APPLICATION OF GEOPHYSICAL AND HYDROCHEMICAL TECHNIQUES FOR SEAWATER INTRUSION ASSESSMENT IN COASTAL AQUIFER: A CASE STUDY AT AMPHOE CHA-AM, CHANGWAT PHETCHABURI

Abstract

Seawater intrusion in coastal area is one of the important environmental problems that negatively affect on groundwater resources in the future. In this research, the study area is located in Amphoe Cha-Am, Changwat Phetchaburi, Thailand, and appears to be affected by this problem. The purposes of this research are to integrate the geophysical investigation with hydrogeological and hydrochemical data for further evaluating seawater intrusion into coastal aquifers in this area. The study conducted 80 of VES survey and then selected 47 VES to create 4 pseudo cross-section lines from West-East, running perpendicular to the coastal line, by using IPI2WIN software. The geophysical results were described together with hydrochemical analysis of 57 groundwater samples. The finding found that seawater intrusion occurs in Qcl aquifer with an average depth of 50-60 meters and presents more obviously near the coastal line. Resistivity value of less than 5 Ωm represents that the aquifer getting highly contaminated by seawater intrusion as well as of the range between 5-10 Ωm represents that the aquifer getting partially contaminated by seawater intrusion. According to the piper diagram, groundwater can be divided into 5 hydrochemical facies as follows: Ca-Na-HCO3 and Ca-HCO3-Cl (the facies getting low impact from seawater intrusion), Ca-Na-HCO3-Cl (the facies getting moderate impact from seawater intrusion), Na-Cl (the facies getting a high impact from seawater intrusion). Furthermore, the area has low resistivity value, it corresponds to the high value of EC, and the facies is usually Na-Cl. Finally, the finding in this study found that the northern part of the study area has been faced seawater intrusion relative higher impact than other areas and seawater laterally intrude about of 8 kilometers inland.