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Page Title:
Maximum flux estimation (steady state) |
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 Deff
(B17)
(C0 Cw)
Kcap (C0 Cw)
F
Leff
where
F = chemical flux, ng cm-2 sec-1
Deff = effective binary diffusivity of chemical in cap, cm2/sec
= sediment porosity (void volume/total volume)
Leff = effective cap thickness
C0 = pore water concentration in sediment beneath cap including dissolved
and sorbed to colloidal species, ng/cm3
Cw = total contaminant concentration in overlying water, ng/cm3
Kcap = effective mass transfer coefficient through cap, cm/sec
The effective diffusion coefficient is generally estimated by the equation of
Millington and Quirk (1961)
4/3
Dw
Deff
(B18)
where
Dw = molecular diffusivity of compound in water
= void fraction or porosity of sediment
Millington and Quirk suggest the factor 4/3 to correct for the reduced area and
tortuous path of diffusion in porous media.
In general, the chemical flux is influenced by bioturbation and a variety of
water column processes. Figure B1 shows the definitions of fluxes in a capped
system at this pseudo steady state. The flux of chemical through each layer is
equal to the sum of the rate of evaporation and flushing. Mathematically, in
terms of mass transfer coefficients, one has:
Kcap As(C0
Cbio)
Kbio As (Cbio
Csw)
M
Kov As C0
(B19)
Q) Cw
Kbl As (Csw
Cw)
(Ke Ae
where
M = rate of chemical loss from system, mg/day = F*As
Kov = overall mass transfer coefficient, cm/day
B10
Appendix B Model for Chemical Containment by a Cap
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