Solute transport and storage mechanisms in wetlands of the Everglades, south Florida

Abstract

[1] Solute transport and storage processes in wetlands play an important role in biogeochemical cycling and in wetland water quality functions. In the wetlands of the Everglades, there are few data or guidelines to characterize transport through the heterogeneous flow environment. Our goal was to conduct a tracer study to help quantify solute exchange between the relatively fast flowing water in the open part of the water column and much more slowly moving water in thick floating vegetation and in the pore water of the underlying peat. We performed a tracer experiment that consisted of a constant-rate injection of a sodium bromide (NaBr) solution for 22 hours into a 3 m wide, open-ended flume channel in Everglades National Park. Arrival of the bromide tracer was monitored at an array of surface water and subsurface samplers for 48 hours at a distance of 6.8 m downstream of the injection. A one-dimensional transport model was used in combination with an optimization code to identify the values of transport parameters that best explained the tracer observations. Parameters included dimensions and mass transfer coefficients describing exchange with both short (hours) and longer (tens of hours) storage zones as well as the average rates of advection and longitudinal dispersion in the open part of the water column (referred to as the ''main flow zone''). Comparison with a more detailed set of tracer measurements tested how well the model's storage zones approximated the average characteristics of tracer movement into and out of the layer of thick floating vegetation and the pore water in the underlying peat. The rate at which the relatively fast moving water in the open water column was exchanged with slowly moving water in the layer of floating vegetation and in sediment pore water amounted to 50 and 3% h À1 , respectively. Storage processes decreased the depth-averaged velocity of surface water by 50% relative to the water velocity in the open part of the water column. As a result, flow measurements made with other methods that only work in the open part of the water column (e.g., acoustic Doppler) would have overestimated the true depth-averaged velocity by a factor of 2. We hypothesize that solute exchange and storage in zones of floating vegetation and peat pore water increase contact time of solutes with biogeochemically active surfaces in this heterogeneous wetland environment.

Characteristics of surface-water flows in the ridge and slough landscape of Everglades National Park: implications for particulate transport

“…Our results corroborate these observations and suggest that future research should focus on developing expressions that describe changes in flow regime due to the presence of periphyton coatings, surface periphyton mats, and litter. In addition, our field data strongly indicate that future modeling efforts must also take the presence of floating biomass, such as Utricularia, into consideration as has been recently attempted by Harvey et al (2005).…”

“…Further, Neumeier & Ciavola (2004) documented flow reductions correlated with local vegetation density and the absence of a logarithmic velocity profile in a Spartina maritima marsh. Additional studies have suggested that differences in canopy morphology can affect flow speed and flow energy due to variations in the way the plant material is distributed in the water column (Leonard & Reed 2002;Neumeier & Ciavola 2004;Harvey et al 2005). The variations in flow have potential to increase the heterogeneity of particulate concentrations in the water column and to promote dispersion of both suspended particulates and dissolved materials (Nepf et al 1997;Harvey et al 2005).…”

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

“…In-stream transport of tracers has increasingly been supplemented with measurements of tracer transport into hyporheic and surface water storage zones (Harvey and Fuller, 1998;Harvey et al, 2005). The goal is to help isolate characteristics of STS and HTS environments, although the comparisons are often limited by the small number of pointscale observations, which may not be representative of storage processes in the reach as a whole.…”

“…A drawback of such approaches is that rigorous data collection is required to obtain enough measurements of streambed topography, hydraulic conductivity and boundary conditions (Gooseff et al, 2006;Wondzell et al, 2009a). Briggs et al (2009) and Harvey et al (2005) utilized the two-zone transient storage model -developed by Choi et al (2000) -to differentiate STS from HTS by measuring tracer breakthrough in the STS and then, utilizing a transient storage model, partially parameterized with STS to determine HTS by inverse modeling. Drawbacks to this approach include (1) additional data collection (e.g., velocity and concentration time series), and (2) the parameterization of two additional parameters, transient storage area and the mass exchange coefficient.…”

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

“…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.…”