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.