Gravity survey is one of the passive geophysical techniques commonly used to delineate geological formations, especially in determining basement rock and the overlying deposit. Geologically, the study area is made up of thick quaternary alluvium deposited on top of the older basement rock. The Muda River basin constitutes, approximately, of more than 300 m of thick quaternary alluvium overlying the unknown basement rock type. Previous studies, including drilling and geo-electrical resistivity surveys, were conducted in the area but none of them managed to conclusively determine the basement rock type and depth precisely. Hence, a regional gravity survey was conducted to determine the thickness of the quaternary sediments prior to assessing the sustainability of the Muda River basin. Gravity readings were made at 347 gravity stations spaced at 3–5 km intervals using Scintrex CG-3 covering an area and a perimeter of 9000 km2 and 730 km, respectively. The gravity data were then conventionally reduced for drift, free air, latitude, Bouguer, and terrain corrections. These data were then consequently analyzed to generate Bouguer, regional and total horizontal derivative (THD) anomaly maps for qualitative and quantitative interpretations. The Bouguer gravity anomaly map shows low gravity values in the north-eastern part of the study area interpreted as representing the Main Range granitic body, while relatively higher gravity values observed in the south-western part are interpreted as representing sedimentary rocks of Semanggol and Mahang formations. Patterns observed in the THD anomaly and Euler deconvolution maps closely resembled the presence of structural features such as fault lineaments dominantly trending along NW-SE and NE-SW like the trends of topographic lineaments in the study area. Based on power spectral analysis of the gravity data, the average depth of shallow body, representing alluvium, and deep body, representing underlying rock formations, are 0.5 km and 1.2 km, respectively. The thickness of Quaternary sediment and the depth of sedimentary formation can be more precisely estimated by other geophysical techniques such as the seismic reflection survey.
During the periods of strong winter monsoon, the occurrence of extensive floods in the low-lying areas mostly in the east coast part of the Peninsula Malaysia continually results in huge destruction to public and private utilities, damage to farmlands, and in addition, leads to loss of lives with outbreak of epidemic which prompted evacuations of hundreds of thousands from their homes and communities. This paper presents research results on the impacts of subsurface water on the east coast Peninsula Malaysian floods, therefore complementing the existing data on the flooding events in this area. In our finding, we present the vulnerability of the aquifer units in the flood-affected communities to floods during intense precipitations and then proffer suggestions to prevent near future occurrence. The partial coverage of the alteration zone that is completely nonexistent in this area should have provided the needed essential storage and transmissivity if present in the east coast Peninsula Malaysia environments. Most of the aquifer units in this part are unconfined and are opened to surface water. We applied electrical resistivity tomography methods of geophysical prospecting and borehole lithologic logs to study the subsurface structures prone to flooding in addition to the information on the rainfall data. However, the GOD vulnerability Index rating method was modified and improved upon with the additions of fluctuations of borehole water heights FBWH and the river bank heights RBH together with the topography parameters to produce the GOD-FBRT Index that better refine the method by considering the literal movement of fluids within the subsurface. The configuration of the Kelantan River basin land system as delineated by this study makes the entire east coast areas vulnerable to flooding with the degree of vulnerability as computed in the study area that ranged between 0.1 (very low) and 1.0 (very high).
<p>This study aims to clarify the groundwater recharge and flow processes in a basin located in a tropical humid area with a complex geological setting. We conducted intensive sampling campaigns for river water and groundwater during three different time periods in the relatively dry season (September 2019, January-March 2020, and August-September 2022). The 40 river water samples and 25 groundwater samples were taken from the upper-stream area with an altitude of 527 m to the downstream area with an altitude of 10 m. The oxygen-18 (&#948;<sup>18</sup>O) and deuterium stable isotopic compositions and inorganic constituent concentrations were determined on all water samples. We also used the monthly stable isotopic compositions in rainwater observed at a location with an altitude of 26 m, 15 km apart from the Klang River basin, by the Global Network of Isotopes in Precipitation database (GNIP), IAEA. These chemical compositions were used as tracers to investigate the groundwater recharge and flow system. The deep groundwater shows a lower &#948;<sup>18</sup>O than the volume-weighted mean of rainwater and higher ion concentrations, whereas the shallow groundwater shows a higher &#948;<sup>18</sup>O and lower ion concentrations. This suggests that the deep groundwater with low &#948;<sup>18</sup>O seems to be recharged in the mountainous area with an altitude ranging from 70 to 1421 m. Additionally, we conducted a principal component analysis and cluster analysis using inorganic constituent concentrations and stable isotopic compositions, showing that the deep and shallow groundwater samples are classified into two groups. This shows that the deep groundwater in the downstream area is recharged mainly in the mountainous areas with the highest altitude of 1421 m, and the shallow groundwater is recharged partly in the hilly areas with the highest altitude of 250 m. We believe our study serves new findings on the groundwater flow system in mega-cities of tropical climate regions.</p>
Forecast of groundwater potential zones is essential, especially in areas where surface water is not sufficient during the dry season, such as the Tampin District, Negeri Sembilan, Malaysia. In the literature, the lineament parameter is often combined with a groundwater assessment model such as the DRASTIC model. However, most of these practices do not follow the procedure of assigning weight to the parameter as used in the model, so many researchers assign different ranges of weights to the lineament parameter. Therefore, this study focuses on how to systematically incorporate a lineament density map into the existing DRASTIC model based on the specific range of weight specified by the model; thus, the inclusion of more geological input will improve the model performance. DRASTIC is an abbreviation for the parameters used in the model: Depth to water; Recharge; Aquifer media; Soil media; Topography; Impact to vadose zone; and Conductivity (hydraulic). The addition of the lineament density map has successfully improved the performance of the DRASTIC model from 50 to 80% based on the distribution of 30 producing wells in the Tampin District, where the geology and lineament density play major roles in determining the potential groundwater area.
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