Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 150

5210B * ), the oxygen consumed after 60-90 days of incubation (5210C), the continuous oxygen
uptake (5210D), and the use of ammonia for estimating nitrogenous demands are provided in
Standard Methods for the Examination of Water and Wastewater (APHA 2005).
While the amount of oxygen consumed is important, the rate at which it is consumed is also
critical for estimating the impacts of BOD on DO in rivers and streams. The rate of decrease in the
BOD concentration and the corresponding increase in the oxygen demand exerted (Figure 5.26) are
typically exponential and can be described as a irst-order process with a rate in units of days −1 . The
typical rates for untreated sewage range from 0.2 to 0.5 day −1 , while the rates for sewage following
primary treatment range from 0.1 to 0.3 day −1 (Chapra 1997). The more highly treated or resistant
the materials are to decay (refractory), the lower the rate and the greater the ratio of BOD U and
BOD 5 will be.
Consider a irst-order decay rate of 0.1 day, where after 1 day approximately 90% of the BOD
would remain, and 10% of the BOD would be exerted. Similarly, on the second day, another 10% of
the BOD remaining would be exerted. The time required for 50% of the BOD to decay (and/or be
exerted) would be about 7 days [ln(0.5)/0.1)], and the time required for 90% of the BOD to decay (or
demand exerted) would be 23 days [ln(0.1)/0.1]. Assuming a constant discharge and a representative
low velocity of 1 ft. s -1 , the distance downstream where the BOD concentrations would be half and
10% of the initial concentrations would be 113 and 376 mi., respectively. So, depending on the rate
of decomposition, the impact of a discharge can be far downstream of the source and varies with
both the rates of low of the system and the rates of deoxygenation.

5.5.4 p roductIVIty and r eSpIratIon
The productivity of aquatic plants can add oxygen over the course of a day, while respiration con-
sumes oxygen, resulting in sinusoid-like diel variations in DO concentrations. Depending on plant
densities and other factors, the diel variations can be quite large, as illustrated in Figure 5.27 for
the Cahaba River, Alabama. DO variations also occur as plant densities vary over seasons or years.
A variety of primary producers may occur in streams and rivers. In deeper and more quiescent
waters, loating or planktonic plants that are suspended in the water column and carried by currents

Dissolved oxygen (mg L -1 )

18

16

14

12

10

8

6

4

2

0

June 10 June 24 June 08 July 22 August 05

Time - June 1999-August 1999

FIGURE 5.27 Diurnal luctuation of dissolved oxygen concentrations at Piper Bridge (blue) and Shelby
County Highway 52 (green) in the Cahaba River, Alabama. (From ADEM, Final nutrient total maximum daily
loads (TMDLs) for the Cahaba River watershed, Alabama Department of Environmental Management, Water
Quality Branch, Water Division, Montgomery, AL, 2006. With permission.)

* Numbers correspond to standard laboratory procedures from APHA.