Water Quality Management - River Dynamics and Integrated River Management - page 568

The SOD rate is typically assumed to be uniform along a reach. Therefore, the presence and
magnitude of benthic deposits becomes another factor in reach selection. The SOD rate, S B , is typically
specified in grams per square meter per day, and to convert this into a distributed sink of DO it is divided
by the flow depth as follows:
S b = S B / D (9.23)
where the units of S b are in milligrams per liter per day. The effect on the DO deficit of this distributed
sink of DO can be computed as follows:

S

Kt D (1 e )

b

(9.24)

SOD t

K

a

where D SOD t = equals to the dissolved oxygen deficit resulting from SOD.
SOD measurements —SOD rates can be measured in the laboratory and in the field ( in-situ ). The
laboratory measurements are done by removing a sample of the river bed (preferably undisturbed) and
placing the sample in a large container with oxygenated water. DO reduction over time is a measure of
the uptake of the river bed. The in-situ measurements involve careful submersion of a sediment chamber
that is sealed to the bed, minimally disturbing the sediments, and through which a gentle flow is water is
introduced (Fig. 9.8). The DO concentration in the flowing water is measured over time to determine the
SOD rate. If done carefully, the laboratory and in-situ methods for measuring SOD rates can yield similar
results. For example, throughout the Passaic River, New Jersey, U.S. in situ measurements in 1983 ranged
from nondetectable to 2.43 g/m 2 /day and laboratory measurements ranged from 1.46 to 4.03 g/m 2 /day
both at 20 ć (New Jersey Department of Environmental Protection, 1987).

Fig. 9.8 The left photograph shows a cylindrical sediment oxygen demand (SOD) chamber being prepared for
insertion in the sediment bed, and the right photograph shows the on-shore chamber to which water is pumped from
the SOD chamber for continuous monitoring of the dissolved oxygen by the probe inserted in the top of the on-shore
chamber (photos by Cheng Liu)

Several limitations are encountered when using SOD rate measurements in modeling for waste-load
allocation. First, SOD rate measurements are point values, whereas SOD rates are applied reachwise in
water-quality models. Second, there is a tendency to measure SOD at points where it is suspected to be
high, i.e. field personnel are inclined to sample in sludge “banks” they can visibly identify. Third, SOD is
a dynamic process that varies with time. Finally, disturbance of the bed during sampling can increase
water-sediment interaction, and, thus, the measured SOD rates. All of these factors influence the reliability
of using measured SOD rates in water-quality models applied to waste-load allocation. Measured SOD
rates also can be useful in evaluating the long-term improvements in water quality in a river system. For
example, Fig. 9.9 shows the reductions in SOD rates in the Chicago Waterway System from 1976 to