Water Resources Research Act Program

Details for Project ID 2011AZ436B

Hydrology Versus Ecology: The Effectiveness of Constructed Wetlands for Wastewater Treatment in a Semi-arid Climate

Institute: Arizona
Year Established: 2011 Start Date: 2011-03-01 End Date: 2012-10-31
Total Federal Funds: $10,000 Total Non-Federal Funds: $22,771

Principal Investigators: Daniel Childers, Laura Turnbull

Abstract: Urbanization is a major driver of land-use change world-wide, including semi-arid areas of the southwestern USA, such as Phoenix, and is associated with an increase in the volume of municipal wastewater that is tied to the growing human population in the city. Increasingly, wetlands are being constructed for tertiary wastewater treatment (i.e., nutrient removal from effluent). The use of constructed wetlands to improve the quality of wastewater effluent is relatively uncommon in arid ecosystems, however, and lessons learned from their use in more mesic settings may not translate well to dryland settings. We seek to address two pertinent questions about these aridland wetland treatment ecosystems: 1) Is wetland uptake and transformation of bioactive solutes [by plants and soil microbes] sufficient to counteract the effects of evapoconcentration to yield a net improvement in the quality of wastewater? 2) What are the relative effects of surface water evapoconcentration and soil evapoconcentration on the short and long-term ability of constructed wetlands to improve wastewater quality? In semi-arid climates, high evaporation rates will concentrate solutes in the water column while high evapotranspiration rates (by wetland plants) will concentrate solutes in the soils of constructed wetlands. The concentration of bioactive and non-bioactive solutes via these processes may exceed the ability of wetland biological processes to transform and remove bioactive solutes, thus reducing the treatment efficacy of the wetland. These same processes will also concentrate solutes that are not biogeochemically active, particularly in soils, perhaps to the point that wetland plant function and thus treatment efficacy are adversely affected. The extent to which evaporation and evapotranspiration concentrate solutes in wetland soils and water will vary based on factors that include the volume of wastewater discharge into the wetland, water residence time in the wetland, and time of the year. Our research will be guided by four primary objectives: 1) to derive the hydrological budget for the Tres Rios constructed wetland ecosystem in Phoenix AZ; 2) to derive solute budgets for the Tres Rios constructed wetland ecosystem, for bioactive and non-bioactive solutes; 3) to quantify the relative difference between evapoconcentration and wetland-assimilation of solutes; and 4) to quantify wetland ecosystem/biogeochemical function. Flow volume into and out of three flow regulating wetlands will be monitored, and influent and effluent samples will be analyzed for bioactive solutes (N, P and dissolved organics) and non-bioactive solutes (Cl-, Na+, Ca+2, Al+3). Evaporation from the wetlands will be estimated using data from a meteorological station at Tres Rios. Leaf evapotranspiration will be measured using a hand held infrared gas analyzer, then scaled up to the wetland plant community. Vegetation samples will be taken to determine vegetative uptake of bioactive solutes, and soil samples will be collected to determine the concentration of solutes in soils. We will monitor wetland vegetation productivity (as biomass accrual) using non-destructive allometric techniques. As a primary outcome of this research we will determine the efficacy of semi-arid constructed wetlands to improve water quality given the potentially confounding effects of evapoconcentration. This improved understanding of constructed wetlands function for the improvement of wastewater effluent will be of direct benefit to municipalities, since all municipalities are required to meet water quality regulations set by the EPA.