The Classification and Process of SGD

Due to the dual effects of land and ocean driving forces, it includes two components namely the pure supply from the inland (generally fresh water) and the circulation of sea water (that is, the water flowing back to the ocean after entering the offshore aquifer through the sea land interface under the action of sea tide, wave, density difference of saltwater and fresh water and other factors (Figure 2). Thus, SGD can be divided into SFGD and SRGD (Figure 2).
SFGD is the groundwater formed by the infiltration of atmospheric precipitation, which comes from the mainland and consists of phreatic water and confined water. The SFGD has experienced sufficiently water rock interaction before its infiltration into the sea. So, with a large number of soluble substances dissolved in the underground water, its material composition mainly comes from the dissolution and filtration of soil, sediment and rock. The driving force of SFGD is local hydraulic gradient. Since the ground water level on the land is higher than the sea level, the head difference drives the ground water in the aquifer on the land to discharge to the seafloor (Burnett et al. , 2003; Moore, 2010; Santos et al. , 2012; Li and Jiao, 2013). The deep confined aquifers in some coastal areas can extend far to the bottom of the sea, which means that the land-based underground fresh water can discharge far away from the shore.
SRGD comes from seawater and returns to seawater. The driving forces are mainly tidal pump, wave action and density difference (Burnett et al. , 2003; Moore, 2010; Santos et al. , 2012; Li and Jiao, 2013). When the tide is rising, the upward seawater under the action of tide pump seeps into the aquifer and becomes underground water. It seeps out from the aquifer and returns to the ocean when the tide is ebbing. The circulating sea water under the action of wave mainly occurs along the sandy beach. Compared with the action of tide pump, its circulating depth is much smaller, but its circulating frequency is much higher, which is shown as follows rapid replacement of pore water in surface sediments. The circulating seawater driven by density difference occurs under the mixed zone of saltwater and fresh water in the coastal aquifer. A salt wedge is usually formed in the aquifer, as the density of seawater is higher than that of fresh groundwater (Robinson et al. , 2007).
Besides the flow exchange between surface and subsurface water, circulating seawater and land-based underground both form a net dissolved matter input to the ocean, increasing the amount and types of dissolved substances in the seawater (Dzhamalov and Safronova, 2002). When the sea water seeps into the aquifer, it not only mixes and reacts with the ground fresh water, but also dissolves the soluble substances in the aquifer rock and soil. Therefore, the significance of circulating seawater in aquifer mainly reflected by its geochemical cycle and landy material input.