EPSL
ELSEVIER
Earth and Planetary
Science Letters 144 (1996) 591-604
Estimating groundwater discharge into the northeastern Gulf of
Mexico using radon-222
Jaye E. Cable *, William C. Burnett ‘, Jeffrey P. Chanton, Georges L. Weatherly
Drpartrwrr~
ofOcrano,~raphy.
Flwida
Stutv Unircrsit\,
Tallahassre.
FL 32306-3048.
USA
Received 28 June 1996; revised 26 August 1996; accepted 3 September 1996
Abstract
Submarine groundwater discharge (SGD) may provide important chemical constituents to the ocean, but the dispersed
nature of this process makes locating and quantifying its input extremely difficult. Since groundwater contains 3-4 orders of
magnitude greater radon than seawater, ‘?‘Rn may be a useful tracer of this process if all other sources of radon to bottom
waters can be evaluated. We report development of a SGD tracing tool based on radon inventories in a coastal area of the
northeastern Gulf of Mexico. We evaluated factors that influence the concentration of radon in the water column (i.e.,
production-decay,
horizontal transport, and loss across the pycnocline) using a linked benthic exchange-horizontal
transport
model. Total ““Rn benthic fluxes (2 2420 dpm mm2 day- ’ ) measured with in situ chambers are of the magnitude required
‘22Rn inventories, while estimates of molecular diffusion show that this input is
to support measured sub-pycnocline
relatively small (I 230 dpm m-’ day- ‘1. Using this model approach, together with measurements of the radon inventory,
we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 set _’ into the 620 km2 study area. This
discharge, equivalent to an upward advective velocity of approximately 2-10 cm day- ’ dispersed over this entire study area,
is equivalent to approximately 20 first magnitude springs.
Keykvrds:
ground water: discharge;
Gulf of Mexico; Rn-222
1. Introduction
River discharge to the world’s oceans is widely
recognized as an important contributor to oceanic
chemistry
[ 1,2]. Water recirculation
through the
seafloor caused by macrofaunal irrigation [3,4], hydrothermal vent activity [5], and along continental
* Present address: Department of Fisheries and Aquatic Sciences, 7922 NW 71st Street, University of Florida, Gainesville.
Florida 32653. E-mail: *<
[email protected]
_
’ E-mail: [email protected].
00 12-82 I X/96/$12.00
Copyright
PII S0012~821X(96100173-2
margin brine seeps [4] also affect oceanic chemicai
mass balances. Other poorly understood but potentially significant oceanic sources of chemical components are submarine groundwater discharge (SGD)
and recirculated seawater along continental shelves.
These as yet unquantified sources include groundwater seepage through sediments as well as submarine
spring discharge.
The occurrence of groundwater discharge into the
ocean has been discussed for decades [7] but until
recently its role in coastal ecosystems was little
appreciated. Geochemical
and hydrologic budgets
usually neglect groundwater
contributions
to the
Q 1996 Elsevier Science B.V. All rights reserved.
592
J.E. Cable et al. /Earth
and Planetaryv Science Letters 144
coastal zone due to difficulties associated with locating and quantifying
its input. Fanning et al. [S]
measured unsupported “‘Rn bottom water concentrations on the West Florida Shelf ranging from 0.5
to 2.5 dpm 1-l and suggested groundwater discharge
or diffusion from underlying phosphorite deposits
might be responsible for these relatively high concentrations. We compared Fanning’s “‘Rn contour
plot to a map of known submarine spring distributions along the Florida coastline [9] and found that
the highest ‘*‘Rn activities are clearly found in
regions of greatest submarine spring density. Radon
concentrations
are often 3-4 orders of magnitude
greater in groundwater than in coastal seawater, and
this qualitative relationship suggests that radon should
be a useful tracer of submarine groundwater discharge.
Development
of tracer techniques
to quantify
groundwater discharge would greatly enhance our
ability to assess its potential effect on coastal ecosystems. Previous studies have employed ‘*‘Rn to examine atmospheric evasion [lo], sediment-water
exchange [4], submarine spring discharge chemistry
[I I], water column mixing rates [ 121, and other
processes. For example, 222Rn and salinity have been
used to calculate hydraulic mixing rates between
surface seawater and interstitial waters in a lagoonal
patch reef in Kaneohe Bay, Oahu [13]. We report
results of a study in a small area of the northeastern
Gulf of Mexico evaluating the use of “‘Rn as a
tracer of submarine
groundwater
discharge. The
overall goal was to establish a quantitative relationship between the regional flow of SGD and the
inventory of water column ‘*‘Rn. Since inventories
are relatively easy to determine, this approach allows
a regional assessment of subsurface fluid flow which
could, in principle, be applied to other areas.
2. Geologic setting, sample collection
and analysis
The Floridan Plateau is a broad, flat limestone
and dolomite platform separating the Atlantic Ocean
from the Gulf of Mexico. Florida, the emergent part
of this plateau, contains one of the United States’
largest freshwater reservoirs, the Floridan Aquifer
[9]. The field portion of this investigation took place
in a 620 km* section of the inner continental shelf
Fig. I.
FSUML
seasonal
is about
C19961SYI-604
The study area (box; - 620 km’) is located south of
in the northeastern Gulf of Mexico. K-Tower, the site of
water column sampling and benthic flux measurements.
24 km southeast of FSUML.
(Fig. 1) south of the Florida State University Marine
Laboratory (FSUMLI.
Hydrographic characteristics were obtained from
conductivity, temperature, and pressure readings using a CTD profiler (Seabird Electronics)
at each
water collection station in the study area. In addition,
long-term,
burst-sampling
current
meters
were
moored in 18 m of water at K-Tower (29”40.72N,
84O22.71 W), located about 24 km south of the
FSUML, to determine average current flow characteristics within the area. Shallow (10 meters above
bottom; mab) and bottom (1 mab) current meters
were deployed in an array on 3 occasions: (I) December 199 1 to April 1992 [ 141; (2) July to October
1992; and (31 July to August 1993. During these
sampling periods, current velocity, current direction,
current meter inclination. and temperature were measured every 1.5 min in 32 set sampling bursts. All
tidal frequencies were suppressed in the current meter data using a Gaussian filter (full width-half maximum of 24 h).
From July 1992 to December 1994, 135 bottom
water samples and 71 water column profiles were
collected [15]. Water samples for 222Rn (t,,? = 3.82
day) and “6Ra (t,,? = 1620 yr) analysis were collected in 4-l evacuated glass sampling bottles using a
submersible pump that drew water from depth directly into the bottles. All samples were analyzed for
“‘Rn (and later for ‘I6 Ra) using a standard approach
for “‘Rn extraction from seawater by sparging with
helium [ 16,171. The unsupported radon (total “*Rn
at time of analysis minus 226Ra) was decay-corrected
back to time of sampling to obtain the in situ excess
J.E. Cable et al./
Earth and Planetary
“‘Rn. All “>Rn concentrations
are reported as unsupported and corrected to time of collection.
Pore waters were sampled at K-Tower by SCUBA
divers using a sediment pore water ‘sipper’, which
consisted of a depth-calibrated 53.5 cm long, 6.4 mm
I.D. stainless steel tubing with a perforated bottom
copper tip. Pore water was collected by inserting the
screened copper tip into the sediments to the desired
depth and drawing water into a syringe. Each pore
water sample was transferred from the syringe to an
evacuated 125 ml glass sampling bottle for analysis
at the water surface.
Radon-222 sediment-water
exchange rates were
determined by benthic flux measurements
using in
situ chambers [ 181. Divers deployed four seepage
meter-style steel drums (90.7 1; 0.255 m’) by carefully pushing each chamber about 3 cm into the
sediments. The water inside the chambers was stirred
gently using manually operated propellers to simulate a mixed water column. Initial and final (4-5 h
later) water samples were collected from the benthic
chambers and analyzed for ““Rn and “bRa. The
“‘Rn concentrations
were corrected for decay, and
the benthic flux was calculated as shown in Martens
et al. [ 191. Advective seepage rates from the sediments were measured from the benthic flux chambers or using a seepage meter technique [20].
Sediment
equilibration
experiments
were performed to estimate the amount of pore water ‘“Rn at
equilibrium
with the solid phase sediments. Each
experiment consisted of slurrying approximately 85
g wet sediment aliquots with 300 ml of seawater in
500 ml Erlenmeyer flasks for 30 days. After this
period, radon in the water is assumed to be equilibrated with sediment 2’6Ra, and the activity CC,,) is
calculated using the porosity (4) and wet bulk density ( p,,,) of the sediments.
593
Science Letters 144 (1996) S-604
winds, tides, and density gradients. Although outer
shelf currents such as the Loop Current may influence this circulation,
their effects are small and
uncertain.
Winter (December-April)
hydrographic
profiles displayed a well-mixed water column which
is attributed to atmospheric forcing when cold continental air masses move over the Gulf [14.2 I I. Results showed that temperature, salinity, and density
contours were uniformly distributed with depth, while
temperature and salinity values increased slightly
with increasing distance from shore. Net (non-tidal
or low frequency) u and u current velocities show
that surface layer currents were strongest to the east
with an average II velocity of 3.2 cm set-’ between
December 199 1 and April 1992 [ 14,151. Duplicate
current meter moorings at 17 m deep ( 1 mab) and 50
m apart were highly coherent, showing that currents
moved north-northeast
at more than 2.2 cm set _’
during the winter deployment period.
During the summer (June-August),
the inner continental shelf waters develop a strong pycnocline and
are characterized by estuarine-like circulation [2 1,221.
Surface currents (10 mab) measured between July
and October, 1992, were weak and variable in direction, while in July and August, 1993, the surface
currents had a strong easterly component at 2.2 cm
see-‘. The mean u current velocity for the summers
of 1992-93 was 1.0 cm secY’ with a significant
minimum (LJ= 0.5 cm set- ’ ) found in the bottom
waters during July and August, 1992. A progressive
vector diagram (Fig. 2) demonstrates the calculated
time-series bottom water movement t 1 mab) for this
1992 time period remained confined to a region
similar in scale to the study area until the summer
stratification
broke down in the fall. These data
provide additional documentation
that the most quiescent, stable water column in this part of the northeastern Gulf of Mexico occurs during the summer
months.
3. Results and discussion
3.2. Water column distributions
3.1. Hydrographic characteristics
nental shelf ti’aters
of “‘Rn
and 2’6Ra
of the inner conti-
For the purpose of this study, the inner shelf is
defined as being seaward from the coastal barrier
islands to the 20 m isobath. The circulation on the
West Florida inner shelf is dominated by surface
The formation and degradation of the pycnocline
has a significant effect on the water column distributions of ‘“Rn (Fig. 3). During the summer, the
waters are clearly stratified and “‘Rn activities in
the bottom water reach the relatively high level of
4.10 & 0.13 dpm 1-l (Fig. 3B). The mean summer
594
J.E. Cable et al. /Earth and Planetar? Science Letters 144 t 1996) 591-604
bottom water “‘Rn concentration between 1992 and
1994 was 3.2 & 1.8 dpm ll’ (n = 234; [15]). Comparable bottom water enrichment has been documented
for CH, in the same area [23]. During the summer
stratification,
the bottom waters become efficient
collectors of “*Rn with a source on or under the
seafloor. Without this stratification, the waters would
quickly lose the trace gas signal due to atmospheric
evasion. Based on ‘*‘Rn depletion in surface waters,
Fanning et al. [24] reported an intense sea-air gas
exchange occurring in the waters of the West Florida
shelf due to frequent storm fronts in the Gulf of
Mexico. Thus, a distinct advantage exists in using
the summer, sub-pycnocline
“‘Rn
inventories
to
quantify groundwater flow, since an important loss
term can be neglected.
Radium-226 activities are also shown on these
profiles to illustrate the extent of *“Rn enrichment
or depletion relative to its radioactive parent. Activities of 226Ra measured within our study area between
1992 and 1994 averaged 0.44 + 0.22 dpm 1-l (n =
564), significantly greater than average open Gulf of
Mexico values of 0.106 f 0,028 dpm 1-l reported in
1984 [25]. These higher 226Ra activities provide additional support that another source, such as groundwater, supplies radium and radon to the coastal
ocean.
In the following section, we present a model
which links advective-diffusive
benthic exchange to
horizontal water column transport for conservative
“‘Rn. This model is designed to test the importance
of water column transport on ‘“Rn concentrations
and to estimate the subsurface fluid flux from the
seafloor based on comparisons
of measured and
modeled inventories of “‘Rn in the overlying waters.
3.3. Two-dimensional
mass balance model
3.3. I. General approach
The water column inventory of ‘**Rn represents a
balance between its sources and sinks: (1) benthic
,140
"*lo-i1
-..
100
“...,
KM
Fig. 2. A progressive
K-Tower. Calculated
vector diagram shows net bottom water (1 mab) movement during the July-October
1992 current meter deployment at
water mass movements over consecutive 24 h periods are depicted as distance (km) between dots on the diagram.
J.E. Cable et al. / Earth and Planetary
diffusive-advective
exchange; (2) ‘*‘Rn water column production and decay; (3) horizontal water column advection; (4) eddy diffusion along and across
the pycnocline;
and (5) air-sea exchange. Benthic
diffusive-advective
exchange processes can be further divided into molecular diffusion, sediment irtigation and resuspension by biological organisms or
wave pumping, fluid flow through sediments, and
submarine spring discharge. Results of molecular
diffusion experiments are given in the next section
and demonstrate that this process has a minor effect
on summertime water column *‘*Rn inventories. Although macrofaunal irrigation can enhance exchange
from the sediments, active macrofauna communities
were not observed in this study area or in sediment
cores. Based on control experiments made with seepage meters in the near-shore region of the study area
[26], wave pumping is not thought to have a significant effect on interstitial fluid transport. Some submarine springs are known to occur in the region, but
Activity
(dpm.L.t)
Activity
(dpmL_l)
Science Letters 144 (I%61
u
591-604
c.
SL
Fig. 4. An incremental mass balance of “‘Rn in the sub-pycnocline water column was applied using this box model, which allows
the sub-pycnocline
water column to change with distance up the
continental shelf.
they tend to be small - groundwater seepage appears to be the dominant process delivering SGD
into this area [15]. Since a strong pycnocline develops in the summer, use of a sub-pycnocline
box
model to predict fluid flow from the sediments avoids
the necessity of including air-sea exchange (Fig. 4).
As a water mass moves horizontally
from an
initial boundary to some reference point across the
continental shelf (a Lagrangian approach), three processes will affect the water column radon activities:
(1) decay of initial water column ‘Z’Rn; (2) incremental addition and decay of radon from benthic
advection and diffusion; and (3) exchange of radon
across the pycnocline. All three of these processes
are addressed in the model which links benthic fluxes
to the horizontal water column transport.
The “‘Rn flux balance can be written as:
‘\‘iAo
+
Jben
+ (Ga)Vn - vsC,A,- Jpyc
-(*C,“)V”-(~,,C,~)At=O
Fig. 3. Representative water column profiles at K-Tower demonstrate seasonal “‘Rn distributions with depth (m). Activities (dpm
I-’ ) of total “‘Rn (open squares) and 226Ra (closed diamonds)
are given for: (A) 25 May 1993; (B) 10 August 1993; (C) 16
Septemebr 1993; and (D) 24 February 1994.
595
(1)
where TV,is the low frequency current velocity; C,
represents the initial radon activity entering the region from offshore; A, is the area of the offshore
side of the study area; Jben is the total advective-diffusive benthic flux; A is the “‘Rn decay constant
(1.25 X lo-”
min-‘);
AC,, and AC,, represent
water column radon production and decay, respectively; V, is the volume of water in individual box
segments; C, is the final radon activity horizontally
exiting the box inshore; A, is the area of the exit
side (inshore) of the lz* box; JPYc is the radon eddy
596
J.E. Cuble et al. /Earth
and Planetaq
diffusive flux across the pycnocline; uuP is the water
velocity across the pycnocline; and A, is the area of
the upper surface of the pycnociine.
Most of the
parameters described in the flux balance were measured or easily calculated (Table 1).
Eddy diffusive fluxes across the pycnocline were
calculated from Fick’s first law using the observed
“‘Rn concentration
gradient across the pycnocline
(dC/dz)
as a function of depth and the estimated
vertical eddy diffusivity (K,):
JPYc= K,
i
$
(2)
i
The inversely proportional relationship reported
between the local buoyancy gradient and vertical
eddy diffusivity from 14 GEOSECS profiles of “‘Rn
and “‘Ra demonstrated K, values ranged from 5 to
440 cm” set- ’ near the seafloor [27]. Using their
oceanic buoyancy flux (-4 X 1O-6 cm’ sece3>, K,
values of 3.4 (* 1.8) X lop4 cm2 set- ’ (mean i
1a; Table 1) were calculated from the depth of the
greatest density gradient in seven hydrographic profiles collected in our study area.
Horizontal eddy diffusivities (K, > can also affect
Table I
Mass balance input parameters
Science Letters 144 (1996) B-604
the distribution of “‘Rn in near-bottom ocean waters. Lietzke and Lerman [28] report “‘Rn K, values ranging from 10” to lo6 cm’ set ’ for a
borderland basin off California. We estimated horizontal “*Rn exchange between the study area and
adjacent coastal segments and offshore waters and
found the exchange to be very small, due to the low
concentration
gradients observed in the area. This
low exchange rate implies that SGD occurs even
further offshore, and our groundwater discharge results represent a minimum. Horizontal exchange is
estimated to be the same order of magnitude as
molecular diffusion from bottom sediments.
First, the benthic exchange model used to obtain
J ben is described. Jhen is estimated from the upward
fluid velocity advecting across the sediment-water
interface necessary to balance measured 222Rn inventories. The linked model approach uses four simplifying assumptions:
(1) benthic fluxes are spatially
and temporally uniform; (2) 2’2Rn activities are vertically uniform within the deep layer; (3) the subpycnocline water column geometry is a simple wedge
shape; and (4) horizontal fluid flow in sediments is
small relative to vertical flow.
for the linked model
Description
Input parameters
n
Slope of the continental shelf
Eddy diffusivity, K, (lOel cm’ see- ‘)
Porosity of sediments, 4 (ml cm- ‘) ’
Sediment wet bulk density. pwrt (g cm-‘) b
Mean study area sediment 5’s (lOJ dpm rne3)
K-Tower sediment, C,, (10 dpm m-j) ’
Overlying bottom water, C, (IO4 dpm m-‘1 d
Upper pycnocline “‘Rn (IO” dpm m-‘)
Sub-pycnocline ““Rn, C, (10” dpm mm’)
Submarine spring “‘Rn, C,, (IO’ dpm m-“1
Aquifer ‘*‘Rn, C,, (10’ dpm m--r)
-4.5 x 10-j
3.4 i. 1.8
0.42 t 0.04
1.9 f 0.1
7.1 f 2.6
4.4 f 0.7
0.46 + 0.11
0.11 +0.07
0.21 i 0.13
54.3 + I.0
55.7 i 8.7
13
13
13
3
12
34
31
‘
Reference
7
I
48
[I51
[ISI
(mean i 1D ) were measured from 1992 to 1994.
a Porosity. 4 = ((A X W,/p,,
))/((A X We/p,,)
+ (1 - Wo X A/pdry )) where ps, is the density of seawater; A = [ 1000/((1000 35.174X3/35))]
corrects for the salt content of interstitial fluids: S is the measured salinity of the overlying bottom waters; pdryis the dry
grain density as measured by volume displacement of a weighed aliquot of sediment in a known volume of water; and Wn = [(wet sediment
weight-dry
sediment weight)/wet
sediment weight] is the fraction of water present in sediment.
b Wet bulk density, p,,( =[~xP,,l+[(l-~)Xpdryl.
’ Sediment equilibration activities are in units of disintegrations per minute (dpm) per cubic meter of wet sediment.
d Overlying bottom water “‘Rn used in the benthic exchange model is specific for each sediment equilibration station. The summer
sub-pycnocline “‘Rn mean concentration inside the study area was 0.32 (50.18) X IO” dpm m-j (n = 234; [I5]).
Water column and sediment I” Rn concentrations
J.E. Cable et al./
Earth and Planetar?
3.3.2. Betlthic exchange model
A one-dimensional,
vertical advection-diffusion
model presented by Craig [29] may be adapted to
estimate radon exchange from sediments:
dC
-=
dt
a?c
K,-i)Z’
ac
(3)
+w7z+P+hc
where C is the radon concentration in the sediments;
z is depth positive downwards;
K, is the vertical
diffusivity;
(a’C)/(az’)
and (aC)/(az)
are the
‘“‘Rn concentration
gradients across the sedimentwater interface for diffusion and advection, respectively; o is vertical advective velocity; P is a zeroorder production term; and hC is radioactive decay.
To model radon activity in sediments, K, is set
equivalent to D,, the effective wet sediment diffusion
coefficient, after correcting for temperature ( - log
D,, = (980/T) + 1.59; [30]) and sediment tortuosity
<D, = $D,; [3 I]). The ‘I2 Rn molecular diffusion coefficient (D, = 1.14 X 1O-5 cm’ set- ’ at 18°C; [32])
becomes 6.0 X 1O-6 cm’ set-’
for a sediment
porosity of 0.42 and a typical summer bottom water
temperature of 28S”C. For ‘2’Rn in the sediments, P
is the result of radon present in pore fluids due to
recoil after production by decaying ?26Ra in mineral
grains (P = AC,,). The C,, term is an experimental
“‘Rn sediment equilibration activity (dpm me3 wet
sediment; Table 1) which can be converted to pore
fluid concentrations
by dividing by the fractional
porosity. Advection,
w, and radioactive decay, h,
represent losses from the sediments and are defined
as negative terms in the model. For “‘Rn, the solution to Eq. (3) reduces to:
(C, - C,,q)[esjsinh(
c=
A’:z
r ‘) j
+ C,,
(4)
where C, is the overlying bottom water radon activity (dpm rne3); z,s is a depth in the sediments much
deeper than the depth where C,, initially occurs; z *
is a one-dimensional
mixing parameter described by
Ds/w; and A = [l + 42 * (A/w)]~.~. which includes
radioactive decay and advection [29]. This equation
was used to model the radon concentration
versus
591
Science Letters 144 (1996) 591-604
depth for theoretical diffusion (w = 0 m min- ’ )
profiles of radon deficiency in the sediments using
experimentally
determined C,, values. When advection in the sediments is considered, information regarding the subsurface fluid radon concentration
is
necessary to estimate the fluid flux across the sediment-water
interface based on radon inventories.
We will estimate upper and lower limits of subsurface advective flow based on field measurements and
the benthic exchange model.
3.3.3. Water column transport model
The effect of horizontal water column transport
on “2Rn concentrations
was estimated from measured current velocities and sub-pycnocline
“‘Rn
concentrations.
Solving the mass balance equation
(Eq. (1)) for C, allows the final radon concentration
to vary as a function of current velocity (7,~)across
the shelf:
c, =
YC,- ,A, + Jben + (AC,,)V,
YA,+ I + A,v,,
- (K,$C/dz)A
+ AV,
(5)
Values for Jbeo from the benthic exchange model
are linked to Eq. (5) so that C, may vary as a
function of advection through sediments (w). Initial
offshore water column ‘2’Rn CC;), which is transported into the study area, decays at a rate dependent
on A,, and time.
In addition, because the dominant current direction in the study area is to the north (shoreward), the
model conserves mass by allowing the sub-pycnocline water column height to decrease as a function of
the average slope of the continental shelf (Table I).
The volume of the individual segments in the box
model are incrementally decreased to account for the
decreasing water column height (Fig. 4). The water
balance for this model is written as:
WA, + v,A, = pupA, + u,A,
(6)
where this balance is solved for uuP, the velocity
water moving across the pycnocline:
oA,
of
+ v,(A, - A,,)
V up =
A,
and is incorporated
(7)
into the mass balance (Eq. (5)) to
598
J.E. Cable et al. /Earth
and Planetarv
account for the change in water volume as the water
moves up-slope and the sub-pycnocline
water column height decreases. This volume conversion is not
incorporated into the benthic flux terms because they
are not volume dependent. Thus, the water column
incrementally
gains radon from benthic fluxes and
loses radon to decay and vertical diffusivity across
the pycnocline during its transit across the shelf.
This Lagrangian approach allows simulation of
the final deep layer water column “‘Rn inventories
after accounting for all known sub-pycnocline sources
and sinks. The radon inventories in the bottom layer
can be converted easily to concentrations
since the
geometry of this layer is known. In the following
sections, mass balance results are presented for diffusive and advective-diffusive
conditions. These model
results are then compared to observational
data to
make estimates about the quantity of groundwater
entering the study area.
3.4. Measured benthic fluxes and “‘Rn
incentories
Diffusion of radon into overlying waters causes a
deficiency in sediments which is greatest near the
surface and decreases progressively downward from
the sediment-water
interface. A theoretical profile of
K-Tower ‘**Rn versus depth using Eq. (4) demonstrates the effect of radon diffusion in an undisturbed
sedimentary environment (solid line; w = 0; Fig. 5).
The diffusional
loss of radon into the overlying
waters near the sediment surface creates the near-interface deficiency, while the ‘equilibrium’ value at
depth represents a balance of radon decay with input
via recoil and other processes from the 226Ra present
in the solid phase sediment. Although diffusion can
provide a significant contribution of radon to overlying waters, advective processes can greatly enhance
these diffusional inputs. For example, macrofaunal
irrigation of sediments can mix pore waters to greater
depths and release additional dissolved constituents
to the overlying waters, as was shown for several
California borderland basins of the western Pacific
Ocean [4].
The sandy, shell-hash sediments near K-Tower
contain an average dry sediment 226Ra activity of
0.80 f 0.20 dpm gg’ (n = 5). Based on many observations made by divers, the shelf sediments in the
study area do not appear subject to much macrofau-
Science Letters I44 (1996) 591-604
Activity
0
(dpm.L.’
20
40
wet
sediment)
60
80
0.00
Fig. 5. A theoretical profile of “‘Rn concentrations
(solid line:
w=O; dpm I-’ wet sediment) in K-Tower sediment calculated
using the benthic exchange model (Eq. (4)) shows the “‘Rn
deficiency near the sediment-water
interface. Sediment core equilibration activities (squares) and pore water profile activities (#I,
circles; #2, triangles) from K-Tower are also shown for comparison
nal irrigation. Setting the advective velocity to zero
in Eq. (4) and using the C, and K-Tower sediment
C,, parameters previously described (Table 1). a
diffusive flux of 133 & 7 dpm m-’ day-’ is estimated for this site. In addition, equilibration of the
K-Tower sediment core sections yielded activities
similar to the C,, theoretical value (squares, Fig. 5)
confirming the use of this approach. The flux from
Eq. (4) also compares well with a diffusive flux of
128 f 8 dpm m-’ day- ’ calculated from these results using a Martens et al. [19] depth-independent
equation:
J = (D,h)““(C,,
- C,)
(8)
where J is the flux calculated for the change in
concentration
between the sediments CC,,> and the
overlying water column CC,). Sediment analyses
were also performed at numerous
other stations
within the study area. The mean (_+ 1CT>diffusive
flux for the entire study area based on C,, determinations from sediment equilibration
experiments (n
= 13) and Eq. (8) was 230 & 110 dpm rnp2 day-‘.
Pore water profiles also collected in K-Tower
J.E. Cable et al. /Earth
and Planetary
sediments about 5 m apart confirmed that diffusion
into the overlying waters is relatively small. These
profiles are shown with the sediment core equilibration activities in Fig. 5 (circles, #l; triangles. #2).
Pore water profile #2 displays a trend similar to the
theoretical curve. while profile #l may indicate
excess ‘“Rn. The excess activity could be due to a
number of factors, including the presence of advecting fluids containing high “‘Rn or sediment inhomogeneity.
Both pore water profiles demonstrated “‘Rn concentrations lower than expected near the sedimentwater interface based on the theoretical considerations. This greater deficiency suggests that a physical
process (irrigation or wave pumping) may be affecting the sediments in the upper 7 cm. The total radon
deficiency was estimated in the sediments by first
taking the difference between the K-Tower sediment
C,, and these pore water “‘Rn concentrations. These
deficiencies
were then integrated to the depth at
which pore water “‘Rn approximately equalled sediment C,,. If one assumes that these profiles are
steady-state, fluxes to the overlying water can be
estimated in the same way that estimates are made
by integrating the unsupported “‘Rn water column
activities. These fluxes (350-450 dpm mm’ day- ‘1
indicate that even an external process such as wave
pumping cannot account for the observed water column “‘Rn inventories. They would only support an
inventory of approximately
2,200 dpm m-‘, while
measured summertime inventories in this area were
typically around 30,000 dpm m-‘. Some other process must be contributing
significant amounts of
“‘Rn to this system.
Measured benthic fluxes and calculated diffusive
fluxes did not yield similar results. Benthic flux
chamber measurements showed a mean “‘Rn flux of
2520 + 670 dpm mm2 day-’ (&la;
n=6> at KTower between August and October, 1993. Several
ports on these chambers were open to allow seepage
and provide a measure of the total benthic flux.
Advective fluxes appear to be a very important part
of the total Jhen estimate. The mean seepage velocity
at this site, measured via seepage meters. was 1.2 1
k 0.49 cm day _ ’ ( -t 1(T; n = 8; equivalent to 8.5 f
3.3 ml rnp2 mini ‘), confirming that advective processes enhance the material flux across the sediment-water
interface.
Science Letrem 144 (1996) S-604
599
Inventories of “*Rn measured in the overlying
water column vary seasonally (Fig. 3). During the
summer, radon is significantly more concentrated in
the bottom waters. with a typical summer subpycnocline inventory of 30,000 dpm m-‘. Assuming
the seafloor is the only significant input, these water
column inventories would require a benthic flux of
5.400 dpm rn-’ day-’ - over one order of magnitude higher than the estimated diffusional inputs. It is
clear from these measurements and calculations that
diffusion accounts for a minor portion of the observed water column “‘Rn activities.
The dispersed nature of groundwater flow through
sediments makes precise identification
of seepage
areas difficult. Even though this flow may be very
slow and its occurrence
‘patchy,’ its widespread
distribution
could result in a significant
regional
input of groundwater and recirculated seawater. Advective fluxes from sediments were estimated from
the linked benthic exchange-horizontal
water column transport model presented above. An initial
water column radon concentration CC,) was defined
as a mean of several offshore measurements outside
the study area. Average surface layer “‘Rn activities
measured immediately above the pycnochne during
the summer months were used to calculate the diffusive flux across this density gradient.
The physical limits of the study area were set at
2 1 X 30 km, and we chose a 10 km reference point
for modeling, because it is approximately
centered
along any north-south
(onshore-offshore)
tract in
the study area. An upper limit for current velocities
(3.2 cm sect’ ) used in the model was based on
winter velocities measured at K-Tower between 199 1
and 1993 [ 141. One drawback to our approach is the
requirement that estimates of benthic advective fluxes
must be considered uniform across the shelf. It is
likely that seepage velocities will be underestimated
in one location and overestimated in others. Point
sources, such as submarine springs, would be integrated with areas where little or no actual flow may
be occurring. However. this approach does allow
estimates of regional submarine fluid discharge, the
overall objective of this study.
Our analysis shows that the main factor control-
600
J.E. Cable et al. /Earth
and Planetary Science Letters 144 (1996) 591-604
ling the *“Rn concentration in a water mass moving
north (shoreward) on the continental
shelf for a
given distance (I 0 km) is the magnitude of benthic
inputs. Net current velocities do have a smaller, but
noticeable, effect. When “‘Rn sediment diffusion is
the only benthic contribution (w = 0) to the water
column, the dominant control on the water column
“‘Rn activity is the balance between radioactive
decay and this diffusive input. Thus, the model
allows determination of the average seepage velocity, assuming one can estimate or measure: (1) net
horizontal current velocity; (2) “‘Rn concentrations
in the advecting fluids; (3) sediment porosity; and
By assigning
(4) water column “‘Rn inventories.
values for the sediment fluid radon concentration and
the current velocity, the seepage velocity may be
estimated by successive approximation
until an appropriate balance is established with the measured
water column ‘*‘Rn inventory.
The concentration of “’ Rn in the advecting fluids
is an important yet difficult parameter to evaluate
precisely. Several approaches may be taken to obtain
estimates of “‘Rn in these advecting fluids. For
example, the “‘Rn concentration of waters discharging from a submarine spring located in the near-shore
portion of our study area may be representative of
the groundwater flowing offshore. The relatively high
“‘Rn
activities associated with groundwater
and
springs should provide a minimum estimate of SGD,
since this concentration is at the upper limit of what
might actually be encountered.
Actual pore water
concentrations
are lower because of mixing with
infiltrated seawater. Lanark Spring, located about
100 m offshore, is characterized by a “‘Rn activity
of about 540 dpm I-’ [15]. In addition, groundwater
collected along the coastline from shallow private
wells in the same area showed that 95% of the
measured “‘Rn activities (n = 48) were less than
4000 dpm I-‘, with a geometric mean value of
560 k 90 dpm I-’ [ 181. Since the radon concentration in groundwater probably changes by mixing as
well as decay during transit, lower concentrations are
likely to occur offshore. An alternative to these
spring or aquifer values would be sediment equilibration radon values. These lower limit estimates for
radon in the advecting fluids should give an upper
estimate of the quantity of groundwater entering the
region through the sediments. In areas where dissem-
?_
________.__...-.-~~
___._____.-------
_____..._0=~.~
I-
O
0
0.5
I
15
2
I
25
3
:5
Onshore Current Velocity kmmx-1)
Fig. 6. Simulations of “‘Rn concentration
(dpm I-’ ) “tYS”S
horizontal (onshore) current velocity (cm set-’ ) under various
benthic advective conditions demonstrate the influence of these
processes on “‘Rn concentrations.
Radon in the subsurface pore
fluids are assumed to be equal to C,, (solid lines; 7.1 +2.6X IO”
dpm m-j) and C,( (dashed lines: 54.3 + 1.O X 10’ dpm m-j). All
other conditions set for this simulation are given in Table I.
inated seepage flow or recirculated
seawater are
thought to be the main process by which subsurface
fluids enter continental shelf waters, C,, is probably
the best estimate of subsurface “‘Rn. Direct measurements of seepage performed in the area have
suggested that seepage is probably the dominant
process here [ 151.
The combined effects of horizontal transport and
benthic advection on the final water column “‘Rn
concentration are observed when the final concentration CC,) is plotted versus current velocity (Fig. 6).
Typical observed sub-pycnocline
“‘Rn
activities
(2-4 dpm I-’ ) and summer current velocities (0.51.5 cm set- ’ ) are also shown for comparison. Sediment equilibration CC,,; solid lines) and submarine
spring CC,,; dashed lines) concentrations
were both
used for these simulations. At current velocities less
than 0.5 cm set-‘. “’ Rn inputs are balanced almost
exclusively by decay. As the current velocity increases, 222Rn from benthic inputs has less time to
accumulate in the water column, and these net velocities play a more active role in controlling the radon
concentrations
in the water column. Using both C,,
and C,, scenarios, this model demonstrates that ob-
J.E. Cable et al./
Earth and P1anetar.v Science Letters
served “’ Rn values can be predicted easily, and that
the required upward advective velocities may thus be
estimated.
Using field measurements
of radon inventories
and current velocities as a guide, we estimated the
magnitude of advective flow into the study area
(Table 2). Benthic advective velocities, based on
sediment equilibration
and groundwater
activities,
must range between 2 and 10 cm day- ’ to maintain
a typical summertime water column “‘Rn inventory
of about 30,000 dpm mm ‘. The evidence presented
by the observed water column and benthic flux
measurements strongly suggests that advective transport of “‘Rn through sediments is an important
process in this area.
Based on the range of modeled seepage velocities
and a study area size of 620 km’, a regional flux of
subsurface fluids into the study area is estimated to
range between 180 and 7 IO m3 set- ‘. This input is
of the same magnitude as the Apalachicola River. the
largest river in Florida, which has an average freshwater flow of 720 m’ see- ’ [33]. Wakulla Springs,
one of the largest freshwater springs in Florida.
discharges about 1 IO m3 set- ’ of water into the
Wakulla River [9] and ultimately into the northeastem Gulf of Mexico. We estimated the flow from
Lanark Submarine Spring at 0.041 m3 set- ’ [ 151,
while the estimated discharge from a single vent at
Mud Hole Submarine Springs south of Tampa Bay
was only 0.025 m3 set- ’ [I I]. Thus. the regional
benthic fluid flux entering this area, estimated by
balancing the radon inventories in the overlying water column. is clearly significant - perhaps 50-250
times the size of a first magnitude spring ( 2 IO0 cfs;
2.8 m3 set- ’ ). Although submarine springs were not
observed or reported to discharge as ‘boils’ in this
601
144 119961 S-604
offshore area, we recognize that such a process could
result in significant loss of “‘Rn by atmospheric
degassing. Should this actually occur, however, our
discharge results would be underestimated because
of the unaccounted for loss.
It is important to note that much of the advecting
fluid is probably a mixture of groundwater and recirculated seawater driven by groundwater discharge
processes. This model cannot distinguish the difference in the ultimate sources. However, comparison
to estimated fluxes of known submarine springs suggests that disseminated seepage and recirculated seawater are much more important in this area. AIthough seepage fluid flux velocities (cm day-‘) may
seem small in comparison to the velocity of water
exiting a river or spring, the large spatial distribution
can provide a volumetrically
important source of
fluids and dissolved constituents to the coastal ocean.
The coastal waters in the South Atlantic Bight are
significantly
enriched in Z2hRa and groundwater is
the likely source of this excess radium [34]. Based on
a mass balance approach. Moore calculated that the
groundwater flow may represent up to 40% of the
river flow into this region. A similar discharge estimate based on the “hRa inventory of the subpycnocline waters from the Gulf of Mexico study
area is given here.
The average summertime lZhRa concentration
in
these waters is 0.46 + 0.17 dpm I-’ (n = 168). Since
open Gulf of Mexico waters contain an average of
0. I 1 dpm I - ’ [27], we have an excess of 0.35 dpm
I-’ to account for. River flow in this area is dominated by the Apalachicola River, which has a total
Table 2
Estimates of the benthic advective fluid flow into the 630 km’ study area
-7,
--- Rn source
“‘Rn in advecting
Current velocity
C,, . Groundwater
, Sediment
Total subburface fluid
(cmsec-‘1
560 f 90
0.5
2.5
180
1.5
2.7
190
9.3
670
Wells
C,,,
Velocity of pore
fluids (dpm I-’ )
169 5- 62
0.5
Equilibration
1.5
fluids (cm day
10
’)
flow cm3 set- ’ )
710
The two different estimates used for the fluid “’ Rn concentrations were: (1) groundwater wells: and (7) sediment equilibration experiments.
The estimates are also made for both 0.5 and
inventory of 30.000 dpm rn- ‘,
I .5 cm
set-
’
current velocities and are based on a typical summer sub-pycnocline ‘“Rn
602
J.E. Cable et al./ Earth and Planeta? Science Letters 144 (19961 591-604
radium discharge of 8.08 X 10’ dpm day-’ [35].
Dividing that discharge by the volume of the study
area (620 km’ X 10 m bottom layer thickness = 6.2
X 10” 1) results in a radium flow per day into the
study area of 13 X 1O-4 dpm 1-l dayy’. We estimate the average residence time for water in the
study area at about 24 days, based on the average
summertime
net flow (N 1 cm set-‘>
from the
current meter data. The river contribution
to the
radium balance is thus 13 X lop4 dpm 1-l day-’ X
24 days = 0.03 1 dpm 1~ ’ , less than 10% of the
‘excess’ 226Ra observed.
The net excess that remains to be balanced is thus
0.35 dpm 1-l minus the river contribution (N 0.031
dpm 1-l) or about 0.32 dpm 1-l. We convert this to
a required radium activity per time estimate by scaling the excess to the entire volume of the subpycnocline waters (6.2 X 10” 1) and dividing by the
24 day residence time. This results in an estimate of
8.2 X 10” dpm day-‘, which must be balanced by
another source, most likely subsurface fluid flow.
Diffusion of ‘26Ra from bottom sediments can only
account for approximately
l-3% of this excess for
the time scale being considered. At this point, one
may calculate the discharge by estimating the ‘26Ra
concentration
of the advecting fluids. Taking the
calculated excess 2’6Ra to be balanced (8.2 X 10”
dpm day- ’ ) and dividing by the geometric mean for
‘26Ra in Florida groundwater (excluding the phosphate mining district) of 2 dpm 1-l [36], we arrive at
an estimated discharge into the study area of 500 m3
very close to the middle of the range
set-’ (180-710 m3 set-‘) based on the *“Rn model.
4. Summary and conclusions
Radon-222 is a useful natural tracer of groundwater discharge into the coastal zone because: (1) it is
chemically conservative and easily measured; (2) it
is 3-4 orders of magnitude more concentrated in
groundwater than seawater; and (3) it decays at a
rate comparable to the time scale of many coastal
processes. Regional estimates of coastal subsurface
discharge may be made by application of a box
model based on deep water ’ 2Rn inventories.
In
order to apply this model for estimating fluid dis-
charge, a stable sub-pycnocline
system must be present and some information on current flow and 2’2Rn
concentrations
in the coastal aquifer, sediment pore
waters, water column, and sediments must be available. The radon approach works well in our inner
shelf environment
because the density stratification
prevents
significant
transport
of benthic-derived
species through the pycnocline and the slow circulation ensures that there is little net flow over a time
period comparable to the mean life of *“Rn. As is
common in many coastal areas, the major loss term
for 2”Rn becomes radioactive decay, which can be
easily modeled.
Hydrographic
measurements
of the inner shelf
waters in a small area of the Gulf of Mexico showed
that a two-layered, estuarine-like system develops in
the summer in which the bottom layer moves in a
shoreward direction while the surface mixed layer
moves offshore. This two-layered structure allowed
us to construct a mass balance for 222Rn for just the
sub-pycnocline
layer, eliminating the need to consider atmospheric losses. Based on measured current
velocities and the size of our study area, radon
transport to or from adjacent coastal areas was unimportant under normal flow conditions. The magnitude of horizontal eddy diffusivity is low, similar to
molecular diffusion from sediments, and does not
significantly
influence
the “‘Rn
budget. During
sluggish summer circulation, advective transport of
water masses across the continental shelf influences.
but does not control, the measured water column
“‘Rn concentrations.
Entry of lower-concentration
offshore waters into the region is accounted for in
the model and exchange with adjacent coastal areas
will not result in any significant variation in radon
inventories.
We demonstrated
that water column
transport and diffusion from sediments are insufficient to support the measured “‘“Rn inventories. The
only other reasonable source for this radon is via
sediment pore fluids, relatively concentrated in radon,
advecting from the seafloor, as supported by benthic
flux experiments and direct seepage measurements.
This fluid flow, probably a combination of groundwater and recirculated seawater, is thought to be
driven by the hydraulic head on the coastal aquifers.
Estimates of the magnitude of this flow indicate that
this source is volumetrically important - comparable in discharge to the major rivers of Florida.
J.E. Cable et al. / Earth and Planetary Science Letters 144 (19961591-604
Acknowledgements
Project funding was provided by the National
Science Foundation
(OCE-9 I-01 797) and Earthwatch, Inc. to WCB, JPC. and GLW. The efforts of
Capt. S. Wilson of the R/V Semiptole, G. Bugna, P.
Cable, R. Corbett, and M. Shultz, and many Earthwatch volunteers are gratefully acknowledged
for
their help in completing this work. D. Hunley and J.
Ritch of the FSU Current Meter Facility built, programmed,
and deployed the current meters. R.
Harkema compiled the current meter data after experiments were complete. The FSU support staff and
facilities of the Academic Diving Program, Marine
Laboratory, and Oceanography Machine Shop were
essential to our research. We thank W. Sturges, J.
Carroll, and W.S. Moore for their helpful comments
on earlier versions of this manuscript. [MKI
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