8/10/2019 art%3A10.1007%2FBF00003146

1/19

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

8/10/2019 art%3A10.1007%2FBF00003146

2/19

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.

8/10/2019 art%3A10.1007%2FBF00003146

3/19

239

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

8/10/2019 art%3A10.1007%2FBF00003146

4/19

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.

8/10/2019 art%3A10.1007%2FBF00003146

5/19

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

8/10/2019 art%3A10.1007%2FBF00003146

6/19

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

8/10/2019 art%3A10.1007%2FBF00003146

7/19

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

8/10/2019 art%3A10.1007%2FBF00003146

8/19

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

8/10/2019 art%3A10.1007%2FBF00003146

9/19

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

10/19

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

8/10/2019 art%3A10.1007%2FBF00003146

11/19

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

-C~

10

C

c, n o o o o

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

o o~

-r C-

C-

CD

C-

+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

0'0

0 00

C

CC

l

0

_

-

00

0)

0

V)

~

> >

0C

O Y 0

0

0C0 _X _ El

x Z u u 0:

i-

< 0o -1

nOCO V)

V 0

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

C

0

E- .

0+ C v

0

0

C= CC

mC

C C

8/10/2019 art%3A10.1007%2FBF00003146

13/19

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

8/10/2019 art%3A10.1007%2FBF00003146

14/19

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.

8/10/2019 art%3A10.1007%2FBF00003146

15/19

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

8/10/2019 art%3A10.1007%2FBF00003146

16/19

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

(