Common Methods on SGD
Accessment
In recent years, there are many methods on SGD detection and estimation.
Presently, the commonly used SGD evaluation methods can be sumarized
into four categories: direct measurement method, hydrological model
method, environmental isotope tracer method and geophysical tracer
method.
Direct measurement
This method is to measure SGD directly by means of percolator. In 1977,
Lee (Lee, 1977)invented the manual percolation instrument. Its structure
is very simple, mainly composed of a stainless steel cylinder with a
volume of 208 L and one end closed, a conduit with a double valve switch
and a water collecting bag with a volume of 4 L. At present, the more
advanced automatic percolator is based on thermal pulse technology
(Taniguchi et al. , 2003), acoustic doppler technology or dye
dilution technology(Sholkovitz et al. , 2003). It can record the
change of groundwater discharge with the tide cycle continuously.
However, it costs a lot of time and labors to measure SGD with the
percolator, as it requires the diver to place and recover the percolator
on the seabed. Moreover, the percolator can only measure the local SGD.
When necessary to make out the characteristics and laws of the regional
SGD, a large number of measuring points must be arranged. In addition,
the measurement process of seepage meter is easily disturbed or damaged
by current and wave (Cable et al. , 1997). To a certain extent,
these drawbacks prevent percolator from widely use.
Hydrological model
It refers to calculate groundwater discharge by hydrogeological
mathematical methods to evaluate SGD, based on Darcy’s law, water
balance method and numerical simulation method.
Darcy’s law realizes the leap from the qualitative and semi qualitative
analysis to the quantitative evaluation of groundwater, which is the
basis of the assessment of groundwater. It is used to calculate the
amount of groundwater flowing through the aquifer driven by the
hydraulic gradient (Ma et al. , 2015; Qu et al. , 2017).
Meanwhile the theoretical basis of water balance method is necessary
that the expenses and receipts of water in a certain region or even in
the whole world is balanced, of which SGD is a part. If we know other
items, we can get the amount of SGD according to the balance formula
(Burnett et al. , 2003). However, the reliability of water balance
method depends on the variable precision of each hydrological process,
such as precipitation, evaporation, surface runoff and artificial
exploitation. The evaporation is especially difficult to estimate,
leading to the error which can not be overlooked.
The numerical simulation method is using the finite element method or
the finite difference method to discretize the flow equation and solute
transport equation spatio-temporal, and to get the flow velocity and
salinity on the discrete element. The groundwater discharge can be
calculated according to the seepage velocity distribution on the aquifer
boundary (Chang et al. , 2018; Yu et al. , 2019). However,
at present, the problems faced by this model are: serious lack of
hydrogeology data; inappropriate generalization of parameters;
simplification of complicated driving force from the ocean et al. Since
so, the simulated results usually deviate much from the total discharge
of groundwater evaluated by seepage mete, tracer method and measured
(Oberdorfer, 2003; Moore, 2010).
Environmental isotope
tracer
Geochemical tracing uses natural radionuclides of uranium and thorium
decay series. These natural radionuclides usually have higher
concentration in groundwater with lower reactivity in coastal seawater,
which means chemical behavior is more conservative. The basic theory of
using tracer to evaluate groundwater discharge is the mass balance
principle (Yuan et al. , 2016; Zhang et al. , 2016). The
tracer technology combined the SGD signals in the coastal waters
effectively and ignored the heterogeneity of groundwater discharge. The
results show the average level of SGD on a regional scale (Burnettet al. , 2001).
Geophysical tracing
The resistivity measurement technology based on the electrical
difference between salt and fresh water body and surrounding soil is a
new monitoring method developed in the early 21st century. Compared with
other methods, resistivity method has the advantages of continuity,
spaciality and intuitionistic mapping. It can be used not only for local
monitoring, but also for large-scale detection. In its initial stage, it
is often used as a comparison method, which is often combined with
temperature sensing and tracer method to monitor the groundwater
discharge process (Henderson et al. , 2010). With the development
by Henderson, Dimova and Johnson (Henderson et al. , 2010; Johnsonet al. , 2015), resistivity method gradually shows its technical
advantages and application potential in the aspects of real-time
reflection of water transport, identification of conversion interface,
estimation of local discharge, etc., and has been applied more and more.
However, from the current application examples, this method is also
mainly used for the discharge monitoring of the phreatic aquifer in the
tidal process.