4. CONCLUSION
An accurate diagnosis of SWI vulnerability is required for the
sustainable utilization of groundwater resources in coastal areas.
GALDIT is a SWI vulnerability assessment method that shows
representative values using a statistical test of observed data. This
study differentiated the analysis method by classifying the six
parameters of the existing GALDIT into static and dynamic parameters.
For the static parameters—G (groundwater occurrence), A (aquifer
hydraulic conductivity), and D (distance from the shore)—similar to
the existing method, the annual average or average of the observed
values was used. For the dynamic parameters—L (height to groundwater
level above sea level), I (impact of existing status of seawater
intrusion), and T (thickness of the aquifer)—monthly averages were
used to reflect the observed values that change every month. In the
existing score range, the most vulnerable and least vulnerable values
were divided by quartiles for the assessment of SWI vulnerability. The
existing quartile methods cannot respond sensitively to relatively small
changes in SWI vulnerability because the criteria for dividing scores
are wide. The present study divided the values assigned to each
parameter by the decile method to sensitively reflect the degree of
vulnerability that changes every month in areas where the seasonal
variation of the dynamic index is evident, as in South Korea.
As a result of calculating the GALDIT index by applying the existing
weight, September was selected as the most vulnerable month at a value
of 3.03. In September, the ratio of areas with a score of five or higher
was the highest at 8.87%, and it was the most vulnerable month for I
and the third most vulnerable month for T. Thus, September was the month
most vulnerable to the final GALDIT parameters. In the range of the
existing GALDIT index, 3.03 is a value that indicates no vulnerability.
Consequently, the vulnerability was low because it also increased a
large part of the inland area that was not affected by the impact of the
existing status of seawater intrusion.
In the monthly seawater intrusion assessment, the effective weight of T
was calculated to be high. Because of the nature in South Korea, the
saturated thickness of the aquifer is somewhat high, and most areas are
highly vulnerable. Because of T, which is mostly thick, it is difficult
to indicate the differences in vulnerability even if time variability is
considered. L was determined based on the sea level observed by sea
water level stations near the groundwater level measuring site, instead
of the existing method of indicating the groundwater level based on the
sea level.
The sensitivity analysis results showed that T was the most important
index, as the T score was calculated to be higher than the other
parameters, indicating that it is the most sensitive index. Thus, the
calculated effective weight was likewise high. When performing the
monthly GALDIT assessment in the future, the modification of rating
criteria and weight of each parameter will likewise be necessary. We
attempted to modify the existing GALDIT assessment method within a range
that does not exceed the criteria, but with differences in the
parameters that act sensitively depending on the characteristics of the
study area. The sensitive parameters can have a significant effect on
the SWI or may be overestimated in some cases.
To the best of our knowledge, this study attempted the first monthly
analysis of the GALDIT parameters based on the decile method. If
intensive response measures are applied to vulnerable areas using the
methodology of this study and operational plans are established
considering the vulnerable period, the SWI damage could be effectively
reduced, and sustainable utilization of groundwater in the coastal areas
could be realized.
It may be necessary to improve the theoretical weight and expand the
variable range considering static and dynamic parameters representing
extreme situations due to climate change in the future. Furthermore, a
follow-up study is required to improve the equations for calculating
GALDIT parameters according to site characteristics.