water Article Development of Seawater Intrusion Vulnerability Assessment for Averaged Seasonality of Using Modified GALDIT Method Il Hwan Kim , Il-Moon Chung and Sun Woo Chang *   Citation: Kim, I.H.; Chung, I.-M.; Chang, S.W. Development of Seawater Intrusion Vulnerability Assessment for Averaged Seasonality of Using Modified GALDIT Method. Water 2021, 13, 1820. https:// doi.org/10.3390/w13131820 Academic Editors: Evangelos Tziritis and Andreas Panagopoulos Received: 21 May 2021 Accepted: 24 June 2021 Published: 30 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Land, Water and Environmental Research, Korea Institute of Civil Engineering and Building Technology, Goyang 10223, Korea; kimilhwan@kict.re.kr (I.H.K.); imchung@kict.re.kr (I.-M.C.) * Correspondence: chang@kict.re.kr; Tel.: +82-31-910-0278 Abstract: Climate change and anthropogenic activities are necessitating accurate diagnoses of seawa- ter intrusion (SWI) to ensure the sustainable utilization of groundwater resources in coastal areas. Here, vulnerability to SWI was assessed by classifying the existing GALDIT into static parameters (groundwater occurrence (G), aquifer hydraulic conductivity (A), and distance from shore (D)) and dynamic parameters (height to groundwater-level above sea-level (L), impact of existing status of seawater intrusion (I), and aquifer thickness (T)). When assessing the vulnerability of SWI based on observational data (2010–2019), 10-year-averaged data of each month is used for GALDIT dy- namic parameter for representing the seasonal characteristics of local water cycles. In addition, the parameter L is indicated by the data observed at the sea-level station adjacent to the groundwater level station. The existing GALDIT method has a range of scores that can be divided into quartiles to express the observed values. To sensitively reflect monthly changes in values, the range of scores is divided into deciles. The calculated GALDIT index showed that the most vulnerable month is September, due to relatively low groundwater level. The proposed method can be used to apply countermeasures to vulnerable coastal areas and build water resources management plan considering vulnerable seasons. Keywords: GALDIT; monthly vulnerability; seawater intrusion (SWI); vulnerability assessment; effective weight; densely populated area 1. Introduction Coastal areas host large populations of people owing to their prosperity. Since the 20th century, 21 megalopolises in the coastal areas have grown rapidly to achieve a population of more than eight million, and more than a third of the global population resides within 100 km of the shore [1]. With the increasing area affected by seawater intrusion (SWI) in coastal areas, the available amount of water resources is decreasing due to the aquifer salinization. Furthermore, changes caused by the salinization of coastal aquifers, such as limitations in the cultivation environment of agricultural and marine products, are damaging economic activities [2,3]. The land-use changes due to industrial development increase surface runoff and decrease recharge of the groundwater system. Furthermore, climate change increases rainfall intensity due to change in the rainfall pattern. As the number of days without rain rises, the amount of water resources discharged to the surface increases, while that of recharge to the aquifer decreases. Consequently, groundwater resources gradually de- crease [4–8]. The continuous rise in sea levels accelerates the increase in the SWI range with respect to freshwater body, seawater–freshwater interface and mixing zone [9]. According to the analysis method of Ghyben–Herzberg, the effect of a 1 m rise in the sea level on the freshwater aquifer corresponds to 40 m of freshwater thickness [10–12]. Sherif and Singh [13] claimed that when the sea level rises by 0.5 m, the effect of SWI reaches up to 9 km from the shore. The imbalance between the inflow and outflow from the aquifer can Water 2021, 13, 1820. https://doi.org/10.3390/w13131820 https://www.mdpi.com/journal/water