Sains Malaysiana 38(6)(2009): 851–856

Kemasinan Air Perigi dan Subpermukaan Lembangan Tuba Langkawi dengan Analisis Hidrokimia dan Survei Keberintangan Elektrik Menegak

(Well Water and Subsurface Salinity of Tuba Basin Langkawi by Hydrochemical

Analysis and Vertical Electrical Resistivity Survey)

Umar Hamzah*, Abdul Rahim Samsudin, Abdul Ghani Rafek & Khairul Azlan Razak

Program Geologi, Pusat Pengajian Sains Sekitaran & Sumber Alam

Fakulti Sains & Teknologi, Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor D.E., Malaysia

Received: 31 July 2008 / Accepted: 15 January 2009

ABSTRAK

Lembangan Tuba adalah satu endapan aluvium yang terletak di antara perbukitan granit di bahagian barat Pulau Tuba dan tanah tinggi batuan sedimen Formasi Setul di bahagian barat. Lembangan ini menganjur timur laut ke arah barat daya sepanjang hampir 3 km dan mempunyai kelebaran sejauh 2 km. Satu survei geofizik permukaan menggunakan teknik geoelektrik telah dilakukan untuk mengkaji struktur subpermukaan dan mengesan kehadiran sebarang akuifer serta jenis nya dalam lembangan ini. Di samping itu, kepekatan unsur-unsur dalam air tanah juga dianalisis untuk melihat kebarangkalian berlakunya intrusi air laut ke dalam akuifer pantai. Sebanyak 22 stesen telah dipilih secara rawak di seluruh kawasan kajian untuk dilakukan survei geoelektrik duga-dalam. Sementara itu air bawah tanah telah diambil dari 11 telaga untuk analisis kimia di makmal. Hasil analisis mendapati semua sampel air perigi ini adalah berjenis air tawar. Profil keberintangan pada arah timur laut - barat daya yang menganjur dari Teluk Berembang hingga Teluk Bujur mempunyai nilai keberintangan yang berkisar di antara 4 Ωm sehingga infiniti. Lapisan teratas yang mempunyai ketebalan sekitar 1-3 meter dan berkeberintangan 4 -12 Ωm ditafsirkan sebagai lempung manakala lapisan di bawahnya yang berketebalan 10 hingga 50 meter dengan nilai keberintangan 2 - 480 Ωm ditafsirkan sebagai bahan berpasir dengan air tawar atau pasir berair payau. Keberintangan berjulat ribuan ohm.m hingga infiniti dianggap mewakili batuan dasar granit dan batu kapur. Kedalaman maksimum yang dicapai oleh survei keberintangan ialah sekitar 70 m.

Kata kunci: Analisis kimia; akuifer, air masin; keberintangan elektrik; survei geoelektrik

ABSTRACT

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 Ωm representing 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.

Keywords: Aquifer; chemical analysis; geoelectrical survey; resistivity; saline water

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*Corresponding author; email: umar@ukm.my