Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 397

FIGURE 14.25 U.S. EPA criteria for ammonia.

14.5.3 d IStrIbutIon of n ItroGen
The seasonal distribution of nitrogen in lakes and reservoirs is highly variable, as described by
Wetzel (2001). However, there are some general trends often related to the lake's trophic status. For
example, in eutrophic systems, concentrations of nitrate and ammonia are often relatively low in epi-
limnetic waters, due to their uptake by plants. Below the photic zone, as oxygen is depleted, nitrate
is used as an oxidant (TEA) by heterotrophic bacteria (see Table 14.3). Ammonia is used in cell syn-
thesis. However, the rates of ammoniication, the release of ammonia from decomposing organics,
usually exceed the rate of synthesis, therefore ammonia builds up under anoxic conditions (Gordon
and Higgins 2007). The result of the uptake of ammonia in the epilimnion and ammoniication in the
anoxic hypolimnion often leads to extremes in the vertical distribution of ammonia concentrations.
Seasonal variations in the longitudinal and vertical distribution of nitrate-N and ammonia-N con-
centrations are illustrated in Figures 14.27 and 14.28 using model predictions for Allatoona Reservoir
and Walter F. George Reservoir, Georgia, for July 31, 1995. The concentrations of both nutrients for
Allatoona Reservoir are low in surface waters. For Allatoona Reservoir, there are relatively strong
predicted longitudinal as well as vertical variations in ammonia concentrations, even though the oxy-
gen concentrations for this date are relatively uniform (Figure 14.21). For Walter F. George Reservoir,
a high-low event had a large impact on the nitrogen concentrations on this date (see also Figures 14.18
and 14.19). These predictions illustrate the complexity in making generalizations regarding concen-
tration distributions for nitrogen and the importance of the physical transport on those distributions.
The seasonal variation in the distribution of ammonia for Lanier Reservoir is illustrated in Figure
14.29a and b. For this reservoir and for this year, ammonia concentrations followed but lagged the
trends in the development of the anoxic hypolimnion (Figure 14.17a and b). As the season pro-
gressed, ammonia concentrations increased in anoxic waters in the upper reaches of the reservoir,
gradually extending down into the reservoir. The greatest concentrations were eventually restricted
to the deepest portion of the reservoir during the fall deepening of the epilimnion. The reservoir
completely mixes usually in January.