Introduction

SGD is defined as any and all flows of water from the seabed to the coastal continental margin, regardless of fluid composition and driving force (Burnett et al. , 2003). SGD was discovered and used in undersea springs before the 20th century. However, it was not until the 1980s that scientists began to pay attention to SGD because it also made a difference to groundwater emissions in coastal waters (Burnettet al. , 2003). SGD is an important part of the global water cycle and an access for land to transfer water and materials to the ocean. It affects the ecological environment of coastal areas. Excessive nutrients and other substances carried by SGD are discharged into the coastal waters, which will produce man-made interference to the biogeochemical process in the coastal zone, causing serious damage to mariculture, fishing and other marine economic activities(Kim et al. , 2005; Swarzenski et al. , 2007; Hwang et al. , 2010). SGD has been studied along different coast including sandy, bedrock fissure and karst coast, of which Many research sites are located in the east and west coasts of the United States in North America (Moore, 1996, 2007, 2010; Zarnoch et al. , 2017; Adolf et al. , 2019; Kelly et al. , 2019), and the eastern coastal areas of Brazil in South America(Niencheski and Windom, 2015; Dias et al. , 2016; da Rochaet al. , 2018), the Mediterranean coast of Europe (Aaron et al., 2019; Oehler et al., 2019; Tamborski et al., 2018,2019) and the Baltic coast (Donis et al. , 2017; Krall et al. , 2017; Franzet al. , 2019), The east and west coast of Australia (Lamontagne and Webster, 2019; Maher et al. , 2019; Sadat-Noori and Glamore, 2019), South Korea , Asia (Lee et al. , 2018; Cho et al. , 2019; Kang et al. , 2019) and the coast of Japan (Burnett et al. , 2018; Nikpeyman et al. , 2019; Zhu et al. , 2019).
In recently years, the study of SGD along Chinese coastal arose people’s attention. The coastline in China is about 18000 kilometers long, which is one of the longest countries in the world. The research focuses on the uneven distribution of groundwater input in Jiaozhou Bay, Laizhou Bay and Tolo Bay ((Cen et al. , 2017; Liu et al. , 2018; Yuan et al. , 2018). There exsists a wide and thick distribution area of loose sediments in the estuary of Yellow River, Yangtze River, Pearl River and other rivers. In addition, the water yield of coastal aquifer is good, resulting in the large amount of groundwater input(Xuet al. , 2014; Xia et al. , 2016; Guo et al. , 2017; Liu et al. , 2018). In the east coast of Hainan Island, groundwater input pose a huge impact on the local environment (Wang and Du, 2016), while the input of groundwater in the Minjiang River estuary is very limited (Liu et al. , 2016). However, due to the difficulties in field investigation or research, the requiry of the cooperation of scholars in ocean dynamics, groundwater dynamics, ocean and groundwater geochemistry, isotopic chemistry, etc. the research on SGD in China lags behind such countries and regions, which is seriously divorced from the increasingly serious environmental pollution and ecological deterioration in China’s near sea.
The Bohai Sea and the Yellow Sea are important sea areas in the north of China, whose coastlines account for about one third of the total length of China’s (Figure 1). The coast there is one of the most active areas of economic and social activities. There are Liaodong Bay, Bohai Bay, Laizhou Bay, Jiaozhou Bay and other bays along the coast of the Yellow Sea and the Bohai Sea, as well as six provinces and cities of Liaoning, Hebei, Tianjin, Beijing, Shandong and Jiangsu. These provinces and cities have superior geographical location and are the main industrial and residential areas. In addition, the Yellow Sea and the Bohai Sea are important sea areas in China, and as the junction of land and marine ecosystem, they are one of the most rich-biodiversity regions in the world, which can meet the needs of human’s life and development. Therefore, the coastal zone of the Yellow Sea and the Bohai Sea has also become the most densely populated and economically developed area in the world. In today’s economic globalization, the region also plays an increasingly important role. However, with the development of industry, agriculture and the enhancement of human activities, more land-based pollutants are generated. The pollutants and nutrients contained in SGD can be several times of magnitude higher than surface runoff (Charetteet al. , 2001; Slomp and Van Cappellen, 2004). For example, according to the research, along the Yellow Sea and Bohai Sea, the SGD value of Laizhou Bay is 4.12*107m3/d (Wang et al. , 2015), and that of Jiaozhou Bay is 1.21*107m3/d (Zhang et al. , 2017). To some extent, SGD may cause the deterioration of the marine environment such as the highly eutrophic of coastal water and red tide. In order to understand the process of SGD, this paper introduces this important land sea interaction process with the case study of the Yellow Sea and the Bohai Sea.
This study first describes the formation process of SGD and then introduces the common investigation methods of SGD. The basic purpose is to summarize the research situation of SGD along the Bohai Sea and the Yellow Sea, analyze the environmental impact of SGD on the coastal areas and provide suggestions for further research on SGD in the future.