2.1 Data collection for monthly GALDIT assessment.
GALDIT is a diagnostic method based on the index and ranking that
evaluates the vulnerability of coastal aquifers for six parameters
considering groundwater occurrence (G), aquifer hydraulic conductivity
(A), distance from the shore (D), height of groundwater level above sea
level (L), impact of the existing status of seawater intrusion (I), and
saturated thickness of the aquifer (T) to examine the physical effects
of coastal aquifers on the SWI. This study developed a monthly seawater
intrusion vulnerability assessment method using GALDIT parameters. Data
were collected from the National Groundwater Information Center (2021),
the National Geographic Information Institute (2021), and the Korea
Hydrographic and Oceanographic Agency (2021). The groundwater level,
drill log, and groundwater survey report data were collected from the
National Groundwater Information Center. Digital elevation maps (DEM)
and topographic map data were collected from the National Geographic
Information Institute. Tidal and other data were collected from the
Korea Hydrographic and Oceanographic Agency.
For a monthly vulnerability assessment of the SWI, the parameters that
were relatively static and those that changed temporally were classified
as i) static parameters and ii) dynamic parameters, respectively. For
the static parameter group, G, A, and D were selected. The aquifer type
and A are regarded relatively static parameters in the absence of human
activities. D changes when the sea level rises in the long term;
however, this was excluded in this study, considering that the monthly
change of the coastline due to the sea level is insignificant compared
to the reference value. With regard to D, the data observed to date were
used without considering the future rises in the sea level. D that
exhibited numerous fluctuations were extracted from the average
distances obtained in the last 10 years.
For input data of the parameters that change with time, we employed L,
I, and T. Monthly averaged values were used for the dynamic parameters.
When deriving the input value of the height of the freshwater
groundwater relative to the sea level, the parameter values were
calculated considering that the observed groundwater level and the
standard of sea level also fluctuate by month. T likewise changes
monthly due to the effect of groundwater level. For each index displayed
by month, we compared the parameters that were calculated by applying
the effective weight using the weight and sensitivity analysis of
GALDIT. Figure 1 shows a flowchart for calculating monthly GALDIT
parameters.