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.