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.