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The bottom boundary may be specified as fixed outflow to represent a base aquifer, where the concentration varies with time as mass is transported into the aquifer from the landfill and transported out from beneath the landfill by the base velocity vb.

 

Consideration of the conservation of mass gives the base concentration as:

 

c(τ, z = Hb) = [ f(τ, z=Hb, c)/nbhb - vb c (τ, z=Hb)/nb L ] dτ

 

where,

c(τ, z=Hb) = the concentration in the base aquifer, averaged over the entire thickness of the base,

f(τ, z=Hb, c) = the mass flux into the aquifer,

nb = porosity of the base aquifer,

hb = thickness of the base aquifer,

vb = Darcy velocity in the aquifer and down-gradient edge of the landfill,

L = length of the landfill parallel to the velocity vb.

 

Note that the use of a very large base velocity will give the same results at using a constant base concentration of zero. If the base velocity is zero and the porosity is zero the bottom boundary is effectively a zero flux boundary.