LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 275

more acidic waters, excessive input of aluminum salts
can decrease the lake pH and result in concentrations
of dissolved aluminum in the water column that are
toxic to fish and other biota. For this reason, some
experts recommend against the application of alum in
lakes (e.g., Jørgensen et al., 2005).

eutrophic lakes may cause adverse effects in down-
stream receiving waters.

Oxygenation. Increasing the oxygen content in the
lower portions of the lake reduces the release of phos-
phorus from the benthic sediments. However, although
phosphorus is released from benthic sediments at con-
siderably higher rates under anaerobic conditions, con-
siderable amounts of phosphates and substances
stimulating algal growth are still released from the sedi-
ment under aerobic conditions. Phosphorus release
from benthic sediments typically occurs when the
oxygen concentration in the overlying water becomes
less than 5 mg/L, and anaerobic conditions are typically
associated with oxygen concentrations below 2 mg/L.
The reason why phosphorus release from benthic sedi-
ments is greater under anaerobic conditions than under
aerobic conditions is that oxygenated forms of iron and
manganese in natural waters form an insoluble precipi-
tate with phosphorus, thereby limiting the amount of
dissolved phosphorus under aerobic conditions.

7.6.1.4 Limitation of Internal Loading. Sediments
on the lake bottom are the primary source of internal
phosphorus loading. Conventional measures that can be
taken to control internal loading from benthic sedi-
ments, include covering, dredging, removal of hypolim-
netic water, and artificial destratification.

Covering. Sheeting is sometimes used to cover the
bottom sediment, but the sheeting typically has to be
weighted to prevent it from floating. Sand and gravel
cannot be used to keep the sheeting in place because
they provide a good substrate for macrophyte seedlings
after 1-2 years. The sheeting should be permeable to
gasses to prevent the sheeting from ballooning due to
the release of gases from the sediment. Alternatives
to covering the bottom of the lake with sheeting include
covering the bottom with foil, clay, crushed bricks, or
other inert materials (Jørgensen et al., 2005).

Artificial Destratification. Artificial destratification of a
stratified lake can change the composition of the algal
population, reduce the number of algae and the algal
growth rate, and aerate the lake to compensate for the
oxygen deficit resulting from metabolic activity. With
artificial circulation of the lake, the oxygen content
of the water is increased at all depths. Specifically, at
the sediment-water interface, aerobic conditions are
created, suppressing the release of nutrients from the
sediment.
In general, over the long term, in-lake treatments will
only be effective if accompanied by efforts to reduce
external nutrient loads. In-lake treatments typically
require significant expenditures for equipment, chemi-
cals, and labor.

Dredging. Dredging can only be used to remove the
sediment from small and shallow lakes. Major consider-
ations are the treatment and deposition of the material
removed, and the need to pay particular attention to the
amount of toxic metals present. The practice of remov-
ing sediment from lakes and from shallow areas is more
effective than covering it with synthetic sheeting,
because the nutrients are actually removed from the
lake. It does, however, involve more technical problems
and higher costs, and it is feasible only if a location is
available for the deposition of the sediment removed.
The primary advantage of dredging is its relatively long-
lasting effect. However, dredging operations can cause
extensive damage to the benthic community, which may
be an important food source for fish and may disturb
fish spawning habits if not carefully designed and
implemented.

7.6.1.5 Limitation of Algal Development. Control
measures used to limit algal development in a lake
without changing the phosphorus budget include artifi-
cial mixing, manipulation of the pelagic food web (bio-
manipulation), reduction of the residence time of water
in the lake, and the use of nontoxic natural products to
control algal growth. Commonly used substances to
control algae (algaecides) include copper sulfate penta-
hydrate and other chelated copper compounds. Potas-
sium permanganate has also been found to be effective
for algal control in some cases. Copper sulfate applica-
tion methods and dosages will vary depending on the
lake or reservoir conditions, and caution is required
because copper sulfate addition can have a detrimental
effect on fish and other aquatic life. For this reason,
some experts recommend against the use of copper

Removal of Hypolimnetic Water. This approach removes
water that is within the hypolimnion of a lake, and this
water is usually low in oxygen content and rich in nutri-
ents. Withdrawing the hypolimnetic nutrient-rich waters
selectively using a siphon or deepwater outlet in a dam
can decrease the quantity of nutrients stored and recy-
cled in the water body. However, hypolimnetic with-
drawals may also trigger thermal instability and lake
turnover. In addition, the discharge of these nutrient-
rich, often anaerobic waters from the hypolimnion of