science for a changing world

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

RADIUM ISOTOPES AS SEDIMENT-WATER INTERFACE 

TRACERS: INDIAN RIVER LAGOON, FLORIDA

by

Peter W. Swarzenski

U.S. Geological Survey 

Center for Coastal Geology

600 4th Street South 

Saint Petersburg, FL 33701

Open File Report 00-482

2000

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North 

American Stratigraphic Code. Reference therein to any specific commercial product, process, or service by tradename, trademark, 

manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States 

Government or any agency thereof.
RADIUM ISOTOPES AS SEDIMENT-WATER INTERFACE TRACERS: 

INDIAN RIVER LAGOON, FLORIDA

Introduction

Isotopic disequilibria can readily occur in ground water and coastal waters 

between pairs of isotopes in the 238U, 235U and 232Th decay series. For example, 

disequilibria between 238U and 234\J (i.e., 234U/238U ratios) have been observed to range 

from 0.5 to over 20 in varied ground waters (Osmond and Cowart, 1982; Asikainen, 

1981; Dickson et al., 1983). Laboratory experiments confirm that uranium isotopes are 

fractionated by alpha recoil effects (Fleischer, 1982; Sun and Semkow, 1998), as well as 

by physico-chemical differences in oxidation states and bonding energies (Benes et al., 

1982). Similar processes also effect the fractionation of other isotopes in the U-Th decay 

series (Fig. 1), such as Th, Ra, Pb or Po (Ivanovich and Harmon, 1982). However, it is 

often easier to explain such disequilibria by the chemical properties and long half-lives of 

the intermediate radionuclides. Decay of 232Th to 228Th, for example, occurs via the decay 

of 228Ra, and in this decay sequence each parent-daughter radionuclide exhibits unique 

geochemical behaviors that facilitate isotopic disequilibria (Kadko et al., 1987). Coastal 

bottom sediments older than ~10 years will therefore likely have thorium activity ratio
238-Uranium Decay Series

23Bu 234 U

(4.5E9a) (2.5E5a)

*

1 (1.2m) 

* 1

r

234Th ' 230Th

(24.1 d) {7.5E4 a)

\ r

226 Ra

(i.f

i

222,

(3.8

V

E3a)

Rn

d)

r

(3.11m)

1

232-Thorium Decay Series

_

232Th 2Z8Th

(1.4E10a) (1.9 a)

f

228 Ac

,r x (6 '1h) r

228Ra 224 Ra

(5.7 a) (3.7

1 

2201

(54

d)

r

Rn

6s)

r

216Po

(0.15s) .

235-Uranium Decay Series

236 U

(7E8a)

^

231 Pa

- (3.3E4a) 

r J*

M1 Th

(1.1 d)

1

227Jh

- (18.7 d) 

r >W

* /

227 Ac

(21.8a)

1 r

223 Ra

(11.4 d)

Radionuclide 

(Half-life)

Alpha 

Decay

Beta

Decay

1

1

^

f 

f

219 Rn

(3.96s)

1 '

216Po

ft -7Q me 1

1

Figure 1. Decay chains of the U and Th series isotopes and the half life of each isotope. 

The vertical arrows represent alpha decay and the diagonal arrows represent beta 

decay.

(228Th/232Th) much less than one (Ivanovich and Harmon, 1982), while the overlying 

water column activity ratios can become quite large (5-30).

In the past, only two of the four naturally occurring isotopes of radium were 

considered to be useful in environmental disequilibrium studies (Cochran, 1980; Rama 

and Moore, 1996). As the daughter of ^Th, 226Ra (t/2 = 1600 yr) is usually present in 

excess of its parent in natural waters, and the greater solubility of Ra over Th facilitates 

the upward diffusion of radium across the sediment-water interface (Li et al., 1977; 

Webster et al., 1994,1995; Boynton et al., 1995; Hancock and Murray, 1996). Current
estimates suggest that about 25% of 226Ra in seawater originates from coastal sediments 

via such diffusion/advection (Ivanovich and Harmon, 1982). Radium-228 (t/2 = 5.75 yr), 

is the direct daughter of 232Th, and like 226Ra, is also found in excess of its parent in 

natural waters due to diffusion/advection (Hammond et al., 1990; Ghose et al., 2000). 

The remaining two radium isotopes, 223Ra and 224Ra, have much shorter half-lives of 11.4 

days and 3.7 days, respectively. Due to recent advances in our detection capabilities 

(Moore and Arnold, 1996) can we now begin to utilize these 'new found ' isotopes as 

short-lived water mass or sediment-water interface tracers (Swarzenski, 1999; Hancock et 

al., in press). Collectively, radium isotopes can thus provide a powerful suite of naturally 

occurring tracers that span a time window from days to 1000's of years.

If we assume a constant rate of supply from a parent nuclide, then the activity of a 

particular daughter isotope in solution can be expressed by the first order decay equation:

At =Aafl-e M), (3-1) J

where t is the contact time between a solid and solution, At is the activity at tune t, A a, is 

the activity at secular equilibrium, and X is the decay constant (X, = ui2/t/2). For either 

223Ra or 224Ra, "kt is considerably larger than 1; even in relatively 'young' waters the 

expression (1 - e~**) rapidly approaches 1 (equilibrium) as t exceeds ~ 50 days. In 

contrast, for long-lived 226Ra, ^226^ is much less than 1, and thus (1 - e~**) remains close to 

the value of ^22$. Therefore, where ground-water residence times are small, one will 

likely obtain high values of 223Ra/226Ra ratios relative to equilibrium (223Ra/226Ra = 

0.046) values, i.e., (223Ra/226Ra), will approach (223Ra/226Ra)a/>i226^ One objective of this
paper is thus to determine the utility of radium as a sediment-water interface tracer in the 

northern section Indian River Lagoon, where it is hydrogeologically feasible that 

'recycled' ground water can discharge into lagoon bottom waters.

2 Sampling and Analytical Methods

The field effort for this project was divided into two sampling trips that were 

timed to correspond to dry (May, 1999) and wet (August, 1999) precipitation seasons. 

During a third trip (December, 1999), several previously deployed interstitial water 

samplers (multi-samplers/peepers) were sampled and retrieved. An unusually large 

number of energetic tropical weather systems (Floyd: 09/99, Harvey: 09/99 and Irene: 

10/99) directly affected our study area by increased precipitation and wind-driven storm 

surge. '

A series of ground water samples was collected during the dry and rainy season 

sampling trips from several private and monitoring wells around the perimeter of our 

field site. Each well was first purged and then sampled using either peristaltic or rotary 

vane pumps. To ensure adequate purge volumes, hydrologic parameters such as pH, 

dissolved oxygen, conductivity, salinity and temperature were simultaneously measured.

Twenty one-liter samples of ground water were collected for radium isotopes. 

Water samples from the lagoon proper were collected from a series of shoreperpendicular transects, from several end-member sources (Turnbull Creek, Haulover 

Canal), as well as from numerous mid-lagoon sites. For radium, large volume (50 - 150 

L) lagoon samples were collected approximately 20-cm off the sediment water interface
with a rotary vane pump. Interstitial waters were also collected for radium analyses using 

three unique methods; conventional-type (diameter = 60 cm) seep meters (Lee, 1977), 

multi-samplers consisting of an array of finely screened tubing, and 60.75-L benthic flux 

chambers. Radium was quantitatively removed from all water samples onto pre-weighed 

MnOa impregnated acrylic fiber packets. The efficiency of Ra adsorption was 

periodically monitored by a second Mn column placed hi series below the first column. 

Uncertainties in Ra isotope activities correspond to ICT counting errors.

Ground water and lagoon surface/interstitial water samples were analyzed for the 

two short-lived radium isotopes and 228Th by delayed coincidence alpha scintillation 

counting. Mn fibers were transferred to the laboratory as soon as possible to minimize 

the loss of radium due to radioactive decay. The fibers were rinsed hi distilled and deionized water to remove sea salts, partially air dried, and then placed into a He gas 

recirculating line linked to a 1.1 -L scintillation cell directly connected to 5" photomultiplier tubes (PMT). Scintillation signals were routed via a Ortec PMT base to a 

delayed coincidence computer program that time discriminated alpha decays of 219Rn and 

220Rn produced by the radioactive decay of 223Ra and 224Ra (Moore and Arnold, 1996). 

The Mn fibers were subsequently re-analyzed after about 4-6 weeks to allow for the 

initial excess ^Ra to equilibrate with 228Th, which also quantifiably adsorbed to the 

fiber. Background activities hi each of the two delayed coincidence systems were 

monitored after every sample run. Isotopic standards derived from 227Ac and 232Th were 

analyzed periodically, as well as after each batch sample run (approximately every 

twelfth sample). Activities of 226Ra were measured on a separate water sample either by 

Rn emanation or by y-ray spectrometry.
3.3 Results and Discussion

3.3.1 Hydrography

Indian River Lagoon (IRL) is subtropical in climate, and precipitation generally 

follows a seasonal pattern. A typical dry season period extends from January through 

May. During both sampling trips in 1999, monthly precipitation values were 

considerably greater (~130 -160%) than the 30-yr accumulated rainfall averages. 

Nonetheless, during the dry season (May 1999) sampling trip, salinity in the lagoon was 

typically in excess of seawater (35%o), averaging 38.1%o. Water column temperatures 

fluctuated daily from a low of 23 - 27 °C. The ~ 9% enrichment in lagoon salinities 

relative to open ocean seawater values reflect heightened rates of evaporation of lagoon 

water relative to water mass renewal rates. The salinity of Turnbull Creek, which 

traverses a large wetland north of IRL, was only slightly below 35%o, and further 

suggests minimal fresh water exchange and long water residence times in the lower 

reaches of this distributary wetland system. Haulover Canal forms the only conduit 

between IRL and Mosquito Lagoon, which is another shore-parallel coastal lagoon 

system that does have limited exchange with the Atlantic Ocean. Small-scale baroclinic 

forces facilitate water exchange through Haulover Canal. The salinity of Haulover Canal 

(36.1%o) indicates an integration of water mass mixing and transport processes water 

within these two lagoon systems.

During August 1999 average water column salinities (36.2%o) and water 

temperatures were consistently lower than during the May cruise. In contrast, ground
water samples from the same well sites reflect little change in either temperature or 

conductivity from wet to dry season.

3.3.2 Ground Water

Sources for ground water on the east coast of Florida can consist of several 

discrete components that originate from the Surficial, Intermediate or Floridan Aquifers. 

The Surficial Aquifer is contained within highly permeable Plio-Pleistocene strata and 

usually responds rapidly to local precipitation/evaporation events. There is evidence of 

discontinuous confining layers within the Surficial Aquifer (Toth, 1988), which could 

facilitate vertical ground water mixing. A deeper Eocene (~ 40 million year old) Ocala 

Limestone forms one of the most productive formations of the Floridan Aquifer system. 

Above the Ocala Formation, the clay-rich confining Hawthorn Group generally thins out 

in the vicinity of the study site, and discontinuous, inter-bedded limestone deposits may 

contain ground water of various sources (Surficial to Floridan) (Bermes, 1958).

Regional-scale potentiometric surface maps of the Upper Floridan suggest a positive 

hydrostatic head differential relative to sea level in upper Indian River Lagoon (Figures 

3-2 and 3-3). Based on such maps that rely on a network of variable well data, the 

potentiometric surface of the upper Floridan does not appear to fluctuate by more than +/- 

2 m seasonally.
Atlantic

Potentiometric Surface (m) of the

Upper Floridan Aquifer

(May, 1998)

, aa- , ~..

flj°30'

2USGS

20

8J°00'

o

80°30'

20 40

km

Figure 3-2. Potentiometric surface map of the Upper Floridan Aquifer in the vicinity of 

upper Indian River Lagoon for May, 1998. (After Bradner, 1998)
Potentiometric Surface (m) of the

Upper Floridan Aquifer

(May, 1999)

i; m

87C30'

20

87°00J

0

80°30'

20 40

Figure 3-3. Potentiometric surface map of the upper Floridan Aquifer in the vicinity of 

upper Indian River Lagoon for May, 1999. (After Bradner and Knowles, 1999)

10
3.3.3 Radium Isotopes

The lagoon and ground water radium isotope activities for May and August 1999 

are compiled in Appendices B and C. This discussion will describe and interpret the 

behavior of Ra from the following sample sets: 1) overall trends, 2) endmembers (ground 

water samples, Turnbull Creek and Haulover Canal), 3) shore-perpendicular transects 

(Stations IRL-1 - 8,17 - 20 and 21 - 24), 4) mid-lagoon stations (Stations IRL-11,16,25, 

26,28) and 5) interstitial waters (IRL-4).

The two short-lived radium isotopes (223'224Ra) exhibit slight spatial and temporal 

variability in upper Indian River Lagoon (Table 3-1), and Ra activities were generally 

higher during the rainy season (August). Although mean lagoon-wide 223Ra activities 

remained constant (0.10 dpm/L), Ra activities increased by a factor of 1.2 from May to 

August. As expected, dissolved 228Th activities were low (~3 dpm/lOOL) in the water 

column, and can be largely discounted as an in situ source for dissolved Ra. The 223' 

224Ra activities in two sources of surface water to the lagoon (Haulover Canal and 

Turnbull Creek) were comparable to lagoon water Ra activities. In these inlets, mean 

223Ra activities increased slightly from May to August, while ^Ra remained constant.

During the time of sampling, radium isotope mass balance estimates do not 

suggest significant water exchange through either inlet. This does not imply zero net 

water exchange per se (Smith, 1992), but rather indicates that surface water radium 

activities in the inlets and lagoon waters are insensitive to surface water mass 

movements.

11
Table 3-1. Mean radium and 228Th activities for lagoon, ground water and 

creek/inlet water (May, August 1999).

Sample type Depth "JRa

(m) (dpm/L)

Lagoon waters

Mean

STDEV

1.21

n/a

Ground waters

Mean

STDEV

29.70

n/a

Creek and inlet -waters

Mean

STDEV

n/a

n/a

Lagoon waters

Mean

STDEV

1.07

n/a

Ground waters

Mean

STDEV

29.70

n/a

Creek and inlet waters

Mean

STDEV

n/a

n/a

0.10

0.03

1.38

1.54

0.08

0.05

0.10

0.02

0.87

0.31

0.10

0.05

ex ""Ra "°Th

(dpm/L) (dpm/lOOL)

May, 1999

0.09

0.02

6.86

3.49

0.14

0.11

August, 1999

0.11

0.02

5.28

2.70

0.14

0.09

3.86

1.29

23.77

0.41

3.14

0.18

3.39

0.82

23.87

0.38

2.56

0.31

z"°Ra

(dpm/L)

2.05

0.23

3.59

2.06

2.42

0.60

2.63

0.26

n/a

n/a

3.01

0.73

223/226 

AR*

0.05

0.01

0.46

0.41

0.03

0.01

0.04

0.01

n/a

n/a

0.03

0.01

224/223 

AR

0.98

0.36

9.97

7.41

1.70.

0.29

1.16

0.32

6.88

4.54

1.35

0.26

*AR =activity ratio (unitless)

Ground water 223>224Ra activities from wells surrounding upper Indian River 

Lagoon were typically an order of magnitude greater than lagoon water activities. The 

highest 224Ra activities (-10 dpm/L) were observed at wells GW2,4, which are among 

the deepest wells (~ 37 m) in our field site and likely tap the Upper Floridan. The 

shallower wells (GW3,6) access the Surficial Aquifer and exhibit widely fluctuating 

radium activities (^Ra: 0.39 - 4.61 dpm/L and 224Ra: 1.34- 7.96 dpm/L). At GW4, the 

224Ra/223Ra ratio exceeded a value of 14 during August 1999. Such elevated ratios reflect

12
variations in U/Th isotopic compositions of the source strata (Osmond and Cowart, 1982; 

Rama and Moore, 1996). ,

There is no apparent correlation between well depth and radium activities for 

either field season. The variable radium activities and activity ratios suggest abundant 

lateral and vertical transport between the Upper Floridan and Surficial Aquifers. Well 

GW-3 extends only into the Surficial Aquifer (open depth = 1.5 4.6 m), yet it has an 

isotopic signature that implies a mixture of several ground water sources. The mobility 

of ground water can be considerably enhanced through karst conduits or through 

fractures. Due to rapid sorption reactions, the mobility of radium once released to 

solution by alpha recoil or by decay of Th is likely to be very limited. Thus the source 

for radium in ground water is usually close to the well structure itself (Asikainen, 1981; 

Dickson et al., 1983; Rama and Moore, 1996).

Shore-perpendicular transects (Big Flounder and Duck Roost Cove) within the 

lagoon have highest excess 224Ra activities closest to shore. In contrast, 223Ra activities 

generally increase along these transects toward the middle of the lagoon. A plot of 223Ra 

from samples with water depths less than 1.5 m (removing the mid-lagoon samples) 

suggests a potential correlation with depth (Fig. 3-4). However, such a bottom effect is 

clearly not evident using 223Ra/226Ra ratios, although most values suggest a lowered (i.e., 

depleted) 223Ra activity relative to the secular equilibrium isotopic ratio (223Ra/226Ra = 

0.046). It is possible that this bottom effect may indicate a time-dependent dilution of 

water being exchanged across the sediment- water interface. Because of the short half-life 

of 223Ra and 224Ra, exchange processes at the sediment-water interface may be examined 

with much greater resolution than using only long-lived radium isotopes (228>226Ra).

13
If

O**

(B

secujar 

equilibrium

D

n n n n 

n a

n

n c 

n

a

 

n

223Ra (dpm/L) 

223Ra/226Ra

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

 V

Figure 3-4. Water column (z < 1.5 m) profile of 223Ra activities (dpm/L) and 223Ra/226Ra 

activity ratios (August, 1999). The deep water stations have been excluded here to 

look only at sediment effects and dilution.

To quantify exchange across the sediment-water interface in upper Indian River 

Lagoon, two methods will be presented that provide an indirect measure of flux, albeit on 

very different spatial scales. These results will then be compared to a simple diffusion 

model (Berner, 1980; Santschi et al., 1990; Hammond et al., 1999) using interstitial 

radium profiles.

14
3.3.4 Flux measurements

To estimate flux across the sediment-water interface in upper Indian River 

Lagoon, input and removal functions for water and radium must be well constrained. The 

field site at upper Indian River Lagoon is effectively isolated from adjacent surface water 

bodies except at Haulover Canal, which connects Mosquito River Lagoon to Indian River 

Lagoon. By comparing salinities, Turnbull Creek in essence forms a northward extension 

of Indian River Lagoon. It is expected that only during high precipitation events will 

Turnbull Creek discharge freshened waters to upper Indian River Lagoon. At the 

southern boundary of the study site, dredged materials from the Intercoastal Waterway 

form a 'quasi' sill that limits water exchange with Indian River Lagoon proper. 

Generalized flow in upper Indian River Lagoon can therefore be expressed simply as a 

function of the lagoon area, A and the time-dependent mean water depth, h(t):

Q = A . 

dt

Water levels in this confined region of the lagoon are controlled seasonally by 

winds, precipitation/evaporation and tidal oscillations (Smith, 1992; 1993). Due to the 

small tidal amplitude of upper Indian River Lagoon, water transport through Haulover 

Canal is largely a wind-driven process (Smith, 1992). As suggested, this is a potential 

source and/or sink for lagoon water radium isotopes. Although the mean inlet 223Ra 

activities are comparable to lagoon values for both field seasons (Table 3-1), 224Ra 

activities were consistently higher in Turnbull Creek, suggesting a possible marine source 

(via thorium).

15
The flux of radium in upper Indian River Lagoon is calculated first by simple 

mass balance ('lagoon budget method'). In doing so, the time-dependant mass of Ra in 

the lagoon (VRai) can be modeled as function of the area-integrated flux of Ra across the 

sediment-water interface (AJ) plus the mass of Ra being transported through Haulover 

Canal (QRainiet'), minus the mass of Ra lost due to radioactive decay (AVRciL):

(3-3) 

at

where Fis the volume (m3), Rai is the radium activity in the lagoon (dpm/L), Jis the flux 

(dpm/m2/d) of radium from the lagoon sediment into the water column, and Rainiet is the 

Ra activity at Haulover Canal. The term Jis estimated by adjusting its value until it 

matches the observed lagoon activities,

3.3.5 Benthic Flux Chambers

Exchange across the sediment-water interface may also be constrained on a very 

localized scale by the use of benthic flux chambers (Fig. 3-5). The flux of radium into 

such chambers can be evaluated by assessing the change in Ra activity over the 

deployment duration. We positioned two benthic flux chambers (Boxes 1 and 2) about 3 

cm into the seabed at IRL-6, in about 1 m of water. The two benthic chambers were 

placed about 2 m apart (see Chapter 4). The 45 x 45 x 30 cm boxes were periodically 

hand-stirred to maintain a well-mixed chamber water column. The initial (t = G) activity 

of radium was measured in a water parcel adjacent to the chamber (Table 3-2).

16
sampling ports

Sedimentwater interface

stirrer 

Benthic Flux!Chamber

SEDIMENT COLUMN

Figure 3-5. Placement of abenthic flux chamber into bottom sediments.

Because an unknown amount of ambient lagoon or subsurface water may be 

hydraulically drawn into a benthic chamber during time-series (t = 4,8 hr) sampling, the 

following corrections were applied to Rach (after Hancock et al., in press), assuming that 

the chambers were reasonably well-mixed for the duration of the experiment:

Rach =(RatK -RaL )b

(1-e )

(3-4)

17
Table 3-2. Time series radium activities in two benthic flux chambers.____

Station Date Time 223Ra ex224Ra 228Th 226Ra

_______________(hours)____(dpm/L)____(dpm/L) (dpm/lOOL) (dpm/L)

Big Flounder Transect

IRL6 8/17/99 initial @t = 0 0.12 ± 0.01 0.11 ± 0.01 3.42 ± 0.34 2.34 ± 0.43

Boxl t = 4 2.69 ± 0.11 

t = 8 0.14 ±0.01 0.13 ± 0.01 3.38 ± 0.34 2.93 ± 0.17

initial @t = 0 0.12 ± 0.01 0.11 ± 0.01 3.61 ± 0.36 2.46 ± 0.10

Box 2 t = 4 0.11 ±0.01 0.10 ± 0.01 3.34 ± 0.33 2.50 ± 0.14

t = 8 0.13 ±0.01 0.12 ± 0.01 3.76 ± 0.38 2.65 ± 0.31

where Rach is the radium activity within the chamber, Rats is the radium activity 

measured at the conclusion of the experiment (t = 8) and b is the sample-to-chamber 

volume ratio. Short-lived radium isotope activities increased by about 15% over the 8-hr 

deployment period. Similarly, 226Ra activities increased about 7 - 20% in the two 

chambers during the 8-hr experiment.

For a chamber of height hc, the change in radium activity (RactJ can be expressed 

as a function of the sediment-water interface flux (J):

(3-5)

dt h.c

Setting the initial chamber activity equal to the corresponding lagoon activity (Rach 

RO.L, f = 0) as a boundary condition and assuming a constant value for J (integrated over 

the surface area beneath the chamber) provides the following solution to Eq. 3.5:

18
J-hcARach 

J-hcARaL

(3-6)

which can be rearranged for J, as follows:

j = -£-± £* ± I (3-7)

The calculated fluxes will be compared with results from other techniques in section 

3.5.7 (e.g. Table 3.5). Minimum propagated errors for .7are based on + 1 cr.

3.3.6 Pore Water Profiles

The exchange of radium across the sediment-water interface as well as the shape 

of the interstitial Ra profile depend on the rate of Ra production from sediment-bound Th 

isotopes, and on physico-chemical processes that can remove radium; i.e., decay and 

diffusion/advectiori (Cochran, 1980; Kadko et al., 1987; Hammond et al., 1990). Chapter 

4 summarizes some of the principal processes that can affect the distribution of radium in 

coastal settings (excluding the atmospheric evasion term, which only affects gaseous 

radionuclides such as radon).

It has been shown that horizontal transport from adjacent water bodies is not a 

major input function for radium to our study site. Radioactive decay is an important mass 

balance term for any short-lived radioisotope, and is of course quantifiable. The more 

difficult two terms are diffusion along reasonably static geochemical gradients 

(diagenetically controlled diffusion) and physically-enhanced advection/diffusion that

19
responds to water level fluctuations (tidal pumping, ground water upwelling) and/or 

water mixing. Separating these two processes is important but often a daunting task.

Pore water profiles provide an integrated summary of the subsurface 

biogeochemical reactions. Such profiles, when used in conjunction with other sedimentwater interface techniques (i.e., lagoon budget method and benthic chambers), can 

sometimes provide the necessary information to distinguish water flow across the 

sediment water interface from diffusive fluxes. Pore waters were collected using a minipiezometer ('multi-sampler'), which enables in situ interstitial water to be drawn from 

various sediment depths. Chloride concentrations and other hydrographic parameters of 

these pore waters are described in Chapter 2. Table 3-3 lists the pore water radium 

activities for station IRL-4.

At IRL-4, pore water radium activities for each isotope increased with depth. 

224Ra exhibited the smallest down-core increase in activity (5%), while 226Ra activities 

increased the most (~ 40%). Because radium is continuously being generated in the 

bottom sediments of Indian River Lagoon, the rate of production can define pore water 

Ra profiles. The rate of Ra production in sediments is usually defined in terms of an 

exchangeable component that is available for desorption by ion-exchange processes 

(Webster et al., 1994,1995; Hancock et al., in press). Exchangeable Ra is produced by 

the decay of particle reactive Th and Ac isotopes either at particle surface sites (or 

coatings), or within the mineral structure itself. Alpha recoil processes may also eject Ra 

atoms directly into solution (Fleischer, 1982; Sun and Semkow, 1998), where they are 

scavenged rapidly and then partitioned between sediment surfaces and pore water.

20
Table 3-3. Pore water radium isotope activities at Station IRL4.

Station Depth

(cm)

223Ra

(dpm/L)

xs224Ra

(dpm/L)

228-Tn

(dpm/lOOL)

226Ra

(dpm/L)

Big Flounder Transect

Overlying water*

Pore water

(12/13/99)

39

64

95

124

156

185

0.14 ± 0.01

2.79 ± 0.28

2.51 ± 0.25

3.58 ± 0.36

3.82 ± 0.38

4.21 ± 0.42

4.43 ± 0.44

0.10 ± 0.01

5.89 ± 0.59

4.45 ± 0.45

4.69 ± 0.47

5.13 ± 0.51

7.47 ± 0.75

6.19 ± 0.62

4.70 ± 0.47

10.24 ± 1.02

8.38 ± 0.84

11.92 ± 1.19

12.72 ± 1.27

13.76 ± 1.38

10.87 ± 1.09

2.39 ± 0.09

2.21 ± 0.19

2.53 ± 1.16

2.82 ± 0.24

3.13 ± 0.85

3.20 ± 0.16

3.92 ± 0.014

* mean from IRL-5 dry and rainy season sampling (see Appendices B and C). This site 

was chosen as a water column end-member.

The production rate, P, of exchangeable Ra per unit volume of dry sediment can 

be expressed as a function of the sediment porosity ($), the Ra decay constant (X) and the 

activity of exchangeable Ra (Raexch) per unit volume of saturated sediment (Webster et 

al., 1994,1995):

Eq. 3-8

A measure ofRaexch can be derived from sediment Ra/Th ratios. In upper Indian River 

Lagoon the mean 228Ra/232Th ratio for eight surface sediment samples (excluding the one 

potentially anomalous 228Ra measurement; 1.65 dpm/g) was 2.02 (Chapter 4), roughly 

two tunes above the secular equilibrium value of 1.0. Such enrichment in surficial 

sediment daughter isotopes is attributed mainly to the enhanced solubility of Ra relative 

to Th (upward diffusion of Ra), source rock heterogeneities, redox associated postdepositional mobility of Ra, or a ground-water influence. Such activity ratios observed in

21
Indian River Lagoon are not excessive, especially in dynamic coastal settings (Bollinger 

and Moore, 1993; Webster et al., 1995; Hancock and Murray, 1998).

The pore water flux across the sediment-water interface was calculated using a 

derivation of Pick's first law (Berner, 1980; Aller, 1980; Santschi et al., 1990):

\ Eq.3-9

'2=0

where ^is porosity (interstitial water volume fraction), Ds is a tortuosity-corrected 

molecular diffusion (DS 02} coefficient (Li and Gregory, 1974; Boudreau, 1998; 

Hammond et al., 1999), Rap is the depth dependent pore water radium activity and z is 

depth below the sediment-water interface. The most difficult decision in determining 

calculated fluxes is the choice of concentration gradients for the dissolved pore water 

profiles (Fig. 3-6). We used the bottom and upper most pore water sample to determine 

two concentration gradients that provided a range of flux estimates. The calculated flux 

of dissolved radium out of the seabed could thus still over-estimate the total flux due to 

adsorption/precipitation across the redox boundary.

A mean (May and August) water-column Ra value from a site most proximal to 

IRL-4 was used as the end-member value for flux calculations. A sample calculation for 

the diffusive flux of 223Ra using Eq. 3.9 is presented in Table 3-4. The concentration 

gradient in pore water radium activities from a depth of 185 cm to above the sediment22
IRL4

50

O

Q. 100

0)

a.

150

200

sediment-water interface

A

223Ra

xs224Ra

226Ra

4 

dpm/L

6 8

Figure 3-6. Pore water profiles for 224Ra, ^Ra and 226Ra at IRL4. These interstitial 

waters were collected in situ by mini-piezometer (multi-sampler).

water interface was smallest for 226Ra, which may be explained due to its long half-life 

(i.e., slow production). By contrast, both 224Ra and 223Ra exhibited pronounced increases 

in activity with depth relative to the overlying water column activities. The removal of 

Ra in the surface sediments is attributable to upward Ra diffusion.

To complement such diffusive flux calculations, we also estimated the upper limit 

of subsurface water exchange. This in effect utilizes benthic flux measurements to derive

23
Table 3-4. Calculating diffusive fluxes with 223Ra.

Supporting formula and data: 

'dRa'

/2=0

(f> = 0.90 (Trefryetal., 1992)

Ds = 6.90xlO'5 m2 /day; 0 = 1.211; corrected Ds = 5.7x1 0'5 m2 /day

dRap 4.43-0.14 dpm/L . 4 

 - = - = 2,319 dpm/m 

dz 185-0 cm

J = 0.9(5.7xlO~05 ff2 2 /rf)(2,319 dpm/m*) = 0.119 dpm Irn 1 Id

a rate of upward water transport (cm/day). Based on the deepest pore water 226Ra activity 

at IRL-4 (Table 3-3; 3.93 dpm/L) and using a sediment porosity ($ value of 0.90 (Trefry 

et al., 1992), requires an upward subsurface water flux, Jw of 6 -17 cm/d to sustain the 

observed benthic flux (Jb, Table 3-2; 160 - 480 dpm/m2/d), as follows:

Eq.3-10

j

Although the flow of subsurface water is likely much less than the values derived from 

Eq. 3.10, the range in Jw agrees well with similar results obtained for Indian River 

Lagoon during this study and others (Belanger and Walker, 1990; Belanger and 

Montgomery, 1992).

24
3.3.7 Comparison of the Three Flux Measurements

Lagoon budget and benthic chamber calculations provide an integrated 

measurement of flux of Ra across the sediment-water interface during the sampling 

duration. Such results differ fundamentally from those derived via seepage meters 

('direct' measurements) that produce a localized and at times controversial flux rate. The 

two radium flux methods described here provide information on J over very different 

spatial scales. The two benthic chambers measure flux across the sediment-water 

interface on a small and localized scale. In contrast, the lagoon budget method provides 

an integrated lagoon-wide estimate of the flux across the sediment water interface. This 

latter method is therefore sensitive to integrated sources and sinks for Ra in the lagoon. 

Due to long half-life of 226Ra, the uncertainties associated with the lagoon budget method 

using 226Ra can be quite large. Similar flux measurements using either 223>224Ra are much 

easier to constrain, and uncertainties are correspondingly lower (~20 %, Hancock et al., 

hi press). Table 3-5 lists direct and modeled values for J, in units of dpm/m2/day. 

All three techniques show reasonable agreement in calculated J. Lagoon budget methods 

produced a flux for the two short-lived Ra isotopes that ranged from -7-22 dpm/m2/d. 

By contrast, the same approach using 226Ra produced a 'zero' flux rate, given the 

uncertainties of these methods. This difference may be explained by the vastly different 

half-lives of the three Ra isotopes. Their effectiveness in exchange derivations has to 

match the time frame of the processes being studied. Processes that affect and control the 

water budget hi Indian River Lagoon are likely to occur over daily (e.g., storm event), 

weekly (e.g., precipitation) and monthly (e.g., seasonal weather changes) time scales. A 

combination of short-lived Ra isotopes, normalized to long-lived 226Ra may provide the

25
Table 3-5. Flux measurements using various direct (lagoon budget, benthic

chamber) and indirect (modeled) methods. Minimum uncertainties based on 

+/- la.

Flux from sediment (dpm/m'/d) 

Lagoon budget method Benthic chamber Pore water profile

Isotope

223Ra

224Ra

May August August

Box 1 Box 2

7.4 6.5 19.0 10.4

20.4 22.1 23.0 14.2

226Ra 0.0 0.0 480.2 160.2

December

Minimum Maximum

0.1 0.4

0.2 0.8

0.0 0

best tracer suite. Benthic flux chamber results show strong agreement between 223>224Ra 

isotopes, and these results also agree well with the lagoon budget methods using the same 

Ra isotopes. Benthic chamber measurements using 226Ra produced results that are 

considerably greater than those generated with 223>224Ra, yet these differences may be 

reconciled with more detailed solid phase and pore water sediment work. 

3.4 Summary and Recommendations

Sediments are often the dominant source for radium as well as nutrients in coastal 

waters. This chapter examined the utility of Ra as an effective sediment-water interface 

tracer in upper Indian River Lagoon, where the exchange of water and chemical 

constituents can have an important effect on water column processes. Benthic fluxes are 

estimated using three unique methods, lagoon budget, benthic chamber and pore water 

modeling. The first two yield integrated measurements of flux while the third technique 

generates an indirect flux estimate based on pore water profiles of radium isotopes.

26
Flux estimates showed a wide range that extended up to 480 dpm/m2/d 

Interestingly, the lowest (lagoon budget method) and highest values (benthic flux 

chamber) were observed using 226Ra as a tracer. Using ^Ra, a maximum upward 

subsurface water flow of about 5-17 cm/day would be required to sustain observed 

benthic fluxes. These calculated.values appear to corroborate our direct and calculated 

seepage results and also compare favorably to existing seepage values (Belanger and 

Mikutel, 1985). This work illustrates that the determination of benthic fluxes using 

multiple Ra isotopes with widely variable half-lives provides a useful, complementary 

technique to study exchange processes across the sediment water interface over many 

time scales. - - - ......... ...........

Future work will extend the modeling efforts to include bioturbation and 

advection by collecting solid-phase samples down core. Such efforts will also provide 

crucial information on Ra adsorption-desorption and ion exchange. Subsurface 

temperature profiling in the lagoon will provide a means to link subsurface water to tidal 

oscillations and seasonal variations.

REFERENCES CITED

Aller, R.C., 1980, Quantifying solute distributions in the bioturbated zone of marine 

sediments by defining an average micro-environment: Geochim. Cosmochim. 

Acta,v. 44, p. 1955-1965.

Asikainen, M., 1981, Radium content and the 226Ra/228Ra activity ratio in ground water 

from bedrock: Geochim. Cosmochim. Acta, v. 45, p. 1375-1381.

27
Baliela, I., Teal, J.M., Volkmann, S., Shafer, D., and Carpenter, E.J., 1978, Nutrient and 

particulate fluxes in a salt marsh ecosystem: tidal exchanges and inputs by 

precipitation and groundwater: Limnol. and Oceanogr., v. 23, p. 798-812.

Belanger, T.V., andMikutel, D.F., 1985, On the use of seepage meters to estimate

groundwater nutrient loading to lakes: Water Resources Bull., v. 21, p. 265-272.

Belanger, T.V., and Montgomery, M.T., 1992, Seepage meter errors: Limnol. Oceanogr., 

v. 37, p. 1787-1795.

Belanger, T.V., and Walker, R.B., 1990, Groundwater seepage in the Indian River

Lagoon, Florida, in (eds.), Tropical Hydrology and Caribbean Water Resources:, 

Proc. Int. Symp. Am. Water Resour. Assoc., 367-375.

Benes, P., Obdrzalek, M., and Cejchanova, M., 1982, The physico-chemical forms of 

traces of radium in aqueous solutions containing chlorides, sulfates and 

carbonates: Radiochem. Radioanal. Lett., v. 50, p. 227-242.

Bermes, B.J., 1958, Interim report on geology and groundwater resources of Indian River 

County, Florida:, Bur. of Geol., Inf. Circ. No. 18, 75 p.

Berner, R.A., 1980, Early Diagenesis: A Theoretical Approach: Princeton, NJ, Princeton 

Univ. Press, 241 p.

Bollinger, M.S., and Moore, W.S., 1993, Evaluation of salt marsh hydrology using 

radium as a tracer: Geochim. Cosmochim. Acta, v. 57, p. 2203-2212.

Boudreau, B., 1998, The diffusive tortuosity of fine-grained unlithified sediments: 

Geochim. Cosmochim. Acta, v. 60, p. 3139-3142.

Boynton, W.R., Hagy, J.D., Murray, L., Stokes, C., and Kemp, W.M., 1995, Modeling 

the effect of salinity on the desorption of radium from sediments.: Geochim. 

Cosmochim. Acta, v. 59, p. 2469-2476.

Bradner, L.A., 1998, Potentiometric surface of the upper Floridan aquifer in the St. Johns 

River Water Management District and vicinity, Florida:, USGS, Open File 

Report 99-10.

Bradner, L.A., and Knowles, L., Jr., 1999, Potentiometric surface of the upper Floridan 

aquifer in the St. Johns River Water Management District and vicinity, Florida:, 

USGS, Open File Report 99-0608.

Cochran, J.K., 1980, The flux of 226Ra from deep-sea sediments.: Earth Planet. Sci. 

Lett., v. 49, p. 381-392.

28
Dickson, B.L., Meakins, R.L., and Bland, C.J., 1983, Evaluation of radioactive anomalies 

using radium isotopes in ground waters: J. Geochem. Exploration, v. 19, p. 195- 

205.

Fleischer, R.L., 1982, Alpha-recoil damage and solution effects in minerals: Uranium 

isotopic disequilibrium and radon release: Geochim. Cosmochim. Acta, v. 46, p. 

2191-2201.

Ghose, S., Alam, M.N., and Islam, M.N., 2000, Concentrations of 222Rn, 226Ra and

22^Ra in the surface seawater of the Bay of Bengal: J. Environ. Radioactivity, v. 

47, p. 291-300.

Hammond, D.E., Giordani, P., Berelson, W.M., and Poletti, R., 1999, Diagenesis of

carbon and nutrients and benthic exchange hi sediments of the Northern Adriatic 

Sea: Marine Chem., v. 66, p. 53-79.

Hammond, D.E., Marion, R.A., Berelson, W.M., and Ku, T.-L., 1990, .Ra-228 

- distribution and mixing hi San Nicholas and San Pedro Basins, Southern 

California Borderland: J. Geophys. Res., v. 95, p. 3321-3335.

Hancock, G.J., in press, Estimating the flux of Ra into a shallow estaurine lagoon; 

implications for biogeochemical processes. Geochim. Cosmochim. Acta.

Hancock, G.J., and Murray, A.S., 1998, The source and distribution of dissolved radium 

hi the Bega River estuary, southeastern Australia: Earth Planet. Sci. Lett., v. 138, 

p. 145-155. . .

Hancock, G.J., Webster, I.T., Ford, P.W., and Moore, W.S., hi press, Using radium

isotopes to examine transport processes controlling benthic fluxes into a shallow 

esruarine lagoon.

Ivanovich, M., and Harmon, R.S., 1982, Uranium Series Disequilibrium: Applications to 

Environmental Problems: Oxford, Clarendon Press, 901 p.

Kadko, D., Cochran, J.K., and Mitchell, L., 1987, The effect of bioturbation and

adsorption gradients on solid and dissolved radium profiles hi sediments from the 

eastern equatorial Pacific: Geochim. Cosmochim. Acta, v. 38, p. 703-714.

Key, R., Brewer, R., Stockwell, J., Guinass, N., and Schink, D., 1979, Some improved 

techniques for measuring radon and radium hi marine sediments and hi seawater: 

Marine Chem., v. 7, p. 251-264.

Lee, D.R., 1977, A device for measuring seepage flux hi lakes and estuaries: Limnol. 

Oceanogr., v. 22, p. 140-147.

29
Li, L., Barry, D.A., Stagnitti, F., and Parlange, J.-Y., 1999, Submarine groundwater

discharge and associated chemical input to a coastal sea: Water Resources Res., v. 

35, p. 3253-3259.

Li, Y.-H., and Gregory, S., 1974, Diffusion of ions in seawater and in deep-sea 

sediments: Geochim. Cosmochim. Acta, v. 38, p. 703-714.

Li, Y.-H., Mathieu, G.G., Biscaye, P., and Simpson, H.J., 1977, The flux of Ra-226 from 

estuarine and continental shelf sediments: Earth Planet. Sci. Lett., v. 37, p. 237- 

241.

Mathieu, G., Biscayne, P., Lupton, R, and Hammond, D., 1988, System for measurement 

of 222Rn at low levels in natural waters: Health Physics, v. 55, p. 989-992.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida 

and parts of Georgia, Alabama, and South Carolina:, U.S. Geological Survey, 91

P- PMoore, W.S., 1996, Large groundwater inputs to coastal waters revealed by 22^Ra 

enrichments: Nature, v. 380, p. 612-614.

Moore, W.S., and Arnold, R., 1996, Measurement of 22^Ra and 224Ra in coastal waters 

using a delayed coincidence counter: J. Geophys. Res., v. 101, p. 1321-1329.

Osmond, J.K., and Cowart, J.B., 1982, Ground water, in (eds.), Uranium Series

Disequilibrium, Application to Environmental Problems: Oxford, Clarendon 

Press,.

Pandit, A., and El-Khazen, C.C., 1990, Groundwater seepage into the Indian River 

Lagoon at Port St. Lucie: Florida Scientist, v. 53, p. 169-179.

Rama, and Moore, W.W., 1996, Using the radium quartet for evaluating groundwater 

input and water exchange in salt marshes: Geochim. Cosmochim. Acta, v. 60, p. 

4645-4652.

Santschi, P., Hohener, P., Benoit, G., and Buchholtz-ten Brink, M., 1990, Chemical 

processes at the sediment-water interface: Marine Chem., v. 30, p. 269-315.

Scott, T.M., 1988, The lithostratigraphy of the Hawthorn Group (Miocene of Florida):, 

Florida Geological Survey Bulletin, 147 p.

Scott, T.M., 1992, A Geological Overview of Florida:, Florida Geological Survey, 78 p.

Smith, N.P., 1992, The intertidal volume of Florida's Indian River Lagoon: Florida 

Scient., v. 55, p. 209-218.

30
Smith, N.P., 1993, Tidal and nontidal flushing of Florida's Indian River Lagoon: 

Estuaries, v. 16, p. 739-746.

Sun, H., and Semkow, T.M., 1998, Mobilization of thorium, radium, and radon

radionuclides in ground water by successive alpha recoils: J. Hydrology, v. 205, 

p. 126-136.

Swarzenski, P.W., 1999, Examining freshwater-saltwater interface processes with four 

radium isotopes:, USGS, Open File Report, FS-065-99.

Toth, D.J., 1988, Saltwater intrusion in coastal areas of Volusia, Brevard, and Indian. 

River Counties: Palatka, FL, St. Johns River Water Management District, 

Technical Publication SJ 88-1,160 p.

Trefry, J.H., Feng, H., Trocine, R.P., Metz, S., Gruguric, G., Vereecke, R., and 

Cleveland, S., 1992, Concentrations and benthic fluxes of nutrients from 

sediments in the Indian River Lagoon, Florida (Project Muck, Phase II): Palatka, 

FL, St. Johns River Water Management District, Final Report, 47 p.

Webster, I.T., Hancock, G.T., and Murray, A.S., 1994, On the use of radium isotopes to 

examine pore water exchange in an estuary: Limnology and Oceanography, v. 39, 

p. 1917-1927.

Webster, I.T., Hancock, G.T., and Murray, A.S., 1995, Modeling the effect of salinity on 

the desorption of radium from sediments: Geochim. Cosmochim. Acta, v. 59, p. 

2469-2476.

31