McKay, Donald. "Front matter" Multimedia Environmental Models ...

carbon figure for deeper sediments compared to high 17% for the depositing material
implies that about 75% of the organic carbon is degraded.
It is now possible to assemble an approximate mass balance for the sediment
mineral matter (MM) and organic matter (OM) and thus the organic carbon (OC).
This is given in Table 4.1.
Table 4.1 Illustrative Sediment–Water Mass Balance on a 1 m 2 Area Basis
Mineral matter Organic matter Total
Organic
carbon
cm3 g cm3g cm3g g
Deposition 500 1200 500 500 1000 1700 250
Resuspension 200 480 200 200 400 680 100
OM conversion – – 233 233 233 233 117
Burial (solids) 300 720 67 67 367 787 33
Total burial is 1000 cm 3 /year or 1420 g/year, corresponding to a “velocity” of 1 mm/year.
The sediment thus has a density of 1.42 g/cm 3 or 1420 kg/m 3 .
Assumed densities are: mineral matter 2.4 g/cm 3 , organic matter 1 g/cm 3 .
Organic matter is 50% (mass) organic carbon.
On a 1 m2
basis, the deposition rate is 0.001 m3
per year or 1000 cm3
per year.
With a particle density of 1.7 g/cm3,
this corresponds to 1700 g/year of which 500 g
is OM, and 1200 g is MM. We assume that 40% of this is resuspended, i.e., 200 g
of OM and 480 g of MM. Of the remaining 300 g OM, we assume that 233 g is
digested or degraded to CO2,
and 67 g is buried along with the remaining 720 g of
MM. Total burial is thus 1420 g, which consists of 720 g of MM, 67 g of OM, and
633 g of water. The total volumetric burial rate of solids is 367 cm3/year.
Now,
associated with these solids is 633 cm3
of pore water; thus, the total volumetric
burial rate of solids plus water is approximately 1000 cm3/year,
corresponding to a
rise in the sediment-water interface of 1 mm/year. The mass percentage of OC in
the depositing and resuspending material is 15%, while in the buried material it is
4.2%. The bulk sediment density, including pore water, is 1420 kg/m3.
On a 7 ¥ 105
m2
basis, the deposition rate is 700 m3/year,
resuspension is
280 m3/year,
burial is 257 m3/year,
and degradation accounts for the remaining
163 m3/year.
The organic and mineral matter balances are thus fairly complicated,
but it is important to define them, because they control the fate of many hydrophobic
chemicals.
It is noteworthy that the burial rate of 1 mm/year coupled with the sediment
depth of 3 cm indicates that, on average, it will take 30 years for sediment solids
to become buried. During this time, they may continue to release sorbed chemical
back to the water column. This is the crux of the “in-place contaminated sediments”
problem, which is unfortunately very common, especially in the Great Lakes Basin.
In the simple four-compartment environment, we treat only the solids but, in the
eight-compartment version, we include the sediment pore water. In the interests of
simplicity, we assign a density of 1500 kg/m3
to the sediment in the four-compartment
model.
©2001 CRC Press LLC