Assessment of seawater intrusion using multivariate statistical, hydrochemical and geophysical techniques in coastal aquifer, Cha-am district, Thailand

Abstract. Seawater intrusion in coastal areas is one of the important environmental problems, causing negative impact on groundwater resources in the future. To assess and mitigate the seawater intrusion, the affected aquifers need to be characterized. By integrating geophysical investigation and multivariate statistical analysis of the hydrochemical data, seawater intrusion into coastal aquifers in this area could be evaluated. The study conducted 80 locations of the vertical electrical sounding (VES) survey; then selected 47 VES to create four pseudo cross-section lines in a west-east direction, running perpendicular to the coast in Cha-Am district, Thailand, which is negatively affected by this problem. The geophysical results were described together with the hydrochemical analysis of 57 groundwater samples. The results revealed that seawater intrusion occurred in the Qcl aquifer with an average depth of 50–60 meters and presented more obviously near the coastal line. The resistivity value of <5 Ωm represented highly contaminated areas impacted by seawater intrusion while the range between 5–10 Ωm represented moderately contaminated areas. According to the hydrochemical characteristics, groundwater can be divided into three groups according to the level of impact of seawater intrusion: Ca-Na-HCO₃ and Ca-HCO₃-Cl (slightly impacted), Ca-Na-HCO₃-Cl (moderately impacted), and Na-Cl (highly impacted). The area had a low resistivity value, corresponding to the high value of electrical conductivity (EC), and the hydrochemical facies was generally Na-Cl. The hydrochemical facies evolution diagram (HFED) revealed that most of the samples fell close to the mixing line, demonstrating mixing between seawater and fresh water and that some samples fell in the intrusion phase. According to multivariate statistical analysis, the finding was in agreement with the HFED. There are three main processes: seawater intrusion, natural groundwater recharge, and finally hydro-geochemical interaction. Finally, the findings in this study demonstrated that the levels of seawater intrusion could be classified into three zones depending on the degree of seawater intrusion. Furthermore, the northern part of the study area faced seawater intrusion with a relatively higher impact than other areas, and seawater laterally intruded about eight kilometers inland.