Siltation basins and wetlands
A critical problem with the use of either of these systems is obtaining
the area needed to construct them.
If a portion of the upper reservoir is
modified to form a smaller reservoir, then significant loss of storage capac-
Both the marsh and the siltation basin may require substantial
ity may occur.
maintenance in the forms of harvesting of plants and the removal of
lated silt through dredging.
Wetlands will discharge nutrients during high-flow, low-vegetation peri-
More significantly for potable water reservoirs, they also can discharge
ods.
dissolved organic molecules, which may impart taste and odor, increase the
chloride demand, and perhaps contribute to trihalomethane production.
Lee,
Bentley, and Amundsen (1975) suggest that marsh outflows could be treated with
Willenbring (1985) notes that channelized
a low dose of aluminum sulfate.
flow in a wetland will reduce its removal capacity.
Prereservoir phosphorus removal
An area of significant concern is the potential for adverse effects on
The interested reader is referred to
stream biota from aluminum salts.
Part IV as well as to Burrows
Kennedy and Cooke
and Cooke
et al. (1986) for a discussion of the chemistry of aluminum and the environ-
Briefly, when
mental conditions under which it can be deleterious
biota.
aluminum sulfate is added to natural waters containing bicarbonate-carbonate
alkalinity, a visible precipitate of aluminum hydroxide is formed, and
falls.
The
is sorptive of phosphorus and organic matter, and some mate-
The forms of aluminum that appear in the
rials are trapped with the
Insoluble
predominates between
6 to 8,
water are
and lower
while soluble species predominate at higher
and Al
are considered to be potentially toxic, and
and Al
therefore
must not fall below
6.0.
-Very few studies of the toxicity of aluminum to aquatic biota have been
Collectively, these studies suggest that concentrations of Al
conducted.
-1
are not toxic to
below 0.050 mg Al
gairdneri (rainbow trout). Biesinger and
(Insecta, Chirononidae) and
magna was 3.90 mg
Christensen (1972) found that the 48-hr
for
-1
and a lo-percent reproductive impairment occurred at 0.32 mg Al
when animals were reared in Lake Superior water (alkalinity 50 mg
reared
7.74).
Lamb and Bailey (1981) report that
of T.
40
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