DILUTION AND FLUSHING
Reservoirs and lakes with high concentrations of nutrients may have
Algal blooms, particularly when blue-green
severe blooms of nuisance algae.
algae are involved, interfere with the recreational use of the water and may
dramatically increase potable water treatment costs.
Algal excretory and
decomposition products, along with other sources of dissolved organic matter,
are associated with dissolved oxygen depletions, taste and odor, and with the
appearance of trihalomethanes and other organohalides following chlorination
Dilution is a procedure in which water of low nutrient content is added
for the purpose of lowering the reservoir's concentration of nutrients to a
level at which algal cell growth is limited. Cell washout increases as well.
Flushing, on the other hand, emphasizes cell washout through a sharp increase
in the water exchange rate.
The inflowing water may not necessarily have a
lower nutrient concentration.
The procedures become equivalent when low
nutrient water is added at a rate sufficient to achieve cell washout equal to
algal cell growth rate.
This normally requires a large volume of scarce
In practice, the procedures are differentiated because one
(flushing) emphasizes what goes out of the reservoir without consideration of
nutrient concentration changes and associated changes in cell growth, and the
other (dilution) emphasizes a limitation on algal growth through a decrease in
nutrient concentration as well as through cell washout (Welch 1981). Dilution
can also improve water quality by decreasing the concentration of algal excre-
These procedures can be particularly effec-
tory and decomposition products.
tive for some reservoirs when treatment of upstream nutrient sources is not
Both techniques, but particularly dilution, are limited in their
applicability by the difficulty of finding an additional water source that can
be diverted to the reservoir.
The reader is referred to Uttormark and
and Cooke et al. (1986) for reviews of these