Kemasinan Air Perigi dan Subpermukaan Lembangan Tuba Langkawi denganAnalisis Hidrokimia dan Survei Keberintangan Elektrik Menegak(Well Water and Subsurface Salinity of Tuba Basin Langkawi by HydrochemicalAnalysis and Vertical Electrical Resistivity Survey)

Tuba basin is an alluvial deposit located between granitic hill in the western part of Tuba Island and the Setul formation sedimentary rocks in the eastern site of the island. This basin stretched along 3 km in the NE-SW direction with an estimated width of about 2 km. A geophysical survey using geoelectrical technique was carried out to figure out the subsurface structure, to detect the presence of underground aquifers and to investigate any saltwater intrusion into these aquifers in the basin. Concentrations of several elements in the well water were also analyzed to investigate any occurrence of salt water intrusion into the coastal aquifers. For this purpose, the vertical electrical sounding surveys were carried out at 22 randomly distributed stations in the study area. Water samples were also taken from 11 wells for hydrochemical analysis in the laboratory. Our results showed that all water samples were of fresh water type. Electrical resistivity profile constructed from stations located in NE-SW direction from Teluk Berembang to Telok Bujur shows a wide range of resistivities ranging from 4 Ωm to infinity. The top layer with a thickness of 1-3 m and resistivity values of 4 – 12 Ωm is interpreted as clay zone. This layer is overlying a much thicker layer of 10-50 m with resistivity values of 2 - 280 Ωmrepresenting sandy material that may contain fresh water or sand with brackish water. Layers with resistivity values from thousands ohm.m to infinity are interpreted as either granite or limestone bedrock. Maximum thickness observed in this resistivity survey is approximately 70 m.