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Using natural distributions of short-lived radium isotopes to quantify groundwater discharge and recharge
Radium activity in pore water of wetland sediments often differs from the amount expected from local production, decay, and exchange with solid phases. This disequilibrium results from vertical transport of radium with groundwater that flows between the underlying aquifer and surface water. In situations where groundwater recharge or discharge is significant, the rate of vertical water flow through wetland sediment can be determined from the radium disequilibrium by a combined model of transport, production, decay, and exchange with solid phases. We have developed and tested this technique at three sites in the freshwater portion of the Everglades by quantifying vertical advective velocities in areas with persistent groundwater recharge or discharge and estimating a coefficient of dispersion at a site that is subject to reversals between recharge and discharge. Groundwater velocities (v) were determined to be between 0 and -0.5 cm d-1 for a recharge site and 1.5 ± 0.4 cm d-1 for a discharge site near Levee 39 in the Everglades. Strong gradients in 223Ra and 224Ra usually occurred at the base of the peat layer, which avoided the problems of other tracers (e.g., chloride) for which greatest sensitivity occurs near the peat surfacea zone readily disturbed by processes unrelated to groundwater flow. This technique should be easily applicable to any wetland system with different production rates of these isotopes in distinct sedimentary layers or surface water. The approach is most straightforward in systems where constant pore-water ionic strength can be assumed, simplifying the modeling of radium exchange.
The interface between groundwater and surface water is a zone where the interactions between physical, chemical, and biological processes enhance rates of biogeochemical cycling (e.g., Martens 1987; Wersin et al. 1991; Cirmo and McDonnell 1997; Siegel et al. 2001). In marine environments, radium isotopes have been used to quantify water and solute fluxes through this reaction zone by modeling diffusion across the interface (e.g., Hancock et al. 2000; Nozaki et al. 2001), bioturbation (e.g., Cochran 1980; Sun and Torgersen 2001), tidal flushing of the sediments (e.g., Websteret al. 1994; Rama and Moore 1996), or groundwater discharge (e.g., Moore 1997; Krest et al. 2000; Burnett et al. 2002). 223Ra and 224Ra, with their short half-lives (Table 1), promise to be useful for quantifying rates of exchange over short timescales, as occurs in this reaction zone, but only a few studies have examined the geochemistry of these isotopes in groundwater or pore water (e.g., Webster et al. 1994; Hancock et al. 2000; Sun and Torgersen 2001). Several recent studies have used these isotopes to model coastal or estuarine residence times (Moore 2000; Charette et al. 2001; Kelly and Moran 2002), but many questions remain concerning the processes that control the distribution of radium in these systems. For example, the distribution of radium isotopes in the pore water is often not examined, but might be crucial to determining magnitudes and patterns of groundwater radium discharge or recharge. This is particularly true if there is a salinity gradient present that would affect the partitioning of radium between the dissolved and adsorbed phases (Webster et al. 1995; Moore 1999). If we are to utilize these powerful tracers of water flow in biogeochemically complex systems like estuaries and the coastal ocean, we need to make sure we understand what happens as these isotopes are transported through the reaction zone into the surface water. One place to gain understanding of the reaction zone geochemistry of radium is in freshwater wetlands, where pore-water geochemistry is not complicated by large variations in pore-water salinity.
In freshwater systems such as streams and wetlands, groundwater discharge or recharge and hyporheic exchange between surface water and pore water of streambed sediments are important controls on biogeochemical cycling (Grimm and Fisher 1984; Brunke and Gosner 1997; Cirmo and McDonnell 1997; Mulholland et al. 1997; Drexler et al. 1999). 223Ra and 224Ra could be useful for measuring the rate of this exchange, but little work has been reported in which these isotopes have been used as tracers of water flow in freshwater (Kraemer and Genereux 1998) and none that we know of in freshwater wetlands. Although dissolved radium activities are low in most freshwater systems (King et al. 1982; Moore 1999), using radium to determine groundwater exchange in a freshwater system is potentially less problematic than in estuarine or marine systems, where variable pore-water ionic strengths affect sediment-dissolved partitioning coefficients.
Our freshwater study area, the Florida Everglades, is similar to many estuarine and coastal marine wetlands in that groundwater/surface-water exchange is attenuated by a thin (typically ~1 m thick or less), organic-rich sediment that exists in an almost continuous layer across much of the system. This layer has a lower hydraulic conductivity than underlying sediments, thereby impeding exchange between the surface water and surficial aquifer (Harvey et al. 2002). In other systems, this impeding layer can be of different lithology, but in the Everglades, it is a layer of peat primarily composed of decaying roots and stems of emergent macrophytes. Although vertical water transport does occur across the Everglades peat layer, rates are slow enough that they are difficult to quantify, largely because of very weak hydraulic gradients in most areas of the Everglades (Nemeth et al. 2000). Even modest rates of vertical transport, however, become significant to Everglades water budgets because they often occur over a large area (Choi and Harvey 2000). Common methods for measuring exchange across the peat layer are prone to complications: small hydraulic gradients are difficult to measure over short vertical distances; seepage meters tend to be imprecise at slow rates; radon profiles or emanation rates are complicated by methane bubble ebullition; and chloride profiles commonly exhibit a strong gradient only at the surface of the peat and are affected by other processes (e.g., bubble ebullition) in addition to recharge and discharge.
We present here a method to quantify these slow vertical fluxes through the peat layer by modeling the pore-water profiles of 223Ra and 224Ra. The approach used was to collect field data on natural distributions and production rates of radium in vertical profiles through Everglades peat and to model that data using one-dimensional advective flow models.
1Corresponding author (firstname.lastname@example.org).
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM(TJE)