Recharge and Discharge Measurements in the Everglades using Short-lived
Radium Isotopes
James M. Krest and Judson W. Harvey
U.S. Geological Survey, Reston, VA
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.
223
Ra (t1/2 = 11.4 d) and 224Ra (t1/2 = 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
(Figure 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
 2C
C
Pˆ
n
1  n C *
D

v


λ
C


s
t
Z 2
Z (1  K D ) 1  n
n t
transport
production
decay
exchange
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
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.
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
224
Depth Below Peat Surface (m)
Discharge
0
Ra (dpm 100L-1)
20
40
223
60
0.00
0
0.75
0.75
1.00
1.00
Depth Below Peat Surface (m)
Recharge
0
20
40
60
0.00
0
2
4
6
0.00
c
0.25
1.00
8
S10C-S
Advection
-1
v = 0.50 ± 0.25 cm d
0.25
0.50
0.75
6
b
S10C-S
Advection
-1
v = 2.4 ± 0.6 cm d
0.50
0.50
4
0.00
a
0.25
Ra (dpm 100L-1)
2
d
0.25
S10C-N
Advection
-1
0.0 > v > -0.9 cm d
0.50
0.75
S10C-N
Advection
-1
0.0 > v > -0.43 cm d
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.
1.00
site near Levee 39 in the Everglades (Figure 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.
This technique should be readily 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 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.
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.
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