How to Manage Pests
Most treatment wetlands must be built near the wastewater source because conveyance of wastewater and storm water over long distances is expensive and impractical. Several excellent publications listed in the bibliography highlight the factors to be considered when siting a wetland and describe the engineering principles that should be considered in the design of a wetland for water quality improvement.
Surrounding land uses and the potential for mosquitoes to move from a wetland into residential and commercial zones must be considered when siting a wetland. A conflict will be created over time if suburban sprawl encroaches on treatment wetlands in rural areas. Also, the area circumscribed by a wetland underestimates the potential region affected by mosquitoes because adult mosquitoes effectively disperse up to several miles from their developmental sites.
Buffer zones between human developments and adjacent mosquito habitation sites have been recommended by public health officials outside the United States to extend 1 to 1-1/4 miles (1.5 to 2 km), but larger buffer zones of 3 miles (5 km) or more may be needed in situations where resident mosquito species disperse readily. Strong prevailing winds can move swarms of biting adult mosquitoes up to 10 miles (16 km). Active mosquito abatement is generally carried out within a 1-mile radius of a human residential area if active sites of mosquito production are nearby. There is currently no established criterion in California for determining the size of a buffer zone around a treatment wetland. Typical distances reached by 90 percent of the mosquitoes emerging from a freshwater treatment wetland might range from 1/2 to 3 miles (1 to 5 km), but a buffer zone of this size may not be sufficient to avoid legal abatement.
Poor water quality tends to increase the production of mosquitoes. High levels of organic matter and nutrients, particularly reduced forms of nitrogen such as ammonia, are thought to provide nutrients for the bacteria and algae used as food by mosquito larvae. The decomposition of organic matter and conversion of ammonium to other forms of nitrogen in the nitrogen cycle require considerable amounts of oxygen, which can lead to low dissolved oxygen concentration and can create unsuitable conditions for aquatic mosquito predators such as predatory insects and fish.
Wastewater may require pretreatment before discharge into a treatment wetland, and the level of pretreatment is an important consideration in the size of a treatment wetland. Studies to date indicate that discharge of raw or primary-treated municipal wastewaters into a vegetated lagoon or shallow vegetated wetland can result in mosquito larval abundance from several hundred to over 1,000 larvae per 400-milliliter dip sample.
Pretreatment to secondary standards may limit average densities to fewer than 200 mosquito larvae per sample, but these levels far exceed acceptable mosquito abundance, particularly when humans live nearby. Where threshold values for intervention against mosquitoes are in place for seasonally flooded and treatment wetlands, they range from average densities as low as 0.2 to 0.5 mosquito larvae Culex and other species) per dip sample to 5 mosquito larvae per dip sample. Although pretreatment before discharge into a treatment wetland may reduce mosquito production, it does not guarantee against mosquito presence.
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