source and distribution of dissolved radium in the bega riverestuary, southeastern australia

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

Numerous publications have now described the non-conservative behaviour of radium in the mixing zones of rivers and oceans [l-6]. These studies have shown that the estuarine concentrations of 226Ra increase with increasing salinity to levels greater

* Corresponding author. Fax: +61 6 246 5800. E-mail: han- [email protected]

than both the river and ocean end-members, indicating a net addition of dissolved radium to the estuary. Li et al. [l] considered that this ‘excess’ 226Ra was supplied by river-borne sediments carried into the estuary. In saline water, the competition effects of soluble cations for ion exchange sites on sediment particles results in the desorption of surface-bound radium. Elsinger and Moore [2] determined a system- atic decrease in the 226 Ra concentration of suspended particulate matter (SPM) with rising salinity in the Winyah Bay estuary. Other studies [3-51 concluded

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146 GJ. Hancock, A.S. Murray/Earth and Planetary Science Lerters 138 (1996) 145-155

that estuarine bottom sediments also supply significant fluxes of radium. High concentrations of radium have been measured in near-bottom ocean water and deep-sea sediment porewater [7,8], implying that porewater of bottom sediments is the transfer medium.

Bottom sediments are thought to be the major source of the shorter lived isotopes, 228Ra (half-life 5.7 y) and 224Ra (half-life 3.6 d) to estuarine waters [4,9]. The enrichment of these isotopes in estuarine and near-shore environments is often much greater than the long-lived 226Ra (half-life 1600 y). Moore [4] suggested that this was due to their higher rate of activity regeneration by their insoluble thorium parents in bottom sediments. The high 228Ra/ 226Ra activity ratios (ARs) generated in coastal waters have been used as a tracer of water movement in oceans [lo] and 224Ra has been used to estimate current speeds in the Caribbean Sea [ 11 I.

Bollinger and Moore [ 12,131 measured the flux of 224Ra from marsh sediments and calculated the rate of porewater exchange with marsh surface water. Surface water-porewater exchange processes are im- portant to our understanding of estuarine processes because they affect the fate of nutrients and other particle reactive pollutants. Recently, Webster et al. [ 181 modelled the distribution of radium in the Bega River estuary, southeastern Australia. By matching model-predicted 224Ra and 223Ra surface water data with measurements, they estimated the effective

14&x4’ 36’42

tidal limit

depth to which bottom sediments were flushed by surface water during each tidal cycle.

Despite the extensive use of radium of isotopes as tracers in the marine environment, there has been little attempt to understand the processes governing the release of radium isotopes from marine sediments. In this paper we present the concentration data of all four naturally occurring radium isotopes, 226Ra, 228Ra, 224Ra and 223Ra (half-life 11.4 d) in the surface water and bottom sediment porewater of the Bega River estuary. To the best of our knowledge this is the first estuarine study incorporating measurements of 223Ra. Using these data we establish the source of dissolved radium to the estuary, and gain information on the rates and mechanism of radium release from estuarine sediments, and obtain estimates of the rate of surface water and porewater exchange in the estuary.

2. Site description

The Bega River is located in southeastern New South Wales, Australia. Its estuary comprises an 11 km reach from its tidal limit to where the river enters the Tasman Sea (Mogareka InIet, Fig. 1). During the period of this study the depth of the river ranged from about 1 m in the main channel of the upper region of the estuary to 2-3 m near its mouth. Localised areas up to 14 m deep were found about

tliacxn Lagoc

Fig. 1. A map of the Bega River estuary, showing sample site locations.

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- flow lagoons in the middle estuary, and swamp areas near the mouth. The bottom sediments in the river channel are typically sand and gravel. In the back- flow lagoons and swamps the sediments are fine grained, comprising mainly silt and clay minerals.

Water flow in and out of the estuary is restricted by a sand bar, the position of which is largely governed by the flow of the river. During this study river flow was relatively low (180 Ml/day) and the width of the mouth was only about 50 m. During low flow periods, the movement of water in the estuary is greatly influenced by the tide.

3. Methods

3.1. Sample collection

Water and suspended particulate matter @PM) samples were collected from seven sites along the estuary in November 1991. Samples were collected between the tidal limit and the mouth of the estuary,

Table 1

Filtered water samples from the Bega estuary

generally at low tide. A freshwater sample was collected upstream of the tidal limit (site 01, and a seawater sample was collected from Tathra Wharf (site 8w), about 3 km south of the estuary mouth. Sampling site locations are shown in Fig. 1.

Water samples were collected from about 0.3 m below the surface. A continuous flow centrifuge (CFC) was used ‘in situ’ to separate SPM with a particle size greater than approximately 1 pm. This apparatus enabled the collection of gram quantities of SPM from many hundreds of litres of water.

Bed sediment and porewater samples were collected from the main river channel. One site was sampled in July, 1992, and three other sites were sampled in December, 1992. Bottom sediments were collected to a depth of about 300 mm from areas of the river bed exposed at low tide. Porewater samples were obtained by allowing interstitial water from the surrounding sediments to fill the hole created by the sediment collection. The depth of porewater prior to collection was 100-150 mm. One other bottom sediment sample was collected from Blackfellows La- goon (site 3bl using an Eckman grab sampler.

site COlleaiOIl distance salinity SPM %a ?a %a ‘I’&

Sutface water

0 Nov 1991 1 I,

2 ”

3 *

4 11

5 *

6 I,

7 I,

-0.3 0.1 1.7 0.63 iO.08 1.3 i0.2 0.03 a.02

2.1 0.8 2.0 1.11 ho.13 3.1~0.6 0.12 Go.06

3.6 2.2 3.2 1.61 NO.16 5.8 kO.9 0.23 a.11

4.8 4.4 3.1 1.79 a.14 6.7 ?&.8 0.50M.14

5.1 10.0 5.2 2.6 M.2 11.4zt1.4 1.0 AO.3

7.0 14.9 3.8 2.8 hO.3 13.9i1.8 1.3 i0.3

9.3 20.0 2.9 3.0 i0.2 14.4 *1.5 1.1 iSo.2

11.0 26.7 1.8 2.6 ho.2 12.9h1.5 1.3 HI.2

1.1 ti.4

4.0 M.9

1.9 il.8

9.4 l 1.s

20 *3

21*3

25 i3

28 k3

SW Nov 1991

Porewater

0 Lkc 1992 4 *

5 July 1992

7 Dee 1992

14.0 35.8 0.7 1.30 H).os 0.7 a.1

-0.3 0.1

5.1 5.8

7.0 14.4

11.0 22.2

2.2 +&lo.2 4.4 AO.7 0.08 kOto.05 4.5 HI9

5.5 a.3 13.8 +1.3 1.03 HI.18 26 it2

3.1 HI.2 17.2 Al.9 2.6 MO.4 73 i8

1.75 LtO.20 17.6 12.5 4.5 +0.7 94 *14

0.21 io.04 3.110.3

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.

Sediment samples were solubilised by pyrosul- phate fusion. Radium, thorium and uranium measurements on filtered water and sediment samples were determined by alpha-particle spectrometry following 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).

4. Results

Dissolved radium isotope activity concentrations 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

10 20 30 40

salinity (ppt)

Fig. 2. Dissolved silicon concentrations and salinity in surface

water samples shows largely conservative mixing.

l

i

-. 0 lb i0 i0

salinity (ppt)

Fig. 3. Surface water radium isotope concentrations against salin-

ity. All estuarine concentrations lie above the conservative mixing

line, represented by the dotted line joining seawater (square) and

freshwater.

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 conservative 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 resuspension of bottom lagoon sediments is the most likely source of the additional SPM [16].

Concentrations of dissolved radium are plotted against salinity in Fig. 3. All isotopes show similar non-conservative behaviour in the estuary, with their concentrations lying well above the conservative

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.

The bottom sediment radionuclide data is pre- sented in Table 2. The loss of 226Ra from suspended and bottom sediment within the estuary is illustrated by a plot of the sediment 226Ra/ 230Th AR against salinity (Fig. 4). Th-230 is the parent of 226Ra, and is known to remain strongly bound to particles in saline water. The decrease in the 26Ra/ 230Th AR of fluvial sediment in saline water can, therefore, be used as a 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 (site 01, to a minimum of 0.59 + 0.02 at a salinity of IO ppt (site 4), and changes little with further increases in salinity. The reduction in 226Ra activity corresponds to 55 + 3% of the 226Ra content of the SPM in freshwater, or 35 f 4 mBq/g dry wt. There is also evidence of 226Ra loss from bottom sediments (Fig. 4), with the 226Ra/ 230Th AR decreasing from 1.09 f 0.06 in freshwater, to values around 0.71 in the estuary (Table 2). This decrease corresponds to a

Table 2

Bottom sediment radionuclide concentrations (mBq/g dry wt)

*- 0.4 -1

suspended sediment

0.0 &- II- I

0 5 10 15 20 25 30

salinity (ppt)

Fig. 4. 226Ra, 230 I% AR of suspended and bottom sediments

against salinity. The reduction in the AR is measure of radium

loss from the sediment as a result of exposure to saline water.

loss of about 35% f 7 from the river bed sediments, or 2.7 + 0.5 mBq/g dry wt, which in absolute terms is much less than the SPM. The difference can be attributed to the much larger mean particle size and much lower radionuclide concentration of the river- bed sediments. The bottom sediment sample of fine- grained mud from Blackfellows Lagoon contained radionuclide concentrations and apparent 226Ra losses similar to the SPM. Apparent losses of 22*Ra, as derived from the decrease in the 228Ra/ 232Th AR

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 l 3 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.

150 GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155

(Table 2), are similar to those of 226Ra for both river bed and lagoon sediments, indicating that radium loss is occurring on a time scale which is short compared with the 228Ra half-life.

5. Discussion: The source of dissolved radium

5.1. Surface water samples

The loss of radium from sediments in the Bega estuary coupled with the non-conservative increases in dissolved radium identifies sediments as the source of the additional or ‘excess’ dissolved radium in the estuary. As noted above, net 226Ra desorption from SPM appears to be complete at about 10 ppt salinity (site 4). However, despite increasing dilution by seawater, the 226Ra concentration of the surface water does not decrease above 10 ppt salinity, but remains approximately constant (Fig. 3). This behaviour indicates a continued supply of 226Ra in the higher salinity regions from another source.

The short-lived radium isotopes (224Ra, 223Ra, 228Ra) also increase along most of the estuary, but at a much greater rate than 226 Ra, reaching concentrations many times

P reater than either end-member.

The enrichment of 28Ra and 224Ra relative to 226Ra in estuarine waters has been noted in previous studies [9,11 ,12,17], and is considered to be indicative of a diffusive flux of radium from bottom sediments.

The relative contributions of suspended and bottom sediments to the excess dissolved radium can be estimated from mass balance. We assume that SPM moves conservatively with water, or, if deposition and resuspension of sediment is occurring, SPM moves more slowly than the net water movement. At site 4 (10 ppt salinity) the net 226Ra desorption from SPM was calculated above to be 35 + 4 mBq/g. The mean SPM concentration in this region of the estuary is 4 mg/l, indicating that 0.15 &- 0.01 mBq/l 226Ra has been released to the water column by SPM. This amount is only 8% f 1 of the dissolved excess 226Ra at site 4 (1.8 f 0.3 mBq/l). Calcula- tions at other sites vary only slightly from this value. The remaining excess 226Ra must originate bottom sediments. Similar calculations for the other isotopes show that > 99% of their activity originates from bottom sediments.

zi- 100

3 g 60

E 2 60

H i

0 5 10 15 20 25

salinity (ppt)

Fig. 5. Radium concentrations of porewater against salinity. 223Ra

concentrations have been increased by a factor of 22 (the

238U/ 235U AR in nature).

5.2. Porewater samples

The porewater concentrations of all radium isotopes are plotted against salinity in Fig. 5. In order to present the 223Ra data more clearly, the activities have been multiplied by 22, the approximate 238U/ 235U AR in nature (238U and 235U are the parents of the decay series containing 226Ra and 223Ra, respectively). Both the 224Ra and 223Ra concentrations increase with salinity, and all concentrations are well in excess of the surface water samples from the same site and/or salinity (Table 1). The surface water and porewater samples were collected on different occasions and under different flow conditions but it is considered unlikely that the bottom sediment characteristics of the river had changed, and thus it is also unlikely that the radium content of porewater at a given salinity had changed greatly. The measurements indicate that bottom sediment porewater is the source of 224Ra and 223Ra to surface water. Due to the strong tidal influence on water depth in the estuary, it is considered that surface water-porewater exchange driven by tidal pumping was the primary process controlling the transfer of radium from bottom sediments to surface water at the time of sampling [ 181. Other processes, such as bioturbation and molecular diffusion, are considered to be only minor contributors.

The high porewater activities of 224Ra and 223Ra indicate that the enrichment of these isotopes in

GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 151

estuarine surface water is primarily controlled by two factors: the salinity, and hence the extent of desorption of radium isotopes from bottom sediments into the porewater, and the extent of mixing between surface water and porewater. Both of these factors will result in an increase in the radium concentrations of surface water as it moves towards the mouth of the estuary. Countering these increases will be the effects of dilution by low activity seawater.

The similarity in the shape of all curves in Fig. 3 suggests that surface water-porewater mixing will also account for at least some of the excess dissolved 226Ra md 228 Ra in the Bega River estuary. This conclusion is supported by the porewater concentrations of 228Ra and ‘*’ Ra in the middle and upper estuary, which are higher than the surface water samples, although much less so than for 224Ra and 223Ra. However, unlike 224Ra and 223Ra, the porewater concentrations of 228Ra level off in the lower estuary, and 226Ra decreases (Fig. 5). The porewater concentration of 226 Ra near the mouth of the estuary (site 7) is lower than the corresponding surface water sample collected a year earlier. The fact that 226Ra in porewater is comparable with surface water in the lower estuary, suggests that bottom sediments con- tribute very little 226Ra to surface water in this region.

Elsinger and Moore [2] noted that increased surface water concentrations of 226Ra in an estuary could occur as a result of a decrease in river flow following a period of relatively high flow. They suggested that movement of the salt wedge up the estuary may have released 226Ra from freshwater sediments deposited during or after high flow. This process could explain the relatively high porewater concentrations of 226 Ra in the upper-middle estuary compared with the lower estuary, as the flow hydro- graph of the Bega River was decreasing at the time of the sample collection.

6. The behaviour of 224Ra and 223Ra

6.1. Regeneration of short-lived radium isotopes

The high porewater activities of 224Ra, 223Ra and 228Ra relative to 226 Ra indicate that not only salinity, but half-life influences the concentration of each

isotope in porewater. This is particularly evident in the lower region of the estuary (site 7). Here, a porewater 224Ra/ 226Ra AR of 54 + 5 was measured, - a value _ 40 times the AR of their parent isotopes (228Th and 230Th) in the sediment. There is a similar enrichment of 223Ra relative to 226Ra in this sample. The ingrowth of the activity of a short-lived daughter isotope (A,,) towards the activity of its long-lived parent (ATh) is approximated by:

A Ri3 = A,,( 1 - eeA’)

where:

A = ln2/t,,,

and t1/2 is the half-life of the daughter isotope. A,, can be assumed to constant in sediments. Thus, if the initial activity of the daughter is low (e.g. due to its loss to surface water), then a short-lived daughter isotope will grow back towards equilibrium with its parent more rapidly than a longer lived daughter isotope.

A simple sequential leaching experiment was de- signed to simulate the effect of tidal pumping on bottom sediments and monitor the effect of isotope half-life on the radium content of porewater. Bottom sediments, collected from a freshwater stretch the of Bega River (site 0) were shaken for 1 h with saline water. The suspension was then centrifuged, the supematant filtered and analyzed for radium. More saline water was then added to the original sediment and the whole process repeated 9 times on the same day. After the 10th leaching the sediment was stored for 20 days and a 1 lth leaching performed. Desorbed radium was measured in the Ist, 4th, 7th, 10th and 11 th leachates.

Fig. 6 shows that decreasing amounts of 226Ra, “sRa and 224 Ra were desorbed during each successive leaching, indicating a gradual loss of the ion-exchangeable radium originally present in the freshwater sediment. Due to its low activity concentrations and large uncertainties, the behaviour of 223Ra is not considered. After the 20 day delay, the activity of desorbed 226Ra md 228 Ra continued to fall, whereas desorption of the short-lived isotope, 224Ra, increased. Examination of the 224Ra/226Ra and 228Ra/ 226Ra ARs (Table 3) indicates that the relative proportions of each isotope desorbed during successive leaches remained approximately the same

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 224Ra/ 226Ra AR increased from an initial value of 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 radium and, theoretically, the desorbed 224Ra activity should have returned to the activity of the 1st leach. The lower than expected 224Ra activity in the 11th leachate could be due to the compaction and aggre- gation of sediment particles during centrifugation, reducing the effective surface area for ion exchange.

These results indicate that the isotopic composition of bottom sediment porewater is significantly influenced by both the degree, and the rate, of leaching of the sediments by saline water. The flushing of bottom sediments by saline water each tidal cycle results in the incremental leaching of ion-exchangeable radium from the sediment. If the time scale of this leaching process is comparable to the half-lives of 224Ra and 223Ra, there will also be significant regeneration of these isotopes. For the longer lived isotopes (226Ra and 228Ra), there will be little regeneration.

6.2. Rate of radium removal from bottom sediments

Some indication of the time scale of leaching from bottom sediments can be obtained by compar- ing the concentrations of 224Ra and 223Ra in porewa-

0 A-r-7- 1 :L__.

0 2 4 6 8 10 12

Leach number

Fig. 6. Sequential leaching of radium against leach number,

showing a steady decrease, except for ZZ4Ra after 20 days storage.

Table 3

Sequential leaching experiment: activity ratios of radium isotopes

leached from Bega River sediment

1 2.07 M.13 3.2 *0.2 4 2.171tO.18 3.1 *0.5

7 1.92i0.15 3.9 *0.7

10 2.11 *to.31 3.7 AO.6

11 1.85 ko.14 9.9*1.1

a Leach nos. I-10 were performed on the same day. Leach no.

I1 was performed 20 days later.

ter. The similar relative behaviour of both isotopes is evident in Fig. 5. Table 2 shows that the 224Ra/ 223Ra AR in all three estuarine porewater samples remains approximately constant, and that these ARs are within measurement error of the AR of their

4 arent iso-

topes, estimated by the bottom sediment ’ ‘Th/ 235U AR (also in Table 2). We have assumed a 238 U/ 235 U AR of 22, and secular equilibrium in the 235U series down to 227Th.

The similarity in the sediment 228Th/ 235U AR and the porewater 224Ra/ 223Ra AR indicates that the time scale for the leaching of sediment-bound 224Ra and 223Ra from bottom sediments into the water column is long compared with their half-lives (i.e. weeks-months, or longer). Based on laboratory experiments, 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-exchangeable 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 each tidal cycle. Flushing of bottom sediments by tidal pumping occurs with a period of - 12 h, and so it would take many weeks, based on Webster et al.‘s calculation, to remove most of this pool. Over this period, most of ion-exchangeable radium in the

GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 153

tidal limit --fj-- al,, Q2 4 ~- Qkv

. Qi Qkt

P ‘Q3

P c,

c: c: estuary mouth

P

Fig. 7. The multi-box model, showing the flow of water in the estuary.

sediment would have been regenerated. Under these conditions, we would expect the AR of 224Ra and 223Ra in porewater to remain close to their parent AR of the sediment, in agreement with our observations. Since the buffering capacity of the pool of ion-exchangeable Ra held by bottom sediments is large, we would thus expect the 224Ra and 223Ra concentration of porewater to remain relatively unchanged over many tidal cycles.

7. Surface water-porewater mixing

If the distribution of Ra in the estuary is assumed to have reached steady state, the flux of Ra from porewater should equal the loss of radium in surface water by decay, and by advection to the sea. By determining Ra loss from the water column, the flux of water crossing the sediment-water interface in the Bega estuary can be estimated. To do this we apply a 2-D steady-state box model and use 224Ra and 223Ra data. The estuary is assumed to approximate a channel 11 km long, its width ranging from 130 m in the upper estuary, to 300 m near its mouth, and its depth ranging from 1 to 2 m. This channel has been divided into three adjoining boxes (Fig. 7). The

Table 4

Values of parameters used in the multi-box model

dimensions of each box are summarised in Table 4. Each box, i, has an average salinity, Si, and an average surface water Ra concentration Cl. Each box overlies porewater with an average Ra concentration, CL, which remains constant for a given salinity because of the buffering capacity of bottom sediment. The position of boxes 2 and 3 were chosen such that the average salinity of the box corre- 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 estuary (S , in box 1) did not match the salinity of the porewater sample in this box. The value of CL is, therefore, estimated from the approximate linear relationship between porewater 224Ra and 223Ra, and salinity, shown in Fig. 5. The values of Si and Cf have been determined by averaging the appropriate measurements in Table 1. Each measurement was weighted according to the length of estuary it represented. The rate of exchange of water between adjoining boxes, due to mixing caused by tidal action, is given by Q,,, and the net flow rate of water passing through each box towards the mouth of the estuary (Q,) is given by the flow of river water entering the estuary (180 Ml/day). The salinity and Ra concentration of river water (S, and C,> and

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 l 3 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

154 G.J. Huncock, A.S. Murray/Earth and Planetary Science Letters 138 (1996) 145-155

seawater (S,, and C,,) entering the estuary are obtained from Table 1 (sites 0 and 8~). Given steady-state conditions and salt mass balance, the rate of change in the salt content of box i is zero:

Si- ,(Q, + QiW ‘) + Si+ ,Qfw - SiQfC ’

- si(Qr + Qrw) = 0 (1) We have assumed that the salinity of porewater

equals that of the overlying surface water, and so the effect of Q, on salinity is zero. Eq. 1 reduces to:

Qfw = (Qr + Ql[w ‘)(Si - Si- I)/(Si+ 1 - Si)

Q,. Si and S,, are known, and Qfw is zero. Thus,

Q fw can be determined for box 1, and Qf,,, can be determined for each subsequent box. Similarly, an equation can be written for the rate of change in the Ra activity of box i due to tidal mixing. However, on this occasion, terms describing the net input of Ra from porewater, and the decay of unsupported Ra in the water column must be included:

Cf- ‘(Q, + Qf; ‘) + Cf’ ‘Qfw - CfQf; ’

- C;(Q, + Qiw) + Q;(C; - C:)

- @Vi = 0

where A is the decay constant of the Ra isotope. Thus, the rate of water exchange across the sediment-water interface <QL> can be determined for each box. The flux of porewater moving into each box (Fi) is then Q6/Ai, where Ai is the cross-sec- tional area of sediment of the box. Estimates of Fi, and the values of parameters used to determine them are summarised in Table 4. The uncertainties associated with Fi were determined by propagating errors of Ra measurement. Given these uncertainties, Fi determined using 224Ra and 223Ra are not significantly different. For the Bega estuary, the total daily porewater flux corresponds to about 15% of the estuary volume, and 2.3 times the advective flow.

The depth of sediment (H,) supplying the ob- served flux of porewater over an interval of time t, is given by:

H; = Fit/@

where @ is the sediment porosity (0.40). If we assume that porewater-surface water exchange in the Bega estuary is due entirely to the draining and

filling of sediments caused by tidal action, then r can be set at one tidal period (l/2 d), and Hi calculated (see Table 4). Inasmuch as mixing due to processes other than tidal pumping, such as wave action and bioturbation may also occur, Hi may tend to overes- timate the true mixing depth. Values of Fi, however, are not affected by this assumption.

Using our 224Ra and 223Ra surface water data, Webster et al. [ 181 estimated H, to be 150 mm averaged over the whole estuary. They used a 1-D advection-diffusion equation to model the Ra distribution, and estimated the flux of Ra from bottom sediments using a desorption model based on laboratory experiments. Our box model approach, which uses actual porewater Ra concentrations to determine the bottom sediment flux of Ra, yields an average H, of 260 f 60 mm for the whole estuary. This value was obtained by weighting each HL according to its analytical uncertainty, and the surface area of sediment it represents. Given the analytical uncertainty associated with this value, together with uncertainties introduced into both models by approxima- tions associated with the dimensions of the estuary, the Ra distribution in surface water and porewater, and the sediment composition of the estuary [ 181, the two estimates of H, are probably not significantly different.

8. Summary and conclusions

During low river discharge the distribution of dissolved radium in the Bega River estuary is almost entirely due to the flux of radium from bottom sediments. Radium isotopes accumulate in the porewater of bottom sediments, which is then mixed with surface water. The distribution of radium in the estuary is, therefore, controlled not only by the salinity distribution, but also by the extent of surface water-porewater mixing.

The isotopic composition of radium in bottom sediment porewater is strongly dependent on the extent and rate of leaching of the sediments. Sedi- ments in the lower estuary, which have been leached by highly saline water over many tidal cycles, will release high activities of the short-lived radium iso- to 229e

s compared to 226Ra. The fact that the Ra/ 223Ra AR of estuarine porewater is close to

G.J. Hancock. AS. Murray/Earth and Planetary Science Letters I38 (1996) 145-155 155

the 228Th/ 235U AR of its associated bottom sediment indicates that the time scale for the removal of ion-exchangeable 224Ra and 223Ra from bottom sediments is long compared to their half-lives.

We have used a 2-D steady-state box model and 224Ra and 223Ra concentrations to estimate the flux of porewater across the sediment-water interface. This information will be used to help determine the fate of nutrients and other pollutants in the estuary.

Acknowledgements

We thank Y-H Li, R.F. Stallard, I.T. Webster and an anonymous reviewer for helpful comments on this manuscript. We particularly want to thank Y.-H. Li for his contribution to Section 7. [MKI

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