6.2 Weed Harvesting
A common situation in eutrophic lakes is to have a shallow (<2 m deep) shoreline
filled with both submerged and emergent growths. These are considered unsightly,
interfere with boating, make swimming undesirable, and make fishing nearly impossible. At the same time they provide a breeding ground for fish. Weed harvesting has
been used under the guise of reducing the nutrient inputs to a lake. However, it has
been estimated that they represent only in the order of 1% of the phosphorus content
of the lake. They are usually harvested by a special boat that may not be able to reach
the shallowest portion or certain bays in a lake. Here weeds may be removed by rake
or hand pulling while wading in the shallow water. Also, the weeds harvested must
be removed from the shore, or the nutrients will return to the lake as the weeds
decompose. Harvesting the extensive Eurasian watermilfoil in Lake Wingra, WI,
resulted in the reduction of only a small fraction of the lake’s metabolically active
nutrient pool [21]. This is a relatively expensive treatment for the amount of nutrient
reduction accomplished. It does remove the unsightly and undesirable weeds.
Related to weed harvesting is the use of herbicides to kill the weeds. This must be
applied before the weeds reach full growth and may have to be repeated during the
growing season. Any dead weeds should be removed. The use of herbicides may
have other undesirable environmental impacts, and they are not recommended if the
water is used for drinking.
6.3 Dredging
The principle of dredging is to remove the organic sediments on the bottom of the
lake that add to the nutrient supply when the hypolimnion becomes anaerobic
[22, 23]. This is an expensive technology and is impractical for deep lakes. It also
destroys the natural bottom of the lake. It is somewhat practical in artificial lakes or
reservoirs where the water level can be drawn down (usually during the winter), and
surface equipment such as bulldozers can be used for the dredging. Any dredged
material must be handled in an environmentally safe way. If any hazardous contaminants are shown to be present, this could be costly. Starting in mid-August of 2012,
the US Environmental Protection Agency (US EPA) has targeted up to US$57
million in Great Lakes Restoration Initiative funds for two projects in the Sheboygan
River, focusing on dredging contaminated sediment from the Great Lakes’ river
area. [42]. Dissolved air flotation (DAF) is one of the best processes for treatment of
the dredged materials [43–45]. Appendixes A, B, C, and D document the pollutant
contents of the dredge materials from Ashtabula and Fairport of Ohio, USA. The
pollutants in the dredged materials contain high concentrations of total phosphorus,
nitrogen, oil and grease, chemical oxygen demand (COD), toxic heavy metals, and
toxic volatile solids. Appendix E is a US EPA control technology summary for
7 Lake Restoration
271
A common situation in eutrophic lakes is to have a shallow (<2 m deep) shoreline
filled with both submerged and emergent growths. These are considered unsightly,
interfere with boating, make swimming undesirable, and make fishing nearly impossible. At the same time they provide a breeding ground for fish. Weed harvesting has
been used under the guise of reducing the nutrient inputs to a lake. However, it has
been estimated that they represent only in the order of 1% of the phosphorus content
of the lake. They are usually harvested by a special boat that may not be able to reach
the shallowest portion or certain bays in a lake. Here weeds may be removed by rake
or hand pulling while wading in the shallow water. Also, the weeds harvested must
be removed from the shore, or the nutrients will return to the lake as the weeds
decompose. Harvesting the extensive Eurasian watermilfoil in Lake Wingra, WI,
resulted in the reduction of only a small fraction of the lake’s metabolically active
nutrient pool [21]. This is a relatively expensive treatment for the amount of nutrient
reduction accomplished. It does remove the unsightly and undesirable weeds.
Related to weed harvesting is the use of herbicides to kill the weeds. This must be
applied before the weeds reach full growth and may have to be repeated during the
growing season. Any dead weeds should be removed. The use of herbicides may
have other undesirable environmental impacts, and they are not recommended if the
water is used for drinking.
6.3 Dredging
The principle of dredging is to remove the organic sediments on the bottom of the
lake that add to the nutrient supply when the hypolimnion becomes anaerobic
[22, 23]. This is an expensive technology and is impractical for deep lakes. It also
destroys the natural bottom of the lake. It is somewhat practical in artificial lakes or
reservoirs where the water level can be drawn down (usually during the winter), and
surface equipment such as bulldozers can be used for the dredging. Any dredged
material must be handled in an environmentally safe way. If any hazardous contaminants are shown to be present, this could be costly. Starting in mid-August of 2012,
the US Environmental Protection Agency (US EPA) has targeted up to US$57
million in Great Lakes Restoration Initiative funds for two projects in the Sheboygan
River, focusing on dredging contaminated sediment from the Great Lakes’ river
area. [42]. Dissolved air flotation (DAF) is one of the best processes for treatment of
the dredged materials [43–45]. Appendixes A, B, C, and D document the pollutant
contents of the dredge materials from Ashtabula and Fairport of Ohio, USA. The
pollutants in the dredged materials contain high concentrations of total phosphorus,
nitrogen, oil and grease, chemical oxygen demand (COD), toxic heavy metals, and
toxic volatile solids. Appendix E is a US EPA control technology summary for
7 Lake Restoration
271