3.2 Dynamic GALDIT parameters
3.2.1 Height of groundwater level above sea level (L)
In this section, we examine the analysis results of monthly averages for
the data from 2010 to 2019 regarding the parameters that change
significantly over time, that is, the height of the groundwater level
above sea level (L), the impact of the existing status of seawater
intrusion (I), and saturated thickness of the aquifer (T).
L was most vulnerable to SWI when it was less than 1 m, and the score
was ten in this case. When L exceeds 2.6 m, the score is the lowest at
one, and the theoretical weight is four. This index changes monthly. The
existing calculation method is used to observe L and compare the range
values. In this study, the score was determined by comparing the
measured groundwater level from the sea level, with the monthly sea
level height measured at the sea water level station. The sea level
observation data of the sea water level station were interpolated by
setting the coastline as the domain. L was determined by the minimum
distance to the interpolated coastline. The histogram in Figure 6 shows
the distribution of scores represented by the decile method. The y-axis
on the left is the cumulative ratio of data L according to the rating,
and the secondary y-axis represents the average of the data expressed in
a straight line. As a consequence of calculating the monthly groundwater
height relative to sea level, April exhibited the highest average of the
study area at 1.25, and August had the lowest average at 1.21. Owing to
the nature of Korea’s climate, the groundwater level of the unconfined
aquifer rises during the rainy season or intensive rainfall from June to
August. The sea level also rose the most in August; however, the extent
of increase in the sea level was smaller than that of the groundwater
level, and it was the lowest. The variations in groundwater level
differed depending on the area, albeit the sea level was the lowest in
January and February. In April, when the rainfall is small, and the use
of groundwater is concentrated, the groundwater level drops
significantly, making it a period that is most vulnerable to SWI.
3.2.2 Impact of the existing status of seawater intrusion (I)
For the current SWI situation, we used electrical conductivity, which is
easily obtained from National groundwater monitoring network in KOREA.
Existing studies used the molar ratio of Cl-; however,
observations were irregular, and the requirement for the length of
continuous data could not be satisfied. Chang et al. (2019) used the
electrical conductivity data obtained from the seawater intrusion
monitoring network as input data for the I parameter in the GALDIT
assessment. The present study also used electrical conductivity data
obtained from the National Groundwater Monitoring Network, whose quality
control for observation data is excellent. Based on the rating of Chang
et al. (2019), it was most vulnerable when the electrical conductivity
exceeded 3,000 μs/m, and a score of two was attributed if it was below
200 μs/m. This index can indicate monthly changes, and the theoretical
weight of the current SWI situation is one. The colors in the histogram
in Figure 9 express the rating divided by the decile method. The y-axis
on the left is the cumulative ratio of data I according to the rating,
and the secondary y-axis on the right represents the average of the data
expressed by a straight line. When the thematic map in Figure 7 is
examined with the naked eye, one observes that the change in parameter
values is not large in most areas. However, the area in the north of the
study area shows higher values in March, which remained high until April
and dropped from May. The values increased again in September, slightly
decreased in October, and were maintained at 3–4 in November. The
straight line in the graph in Figure 8 represents the average parameter
value for each month. Upon comparison of the monthly average parameters,
the most vulnerable month is September, with the average of all areas at
3.65, whereas the least vulnerable month was February, with the average
at 3.32.
3.2.3 Saturated thickness of the aquifer (T)
T was determined using drill logs (www.gims.go.kr). The bottom point of
the unconfined aquifer was estimated by analyzing the sample and stratum
composition from the drill logs. T was calculated from the height of the
groundwater level observed in real-time. High-quality data over a
continuous period of 10 years were used among the groundwater level data
observed in real-time. The saturated thickness of the aquifer is most
vulnerable to SWI when it exceeds 10 m, and it is satisfactory when it
is less than 2 m, and the score is one. The theoretical weight of T was
two.
The colors in the histogram in Figure 10 express the ratings divided by
the decile method. The y-axis represents the cumulative ratio of the
area according to the rating, the secondary y-axis represents the scores
of the index, and the values indicated by the broken line indicate the
total average of the study area. The analysis reveals the minimum score
at four, which indicates all T were above 4 m. August was the most
vulnerable month at 9.12, and April had the lowest score of 8.92. T is
an index related to the real-time groundwater level. It is believed that
T increases in July and August when significant recharge occurs due to
rainfall, and it decreases from February to April when there are fewer
rainfall events, and the pumping of groundwater increases.