Water Quality Management - River Dynamics and Integrated River Management - page 585

which is required in relatively large amounts and unless all three are available, growth will be limited.
Plants acquire carbon from carbon dioxide in the atmosphere, and, thus, nitrogen and phosphorus typically
are the limiting nutrients—with nitrogen typically limiting plant growth in seawater and phosphorus
typically limiting plant growth in freshwater lakes and rivers (Welch, 1980). Nutrients are considered to
be pollutants when their concentrations are sufficient to allow excessive growth of aquatic plants,
particularly algae (Masters, 1991, p. 109).
The negative effects of phosphorus are primarily related to excessive and/or nuisance growth of aquatic
plants. However, there are many negative effects associated with nitrogen compounds as follows:
(1) Conversion of ammonium to nitrate consumes large quantities of dissolved oxygen,
(2) Ammonia and nitrate serve as nutrients for excessive growth of algae,
(3) Nitrite and nitrate in the water have been linked to the disease methemoglobinemia,
(4) Ammonia in its unionized form is toxic to fish, and
(5) Chlorine and hypochlorus acid/hypochlorite can react with any ammonia present in the water forming
chloramines, which are more toxic than either chlorine of hypochlorus acid/hypochlorite (Davis and
Masten, 2004, p. 298).
The first effect has been discussed in a simplified way as nitrogenous biochemical oxygen demand in
Section 9.3.3. The second through fourth effects are described in the following subsections of this chapter.
The fifth effect is discussed relative to disinfection performance. This section closes with a discussion of
methods to reduce nutrient loads in rural and urban non-point source/diffuse pollution.
9.3.1 Eutrophication
Eutrophication is a natural process that occurs in all lakes because of the gradual accumulation of silt and
organic matter in the lake. As the accumulating silt and organic debris causes the lake to get shallower and
warmer, more plants take root along the shallower edges, and the lake slowly transforms into a marsh or
bog (Masters, 1991, p. 134). Cultural eutrophication is a process whereby water bodies, such as lakes,
estuaries, or slow-moving rivers receive excess nutrients due to man's activities that stimulate excessive
plant growth including algae, periphyton attached algae, and nuisance plants and weeds. The death of
this excessive growth of plants and the settling of the dead organic matter greatly accelerates the natural
eutrophication process. Throughout the rest of this discussion the term eutrophication refers to cultural
eutrophication, which is a problem to be dealt with by Integrated River Management. Eutrophication is
mainly associated with lakes, but it frequently occurs in impounded and slow moving rivers as shown in
Fig. 9.17. A detailed discussion of general eutrophication problems and eutrophication management,
focusing on examples for estuaries, is given in Section 8.4.
The enhanced plant growth often is referred to as an algal bloom. These blooms eventually die and
decompose, and their decomposition removes oxygen from the water, potentially leading to concentrations
of DO that are insufficient to sustain normal life forms. If anaerobic conditions result, hydrogen sulfide is
formed in the bed sediments and metals such as iron and manganese, which are normally tied up as
precipitates in sediments, are dissolved and released into the water column thorough a process called
anaerobiosis. Algae and decaying organic matter add color, turbidity, odors, and objectionable tastes to
water that are difficult to remove and that may greatly reduce its acceptability as a domestic water source
(Masters, 1991, p. 109). Further, marginal amounts of chlorine, enough for disinfection but not enough
for oxidation, may intensify algal odors and tastes in much the same way as they intensify phenolic tastes,
whereas high concentrations of chlorine will destroy both the organisms and their odorous oils and cell
matter (Fair et al., 1971, p. 670).
The decay of the excess organic matter is not the only water quality problem resulting from eutrophication.
Clark et al. (1977, p. 276) note that oligotrophic (low in nutrients and primary productivity) surface