Contaminant transport modeling - Computational River Dynamics - page 454

model has been adopted to quantify the exchange flux at the bed surface:

S BED = K BED ( C W − C BED ) (12.89)

day − 1 ), C W denotes the con-
centration of a constituent (DO, NH 3 ,PO 4 , etc.) in the water column, and C BED
is the concentration of the corresponding constituent in the sediment bed. This flux
exchange model is often used when the fate and transport of DO, nitrogen, phospho-
rus, etc., in the benthic sediment are computed. The details can be found in Wool et al.
(1995) and DiToro (2001).
Many experiments have been conducted to measure the coefficient K BED in lake sys-
tems (DiToro, 2001). Steinberger and Hondzo (1999) investigated the factors affecting
K BED of DO and established an empirical relation:

where K BED is the diffusional exchange coefficient (m ·

0.012 Uh
ν

0.89

D 0.33

K BED h
D

= (12.90)

where D is the molecular diffusivity.
In addition, the development of bed forms, such as sand ripples and dunes, will
affect the mass transfer at the bed surface. Under unsteady flow conditions, the pore
water in the sediment bed will move in and out, and thus induce additional mass
transfer.

12.2.3 Other biochemical processes

Many other biochemical processes of non-conservative constituents can be modeled
by considering the first-order decay, gravitational settling, and flux exchange at the
bed surface as

DC
Dt

− ω

S BED
h

c
h C

=− KC − (12.91)

where

c is the settling velocity of the constituent, and S BED is the flux of the
constituent entering from the water column to the sediment bed.
For example, first-order decay has been a very good assumption in many modeling
studies of coliform bacteria. The decay coefficient ranges from 0.0004 to 1.1 hr − 1
(Bowie, 1985), but most values are in the range from 0.02 to 0.1 hr − 1 and a median
rate of 0.04 hr − 1 for total coliforms (Huber, 1993). The decay of bacteria is affected
by temperature, salinity, light, etc. The effect of temperature can be accounted for
through Eq. (12.51), with

ω

being about 1.07. Mancini (1978) shows that coliform
mortality increases with the percent seawater:

θ

K sea
K fresh =

0.8 + 0.006 p sea (12.92)

where K sea and K fresh are the first-order decay coefficients in saline and fresh waters,
respectively; and p sea is the percent seawater (e.g., p sea = 100 percent in the ocean).