Wetlands in the Desert

Jan. 1, 2001
An unexpected wetlands area in the Arizona desert is attracting scores of visitors – human, avian, and other. Located near the convergence of the Salt, Gila, and Aqua Fria Rivers, the Tres Rios (Spanish for “three rivers”) constructed wetlands project is designed to improve water quality and provide some side benefits as well. A wastewater treatment plant serving the city of Phoenix and outlying areas discharges treated water to the lower Salt River Basin. Concerned about meeting the increasingly stringent water-quality requirements for discharges into Arizona waterways, the city, which operates the plant, began constructing the wetlands area in 1995 to provide additional treatment for the discharge. Three separate sites now comprise about 12 ac. of free-water surface treatment wetlands. If this EPA-approved, federally funded demonstration project works as well as expected to enable the plant to meet upcoming National Pollutant Discharge Elimination System requirements, the project will eventually expand to include 800 ac., enough to treat the entire 150-mgd discharge from the plant.Secondary goals of the Tres Rios project are to enhance wildlife habitat and provide space for public recreation and education. “Our main goal here is wetlands. We have a lot of water,” laughs Ron Elkins, construction project manager for Tres Rios. “We’re trying to reutilize it and make an area for the public. People see it and think, OThis is 20 minutes from downtown. I was not expecting this.’ We have phenomenal avian life. We have everything from bobcats to coyotes to javalina to quail to ibis – the whole gamut.”With those secondary goals in mind, roads and trails in the Hayfield section of the wetlands were recently improved to control fugitive dust, upgrade the access road, improve the onsite parking area and perimeter roadway around the wetlands, and provide handicap access to the trails. Dust Pro Inc. of Phoenix was the main subcontractor for the soil stabilization work. Owner Lou Snow notes that although this portion of the work covered only about 19 ac., the entire 800-ac. project might eventually include about 10 mi. of trails and roads.Dust Pro applied four different soil stabilizers to the trails, which the city and the public will evaluate over the next year to determine which should be used for future work. Three requirements helped determine the candidate materials. Given the project’s goal of encouraging habitat redevelopment, the city wanted an unpaved surface. “We didn’t want to go out there and blacktop or put down concrete. We wanted it to look as natural as possible but still have amenities for the public to use,” states Elkins. Public access was another priority; the trails need to maintain stability under increasing traffic and smoothness for wheelchair access. “We knew we didn’t want to put down any type of gravel because it’s hard to navigate a wheelchair through it. Members of the city’s Mayor’s Commission on Disability Issues gave us their input on what they felt would be a good surface and slope rates,” he says. Finally, the Tres Rios roads and trails had to meet Maricopa County’s dust control standards. Using these parameters, the city’s transportation and parks and recreation departments recommended four stabilization options: dry cement, lime fly ash, organic resin, and decomposed granite in a polymer-based stabilizer. An independent soils-testing lab took samples and determined how much of each mixture should be mixed into the soil for maximum effectiveness. Each of the four materials was mixed into the soil with a rotary mixer, applied to a 6-in. depth on different areas of trail or road surface, then compacted.Although the road and trail stabilization took only a few days, Snow’s team faced a number of challenges, such as limited equipment access and special precautions for sensitive wildlife habitat – including decibel limits for the machinery.“The trailways were very small, but they were hard to access,” Snow observes. “We had the big mixing machines up on top for the roads, but down through the pathways we were limited because we had a very narrow foot trail, and we couldn’t destroy the vegetation.” Smaller mixing equipment was carried to the trailways. “We ended up having to create a 4-foot-wide trail and stabilize it at the same time. It had to be a smooth, flowing trail to make it handicapped accessible; we couldn’t have areas where people would just jump from rock to rock.” A few detours were also necessary because the city had specified that contractors must protect existing trees, rerouting trails to go around them if necessary. The continuous outflow from the Hayfield site supports about a mile of riparian habitat along the Salt River, with native cottonwood and willow trees and such non-native species as Salt Cedar. Birds – yellow-headed blackbirds, stilts, ibis, and ducks – and water snakes, frogs, and turtles all inhabit the area. Though not natural wetland denizens, about half a dozen rattlesnakes put in an appearance while the trail work was ongoing. “Anytime there’s water in an arid place, there are all kinds of animals,” notes Snow.Thousands of people have visited the area since construction started, and Elkins envisions the improvements and further expansion of the wetlands attracting thousands more. “It’s going to be really nice. It’s been a long time coming. We’re six years into the demonstration mode, and we’ll be another two years in planning and design, and in late 2002 or early 2003 we should be out there turning dirt and getting it going.”