Turquoise infrastructure in cities: How a constructed treatment wetland provides key urban ecosystem services in Phoenix, AZ

Background/Question/Methods:

Green and blue infrastructure are well known natural features in cities. Wetlands are found in virtually all cities, and because wetland ecosystems have both terrestrial and aquatic characteristics they represent both types of infrastructure. And because the colors green and blue together make turquoise, urban wetlands provide “turquoise services”. Urban wetlands designed to treat wastewater effluent are expected to provide the ecosystem service of nutrient and contaminant uptake. Treatment wetlands in hot, arid climates also lose considerable water via transpiration and evaporation, particularly during the hot, dry growing season. This water loss may evapoconcentrate nutrients and solutes, potentially challenging the ability of these wetlands to provide desired ecosystem services. To address this potential challenge, we have been measuring a number of ecosystem-level parameters in a constructed treatment wetland at the City of Phoenix’ Tres Rios Wastewater Treatment Plant since June 2010. The wetland vegetation and soils are sequestering large amounts of nitrogen. Our whole-system nutrient budgets also show considerable nitrogen uptake, but rates are not as dramatic as within the vegetated marsh proper.  At the same time, our plant transpiration and water budget measurements show large water deficits, particularly during the hot summer months.  

Results/Conclusions: Summer plant biomass is typically 1.5 - 3 kg dw m^-2 for the six dominant species (2 species of Typha and 4 species of Schoenoplectus); Typha biomass makes up 80-90% of this. Evaporation and transpiration rates are highest during the hot summer months, with daytime high temperatures over 45°C and humidity as low as 2%.  During this time, evaporation and transpiration account for 90% or more of the water deficit for the entire system, and up to 75% of that is plant transpiration.  Evapoconcentration of solutes driven by plant transpiration is occurring, but the marsh still removes virtually all of the inorganic nitrogen being supplied to it. We estimate that transpiration-driven water losses account for 20-25% of the total water volume overlying the marsh during hot, dry summer days. Thus, plant transpiration is driving a lateral “tide” that replaces the water overlying the vegetated marsh roughly every 4-5 days. This “biological pump” is also drawing nutrients into the marsh, making the Tres Rios treatment wetland more effective at nutrient removal than similar turquoise infrastructure in cooler, mesic cities. Thus the hot, arid climate that we hypothesized would present evapoconcentration challenges to nutrient uptake is actually enhancing this ecosystem service.