EPSLELSEVIER                         Earth and Planetary Science Letters I38 ( 1996) 145- 155    Source and distribution o...
146                  GJ. Hancock, A.S. Murray/Earth     and Planetary Science Lerters 138 (1996) 145-155that estuarine bot...
C.J. Hancock. AS. Murray/Earth        and Planetary Science Letters 138 (1996) 145-155              1472.5 km upstream of ...
148                         GJ. Hancock, AS. Murray/Earth          and Planetary Science Letters 138 (1996) 145-1553.2. La...
G.J. Hancock, AS. Murray/Earth             und Planetary Science Letters 138 (1996) 145-155mixing line joining the two end...
150                   GJ. Hancock, AS. Murray/Earth   and Planetary Science Letters 138 (1996) 145-155(Table 2), are simil...
GJ. Hancock, AS. Murray/Earth   and Planetary Science Letters 138 (1996) 145-155               151estuarine surface water ...
152                       GJ. Hancock, A.S. Murray/Earth       and Planetary Science Letters 138 (1996) 145-155during the ...
GJ. Hancock, AS. Murray/Earth             and Planetary Science Letters 138 (1996) 145-155                                ...
154                   G.J. Huncock, A.S. Murray/Earth   and Planetary   Science Letters 138 (1996) 145-155seawater (S,, an...
G.J. Hancock. AS. Murray/Earth      and Planetary Science Letters I38 (1996) 145-155                            155the 228...
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Source and distribution of dissolved radium in the bega riverestuary, southeastern australia


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

  1. 1. EPSLELSEVIER 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 1995Abstract Measurements of the activities of the four naturally occurring radium isotopes in the surface water and porewater of anestuary have yielded information on the release of radium from sediments and on the extent of surface water-porewaterinteraction in the estuary. Under low-flow conditions, the non-conservative behaviour of dissolved radium in the estuary isalmost 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 estuarineporewater can be explained by the repeated leaching of radium from bottom sediments by saline water, and the rapidregeneration of the short-lived isotope activity from their sediment-bound parent nuciides. The leaching of radium frombottom sediments is apparently occurring on a time scale which is long (weeks-months) compared with the 224Ra and 223Rahalf-lives, indicating that the amount of ion-exchangeable radium adsorbed to the sediments is large compared with theamount dissolved in porewater. By applying a simple 2-D steady-state multi-box model, 224Ra and 223Ra surface water and porewater concentrationshave been used to estimate the daily flux of porewater crossing the sediment-water interface in the Bega estuary. This fluxis found to be about 15% of the estuary volume.Keywords: New South Wales Australia; radium; surface water; pore water1. 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 wasnon-conservative behaviour of radium in the mixing supplied by river-borne sediments carried into thezones of rivers and oceans [l-6]. These studies have estuary. In saline water, the competition effects ofshown that the estuarine concentrations of 226Ra soluble cations for ion exchange sites on sedimentincrease 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 thecock@cbr.dwr.csiro.au Winyah Bay estuary. Other studies [3-51 concluded0012-821X/96/$12.00 0 19% Elsevier Science B.V. All rights reservedSSDI 0012-821X(95)00218-9
  2. 2. 146 GJ. Hancock, A.S. Murray/Earth and Planetary Science Lerters 138 (1996) 145-155that estuarine bottom sediments also supply signifi- depth to which bottom sediments were flushed bycant 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 asdeep-sea sediment porewater [7,8], implying that tracers in the marine environment, there has beenporewater of bottom sediments is the transfer little attempt to understand the processes governingmedium. the release of radium isotopes from marine sedi- Bottom sediments are thought to be the major ments. In this paper we present the concentrationsource 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 theand near-shore environments is often much greater Bega River estuary. To the best of our knowledgethan 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 theactivity regeneration by their insoluble thorium par- source of dissolved radium to the estuary, and gainents in bottom sediments. The high 228Ra/ 226Ra information on the rates and mechanism of radiumactivity 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 of224Ra from marsh sediments and calculated the rate 2. Site descriptionof porewater exchange with marsh surface water.Surface water-porewater exchange processes are im- The Bega River is located in southeastern Newportant to our understanding of estuarine processes South Wales, Australia. Its estuary comprises an 11because they affect the fate of nutrients and other km reach from its tidal limit to where the river entersparticle 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 rangedRiver estuary, southeastern Australia. By matching from about 1 m in the main channel of the uppermodel-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. 3. C.J. Hancock. AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 1472.5 km upstream of the mouth. There are two back- generally at low tide. A freshwater sample wasflow lagoons in the middle estuary, and swamp areas collected upstream of the tidal limit (site 01, and anear the mouth. The bottom sediments in the river seawater sample was collected from Tathra Wharfchannel 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 centrifugeby a sand bar, the position of which is largely (CFC) was used ‘in situ’ to separate SPM with agoverned by the flow of the river. During this study particle size greater than approximately 1 pm. Thisriver flow was relatively low (180 Ml/day) and the apparatus enabled the collection of gram quantitieswidth 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 were3. Methods collected to a depth of about 300 mm from areas of the river bed exposed at low tide. Porewater samples3.1. 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 tosamples 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 1Filtered 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. 4. 148 GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-1553.2. Laboratory analyses The CFC sediment suspension was washed withdemineralised water and dried. All water sampleswere filtered through 0.45 pm membrane filterswithin 24 h of collection. The suspended solidsconcentration of each sample was determined fromthe weight of dry residue on the filter. Dissolvedsilicon was determined on the filtered water samplesby flame AAS. l Sediment samples were solubilised by pyrosul-phate fusion. Radium, thorium and uranium mea-surements on filtered water and sediment samples iwere determined by alpha-particle spectrometry fol-lowing radiochemical separation [14,15]. Dissolved224Ra md 223 Ra activities in water samples werecorrected for decay between collection and analysis(usually less than 3 days). For 224Ra, a correctionwas also made for support by dissolved 228Th. In allcases the 228Th concentrations were less than 0.20mRq/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 andSPM concentrations at each site. The uncertainties inthe 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 similarFig. 2. Dissolved silicon concentrations and salinity in surface non-conservative behaviour in the estuary, with theirwater samples shows largely conservative mixing. concentrations lying well above the conservative
  5. 5. G.J. Hancock, AS. Murray/Earth und Planetary Science Letters 138 (1996) 145-155mixing line joining the two end-members (dashedline). All radium isotope concentrations increasesteadily, 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 decreaserapidly towards seawater concentrations near themouth of the estuary. *- 0.4 suspended sediment The bottom sediment radionuclide data is pre- -1sented in Table 2. The loss of 226Ra from suspendedand bottom sediment within the estuary is illustrated 0.0 &- II- Iby a plot of the sediment 226Ra/ 230Th AR against 0 5 10 15 20 25 30salinity (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 sedimentswater. The decrease in the 26Ra/ 230Th AR of fluvial against salinity. The reduction in the AR is measure of radiumsediment 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 radiumwhich has desorbed [2]. The suspended sediment ARdecreases 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 becreases in salinity. The reduction in 226Ra activity attributed to the much larger mean particle size andcorresponds 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, asthe estuary (Table 2). This decrease corresponds to a derived from the decrease in the 228Ra/ 232Th ARTable 2Bottom 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. 6. 150 GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155(Table 2), are similar to those of 226Rafor both riverbed and lagoon sediments, indicating that radiumloss is occurring on a time scale which is short zi-100compared with the 228Rahalf-life. 3 g 60 E 2 605. Discussion: The source of dissolved radium H i5.1. Surface water samples The loss of radium from sediments in the Begaestuary coupled with the non-conservative increases 0 5 10 15 20 25in 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. 223Raestuary. As noted above, net 226Ra desorption from concentrations have been increased by a factor of 22 (theSPM appears to be complete at about 10 ppt salinity 238U/ 235U AR in nature).(site 4). However, despite increasing dilution byseawater, the 226Ra concentration of the surface wa-ter does not decrease above 10 ppt salinity, but 5.2. Porewater samplesremains 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 activities228Ra)also increase along most of the estuary, but at have been multiplied by 22, the approximatea much greater rate than 226 reaching concentra- Ra, 238U/ 235U AR in nature (238U and 235U are thetions many times reater than either end-member. parents of the decay series containing 226Ra and PThe 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 samplesa 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 collectedtom 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 bottommoves 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 ofmoves 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 sedimentSPM was calculated above to be 35 + 4 mBq/g. porewater is the source of 224Ra and 223Rato surfaceThe mean SPM concentration in this region of the water. Due to the strong tidal influence on waterestuary 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 ofexcess 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 asThe 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. 7. GJ. Hancock, AS. Murray/Earth and Planetary Science Letters 138 (1996) 145-155 151estuarine surface water is primarily controlled by isotope in porewater. This is particularly evident intwo factors: the salinity, and hence the extent of the lower region of the estuary (site 7). Here, adesorption 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 isotopesbetween surface water and porewater. Both of these (228Th and 230Th) in the sediment. There is a similarfactors 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 daughtermouth of the estuary. Countering these increases will isotope (A,,) towards the activity of its long-livedbe 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 willalso 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 thesamples, although much less so than for 224Ra and initial activity of the daughter is low (e.g. due to its223Ra.However, unlike 224Ra and 223Ra,the porewa- loss to surface water), then a short-lived daughterter concentrations of 228Ra level off in the lower isotope will grow back towards equilibrium with itsestuary, and 226Radecreases (Fig. 5). The porewater parent more rapidly than a longer lived daughterconcentration 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 onporewater is comparable with surface water in the bottom sediments and monitor the effect of isotopelower estuary, suggests that bottom sediments con- half-life on the radium content of porewater. Bottomtribute very little 226Ra to surface water in this sediments, collected from a freshwater stretch the ofregion. 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, theface water concentrations of 226Ra in an estuary supematant filtered and analyzed for radium. Morecould occur as a result of a decrease in river flow saline water was then added to the original sedimentfollowing a period of relatively high flow. They and the whole process repeated 9 times on the samesuggested that movement of the salt wedge up the day. After the 10th leaching the sediment was storedestuary may have released 226Ra from freshwater for 20 days and a 1lth leaching performed. Desorbedsediments deposited during or after high flow. This radium was measured in the Ist, 4th, 7th, 10th andprocess 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- Ragraph 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 not6. The behaviour of 224Ra and 223Ra considered. After the 20 day delay, the activity of 226Ra md 228 desorbed Ra continued to fall, whereas6.1. 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 duringbut half-life influences the concentration of each successive leaches remained approximately the same
  8. 8. 152 GJ. Hancock, A.S. Murray/Earth and Planetary Science Letters 138 (1996) 145-155during the first day, but after the 20 day delay, the Table 3 Sequential leaching experiment: activity ratios of radium isotopes224Ra/ 226Ra AR increased from an initial value of leached from Bega River sedimentabout 3.2, to a value of 9.9 f 1.1. The increase canbe explained by ingrowth of 224Ra activity in thesediment back towards secular equilibrium with itssediment-bound parent 228Th. Thorium desorptionfrom the sediment was negligible compared to ra- 1 2.07 M.13 3.2 *0.2dium and, theoretically, the desorbed 224Ra activity 4 2.171tO.18 3.1 *0.5should have returned to the activity of the 1st leach. 7 1.92i0.15 3.9 *0.7The lower than expected 224Ra activity in the 11th 10 2.11 *to.31 3.7 AO.6leachate could be due to the compaction and aggre-gation of sediment particles during centrifugation, 11 1.85 ko.14 9.9*1.1reducing 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, ofleaching 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 iscycle results in the incremental leaching of ion-ex- evident in Fig. 5. Table 2 shows that the 224Ra/ 223Rachangeable radium from the sediment. If the time AR in all three estuarine porewater samples remainsscale of this leaching process is comparable to the approximately constant, and that these ARs are withinhalf-lives of 224Ra and 223Ra, there will also be measurement error of the AR of their arent iso- 4significant regeneration of these isotopes. For the topes, estimated by the bottom sediment ’ ‘Th/ 235Ulonger lived isotopes (226Ra and 228Ra), there will be AR (also in Table 2). We have assumed a 238 235U/ Ulittle 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 224Rafrom bottom sediments can be obtained by compar- and 223Ra from bottom sediments into the watering 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 etFig. 6. Sequential leaching of radium against leach number, al.‘s calculation, to remove most of this pool. Overshowing a steady decrease, except for ZZ4Ra after 20 days storage. this period, most of ion-exchangeable radium in the
  9. 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 an223Rain porewater to remain close to their parent AR average surface water Ra concentration Cl. Each boxof 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 salinitychangeable 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 chosenof 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 the7. 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, andporewater should equal the loss of radium in surface salinity, shown in Fig. 5. The values of Si and Cfwater by decay, and by advection to the sea. By have been determined by averaging the appropriatedetermining Ra loss from the water column, the flux measurements in Table 1. Each measurement wasof 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 waternel 11 km long, its width ranging from 130 m in the passing through each box towards the mouth of theupper estuary, to 300 m near its mouth, and its depth estuary (Q,) is given by the flow of river waterranging from 1 to 2 m. This channel has been entering the estuary (180 Ml/day). The salinity anddivided into three adjoining boxes (Fig. 7). The Ra concentration of river water (S, and C,> andTable 4Values 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. 10. 154 G.J. Huncock, A.S. Murray/Earth and Planetary Science Letters 138 (1996) 145-155seawater (S,, and C,,) entering the estuary are filling of sediments caused by tidal action, then r canobtained from Table 1 (sites 0 and 8~). Given be set at one tidal period (l/2 d), and Hi calculatedsteady-state conditions and salt mass balance, the (see Table 4). Inasmuch as mixing due to processesrate 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 mmequals that of the overlying surface water, and so the averaged over the whole estuary. They used a 1-Deffect 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, whichQ fw can be determined for box 1, and Qf,,, can be uses actual porewater Ra concentrations to determinedetermined for each subsequent box. Similarly, an the bottom sediment flux of Ra, yields an averageequation can be written for the rate of change in the H, of 260 f 60 mm for the whole estuary. ThisRa activity of box i due to tidal mixing. However, value was obtained by weighting each HL accordingon this occasion, terms describing the net input of Ra to its analytical uncertainty, and the surface area offrom 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 significantlywhere 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 foreach box. The flux of porewater moving into each 8. Summary and conclusionsbox (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 ofand the values of parameters used to determine them dissolved radium in the Bega River estuary is almostare summarised in Table 4. The uncertainties associ- entirely due to the flux of radium from bottomated 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 withdetermined using 224Ra and 223Ra are not signifi- surface water. The distribution of radium in thecantly 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 surfaceestuary 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 bottomserved flux of porewater over an interval of time t, is sediment porewater is strongly dependent on thegiven by: extent and rate of leaching of the sediments. Sedi- ments in the lower estuary, which have been leachedH; = Fit/@ by highly saline water over many tidal cycles, willwhere @ 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 229ethe Bega estuary is due entirely to the draining and Ra/ 223Ra AR of estuarine porewater is close to
  11. 11. G.J. Hancock. AS. Murray/Earth and Planetary Science Letters I38 (1996) 145-155 155the 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 andments 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 shelfThis 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 saltan 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 marshfor 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.