in laboratory systems at
7.8, were unaffected in acute tests at doses from
-1
6.5 to 77.8 mg Al
. Dissolved aluminum
remained below 0.1
Al
and the
was used by the larvae for tube building.
In chronic tests
at
6.8, Al
remained at the same low level over the same dose range.
How-
ever, mortality occurred at every dose, and pupation did not occur over the
Narf (1978) found that there were no apparent effects to
55-day study.
--
and Freeman (1973) found
benthic insects in several lake treatments.
-1
produced behavioral problems in rainbow
that a concentration of 0.52 mg Al
trout after several weeks of exposure, whereas a concentration of 0.052 mg Al
Buergel and Soltero (1983) found
produced no
or short-term effects.
hyperplasia or necrosis, or
no mortality, physiological stress,
-1
tion of rainbow trout growth after a dose of 12.2 mg Al
to hard-water
Medical Lake, Washington.
The deposition of
in the stream may pose some hazard to aquatic
organisms and may have an adverse appearance.
High flows should displace the
deposit to the reservoir, and offer the benefit of treating
rich reservoir sediments with a substance that may stop internal phosphorus
release at the deposition site.
The negative features of the use of treatment
plant-generated aluminum hydroxide sludges have already been discussed.
Summary
Prereservoir treatments are a partial substitute for watershed manage-
ment and advanced waste treatment.
The object is to detain or remove loads of
nutrients, organic matter, and silt by settling basins, marsh filtration, or
the addition of nutrient-precipitating chemicals to the stream.
With the pos-
sible exception of dry dams, these methods have not been widely employed, as
yet, but a review of case histories demonstrates their potential effective-
ness.
These methods can be costly, although reliable cost estimates are
uncommon.
Also, there are problems with land acquisition to build such
basins, with the discharge of nutrients and organic matter from marshes during
high flows, the requirement for periodic silt removal, and the potential for
creating toxic conditions through the addition of aluminum salts.
Table 2 is
a summary of this method.
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