Long-term monitoring and research of the ecology of the Tres Rios constructed treatment wetland, Phoenix, AZ, ongoing since 2011

Project Description In order to better understand the water, nutrient and treatment dynamics of aridland
constructed treatment wetlands, we have developed datasets tracking primary productivity,
water quality, and water budget dynamics at the Tres Rios wetlands, operated by the City
of Phoenix Water Services Department, since Summer 2011. The 3-cell Tres Rios Wetlands
were completed in 2010 and are associated with the 91st Avenue Wastewater Treatment Plant,
the largest in Phoenix. This project is focused on the largest of the three wetlands
treatment cells, FRW3, which was the first to be planted and become operational as of
Summer 2010. Each wetland cell is bounded by roads (the "shoreline"), and FRW3 in
particular is 42 ha in size, approximately half of which is open water and half of which
is fringing vegetated marsh. Water depth was consistent across the marsh (approximately
25cm), while effluent inflow to the cell varied seasonally from 95,000 to over 270,000 m3
d-1. Measurements were taken along two gradients representing the two hydraulic pathways
of the system: whole-system inflow and outflow, as well as the inflow (open water-marsh
interface) and outflow (shoreline) of the vegetated marsh at evenly distributed locations
across the system. Abstract Constructed treatment wetlands provide cost effective and ecosystem-service based
solutions to the problem of urban water reuse. They are a particularly attractive option
for water reuse in arid urban systems where water resources are scarce, and understanding
how they function in these environments is critical to facilitating sustainable water use
practices. Although constructed treatment wetlands are well established and studied in
mesic climates, how they function in and respond to hot, arid climates is comparatively
not well understood. Specifically, large atmospheric water losses via evaporation and
plant transpiration may comprise a much larger component of the whole-system water budget
than in mesic climates. Additionally, given the primary role that emergent macrophytes
play in nutrient removal, particularly nitrogen removal, the effects of plant community
composition and primary productivity patterns on system performance in the context of
aridland constructed treatment wetlands have not been extensively studied. Our goal was to develop a model of how these "working wetlands" perform in arid
climates by developing and comparing nutrient and water budgets. At the Tres Rios
constructed treatment wetland in Phoenix, AZ, USA, we measured atmospheric water losses
via evaporative pathways (plant transpiration and open water evaporation) as well as
inorganic N fluxes between the whole system and the vegetated marsh areas. Total water
losses via evaporative pathways peaked at 300,000 m3/mo-1 (714 L H2O m-2 mo-1) in the hot
summer months and represented more than 70% of the whole-system water budget over a 27
month time period. These evaporative losses are nearly an order or magnitude higher than
rates observed in mesic systems. Peaks in above-ground biomass ranged from 1586±179 to
2666±164 gdw m-2, with Typha spp. accounting for up to 2/3 of total biomass. Overall, the
vegetated marsh removed almost all of the inorganic N supplied to it, and large
transpirative water losses were observed to move large volumes of replacement water into
the marsh via a plant-mediated "biological tide." This process providing additional
opportunities for soil microbes and emergent macrophytes to process target solutes, and
potentially enhancing the treatment efficacy of the aridland Tres Rios constructed
treatment wetland relative to more humid and mesic systems.