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.