Sains Malaysiana 35(1):11-18 (2006)

Soil and Groundwater Investigation in Kuala Selangor Coastal

Plain using Geoelectrical and Geochemical Surveys

(Penyiasatan Tanih dan Air Bawah Tanah di Dataran Pantai Kuala Selangor

menggunakan Survei Geoelektrik dan Geokimia)

Umar Hamzah, Abdul Rahim Samsudin

& Edna Pills Malim

Pusat Pengajian Sains Sekitaran dan Sumber Alam

Fakulti Sains dan Teknologi

Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor D.E. Malaysia

ABSTRACT

An integrated geoelectrical and geochemical techniques were used to investigate and delineate different types of groundwater in Kuala Selangor-Batang Berjuntai alluvial aquifer. Previous hydrogeological borehole investigations showed that this aquifer contained several types of groundwater in relation to its salinity. The high salinity of the groundwater in some areas was believed to be due to either saltwater intrusion from the nearby sea or river infiltration during tide season. Both vertical electrical sounding (VES) and 2-D electrical resistivity imaging methods were employed to study and map the subsurface variation of resistivity in the area. For each measurement, a total spread length of 200 m was obtained with vertical depth penetration of about 60 to 100 m. Chemical analysis data of the groundwater samples taken from both shallow and deep boreholes were used in the interpretation of the groundwater interfaces. A total of 45 VES stations were succesfully established along three parallel roads with direction almost perpendicular to the coastal line. The distance between stations vary from 1 - 2 km with a maximum length of about 60 km surveyed line. Results of the vertical electrical soundings and resistivity imaging as well as the chemistry of the groundwater samples showed that groundwater in the study area can be grouped into three types i.e. fresh water (resistivity >100 Ωm.), brackish water (5-100 Ωm.) and salt water (resisitvity <5 Ωm.). The subsurface resistivity sections derived from the VES study suggest that the boundary between fresh and salt water was located between 4 to 8 km from the present coastal line. This result appears to agree well with the groundwater interfaces obtained from the chloride concentration map. Values greater than 300 ppm was considered to be representing zone of brackish water whilst values greater than 800 ppm represents saline water zones.

Keyword: Groundwater aquifer, geoelectrical survey, geochemical analysis, water salinity, boreholes

ABSTRAK

Teknik-teknik geoelektrik dan geokimia telah digunakan secara integrasi untuk menyiasat dan menentukan jenis air bawah tanah yang terdapat di dalam akuifer aluvium di kawasan Kuala Selangor-Batang Berjuntai. Mengikut penyiasatan terdahulu yang dibuat berdasarkan kajian hidrogelogi lubang gerudi, air bawah tanah yang terdapat dalam akuifer di kawasan kajian mempunyai kemasinan yang berbeza-beza. Kemasinan air yang tinggi di beberapa tempat dipercayai terhasil daripada intrusi air laut atau infiltrasi air sungai semasa air pasang. Pemetaan perubahan keberintangan di kawasan kajian telah dilakukan dengan menggunakan teknik duga-dalam elektrik dan pengimejan keberintangan elektrik 2-D. Panjang rebakan bagi setiap pengukuran ialah 200 m yang akan menghasilkan kedalaman penyiasatan secara menegak disekitar 60 hingga 100 m. Data analisis kimia sampel air bawah tanah yang diambil dari lubang gerudi cetek dan dalam telah digunakan untuk mentafsirkan sempadan jenis air bawah tanah. Pengukuran duga-dalam elektrik telah dilakukan pada 45 stesen yang terletak di sepanjang tiga jalan raya yang selari tapi pada arah serenjang dengan garis pantai. Jarak di antara setiap stesen pembacaan ialah 1-2 km dan jumlah panjang keseluruhan garis survei ialah 60 km. Hasil kajian duga-dalam menegak dan pengimejan keberintangan serta sampel kimia air bawah tanah di kawasan kajian boleh dibahagikan kepada tiga kumpulan iaitu air segar (keberintangan > 100 Ωm.), air payau (keberintangan 5­100 Ωm) dan air masin (keberintangan <5 Ωm.). Keratan rentas keberintangan subpermukaan yang dibina daripada kajian duga-dalam elektrik mencadangkan bahawa sempadan di antara air masin dan segar terletak di antara 4-8 km daripada garis pantai. Hasil ini hampir menyamai sempadan antara muka air bawah tanah yang diperolehi dari peta kepekatan klorida. Kepekatan melebihi 300 bahagian persejuta dianggap mewakili zon air payau manakala kepekatan melebihi 800 bahagian persejuta mewakili zon air masin.

Kata kunci: Akuifer air bawah tanah, survei geoelektrik, analisis geokimia, kemasinan air dan lubang gerudi

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