Conservative and reactive solute transport in constructed wetlands

Abstract

[1] The transport of bromide, a conservative tracer, and rhodamine WT (RWT), a photodegrading tracer, was evaluated in three wastewater-dependent wetlands near Phoenix, Arizona, using a solute transport model with transient storage. Coupled sodium bromide and RWT tracer tests were performed to establish conservative transport and reactive parameters in constructed wetlands with water losses ranging from (1) relatively impermeable (15%), (2) moderately leaky (45%), and (3) significantly leaky (76%). RWT first-order photolysis rates and sorption coefficients were determined from independent field and laboratory experiments. Individual wetland hydraulic profiles influenced the extent of transient storage interaction in stagnant water areas and consequently RWT removal. Solute mixing and transient storage interaction occurred in the impermeable wetland, resulting in 21% RWT mass loss from main channel and storage zone photolysis (10%) and sorption (11%) reactions. Advection and dispersion governed solute transport in the leaky wetland, limiting RWT photolysis removal (1.2%) and favoring main channel sorption (3.6%). The moderately leaky wetland contained islands parallel to flow, producing channel flow and minimizing RWT losses (1.6%).

Unit Process Wetlands for Removal of Trace Organic Contaminants and Pathogens from Municipal Wastewater Effluents

“…In wetlands, the relatively hydrophilic fluorescent dye, rhodamine WT, sorbs significantly to plants and sediments (Lin et al, 2003;Keefe et al, 2004). At neutral pH values, rhodamine WT contains both positively and negatively charged functional groups and is, thus, likely to be adsorbed via specific interactions with charged functional groups on the sorbents (Kasanavia et al, 1999).…”

“…Short-circuiting is the result of preferential flow paths through a wetland, which are caused primarily by uneven plant distribution and channelized flow (Kjellin et al, 2007;Lightbody et al, 2008). Short-circuiting results in water having a range of residence times in a wetland, reducing the wetland's treatment efficiency (Keefe et al, 2004;Wö rman and Kronnä s, 2005). This is especially detrimental for wetlands designed to remove waterborne pathogens, which require reductions in a concentration of several orders of magnitude to provide effective treatment and, thus, are severely compromised by even a modest amount of short-circuiting.…”

“…While tracer studies provide an understanding of how far the system deviates from the ideal, more complicated models are necessary to predict contaminant treatment efficiency, as water flowing via different paths may be subjected to diverse biogeochemical conditions, resulting in variable treatment (Kadlec, 2000;Harvey et al, 2005). For example, Keefe et al (2004) modeled the reactive transport of rhodamine water tracer (WT) in three wetlands using a solute transport model with transient storage. Results showed that rhodamine WT loss rates via photolysis and sorption differed in storage and main channel zones, with sorption mass transfer rates being a factor of two higher in storage zones than in the main channel, and photolysis rates in the storage zones being almost an order of magnitude lower than those occurring in the main channel.…”

A comprehensive one-dimensional numerical model for solute transport in rivers

“…After the introduction of the TSM, transient storage processes have been studied in a variety of small mountain streams, as well as large rivers, and it was shown that simulation results of tracer study data considering the transient storage impact have good agreement with observed data. Also, it was shown that the interaction between the main channel and storage zones, especially in mountain streams, has a great effect on solute transport behavior (D'Angelo et al, 1993;DeAngelis et al, 1995;Morrice et al, 1997;Czernuszenko et al, 1998;Chapra and Runkel, 1999;Chapra and Wilcock, 2000;Laenen and Bencala, 2001;Fernald et al, 2001;Keefe et al, 2004;Ensign and Doyle, 2005;Van Mazijk and Veling, 2005;Gooseff et al, 2007;Jin et al, 2009). In this study, a comprehensive model, called TOASTS (third-order accuracy simulation of transient storage), able to obviate shortcomings of current models of contaminant transport, is presented.…”

Pool Effects on Longitudinal Dispersion in Streams and Rivers

“…The former includes either various accidental spills, or deliberate discharge of the effluent of wastewater treatment plant (WWTP) [1]. The latter refers to surface runoff which is recognized as a major source of pollutants to rivers and streams.…”