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24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Aquifer Management Meeting, 4 –8 July 2016, Cairns, Australia

Seasonal distribution of radium isotopes and submarine groundwater discharge in Bay, China

Xuejing Wang1, Hailong Li1, Chunmiao Zheng1 1School of Environmental Science and Engineering, South University of Science & Technology of China, Shenzhen, China

ABSTRACT Seasonal patterns of radium isotope distributions can be used to indicate changes in SGD through the season. Here we report the distributions of two short-lived radium isotopes (223Ra and 224Ra) measured in the seawater of Laizhou Bay, China in the wet and dry seasons, respectively. The primary source of the radium isotopes is shown to be SGD. The higher inventories of radium isotopes imply higher fluxes of SGD to the bay during the wet season because of the little difference of the activities of these isotopes in SGD with season. The large volume of SGD in wet season confirms its importance in supplying considerable quantity of nutrients.

INTRODUCTION As a major component of the hydrological cycle, submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and an important pathway for dissolved material transport from land to ocean. Natural radium isotopes are recognized as ideal tracers for effective and efficient assessment of SGD in local scales and global scales since they are conservative chemically and widely enriched in SGD (Moore, 1996; Kim et al., 2005; Moore et al., 2008; Kwon et al., 2014; Wang et al., 2015). Thus, the seasonal patterns of radium isotope distributions can be used to indicate changes in SGD through the season.

Laizhou Bay is subject to a variety of environmental stresses, and hence provides an archetype of a semi-enclosed bay for which ecological functioning is a sensitive issue. Wang et al. (2015) used the salinity and 226Ra as tracers to quantify submarine fresh groundwater discharge and SGD in Laizhou Bay, China. The early study was based on data collected in August 2012, regarding as a wet season. The main purposes of this study were to investigate the spatial and temporal distributions, inventories of radium isotopes (223Ra and 224Ra) in Laizhou Bay and use these seasonal patterns of radium isotope distributions to indicate changes in SGD.

METHODOLOGY Laizhou Bay is one of the three bays in the , China (Figure 1). The natural coastline of Laizhou Bay extends from Qimu Cape to the Estuary. Our filed work was launched in wet season (August, 2012) and dry season (May, 2014). We collected 8 coastal groundwater and 6 river water samples along the shoreline and 44 seawater samples (1-2 m below seawater surface) for Ra extraction. Radium isotopes were collected using the methods established by Moore (1976). The short-lived radium isotope 223Ra and 224Ra were counted using a two-channel radium delayed coincidence counting system (RaDeCC), and the expected error of measurements is 10% (Moore and Ralph, 1996).

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24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Aquifer Management Meeting, 4 –8 July 2016, Cairns, Australia

Figure 1. Map of Laizhou Bay and sampling stations.

RESULTS Figure 2 shows the changes of the activities of 223Ra and 224Ra in seawater and groundwater with salinity in different seasons, respectively. The seawater salinities were higher in the dry season (May, 2014) than in the wet season (August, 2012), where the activities of the radium isotopes were comparatively low. From Fig. 2a and 2b one can observe a more systematic distribution that Ra activity slightly decreases when the salinity increases. The coastal groundwater with relatively high salinity have higher activities of radium isotopes, however there is the little difference of the activities of these isotopes in coastal groundwater with season (Fig. 2c, 2d).

Figure 2. Plot of 223Ra and 224Ra versus salinity in seawater (a, b) and groundwater samples (c, d).

The horizontal spatial distribution of 223Ra and 224Ra in different seasons was showed in Figure 3. The activity distributions of these two isotopes in different seasons have the

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24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Aquifer Management Meeting, 4 –8 July 2016, Cairns, Australia

following similar features: (1) the activities were significantly higher in the west and south than in the east and north of the Bay; and (2) the activities were very high in the estuary and near-shore areas and they generally decreased with the offshore distance. The inventory of Ra in the bay can be estimated based on measurements of radium concentration and water depth at each sampling station. We divided the study area into 91 sections as a representation of the mass-balance box geometry. We calculated the mass of Ra in each box as the product of the area of the corresponding section, the averaged seawater depth and radium concentration within each compartment and derived a total estimated mass for all increments. We obtained the inventories of 223Ra and 224Ra in the dry season are 72.5% and 65.5% of these inventories in the wet season, respectively.

Figure 3. Contour plots of 223Ra and 224Ra in wet season (a, b) and dry season (c, d).

CONCLUSIONS The activities and the horizontal distributions of two short-lived radium isotopes (223Ra and 224Ra) with season in Laizhou Bay were characterized. The activities and the inventories of these isotopes are significantly higher in the wet season than in the dry season. The primary source of the radium isotopes is shown to be SGD. Because there is the little difference of the activities of these isotopes in SGD with season, the higher inventories of radium isotopes imply higher fluxes of SGD to the bay during the wet season. The larger volume of SGD in wet season confirms its importance in supplying considerable quantity of nutrients because of the enrichment of nutrients in SGD.

ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China (Grant No. 41430641) and SUSTC Presidential Postdoctoral Fellowship. We thank Tao Zheng, Zhenfei Xu, Long Xi, Shengtao Zheng and Zhigang Cheng for their field work and laboratory test.

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24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Aquifer Management Meeting, 4 –8 July 2016, Cairns, Australia

KEYWORDS: submarine groundwater discharge; radium isotopes; Laizhou Bay

REFERENCES Kim, G., J. W. Ryu, H. S. Yang, and S. T. Yun. 2005. Submarine groundwater discharge (SGD) into the revealed by 228Ra and 226Ra isotopes: Implications for global silicate fluxes, Earth and Planetary Science Letters, 237(1/2): 156-166.

Kwon, E.Y., G. Kim, F. Primeau, W.S. Moore, H.M. Cho, T. DeVries, J.L. Sarmiento, M.A. Charette, and Y.K. Cho. 2014. Global estimate of submarine groundwater discharge based on an observationally constrained radium isotope model, Geophysical Research Letters, 41: 8438-8444.

Moore, W.S. 1976. Sampling 228Ra in the deep ocean. Deep Sea Research and Oceanographic Abstracts, 23(7): 647-651.

Moore, W.S. and R. Arnold. 1996. Measurement of 223Ra and 224Ra in coastal waters using a delayed coincidence counter. Journal of Geophysical Research, 101: 1321-1329.

Moore, W.S. 1996. Large groundwater inputs to coastal waters revealed by 226Ra enrichments. Nature, 380(6575): 612-614.

Moore, W.S., J. L. Sarmiento, and R.M. Key. 2008. Submarine groundwater discharge revealed by 228Ra distribution in the upper Atlantic Ocean, Nature Geoscience, 1(5): 309-311.

Wang, X.J., H.L. Li, J.J. Jiao, D.A. Barry, L. Li, X. Luo, C.Y. Wang, L. Wan, X.S. Wang, X.W. Jiang, Q. Ma, and W.J. Qu. 2015. Submarine fresh groundwater discharge into Laizhou Bay comparable to the Yellow River flux. Scitific Reports, 5, 8841.

Contact Information: Xuejing Wang, South University of Science & Technology of China, Environmental Science and Engineering, No 1088, Xueyuan Rd., Nanshan District, Shenzhen, 518055, China; Email: [email protected]; [email protected]

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