension of the contaminated aquifer source zone parallel to flow (cm)
L_y  is the horizontal dimension of the contaminated aquifer source zone perpendicular to flow (cm)
L_z  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)
 ß_s  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.