Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 148

FIGURE 5.24 Reaeration in low over a weir. (Photograph by J.L. Martin.)

A variety of equations have been developed to predict reaeration from currents, winds, and over
structures, and the reader is referred to Thomann and Mueller (1987), Chapra (1997), and Duan
et al. (2009) for detailed explanations. Reaeration is frequently measured in the ield, and ield mea-
surements have formed the basis for many of the commonly used reaeration equations.
Some of the methods previously used to measure reaeration are no longer an option. For example,
Owens et al. (1964) and others deoxygenated water by adding sodium sulite in order to estimate
its reaeration. Tsivoglou (1967), Tsivoglou and Wallace (1972), and Wilhelms (1980) measured gas
transfer at the water surface using injections of radioactive krypton-85 to mimic oxygen exchange
while using tritium as a tracer to quantify the effects of mixing and dilution.
The method of using a continuous injection of a conservative tracer coupled with a nonconserva-
tive tracer to mimic oxygen is still used today for measuring reaeration; however, more commonly,
rhodamine WT dye is used as the conservative tracer while nonradioactive krypton gas is used to
mimic oxygen (Kilpatrick et al. 1989; USEPA SESD-EAB 2007). Since the noble gas krypton is inert
and is already a component of air, it does not pose any ecological threat. This method also requires
estimates of transport, thereby producing estimates of low, time of travel, mean reach velocity, and
depth. Other similar techniques have used chlorine (Cl − ) or bromide (Br − ) as a conservative tracer
and propane or sulfur hexaluoride (SF 6 ) as a nonconservative tracer (Fellows et al. 2001; Grace
and Walsh unpub., cited in Grace and Imberger 2006; LINX 2004). Other methods commonly used
involve open water methods such as those based on diel variations in oxygen concentration. Grace
and Imberger (2006) describe a variety of methods to estimate reaeration. Another method used for
measuring reaeration is the loating diffusion dome method (Copeland and Duffer 1963; Hall 1970),
illustrated in Figure 5.25, which Koenig and Murphy (2001) reported to be fast and inexpensive, but
not as accurate as the tracer techniques.

5.5.3 b IocHeMIcaL o xyGen d eMandS
The decomposition of organic materials discharged into streams and rivers results in the subsequent
decrease in DO concentrations, or deoxygenation, as a result of aerobic decomposition. The goal
of wasteload allocations and permitting of discharges is to determine an allowable concentration
of organic materials that could be discharged while still resulting in DO concentrations that meet
standards and criteria. So, initially, a relationship between concentrations of organic material and
consequent reductions in oxygen is needed. This could be accomplished with stoichiometry using a
balanced chemical reaction and if the composition of the organic matter were known (e.g., as glu-
cose, C 6 H 12 O 6 ). However, in the United States, a more common approach is to measure the oxygen