Summary

As an increasing proportion of the global human population inhabits urban areas. As cities continue to grow in size and number, demand for services and resources will also grow. In particular, rapidly-expanding urban areas located in arid environments are experiencing unprecedented demands and stresses on water-related infrastructure, resources and services. In recent years, constructed treatment wetlands have become a popular choice for managers as they represent a low-cost and low-maintenance method to treat and reclaim wastewater, thus alleviating stress on existing water-related infrastructure. While performance and service provisioning in constructed treatment wetlands in mesic climates have been well documented, how these systems perform and deliver services in hot, dry environments has yet to be rigorously studied.

To enhance our understanding of the water, nutrient and, ultimately, treatment dynamics of constructed treatment wetlands in aridland, urban environments, CAP LTER is developing robust water and nutrient budgets for the Tres Rios constructed wetland in Phoenix, Arizona. Completed in 2010 and operated by the City of Phoenix Water Services Department, the Tres Rios Wetlands are a tertiary wastewater treatment facility associated with the City of Phoenix’s 91st Avenue Waste Water Treatment Plant—the largest municipal treatment plant in Phoenix. Water inflows from the treatment plant to the Tres Rios Wetlands varies seasonally from 95,000 to over 270,000 m³ d^-1. Water losses via open water evaporation and plant transpiration are several orders of magnitude higher than annual precipitation inputs in Phoenix. A main goal of our research is to understand how these high rates of water losses to the atmosphere affect the water and nutrient budgets of the wetland ecosystem and potentially affect the services it can provide.

The Wetland Ecosystem Ecology Lab group, which is part of CAP, began monitoring the Tres Rios wetlands in summer 2011. Approximately 42 ha in total area, this wetland system is composed nearly equally of large open-water areas (21 ha) and a fringing vegetated marsh (21 ha). Measurements and samples are taken along two gradients representing the two hydraulic pathways of the treatment cell: whole-system, from inflow to outflow, and within the vegetated marsh proper, from the open water-marsh interface to the shoreline. We use a nutrient budget approach to determine inorganic nitrogen removal by the entire system and the vegetated marsh, and a water budget approach to determine the proportional contribution of atmospheric water losses via transpiration and evaporation to the whole system water budget. Initial results from the first five years of study suggest that plant transpiration, particularly during the hot, dry summer months, is a large enough water loss that it drives a lateral flow of surface water from open water areas into the vegetated marsh. We call this phenomenon a “biological tide” and note that this kind of plant-mediated control of surface hydrology in a wetland has never before been documented. By bringing new nitrogen into the marsh, this “biological tide” is actually making the Tres Rios Wetlands more efficient at nutrient removal than similar in cooler or more mesic climates. Please refer to Sanchez et al. (2016) and Weller et al. (2016) for details on methodology and additional results of this project.

Site map with marsh transect locations (red lines) and points of water inflow and outflow (blue arrows). Dark color is open water (total system size ≈ 42 ha).

Banner photo by Julia Hernandez.