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 be expected while higher nitrate concentrations would be expected during oxic periods. Distribution of
total phosphorus concentrations during the summer growing season indicate maximum values occur in
late summer, coincident with anoxia maxima in the upstream impoundment (Figure 1.3.19). In general,
during thermal stratification and increased concentrations in the releases, ammonia nitrogen
concentrations decrease with distance from dam and nitrate concentrations increase with distance from
dam (Figure 1.3.20). Observations of concentrations at a low and high flow indicate that ammonia
concentrations remaining higher for a greater distance during high flow before decreasing. Nitrate
concentrations increase with distance and the rate of increase is greater during the low flow. Possible
mechanisms for these trends include biological activity and physicochemical processes that vary with
flow.
0.8
0.4
NO3H
NH3H
NO3L
0.6
0.3
NH3L
0.4
0.2
0.2
0.1
0.0
0.0
0
2
4
6
8
10
0
2
4
6
8
10
Distance (km)
Distance (km)
Figure 1.3.20 Spatial trends in ammonia and nitrate concentrations in reservoir
tailwaters during high (H) and low (L) flow
(Nimrod Lake, AR).
Particulate material, provided to the river from runoff and resuspension, impacts river water
quality by providing sites for the adsorption and desorption of chemical constituents. Particulates may
also be relatively inert and concentrations of suspended material will change as a function of grain size
and flow. Major particulate substances include clay minerals, weathered minerals, and organic matter
(carbon) such as leaves and twigs (pieces to logs). In general, particulate matter concentrations
increase with discharge to the impoundment. The increases can be substantial, e.g. 572 and 1440 times
above minimum discharge for particulate organic matter have been reported (Perry and Perry 1991).
However, the impoundment buffers the transport of particulate material and suspended solids in the
tailwater are typically lower and less variable than upstream values (Figure 1.3.21).
In most rivers and streams (of which tailwaters are a subset) the turbulent mixing ensures a
relatively uniform distribution of dissolved substances, although discontinuities may exist especially at
confluences, near-bank boundaries, and in deep rivers. Turbulence usually results in an equilibrium
between dissolved gases and the atmosphere. Exceptions to this occur in areas of high productivity and
reservoir tailwaters where supersaturation occurs. Major dissolved substances follow the "bicarbonate
1.3-23
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