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Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

1. Introduction

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Studies of interactions between surface water and ground water often target interactions that occur on timescales of days to months. For example, watershed hydrologists are typically interested in the relatively fast exchanges that occur, such as hillslope subsurface stormflow and bank storage of river channel water. These processes exert a major effect on the magnitude and timing of precipitation runoff from the watersheds and routing of that water through the channel network (National Research Council, 2002). Ecologists and biogeochemists have found reason to study even shorter timescale (minutes to hours) interactions between streams and the adjacent alluvial materials of the hyporheic zone that affect fate and transport of dissolved constituents, (Jones and Mulholland, 2000).

In a large wetland ecosystem such as the Everglades, there is considerable interest in both short and long timescales of surface-water and ground-water interactions. Short timescale interactions in the Everglades involve vertical exchange between wetland surface water and peat porewater (Krest and Harvey, 2003; Harvey et al., 2005), while longer timescale interactions between wetland surface water and the underlying sand and limestone aquifer also are of interest (e.g. Choi and Harvey, 2000; Bolster et al., 2001; Price et al., 2003; Wilcox et al., 2004). Previous investigations of interactions between surface water and shallow ground water in the Everglades were usually conducted near levees (Swayze, 1988; Meyers et al., 1993; Genereux and Slater, 1999; Bolster et al., 2001; Sonenshein, 2001; Nemeth and Solo-Gabriele, 2003). In general, interactions between surface water and ground water are much less well understood in the interior areas of the Everglades. For example, the effect that levees have in causing local increases in recharge and discharge within adjacent wetlands is generally confined to within a kilometer (Harvey et al., 2004). Municipal pumping wells appear to be important at greater distances (Wilcox et al., 2004), but in general there is comparatively little information about recharge and discharge in the vast areas of the Everglades interior.

Recent measurements of hydrogen and helium isotopes in ground water beneath the interior areas of the Everglades have provided new insights about interactions between surface water and ground water in these remote areas. For example, ground waters in the top 30 m of the Surficial aquifer in the southern Everglades have isotopically determined residence times that range from years to decades in the shallow aquifer, while ground water in the deeper parts of the aquifer are much older (beyond the detection range for these isotopes; Price et al., 2003). Recharge and discharge fluxes across the surface of the interior wetlands have recently been estimated by modeling vertical transport of naturally occurring, short-lived, radium isotopes in peat porewater (Krest and Harvey, 2003). Another method to determine recharge and discharge fluxes across the peat surface is measure the gradient in hydraulic head vertically through the peat and combine those data with bail test estimates of the hydraulic conductivity of peat as a means to compute recharge and discharge fluxes (Harvey et al., 2004). That approach indicated relatively high values of recharge and discharge (on the order of cm per day) that could not be explained by the effects of levees on ground-water flow. Other factors that could control recharge and discharge in the remote interior areas of the wetlands include seasonal and interannual variation in precipitation, as well as the effects of surface-water gravity waves created by pumping and spillway operations. For example, water releases through levee spillways cause the propagation of gravity waves toward interior areas of the wetland, which appear to drive alternating periods of discharge and recharge as they pass by locations in the interior wetlands (Harvey et al., 2004). Use of ground water geochemical tracers could further improve understanding of recharge and discharge in the Everglades. Improved models of surface-water and ground-water exchange are also needed as the basis for improved water quality models.

For the present study, concentrations of naturally occurring tritium were measured in ground water of Water Conservation Area 2A and used as the basis for quantifying long-term average recharge and discharge in the remote areas of the WCA-2A basin interior. The modeling of water and tracer flow was intentionally kept simple so that chemical sub-models could be easily added in the future to address water quality issues in the Everglades. A second objective was therefore to take a step towards evaluating whether the simple model of coupled surface-water and groundwater flow used here could be used in the future as a valid framework for modeling solute transport and reaction processes in the Everglades.

Tritium measurements came from 25 research wells in Water Conservation Area 2A that were screened at various levels in the Surficial aquifer. Measurements of 3H/3He were successful at a few of those wells, and those results offered an important check on ground-water residence times inferred from modeling tritium. The depth of the layer of Everglades ground water that exchanges with surface water was estimated directly from the distribution of tritium in ground water, while the more uncertain hydrologic parameters (average residence time of ground water and recharge and discharge fluxes) were estimated by adjustment to match the average tritium concentration in that layer. What is distinctive about the present study is that long-term average recharge and discharge fluxes were quantified in a remote interior area of the Everglades. Previous use of environmental tracers in Everglades ground water have either been in the vicinity of levees (e.g. Meyers et al., 1993; Wilcox et al., 2004), or when implemented in remote areas have generally stopped short of the goal of estimating recharge and discharge, instead reporting only the estimated residence time of ground water (e.g. Price et al., 2003).

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