Preliminary Assessment of Seawater Intrusion in Phuket Island, Thailand

Abstract

Seawater intrusion can gradually cause a severe problem by contaminating freshwater aquifers and causing a lack of fresh water. Coastal areas of Phuket Island are under risk of seawater intrusion due to many factors, e.g., sea level rise, over-use of groundwater, and so on. The main purpose of this study was to assess the seawater intrusion problem, which refers to the encroachment of seawater into underground freshwater aquifers, in the coastal area of Phuket. To map the seawater intrusion in the coastal areas of Phuket, an analysis of existing groundwater chemistry data (2003-2016), obtained from groundwater observation wells and network monitoring wells, was carried out. Chloride (Cl-) and Total Dissolved Solids (TDS) concentration were the two main indicators of seawater intrusion. The average Cl- and TDS concentration are highly correlated in a cross-plot in 8 districts (Chalong, Rawai, Karon, Kamala, Mai Khao, Choeng Thale, Sisunthon and Thep Krasitti) of Phuket, and the risk areas with seawater intrusion problem were identified by these levels exceeding threshold numbers (500 and 1,000 mg/L for Cl concentration and TDS, respectively). A map of Phuket seawater intrusion was created by GIS techniques, by overlaying the based maps for Cl concentration, TDS concentration, groundwater extraction, and transmissivity. The map indicates that the coastal areas in Mai-Khao sub-district has serious seawater intrusion, while the coastal areas of Kamala, Rawai, and Chalong sub-districts have only moderate seawater intrusion. To map the subsurface cross-section in the area seawater intrusion in those sub-districts, ten Electrical Resistivity Imaging (ERI) was successfully generated by using a Dipole-Dipole array and Werner Array. Then those data files were input in EarthImager2D and 3D software to do inversion vii using smooth model in inversion method. In addition, four stations of Vertical Electrical Sounding (VES) using Schlumberger array were done near to the groundwater producing wells and monitoring wells in Kamala sub-district in the aim of verifying the correlation between groundwater salinity indicators- Cl- and TDS- getting from groundwater producing and monitoring wells and apparent resistivity values obtaining from VES. A relationship between the resistivity and the seawater was demonstrated by laboratory testing to confirm that ERI is a feasible and efficient tool for delineating seawater intrusions, and an empirical model between resistivity and TDS and Cl concentrations in the study area was established. Seawater intrusion in the western part of Phuket, Kamala sub-district, can be seen to intrude in a freshwater aquifer with the expansion approximately 400 m inland and in the depth around 4 m below land surface. Similar to Kamala, Chalong sub-district, seawater intrude in a freshwater aquifer with the expansion approximately 150 m inland and in the depth around 3 m below land surface. In terms of Rawai, bedrock zone has been found in the shallow depth around 3 m above mean sea level and the saline zone has been found near the bedrock zone in shallow depth as well. Regarding Mai Khao, the area which is located around 500 m from the beach is completely covered by seawater intrusion. An empirical relationship between the resistivity and the seawater concentration (TDS and Cl) of the study area was established to identify fresh, brackish, and saline zones. Finally, the correlation of Cl- and TDS with an apparent resistivity of soil, soil sample in laboratory testing and fence diagram of subsurface layers were presented to confirm that ERI is a feasible and an efficient tool for quantitative and qualitative studies of seawater intrusion.