Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Phetchaburi Province, Thailand

Seawater intrusion in coastal regions is an imperative ecological issues, and results in an undesirable consequence on future groundwater resources. To assess and mitigate seawater intrusion, the affected aquifers need to be characterized. By integrating geophysical investigation and multivariate statistical analysis of hydrochemical data, seawater intrusion into coastal aquifers at Cha-Am District in Phetchaburi Province (Thailand), as a model area that is adversely affected by this problem, was evaluated. The vertical electrical sounding (VES) survey study was conducted at 80 locations, and then 47 VES locations were used to create four pseudo cross-section lines in a west-east direction. The geophysical results were described together with the hydrochemical analysis of 57 groundwater samples revealed that seawater intrusion occurred in the Quartemary colluvial sediment aquifer at an average depth of 50-60 m. A resistivity value of < 5 Gm represented areas that were highly impacted by seawater intrusion, while a range between 5 and 10 Omega m represented moderately contaminated areas. The hydrochemical characteristics, divided the groundwater into three groups according to the level of impact of seawater intrusion: (i) Ca2+-Na+-HCO3+ and (ii) Ca2+-HCO3--Cl-; (slightly impacted) and (iii) Na+-Cl- (highly impacted). 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. Multivariate analysis was in agreement with the HFED. Furthermore, the northern part of the study area faced seawater intrusion with a relatively higher impact than other areas.