226Ra and228Ra tracer study on nutrient transport in east coastal waters of Hainan Island, China

Ni SU, Jin-zhou DU*, Tao JI, Jing ZHANG

State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, P. R. China

226Ra and228Ra tracer study on nutrient transport in east coastal waters of Hainan Island, China

Ni SU, Jin-zhou DU*, Tao JI, Jing ZHANG

State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, P. R. China

Material fluxes (e.g., nutrients) from coastal waters to offshore areas play an important role in controlling the water quality of the adjacent sea areas not only by increasing nutrient concentration but also by changing nutrient structures. In this study, naturally occurring isotopes,226Ra and228Ra, were measured with the alpha spectrometry in the Wenjiao-Wenchang and Wanquan estuaries and adjacent sea areas along the east coast of Hainan Island. The excess226Ra and228Ra activities were observed by comparison with the values derived from the conservative mixing of freshwater and seawater end-members in both estuaries. Using a one-dimensional diffusion model, the horizontal eddy diffusion coefficient of 3.16 × 105cm2/s, for nutrients diffusing from their sources, was derived from228Ra activities. Consequently, the corresponding nutrient fluxes flowing into the coastal waters were assessed. The results can provide useful information for the study of the mixing and exchange processes of coastal waters as well as dissoluble pollutant transport in this sea area.

Ra isotope;226Ra and228Ra tracers; horizontal eddy diffusion coefficient; nutrient flux; east coastal waters of Hainan Island

1 Introduction

Radium (Ra) is an alkaline earth element that is strongly absorbed onto particles and sediments in rivers. With the increase of the ionic strength and the decrease of the particle concentration in estuary regions, most Ra exists as dissolved Ra2+in saline, low turbidity seawater due to desorption from suspended particles and diffusion from sediments (Key et al. 1985; Yang et al. 2002; Lee et al. 2005; Gonneea et al. 2008). Submarine groundwater discharge with high Ra concentration also seeps into the estuarine and nearshore waters (Burnett et al. 1990; Krest et al. 1999; Moore 1997, 1999; Yang et al. 2002; Peterson et al. 2008; Nakada et al. 2011).

The coastal ocean exchanges large amounts of nutrient and energy with the open oceanbecause virtually all terrigenous materials, including water, sediments, dissolved particulate nutrients, and trace elements, enter the coastal ocean with surface or sub-surface runoff. However, the transport of these chemicals from the coast to the open ocean is difficult to quantify because of the complex temporal and spatial variability within these systems (Moore 2000). Ra is a useful tool for tracing the water movement rate in the ocean and, consequently, for investigating the characteristics of material (e.g., nutrient and pollutant) transport to adjacent sea areas (Moore 2000; Charette et al. 2007; Colbert and Hammond 2007).

Okubo (1971) investigated the relationship between two parameters of diffusion, the time and length scales, using the examined data from instantaneous dye release experiments in the upper mixed layer of the North Sea. The study provided a practical means for predicting the horizontal diffusion coefficient of substances from an instantaneous source. Alternatively, Ra isotopes are considered good tracers to study biogeochemical processes on different time scales in the coastal and adjacent sea areas (Kaufman et al. 1973; Yamada and Nozaki 1986; Schmidt and Reyss 1996; Moore 2000; Charette et al. 2007; Colbert and Hammond 2007; van Beek et al. 2008). Mixing parameters like the horizontal eddy diffusion coefficient in coastal regions were derived from the228Ra activity measured in surface layers (Somayajulu et al. 1996; Huh and Ku 1997; Rengarajan et al. 2002; Men et al. 2006). In recent years, there have been few studies on the horizontal eddy diffusion coefficient in the South China Sea and the corresponding nutrient transport from the coast to adjacent sea areas. Huang et al. (1997) studied the distribution of228Ra in the surface water of the northeastern South China Sea, and estimated the horizontal eddy diffusion coefficient based on the exponential relationship between the228Ra activity and the distance offshore.

There are diverse ecosystems in the east coastal waters of Hainan Island. According to the surveys in recent years, the coral reef ecosystem in some regions is being degraded and the living space is shrinking due to the transport of terrigenous pollutants to the local waters. This work aimed to study the distribution characteristics of226Ra and228Ra in estuaries and adjacent sea areas along the east coast of Hainan Island, China. The behavior of the two Ra isotopes was investigated during estuarine mixing. A one-dimensional diffusion model was applied to fit the228Ra data for estimating the horizontal eddy diffusion coefficient in the coastal area.

2 Sampling and methods

2.1 Study site

Hainan Island is located in the southern part of China, across the Qiongzhou Strait from the Leizhou Peninsula of Guangdong Province. On its eastern side, it borders the South China Sea (Fig. 1). Our study area was focused on the east coastal area of Hainan Island, including two estuaries, the Wenjiao-Wenchang and Wanquan estuaries. The Wenjiao and Wenchang rivers, with total lengths of 56 km and 37 km, respectively, directly connect withthe Bamen Bay, which has a surface area of 40 km2and an average water depth of 1 to 2 m. The two rivers have a total runoff of 8 × 108m3/year and a mean suspended sediment load of 1.0× 105t/year (Wang et al. 2006). The Wanquan River is the third largest river in Hainan Province, with a total length of about 160 km and a drainage area of about 3.6 × 103km2. The runoff of the Wanquan River shows a significant seasonal variation, with an annual average runoff of 5.2 × 109m3/year and a mean suspended sediment load of 3.9 × 106t/year (Wang 2002; Wang et al. 2006). In our study area, a significant feature is the notable diversity of habitats, such as rocky shores, sandy beaches, mangroves, sea grass beds, and especially, coral reefs. Frequent tropical cyclones strike the island in August and September every year (Mao et al. 2006).

Fig. 1 Sampling sites in study area along east coast of Hainan Island

2.2 Methods

Most of the coastal and estuarine water samples in this study were collected in August 2007. We also collected samples at the sampling sitesW21-W29in the Wenjiao-Wenchang Estuary in July 2008 after a typhoon event.

Samples were collected and enriched as follows: (1) About 20 L of water was collected using a submerged pump installed at a water depth of 0.5 m and filtered immediately through cellulose filters (pore size: 0.45 μm). It was then stored in pre-cleaned polyethylene containers. (2) 4.00 dpm of229Th-225Ra solution (Eckert and Ziegler Isotope Products, 7229) was added as an internal tracer while the sample solution was stirred and acidified to reach a pH value of 2, and then allowed to stand for about 6 h to equilibrate with seawater. (3) NH4OH, KMnO4and MnCl2solutions were added to form an amorphous dark brown suspension of MnO2at a pH value of 9. (4) The suspension was stirred for 0.5 to 1 h, and allowed to settle for more than 12 h. (5) The precipitate was separated from the supernatant and dissolved in HNO3and H2O2solutions (Dimova et al. 2007).

The following separation and purification procedures were described by Hancock and Martin (1991). Briefly, the Pb(NO3)2solution, dilute H2SO4, and solid K2SO4were added to the acidic solution mentioned above to form the Pb(Ra)SO4co-precipitate. The co-precipitate was centrifuged and redissolved in the ethylene diamine tetraacetic acid (EDTA) solution. Then, this solution was transferred through an anion exchange column (DOWEX 1X8-200, 100-200 meshes, chloride form, 50 mm in height, and 7 mm in diameter) for desulfidation, and this column was washed in 13-mL EDTA solution with a concentration of 0.01 mol/L and a pH value of 10. After that, thorium (Th) and actinium (Ac) were retained on the column (Th separation). Later, the solution was transferred from the anion column onto a cation exchange column (DOWEX 50WX8-200, 200-400 meshes, 80 mm in height, and 7 mm in diameter) to elute plumbum (Pb) and residual Th and Ac. Finally, Ra isotopes were electrodeposited onto a stainless-steel disc and determined by the alpha spectrometry (Canberra 7200-08).226Ra is an alpha-emitting isotope, while228Ra is a beta-emitting isotope. Hence, the226Ra activity was calculated immediately after the counting, but the228Ra activity could only be obtained indirectly through the activity of its daughters228Th and224Ra after the disc was stored for more than six months.

3 Results and discussion

We present the226Ra and228Ra data andRaactivity ratio (henceforth denoted as [228/226]) in two estuaries (sampling sitesW1-W16in the Wenjiao-Wenchang Estuary andB1-B23in the Wanquan Estuary) and offshore sea waters (sampling sitesT1,T3, andT5on transectT, andT1′,T3′, andT5′on transectT′) in August 2007 and in the Wenjiao-Wenchang Estuary (sampling sitesW21-W29) in July 2008 after a typhoon event. The sampling siteB23is on upper reach of the Wanquan River and not shown in Fig. 1. The detailed sampling information and the Ra isotope activities are shown in Table 1.

Table 1 Sampling sites, water depth, salinity, and analytical results of Ra isotopes in estuaries and adjacent sea areas along east coast of Hainan Island, China

3.1 Distribution of226Ra and228Ra in Wenjiao-Wenchang Estuary

Sampling sites in the Wenjiao-Wenchang Estuary are shown in Fig. 1. In August 2007, the226Ra and228Ra activities were 13.0-34.3 dpm/100L and 34.7-140 dpm/100L, respectively. In July 2008, as influenced by the typhoon event, the226Ra and228Ra activities decreased to 12.2-23.8 dpm/100L and 24.6-71.1 dpm/100L, respectively. The low Ra concentrations may be due to the strong turbulent mixing and seawater dilution caused by the typhoon. As expected, the maximum desorption event in July 2008 occurred in the region with a lower salinity range than that of the region in August 2007 (Figs. 2(a) and 2(c)).226Ra and228Ra concentrations were well above the conservative mixing line between the river and seawater end-members, especially in August 2007. This distinct convex curvature indicated the excess addition of Ra into the dissolved phase. Ra sources include the desorption from riverine suspended particles, the diffusion from bottom sediments, and most importantly, the submarine groundwater discharge (Hussain et al. 1999; Krest et al. 1999; Moore 1999; Yang et al. 2002; Peterson et al. 2008; Nakada et al. 2011).

[228/226] followed the same pattern as the228Ra activity in the Wenjiao-Wenchang Estuary and varied greatly with the salinity, as shown in Figs. 2(a) through (d). In August 2007, [228/226] in freshwater was lower than 2, and it increased sharply to the maximum value of 5.8 at moderate salinity, and then decreased through the high-salinity region in the mixing zone. In July 2008, [228/226] increased from the value lower than 2 in freshwater as well to the maximum value of 4.1 and then decreased with the increasing salinity. Although the two isotopes have similar chemical properties, the difference in their half-lives will affect the production rates because of their respective parents. Short half-lives result in higher production rates (Beck et al. 2007). Thus, we know that the growth rate of228Ra is higher than that of226Ra, accounting for [228/226] of more than 1 in most cases. Meanwhile, if we assumed that the desorption of226Ra and228Ra from suspended particles was similar and the contribution from submarine groundwater discharge was negligible, the diffusion of these two isotopes from bottom sediments have been much different to account for the large difference in [228/226]. However, this may not have been the case, as the diffusion from sediments cannot cause such a large difference in [228/226]. It was reported that in estuaries with salinity ranging from 2 to 20, where Ra isotopes were excessively released into the seawater, the maximum values of [228/226] were within the range of 3 to 5 (Elsinger and Moore 1983; Rengarajan et al. 2002). In this study, the maximum [228/226] observed was close to 6 in 2007 and greater than 4 in 2008 in the Wenjiao-Wenchang Estuary. Such results most likely suggested discharge of submarine groundwater with unusually high [228/226] (Swarzenski 2007; Charette 2007), which should be further investigated.

Fig. 2226Ra and228Ra activities and [228/226] vs. salinity for Wenjiao-Wenchang Estuary

3.2 Distribution of226Ra and228Ra in Wanquan Estuary

Unlike the natural topography in the Wenjiao-Wenchang Estuary, the topography is quite complex in the Wanquan Estuary. There are small sandbars close to the mouth, separating the river flow into southern and northern branches. Sampling sites in the Wanquan Estuary were located in the southern branch (Fig. 1). The226Ra and228Ra activities and [228/226] there were 7.04-18.9 dpm/100L, 8.37-32.6 dpm/100L, and 1.0-2.4, respectively, which were significantly lower than those in the Wenjiao-Wenchang Estuary. The relationship between the226Ra or228Ra activity and salinity also demonstrated non-conservative mixing, except for one sampling point where salinity was 33.9 (Figs. 3(a) and (b)). This implies that the impact of groundwater discharge in the Wanquan Estuary on the Ra concentration might not be as significant as that in the Wenjiao-Wenchang Estuary. At the sampling point with the salinity of 33.9, we observed an increase of the228Ra activity but a decrease of the226Ra activity, probably indicating the extra input of228Ra to the mixing zone. A similar phenomenon was found in the Yangtze Estuary by Elsinger and Moore (1984). We suspect that the diffusion from bottom sediments other than groundwater discharge provided228Ra to the water body, while addition of226Ra from these sources was masked by water dilution.

Fig. 3226Ra and228Ra activities and [228/226] vs. salinity for Wanquan Estuary

3.3 Estimation of horizontal eddy diffusion coefficient

228Ra in ocean surface waters is continuously supplied from the continental shelf sediments by the decay of its parent232Th. Horizontal mixing of the water body promotes the transport of228Ra from the surface mixing layer offshore into the open ocean. It is possible to use228Ra as a tracer to estimate the exchange rate of coastal waters. If net advection can be neglected, and the system is in a steady state, the distribution pattern of the228Ra activity versus distance offshore can be expressed with a simple one-dimensional diffusion model as follows (Moore 2000):

whereAis the228Ra activity (dpm/100L);xis the distance offshore (km);λis the decay constant of228Ra, andλ= 0.12/year; andKhis the horizontal eddy diffusion coefficient (cm2/s). Under boundary conditionsA=A0atx= 0 andA→0 atx→∞, the solution of Eq. (1) is then expressed as

Eq. (2) describes the decrease of228Ra with increasingxas an exponential function. ThusKhcan be calculated from the slopeSof a plot of lnAversus the distance offshoreFig. 1 shows two transectsTandT′ in the adjacent sea areas. Profiles of the226Ra and228Ra activities as a function of the distance offshore for these two transects are illustrated in Fig. 4.

Moore (2000) showed a significant decrease of Ra isotopes with the distance offshore within 50 km from the coast. However, in this study, we saw a slightly decrease of the226Ra and228Ra activities at transectT(Fig. 4(a)). It has been reported that228Ra is known to build up to high concentrations in extended continental shelf areas (van der Loeff et al. 1995; Kim et al. 2005), which would constitute an alternative source of228Ra from neighboring regions. Fig. 4(b) shows an increase of the228Ra activity further offshore at transectT′ that causes a poor linear regression result (R2=0.036 7). This suggests an additional228Ra source that contributed to theoffshore areas. In this case, the mixing model assumptions cannot be met, and thus, transectT′was not included for the diffusion coefficient calculation. However, the estimate at transectTseemed more robust, and any subsequent226Ra or nutrient flux calculations would only be based on this transect.

Fig. 4226Ra and228Ra activities as a function of distance offshore for transectsTandT′

Fig. 5 shows lnAas a function of the distance offshore for transectsTandT′. The slope of the regression line was –0.011 dpm/(100L·km) at transectT, and theKhvalue of 3.16 × 105cm2/s was obtained from Eq. (2) based on the mixing model assumptions.

Fig. 5 lnAas a function of distance offshore for transectsTandT′

Table 2 summarizes the horizontal eddy diffusion coefficients calculated using the228Ra activity in some regions. Huang et al. (1997) estimated the horizontal eddy diffusion coefficient of 2.3 × 106cm2/s in the northeastern South China Sea. Clearly, the estimatedKhvalue in this study was an order of magnitude less than that reported by Huang et al. (1997).

For a conservative tracer, its flux can be estimated fromKhand the offshore concentration gradient (Moore 2000). The gradient of226Ra was 0.096 dpm/(100L·km) in the study area. Thus, the corresponding offshore226Ra flux for unit cross-sectional area was 2.62 × 109dpm/(km2·d). In the study area, there was a better pycnocline at the water depth of about 10 m, which inhibited Ra transport from the bottom to overlying water. Therefore, we assumed that Ra was transported offshore in this 10-m deep surface layer; thus, the offshore flux of226Ra for unit length was 2.62 × 107dpm/(km·d). This value can be used to balance the226Ra flux to the ocean. Moore (2000) estimated the226Ra flux from the coastline to the ocean and concluded that a substantial volume of groundwater discharge was required to balance the Ra budget. Moreover, application of the228Ra-derivedKhcan also help assess the nutrient fluxes to the open South China Sea.

Table 2 Horizontal eddy diffusion coefficients in some regions, calculated with228Ra activity

3.4 Horizontal diffusion flux of nutrients from coast to offshore area

One important pathway for terrigenous nutrient transport from land to the open ocean is via the diffusion process. In this study, we used the nutrient concentrations at transectT′as an example. Based on the offshore nutrient gradients (, and) and theKhvalue estimated in section 3.3, the nutrient fluxesQfrom the coast to the open South China Sea can be calculated as follows:

whereιis the horizontal nutrient gradient, which was 0.46 μmol/(m3·m) for, 0.03 μmol/(m3·m) for, and 1.03 μmol/(m3·m) forin this study. The horizontal eddy diffusion coefficientKhwas 3.16 × 105cm2/s. Using Eq. (3), the nutrient fluxes in our study area were 1.3 mol/(m2·d) for, 0.082 mol/(m2·d) for, and 2.8 mol/(m2·d) for. The N:P ratio of 16 was the same as the Redfield ratio. The changes of the nutrient concentration could result in the shift in theN:Pratio during the diffusion process. Chen et al. (2001) reported that in the wet season of six months, the Kuroshio current imported (288 ± 26) × 109mol of N, (20.6 ± 1.9) × 109mol of P, and (412 ±37) × 109mol of Si into the South China Sea through the Bashi Channel. In this study, if the nutrients were transported offshore in the 10-m deep surface layer, the total amounts of nutrients for the study area with a 100-km coastline within six months were 2.3 × 108mol of N, 1.5 × 107mol of P, and 5.1 × 108mol of Si. Based on a comparison with the estimates made by Chen et al. (2001), the horizontal transport in this work supplied a small portion of the nutrients into the South China Sea. However, its potential impact on the east coast of Hainan Island cannot be ignored, and the variation of the N:Pratio should be further investigated. Moreover, other nutrient fluxes from the coastal to offshore areas can also be estimated by the diffusion coefficient obtained by this study, which could be helpful to understanding the way aquatic environmental pollution along the coast affects offshore areas.

4 Conclusions

The activity of Ra isotopes (226Ra and228Ra) showed non-conservative behavior in the mixing zone of the Wenjiao-Wenchang and Wanquan estuaries, suggesting an extra input of Ra into the Estuary, and especially the possible contribution from submarine groundwater discharge. It should be mentioned that the potential lateral source of Ra contributing to the offshore area at transectT′ meant that the model assumptions could not be met there. Consequently, we could estimate the eddy diffusion coefficient only from the data at transectT. From the distribution of the228Ra activity with the distance offshore at transectT, the horizontal eddy diffusion coefficient was obtained, which was 3.16 × 105cm2/s, and the associated nutrient fluxes were 1.3 mol/(m2·d) for, 0.082 mol/(m2·d) for, and 2.8 mol/(m2·d) for. Although these estimates only considered a small part of nutrient inputs into the South China Sea, they can still provide a firm basis for the future research when we are committed to reducing their influences on the water quality by controlling the nutrient structures in the coastal ecosystem of east coast of Hainan Island.

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This work was supported by the Natural Science Foundation of China (Grant No. 41021064), the Sino-German Cooperation Project of Ministry of Science and Technology of China (Grant No. 2007DFB20380), and the Ph. D. Program Scholarship Fund of East China Normal University (Grant No. 2010047).

*Corresponding author (e-mail:jzdu@sklec.ecnu.edu.cn)

Received Aug. 14, 2010; accepted May 23, 2011