supply intakes and provide precipitates and oxide coatings on streambed substrates that may impact the
biota. Elevated concentrations of reduced metals may be toxic to the biota as well. Hydrogen sulfide is
indicative of anoxia in the upstream reservoir and is primarily problematic as the degassing of dissolved
sulfide as hydrogen sulfide. This gas is readily derived upon oxidation, quite noxious, and potentially
toxic and corrosive at low concentrations.
Metals are often in elevated concentrations in release water especially from hypolimnetic
releases. Iron, manganese, and sulfur (considered here with metals) are the constituents of most
concern due to their increased mobility associated with oxygen depletion. Iron is readily oxidized upon
release (exerting a chemical demand on dissolved oxygen). Oxidized iron forms a particulate (and may
complex other metals and organics) and is transported or deposited in the downstream area as a
function of physical processes described earlier. Recent investigations indicate that biological processes
and substrate interactions may play an important role in removing iron from the release water.
Manganese oxidation (more probably biological removal) is much slower and often function of
substrate availability. Hydrogen sulfide is gaseous and is rapidly removed to the atmosphere (hence the
rotten egg or sulfur smell in the immediate tailwater).
Examples of metals dynamics in reservoir releases are depicted in Figure 1.3.16 and Figure
1.3.17. Concentrations of total iron and manganese increase coincident with thermal stratification with
maximum concentrations occurring in late summer and lower discharge. The temporal pattern for total
manganese is less variable annually than the pattern for total iron. Figure 1.3.17 depicts dissolved and
particulate (calculated as the difference between measured total and dissolved fractions) concentrations
of iron and manganese. In general, iron is most often observed in the particulate form and manganese is
most often observed as dissolved. This is typical for most reservoir releases and indicative of the higher
oxidation reduction potential for iron. The higher oxidation reduction potential for iron allows for a
higher oxidation rate and increased formation of particulates which may contribute to the variability in
tailwater concentrations. The spatial dynamics of iron and manganese in reservoir tailwaters are a
function of flow (Figure 1.3.18) and vary by site (Nix et al. 1991). In general, higher concentrations
are observed further downstream during higher flow and are then observed to decrease with distance
(triangles, Figure 1.3.18). Conversely, during lower flow, concentrations tend to decrease quickly for
iron and almost linearly or as a first order process for manganese (circles, Figure 1.3.18). As observed
for seasonal patterns, the response of total and dissolved iron compared to total and dissolved
manganese is quite different with manganese remaining primarily in the dissolved fraction and iron
occurring in the particulate (difference between total and dissolved) fraction.
Although reservoirs act as sediment and nutrient traps (sinks), mobilization of sedimentary
nutrients during oxygen depletion can result in increased nutrient concentrations in release water.
1.3-19
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