3.1.2  Leaching

 The leaching process is assumed to occur by advective transport of the contaminant out of the downgradient face of the source zone.  This means that the rate of loss of mass at any given time is given by the volumetric flux of water out of the source zone face multiplied by the concentration of the contaminant in the water at that time:^(a)



where    Q_w is the volumetric flux of water flowing through the aquifer source zone or vadose zone source zone, or out of the bottom of a pond/surface impoundment source zone (cm³ yr^-1).

The volumetric water flux, in turn, can be expressed in terms of the Darcy water flux density (i.e., Darcy velocity) and the area of the downgradient face of the source zone:



where    q_w is the Darcy flux density of water flowing through the aquifer or vadose zone (cm yr^-1).

By substituting Equation 3.7 into Equation 3.6, the mass flux equation for loss from the source zone by leaching can be expressed as



Again, this equation applies if leaching is the only loss process, and also if decay/degradation is occurring simultaneously.

 When a NAPL phase exists in the source zone, the aqueous concentration of the contaminant is controlled by the composition of the NAPL phase.  In this case, the source-term release module calculates the value of C_wi in Equation 3.8 by the phase partitioning theory described in Section 2.2.4 (which uses different methods, depending on the mole fraction [or concentration] of the contaminant in the NAPL phase).

When no NAPL phase exists in the source zone, the aqueous concentration can be calculated by a simple phase partitioning relation (i.e., Equation 2.7 in Section 2.2.3).  Substituting Equation 3.1 into Equation 2.7, and explicitly noting that the volumetric water content is equal to the total porosity for a contaminated aquifer source zone, the aqueous concentration can be given by



Now, substituting Equation 3.9 into Equation 3.8, the mass flux equation for loss from the source zone by leaching (when no NAPL is present) can be expressed as



Recall that the retardation factor, R_i,  that appears in Equation 3.10 was defined in general terms by Equation 2.5.  It is worthwhile to note that for the contaminated aquifer source zone, the volumetric water content and solid-sorbent bulk density in Equation 2.5 are equal to the total porosity,  q_t, and soil bulk density, ß_s, respectively.

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(a)

Note that, in a strict sense, the variable Qw should really be referred to as the aqueous solution flux (which contains water and dissolved constituents). However, because aqueous solutions are typically volumetrically dilute with respect to their dissolved constituents, it is quite common to refer to the aqueous solution flux as the water flux.