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.