Since construction of the Sweetwater Wetlands began in 1996, the City of Tucson has significantly reduced the mosquito population. The site includes 17.5 acres of constructed wetlands, 14 acres of recharge basins, and a recurring mosquito problem. The Wetlands was initially constructed to treat filter backwash water from the City's Reclaimed Water Treatment Plant.
The treatment plant, built in 1984, produces reclaimed water by filtering secondary effluent through granular silica/carbon filters. The filters plug up periodically and plant operators must backwash the filters to clean them. The wetlands is designed to receive and treat the backwash water produced during this process. The backwash water, high in suspended solids, drains by gravity through settling basins and wetland ponds, and into adjacent recharge basins.
The wetland ponds contain areas of deep, open water alternating with zones of shallow water planted with bulrush and cattail. Over the years, the shallow vegetated zones have been prone to produce mosquitoes which are potential carriers of encephalitis virus.To avoid a potential public health risk, the City has a mitigation program to control the mosquito population.
Tucson Water staff monitor mosquito populations weekly at the Sweetwater Wetlands by setting carbon dioxide traps. University of Arizona Entomology Department staff analyze the trap populations and provide species identifications and total numbers. The City used to have granular mosquito larvicide applied weekly to the water using a remote-controlled helicopter. In 2003, Water staff switched to a ground-based, hydroseed sprayer for the application of mosquito larvicide. The larvicide is mixed into a water-based slurry so it can be sprayed from a high-pressure water cannon over the dense aquatic vegetation. Although this method of application improves the amount of larvicide contacting the water along the pond edges, the hydroseed sprayer is unable to reach the central areas of the bulrush and cattail.
In 2004, staff contracted for the use of a Kawasaki ARGO to distribute additional larvicide out into the central areas of emergent vegetation. The ARGO is a "tracked Aquatic Watercraft" capable of entering the heavy vegetation and traversing across it. A mechanical spreader distributes the granular larvicide from the back of the vehicle. The combined use of the hydroseeder and the ARGO results in more complete distribution of larvicide across the thick vegetation The larvicide is specific to mosquito larvae (designed to dissolve at the pH of the mosquitoes' mid-gut) and is therefore non-toxic to the many beneficial aquatic insects at the Wetlands.
During periods of higher trap counts, the City contracts to have the Wetlands treated using a truck-mounted aerial fogger that produces an Ultra Low Volume (ULV) mist. The product applied is a synthetic pyrethroid (Anvil 2+2) with a very low toxicity that has been approved by the EPA for use in aquatic environments. The mist droplets are micron-sized and the ULV application rate is less than one ounce per acre.