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.