Hydrology versus ecology: The effectiveness of constructed wetlands for improving wastewater quality in an arid climate

Background/Question/Methods

Increasingly, wetlands are being constructed for tertiary wastewater treatment, to “polish” treated wastewater. In arid climates, high evaporation rates will concentrate solutes in the water column while high transpiration rates will concentrate solutes in wetland soils. The evapoconcentration 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. Because of the amplification of evaporation and transpiration in arid systems lessons learned from the use of constructed wetlands in more mesic settings may not translate well to arid settings. In this study, we seek to answer the following question: Is wetland uptake and transformation of bioactive solutes sufficient to counteract the effects of evapoconcentration in arid climates to improve wastewater quality? To answer this question, we conducted research in the Tres Rios constructed wetland system in Phoenix, AZ, which is one of the first constructed wetlands in an arid climate. We derived hydrological and solute budgets, monitored wetland primary production (as biomass accrual) and transpiration rates, and measured wetland soil and plant tissue for nutrient uptake.

Results/Conclusions

Transpiration rates were highest during the hot summer months when the plants were responsible for a water loss rate equivalent to 4 – 6 cm water depth per day. Water loss from the entire wetland due to evaporation and transpiration was about 50% lower in winter months. Water samples collected along 10 sampling transects in the wetland showed average declines in inorganic nitrogen concentrations at the patch scale, from the water’s edge to the interior of the vegetation, of 73% for ammonium and 80% for nitrate+nitrite. Chloride concentrations increased an average of 15%, suggesting that evapoconcentration of non-bioactive solutes is occurring within the vegetated portions of the wetland. Nutrient budgets show that in summer, the wetland retains 90% of ammonium, 49% nitrate, and 48% nitrite. In winter months, retention of ammonium decreases by 50%, while retention of other forms of inorganic nitrogen decreases only marginally. Derived budgets show no significant difference between input and output of chloride, indicating that evapoconcentration has no significant effect at the whole system scale. Results indicate that even in arid climates, constructed wetlands do improve water quality; however, the longer term implications of “evapoconcentration hot spots” of non-bioactive solutes have yet to be determined.