A constructed treatment wetland provides key urban ecosystem services even in a hot, dry climate

Background/Question/Methods:    Wetlands being used 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 the nutrients and solutes that the wetland ecosystem is expected to process and sequester, perhaps making it more difficult for the system to provide desired ecosystem services. To address this potential challenge, we have been measuring a number of ecosystem-level parameters—including plant production, transpiration, nutrient retention, and evapoconcentration—in a constructed treatment wetland at the City of Phoenix’ Tres Rios Wastewater Treatment Plant since June 2010. The wetland vegetation and soils appear to be sequestering—and presumably transforming—large amounts of nitrogen: Water samples collected along ten marsh transects show average declines in inorganic nitrogen concentrations > 80% for ammonium, nitrate+nitrite, and total nitrogen. Our whole-system nutrient budgets also show considerable nitrogen uptake, but rates are not as dramatic as within the vegetated marsh proper.  This “working” urban wetland is also a field collaboratory where students are learning how to conduct field research, how to work collaboratively, and how to mentor each other. 

Results/Conclusions:    Summer plant biomass typically exceeds 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. Transpiration rates are highest during the hot summer months, with daytime high temperatures over 45°C and humidity as low as 2%.  Summer transpiration rates of 4 – 6 cm water depth day^-1 are responsible for nearly 90% of the water deficit for the entire treatment wetland.  Chloride content increased an average of 15% along the marsh transects, suggesting that evapoconcentration of solutes driven by plant transpiration is occurring. We estimate that transpiration-driven water losses account for >50% 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 two days. This “hydraulic pump” is also drawing nutrients into the marsh, making the Tres Rios treatment wetland more effective at nutrient removal. The hot, arid climate that we hypothesized would present evapoconcentration challenges to nutrient uptake is actually enhancing this ecosystem service.