Observations of short-circuiting flow paths within a free-surface wetland in Augusta, Georgia, U.S.A

Velocity heterogeneity is characteristic of wetland systems and results in some influent water remaining in the wetland for less than the expected residence time on the basis of volume and flow rate. This phenomenon, known as short-circuiting, alters the distribution of the chemical and biological transformations that occur within the wetland. Field observations in a 1.5-km2 constructed treatment wetland in Augusta, Georgia were used to quantify the size, distribution, velocity, and transport potential of fast flow paths, which cause short-circuiting within wetlands. The flow paths were identified by a tracer study and velocity measurements. In each of the three cells examined, between three and six fast flow paths were found, most less than 4 m wide. These flow paths had an area-averaged velocity on the order of 1 cm s21, at least 10 times the velocity observed where water passed through vegetation. Within different cells in this wetland, 20–70% of the flow had a residence time less than oneeighth of the nominal residence time. With this degree of short-circuiting, uniform flow is a poor approximation for the flow through the wetland. In addition, tracer studies were used to make direct measurements of mixing within open-water deep zones. Lateral mixing was sensitive to the direction of the wind. The average daily maximum temperature in the densely vegetated slow-flow zones was 2.0uC cooler than that at the surface of the open water zones and 0.9uC cooler than that of the fast-flow zones. The importance of short-circuiting flow paths in this relatively simple wetland suggests that this phenomenon is likely of even greater significance in natural wetlands because they are typically much more complex. There is increasing interest in the internal circulation of both natural and constructed wetlands to better understand and model their ecology, nutrient and pollutant removal, and water-storage abilities (Hopkinson et al. 1988; Mitsch and Gosselink 2000). Models for the marsh regions within wetland systems often assume uniform vegetation, which is represented by uniform hydraulic roughness and dispersion constants (Jadhav and Buchberger 1995; Bolster and Saiers 2002; Persson 2005). When no dispersion is present, this assumption results in plug flow, in which all water entering the marsh spreads out across the width and remains in the marsh for exactly the hydraulic residence time t 5 V/Q, where V is the marsh volume and Q the volumetric flow rate through it. However, velocity heterogeneity is nearly always present in wetlands and results in some influent water remaining in the wetland for a time much shorter than the mean residence time. For example, in a 0.81-km2 wetland in Florida, 32% of the flow exits the wetland in only 0.3t (Keller and Bays 2002). This phenomenon, known as short-circuiting, results in zones of different local residence time that can generate distinct biogeochemical zones (Harvey et al. 2005), which in turn may influence the diversity of available habitats. In addition, biogeochemical processes and uptake rates depend on local water velocities (Eriksson 2001). In treatment wetlands, such heterogeneity nearly always results in reduced contaminant removal (Wörman and Kronnäs 2005). Understanding and accounting for short-circuiting is therefore vital for constructing accurate and useful models of wetland systems (Bolster and Saiers 2002). Fast flow paths, which carry short-circuiting water, often result when a portion of the wetland lies below the average marsh elevation. An extreme example is a tidal creek system, in which flow is dominated by transport within incised creek channel networks that are clearly visible in aerial photographs (Arega and Sanders 2004). In many natural wetlands, macrophytes fringe a central, deeper open channel, with a large fraction of the flow carried by that channel (Cooper 1994). For example, Stern et al. (2001) observed that a central channel carried more than 50% of the flow in a natural wetland in Westchester, New York. Similarly, within the ridge and slough landscape in the Everglades National Park in Florida, water velocities are slower within elevated tree islands than in the surrounding deeper marshes (Bazante et al. 2006). Constructed wetlands may also have deep channels that create preferential flow paths. For example, within the Everglades Nutrient Removal Project in Florida, abandoned agricultural ditches and borrow canals oriented parallel to flow 1 To whom correspondence should be addressed. Present address: St. Anthony Falls Laboratory, University of Minnesota, 2 Third Ave. SE, Minneapolis, MN 55414 ([email protected]). 2 Present address: Department of Civil and Environmental Engineering, University of Washington, More Hall, Box 352700, Seattle, Washington 98195-2700.