IDENTIFICATION OF SEAWATER INTRUSION USING GEOPHYSICAL METHODS IN THE MANDALIKA, LOMBOK, INDONESIA

Abstract

Seawater intrusion is an event of infiltrating seawater into aquifers caused by constituent rocks that have high porosity, such as alluvial rocks. The Mandalika Lombok Indonesia is located on an alluvial plain that is very vulnerable to the occurrence of seawater intrusion. This study aims to identify areas that have been potentially affected by seawater in the Mandalika and surrounding areas. The method used is a geophysical method that combines geoelectric resistivity and gravity methods. For the data processing, we used several computer programs, namely Res2Dinv and Oasis Montaj for geoelectric data and Oasis Montaj and Grav3D for gravity data. The results of data processing are resistivity values of (0. 2 – 1022.9) Ωm, residual anomaly values of (-2.7 – 2. 3) mGal, and 3D model density values of (1 – 3) gr/cm³. Based on a cross-section of 2D resistivity, the aquifer is at a depth of (2 – 12) meters. There are three main layers consisting of the upper layer (Alluvium) with resistivity (18 – <100) Ωm and density value of 1.2 gr /cm³, the middle layer (clay sand as a shallow aquifer layer) with resistivity <18 Ωm and density value of 1.7 gr /cm³, then the lower layer (basement) clay with resistivity (100 – 3000) Ωm and density value of 2.0 gr/cm³. The potential intrusion areas are located in the western part, which includes the coast of Kuta Beach and Seger Beach, and the eastern part includes the coastal area of Tanjung Aan, the Gerupuk hamlet, and the Mertak village.