This document describes a study of radium isotopes in the Bega River estuary in Australia. Measurements of radium-224, radium-223, radium-226, and radium-228 were made in surface water and sediment porewater to understand the source and distribution of dissolved radium in the estuary. The results show that bottom sediments are a major source of radium to the estuary surface waters, with radium accumulating in porewater and mixing into surface waters via tidal exchange. A model was applied to estimate the daily flux of porewater crossing the sediment-water interface, about 15% of the total estuary volume.

1. EPSL
ELSEVIER Earth and Planetary Science Letters I38 ( 1996) 145- 155
Source and distribution of dissolved radium in the Bega River
estuary, Southeastern Australia
G.J. Hancock *, A.S. Murray
CSIRO Division of Water resources, GPO Box 1644, Canberra, ACT. 2601, Australia
Received 7 March 1995; accepted 15 November 1995
Abstract
Measurements of the activities of the four naturally occurring radium isotopes in the surface water and porewater of an
estuary have yielded information on the release of radium from sediments and on the extent of surface water-porewater
interaction in the estuary. Under low-flow conditions, the non-conservative behaviour of dissolved radium in the estuary is
almost entirely due to the flux of radium from estuarine bed sediments.Radium accumulates in bottom sediment porewater,
and is then mixed with estuarine surface water, probably as a result of tidal action.
It is shown experimentally that the enrichment of the short-lived isotopes ( 224Ra and 223Ra) relative to 226Ra in estuarine
porewater can be explained by the repeated leaching of radium from bottom sediments by saline water, and the rapid
regeneration of the short-lived isotope activity from their sediment-bound parent nuciides. The leaching of radium from
bottom sediments is apparently occurring on a time scale which is long (weeks-months) compared with the 224Ra and 223Ra
half-lives, indicating that the amount of ion-exchangeable radium adsorbed to the sediments is large compared with the
amount dissolved in porewater.
By applying a simple 2-D steady-state multi-box model, 224Ra and 223Ra surface water and porewater concentrations
have been used to estimate the daily flux of porewater crossing the sediment-water interface in the Bega estuary. This flux
is found to be about 15% of the estuary volume.
Keywords: New South Wales Australia; radium; surface water; pore water
1. Introduction than both the river and ocean end-members, indicat-
ing a net addition of dissolved radium to the estuary.
Numerous publications have now described the Li et al. [l] considered that this ‘excess’ 226Ra was
non-conservative behaviour of radium in the mixing supplied by river-borne sediments carried into the
zones of rivers and oceans [l-6]. These studies have estuary. In saline water, the competition effects of
shown that the estuarine concentrations of 226Ra soluble cations for ion exchange sites on sediment
increase with increasing salinity to levels greater particles results in the desorption of surface-bound
radium. Elsinger and Moore [2] determined a system-
atic decrease in the 226 concentration of suspended
Ra
* Corresponding author. Fax: +61 6 246 5800. E-mail: han- particulate matter (SPM) with rising salinity in the
cock@cbr.dwr.csiro.au Winyah Bay estuary. Other studies [3-51 concluded
0012-821X/96/$12.00 0 19% Elsevier Science B.V. All rights reserved
SSDI 0012-821X(95)00218-9

2. 146 GJ. Hancock, A.S. Murray/Earth and Planetary Science Lerters 138 (1996) 145-155
that estuarine bottom sediments also supply signifi- depth to which bottom sediments were flushed by
cant fluxes of radium. High concentrations of radium surface water during each tidal cycle.
have been measured in near-bottom ocean water and Despite the extensive use of radium of isotopes as
deep-sea sediment porewater [7,8], implying that tracers in the marine environment, there has been
porewater of bottom sediments is the transfer little attempt to understand the processes governing
medium. the release of radium isotopes from marine sedi-
Bottom sediments are thought to be the major ments. In this paper we present the concentration
source of the shorter lived isotopes, 228Ra (half-life data of all four naturally occurring radium isotopes,
5.7 y) and 224Ra (half-life 3.6 d) to estuarine waters 226Ra,228Ra,224Raand 223Ra(half-life 11.4 d) in the
[4,9]. The enrichment of these isotopes in estuarine surface water and bottom sediment porewater of the
and near-shore environments is often much greater Bega River estuary. To the best of our knowledge
than the long-lived 226Ra (half-life 1600 y). Moore this is the first estuarine study incorporating mea-
[4] suggested that this was due to their higher rate of surements of 223Ra.Using these data we establish the
activity regeneration by their insoluble thorium par- source of dissolved radium to the estuary, and gain
ents in bottom sediments. The high 228Ra/ 226Ra information on the rates and mechanism of radium
activity ratios (ARs) generated in coastal waters have release from estuarine sediments, and obtain esti-
been used as a tracer of water movement in oceans mates of the rate of surface water and porewater
[lo] and 224Ra has been used to estimate current exchange in the estuary.
speeds in the Caribbean Sea [ 11I.
Bollinger and Moore [ 12,131 measured the flux of
224Ra from marsh sediments and calculated the rate 2. Site description
of porewater exchange with marsh surface water.
Surface water-porewater exchange processes are im- The Bega River is located in southeastern New
portant to our understanding of estuarine processes South Wales, Australia. Its estuary comprises an 11
because they affect the fate of nutrients and other km reach from its tidal limit to where the river enters
particle reactive pollutants. Recently, Webster et al. the Tasman Sea (Mogareka InIet, Fig. 1). During the
[ 181 modelled the distribution of radium in the Bega period of this study the depth of the river ranged
River estuary, southeastern Australia. By matching from about 1 m in the main channel of the upper
model-predicted 224Ra and 223Ra surface water data region of the estuary to 2-3 m near its mouth.
with measurements, they estimated the effective Localised areas up to 14 m deep were found about
14&x4’
36’42
tidal
limit
tliacxn
Lagoc
Fig. 1.A map of the Bega River estuary, showing sample site locations.

3. C.J. Hancock. AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 147
2.5 km upstream of the mouth. There are two back- generally at low tide. A freshwater sample was
flow lagoons in the middle estuary, and swamp areas collected upstream of the tidal limit (site 01, and a
near the mouth. The bottom sediments in the river seawater sample was collected from Tathra Wharf
channel are typically sand and gravel. In the back- (site 8w), about 3 km south of the estuary mouth.
flow lagoons and swamps the sediments are fine Sampling site locations are shown in Fig. 1.
grained, comprising mainly silt and clay minerals. Water samples were collected from about 0.3 m
Water flow in and out of the estuary is restricted below the surface. A continuous flow centrifuge
by a sand bar, the position of which is largely (CFC) was used ‘in situ’ to separate SPM with a
governed by the flow of the river. During this study particle size greater than approximately 1 pm. This
river flow was relatively low (180 Ml/day) and the apparatus enabled the collection of gram quantities
width of the mouth was only about 50 m. During low of SPM from many hundreds of litres of water.
flow periods, the movement of water in the estuary is Bed sediment and porewater samples were col-
greatly influenced by the tide. lected from the main river channel. One site was
sampled in July, 1992, and three other sites were
sampled in December, 1992. Bottom sediments were
3. Methods collected to a depth of about 300 mm from areas of
the river bed exposed at low tide. Porewater samples
3.1. Sample collection were obtained by allowing interstitial water from the
surrounding sediments to fill the hole created by the
Water and suspended particulate matter @PM) sediment collection. The depth of porewater prior to
samples were collected from seven sites along the collection was 100-150 mm. One other bottom sedi-
estuary in November 1991. Samples were collected ment sample was collected from Blackfellows La-
between the tidal limit and the mouth of the estuary, goon (site 3bl using an Eckman grab sampler.
Table 1
Filtered water samples from the Bega estuary
site COlleaiOIl distance salinity SPM %a ?a %a ‘I’&
Sutface water
0 Nov 1991 -0.3 0.1 1.7 0.63 iO.08 1.3 i0.2 0.03 a.02 1.1 ti.4
1 2.1 0.8 2.0 1.11 ho.13 3.1~0.6 0.12 Go.06 4.0 M.9
I,
2 ” 3.6 2.2 3.2 1.61 NO.16 5.8 kO.9 0.23 a.11 1.9 il.8
3 * 4.8 4.4 3.1 1.79 a.14 6.7 ?&.8 0.50M.14 9.4 l1.s
4 11 5.1 10.0 5.2 2.6 M.2 11.4zt1.4 1.0 AO.3 20 *3
5 * 7.0 14.9 3.8 2.8 hO.3 13.9i1.8 1.3 i0.3 21*3
6 I, 9.3 20.0 2.9 3.0 i0.2 14.4 *1.5 1.1 iSo.2 25 i3
7 I, 11.0 26.7 1.8 2.6 ho.2 12.9h1.5 1.3 HI.2 28 k3
SW Nov 1991 14.0 35.8 0.7 1.30 H).os 0.7 a.1 0.21 io.04 3.110.3
Porewater
0 Lkc 1992 -0.3 0.1 2.2 +&lo.2 4.4 AO.7 0.08 kOto.05 4.5 HI9
4 * 5.1 5.8 5.5 a.3 13.8 +1.3 1.03 HI.18 26 it2
5 July 1992 7.0 14.4 3.1 HI.2 17.2 Al.9 2.6 MO.4 73 i8
7 Dee 1992 11.0 22.2 1.75 LtO.20 17.6 12.5 4.5 +0.7 94 *14

4. 148 GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155
3.2. Laboratory analyses
The CFC sediment suspension was washed with
demineralised water and dried. All water samples
were filtered through 0.45 pm membrane filters
within 24 h of collection. The suspended solids
concentration of each sample was determined from
the weight of dry residue on the filter. Dissolved
silicon was determined on the filtered water samples
by flame AAS.
l
Sediment samples were solubilised by pyrosul-
phate fusion. Radium, thorium and uranium mea-
surements on filtered water and sediment samples
i
were determined by alpha-particle spectrometry fol-
lowing radiochemical separation [14,15]. Dissolved
224Ra md 223
Ra activities in water samples were
corrected for decay between collection and analysis
(usually less than 3 days). For 224Ra, a correction
was also made for support by dissolved 228Th. In all
cases the 228Th concentrations were less than 0.20
mRq/l and the correction was small (usually < 2%
of the 224Ra activity). -.
0 lb i0 i0
salinity (ppt)
Fig. 3. Surface water radium isotope concentrations against salin-
4. Results ity. All estuarine concentrations lie above the conservative mixing
line, represented by the dotted line joining seawater (square) and
Dissolved radium isotope activity concentrations freshwater.
are shown in Table 1 together with the salinity and
SPM concentrations at each site. The uncertainties in
the radionuclide measurements are due to counting statistics only, and correspond to 1 standard devia-
tion.
Dissolved silicon concentrations are plotted
against salinity in Fig. 2 and show only small devia-
tions from the linear relationship typical of conserva-
tive behaviour. It would appear that the biological
removal of silica (and by implication, radium) by
diatoms was not significant at the time of this study.
SPM concentrations were extremely low at all
sites (maximum 5.2 mg/l, Table l), probably due to
the low-flow conditions at the time of sampling.
SPM shows a non-conservative increase towards the
middle of the estuary. It is suggested that resuspen-
sion of bottom lagoon sediments is the most likely
source of the additional SPM [16].
10 20 30 40
Concentrations of dissolved radium are plotted
salinity (ppt) against salinity in Fig. 3. All isotopes show similar
Fig. 2. Dissolved silicon concentrations and salinity in surface non-conservative behaviour in the estuary, with their
water samples shows largely conservative mixing. concentrations lying well above the conservative

5. G.J. Hancock, AS. Murray/Earth und Planetary Science Letters 138 (1996) 145-155
mixing line joining the two end-members (dashed
line). All radium isotope concentrations increase
steadily, reaching a maximum in the middle estuary
(14-20 ppt), before levelling off. No data are avail-
able for the area between site 7 (27 ppt> and the sea,
but presumably the activities of all isotopes decrease
rapidly towards seawater concentrations near the
mouth of the estuary. *- 0.4 suspended sediment
The bottom sediment radionuclide data is pre- -1
sented in Table 2. The loss of 226Ra from suspended
and bottom sediment within the estuary is illustrated 0.0 &- II- I
by a plot of the sediment 226Ra/ 230Th AR against 0 5 10 15 20 25 30
salinity (Fig. 4). Th-230 is the parent of 226Ra, and is salinity (ppt)
known to remain strongly bound to particles in saline Fig. 4. 226Ra, 230I% AR of suspended and bottom sediments
water. The decrease in the 26Ra/ 230Th AR of fluvial against salinity. The reduction in the AR is measure of radium
sediment in saline water can, therefore, be used as a loss from the sediment as a result of exposure to saline water.
measure of the fraction of sediment-bound radium
which has desorbed [2]. The suspended sediment AR
decreases from a value of 1.31 &-0.08 in freshwater loss of about 35% f 7 from the river bed sediments,
(site 01, to a minimum of 0.59 + 0.02 at a salinity of or 2.7 + 0.5 mBq/g dry wt, which in absolute terms
IO ppt (site 4), and changes little with further in- is much less than the SPM. The difference can be
creases in salinity. The reduction in 226Ra activity attributed to the much larger mean particle size and
corresponds to 55 + 3% of the 226Ra content of the much lower radionuclide concentration of the river-
SPM in freshwater, or 35 f 4 mBq/g dry wt. There bed sediments. The bottom sediment sample of fine-
is also evidence of 226Raloss from bottom sediments grained mud from Blackfellows Lagoon contained
(Fig. 4), with the 226Ra/ 230Th AR decreasing from radionuclide concentrations and apparent 226Ralosses
1.09 f 0.06 in freshwater, to values around 0.71 in similar to the SPM. Apparent losses of 22*Ra, as
the estuary (Table 2). This decrease corresponds to a derived from the decrease in the 228Ra/ 232Th AR
Table 2
Bottom sediment radionuclide concentrations (mBq/g dry wt)
site 2.38
U =‘Th =Ra =*Th “8Ra =‘Th ‘26Ra/23”Th
228Ra/232Th
““Th/=fv =“Ral=‘Ra
L
in
pCtrewaterb
River bed
0 7.4 AO.9 6.9 k0.4 7.6kOo.2 8.8&0.4 8.6i0.3 8.7kO.2 1.09~tO.06 0.97kO.06 26 *3 60 ~~40
4 6.5 *I .4 6.8 io.5 5.OkO.2 9.5 kO.5 5.8M.5 5.4 iO.2 0.73 ztO.06 0.61 MO.06 18*4 26 k4
5 10.1 il.1 11.9kO.7 8.2ti.2 14.6*0.7 8.3106 10.5iO.3 0.69kO.04 0.57*0.05 23 *3 28 zt3
7 5.4 *1.5 6.5 AO.3 4.7k0.2 8.0 i0.3 6.2 M.4 6.3 ho.2 0.72hO.04 0.78 No.05 25 *7 21k2
Lagoon
3b 77 l3 82 h4 41*1 112zt4 57*1 66 *3 0.50 iO.03 0.5 1 ztO.02 19*1
_I
’ 235U activity calculated assuming a 238U/ 235U AR of 22. bPorewater activity ratios derived from data in Table I.

6. 150 GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155
(Table 2), are similar to those of 226Rafor both river
bed and lagoon sediments, indicating that radium
loss is occurring on a time scale which is short
zi-100
compared with the 228Rahalf-life.
3
g 60
E
2 60
5. Discussion: The source of dissolved radium H i
5.1. Surface water samples
The loss of radium from sediments in the Bega
estuary coupled with the non-conservative increases 0 5 10 15 20 25
in dissolved radium identifies sediments as the source salinity (ppt)
of the additional or ‘excess’ dissolved radium in the
Fig. 5. Radium concentrations of porewater against salinity. 223Ra
estuary. As noted above, net 226Ra desorption from
concentrations have been increased by a factor of 22 (the
SPM appears to be complete at about 10 ppt salinity 238U/ 235U AR in nature).
(site 4). However, despite increasing dilution by
seawater, the 226Ra concentration of the surface wa-
ter does not decrease above 10 ppt salinity, but 5.2. Porewater samples
remains approximately constant (Fig. 3). This be-
haviour indicates a continued supply of 226Ra in the The porewater concentrations of all radium iso-
higher salinity regions from another source. topes are plotted against salinity in Fig. 5. In order to
The short-lived radium isotopes (224Ra, 223Ra, present the 223Ra data more clearly, the activities
228Ra)also increase along most of the estuary, but at have been multiplied by 22, the approximate
a much greater rate than 226 reaching concentra-
Ra, 238U/ 235U AR in nature (238U and 235U are the
tions many times reater than either end-member. parents of the decay series containing 226Ra and
P
The enrichment of 28Ra and 224Ra relative to 226Ra 223Ra, respectively). Both the 224Ra and 223Ra con-
in estuarine waters has been noted in previous stud- centrations increase with salinity, and all concentra-
ies [9,11 ,12,17], and is considered to be indicative of tions are well in excess of the surface water samples
a diffusive flux of radium from bottom sediments. from the same site and/or salinity (Table 1). The
The relative contributions of suspended and bot- surface water and porewater samples were collected
tom sediments to the excess dissolved radium can be on different occasions and under different flow con-
estimated from mass balance. We assume that SPM ditions but it is considered unlikely that the bottom
moves conservatively with water, or, if deposition sediment characteristics of the river had changed,
and resuspension of sediment is occurring, SPM and thus it is also unlikely that the radium content of
moves more slowly than the net water movement. At porewater at a given salinity had changed greatly.
site 4 (10 ppt salinity) the net 226Ra desorption from The measurements indicate that bottom sediment
SPM was calculated above to be 35 + 4 mBq/g. porewater is the source of 224Ra and 223Rato surface
The mean SPM concentration in this region of the water. Due to the strong tidal influence on water
estuary is 4 mg/l, indicating that 0.15 &-0.01 mBq/l depth in the estuary, it is considered that surface
226Ra has been released to the water column by water-porewater exchange driven by tidal pumping
SPM. This amount is only 8% f 1 of the dissolved was the primary process controlling the transfer of
excess 226Ra at site 4 (1.8 f 0.3 mBq/l). Calcula- radium from bottom sediments to surface water at
tions at other sites vary only slightly from this value. the time of sampling [ 181. Other processes, such as
The remaining excess 226Ra must originate bottom bioturbation and molecular diffusion, are considered
sediments. Similar calculations for the other isotopes to be only minor contributors.
show that > 99% of their activity originates from The high porewater activities of 224Ra and 223Ra
bottom sediments. indicate that the enrichment of these isotopes in

7. GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 151
estuarine surface water is primarily controlled by isotope in porewater. This is particularly evident in
two factors: the salinity, and hence the extent of the lower region of the estuary (site 7). Here, a
desorption of radium isotopes from bottom sedi- porewater 224Ra/ 226RaAR of 54 + 5 was measured,
-
ments into the porewater, and the extent of mixing a value _ 40 times the AR of their parent isotopes
between surface water and porewater. Both of these (228Th and 230Th) in the sediment. There is a similar
factors will result in an increase in the radium con- enrichment of 223Rarelative to 226Ra in this sample.
centrations of surface water as it moves towards the The ingrowth of the activity of a short-lived daughter
mouth of the estuary. Countering these increases will isotope (A,,) towards the activity of its long-lived
be the effects of dilution by low activity seawater. parent (ATh) is approximated by:
The similarity in the shape of all curves in Fig. 3
A Ri3 = A,,( 1 - eeA’)
suggests that surface water-porewater mixing will
also account for at least some of the excess dissolved where:
226Ra md 228
Ra in the Bega River estuary. This
A = ln2/t,,,
conclusion is supported by the porewater concentra-
tions of 228Ra and ‘*’Ra in the middle and upper and t1/2 is the half-life of the daughter isotope. A,,
estuary, which are higher than the surface water can be assumed to constant in sediments. Thus, if the
samples, although much less so than for 224Ra and initial activity of the daughter is low (e.g. due to its
223Ra.However, unlike 224Ra and 223Ra,the porewa- loss to surface water), then a short-lived daughter
ter concentrations of 228Ra level off in the lower isotope will grow back towards equilibrium with its
estuary, and 226Radecreases (Fig. 5). The porewater parent more rapidly than a longer lived daughter
concentration of 226 near the mouth of the estuary
Ra isotope.
(site 7) is lower than the corresponding surface water A simple sequential leaching experiment was de-
sample collected a year earlier. The fact that 226Ra in signed to simulate the effect of tidal pumping on
porewater is comparable with surface water in the bottom sediments and monitor the effect of isotope
lower estuary, suggests that bottom sediments con- half-life on the radium content of porewater. Bottom
tribute very little 226Ra to surface water in this sediments, collected from a freshwater stretch the of
region. Bega River (site 0) were shaken for 1 h with saline
Elsinger and Moore [2] noted that increased sur- water. The suspension was then centrifuged, the
face water concentrations of 226Ra in an estuary supematant filtered and analyzed for radium. More
could occur as a result of a decrease in river flow saline water was then added to the original sediment
following a period of relatively high flow. They and the whole process repeated 9 times on the same
suggested that movement of the salt wedge up the day. After the 10th leaching the sediment was stored
estuary may have released 226Ra from freshwater for 20 days and a 1lth leaching performed. Desorbed
sediments deposited during or after high flow. This radium was measured in the Ist, 4th, 7th, 10th and
process could explain the relatively high porewater 11th leachates.
concentrations of 226 in the upper-middle estuary
Ra Fig. 6 shows that decreasing amounts of 226Ra,
compared with the lower estuary, as the flow hydro- “sRa and 224 were desorbed during each succes-
Ra
graph of the Bega River was decreasing at the time sive leaching, indicating a gradual loss of the ion-ex-
of the sample collection. changeable radium originally present in the freshwa-
ter sediment. Due to its low activity concentrations
and large uncertainties, the behaviour of 223Rais not
6. The behaviour of 224Ra and 223Ra considered. After the 20 day delay, the activity of
226Ra md 228
desorbed Ra continued to fall, whereas
6.1. Regeneration of short-lived radium isotopes desorption of the short-lived isotope, 224Ra, in-
creased. Examination of the 224Ra/226Ra and
The high porewater activities of 224Ra, 223Ra and 228Ra/ 226Ra ARs (Table 3) indicates that the rela-
228Rarelative to 226 indicate that not only salinity,
Ra tive proportions of each isotope desorbed during
but half-life influences the concentration of each successive leaches remained approximately the same

8. 152 GJ. Hancock, A.S. Murray/Earth and Planetary Science Letters 138 (1996) 145-155
during the first day, but after the 20 day delay, the Table 3
Sequential leaching experiment: activity ratios of radium isotopes
224Ra/ 226Ra AR increased from an initial value of
leached from Bega River sediment
about 3.2, to a value of 9.9 f 1.1. The increase can
be explained by ingrowth of 224Ra activity in the
sediment back towards secular equilibrium with its
sediment-bound parent 228Th. Thorium desorption
from the sediment was negligible compared to ra- 1 2.07 M.13 3.2 *0.2
dium and, theoretically, the desorbed 224Ra activity 4 2.171tO.18 3.1 *0.5
should have returned to the activity of the 1st leach.
7 1.92i0.15 3.9 *0.7
The lower than expected 224Ra activity in the 11th
10 2.11 *to.31 3.7 AO.6
leachate could be due to the compaction and aggre-
gation of sediment particles during centrifugation, 11 1.85 ko.14 9.9*1.1
reducing the effective surface area for ion exchange.
These results indicate that the isotopic composi-
a Leach nos. I-10 were performed on the same day. Leach no.
tion of bottom sediment porewater is significantly I1 was performed 20 days later.
influenced by both the degree, and the rate, of
leaching of the sediments by saline water. The flush-
ing of bottom sediments by saline water each tidal ter. The similar relative behaviour of both isotopes is
cycle results in the incremental leaching of ion-ex- evident in Fig. 5. Table 2 shows that the 224Ra/ 223Ra
changeable radium from the sediment. If the time AR in all three estuarine porewater samples remains
scale of this leaching process is comparable to the approximately constant, and that these ARs are within
half-lives of 224Ra and 223Ra, there will also be measurement error of the AR of their arent iso-
4
significant regeneration of these isotopes. For the topes, estimated by the bottom sediment ’ ‘Th/ 235U
longer lived isotopes (226Ra and 228Ra), there will be AR (also in Table 2). We have assumed a 238 235U/ U
little regeneration. AR of 22, and secular equilibrium in the 235Useries
down to 227Th.
6.2. Rate of radium removal from bottom sediments The similarity in the sediment 228Th/ 235U AR
and the porewater 224Ra/ 223RaAR indicates that the
Some indication of the time scale of leaching time scale for the leaching of sediment-bound 224Ra
from bottom sediments can be obtained by compar- and 223Ra from bottom sediments into the water
ing the concentrations of 224Ra and 223Rain porewa- column is long compared with their half-lives (i.e.
weeks-months, or longer). Based on laboratory ex-
periments, Webster et al. [18] calculated that, at a
salinity of 50% seawater, no more that 1% of the
total pool of ion-exchangeable radium in bottom
sediments from the Bega estuary is dissolved in
porewater. This calculation is in accordance with
other experimental results [ 16,191, which have shown
that at high solid/liquid ratios most of the ion-ex-
changeable radium in a sediment-water system is
adsorbed to the solid phase. Thus, only a small
fraction of the total pool of ion-exchangeable radium
held in bottom sediments is lost to the water column
0 A-r-7- 1 :L__.
each tidal cycle. Flushing of bottom sediments by
0 2 4 6 8 10 12
tidal pumping occurs with a period of - 12 h, and
Leach number so it would take many weeks, based on Webster et
Fig. 6. Sequential leaching of radium against leach number, al.‘s calculation, to remove most of this pool. Over
showing a steady decrease, except for ZZ4Ra after 20 days storage. this period, most of ion-exchangeable radium in the

9. GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 153
.
--fj-- al,, Q2 4 ~- Qkv ‘Q3 Qkt
tidal limit
Qi
c,
P P
estuary mouth
P c: c:
Fig. 7. The multi-box model, showing the flow of water in the estuary.
sediment would have been regenerated. Under these dimensions of each box are summarised in Table 4.
conditions, we would expect the AR of 224Ra and Each box, i, has an average salinity, Si, and an
223Rain porewater to remain close to their parent AR average surface water Ra concentration Cl. Each box
of the sediment, in agreement with our observations. overlies porewater with an average Ra concentration,
Since the buffering capacity of the pool of ion-ex- CL, which remains constant for a given salinity
changeable Ra held by bottom sediments is large, we because of the buffering capacity of bottom sedi-
would thus expect the 224Ra and 223Raconcentration ment. The position of boxes 2 and 3 were chosen
of porewater to remain relatively unchanged over such that the average salinity of the box corre-
many tidal cycles. sponded to the salinity of the porewater sample
collected in that region of the estuary (Table 1 b 1.
The average salinity of the remaining area of the
7. Surface water-porewater mixing estuary (S , in box 1) did not match the salinity of the
porewater sample in this box. The value of CL is,
If the distribution of Ra in the estuary is assumed therefore, estimated from the approximate linear rela-
to have reached steady state, the flux of Ra from tionship between porewater 224Ra and 223Ra, and
porewater should equal the loss of radium in surface salinity, shown in Fig. 5. The values of Si and Cf
water by decay, and by advection to the sea. By have been determined by averaging the appropriate
determining Ra loss from the water column, the flux measurements in Table 1. Each measurement was
of water crossing the sediment-water interface in the weighted according to the length of estuary it repre-
Bega estuary can be estimated. To do this we apply a sented. The rate of exchange of water between ad-
2-D steady-state box model and use 224Ra and 223Ra joining boxes, due to mixing caused by tidal action,
data. The estuary is assumed to approximate a chan- is given by Q,,, and the net flow rate of water
nel 11 km long, its width ranging from 130 m in the passing through each box towards the mouth of the
upper estuary, to 300 m near its mouth, and its depth estuary (Q,) is given by the flow of river water
ranging from 1 to 2 m. This channel has been entering the estuary (180 Ml/day). The salinity and
divided into three adjoining boxes (Fig. 7). The Ra concentration of river water (S, and C,> and
Table 4
Values of parameters used in the multi-box model
2% z’Ra
Box length width depth S, c c: C,l F,’ Y’ C, CP FP I-J’
Ocm) (m) Cm) @pt) Wd) M&/L) WW (LIm’/d) @I@ (mW-) (mBSn) Wm?‘d) (mm)
1 5.3 130 1 2.0 29 5.3il.l 13.0*1.5 18OeO 22ok80 0.23 iO.06 0.34 M 10 250 A370 330 ++I50
2 3.6 160 1.5 14.4 333 21 l3 73 *a 170 *70 220 f90 1.15 ~0.25 2.6M.4 260 i210 310 *260
3 2.1 300 2 22.2 297 26+=3 94 *14 31Oi70 390 ill0 1.25 AO.25 4.5 aI 7 220 *70 280 t180

10. 154 G.J. Huncock, A.S. Murray/Earth and Planetary Science Letters 138 (1996) 145-155
seawater (S,, and C,,) entering the estuary are filling of sediments caused by tidal action, then r can
obtained from Table 1 (sites 0 and 8~). Given be set at one tidal period (l/2 d), and Hi calculated
steady-state conditions and salt mass balance, the (see Table 4). Inasmuch as mixing due to processes
rate of change in the salt content of box i is zero: other than tidal pumping, such as wave action and
bioturbation may also occur, Hi may tend to overes-
Si- ,(Q, + QiW ‘) + Si+ ,Qfw - SiQfC ’ timate the true mixing depth. Values of Fi, however,
- si(Qr + Qrw) = 0 (1) are not affected by this assumption.
Using our 224Ra and 223Ra surface water data,
We have assumed that the salinity of porewater
Webster et al. [ 181 estimated H, to be 150 mm
equals that of the overlying surface water, and so the
averaged over the whole estuary. They used a 1-D
effect of Q, on salinity is zero. Eq. 1 reduces to:
advection-diffusion equation to model the Ra distri-
Qfw = (Qr + Ql[w‘)(Si - Si- I)/(Si+ 1 - Si) bution, and estimated the flux of Ra from bottom
sediments using a desorption model based on labora-
Q,. Si and S,, are known, and Qfw is zero. Thus,
tory experiments. Our box model approach, which
Q fw can be determined for box 1, and Qf,,, can be uses actual porewater Ra concentrations to determine
determined for each subsequent box. Similarly, an
the bottom sediment flux of Ra, yields an average
equation can be written for the rate of change in the
H, of 260 f 60 mm for the whole estuary. This
Ra activity of box i due to tidal mixing. However,
value was obtained by weighting each HL according
on this occasion, terms describing the net input of Ra
to its analytical uncertainty, and the surface area of
from porewater, and the decay of unsupported Ra in sediment it represents. Given the analytical uncer-
the water column must be included:
tainty associated with this value, together with uncer-
Cf- ‘(Q, + Qf; ‘) + Cf’ ‘Qfw - CfQf; ’ tainties introduced into both models by approxima-
tions associated with the dimensions of the estuary,
- C;(Q, + Qiw) + Q;(C; - C:) the Ra distribution in surface water and porewater,
and the sediment composition of the estuary [ 181, the
- @Vi = 0
two estimates of H, are probably not significantly
where A is the decay constant of the Ra isotope. different.
Thus, the rate of water exchange across the sedi-
ment-water interface <QL> can be determined for
each box. The flux of porewater moving into each 8. Summary and conclusions
box (Fi) is then Q6/Ai, where Ai is the cross-sec-
tional area of sediment of the box. Estimates of Fi, During low river discharge the distribution of
and the values of parameters used to determine them dissolved radium in the Bega River estuary is almost
are summarised in Table 4. The uncertainties associ- entirely due to the flux of radium from bottom
ated with Fi were determined by propagating errors sediments. Radium isotopes accumulate in the pore-
of Ra measurement. Given these uncertainties, Fi water of bottom sediments, which is then mixed with
determined using 224Ra and 223Ra are not signifi- surface water. The distribution of radium in the
cantly different. For the Bega estuary, the total daily estuary is, therefore, controlled not only by the salin-
porewater flux corresponds to about 15% of the ity distribution, but also by the extent of surface
estuary volume, and 2.3 times the advective flow. water-porewater mixing.
The depth of sediment (H,) supplying the ob- The isotopic composition of radium in bottom
served flux of porewater over an interval of time t, is sediment porewater is strongly dependent on the
given by: extent and rate of leaching of the sediments. Sedi-
ments in the lower estuary, which have been leached
H; = Fit/@
by highly saline water over many tidal cycles, will
where @ is the sediment porosity (0.40). If we release high activities of the short-lived radium iso-
assume that porewater-surface water exchange in to s compared to 226Ra. The fact that the
229e
the Bega estuary is due entirely to the draining and Ra/ 223Ra AR of estuarine porewater is close to

11. G.J. Hancock. AS. Murray/Earth and Planetary Science Letters I38 (1996) 145-155 155
the 228Th/ 235U AR of its associated bottom sedi- 161D.G. Moore and M.R. Scott, Behaviour of 226Ra in the
Mississippi River mixing zone, J. Geophys. Res. 91, 143 17-
ment indicates that the time scale for the removal of
14329, 1986.
ion-exchangeable 224Ra and 223Rafrom bottom sedi-
[71 B.L.K. Somayajulu and T.M. Church, Radium, thorium and
ments is long compared to their half-lives. uranium isotopes in the interstitial water from the Pacific
We have used a 2-D steady-state box model and Ocean sediment, J. Geophys. Res. 78, 4529-4531, 1973.
224Ra and 223Ra concentrations to estimate the flux [81 J.K. Cochran, The flux of 226Ra from deep-sea sediments,
Earth Planet. Sci. Lett. 49, 381-392, 1979.
of porewater across the sediment-water interface.
[91 D.M. Levy and W.S. Moore, 2’4Ra in continental shelf
This information will be used to help determine the waters, Earth Planet. Sci. Leti. 73, 226-230, 1985.
fate of nutrients and other pollutants in the estuary. [lOI W.S. Moore, J.L. Sarmiento and R.M. Key, Tracing the
Amazon component of surface Atlantic water using 228Ra,
salinity and silica, J. Geophys. Res. 91, 574-2580, 1986.
Acknowledgements 1111 W.S. Moore and J.F. Todd, Radium isotopes in the Orinoco
estuary and eastern Caribbean Sea, J. Geophys. Res. 98,
2233-2244, 1993.
We thank Y-H Li, R.F. Stallard, I.T. Webster and [I21 M.S. Bollinger and W.S. Moore, Radium fluxes from a salt
an anonymous reviewer for helpful comments on this marsh, Nature 309, w-446, 1984.
manuscript. We particularly want to thank Y.-H. Li iI31 M.S. Bollinger and W.S. Moore, Evaluation of salt marsh
for his contribution to Section 7. [MKI hydrology using radium as a tracer, Geochim. Cosmochim.
Acta 57, 2203-2212, 1993.
[I41 G.J. Hancock and P. Martin, The determination of radium in
environmental samples by alpha-particle spectrometry. Appl.
References Rad. Isot. 42, 63-69, 1991.
[151 P. Martin and G.J. Hancock, Routine analysis of naturally
[l] Y.H. Li, G. Mathieu, P. Biscaye and H.J. Simpson, The flux occurring radionuclides in environmental samples by alpha-
of 226Ra from estuarine and continental shelf sediments, particle spectrometry, Research Rep. 7, Supervising Scientist
Earth Planet. Sci. Lett. 37, 237-241, 1977. for the Alligator Rivers Region, AGPS, Canberra, 1992.
[2] R.J. Elsinger and W.S. Moore, 226Ra behaviour in the Pee [161 G.J. Hancock, The effect of salinity on the sediment concen-
Dee River-Winyah Bay estuary, Earth Planet. Sci. Lett. 48, trations of radium and thorium, M.Sc. Thesis, Australian
239-249, 1980. National Univ., 1993.
[3] R.J. Elsinger and W.S. Moore, 226Ra and 228Ra in the [I71 R.J. Elsinger and W.S. Moore, 224Ra, ‘**Ra and 226Ra in
mixing zones of the Pee Dee River-Winyah Bay, Yangzte Winyah Bay and Delaware Bay, Earth Planet. Sci. Lett. 64,
River and Delaware Bay estuaries, Estuarine Coastal Shelf 430-436, 1983.
Sci. 18, 601-613, 1984. [181 1.T. Webster, G.J. Hancock and A.S. Murray, On the use of
[4] W.S. Moore, Radium isotopes in Chesapeake Bay, Estuarine radium isotopes to examine pore water exchange in an
Coastal Shelf Sci. 12, 713-723, 1981. estuary, Limnol. Oceanogr. 39(S), 1917- 1927, 1994.
[5] R.M. Key, R.F. Stallard, W.S. Moore and J.L. Sarmiento, [ 191 P. Benes, Radium in continental surface water, in: The
Distribution and flux of 226Ra and “‘Ra in the Amazon Environmental Behaviour of Radium, Vol. 1, pp. 373-418,
River estuary, J. Geophys. Res. 90, 69%-7004, 1985. 1990.