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 20.5 cm d21 for a recharge site and 1.5 6 0.4 cm d21 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 surface—a 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., Webster et 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 1 Corresponding author ([email protected]).