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 Upw Leff
Pe
(B13)
Deff
where
Upw = advective velocity (Darcy or superficial velocity) in the sediment
If the magnitude or absolute value of the Peclet number is much greater than one,
advection dominates over diffusion/dispersion, while the opposite is true for
absolute values much less than one. Advection directed out of the cap will speed
contaminant release, while advection directed into the sediment will effectively
lengthen the cap.
The average groundwater flow velocity is estimated from the sediment con-
ductivity (K, centimeters/second) or permeability (K, square centimeters) and the
local hydraulic gradient.
kg
h
h
K
Upw
(B14)
z
z
where
= density of water (~1 g/cm3)
= viscosity of water (~0.01 g cm-1 sec-1)
h
= local gradient in hydraulic head with distance into sediment
z
The minus sign recognizes that the groundwater flow is to regions of lesser
hydraulic head. The average groundwater flow is the volumetric seepage rate
(volume/time) divided by the sediment-water interfacial area. Thus, lakes with
large sediment-water interfacial areas tend to exhibit less potential for advective
influences than small streams. Estuarine systems subject to significant tidal
fluctuations may also exhibit significant advective transport. Losing streams, in
which the advective transport is into the sediment, may exhibit advection but
may not be important since the direction of transport is away from the sediment-
water interface and long travel distances may be required to impact groundwater
of significance. Similarly, advection may be less important in wetlands subject
to frequent cycles of flooding followed by infiltration due to the downward
vector of advection. The presence of a cap will tend to reduce any advective
transport by preferentially channeling flow to uncapped sediment. The perme-
ability of the cap materials may also be selected or modified to minimize
advection.
B7
Appendix B Model for Chemical Containment by a Cap
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