art%3A10.1007%2FBF00003146
Transcript of art%3A10.1007%2FBF00003146
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Biogeochemistry
1 : 237-255,
1990
1990 Kluwer Academic
Publishers.
Printed
n
the
Netherlands
Radium
in the Suwannee
River
and
Estuary
Spring
and
river
input to the Gulf
of Mexico
WILLIAM C.
BURNETT,'
JAMES
B. COWART
2
&
SUCHINT
DEETAE
3
Dept. of
Oceanography,
Florida
State University,
Tallahassee,
FL 32306;
USA;
2
Dept.
of
Geology, Florida
State
University,
Tallahassee, FL
32306; U.S.A.;
3Dept. of
Marine
Science,
Faculty
of
Fisheries,Kasetsart
University,
Bangkok, Thailand
Key
words: radium,
Suwannee River,
Gulf of
Mexico, submarine
springs,
isotopes, Florida
Abstract.
A two-year
study of radium
in
the
Suwannee River has
shown
that
groundwater
discharge,
via
springs,
is
a
very
important
source
of radium both
to the river
and to offshore
Gulf
of Mexico waters.
Dissolved
radium is maintained
within
relatively
narrow
limits in the
river by uptake
into suspended
particles.
In the estuary,
dissolved
radium versus
salinity
profiles
show
distinctive
nonconservative
behavior
with
radium
in
significant
excess of its
linear mixing value
at
mid-salinities. Unlike the
situation
in
many other
estuaries, however,
desorption
of radium
from particles
cannot
account
for most
of
the
observed
excess.
Thus,
the
anomalously
high
radium characteristic
of
much
of the west
Florida
shelf
apparently does
not
have
a riverine source.
Direct
effusion
of
high-radium
groundwater
into these
coastal
waters
is
thought
to
be
the major supplier
of
radium,
and
perhaps other elements
as
well.
Introduction
The principal
delivery
pathways
of
uranium-series
isotopes
to the oceans
are:
atmospheric
deposition, river
runoff,
and
in
situ
production in
ocean
water
(Krishnaswami
&
Lal 1982).
In the
case of
radium, there
appears to
be two
significant
pathways,
river
input
and
diffusion
from
bottom
sedi-
ments
(Cochran
1982).
In some
situations,
it
is possible
that other
sources
are important
as
well. The
waters
of the
west
Florida
shelf
have
been
documented
as
an area
where
226
Ra
and
222
Rn
concentrations
are
in
sig-
nificant
excess
of
their open-ocean
values
as well as
having
concentrations
higher
than
typical
of
other shelf
areas (Fanning
et
al.
1982).
These
excess
values
were
attributed
by
Fanning
and
his colleagues
to
input
of
radium-rich
waters
from rivers
that
drain
the uranium-rich
phosphatic
strata
of Florida.
A later
study by
the
same
group
showed that
the radium
and
radon enrich-
ments
extend
far to
the north
of
their
original
study
area,
including
areas
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238
which
should not be influenced
by
the
occurrence
of phosphate
mineraliza-
tion (Fanning
et
al.
1987).
The possibilities for radionuclide enrichment of
the shelf
waters
were
thus
limited
to
diffusion
from
bottom
sediment
and
direct
discharge of groundwater into
the waters of the continental shelf.
We have made a study of radium
in the Suwannee River and estuary, one
of the most
important rivers in
Florida
in terms of discharge
and
radium
input into
the Gulf
of
Mexico. This
paper
summarizes the
results of two
years
of
monthly
sampling at
seven stations
in the river and
several sampling
profiles
within
the estuary.
Our
study
included measurements of particulate
as
well
as soluble
radium, thus allowing
us
to evaluate the
contribution
of
not
only
the 'river-borne
radium', but 'desorbed-radium'
as well. Our
results
will
show
that
although desorption
of
radium apparently
does
occur
within
the
estuary,
it is insufficient to account
for
the high
concentrations
found in
the
offshore
waters. We conclude,
based on this evidence,
and the nature of
the offshore sediment,
that
the
only plausible
source for the excess radium
in
this area
is from
submarine
springs
and seeps. If this
is the
case
in the
other offshore
areas around Florida, as
we suspect that it
is,
the direct
effusion of groundwater into nearshore
waters may
constitute an
important
source,
not only
of radionuclides,
but of
several
other
classes of
elements as
well. Uranium-series
isotopes, therefore,
may
be useful
tracers of
the
influ-
ence
of groundwater on
the coastal ocean.
Radium in ground
and
surface waters
The
occurrence of radium
isotopes in
natural
waters
is
a
function of the
content
of
their
parents
in the
host
matrix, the geochemistry of
radium
and
its
parent
isotopes,
and
the
half-lives
of
the various
radium
isotopes. When
the aquifer
matrix
is weathered and
leached
by groundwater,
uranium and
radium
can
be
readily mobilized,
transported,
and deposited quite
far from
their
source.
Radium
may
enter
groundwater more readily
than
its
radioac-
tive
parents,
uranium and
thorium,
due
to
chemical
and
crystallographic
differences between these
radionuclides.
The mechanisms
that
cause radium to enter
groundwater
are:
- dissolution
of
aquifer
solids;
-
direct alpha
recoil
across
the solid-liquid
boundaries
during
its
forma-
tion
by
radioactive
decay;
and
-
by desorption from particle
surfaces.
The
alpha
recoil process
is a
prime factor
in
the
higher
activity of
progeny
isotopes compared
with their parents
(Osmond & Cowart
1982;
Hess
et al.
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1985). The transport
of radium
is apparently
retarded, however, by
adsorp-
tion
onto
aquifer surfaces.
For
example,
in spite of
the much longer
half-life
of
226
Ra
(1620
years),
the
transport
distance
of
this isotope
is
usually
considerably
shorter than
its
daughter,
222
Rn, with a half-life
of
only 3.8
days
(King
et
al. 1982). Radium
removal from waters
of
Connecticut aquifers was
shown
to be
rapid, as short
as
a
few minutes, with equilibrium between
adsorption and desorption
being quickly
established (Krishnaswami
et al.
1982).
Although the partition
coefficient of
radium in normal
aquifer
con-
ditions strongly favors
the solid
phase,
groundwater generally has
radium
concentrations
several times to
orders of magnitude
higher
than surface
waters
and
seawater.
Groundwater
in
central
and north Florida
has
been
reported
as
having
a
total range
of
0.1-200dpmL
-
'
226
Ra
(Irwin & Hutchinson
1976). The
geometric mean of
226
Ra in groundwater
outside the central
Florida mining
district
has been
reported
at
about 2dpmL
-
',
several
times the U.S.
geometric
mean of 0.3dpmL-'
(Kaufman &
Bliss 1977). Within
the
phosphate
area,
the values are
higher
yet,
with
a range of 3.3-33
dpm
L
-
'.
The
high
activity of
226
Ra
in central
Florida
groundwaters has been
attributed
by
Humphreys (1984) as
due to secondary accumulation
of
uranium
within
the
aquifer
and
to
the
high
content
of total
dissolved
solids
in those waters.
The data
available
for radium
in river water suggest
that activities
are
normally
less
than about 0.1 dpmL
-
'
(Rona
& Urry 1952; Moore
1967;
Bhat &
Krishnaswami
1969). A
compilation of previously
published
data on
radium in
world
rivers,
including
several of those
which
discharge
into
the
Gulf
of Mexico (Scott 1982),
showed
that
rivers
which
drain arid areas
(Rio
Grande and Pecos),
or traverse uranium-enriched
matrix
strata (South
Texas rivers),
and phosphate
deposits (Suwannee River), are enriched
in
radium
2
to 3
times
the
world
average.
Our
study
of
the
Suwannee
River,
reported here and in more
detail in Deetae (1986)
and Deetae
& Burnett
(1987),
suggests
that while radium
in
the
river
is
indeed
high, there
is
no
direct relationship
to
surface
drainage
of the phosphate
deposits.
Hydrogeology
of the
Suwannee
River
and
estuary
system
The Suwannee
River
is the
second
largest
river in
Florida
(Fig. 1) with
an
average
flow
of about
311
m
3
s-
'
to
the
Gulf of
Mexico
(Kenner et
al.
1975).
As
one of the
few
remaining free-flowing
rivers in the
southeastern
United
States,
the Suwannee
is still relatively
pristine because of
the
generally
undeveloped
nature of
its
drainage basin (FDER
1985). The
river
originates
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240
,f
O0
o
WIDTH
OF
DOTTED RIBBON SHOWS
AVERAGE STREAM FLOW
FrTf Kin.
/
0
40
80
Fig.
1. Major Florida rivers
which
drain into the Gulf of
Mexico. All of these rivers
were
sampled
and analyzed for
226
Ra during
this
study.
in the
Okefenokee
Swamp in southern Georgia and flows southward
to
the
Gulf of Mexico with a river length of
approximately 380 km. The
primary
economy within
the drainage
basin is
agriculture
although
a significant
por-
tion
of
the
work
force
is
also involved
in
construction,
mining,
and
manufac-
turing
(FDER 1985). One of he largest phosphate
mining and processing com-
plexes in
Florida
is
located near
the
banks of
he
northern
part of he Suwannee
River
in
Hamilton County.
Two of our
river
stations were
located just
upstream and downstream
of the principal
drainage
from these
operations.
The flow of the Suwannee River increases systematically
downstream,
being
fed
by
three tributaries (Alapaha, Withlacoochee, and Santa Fe
Rivers) and a series of
at least 50 springs. The largest of
these springs are
represented
by 9
first
magnitude
springs, each
having
an average discharge
greater
than
2.8m
3
s
-
'
(100
cubic
feet
per
second). These
springs act
as a
direct
connection
between
underground aquifers
and the
Suwannee
River
(Rosenau
et
al.
1977). By the
time
the
waters of the river reach the Gulf
of
Mexico, a
very significant
fraction
has been
contributed
via
spring input.
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241
South of the Okefenokee
Swamp,
Suwannee
River water
is acidic,
soft,
and tea-colored as it flows
into Florida. Up
until the approximate location
of
White
Springs,
Florida,
the river
is
superimposed on thick
(up
to
100
m)
Miocene
deposits
of sandy
clay, clayey
sand
(including
phosphorite), sand-
stone and
limestone. The
flow for
this
upstream portion of the
river
is
essentially
derived
from
surface
runoff.
Beginning
at
White Springs, the river
channel deepens, its
banks become steeper and
higher as
it cuts
into the
Suwannee
Limestone of the Floridan aquifer. Between White Springs
and
Ellaville,
the Alapaha and Withlacoochee Rivers
contribute
about
15
and
24%,
respectively,
to the
average
flow.
From
this
point downstream,
the
Suwannee River receives
significant
amounts
of spring water
input
although
during
high
stages,
the river
water may provide direct
recharge
to
the
Floridan aquifer. Below
Ellaville,
the river
enters a region of
thin sandy soil
overlying
the Ocala Limestone, also part of
the Floridan aquifer. This
region
is
characterized
by low
relief, few tributaries and
increasingly
numerous
springs
that significantly
change the flow and quality
of the
river
water. Near
the town of
Branford,
the river
channel
broadens
between
low, marshy
banks, typical of the river
from
this
point to the Gulf of Mexico. The
third
major tributary, the Santa Fe
River, enters the river
approximately
16km
below
Branford and contributes
about
15% of
its
flow.
Manatee
Springs,
the last first
magnitude
spring
which
contributes
to
the Suwannee River,
is
located
about 37
km from the
Gulf of
Mexico.
At the river mouth,
the
river
separates
into two main channels (East and
West
Pass)
and the river enters
the
Gulf of Mexico
through
these
channels and numerous tidal creeks.
Methods
In
the river
portion
of
this
study, water
samples
were collected
just
under
the
surface at
fixed station
locations (Fig. 2)
which were part of a
sampling
network for
a two-year environmental
assessment
of the
river by
the State
of Florida (FDER 1985).
Samples were collected at all
stations
on a monthly
basis
for
two years
beginning
in
January,
1982.
For
estuarine
samples,
collection was
based on the prevailing
salinity gradient.
A
hand-held refrac-
tometer
and a portable
inductive salinometer were
used
to
measure the
salinity
in
the field.
More precise analyses
of salinity were performed
later
in
the laboratory.
River samples
(about
20
liters
for
22
6
Ra
analysis)
were
returned to
the
laboratory
the same day as collected
and
each sample
was
filtered through
three
in-line
filters
consisting of a
Whatman glass microfiber
(934-AH),
GF/F, and Millipore
AA (0.45 #m),
respectively.
Filtering of
the
estuarine
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242
GEORGIA
W tE
S
*
Major
spring
o
Other springs
0 Station number
10 20
30
i
~
Km
Fig.
2. Index map
of
the
Suwannee River showing
sampling
stations
and
locations of known
springs.
samples followed a
similar procedure
but
was performed in the
field.
All
of
the particulate
data in
this study were
derived
from
the
combined amount
of
particulate
material remaining
on
all filters
used
during
the
filtering
process.
The
activity of
226
Ra
was
determined
by
three different techniques. Two
of
these
methods were based on the quantitative
extraction
of radium
from
water by
Mn-impregnated
acrylic
fiber.
For samples from the January-May,
1982
sampling period, a slightly modified method
based
on
gamma-counting
peaks
of
226
Ra daughters from a BaSO
4
precipitate
was
used
(Michel
et al.
1981; Kim
& Burnett
1983). Although this method gave satisfactory
results,
we found
that processing
time
was
reduced by collecting M n-fibers
as
above,
and
then
sealing
the
still wet fibers in
125
ml
Erlenmeyer
flasks which were
later used
for
direct
radon emanation after
an appropriate ingrowth
period
of
about 3
weeks
(Moore 1981).
The
third method of
226
Ra
analysis, used for
a
few
subsets
of
the
samples
.1
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243
analyzed as
above, was also
based on randon emanation,
but radon
was
purged directly
from
the samples rather than
from Mn-fiber. For
these
'whole
bottle'
analyses,
approximately
20
liters
of
unfiltered
water
were
sealed in
large glass wine-making
bottles and purged
with helium
to
remove
any excess radon initially present.
After about a 3-week
ingrowth
period, the
supported
radon
gas was purged, collected
and
loaded
into a
Lucas cell for
alpha scintillation counting. The 'whole bottle'
measurements reported
here
should
include some particulate as
well
as soluble radium.
Results
and discussion
Soluble
and
particulateradium in
the
river
The overall mean soluble
226
Ra activity
at all
stations for
the 2-year
study
period
was
22.4 + 8.4
dpm
100
L-'. No measurable
change in
radium con-
centration
was
observed at the stations
located just upstream
from and
downstream from
the
phosphate
mining operations
near
White
Springs.
Although there
was
no systematic
trend with location,
the
upstream
stations
did
tend to
be lower (18.9
dpm
100
L-'
mean for stations upstream of White
Springs)
compared
to downstream
locations
(27.0
dpm 100
L-' mean
for
stations
south of Branford).
This
increase
is
probably
due to the
increased
importance
of spring input further downstream.
Radium
analyses of several
of the first-magnitude
springs which
flow into
the Suwannee River
showed
that these springs contain relatively
high
226
Ra (Table
1). The concentrations
in these springs progressively
increased in a
downstream
(southwest) direc-
tion
as deeper aquifers
become more important
in
supplying spring water
to
the river. If
this
trend continues
out
onto the
continental shelf, the waters
would
be
expected to
be relatively
rich
in radium.
Since
soluble
radium
in the
Suwannee River is a consequence of mixing
of surface
drainage (low radium) and spring water
(high radium), variations
in radium activity related to discharge may be expected. In
fact,
although
there
is some
relationship
(Fig. 3a), it is obviously
not the
only
controlling
mechanism.
We
observed that
radium concentrations remain
in relatively
narrow limits even during
extreme fluctuations of
flow.
The range
in
226
Ra
at
station 6, for example, was 13.9
to 37.2dpm100L-', about
a 3-fold
variation. Discharge at the
same
station during
the same period varied
by
well
over
2
orders
of
magnitude. When
plotted
as
a
time-series
diagram, the
data of
station 6
appear
to
show a
cyclicity
with a period of approximately
6-7 months
(Fig.
3b). High radium
occurs in the early winter
and
summer,
while low
values
appear in the early spring and fall. The cycles
are
not
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244
Table 1. Concentration of
226
Ra
and discharge to
the
Suwannee River
in first
magnitude
springs
measured during
this
study.
Spring
Location
Flow
226
Ra
226Ra
m
3
s-
1
dpm
100
L-`
Discharge
1010 dpm yr-
Blue Lat
30
28'
49
N.
3.3 15.5
+
0.7
1.59
Long
83
14'
40
W.
Falmouth
Lat
30
21' 40 N. 4.5 34.9 +
1.1 4.92
Long
830 08' 07
W.
Troy Lat
300
00'
21 N.
4.7
34.8 + 0.9 5.16
Long 820
59' 51 W.
Ichetucknee Lat
290
59'
02
N.
10.2
30.0
+
0.9 9.64
Long 82
45' 43
W.
Fannin Lat 29
35' 15
N.
2.9 53.9 +
1.3 4.96
Long 82
56'
08 W.
Manatee Lat
29
29' 22 N. 5.1 91.7 + 1.6 14.80
Long 82
58'
37 W.
Total 41.10
obviously related
to
rainfall
or
discharge
patterns
in
the
drainage
basin
and
may
be
related
to
some
other
process.
Analysis of
Suwannee
River
suspended
particulates for
226
Ra
showed
them to be quite high
in radium
from
stations
with a
small
amount
of total
suspended
sediment (Fig.
4). The
fraction of
radium
contained
in the
particulate, as
opposed to
the soluble fraction was
fairly
constant
at all
stations,
usually varying
between
25-35 . Thus,
the
distribution
of radium
between the soluble
and
particulate phases
may be controlled
by particle
interaction
processes.
The distribution
coefficient, KD,
shows that
226
Ra is
more favored
in
the
solid
phase
by
0.5-1.5
x
105
compared
to
the
solution
phase. Although
we were unable
to
positively
identify the solid
phases
in
these
particulate
samples because of diffuse
X-ray diffraction patterns,
it is
likely that the principal
carrier phase for radium
is a
clay
mineral with high
adsorption
and/or ion-exchange
capacities. The
difficulties
identifying and
analyzing the high-radium particulates
were compounded
by
their
very
low
abundance,
usually less than about
2-3 mg
L-'
in
the down-stream
loca-
tions.
A
sample
of sandy
sediment recovered from the
junction of
the
East and
West Passes
at
the
mouth of
the
river
was
size
fractionated
by sieving
techniques
and radiochemically
analyzed.
Results show that although the
fine (< 0.63p
m) fraction constitutes
a very
small
amount (0.06%)
of the
total
sediment,
it
is very concentrated
in
226
Ra
and
2 0
Pb, having many
times
the equilibrium
activity of
238
U (Fig. 5).
This is internally consistent
with the
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245
fl
0
0
E
ct
C'
Nr
Discharge m
3
/sec.)
0
E
c
c
u
gI
E
0
0
0
To
MONTHS
Fig. 3. (a)
Dissolved
22 6
Ra versus discharge
at
station 6
based on monthly
sampling
for
two
years beginning
January,
1982.
(b)
Time-series
plot
of
same
results
as
in
(a).
river
particulate data, and
suggests
that
fine-grained particles are an
impor-
tant
transport path of radium
to
the
estuary.
The activity of
226
Ra on
Suwannee River particulates
are consistently
greater
than
10 dpm
g-',
and
-
8/10/2019 art%3A10.1007%2FBF00003146
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246
346
E
O
c 4
0
10
20
30
40
Suspended
Load mg
/ I
Fig. 4. Specific
activity of
22
6
Ra
in
suspended
particles
versus concentration
of suspended
sediment
in the Suwannee
River.
Results
are for
all stations
collected
in June,
1983 and July,
1984.
E
Q
0
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8/10/2019 art%3A10.1007%2FBF00003146
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247
is
often
considerably greater. This
is
high
compared
to
an average
activity
of only
3.4
dpm
g-' in M ississippi
River particles
(Moore & Scott 1986)
and
a
range
in
the
Amazon
of
1.7-2.6dpmg-'
(Dion
1983;
Key et
al.
1985).
However,
the total amount
of suspended
material
is
far
less
in
the
Suwannee,
thus diminishing the overall
effect of
these
particles as
radium
transporters
to
the Gulf of
Mexico.
All
major
Florida
rivers
that drain
into the
Gulf
of Mexico (Fig.
1)
were
sampled in May,
1986
and
analyzed for
soluble and particulate
radium
(Table
2). These results
show
that
rivers
in
Northwest
Florida (Escambia,
Choctawhatchee,
Apalachicola, and
Ochlockonee)
have low concentrations
of
radium
compared to those
found
in
central
and
southwest
Florida. This
is
most
likely
a
consequence
of
the higher
contribution
of
springs to
the
flow
of the central and
southern
rivers
although the presence of
high-grade
phosphate ore in
the region may
be important in some
cases.
Crude estimates
of
the
radium discharge
to the
Gulf
of
Mexico for
all
the
rivers
of Table
2
show that
the Suwannee
has
the
highest
radium flux of
the
rivers
investigated. The
Suwannee
alone accounts
for
about 25%
of the
riverine
radium
flux
to
the Gulf
of Mexico from
Florida.
All
Florida rivers
contribute
an estimated
1.5 x 10
t 3
dpmyr
-
' compared
to about
3.1
x
1014dpmyr-'
for
the Mississippi
River
alone (Moore
& Scott
1986).
In
spite
of
this
lesser
input,
radium concentrations are much
higher
on the
west
Florida
shelf than
elsewhere
(Fanning
et
al. 1982).
Reasonable
sources
for
this excess
radium include
release
from particles entering
the Gulf,
diffusion
from bottom sediments,
and direct
effusion of waters
enriched in
radium via submarine
springs and seeps.
Radium
entry
into
the estuary
During
our
study,
five
sampling
trips
were
made to
the Suwannee
estuary.
In addition, one profile
was made for the
estuary
of the
Ochlockonee River,
another coastal plain
river in
Northwest Florida (Fig.
1).
When
dissolved
(filtered Mn fiber)
226
Ra
is plotted against
salinity, typical
nonconservative
profiles
result in
most
cases
(Fig. 6).
The
only
exceptions
are
the March
and
June,
1983 profiles which show
apparent conservative
behavior. These
samplings
were made during
periods of relatively
high
discharge.
A
radium
maximum, if
it does occur
during
these
periods, may be located
further
offshore.
Conservative
mixing
curves
were also observed in
the
Pee
Dee
River-Estuary
during
a
period
of
high
discharge
by
Elsinger
&
Moore
(1980). Most of
our
profiles are similar
to
those observed
in
other
estuaries
except
that
the maximum
226
Ra
concentrations are
much
higher, up
to
125
dpm 100 L
-
'.
The high
concentrations
are
in
agreement, however, with
-
8/10/2019 art%3A10.1007%2FBF00003146
12/19
m
mQ W)\~ m
_, C- t
00
r~C, -0,
q
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00 t
t-
0
o 'It x
00
+1+1
71 +1+1
+1 1
,
- a, C)__ _ O o
o 1 o +1
+1
_- rA _
-
'0
O00~ 00 0
Z Z
o+I
1 +1
+1 +1
+1l
O r r'i n-
Z 1C
r- V-
+1
1
+l + +1 +1 +1 +l
+ + +1
0
+.
l
--
r
0
00
Z
.6 cq In.Otm
W - --
v) oo mr
.C,
V _C/ ' -0 C ( _
00 C-
-Z
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-r C-
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CD
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+l .I
o
- O < _ q t . m
0oC -,
C Nf~000000
C2
0 Cl O
O X 00
C-
C7,,,
I Z ,= m > ,n -V)
,ob e X
_ _C-
c)
s) acO
00
00
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0 00
C
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l
0
_
-
00
0)
0
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0C0 _X _ El
x Z u u 0:
i-
< 0o -1
nOCO V)
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X~O D O O O CO CoNO~
m
-. v: 3 0
I
L)
u Z~
248
C >~
_
O :>s
C>
Cd t C6,
o
0
,= It
CO 0
Z
mta
S
OO
00
..
,)
r,
U
.s
'0
-0
.0
V
_ 0
=
0
_
,1
o
E
~
:a
t0
0)
CO
O
oo
c
0)
._
E
E
x
o
K
CO .
C
CO
O
.
C
--
C
00)
C)
o 0
C
E0
E
Co
3
E
.g
C
d
C0E
0CC
.40
00
-C
F;
I
0 I
.3
= m t
E
V-
n
0
E
iY
G
a
0)
0
0
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0
E- .
0+ C v
0
0
C= CC
mC
C C
-
8/10/2019 art%3A10.1007%2FBF00003146
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249
120
-
December
2 1982
80
/ 143
m3/
40
-j
0
0
E
0
N
120
20
80
40
120
80
40
n
sec.
March 3 1983
612
m3/sec.
K.
October
6-9, 1983
173
m /sec.
OCHLOCKONEE
RIVER ESTUARY
January
4, 1984
0 10 20 30
0
10
20 30
Salinity ( .)
Fig.
6.
Dissolved
226
Ra versus salinity in
the Suwannee River estuary for five sampling periods
and one profile of the
Ochlockonee River estuary. The discharge, measured at the most
downstream station,
is given for each Suwannee
profile.
Triangles represent stations located
in the
East
Pass
while circles
represent
the
West
Pass
portion
of
the
estuary.
the
observations
of Fanning et al.
(1982, 1987) of high radium in the waters
of the west Florida
shelf.
The
question
we
would like
to
examine
here is the source of
this
radium.
Specifically,
can
the excess radiumobserved in
the Suwannee Riverestuary be
explained by desorption of
226
Ra
from particles entering the Gulf of Mexico?
This
argument
has
been
successfully applied
to the
study of
radium in
several
estuaries including the Pee Dee, Hudson, Amazon, and
Mississippi Rivers
(El-
singer &
Moore
1980; Li
&
Chan
1979;
Key
et al.
1985;
Moore
&
Scott
1986).
Basically,
the concept states
that radium
occupying
particle
adsorption and/or
ion exchange sites will exchange with divalent cations encountered upon
enter-
ing the
sea.
Thus, an enrichment in dissolved
226
Ra is observed and data points
occur above
an ideal conservative mixing
line when
plotted against salinity.
June
22,
1983
m sec88
m
3/sec.
Z~J
July
10-12, 1984
286
m
/sec.
. i
_ A I
. I
.
I
.
I
I
fn I
I
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8/10/2019 art%3A10.1007%2FBF00003146
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250
E
C.
o
0
cJ
o
0
0
C )
Salinity
(%o)
Fig. 7.
Specific activity of
226
Ra
in particles collected from
estuarine
samples versus
salinity
for
all Suwannee River
estuary profiles.
Upon
initial examination,
our
results
of particulate radium in the Suwan-
nee estuary
appear
to
support
the
desorption hypothesis
as there
is
a
significant drop in the
226
Ra specific
activity
through
the
estuary
(Fig.
7).
The
226
Ra
concentration on
particles at
0 o salinity (located
at
the
junction of
the
East and West
Passes, where
the
sediment
sample of
Fig.
5
was collected)
was extremely
variable with higher
activities
occurring when
the concentra-
tion of
particles
was lowest.
The
offshore
particulates were consistently
lower. Although
these
results
support the desorption
mechanism,
we
feel
that this is not
the
dominant process
here
because:
- most desorption
apparently
occurs at low salinities
whereas
maximum
226
Ra in the
salinity
profiles occurs
at higher
values,
between 15-20%o;
-
there
are
not
sufficient
river-borne particles
to
account for
the
excess
observed.
Suspended matter
in
the
Suwannee
River
estuary
increases with salinity, an
opposite trend
compared to most rivers. The river-end member suspended
load is only about 4mgL
', much
lower than
the
10's to 100's
of mgL
-
`
observed
in
many
of the world's rivers.
Dissolved radium
in the Suwannee River estuary is
a composite from
several sources:
-
dissolved
radium in the river;
-
dissolved
radium
in seawater;
-
desorbed
radium from
river-derived particles;
addition from diffusive
flux from
bottom
sediments;
and
- direct
effusion
of radium from submarine
springs
and
seeps.
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8/10/2019 art%3A10.1007%2FBF00003146
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251
Table
3.
Results
and
parameters
used
to calculate
the relative
amounts of
dissolved an d
desorbed
radium
from
the Suwannee
River
to the Gulf of
Mexico.
The
March
sampling
was
not
used because
of
an
incomplete
profile.
Parameters
Units
20 Dec
82
22 Jun
83 6 Oct
83 10 Jul
84
Overall
Flow rate
Sampling day
(m
3
s
i) 142.7
288.2
172.5
286.0
311.1
Suspended
load
(mg
L
-
') 0.97
2.60
1.50
2.30
1.84
Dissolved
226
Ra
(dpm
100L
-
')
River
end
33.1
32.2 31.6
31.1
32.0
Max.
Sal. end
98.8
103.8 11.1
44.3
64.5
Max.
Conc.
99.3 125.4
81.1 87.1
98.2
Suspended
226
Ra
(dpm
g-)
River end
114.0
50.4
102.9
39.2 76.6
Max.
Sal.
end
1.2
22.6 5.2
1.5
7.6
Desorbed
112.8
27.8 97.7
37.7 69.0
Desorbed
226Ra
(dpm 100
L-
1
)
10.9
7.2
14.7
8.7
12.7
Yearly
Discharge
Dissolved
river
226
Ra (1012dpmyr-')
1.5
2.9
1.7
2.8
3.1
Desorbed
river
2
26
Ra (1012dpmyr
')
0.5
0.7
0.8
0.8
1.3
Total
river
2
26
Ra
(1012 dpm yr-')
2.0 3.6
2.5
3.6 4.4
We
evaluated the
desorbed
radium
contribution
to the estuary
by subtract-
ing
the
radium concentrations
measured
for particulates collected
at
the
high salinity
end
of
our
profiles
from those
measured in
the river.
This
'desorbable'
radium
concentration
is
then multiplied
by
the suspended
sediment
concentration
to arrive
at an amount
of
desorbed radium
which
may
be
directly
compared to the
contribution of soluble
radium from the
river,
either
as a
concentration or
a flux
(Table
3).
Although
we
have
drawn
our trend
lines in
the
estuarine
profiles (Fig.
6)
through the low
and
high
salinity
ends
of each profile,
it
is more informative
to consider
open
Gulf
of
Mexico
2
26
Ra concentrations
as the
potential seawater
contributor.
An
average
of six
outer shelf
and
open Gulf
of
Mexico
values
reported
by
Moore
and Scott
is 12.6dpm 100L
-
',
much lower
than the high salinity
ends
of most of
our estuarine profiles.
With
a typical
maximum
concentra-
tion of
radium in the
estuary of 98.2
dpm
100
L
-
', and
mixing between
a
river
end-member
at
32.0dpmlOOL
' and a
12.6dpmlO0L
-
'
offshore
component,
an
additional
input
of
approximately
60dpm100L
-
'
is
required.
The radium
desorbed
from
river-borne
particulates
can only
ac-
count
for
about
20%
of
this excess.
When expressed
as a
flux, our
estimates (Table
3)
show that the
total
river
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252
output
(dissolved
plus desorbed) of
226
Ra varied from 2.0-3.6
x
10'2dpm
yr-'. An estimated long-term average of
about 4.4
x
1012dpmyr
-
'
is
higher
because the
river flow was generally below
average
during
our
study.
These
calculations show that the contribution of desorbed radium to the
total river output
is
significant at
about 20-30% of the
total.
However,
this
is insufficient
to account
for
the excess
226
Ra
measured in
the
estuary. The
only other reasonable sources for radium are diffusion
from bottom sedi-
ments
and/or
input from
direct groundwater discharge
into
the coastal zone.
The
sediments in the Suwannee
River estuary consist of
quartz and car-
bonate
sands
and biogenic materials
including
oyster
reefs. We
have
analyzed several samples
of this
material
and have found it
to
be uniformly
low
(