Chinese Journal of Oceanology and Limnology   2015, Vol. 33 Issue(6): 1402-1412     PDF       
http://dx.doi.org/10.1007/s00343-015-4360-y
Shanghai University
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Article Information

KONG Fancui (孔凡翠), SHA Zhanjiang (沙占江), DU Jinzhou (杜金洲), SU Weigang (苏维刚), YU Chenguang (于晨光), ZHAO Shunli (赵顺利), HU Jufang (胡菊芳), YE Mei (冶梅)
Analysis of the distribution characteristics of 226Ra and 228Ra and their sources in the western part of Qinghai Lake
Chinese Journal of Oceanology and Limnology, 2015, 33(6): 1402-1412
http://dx.doi.org/10.1007/s00343-015-4360-y

Article History

Received Dec. 11, 2014
accepted in principle Feb. 21, 2015;
accepted for publication Jun. 25, 2015
Analysis of the distribution characteristics of 226Ra and 228Ra and their sources in the western part of Qinghai Lake
KONG Fancui (孔凡翠)1,2,3, SHA Zhanjiang (沙占江)1,4 , DU Jinzhou (杜金洲)5, SU Weigang (苏维刚)1,2, YU Chenguang (于晨光)1,2, ZHAO Shunli (赵顺利)1,2, HU Jufang (胡菊芳)1,2, YE Mei (冶梅)4       
1 Qinghai Institute of Salt Lakes, Chinese Academy of Science s, Xining 810008, China;
2 University of Chinese Academy of Sciences, Beijing 100049, China;
3 Zoucheng Bureau of Land and Resources, Zoucheng 273500, China;
4 Life and Geographical Sciences College and Education Ministry Key Laboratory of Environments and Resources in the Tibetan Plateau, Qinghai Normal University, Xining 810008, China;
5 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China
ABSTRACT:The 226Ra and 228Ra activities of Qinghai Lake surface water, groundwater, river water, suspended particles, and bottom sediments were measured in a gamma-ray spectrometer. The sources of 226Ra and 228Ra were discussed according to their distribution characteristics. 226Ra and 228Ra activities (dpm/(100 L)) ranged from 14.13±0.22 to 19.22±0.42 and 17.72±0.66 to 30.96±1.47 in the surface water of the North Bay, respectively, and from 7.88±0.24 to 33.80±0.47 and 15.73±0.74 to 57.31±1.44, respectively, in the South Bay. The surface water near the estuary had a lower salinity and had a higher concentration of radium isotopes than the samples collected further away. The farther offshore the sample, the higher the salinity was, and the lower the radium isotope activity. The distribution of radium activities in the western part of Qinghai Lake is controlled by several factors, including Buha River runoff, desorption from suspended particles derived from the river, groundwater discharge, and a small amount of diffusion from the sediment.
Keywords: 226Ra     228Ra     surface water of Qinghailake     groundwater     river water     Qinghai Lake    
1 INTRODUCTION

Radium isotopes are often used as tracers in geochemical analyses based on their uniquely conservative rates of isotope fractionation. There are four naturally occurring radium isotopes: 226 Ra(T 1/2 =1, 600 yr), 228 Ra(T 1/2 =5.75 yr), 224 Ra(T 1/2 =3.66 d) and 223 Ra(T 1/2 =11.4 d). The 226 Ra and 228 Ra isotopes are produced by the decay of 238 U and 232 Th, respectively. Although these two isotopes have the same physical and chemical properties, their nuclear properties are different. They also differ in their halflife, their sources, and their geochemical behavior in aquatic environments. In marine research, radium isotopes are used for the quantitative study of the water-rock exchange rate(Moore, 1984; Suksi et al., 2001; Porcelli and Swarzenski, 2003), water mixing rates(Zhang et al., 2007), water residence time and transport rate(Moore, 2000; Dulaiova and Burnett, 2008; Moore et al., 2008, Peterson et al., 2008), groundwater discharge fl ux(Moore, 1996; Moore, 2003; Kim et al., 2005; Burnett et al., 2006; Swarzenski et al., 2007; Burnett et al., 2008; Liu et al., 2013; Su et al., 2014), submarine groundwater discharge(SGD)-derived nutrients NO 3 ˉ, PO 4 , SiO 4 , dissolved organic nitrogen, and total dissolved nitrogen fl ux(Swarzenski et al., 2007; Lee et al., 2009; Ji et al., 2012a; Su et al., 2013), interstitial water/overlying water exchange rate(Webster et al., 1995; Huang et al., 1997; Krest and Harvey, 2003), and deposition rate and chronology(Dukat and Kuehl, 1995; Su and Huh, 2002; Watters et al., 2006).

Adsorption of radium onto solid particles in freshwater environments depends on the properties of the particles(Burnett et al., 2003). Radium usually adsorbs onto suspended particles found on the surface in underground freshwater aquifers and in the surface water of rivers and lakes. However, when radium infiltrates into a salt water environment, it desorbs from the particle surface and is released into the water, mainly through ion exchange action due to the high ionic strength of salt water(Elsinger and Moore, 1983). When river water meets and mixes with sea water in a river estuary, the radium on suspended particles desorbs because of the rising water salinity. Therefore, radium activity is usually high in the brackish mixed water of estuaries. A large number of studies have shown that as a result of desorption from suspended particles, radium does not behave conservatively in water(Krest and Moore, 1999; Gonneea et al., 2008). At the same time, the infl ux of submarine groundwater and sediments adds considerable amounts of radium into the estuary. There are numerous similar research fi ndings, but the studied areas were mostly semi-closed or enclosed areas concentrated in estuaries, bays and lagoons. Studies on lakes are rare; however, inl and lake ecosystems are important and cannot be ignored.

The western part of Qinghai Lake was used as the study area in this work. We analyzed surface water samples collected from the lake, groundwater and river to study the distribution and changes of the 226 Ra and 228 Ra isotopes. The physicochemical properties of the samples were determined, and the effects of the sources of radium isotopes on their distribution in Qinghai Lake water are discussed.

2 GEOLOGICAL ENVIRONMENT BACKGROUND AND SAMPLING METHODS 2.1 The geological environment

Qinghai Lake is located in the northeastern part of the Qinghai-Tibet Plateau(36°15′–38°20′N; 97°50′– 101°20′E) and is the largest saline lake in China with a watershed area of 29 660 km 2 . It is a one-fault basin with new tectonic movement in the Late Himalayan and is mainly controlled by the north west-west, north north-west and nearly north south-trending tectonic fractures. The northwestern part of the lake is surrounded by high mountains, forming a closed inl and basin, and the southeastern part is a plain(Yuan et al., 1990; Bian et al., 2000). A mountainous area accounts for approximately 68.6% of the watershed area. There are steep mountains, water-gathering valleys and many glacial terrains in the area. The Buha, Shaliu, Haergai, Wuhaalan(Quanji) and Heima Rivers are located around the Qinghai Lake, mainy in the northwest region. Their total annual runoff is 1.34×10 9 m 3, accounting for 83% of the total surface runoff into the lake(Chinese Academy of Sciences, 1994). For the general distribution of groundwater in the Qinghai Lake basin, the mountain range is the supply zone, the pluvial and fl ood plains before the piedmont are the infiltration runoff zone, and the lakeside plain is the groundwater runoff discharge belt. Surface runoff eventually supplies water to Qinghai Lake, but part of the runoff evaporates. Part of the underground runoff becomes a stream that fl ows into Qinghai Lake, and this runoff passes an overfl ow zone of groundwater. Additionally, part of the confi ned groundwater supplies Qinghai Lake through a fault zone in the bottom of the lake(Chinese Academy of Sciences, 1994).

2.2 Sampling methods

Surface water samples were collected from the Qinghai Lake using a bucket. Groundwater samples were pumped from civil wells and springs. We also obtained water samples from the rivers that flow into Qinghai Lake. In the North Bay, we collected seven surface water samples(50 L), five groundwater samples(25 L), and one river water sample(50 L)in July 2012. In July 2013, 16 surface water(50 L)samples, six groundwater(25 L)samples and two river water(50 L)samples were collected from the South Bay.

Mirroring the low activity level of radium isotopes in nature, the 226 Ra and 228 Ra activity in water is usually in the range of 10 -3 –10 -2 Bq/m 3(Liu et al., 1999), while 226 Ra activity in sediments is below 10 -2 Bq/g(Wu et al., 1983). Therefore the samples must be separated and enriched to enable isotope measurements. We immediately filtered all samples through cellulose acetate membranes with a pore size of 0.45 μm. The water samples were filtered through a column of Mn-fi ber(approximately 15 g)to adsorb the radium isotopes. The flow rate was approximately 2 L/min(Moore and Krest, 2004). We then removed the Mn-fi ber from the column and washed it with deionized water to remove the salt residue. To measure the long-lived 226 Ra and 228 Ra isotopes, a specific procedure is used:(1)the Mn-fiber was placed in a clean beaker with 200 mL of 1 mol/L hydroxylamine. HCl and 100 mL of 1 mol/L HCl was added followed by heating;(2)the solution was filtered after the Mnfi ber was boiled and turned white, and 30 mL HCl and deionized water were added to the beaker to clean the white Mn-fiber and the beaker. The clear liquid was added to the solution;(3)the mixture with a magnetic stirrer bar was placed onto a heated magnetic stirrer. Ba(Ra)SO 4 was then co-precipitated(approximately 1 h later)using 5 mL of saturated Ba(NO 3)2 and 25 mL of 1 mol/L Na 2 SO 4 solution(5 minutes later);(4)the co-precipitate was centrifuged, and the upper clear liquid was removed. The co-precipitate was dried and sealed for 20 days. High Purity Germanium gamma spectrometer was adopted in this research, which is produced by ORTEC Company of United States, and the type of detector is GMX45P4. The co-precipitate was then placed into a gamma spectrometry instrument to measure 226 Ra and 228 Ra. The 214 Pb(294, 352 keV peaks) and 214 Bi(609 keV peak)were used to determine 226 Ra and 228 Ac(338 and 911 keV peaks)was used for 228 Ra. The detector was calibrated using certified reference materials(batch number: 08121)obtained from the National Institute of Metrology, China, made the same way as the actual samples. The counting time for each sample was 24 h. The uncertainties of 226 Ra and 228 Ra were 2.43% to 13.5% and 2.06% to 15.0%, respectively.

3 RESULTS 3.1 Distribution of 226 Ra and 228 Ra in the surface water of Qinghai Lake

The activities of the Ra isotopes in surface water, groundwater and river water are shown in Table 1. The 226 Ra and 228 Ra activities(dpm/(100 L))ranged from 7.88±0.24 to 33.80±0.47 for 226 Ra, and 15.73±0.74 to 47.05±0.78 for 228 Ra in the surface water of the North Bay, and from 14.13±0.22 to 19.22±0.42 for 226 Ra, and 17.72±0.66 to 30.96±1.47 228 Ra in the South Bay. A salinity gradient from 12.02 g/L to 14.71 g/L along the Qinghai Lake was observed during the sampling, with lower salinity along the coast, increasing towards the center of the lake. Figures 2a and 2b show the activity distribution of 226 Ra and 228 Ra for the surface water in Qinghai Lake. The activity of 226 Ra in the South Bay was greater than that in the North Bay, and the activity of 228 Ra in the North Bay nearly the same as in the South Bay. However, the activities of 226 Ra and 228 Ra in the surface water near the estuary were higher than their activities farther away at a lower salinity. Their distributions in the North and South Bays are similar. The farther offshore, the higher the salinity(Fig. 2c), but the activities of 226 Ra and 228 Ra also decrease(Fig. 2d). The Quanji River, Shaliu River and Buha River empty into the North Bay(the northwestern part of Qinghailake) and South Bay(the southwestern part of Qinghailake)of the lake. Early investigations concluded that the origin of this excess amount of Ra was due to desorption from river-borne suspended particles, Ra diffusion from bottom sediments(Li et al., 1977; Elsinger and Moore, 1980; Moore, 1981; Key et al., 1985) and strong coastal groundwater discharge(Miller et al., 1990; Hussain et al., 1999; Krest and Moore, 1999; Moore, 1999; Yang et al., 2002; Ji et al., 2012b). The Ra activity of surface water along the coast is less than that in the inner lake(stations QH-SS4, QH-SS6, QH-NS2 and QH-NS7, far away from the estuary), which is most likely attributable to the lower groundwater discharge and lesser diffusion from the bottom sediments(Hwang et al., 2005). This fi nding may signify that the activities of long-lived 226 Ra and 228 Ra can only change by mixing with lower activity inner bay water after desorption is complete(Moore, 2003; Breier and Edmonds, 2007). Figure 3a shows the general decrease of 226 Ra and 228 Ra activities in Qinghai Lake with increasing water depth. However, the 226 Ra and 228 Ra activities showed a small fl uctuation in the surface water(Fig. 3b), which may be caused by the mixing of water due to hydrodynamic processes.

Table1 Sampling data for all the water samples

Fig. 1 The study area and sampling stations

Fig. 2 The 226 Ra and 228 Ra activities in surface water of Qinghai Lake

Fig. 3 The 226 Ra and 228 Ra activities vertical distribution in the Qinghai Lake
Panel a shows 226 Ra and 228 Ra activities at different depths, panel b shows a three-dimensional diagram of 226 Ra and 228 Ra activities at different depths and distances offshore of Qinghai Lake.
3.2 226 Ra and 228 Ra radioactivity in groundwater

Underground water samples were collected from drinking wells and springs. The salinity of the water samples was low, ranging from 0.33 to 3.86 g/L. The depth of most wells was approximately 2–13 m, suggesting that the wells contained shallow groundwater from the surficial aquifer, which was primarily recharged by rainfall and groundwater infiltration. The range of 226 Ra and 228 Ra activities(dpm/(100 L))was 7.88±0.24 to 31.13±0.47 in the groundwater of the northern region of Qinghai Lake, and 16.41±1.50 to 57.54±2.17, respectively, and 3.89±0.66 to 29.91±0.34, and 11.20±2.77 to 61.78±0.98 in the southern region of the lake, respectively. The radium isotope activities in the groundwater within a large range, and they were mostly higher than that in surface water and river water(Fig. 4), but lower than that in pore water. Most 226 Ra activities in the groundwater were comparable to the 226 Ra activities in surface waters along the coast near the estuary, and individual 226 Ra activities in the groundwater were 30 times that in the surface water(Table 1).

Fig. 4 The relation of 228 Ra and 226 Ra activities
3.3 226 Ra and 228 Ra radioactivity in river water

The Buha and Quanji Rivers fl ow into Qinghai Lake, and rainfall and groundwater recharge the rivers. Therefore we collected river water from the Buha River and Quanji River. Table 1 shows that the salinity of the river water is very low, ranging between 0.28 g/L and 1.02 g/L. The range of 226 Ra and 228 Ra activities(dpm/(100 L))in river water was 5.83±0.17 to 21.63±0.34 and 16.09±0.49 to 31.58±1.34, respectively. The 226 Ra activity in river water was less than in surface water; however, the 228 Ra activity in river water was greater than in the surface water but lower than in the groundwater. The radium activities in the river near the Buha estuary were lower than in the river farther from the mouth. This difference in radium activity was likely caused by the different residence times of the river water(Su, 2013).

4 DISCUSSION 4.1 Analysis of the radium sources in Qinghai Lake water

Moore(1996)presented the potential sources of radium to coastal water to include(1)ocean water, (2)river water, (3)desorption from riverine sediment, (4)erosion of terrestrial sediments and (5)groundwater. There are two types of water enriched in radium. The fi rst, represented by beach springs and offshore submarine springs, is called submarine groundwater discharge, which is enriched in 226 Ra relative to 228 Ra. This source is presumed to originate in deeper limestone artesian aquifers. The second source of radium is desorption from river-borne particles and estuarine sediments and diffusion from bottom sediments(Moore, 2006).

Although the geochemical behavior of 226 Ra and 228 Ra should be identical, the different half-life affects the rates of their activity. Thus, different sources of Ra produce different 228 Ra/ 226 Ra activity ratios. Figure 4 is a plot of 228 Ra vs. 226 Ra for all samples examined. If there were only one source of radium and this source was being diluted by Qinghai Lake water, all samples would fall on a single line in this plot. The linear relationship of 228 Ra and 226 Ra for surface water was notably close to that of pore water, and was different from river water and groundwater. This indicates the presence of complex sources of radium for surface water from groundwater, river water and lake water(Moore, 2008). The distribution of 226 Ra relative to salinity(Fig. 5)displays increasingly nonconservative behavior in the Buha River estuarine mixing zone. This fi nding shows that the sources of radium in Qinghai Lake water are complex. The sources of the extra Ra include desorption from riverborne particles and estuarine sediments, and submarine groundwater discharge. These source terms will be discussed and evaluated in the following section.

Fig. 5 228 Ra and 226 Ra activities versus salinity for all the water samples

The slope of the 228 Ra vs. 226 Ra trend from the groundwater samples is a 228 Ra/ 226 Ra activity ratio(AR)of 1.90(n =6)in the North Bay obtained from a linear regression line of the 228 Ra vs. 226 Ra plot for the groundwater samples, while the slope was 1.97(n =5)in the South Bay(Fig. 4). These similar slopes indicate a rather homogenous source, consistent with a shallow seepage from a surficial aquifer(Su et al., 2011). Sample QH-SG5 had a significantly lower value than the slope from the surface water samples, suggesting that the average groundwater values cannot explain the 228 Ra/ 226 Ra activity ratio in the surface water. Most groundwater samples have approximately the necessary 228 Ra/ 226 Ra AR to explain the surface water activity ratio. Therefore the higher slope for groundwater compared to the slope for the surface water samples may indicate a primary radium source for the bay water.

Various sources of Ra produce different 228 Ra/ 226 Ra activity ratios, but identical sources should have similar ratios. The strong linear relationship between 228 Ra and 226 Ra activities in the surface water and deep water(Fig. 4)implies that the Ra in deep water samples has a single source. The Buha River discharges into Qinghai Lake, and this discharge is one of the sources for the near-shore water with a lower salinity. The higher salinity of Qinghai Lake occurs when the tide rises and salinity is lowered when the tide ebbs. The salty water in the Qinghai Lake has desorbing effect to the bay, suggesting that it can be another radium source.

There are two contributions to radium in the river: dissolved radium and desorbed radium. The main input river is the Buha River in the western region of Qinghai Lake, and it accounts for about half of all the river runoff. The average dissolved 226 Ra and 228 Ra activities(dpm/(100 L))were 11.54±0.26 and 24.44±0.84 in the Buha River, respectively, and 5.83±0.17 and 16.09±0.49 in the Quanji River. The radium on the riverine suspended particles can be desorbed and added to the dissolved phase when these particles encounter seawater(Li et al., 1977; Krest and Moore, 1999). Along with the continuous leaching from suspended particles of the Buha River using without radium water, the dissolved 224 Ra continues to increase, but the dissolved 224 Ra tends toward stability after sixth leach(Fig. 6), which illustrates that radium in the riverine suspended particles can be desorbed into river water.

Fig. 6 The leaching experiment of dissolved 224 Ra activities

We can calculate the desorbable radium activity by measuring the radium associated with suspended particles in a river. The desorbed activity of radium(dpm/(100 L))was measured in riverine suspended particles. The maximum desorbed 226 Ra and 228 Ra activities were 0.059±0.000 2 and 0.155± 0.001 1 dpm/g, respectively(Table 2). This desorption was measured at 60 g/(100 L)suspended particulate matter. The sum of the dissolved and desorbed riverine inputs of 226 Ra and 228 Ra were 11.60±0.26 and 24.60±0.84 dpm/(100 L)in the Buha River, respectively, and 5.90±0.17 and 16.25±0.49 dpm/(100 L)in the Quanji River, respectively. If conservative mixing only occurred between the river and lake water end-members from this point, respectively. If conservative mixing only occurred between the river and lake water end-members from this point, the observed maximum 226 Ra and 228 Ra activities(dpm/(100 L))of 33.80±0.47, 57.31±1.44 in the surface water would not be possible. Therefore, we need to fi nd an additional source to explain the excess radium in the lake.

Table2 Desorption of 226 Ra and 228 Ra from riverine suspended particles in the river

Another possible source of radium in the water column is from diffusive fl ux through the sediment pore spaces. The sources of 226 Ra and 228 Ra to coastal waters include desorption from sediments and discharge from salty groundwater(Moore, 1996). After the release of radium from sediments, the activity of 226 Ra and 228 Ra on particles is regenerated only slowly. In the deep seas, 226 Ra is supplied by desorption of regenerated activity. The regeneration of 226 Ra activity in sediments from which it was desorbed proceeds as follows:

A 226 Ra= A 230 Th(1– e - λ 226 t),
where λ 226 is the decay constant of 226 Ra(4.33×10 -4 /yr) and A is activity of the parent and daughter nuclides. Thus, 500 years are required to regenerate 20% of the desorbed activity(A 226 Ra/ A 230 Th=0.2). Therefore, marine sediments that have desorbed 226 Ra contribute little to the enrichment of 226 Ra in the coastal region(Moore, 1996). Because of desorption of 226 Ra by marine sediments, the 226 Ra activity showed an increasing trend with depth, but 226 Ra and 228 Ra activities decreased with depth in Qinghsai Lake, which may indicate that the lake sediments that had desorbed 226 Ra contributed little to the 226 Ra enrichment in the surface water. Regeneration as a source was thus negligible compared to the river input. Because Ra desorption from suspended particles from the river is much more important than desorption from the resuspended sediments and diffusion of regenerated Ra from sediment pore spaces, many studies only consider the radium contribution from the river(Su et al., 2011; Wang et al., 2014).

4.2 The end-members value for radium balance

The different sources of Ra can be identifi ed by measuring the Ra isotopic composition. Thus, we can estimate the contributions of sources using a hybrid model based on different Ra sources. The long-lived isotopes may be used to estimate the relative SGD inputs to the study area from near-shore high AR and offshore low AR sources(Moore, 2003). We used a similar method by establishing a water, 226 Ra and 228 Ra balance to estimate the river water, groundwater and lake water three-end-member contributions fraction for Qinghai Lake. To identify how the three sources infl uence the bay area, we used the three-endmember mixing model developed by Moore(2003). To avoid the complexity of the decay of the shortlived Ra isotopes, 226 Ra and 228 Ra were used to identify the different sources of the radium(Moore, 2003; Dulaiova et al., 2006). The following equations were used to establish a three-end-member mixing model:

where f is the fraction of river(R), bottom water of lake(L), and groundwater(G)end-members. Ra R is the 226 Ra or 228 Ra activity in the river water endmember, Ra L is the 226 Ra or 228 Ra activity in the lake water end-member, Ra G is the 226 Ra or 228 Ra activity in the groundwater SGD end-member, and Ra M is the measured 226 Ra or 228 Ra activity in the sample.

We can obtain the fraction of the three-endmembers by using the above three equations.

If we substitute the average 226 Ra and 228 Ra activities(dpm/(100 L))of groundwater as end members into the formulas, the results are negative for many samples of groundwater fractions. This is because 226 Ra and 228 Ra in the groundwater have a wide range of activity [(3.89±0.24)–(55.90±1.40), (11.20±0.74)–(719.33±5.08)dpm/(100 L)], indicating that the few existing samples are not representative of groundwater and that we need to determine more appropriate end-member values. The radium concentration in groundwater is higher than that in surface water, and the 226 Ra/ 228 Ra values of groundwater are close to that of the surface water. The QH-NG6(228 Ra=57.31±1.44, 226 Ra=31.12±0.47) and QH-SG2(228 Ra=27.52±2.28, 226 Ra=11.13±0.67)samples meet the previously discussed requirements. Therefore we used these samples as groundwater endmembers for the North Bay and the South Bay, respectively. The radium at the bottom of the lake may represent the lake end-member values. Thus, we used samples QH-V1-18(228 Ra=13.41±1.15, 226 Ra=10.17±0.30), and QH-SS10(228 Ra =17.28±0.87, 226 Ra=14.81±0.40)as lake end-member values for the North Bay and South Bay, respectively. The average dissolved river end-members are the Quanji sample(228 Ra=23.40±1.35, 226 Ra=11.33±0.35) and the Buha2 sample(228 Ra=31.58±1.34, 226 Ra=21.63±0.34)for the North Bay and South Bay, respectively. The maximum desorbable activity(dpm/(100 L))of 228 Ra and 226 Ra in Buha River are 9.66±1.06 and 8.83±0.26, respectively, and they are 4.55±0.53 and 4.15±0.13 in Quanji River. In the north bay, f L =0.69–0.77, f GW =0.08–0.19, and f R =0.04–0.23. The average fractions are f L =0.73, f GW =0.13, and f R =0.14. In the south bay, f L =0.51–0.85, f GW =0.07–0.21, and f R =0.04–0.38. The average fractions are f L =0.69, f GW =0.12, and f R =0.19, where, f R included the dissolved radium and desorbed radium from the riverine suspended particles. The radium contribution of the groundwater is less than the contribution of river water, and this is different from a coastal lagoon(Su et al., 2011; Ji et al., 2012a). The contribution for the north region of Qinghai Lake is greater than the south region, maybe due to the large portion of groundwater in the northern region of the lake. The contribution of the Buha River is larger than the contribution of the Quanji River, which is reasonable because its greater runoff.

5 CONCLUSIONS

From the distribution patterns of the 226 Ra and 228 Ra tracer in lake surface water, groundwater, river water, pore water, suspended particles, and bottom sediments, we draw the following conclusions:

Due to buoyant river water mixing with lake buoyant water, radium isotopes in the surface water near the estuary were present at higher concentration than further away where salinity was lower. The farther offshore the sample, the higher the salinity is, but the radium isotope activity decreases. Radium activities decrease with increasing depth of lake water. The 226 Ra and 228 Ra activities as measured from desorption of river suspended particles and sediment diffusion is low.

The 226 Ra and 228 Ra contribution of groundwater is less than the contribution of river water. The contribution for the north region of Qinghai Lake is greater than the southern part, and the contribution value of the Buha River is larger than the contribution of the Quanji River because its larger runoff. The distribution of radium activities in the waters of the western part of Qinghai Lake are primarily controlled by the Buha River runoff, groundwater discharge and , to a lesser degree, by desorption from suspended particles from the river and diffusion from the sediment.

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