Sains Malaysiana 39(4)(2010): 525–532

A Numerical Modelling of Seawater Intrusion into an Oceanic Island Aquifer, Sipadan Island, Malaysia

(Pemodelan Berangka Kemasukan Air Laut ke dalam Akuifer Pulau Lautan, Pulau Sipadan, Malaysia)

Mohd Harun Abdullah & Sarva Mangala Praveena*

Water Research Unit, School of Science and Technology Universiti Malaysia

Sabah Locked Bag 2073 88999 Kota Kinabalu, Sabah, Malaysia

Ahmad Zaharin Aris

Department of Environmental Sciences

Faculty of Environmental Studies, Universiti Putra Malaysia

43400 UPM Serdang, Selangor D.E., Malaysia

Received: 8 July 2009 / Accepted: 23 March 2010

ABSTRACT

The primary source of water on many small islands is the fragile freshwater lens that floats on saline water in its shallow aquifer. The management of such a limited groundwater resource on these islands is seriously constrained by the occurrence of seawater intrusion. Sipadan Island, the renowned and only oceanic island in Malaysia, had experienced in the over-extraction of its groundwater for more than ten years to cater for freshwater demand associated with tourism activities. This paper discusses the output of modelling of seawater intrusion into the island’s aquifer using SEAWAT-2000. The findings indicated that the island’s coastal aquifer has been encroached by seawater. The infiltration of isochlor (chloride concentration) of 2.5 and 45% of seawater and freshwater mixing ratios has moved 63.4 m and 12.7 m inland from the coastline, respectively. The upconing event at the pumping well, as simulated by the three-dimensional model, showed that 14.5% of seawater-freshwater mixing ratio took place below the bottom of each well. Intensive and unregulated exploitation of groundwater from such an unconfined aquifer of the island by pumping wells contributed to the upconing. In order to protect the fresh groundwater resources in the study area from seawater intrusion, adjustment of groundwater pumping rate is needed. This study showed that the model is useful in demonstrating the mechanism and movement of freshwater-seawater interface in the island, and thus provide a powerful management tool for such an aquifer.

Keywords: Groundwater; numerical modelling, oceanic island; seawater intrusion

ABSTRAK

Sumber air primer bagi pulau-pulau kecil adalah lensa air tawar bawah tanah yang terapung di atas air masin di dalam akuifer. Pengurusan sumber air bawah tanah yang terhad di pulau-pulau ini dibebani oleh penyerapan air laut. Pulau Sipadan merupakan pulau yang terkenal dan satu-satunya pulau lautan di Malaysia, mengalami pengekstrakan air bawah tanah secara berlebihan lebih dari sepuluh tahun bagi menampung permintaan air bawah tanah yang dikaitkan dengan aktiviti pelancongan. Makalah ini membincangkan output model kemasukan air laut ke dalam akuifer pulau menggunakan SEAWAT-2000. Peresapan isoklor 2.5 dan 45% (kepekatan klorida) nisbah percampuran air laut dan air tawar, masing-masing adalah 63.4 m dan 12.7 m dari laut ke daratan. Pembentukan upconing di setiap telaga pengepaman seperti yang disimulasi oleh model menunjukkan nisbah percampuran air laut dan air tawar adalah sebanyak 14.5% yang berlaku di bahagian bawah telaga pengepaman. Pengunaan air bawah tanah secara intensif dan tidak terkawal dari akuifer pulau ini menyebabkan pembentukan upconing. Usaha untuk melindungi sumber air bawah tanah di kawasan kajian daripada kemasukan air laut memerlukan pengubahsuaian kadar pengepaman. Kajian ini menunjukkan model tersebut adalah berguna dalam menerangkan mekanisme dan pergerakan antara muka air tawar dan air laut pulau serta sebagai alat pengurusan yang berguna bagi jenis akuifer seperti ini.

Kata kunci: Air bawah tanah; kemasukan air laut; model berangka; pulau di lautan

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*Corresponding author; email: smpraveena@gmail.com