Journal of Environmental Radioactivity 128 (2014) 38e46

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Concentrations and fluxes of dissolved uranium in the Yellow River estuary: seasonal variation and anthropogenic (Water-Sediment Regulation Scheme) impact Sui Juanjuan a, b, Yu Zhigang a, b, Xu Bochao a, b, Dong Wenhua a, b, Xia Dong c, d, Jiang Xueyan a, b, * a

Key Laboratory of Marine Chemistry Theory and Technology (MCTL), Ministry of education, Ocean University of China, Qingdao 266100, PR China College of Chemistry and Chemical Engineering, Ocean University of China, No. 238, Song Ling Road, Laoshan, Qingdao 266100, PR China College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, PR China d Key Laboratory of Marine Environment and ecology, Ministry of education, Ocean University of China, Qingdao 266100, PR China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 December 2012 Received in revised form 7 November 2013 Accepted 10 November 2013 Available online 30 November 2013

The Water-Sediment Regulation Scheme (WSRS) of the Yellow River is a procedure implemented annually from June to July to expel sediments deposited in Xiaolangdi and other large middle-reach reservoirs and to scour the lower reaches of the river, by controlling water and sediment discharges. Dissolved uranium isotopes were measured in river waters collected monthly as well as daily during the 2010 WSRS (June 19eJuly 16) from Station Lijin (a hydrologic station nearest to the Yellow River estuary). The monthly samples showed dissolved uranium concentrations of 3.85e7.57 mg l
1 and 234U/238U activity ratios of 1.24e1.53. The concentrations were much higher than those reported for other global major rivers, and showed seasonal variability. Laboratory simulation experiments showed significant uranium release from bottom and suspended sediment. The uranium concentrations and activity ratios differed during the two stages of the WSRS, which may reflect desorption/dissolution of uranium from suspended river sediments of different origins. An annual flux of dissolved uranium of 1.04  108 g y
1 was estimated based on the monthly average water discharge and dissolved uranium concentration in the lower reaches of the Yellow River. The amount of dissolved uranium (2.65  107 g) transported from the Yellow River to the sea during the WSRS constituted about 1/4 of the annual flux. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Yellow River Water-Sediment Regulation Scheme (WSRS) Dissolved uranium 234 U/238U activity ratio

1. Introduction Naturally occurring uranium-series nuclides have been used extensively as tracers and chronometers in many geological, geochemical, and oceanographic studies (Ivanovich and Harmon, 1992; Chen et al., 1997; Skwarzec et al., 2002; Durand et al., 2005; Dosseto et al., 2008; Krishnaswami and Cochran, 2008; Ryu et al., 2009). Natural uranium (U) has three isotopes: 238U, 234 U and 235U. In terms of radioactivity, 238U and 234U are equally abundant, whereas 235U only has w4.5% of 238U. Three main transport pathways supply U to the ocean: river runoff, atmospheric precipitation of terrigenous (rock) material and direct groundwater discharge. The dominant supply is river runoff

* Corresponding author. Key Laboratory of Marine Chemistry Theory and Technology (MCTL), Ministry of education, Ocean University of China, Qingdao 266100, PR China. Tel.: þ86 532 66786353. 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.11.003

(Palmer and Edmond, 1993; Chabaux et al., 2001; Dunk et al., 2002). The dissolved U concentrations in major world rivers range from 0.02 to 10.3 mg l
1 and vary seasonally (Scott, 1982; Pande et al., 1994; Schmidt, 2005; Jiang et al., 2009; Ryu et al., 2009; Skwarzec et al., 2010). Palmer and Edmond (1993) determined dissolved U in forty major rivers around the world and obtained an average concentration of 0.31 mg l
1. Disequilibrium between 238U and 234U in natural waters and sediments has been a common phenomenon in the earth’s lithosphere and hydrosphere. The disequilibrium, thought to reflect preferential leaching of 234U relative to 238U during watererock interactions (Osmond and Ivanovich, 1992), is well documented for surface waters. Variations in 234U/238U activity ratios of surface waters appear to be closely related to the weathering of bedrock, and can be used to trace and characterize the origin of elemental fluxes carried by rivers to the ocean (Riotte and Chabaux, 1999). The 234U/238U activity ratios generally range from 1.1 to 1.3 in river waters (Riotte and Chabaux, 1999; Chabaux et al., 2001; Jiang et al., 2009).

S. Juanjuan et al. / Journal of Environmental Radioactivity 128 (2014) 38e46

The Yellow River is the second largest river in China, originating from the Qinghai-Tibetan Plateau and discharging into the Bohai Sea. It flows over a total distance of 5464 km and drains a basin area of about 752,000 km2 (Fig. 1). In its middle reaches, the Yellow River cuts through the Loess Plateau which occupies almost 40% of the total Yellow River drainage basin (Zhang et al., 1990). The Loess Plateau is the most physically eroded region in the world (Huang et al., 1992), making Yellow River the most turbid major river in the world, second only to the Ganges-Brahmaputra system in terms of sediment transport (Zhang et al., 1995; Milliman and Meade, 1983; Wang et al., 1986). The high concentration of dissolved U in the Yellow River and its estuary has been noted (Liu, 1988; Palmer and Edmond, 1993; Cheng and Zhang, 1999; Chabaux et al., 2001; Jiang et al., 2009). Reported U concentrations in unfiltered Yellow River waters range between 1.07 and 28.8 mg l
1, averaging w7 mg l
1 (Liu, 1988). These are high values second only to those of the Ganges-Brahmaputra. Together, these Asian river systems contribute significantly to the riverine U input to the ocean. Excluding these systems, the global average concentration of dissolved U in rivers would be reduced from 0.31 mg l
1 to 0.19 mg l
1 (Palmer and Edmond, 1993). Weathering of the Loess Plateau is of prime importance in affecting the dissolved U in the Yellow River. Uranium from the loess deposits enters the river via two major pathways: (1) desorption or dissolution from the suspended loess particles flushed into the river by storm water runoff, and (2) direct leaching of the loess; the leached U reaches groundwater that flows into the river (Jiang et al., 2007). In the Yellow River estuary, the dissolved U concentrations ranged from 3.26 to 5.95 mg l
1 and a non-conservative mixing behavior of U was noted (Zhou and Xu, 1986; Jiang et al., 2007). Over the past many decades, rivers around the world have been altered by human activities, by either a decreased sediment load (hence flux) due to dam-building for flood control and water diversion, or an increase of sediment flux to the sea caused by intensive agriculture and deforestation (Walling and Fang, 2003). The Yellow River provides a fitting example illustrating the effect of human activities on altering the natural state of a large river (Wang et al., 2010). Since late Holocene, the Yellow River has annually delivered more than 1 Gt of sediments to the sea (Milliman et al., 1987; Saito et al., 2001; Wang et al., 2007). Over the past 60 y, a series of dams and reservoirs have been built in the main stream of the Yellow River and the sediment loads decreased rapidly in response to climate changes, dam constructions and soilconservation practices in the Yellow River basin (Wang et al., 2006, 2007). In recent years (2000e2006), the river delivers only 150 Mt of sediment annually to the sea, which is about 15% of the pre-1950s level. The sediment deposit reduces the flood retention

39

capacity of reservoirs and flood delivery capacity of river channels (Zhang and Shi, 2001). In order to ease the siltation of the lower river channel and to transport the sediment deposited in the Xiaolangdi Reservoir (XLD, 870 km from the Station Lijin; see Fig. 1) as well as the sediment derived from the channel erosion to the sea via floodwater, the Yellow River Conservancy Committee (YRCC) began to implement the Water-Sediment Regulation Scheme (WSRS) in June 2002 (Wang et al., 2005). By arranging XLD and other large reservoirs on the main stream of middle reaches, a controlled release of floodwaters from the XLD has been used to expel sediments deposited in the reservoir and to scour the lower reaches of the Yellow River (Wang et al., 2005). This procedure, carried out for about 20 d in every summer, has effectively increased the water discharge since 2002 (Fig. 2). According to the principle of the WSRS of 2010, it can be divided into two stages: (1) floodwaters released from XLD scoured the riverbed sediment deposited in the lower reaches of the Yellow River; (2) muddy flow containing sediments of the XLD reservoir was delivered to the sea to ease the siltation and improve the storage capacity of the reservoir (Ma et al., 2011). Marked changes of the Yellow River water and sediment discharges during implementation of the scheme have been documented by many studies (Li, 2002; Wang et al., 2005, 2010). In this report, we study the effects of such rapid changes in the river basin on the concentrations and fluxes of dissolved U to the sea. For the study, monthly samples were collected from Station Lijin (Fig. 1) in year 2010 and daily samples were collected during the WSRS of 2010. The U concentrations and 234U/238U activity ratios of water samples were determined and the variability of the dissolved U concentrations in different seasons of 2010 was described. This is the first attempt to evaluate the effect of the WSRS on U concentrations and fluxes from the Yellow River to the sea. 2. Study area The main stream of the Yellow River can be divided into three segments: the upper, middle and lower reaches, separated by the Toudaoguai and Huayuankou hydrometric stations (Fig. 1). High suspended sediment concentration and low water discharge are characteristic features of the Yellow River. The river traverses the fluvial plain of northern China at an altitude of 50e100 m in the lower reaches, where 33% of the suspended sediments settle to the riverbed (Long and Xiong, 1981; Milliman and Meade, 1983). Due to heavy sedimentation, most of the riverbed sits 5e10 m higher than the alluvium beyond the river banks (Huang et al., 1992) and the lower reaches of the Yellow River is often called “hanging river”. Station Lijin, located at about 100 km from the river mouth, is the

Fig. 1. Map of the Yellow River, showing the Xiaolangdi reservoir (XLD) and the sampling site of Station Lijin. The shaded area represents the Chinese Loess Plateau (after Gu et al., 1997). CXiaolangdi Reservoir (XLD); 7hydrographic station; city.

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Fig. 2. Annual water discharge of the Yellow River from 1950 to 2010 (Xu et al., 2013).

last hydrometric station before the Yellow River enters the sea (Fig. 1). It is virtually uninfluenced by tides, because tidal dynamics at the Yellow River mouth are weak with a tidal reach of only 20 km in non-flood seasons (Wang et al., 2001). Seasonal variability of water and sediment discharges exists in the estuary of Yellow River. Most of the annual water and sediment discharge occur during the summer monsoon period (JuneeOctober). 3. Sample collection and analysis 3.1. Sampling and sample preparation Since the 1950s, the Yellow River has been well gauged. Successive records of water and suspended sediment at major gauging stations along the main river course are available in the YRCC, from which we acquired detailed information about the water discharges in the Yellow River. Monthly samples were collected near the middle section of a pontoon bridge at Lijin on a day around 20th every month, 2010. The sampling locations were selected to be far away from tributary entries and any apparent sources of pollution. Daily samples were collected near the middle section of Lijin Huanghe Bridge (which is only a few hundred meters away from the Lijin pontoon bridge) during June 19eJuly 16, 2010, as the Lijin pontoon bridge was dismantled due to the increasing water discharge during the WSRS. Surface water samples of 10e15 l were taken using a clean plastic pail and then stored in acid-cleaned plastic containers. The water samples were filtered by 0.45 mm membrane filters on the sampling day. Both the water and suspended sediment samples were transported back to the laboratory within 2e5 d after collection. 3.2. Radiochemical method and measurement Uranium analyses followed the procedures of Luo et al. (1987) and Ku et al. (1998) with minor modifications. The specifics in the sample treatment and detection method were detailed in Jiang et al. (2007). A known volume of filtered water (10e15 l) was acidified to pH 1 with ultrapure hydrochloric acid and then added a 232 U spike solution and 100 mg iron (as NH4Fe(SO4)2). After at least 24 h for the spike equilibration, U was co-precipitated with the precipitation of Fe(OH)3 at pH 8 by the addition of NH3$H2O. The precipitate was dissolved in 8 N HCl and the U contained therein was purified through an anion exchange column (Biorad AG1-X8,

100e200 mesh, about 10 ml resin preconditioned with 8 N HCl). As U and Fe were both retained on the resin, an 8 N NH4NO3 solution was passed through the column to remove Fe until the color of the discharging solution did not change to red when tested with KSCN. Uranium was then eluted with 50 ml of 0.1 N HNO3, extracted into a TTA-benzene solution at pH 3 and deposited onto a stainless steel disc for counting in an a spectrometer system. After suspended sediment samples retained by the filters were dried at 105  C for 24 h, they were ground into powder. A known weight dry sample (2 g) was spiked with 232U and digested with HNO3eHFeHClO4 using an electric heating panel to affect total dissolution. The dissolved samples were analyzed for U isotopes following procedures similar to those for the water samples. The activity of U isotopes (234U and 238U) was determined by a spectrometry equipped with silicon surface barrier detectors with a 314 mm2 active area. The detection limit of measurements was 0.1 mBq and the uncertainty (n ¼ 4, P ¼ 0.95) was about 7%. The precision of uranium concentration based on repeat measurements of samples was 3%. The 232U spike solution used in the experiments can be traced to standards used in the international calibrations, e.g., the Harwell uraninite (Harmon et al., 1979; Ivanovich et al., 1984). The errors quoted for all analyses are 1 standard deviation based on counting statistics only. 3.3. Laboratory simulation experiments In order to assess the leachable U from the suspended particles to the river water during the WSRS, a series of equilibration experiments was conducted in the laboratory. During the first stage of the WSRS, some of the riverbed sediment in the lower reaches, which was exposed to the air during dry seasons, could be submerged, stirred up and then transported to the sea by the floodwater released from the XLD reservoir. So the riverbed sediments collected at Zhengzhou and Jinan (Fig. 1, both stations are located between Xiaolangdi and Lijin) on July 7, 2009 and July 11, 2009, respectively, were used in the experiment. While in the second stage of the WSRS, the suspended sediments in the lower reaches were mainly from the silted sediment of the XLD reservoir. Therefore, the suspended sediment collected in the Yellow River several kilometers downstream from the XLD reservoir on July 5, 2012 has been employed. To keep the original physicochemical property, all the sediment samples were frozen upon collection and wet sediments were used in the experiments. Filtered Yellow River water collected at Station Lijin on May 4, 2013 was employed in the equilibration experiment. The equilibration experiments followed the design of Shulkin and Bogdanova (2003) and Jiang et al. (2009). The masses of sediment used in the experiments can be calculated based on the water contents in the sediments and the average suspended sediment concentrations which were 10 kg m
3 and 32 kg m
3 during the first and second stages of the WSRS, respectively. Therefore, 129 g of Zhengzhou wet riverbed sediment and 140 g of Jinan wet riverbed sediment were respectively added to a plastic container filled with 10 l filtered Yellow River water to obtain the average suspended sediment concentration of first stage (10 kg m
3). To simulate the suspended sediment concentration of the second stage (32 kg m
3), 528 g of Xiaolangdi wet suspended sediment and 10 l filtered Yellow River water were employed. The solid-water slurry in each of the containers was electronically stirred to ensure mixing of the two phases. Each of the mixtures was filtered through 0.45 mm membrane filters after 24 h of stirring. The concentrations and 234U/238U activity ratios of dissolved U in the experiments were determined by a spectrometry following the aforementioned procedures (Section 3.2)

S. Juanjuan et al. / Journal of Environmental Radioactivity 128 (2014) 38e46

41

4. Results 4.1. Monthly variation of dissolved U concentrations and activity ratios

234

U/238U

Monthly dissolved U concentrations and 234U/238U activity ratios in the lower reaches of the Yellow River in 2010 are given in Table 1. The concentrations of dissolved U ranged from 3.85  0.18 (September) to 7.57  0.66 mg l
1 (January), with an average value of 5.93  0.32 mg l
1. The 234U/238U activity ratios of dissolved U varied from 1.24  0.07 in September to 1.53  0.07 in January, with a mean value of 1.40  0.05. These results are similar to the values reported by Jiang et al. (2009) at the Station Lijin. Both the dissolved U concentrations and the 234U/238U activity ratios of the Yellow River are much higher than the mean values for global rivers of 0.31 mg l
1 (Palmer and Edmond, 1993) and 1.17 (Chabaux et al., 2001), respectively. Monthly variations of dissolved U concentrations and 234 238 U/ U activity ratios are related closely to the water discharge and the suspended sediment concentration. The U concentrations and 234U/238U activity ratios in the flood season (JuneeOctober) were lower than those in the dry seasonea relation that is opposite to the variations of water discharge and suspended sediment (Fig. 3). 4.2. Variation of water and suspended sediment at Station Lijin during the WSRS Since the WSRS can be divided into two stages, there was a turning point between the two stages caused by the large increase of suspended sediment at the Station Lijin. The process of the WSRS was well indicated by the variation of river water discharge and suspended sediment at Station Lijin (Fig. 4(c) and (d)). The WSRS in 2010 began with opening the floodgate of the XLD Dam and drawing off the water on June 19th (Ma et al., 2011). Before June 22nd, both water discharge and suspended sediment were relatively low at the Station Lijin. The water discharge started to increase rapidly on June 23rd and peaked on June 26th at Lijin, and then followed by two spikes of 3520 m3 s
1 and 3290 m3 s
1 on July 3rd and 8th, respectively. July 8th was the turning point of the WSRS in 2010. After July 8th, the water discharge decreased rapidly before reaching its lowest value on July 15th. The suspended sediment at Station Lijin during the WSRS behaved differently from the water discharge. It began with a relatively low load of 0.73 kg m
3 on June 19th, and then kept a constant value between June 23rd and July 8th. Suspended sediment increased rapidly from

Fig. 3. Monthly variations of dissolved U concentrations (a), 234U/238U activity ratios (b), water discharges (c), and suspended sediment (d) at Station Lijin of the Yellow River in 2010.

July 8th, and reached its highest value of 63.2 kg m
3 with a relatively lower water discharge on July 10th, which were caused by the artificial hyperpycnal flow released from the XLD reservoir (Zhang and Huang, 2008). Water discharge and suspended sediment decreased to a normal value on July 15th, indicating the end of WSRS in 2010. 4.3. Dissolved and particulate U concentration during the WSRS The U concentrations and 234U/238U activity ratios of the Yellow River water at the Station Lijin during the WSRS are shown in Fig. 4. The concentrations of dissolved U ranged between 4.71  0.17 (on June 21st) and 7.73  0.38 mg l
1 (on July 10th), with a mean value of 5.56  0.21 mg l
1. The dissolved U concentration varied widely within a short period, with a peak on July 10th. The 234U/238U activity ratios varied from 1.33  0.03 to 1.49  0.03, with an average

Table 1 Monthly dissolved U concentrations and 234U/238U activity ratios of the Yellow River in 2010. Date

Water discharge Suspended sediment Dissolved U (m3 s
1) (kg m
3) Concentration (mg l
1)

January 272 February 216 March 187 April 150 May 168 June 919 July 1460 August 1850 September 1070 October 627 November 282 December 101

0.518 0.542 0.636 0.833 0.643 10.3 14.2 14.8 3.15 1.69 0.738 0.294

7.57 6.75 6.37 6.40 6.17 5.34 5.20 5.88 3.85 5.55 5.87 6.15

           

0.66 0.40 0.39 0.48 0.24 0.17 0.18 0.27 0.18 0.35 0.28 0.27

234

U/238U

1.24 1.53 1.42 1.42 1.44 1.49 1.34 1.30 1.35 1.50 1.38 1.37

           

0.07 0.07 0.06 0.07 0.04 0.03 0.03 0.04 0.05 0.07 0.05 0.04

Fig. 4. Daily variations of dissolved U concentrations (a), 234U/238U activity ratios (b), water discharges (c), and suspended sediment (d) at Station Lijin of the Yellow River during the WSRS in 2010. Vertical bars in the graph indicate that July 8th was the turning point of the scheme.

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value of 1.39  0.04. The variation range of 234U/238U activity ratios of the first stage of the WSRS is larger than that of the second stage. The U concentrations and the 234U/238U activity ratios in the suspended sediment of the Yellow River during the WSRS ranged from 0.78  0.05 to 2.40  0.15 mg g
1 and 0.87  0.05 to 1.20  0.05, respectively (Table 2). 4.4. Results of laboratory simulation experiments The U concentrations and 234U/238U activity ratios of sediment samples employed in the simulation experiments are presented in Table 3. The dissolved U concentration and 234U/238U activity ratio of the Yellow River water used in the simulation experiments initially were 3.63  0.19 mg l
1 and 1.43  0.06, respectively. After stirring for 24 h, the concentrations and the 234U/238U activity ratios of dissolved U in the equilibrated solutions of the three simulation experiments (Zhengzhou, Jinan and XLD) became 4.56  0.27 mg l
1 and 1.46  0.06, 4.42  0.21 mg l
1 and 1.44  0.05, and 5.03  0.27 mg l
1 and 1.32  0.05, respectively (Table 3). They indicate a significant removal of U from particle to the river water in the experiments. 5. Discussion 5.1. Seasonal variation of dissolved U concentration in the lower reaches of the Yellow River Seasonal variability of dissolved U concentration has been observed in the lower reaches of the Yellow River. On the whole, the river water has higher dissolved U concentration in winter than that in summer. In 2010, the highest U concentration occurred in January and lowest in September (Fig. 3(a)).

Because the riverbed in the lower reaches of the Yellow River sits 5e10 m higher than the alluvium beyond the river banks (Huang et al., 1992), the import of runoff and underground water can be ignored. The substantial variability of U concentration and 234 238 U/ U activity ratio can be interpreted in terms of rainfall, desorption or dissolution from the suspended river sediment and diffusion from pore water in bottom sediment. In the winter, the water discharge of the Yellow River is at a low level (Fig. 3(c)). The relatively low flow rate allows sufficient sedimentewater interaction and the high dissolved U concentration is mainly sourced from desorption or dissolution of the river suspended sediment of loess origin. Weathering process primarily affects the dissolved U concentration in the river water (Jiang et al., 2009). Rivers that drain regions with arid climates are most likely to have 234U/238U activity ratios greater than those that drain regions with humid climate, which can be testified by the relatively higher 234U/238U activity ratio of the Yellow River water (Fig. 3(b)). A major portion of the 234 U is released from superficial soil horizons and preferentially leached from a-recoil damage sites in the middle reaches of the river due to severe weathering of the Loess Plateau. In summer, the climate of the Loess Plateau is controlled basically by the East Asia summer monsoon which brings moisture-laden air masses from tropical oceans (Ding et al., 2001), resulting in frequent rain storms in this area. The lower dissolved U concentration in the lower reaches of the Yellow River seems to occur because of increasing water discharge during the flood season (Fig. 3(a) and (c)). Heavy rainfall could well dilute the U concentration of the river water (Riotte and Chabaux, 1999). After the WSRS, most of the sediments in the channel of lower reaches were scoured into the river mouth, as shown by the fact that the suspended sediments decreased from 14.8 kg m
3 in August to 3.15 kg m
3 in September. Since approximately 90% of the sediment transported to the river mouth is derived from the Loess

Table 2 Data on water discharge, suspended sediment, dissolved and particulate U concentrations and Date (month-day)

6-19 6-20 6-21 6-22 6-23 6-24 6-25 6-26 6-27 6-28 6-29 6-30 7-01 7-02 7-03 7-04 7-05 7-06 7-07 7-08 7-09 7-10 7-11 7-12 7-13 7-14 7-15 7-16 a b

Dry suspended sediment. Data unavailable.

234

U/238U activity ratios in the Yellow River during the WSRS in 2010.

Water discharge (m3 s
1)

Suspended sediment (kg m
3)

Dissolved U

318 302 335 512 1540 2440 2820 2980 3020 3030 2990 2980 3060 3330 3520 3470 3060 2750 3040 3290 1990 1510 866 637 1070 402 201 e

0.730 0.613 0.746 4.77 11.6 13.4 14.3 13.0 13.9 12.6 12.5 10.5 11.7 10.9 11.0 11.8 10.6 9.56 10.8 12.0 26.9 63.2 56.4 32.5 24.8 13.5 9.40 e

5.34 5.48 4.71 5.36 5.30 5.10 5.23 5.45 6.07 5.32 5.59 5.56 5.79 5.25 5.45 5.23 5.39 5.50 5.54 5.20 5.82 7.73 6.22 6.05 5.47 5.69 5.63 5.20

Concentration (mg l
1)                            

0.17 0.22 0.17 0.23 0.19 0.22 0.17 0.19 0.17 0.22 0.20 0.20 0.23 0.19 0.17 0.19 0.21 0.24 0.20 0.22 0.22 0.38 0.28 0.22 0.18 0.28 0.20 0.18

Particulate U 234

U/238U

1.49 1.42 1.39 1.43 1.38 1.41 1.41 1.38 1.39 1.37 1.39 1.46 1.43 1.40 1.33 1.45 1.41 1.33 1.46 1.33 1.36 1.41 1.37 1.37 1.38 1.35 1.35 1.34

                           

0.03 0.04 0.04 0.04 0.04 0.04 0.03 0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.03 0.05 0.03 0.03

Concentrationa (mg g
1) 1.83 1.55 1.46 1.32 1.52 1.48 1.42 1.42 1.07 eb 0.85 1.60 1.12 1.31 1.35 1.19 0.78 1.42 1.25 1.38 2.40 1.94 2.23 2.13 1.70 1.97 2.05 2.26

        

0.06 0.06 0.09 0.06 0.08 0.07 0.06 0.05 0.04

                 

0.06 0.10 0.09 0.07 0.06 0.09 0.05 0.10 0.08 0.11 0.15 0.08 0.11 0.13 0.07 0.10 0.07 0.09

234

U/238U

1.04 1.06 1.02 1.07 1.08 1.07 1.20 1.05 0.90 e 1.08 0.87 1.05 1.09 1.03 1.06 0.99 0.91 1.02 0.93 1.14 1.02 1.12 1.03 1.10 1.03 1.03 1.04

        

0.03 0.04 0.06 0.05 0.05 0.05 0.05 0.03 0.04

                 

0.08 0.05 0.09 0.06 0.05 0.08 0.07 0.06 0.06 0.07 0.05 0.04 0.04 0.05 0.04 0.04 0.03 0.03

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Table 3 Features of materials employed in the simulation experiments and the results of experiments. Sampling location

Lijin Zhengzhou Jinan Xiaolangdi a b c

Sample

Yellow River water Riverbed sediment Riverbed sediment Suspended sediment

Water contenta (%)

c

e 22.64 28.46 39.34

Particulate U

Dissolved U in the equilibrated solution

Concentrationb (mg g
1)

234

e 1.51  0.20 1.18  0.07 1.83  0.10

e 0.96  0.06 0.97  0.05 1.05  0.05

U/238U

Concentration (mg l
1) 3.63 4.56 4.42 5.03

   

0.19 0.27 0.21 0.27

234

Uranium released from particulate (mg g
1)

U/238U

1.43 1.46 1.44 1.32

   

0.06 0.06 0.05 0.05

e 0.0714 0.0558 0.0264

Wet sediment sample. Dry sediment sample. Data unavailable.

Plateau in the middle reaches (Wang et al., 2006, 2007), desorption/ dissolution of U from weathered loess is the main factor resulting in high dissolved U concentration in the middle and lower reaches of the Yellow River (Jiang et al., 2009). The lowest dissolved U concentration occurring in September may be the result of less desorption/dissolution due to the lower suspended sediment concentration, as well as a consequence of dilution by the high water discharge (Fig. 3). 5.2. Variability of dissolved U concentrations and 234U/238U activity ratios influenced by the WSRS While the water discharge and suspended sediment at Station Lijin showed difference in the two stages of the WSRS, the dissolved U concentration and 234U/238U activity ratio also differed in the two stages (Fig. 4). In the first stage of the Scheme, the dissolved U concentration showed a moderate variation ranging from 4.71  0.17 to 6.07  0.17 mg l
1. During the second stage, the dissolved U concentration had a range of 5.20  0.18 to 7.73  0.38 mg l
1 showing a distinct spike of 7.73 mg l
1 on July 10th. The difference seems due to the changes of the water discharge and suspended sediment concentration. In the first stage, the Yellow River had higher water discharge and lower suspended sediment concentrations. But the water discharge decreased and suspended sediment increased during the second stage. The July 10th dissolved U concentration spike of 7.73 mg l
1 coincides with the highest suspended sediment. Fig. 5 shows that although no distinct relationship is seen between the dissolved U concentration and water discharge, a positive correlation (R2 ¼ 0.66) exists between the dissolved U concentration and the suspended sediment during the WSRS in 2010. This suggests that desorption or/and dissolution from the river suspended sediments may be an important source of dissolved U. Our

laboratory simulation experiment appears to corroborate this suggestion. The results indicate that a significant amount of U may have been desorbed from the suspended sediment in the Yellow River water (Table 3). Particle size may also play a role in influencing the concentration of dissolved U in the river water. The proportion of fine particles such as clay and silt during the second stage doubled that of the first stage (Du, 2011). Fine particles provide larger surface areas and thus can cause more desorption and dissolution of U. Relationships between the water discharge and suspended sediment were often used to identify the sources of river suspended sediment: an anticlockwise or negative hysteresis is often seen as an indicator of outer sources (such as suspended sediment derived from a local rainstorm in the drainage basin) and a clockwise or positive hysteresis represents within-channel sediment sources and/or sediment flushing (Steegen et al., 2000; He et al., 2010). Plots of suspended sediment versus water discharge (Fig. 6) during the two stages of the WSRS show that a clockwise (or positive) hysteresis occurs in the first stage of the scheme while an anticlockwise one occurs in the second stage. During the first stage of the WSRS, the sediments on the riverbed of the lower reaches were stirred up by the flushing water released from the XLD and the suspended sediments increased with the increasing river water discharge. This scenario is conformed to the character of a positive hysteresis. Dissolved U desorbed from the suspended sediments with different origins may have different 234U/238U activity ratios. Therefore, the 234U/238U activity ratios of dissolved U observed at the Station Lijin during the first stage varied to a relatively large extent (Fig. 4(b)). During the second stage of the WSRS, the suspended sediment in the river was mainly from the XLD carried by artificial hyperpycnal flow (Ma et al., 2011). As most of the sediment input from the loess region to the Yellow River were trapped by the XLD reservoir (Wang et al., 2010), it is likely that suspended

Fig. 5. Dissolved U concentration versus water discharge (a) and suspended sediment (b) in the Yellow River during the WSRS in 2010.

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S. Juanjuan et al. / Journal of Environmental Radioactivity 128 (2014) 38e46

Fig. 6. Suspended sediment versus water discharge in the two stages of the WSRS. The direction of arrows indicates the sequence of sampling. (a) Clockwise hysteresis is observed during the first stage of the WSRS (June 19theJuly 8th); (b) Anticlockwise hysteresis is observed during the second stage of the scheme (July 8the19th).

sediments in the second stage were derived rather homogeneously from the loess through local rainstorm in the drainage basin. After the peak suspended sediment of July 10th, the suspended sediments decreased with decreasing river water discharge in the lower reaches and the anticlockwise hysteresis was observed during the second stage of the scheme. It is not difficult to understand that the 234U/238U activity ratio of dissolved U in the second stage varied in a small range, because sediments with a relatively homogenous source may have similar 234U/238U activity ratios. This opinion is also supported by the evidence that the 234U/238U activity ratio of the particulate U during the second stage varied in a smaller range (1.02e1.14) than that of the first stage (0.87e1.20; Table 2).

the water to be 4.50  10
2 Bq l
1 and 6.44  10
2 Bq l
1, respectively. Based on the equations above, the 234U/238U activity ratios of leachable U from sediments at Zhengzhou, Jinan and XLD stations can be calculated as 1.57, 1.48 and 1.03, respectively. The high values occurring at Zhengzhou and Jinan may come from weathering of sediments exposed to the air during the dry season. Alpha recoil fractionation of U isotopes resulting in preferential leaching of 234U over 238U is known to prevail to a larger extent in arid regions (Scott, 1982). As sediments deposited in the bottom of XLD reservoir were submerged before the WSRS, U released from these sediments may show little isotopic fractionation.

5.4. Annual flux of U from the Yellow River to the sea 5.3. Leachable U from the suspended sediment 234

238

Knowledge of the U/ U activity ratio may provide information about the mechanisms and processes of U transport and origin (Camacho et al., 2010). The concentrations and 234U/238U activity ratios of dissolved U in the Yellow River water used in our previously described simulation experiments underwent changes (Table 3). The concentration and 234U/238U activity ratio of leachable U can be calculated from the following equations:

½UL ¼ ½UE
½UY ð234 U=238 UÞE ¼

(1)

ð234 UÞL þ ð234 UÞY ð238 UÞL þ ð238 UÞY

(2)

where [ ] and ( ) represent mass and radioactivity concentration, respectively; the subscripts L, E and Y denotes leachable U, equilibrated solution, and Yellow River water, respectively. Because the 234 U concentration is negligible compared to the 238U concentration in solution, 238U concentration is approximately equal to the U concentration. Therefore, 238U radioactivity in the Equation (2) can be obtained by multiplying U concentration by 1.24  104 Bq/g (specific activity of 238U):

ð238 UÞ ¼ ½U  1:24  104

(3)

ð234 UÞ ¼ ð234 U=238 UÞ  ð238 UÞ

(4)
1

Given dissolved U concentration (UY, 3.63 mg l ) and U/ U activity ratio (1.43) in the Yellow River water, we can calculate the dissolved 238U radioactivity (238U)Y and 234U radioactivity (234U)Y in 234

238

There are no tributaries, industrial wastewater discharges and domestic sewage input into the segment of the Yellow River from Station Lijin to the river mouth. Therefore, the flux of U at Lijin may represent the river contributions to the sea. Dunk et al. (2002) evaluated the global oceanic budget of U for the Holocene. Among the various source and sink parameters controlling the budget, they considered the riverine supply to represent the best constrained parameter. Water discharge of 2010 (1.89  1010 m3) from the Yellow River to the sea is close to the average discharge (1.83  1010 m3) from 2003 to 2010 (Table 4). Thus dissolved U flux to the sea in 2010 can be taken as representing such a flux for recent years. As shown in Fig. 7, monthly fluxes of dissolved U from the Yellow River to the sea in 2010 have been calculated by multiplying dissolved U concentration by average water discharge of the given month. By summing the monthly fluxes, the annual flux of dissolved U from the Yellow River to the sea can be evaluated as 1.04  108 g y
1, which is about 1% of the global U flux (1  1010 g y
1; Borole et al., 1982) to the sea. The seasonal flux of dissolved U varied greatly. The average flux of dissolved U in the flood season was 1.61  107 g month
1, which was more than 4 times higher than that of the dry season (0.33  107 g month
1). The total water discharge of the Yellow River was 1.89  1010 m3 and the total sediment load was 1.63  1011 kg. Table 4 Annual water discharges from the Yellow River to the sea from 2003 to 2010. Year

2003 2004 2005 2006 2007 2008 2009 2010 Average

Annual water 1.93 discharge (1010 m3)

1.99

2.07

1.92

2.02

1.46

1.33

1.89

1.83

S. Juanjuan et al. / Journal of Environmental Radioactivity 128 (2014) 38e46

45

WSRS was 2.65  107 g, accounting for about a quarter of the annual uranium flux. Acknowledgments We are grateful to Dr. Qingzhen Yao for providing the data of water discharges and sediment concentrations and Xiaojing Wang, Juntao Du for collecting some of the daily samples during the WSRS. We are also very grateful to Professor Teh-Lung Ku from University of Southern California for improving English of the manuscript and the two anonymous reviewers for their constructive comments on earlier versions of the manuscript. This study was funded by the Natural Science Foundation of China (NSFC grants 40976044, 41376085 and 41206064), and the Natural Science Foundation of Shangdong Province (No. ZR2011DM010). This is MCTL contribution No. 37. References Fig. 7. Monthly dissolved U fluxes from the Yellow River to the sea in 2010.

Summing the daily U fluxes during the WSRS gives the amount of dissolved and particulate U delivered from the Yellow River to the sea over this period as 2.65  107 g, 1.03  108 g, respectively. Although particulate U flux is four times the dissolved U flux during the WSRS, most of the suspended sediment can be deposited at the river mouth. Therefore, in the evaluation of the global oceanic budget of U, more weight should be placed on the estimated dissolved U flux of the Yellow River than the particulate U flux estimate. The WSRS represents an important period in terms of annual delivery of water and sediment (and material contained therein as well) from the Yellow River to the sea. In 2010, the total amounts of water and sediment discharge during the WSRS were 4.79  109 m3 and 7.06  1010 kg, respectively. The percentages of total river water discharge, sediment load and dissolved U during the period of the WSRS in the annual flux of the Yellow River were 25.3%, 43.3% and 25.5%, respectively. The dissolved U transported in this short period (28 d) is about a quarter of annual dissolved U flux, which is similar to the water discharge but less than the percentage of sediment load. These findings imply that the WSRS in the Yellow River is also an important period for dissolved U transport. 6. Conclusions The dissolved U concentrations in the Yellow River water measured at Lijin in the lower reaches of the Yellow River are much higher than the average value of global major rivers. Desorption or/ and dissolution from the suspended sediments is an important source of the high dissolved U concentration in the Yellow River water. Dissolved U concentration in the lower reaches of the Yellow River has high seasonal variability: higher dissolved U concentration in the dry season and lower in the flood season. The Water-Sediment Regulation Scheme in the Yellow River is an important project of artificially scheduling water and sediment around the world, which has influenced the dissolved U concentrations and 234U/238U activity ratios in the lower reaches. The behaviors of dissolved U were different during the two stages of WSRS and are largely related to the U desorption from suspended sediment of different origins. The flux of dissolved U from the Yellow River to the sea was estimated as 1.04  108 g y
1 based on the monthly sampling data in 2010. More than three quarters of U was transported in the flood season. The amount of dissolved U transported to the sea during the

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