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Journal of Environmental Radioactivity
Volume 78, Issue 2, October 2004, Pages 199-216

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doi:10.1016/j.jenvrad.2004.05.004    How to Cite or Link Using DOI (Opens New Window)  
Copyright © 2004 Elsevier Ltd. All rights reserved.

Size-fractionated thorium isotopes (228Th, 230Th, 232Th) in surface waters in the Jiulong River estuary, China

Lei Zhang, Min ChenCorresponding Author Contact Information, E-mail The Corresponding Author, Weifeng Yang, Na Xing, Yanping Li, Yusheng Qiu and Yipu Huang
Department of Oceanography, Xiamen University, Xiamen 361005, China
Received 1 October 2003;  Revised 1 April 2004;  accepted 4 May 2004.  Available online 26 June 2004.



Abstract

Thorium isotopes (228Th, 230Th, 232Th and 234Th) are useful tracers for studying particle dynamics and trace element scavenging in marine environments. In this study, surface waters were collected along a salinity gradient from the Jiulong River estuary, China, for determination of activity concentrations of 228Th, 230Th and 232Th in different size fractions, namely, the >53 μm, 10–53 μm, 2–10 μm, 0.4–2 μm, 10 kDa–0.4 μm and the <10 kDa fractions. Our results indicated that the activity concentrations of 228Th, 230Th and 232Th in the Jiulong River estuarine waters were significantly higher than most of the previously reported values in coastal and oceanic seawaters, suggesting a higher lithogenic U and Th contribution from the Jiulong River Basin. When normalized to the particulate mass concentration, the activity concentrations of the three thorium isotopes decreased with increasing particle size, demonstrating the important role of surface areas of particles in controlling the scavenging of thorium from the water column. The partitioning of three thorium isotopes showed a common characteristic, i.e., the >53 μm fraction had the least share (0–1%), while the 10–53 μm fraction had the largest share of Th isotopes. The average value of the 230Th/232Th activity ratio (230Th/232Th)A.R. increased from 0.8 in the >53 μm fraction to 3.7 in the 10 kDa–0.4 μm fraction, indicating that the radiogenic Th isotopes are preferentially scavenged by the small size particles. (230Th/232Th)A.R. in the <10 kDa and 10 kDa–0.4 μm fractions were similar, however, suggesting a similar chemical composition and/or equilibrium partitioning between the low molecular weight and colloidal Th. It was very interesting to note that the geochemical behaviors of the three Th isotopes were different from each other. Dissolved 228Th had the highest concentration in the mid-salinity region, showing a non-conservative behavior with additional input. In contrast, dissolved 232Th showed a concave profile, indicating a net removal of 232Th during the mixing of fresh water with seawater. The behavior of Th isotopes in the <10 kDa fraction followed those in the dissolved phases. The difference in geochemical behaviors among three Th isotopes was ascribed to their different sources in the estuary.



1. Introduction

Understanding the particle dynamics in coastal waters has strong implications for carbon biogeochemical cycles and the fate of pollutants. The transport and deposition of particle-associated contaminants in a river estuary are very complex processes, involving physical, chemical and biological factors. Naturally occurring radionuclides that associate strongly with particles are useful tracers for investigation of biogeochemical processes that affect the dynamics of particles and the fate of particle-reactive pollutants because sources of certain radionuclides are well characterized and different decay half-life can be used to study transport and deposition processes over a range of time-scales.

Thorium isotopes (234Th, 232Th, 230Th and 228Th), due to their strong affinity to particle surfaces and the wide range of half-lives (234Th, T1/2 = 24.1 d; 228Th, T1/2 = 1.91 a; 230Th, T1/2 = 7.52 × 104 a; 232Th, T1/2 = 1.39 × 1010 a), have been widely used as tracers for particle dynamics in marine environments. For example, the disequilibria between the parent–daughter pairs, 234Th/238U, 230Th/234U and 228Th/228Ra, have been widely used to estimate the residence times of trace metals and particles in seawater over a range of time-scales (e.g., Coale and Bruland, 1985; Buesseler et al., 1992; Cochran et al., 1993; Cochran et al., 2000 and Chen et al., 1996). Besides, thorium isotopes with longer half-lives such as 232Th and 230Th, can bring additional and distinct information about the source of particles in marine environments (Roy-Barman et al., 1996). Important processes affecting the distribution of Th isotopes (232Th, 230Th and 228Th) in estuaries include: (1) the input of dissolved and particulate Th species from river (for 232Th, 230Th and 228Th); (2) lateral transport of Th species (230Th, 228Th) from shelf/slope areas where they were introduced into the water column by the decay of parent nuclides such as 234U and 228Ra; (3) re-suspension of bottom sediments (for 232Th, 230Th and 228Th); and (4) removal from water column by scavenging and particle sedimentations (for all three Th isotopes). In order to better use Th isotopes to evaluate the dynamics of estuarine particles, their distributions and partitioning among dissolved and different size-fractionated particles need to be better understood.

Most of the previous studies have focused on the size-fractionated 234Th in marine particles (Baskaran et al., 1992; Moran and Buesseler, 1993; Murray et al., 1996 and Guo et al., 2002), whereas there are few studies on the other thorium isotopes, such as 228Th, 230Th and 232Th (Shaw et al., 1998; Luo and Ku, 1995 and Smoak et al., 1999). Detailed size fractionation of multiple Th isotopes in both ‘dissolved’ and particulate phases is scarce. Based on 234Th/238U and 228Th/228Ra disequilibria, Cochran et al. (1993) estimated the aggregation and disaggregation rates of marine particles and the removal rates of large particles in the water column. Recently, Guo et al. (2002) determined the scavenging of 234Th and its phase partitioning among different size fractions, namely, <0.5, 0.5–1, 1–10, 10–53 and >53 μm, in the Gulf of Mexico. Their results showed that the 10–53 μm particulate phase had the largest share of particulate 234Th, and there was a positive correlation between particulate organic carbon (POC)-normalized 234Th/POC ratio and total acid polysaccharides content. Roy-Barman et al. (2002) measured the total, filtered (<0.2 μm) and ultrafiltered (<1 kDa) 230Th and 232Th contents in the western Mediterranean Sea. Their results indicated that (230Th/232Th)A.R. in the <1 kDa phase was similar to those in the <0.2 μm phase, suggesting that an equilibrium was reached between the truly dissolved and the colloidal Th.

Until recently, we knew little about the geochemical behavior of 228Th, 230Th and 232Th in the estuarine region. Anderson et al. (1995) measured the concentrations of dissolved 230Th and 232Th in fresh and brackish waters in the Baltic Sea. The 230Th/232Th atomic ratios in their estuarine waters were 10–100 times higher than those in the river waters and the offshore seawaters, indicating that both Th isotopes enter the estuary as a mixture of two carrier phases. Balakrishna et al. (2001) reported the concentrations of 230Th and 232Th in suspended particulate matter and bottom sediments in the southwest coast of India. Santschi et al. (1979) determined the total, particulate, dissolved and colloidal forms of 228Th, 230Th, 232Th and 234Th in the water column in Narragansett Bay, and suggested that the removal of all Th isotopes was controlled mainly by the particle concentrations in the water column and the re-suspension of sediments.

In this study, surface waters were collected at five stations in the Jiulong River estuary, Fujian, China. The phase partitioning of 228Th, 230Th and 232Th among different size fractions, namely, <10 kDa, 10 kDa–0.4 μm, 0.4–2 μm, 2–10 μm, 10–53 μm and >53 μm, was examined. The objective was to understand the geochemical behavior of the size-fractionated thorium isotopes in the estuarine waters.

2. Materials and methods

2.1. Study area

The Jiulong River is the second largest river in Fujian Province, China. The river has a catchment area of 14,700 km2, and an annual average river discharge of 14,800 × 106 m3, which flows into the coastal region of Xiamen and further into Taiwan Strait (Fig. 1). The lithology of the drainage area consists largely of granite. Long-term maximum and minimum discharge occurs in June and September, respectively, resulting in a seasonal variation of salinity in the estuarine mixing zone. The Jiulong River inlet and its estuarine system has an area of about 85 km2, which contains a water volume of 550 × 106 m3 estimated by bathymetry. Tide is a major factor affecting the mixing of freshwater and seawater, along with wind and advective exchange.


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Fig. 1. A map of the Jiulong River estuary and sampling stations.

2.2. Sample collection

Surface waters were collected from not, vert, similar0.5 m depth at five stations in the Jiulong River estuary using a peristaltic pump on March 12, 2002, onboard the R/V Ocean 1. Sampling stations were designed to cover different salinities along a transect from the river mouth to offshore. Details of the sampling locations and the ancillary data are listed in Table 1. It should be pointed out that the salinity at the most offshore station (S = 31.4, JLJ 1) was slightly lower than that in the open sea (not, vert, similar33). Before the sample collection, all of the tubes and containers were pre-cleaned with 1 M HCl and Milli-Q water in the Lab. About 30 l of seawater at each station was pumped through the tubing and discarded. In addition, the containers were rinsed with in situ seawater three times before collecting the samples.

Table 1. Sampling locations in the Jiulong River estuary Image

Surface waters were size-fractionated by filtration and ultrafiltration for 232Th, 230Th and 228Th measurements. Briefly, not, vert, similar500 l waters were directly passed, at 10–12 l/min, through a 300-mm diameter stainless steel filter holder with a 53 μm Nylon screen. The prefiltered seawater (<53 μm, not, vert, similar120 l) and the screen with the >53 μm particles were taken back to the lab for further treatment. Sample collection was completed within 1 h at each station. Once in the lab, the screens were soni-cleaned for 2 min in 200 ml of filtered seawater (<0.4 μm) to resuspend the large particles. The resulting solution was filtered through a 0.4 μm polycarbonate membrane for collecting the >53 μm particles. Subsamples (not, vert, similar20 l) of the prefiltered seawater (<53 μm) were sequentially passed through a series of polycarbonate filters with the nominal pore sizes of 10 μm, 2 μm and 0.4 μm. The three filters resulted in three different size fractions, namely 10–53 μm, 2–10 μm, 0.4–2 μm. As for the colloidal Th isotopes analysis, large volume seawaters were needed due to the low concentration of Th isotopes in the colloidal fraction. About 100 l of the <53 μm seawaters were first filtered through a 0.4 μm polypropylene cartridge. The resulting <0.4 μm solutions were further size-fractionated into the colloidal (10 kDa–0.4 μm) and low molecular weight fractions (<10 kDa) by a cross-flow ultrafiltration system with a 10,000 NMW hollow fiber cartridge (H10P10, Millipore). The system was cleaned by soaking in 0.2 M HCl and then thoroughly rinsed with Milli-Q water before sampling and between samples. Details for the colloidal Th isotopes collection were described in Chen et al. (2000). The concentration factors for ultrafiltration were 31–44. The colloidal Th concentration (CTh) was calculated as follows:


CTh=(RThUTh)/CF

where RTh, UTh are the thorium concentrations measured in the retentate and integrated permeate samples, respectively, and CF is the concentration factor.

In the following discussion, according to the traditional definition, the >0.4 μm and the <0.4 μm fractions were defined as particulate and dissolved phases, respectively. The 10 kDa–0.4 μm fraction was defined as the colloidal phase, and the <10 kDa fraction was called the low molecular weight (LMW) phase.

2.3. Measurements of thorium isotopes

In this study, both uranium (238U, 234U) and thorium isotopes (234Th, 232Th, 230Th and 228Th) in the samples were determined, but only three of the thorium isotopes (232Th, 230Th and 228Th) are reported here.

For the thorium analysis in solution samples (i.e. LMW and 10 kDa–0.4 μm colloidal fraction), both 232U–228Th and 229Th yield tracers and 60 mg of Fe carrier (as FeCl3) were added to the samples after acidification. The samples were thoroughly mixed by bubbling with nitrogen gas for not, vert, similar10 min, and then allowed to stand for not, vert, similar6 h for spikes to equilibrate with seawaters. About 40 ml of concentrated NH4OH were added to precipitate Fe(OH)3, which co-precipitate with Th. The sample was stirred for 20 min, and then allowed to settle. The precipitate was separated from the supernatant by centrifugation. The resultant Fe(OH)3 was re-dissolved in concentrated HCl to make a 9 M HCl solution for further column chromatography work.

Detailed procedures for Th separation and purification were described in Cai et al., 2002 and Chen et al., 2003. Briefly, separation of U and Th was accomplished by passing the solution through a chloride-form anion resin column (Bio-Rad AG 1X8), preconditioned with 9 M HCl. For Th purification, the effluent containing the Th fractions was taken to dryness, dissolved in 40 ml 8 M HNO3, and then passed through a nitrate-form AG 1X8 column, preconditioned with 70 ml of 8 M HNO3. After washing with 6 column volumes of 8 M HNO3, Th was eluted with 90 ml 9 M HCl, evaporated down to one drop, extracted into a TTA-benzene solution and deposited on a stainless steel disk for α/β measurement.

The particulate samples were transferred to Teflon beakers for spiking and digestion, according to procedures given by Anderson and Fleer (1982). After digestion, the above procedure for dissolved Th was used for Th purification.

Radioactivities of 232Th, 230Th, 229Th and 228Th in the stainless steel disk were measured by alpha spectrometer with silicon surface barrier detectors (EG&G) at 4013, 4687, 4845 and 5430 keV, respectively. The peak areas of 228Th were corrected for the contribution of its daughter nuclide 224Ra (branch ratio 4.9%). Due to the addition of 228Th yield tracers in our procedure, the activities of 228Th in the samples were obtained by subtracting the added amount of 228Th from the measured activities. This treatment will cause a large error for 228Th data. The yield of thorium isotopes during the whole treatment was obtained by the measured and the added activities of 229Th. All of the reported errors were propagated from one sigma counting uncertainty.

2.4. Measurements of total particle concentration (TSM) and salinity

Salinity was measured on-board ship using a refractometer (ATAGO, S-10E). For TSM analysis, about 2 l of seawater was sequentially passed through a series of pre-weighted polycarbonate filters with the nominal pore sizes of 10 μm, 2 μm and 0.4 μm. After filtration, the filters were rinsed with not, vert, similar50 ml distilled water to remove sea salts, and dried to constant weight at room temperature in a vacuum desiccator. Total particle concentration was calculated by dividing the particle weight by the water volume filtered.

3. Results

3.1. Volume activity concentrations of 228Th, 230Th and 232Th

The volume activity concentrations of 228Th, 230Th and 232Th in different size fractions are listed in Table 2. The volume activity concentrations of total 228Th (the sum of all size fractions) ranged from 6.3 to 21.8 mBq/l with an average of 14.5 mBq/l. Most of 228Th was present in the particulate phase. The particulate and dissolved 228Th ranged from 4.3 to 19.5 mBq/l (with an average of 12.7 mBq/l) and from 0.5 to 3.0 mBq/l (averaging 1.8 mBq/l), respectively. For comparison, some of the previous published 228Th data are listed in Table 3. It was obvious that, no matter in particulate or dissolved fraction, concentrations of 228Th in the Jiulong River estuary were about 100-fold higher than those in the open ocean and coastal seawaters. However, it should be pointed out that most of the published 228Th data were obtained from regions with high salinity. We did not find data from estuaries for comparison.

Table 2. Size-fractionated concentrations of 228Th, 230Th, 232Th and TSM in surface waters in the Jiulong River estuary

Image

BD and ND means below detection and no data, respectively. In the calculation of total activities, BD was treated as 0.

Table 3. Concentrations of 228Th, 230Th, 232Th from published datasets

Image

ND means no data.

The volume activity concentrations of dissolved 230Th (<0.4 μm) ranged from 0.05 to 0.74 mBq/l with an average of 0.30 mBq/l. Our results were consistent with those obtained in fresh and brackish waters in the Baltic Sea (Anderson et al., 1995), but were 10–100 fold higher than those obtained from the regions with high salinity ( Table 3). Due to the high TSM concentrations in our study area, the volume activity concentrations of particulate 230Th (>0.4 μm) were much higher than those in the open ocean and coastal waters (Table 3). The high dissolved and particulate 230Th concentrations may result from the inputs of the lithogenic materials from the Jiulong River Basin.

Similar to 228Th and 230Th, most of the 232Th (not, vert, similar98%) in the estuarine waters were in the particulate phase. The volume activity concentrations of particulate 232Th were 1.0–15.7 mBq/l with an average of 8.2 mBq/l, which were 1000-fold higher than those in the open ocean and the coastal sea (Table 3). The volume activity concentrations of dissolved 232Th ranged from 0.08 to 0.26 mBq/l with an average of 0.14 mBq/l. These values were not, vert, similar10-fold higher than those in the open ocean, and close to the values found in the fresh and brackish waters in the Baltic Sea, but obviously lower than those previously reported for nearshore seawaters in China (Table 3). Huh et al. (1989) summarized the volume activity concentrations of dissolved 232Th in seawaters, and found that the reported concentrations had a wide range (from 10−6 to 10−1 mBq/l), which was ascribed to the different methods for 232Th measurement and contamination in the sample collection.

The volume activity concentrations of colloidal 228Th, 230Th and 232Th in the Jiulong River estuarine waters ranged from 0.15 to 0.55, from 0.02 to 0.10 and from 0.01 to 0.03 mBq/l, respectively. Colloidal fractions (>10 kDa) only contributed 16.5% (228Th), 16.7% (230Th) and 11.0% (232Th) of the dissolved phase (<0.4 μm). It was obvious that most of the dissolved Th isotopes in the estuarine waters existed in the low molecular weight fraction (<10 kDa) via association with low molecular weight organic and inorganic ligands. Roy-Barman et al. (2002) measured the colloidal 232Th and 230Th concentrations in the western Mediterranean Sea and found that 36–78% (232Th) and 47–77% (230Th) of the dissolved Th were in the <1 kDa fraction. The fractions associated with colloids in the western Mediterranean Sea were two or three times higher than those in our study, partly because of the difference in membrane cutoff used for colloid collection. To our knowledge, most of the previous colloidal Th studies focused on 234Th, and our study was the first to report the colloidal 228Th concentration in estuarine waters. For comparison, Table 4 summarized the fraction of the colloidal 234Th (10 kDa–0.2 or 0.4 μm) in the dissolved phase. The percentages of colloidal 234Th in the dissolved phase varied significantly among different locations, ranging from 0.04 to 78%. Our results were within this range, and agreed well with the data from the Atlantic (Guo et al., 1997), the Buzzards Bay ( Moran and Buesseler, 1993) and the coastal of Texas ( Baskaran and Santschi, 1993).

Table 4. Percentages (Fc/d) of colloidal 234Th in the dissolved phases

Image

3.2. Mass activity concentrations of thorium isotopes

Since the concentrations and the chemical constitutes of the suspended particles in the estuarine water vary largely, it is difficult to compare the volume specific concentrations of particulate phases obtained from different locations and different seasons. Here we adopted the mass activity concentration to represent their concentrations in the particulate phases. The mass activity concentrations of particulate 228Th, 230Th and 232Th in the estuarine waters ranged from 0.15 to 0.62 (average 0.30), from 0.11 to 0.22 (average 0.15) and from 0.12 to 0.19 (average 0.14) Bq/g, respectively. The mass activity concentrations of 232Th in the study area were significantly higher than the average content in the soils around the world (0.04 Bq/g; Bowen, 1979) and those in the southwest coast of India (0.04 Bq/g; Balakrishna et al., 2001), indicating a high terrestrial input of uranium and thorium from soils of the drainage basin in Fujian province, China. Indeed, results from measurements of natural radioactivity in the waters, soils and organisms around China indicated that 232Th content of soils in Fujian province were the highest in China, with an average of 0.10±0.05 Bq/g (Study Group for National Environmental Radioactivity, 1992a and Study Group for National Environmental Radioactivity, 1992b), consistent with our high specific Th mass concentrations observed for the Jiulong River estuary.

3.3. Partitioning of thorium isotopes among different size fractions

The size-fractionated dissolved and particulate 228Th, 230Th and 232Th in the Jiulong River estuarine waters are shown in Fig. 2. In general, the >53 μm fraction had the lowest percentage (0–1%) in bulk particulate phase at all stations. The 10–53 μm particles had the largest shares (68% of 228Th, 77% of 230Th, 91% of 232Th) except at the highest salinity station (JLJ 1). The partitioning of thorium isotopes between particulate fractions seemed to follow the partitioning of particle concentrations. In our study area, the TSM concentrations of the 10–53 μm particles were the highest, and the >53 μm particles were the least abundant (Table 2). It is interesting to note that the 10–53 μm fraction also had the largest share of 234Th in the Gulf of Mexico (Guo et al., 2002). It appears that the 10–53 μm particles are important for thorium scavenging and removal in water column.


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Fig. 2. Partitioning of 228Th, 230Th and 232Th among different size fractions in surface waters in the Jiulong River estuary.

Along the sampling section, the percentages of dissolved Th isotopes in the total Th concentration increased with increasing salinity. The values changed from 6% (228Th), 1% (230Th) and 2% (232Th) at the low salinity station (JLJ 5) to 32% (228Th), 33% (230Th) and 21% (232Th) at the high salinity station (JLJ 1). The spatial variability of Th percentages in the LMW fraction followed that in the dissolved phase. The variability of 228Th, 230Th and 232Th was similar to that of 234Th, which mostly occurred in particulate phase in coastal seawater but in dissolved phase in the open ocean (Huang and Chen, 2001).

4. Discussion

4.1. High Th characteristic

Our results showed that the contents of 232Th, 230Th and 228Th in the surface water in the Jiulong River estuary were higher than those reported previously for other coastal seas, in terms of volume activity concentrations or mass activity concentrations. The high Th concentration in the study area can be attributed to the high particle concentrations in seawater with high terrestrial U and Th inputs from the drainage basin. The TSM concentrations (40–110 mg/l) in the Jiulong River estuary are also significantly higher than those in other coastal areas, such as Narragansett Bay and the Baltic Sea. High TSM concentrations resulted in the high volume activity concentrations of Th in the Jiulong River estuarine waters. Furthermore, most of the soils in the Jiulong River come from the weathering of granite, a rock with high uranium and thorium contents, resulting in the high mass activity concentrations of Th.

Among the three thorium isotopes, dissolved 228Th had the highest concentration, followed by 230Th and 232Th. This order was consistent with previous reports (e.g., Santschi et al., 1979; Anderson et al., 1995 and Roy-Barman et al., 2002). The order of the volume activity concentrations in the particulate phase (>0.4 μm) follows: 228Th>230Th≈232Th. For comparison, Hirose and Sugimura (1993) also found the same trend in the northwest Pacific surface water. The difference in volume activity concentrations among three Th isotopes was due to the difference of their half-lives and their source functions. The 228Th was produced by the decay of its mother nuclide 228Ra, which has high concentration in estuarine waters due to desorption from particles during the mixing of fresh water with seawater. Besides, the short half-life of 228Th implies that it turns over more quickly than 230Th and 232Th in seawater. Both factors result in the high 228Th concentration in estuarine waters. In contrast to 228Ra, 234U, the mother nuclide of 230Th, consistently shows a conservative behavior in estuarine waters, and the half-life of 230Th is longer than that of 228Th. The major source of 232Th in seawater is the input of the weathering of terrestrial matter. Most of the 232Th will be deposited to the sediments in the river mouth, causing the relatively low 232Th concentration in the surface water compared to 230Th and 228Th.

Fig. 3 summarizes the mass activity concentrations of Th isotopes among different size particles. In general, 228Th, 230Th and 232Th contents decreased with the increasing size of the particle, except for 228Th and 230Th in the >53 μm fraction (Fig. 3). The mass activity concentrations of 228Th and 230Th in the >53 μm fraction were higher than those in the 10–53 μm and the 2–10 μm fractions, which was mostly due to the values measured at station JLJ 1 where the TSM was significantly lower. The mass activity concentrations of Th isotopes decreased with increasing particle size demonstrating that specific surface area is an important factor controlling the scavenging of Th. The small particles generally have higher specific surface areas and could absorb more Th from seawaters.


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Fig. 3. The average values of the mass activity concentrations of 228Th, 230Th and 232Th in different size particles.

4.2. Geochemical behavior of dissolved 228Th, 230Th and 232Th

The geochemical behavior of dissolved (<0.4 μm), LMW (<10 kDa) and colloidal (10 kDa–0.4 μm) fractions were different for 228Th, 230Th and 232Th in the Jiulong River estuary (Fig. 4). The volume activity concentrations of dissolved 228Th increased from the minimum (0.55 mBq/l) at station JLJ 5 (S = 1.1) to the maximum (3.0 mBq/l) at station JLJ 2 (S = 18.5), and then decreased with increasing salinity. Dissolved 228Th showed non-conservative behavior with an additional input in mid-salinity regions (Fig. 4a). This non-conservative behavior of dissolved 228Th is attributed to the desorption behavior of 228Ra, the parent radionuclide of 228Th, from particles in the estuarine waters. The distribution of dissolved 228Ra in the Jiulong River estuary was obtained in the same cruise. The 228Ra also showed non-conservative behavior and the maximum concentration was observed at the stations with salinity around 12–18, indicating desorption of radium from river-borne suspended particles (data not shown).


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Fig. 4. Distributions of dissolved, colloidal and low molecular weight 228Th (a), 230Th (b) and 232Th (c) in surface waters in the Jiulong River estuary.

The volume activity concentrations of dissolved 230Th, in general, increased with increasing salinity, varying from 0.05 mBq/l at station JLJ 5 (S = 1.1) to 0.74 mBq/l at station JLJ 1 (S = 31.4) (Fig. 4b). Although there were some fluctuations along the salinity gradient, dissolved 230Th in the study area showed conservative behavior. This apparent conservative behavior may result from a net effect between the input and the removal of 230Th in the estuary. The sources of dissolved 230Th in estuarine waters included the input of the lithogenic matter and the radiogenic production from 234U, and the fate of the 230Th was controlled by scavenging and removal from the water column by marine particles. Since the activities of 234U increased linearly with increasing salinity in the Jiulong River estuary (Chen et al., 1999), the production rates of 230Th from 234U will increase from the river mouth to the offshore stations. Particle scavenging and removal were reduced in the offshore regions due to the lower concentrations of TSM. Both processes resulted in the increase of dissolved 230Th from river mouth to the open ocean.

The behavior of dissolved 232Th was different from those of 228Th and 230Th in the Jiulong River estuary. The volume activity concentrations of dissolved 232Th decreased from 0.14 mBq/l at station JLJ 1 (S = 1.1) to 0.08 mBq/l at station JLJ 3 (S = 12.4), and then increased to 0.26 mBq/l at station JLJ 5 (S = 31.4), showing a typical removal behavior in the estuary (Fig. 4c). It is well known that 232Th is the only non-radiogenic thorium isotope and is the earliest ancestor of Th decay series. Its pathway into the sea included the input from rivers and aerosols (Huh et al., 1989). Considering that the scavenging or removal is the only factor controlling dissolved 232Th concentrations in estuarine waters, 232Th should behave non-conservatively with a net removal.

Fig. 4 shows that the changing of LMW (<10 kDa) Th along the salinity gradient followed those of dissolved phases. This was reasonable considering that most of the dissolved Th was in the LMW fractions. As to the colloidal fractions, Th concentrations changed little with salinity, and in general, their concentrations had a mirror relationship with those of LMW fractions.

4.3. (230Th/232Th)A.R.

The average values of (230Th/232Th)A.R. in all size fractions are summarized in Fig. 5. In general, (230Th/232Th)A.R. decreased with increasing particle size. (230Th/232Th)A.R. in the LMW and colloidal fractions ranged from 2.8 to 3.7. These values were consistent with those in the region of Cape Hattersa (3.6; Guo et al., 1995), the Baltic Sea (3.7; Anderson et al., 1995) and the Western Mediterranean Sea (1.7–7.4; Roy-Barman et al., 2002), but higher than those in the Narragansett Bay (average 1.3; Santschi et al., 1979). (230Th/232Th)A.R. in the 10–53 μm and >53 μm fractions were low (not, vert, similar0.8), and close to the values reported for Chinese coastal sediments (Huang et al., 1991).


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Fig. 5. The averages of (230Th/232Th)A.R. in different size fractions in the Jiulong River estuarine surface water.

232Th in estuarine waters only comes from the input of terrestrial matters, while 230Th comes from both radiogenic production from 234U and terrestrial input. The source difference between both isotopes made the (230Th/232Th)A.R. a potential indicator for distinguishing the contribution of terrestrial/oceanic source. In most cases, the low (230Th/232Th)A.R. indicates a large terrestrial contribution, and vice versa. In this study, the (230Th/232Th)A.R. decreased with increasing particle size, indicating that radiogenic productive 230Th are preferentially adsorbed onto the small particles.

Statistically, the values of (230Th/232Th)A.R. in the LMW (<10 kDa) fraction were not significantly different from those in the colloidal (10 kDa–0.4 μm) fraction. It was remarkable that the radiogenically produced 230Th is not strongly enriched in the <10 kDa fractions compared to the lithogenic 232Th. Therefore, the colloidal material does not seem enriched in lithogenic material and/or an equilibrium exists between the LMW and colloidal Th to ensure the similarity of (230Th/232Th)A.R..

5. Conclusion

In this study, size-fractionated 228Th, 230Th and 232Th concentrations in surface water in the Jiulong River estuary were reported. Our results showed that the activity concentrations of Th isotopes were higher than those reported in coastal or oceanic regions. The high Th concentrations in the Jiulong River estuary were largely related to the lithological constitutes in Fujian province, China. The partitioning of Th isotopes among size-fractionated fractions showed that most of the Th existed in the particulate phases, in which the 10–53 μm fraction had the largest share and the >53 μm fraction had the least. In dissolved phase, most of the Th occurred in the LMW (<10 kDa) fraction. The mass activity concentrations of 228Th, 230Th and 232Th, as well as the (230Th/232Th)A.R., decreased with increasing particle size, indicating the important role of specific surface area of particles in controlling the scavenging of Th from seawater. The geochemical behavior of dissolved Th in the study estuary was different for three Th isotopes. Dissolved 228Th showed a non-conservative behavior with source inputs in the mid-salinity regions, while dissolved 230Th behaves quasi-conservatively. The concave of dissolved 232Th in the estuary indicated a net removal during the mixing between fresh water and seawater.


Acknowledgements

We thank the two anonymous reviewers for their insightful and constructive comments on this work. We are grateful to Laodong Guo for his constant support and his constructive comments. We thank the captain and the crew of the R.V. Ocean 1 for their assistance during the sampling. This study was supported by the Chinese National Natural Science Foundation (No. 40076024) and Chinese COMRA Foundation (No. DY105-02-04 and No. DY105-02-01).


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Journal of Environmental Radioactivity
Volume 78, Issue 2, October 2004, Pages 199-216
 
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