Figure 1. Idealized profiles showing the expected radium activities in surface water and peat pore-water for the cases of groundwater recharge (top) and groundwater discharge (bottom). Mixing due to dispersion could look similar to either of these profiles. [larger image]

The Everglades peat layer acts as an interface between groundwater and surface water, a zone where the interactions between physical, chemical and biological processes are enhanced, influencing the cycling of elements between water and sediments. Common methods for measuring exchange across the peat layer are prone to complications. For example, hydrologic approaches yield results with high variances when small hydraulic gradients are encountered, especially over short distances. Likewise, direct seepage-meter measurements tend to be imprecise at low seepage rates. Furthermore, some geochemical approaches (e.g. radon and chloride) rely on the measurement of fine scale gradients that are frequently affected by processes independent of recharge or discharge (e.g. methane ebullition or mechanical disturbance of the surface sediments). We present here a new method to quantify these slow vertical fluxes through the peat layer by modeling the pore-water profiles of ²²³Ra and ²²⁴Ra.

²²³Ra (t_1/2 = 11.4 d) and ²²⁴Ra (t_1/2 = 3.7 d) are naturally occurring isotopes of radium which are useful tracers for quantifying rates of groundwater recharge and discharge in wetlands, particularly for time scales of a few days to weeks. Near the interface between peat sediments and the underlying aquifer, or near the interface between the peat and overlying water, pore-water radium activities are commonly different than the amount expected from the radium production rate (fig. 1). This disequilibrium results from vertical transport of radium by pore water. In situations where groundwater recharge or discharge is significant, the rate of vertical water flow can be determined from this disequilibrium using a combined model of radium transport, production, decay, and exchange with solid phases:

C

=

D

2C

- v

C

+

-

C

+

C*

t

Z2

Z

f KD + (1 - f)

t

KD

transport

production

decay

exchange

where C is the number of dissolved radium atoms per volume of water, C* is the number of adsorbed radium atoms per mass of dry sediment, t is time, Z is depth below the peat surface, D is the hydrodynamic dispersion coefficient, v is the pore-water advective velocity, f is the mass of dry sediment per mass of bulk sediment, K_D is the ratio of adsorbed to dissolved radium, is the production rate exchangeable radium (adsorbed plus dissolved), is the radium decay constant, and is the pore water density.

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 (Krest and Harvey 2003). 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 (fig. 2). Our approach has a distinct advantage in the Everglades because 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 in which gradients are readily disturbed by processes unrelated to groundwater flow.

Figure 2. Pore-water radium activities as a function of depth. a) 224Ra, and b) 223Ra activities at site S10C-S are highest at the base of the peat and decrease upwards as the excess radium in discharging groundwater decays to a level supported by its equilibrium production and exchange with the adsorbed fraction. c) 224Ra and d) 223Ra activities at S10C-N are elevated only in the upper portion of the peat, suggesting that recharge occurs at this site. [larger image]

This technique should be readily applicable to any wetland system that has different production rates of these isotopes in distinct sedimentary layers or surface water. The approach is most straightforward in freshwater systems because constant pore-water ionic strength can usually be assumed, which simplifies the modeling of radium exchange with solid phases. In estuarine or marine systems, changing ionic strength could be addressed with additional data and an extended model.

REFERENCES

Krest, J. M. and J. W. Harvey (2003). "Using natural distributions of short-lived radium isotopes to quantify groundwater discharge and recharge." Limnology and Oceanography 48(1): 290-298.

Contact: Krest, James M., U.S. Geological Survey, 12201 Sunrise Valley Drive, Mail Stop 430, Reston, VA 20192; Phone: (703) 648-5472; Fax: (703) 648-5484; jmkrest@usgs.gov; Hydrology and Hydrological Modeling

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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