Finally Remark

The compilation of studies in the Bohai Sea shows that the flux of SGD has been analyzed and calculated in all the five regions, and it is proved that the input of SGD is much larger than the total inflow of each river. In addition, in some areas, the impact of SGD on the environment was also considered. They investigated that the N/P ratio of SGD input was higher than that of river input and the utilization ratio of N and P of phytoplankton in coastal zones. If N and P are added to coastal waters through SGD, the original N/P ratio and N as restrictive nutrient elements in coastal waters will be gradually changed, which not only directly leads to eutrophication and phytoplankton explosive reproduction, but also may change the population structure of phytoplankton in coastal waters.
However, we have noticed that there are still a lot of areas that have not been studied along the coast of the Bohai Sea such as the coastal areas of Jiangsu province, Tianjin province, Hebei province, Dalian City and Jinzhou City in Liaoning province. This indicates that in the future, we should focus on promoting SGD research in the direction of distribution and scale.
In recent ten years, although great progress has been made in SGD research in the Yellow Sea and Bohai Sea area, some problems have not been solved well
Strengthen the study of biogeochemical cycle in underground estuaries. Because of the wide range and variation in the concentration of nutrients, carbon, and metals in aquifers, it is necessary to understand the major controlling factors for these components in coastal aquifers and the chemical reactions that may occur in the concentration of these components along the SGD flow path from land to ocean. The greatest difficulty in the study of underground estuaries is the stratified sampling of groundwater at different depths. There are usually few wells available in the coastal zone. Although there are many fixed wells, they are unable to meet geochemical cycling studies owing to small pore water samplers do not reach the desired depth.
In the Bohai and Huanghai area, the influence of SGD on carbon cycle should be strengthened. In the biosphere of the global carbon cycle, carbon is mainly distributed on land, sea and atmosphere. Relative to the surface water, groundwater of total dissolved inorganic carbon (DIC), the content of CO2 and CH4 is higher. Moore used Ra flux in groundwater, the ratio of DIC/Ra and DOC/Ra to evaluate DIC and DOC (total dissolved organic carbon) input by SGD. The results showed that the flux is greater than quantitative determine SGD freshwater rivers carry input component quantitative evaluation of the new method and means. Therefore, it is very important to study the effect of SGD on the carbon cycle along the Coast of The Bohai Sea and the Yellow Sea.
The comprehensive influence of sea level fluctuation and human activities on the large-scale hydrogeological system of coastal zone should be studied. SGD in coastal aquifers is affected by sea level rise, groundwater exploitation, dredging of ports, changes of coastline and other factors. However, the comprehensive effects of these factors on coastal aquifers and SGD have not been fully studied. In addition, studies on the Yellow Sea and Bohai Sea region mainly focus on important river mouth areas and small scale bays, and there are few large-scale SGD studies, which makes the understanding of SGD insufficient.
Study the hydrogeological conditions of the coastal zone and different types of coastal zone. In the region of Bohai Sea and Yellow Sea, the research focuses on the sandy coast, and the research on other types of coast is less. In addition, the subsurface flow conditions have not been fully described and quantitative analysis, which makes the traditional methods have many shortcomings. In the process of SGD investigation, it is necessary to have a sufficient understanding of the coastal hydrogeological conditions, which is of great significance to distinguish SFGD and SRGD.
In order to describe SGD in more details in the Yellow and Bohai Sea region, great efforts must be made to study considering representative coastal aquifer types, climatic conditions and environmental dynamics, and to promote cooperation among experts in related disciplines. We believe that the intersection and fusion of various disciplines will have a profound impact on SGD research, and can better understand the interaction process of seawater and groundwater, which is of great significance for the study of the hydrologic cycle in the Yellow Sea and Bohai Sea region