Confirming a plant-mediated "biological Tide" in an aridland constructed treatment wetland

As urban populations grow and the need for sustainable water treatment increases, urban constructed treatment wetlands (CTWs) are increasingly being used and studied. However, less is known about the effectiveness of this "turquoise infrastructure" in arid climates. In a recent publication, we presented evidence of plant-mediated control of surface hydrology, using a water budget approach, in a CTW in Phoenix, Arizona, USA. We also demonstrated how this transpiration-driven wetland surface flow made this treatment marsh more effective at pollutant removal than its counterparts in cooler or more mesic environments. Water budget-based calculations estimated that nearly 20% of the water overlying the marsh was transpired daily by the plants (40-60 Lm2d1) during the hottest summer months. We estimated the associated water velocity to be about 40 cm/h. In this paper, we report on hydrodynamic experiments that confirmed the existence of this phenomenon that we refer to as the "Biological Tide," and verified the rate at which transpiration by the marsh vegetation moves surface water into the biogeochemically active marsh. We combined a water budget-based approach and dye tracer experiments to quantify and confirm this phenomenon. Because of the low velocities estimated from our water budget approach (a few cm/h), we used a fixed-wall flowthrough marsh flume to limit the lateral dye movement during the tracer experiments. We measured actual flow rates of 7-50 cm/h (with 5-8% precision) during these experiments, which closely conformed to the values estimated from our water budget-based approaches. The flow was largely dispersive due to the extensive impedance imparted by dense plant stems in the marsh. From these summer flow rates, we calculated that residence time of the water overlying the marsh in this CTW averaged about 13 d, but could be as low as four days. Again, these values were reasonably close to the 15-20% replacement rate for marsh water we estimated using the water budget approach. This is the first time, to our knowledge, that plant-mediated biological control of surface hydrology in a wetland, without connection to groundwater, has been reported.