Riset Geologi dan Pertambangan Volume 33 No 2 (2023)

The northern coastal area of Semarang City has significant problems with a decrease in groundwater quality due to seawater intrusion. Increasing groundwater extraction and subsidence due to the city growth worsen the condition. It is necessary to collect information on the contaminated location to develop a strategy to decrease the spreading of seawater intrusion. This study aims to identify the presence of seawater intrusion zones in groundwater and estimate the spread distance of seawater intrusion to the land. We applied the electrical resistivity tomography method to obtain the image of the subsurface. A geo-electric survey with a dipole-dipole multi-electrode configuration was completed along the West Flood Canal. The subsurface model result indicated that the identified seawater intrusion zone is associated with a low resistivity value of less than 3 Ohm.m. The zone is about 0 – 70 meters deep near the coast and is thinning to about 2600 meters to the south. The result confirms that seawater has penetrated long distances to the land.

Landslide disasters frequently occur in Tasikmalaya Regency, with Salawu Village being at a medium to high ground movement vulnerability zone. Steep slopes and quite high rainfall dominate this area. This study aims to identify subsurface rock types and model the slip plane by applying the dipole-dipole configuration of the 2D geoelectrical method. The result can be used as a mitigation measure to minimize losses if a landslide disaster occurs. We measured the resistivity in five lines with spacing between electrodes of 5 m with a total length of 275 m. The data processing results indicate three classes of resistivity values: a resistivity value under 25 Ωm is interpreted as sandy clay, a value in the range of 20-105 Ωm as clayey/ tuffaceous sand, and a resistivity of more than 80 Ωm as volcanic breccia. The slip area is sandy clay with a slipping mass of clayey/ tuffaceous sand and volcanic breccia. Two of the surveyed lines have safety factor values less than 1.00 (the slope is approximately 40°, and the slip plane angle is between 33° and 35°), which means that the area has an unstable slope. The small safety factors should be a serious concern because landslides can occur at any time. Moreover, breccia rock is one of the landslide materials that would have a very destructive impact. To confirm the results of this study, in situ geomechanical tests are needed in this area.

Landslides occurred in the Cihanjuang area, Cimanggung, Sumedang Regency, West Java on January, 9^th 2021 with 40 fatalities and several buildings severely damaged. This area is in the form of hills with a fairly steep slope and housing is built on it. Geologically, the research area is composed of Undecomposed Young Volcanoes (Qyu) in the form of tuffaceous sand, lapilli, breccias, lava, and agglomerates. The research aims to determine the potential and volume of landslides. The research method used is conducting geotechnical testing and carrying out topographical mapping. Geotechnical tests carried out were water content, specific gravity, sieve analysis, hydrometer, and direct shear strength. The geotechnical test results showed that water content: 16.37% – 31.26%, cohesion: 59.2 kPa – 112.7 kPa, and internal friction angle: 6.30° – 30.50°, percentage of gravel and sand 11.10 % – 34.90 %, percentage of silt and clay 65.10 % – 88.90 %. According to USCS, this soil sample includes the MH-CH-OH classification Topographical mapping was carried out covering an area of 7.006 hectares. The internal friction angle is relatively steep, and the percentage of gravel and sand is quite high, so water infiltration becomes easy, so the Cimanggung area has a high potential for landslides. From the topographic mapping, the volume of the landslide was around 161.981 m³. To maintain slope stability and avoid landslides, apart from planting strong-rooted plants in areas with high elevations, they also manage the drainage by making waterways on the northwest and southeast sides of the steep slopes to the main road.

Salaman District is an area with an area of 68.87 km², which is included in the Magelang Regency area. The southwest part of this area features hilly topography, forming the slope of the Menoreh Hills. In contrast, the central and eastern areas have predominantly flat topography, with breccia rocks distributed widely. The district has a population of 74,429 people, and the majority earn their livelihoods as farmers. The primary commodities include cassava, as well as fruit crops like mango and durian. Other commodities originating from this area are teak wood, bamboo, mahogany, sengon and waru. From January 2020 to April 2021, rainfall in Magelang Regency ranged from 20 mm to 531 mm. Based on data obtained from the BPPD Magelang Regency website, Salaman District is one of the sub-districts that had the highest intensity of landslides in that period, with a total of 67 landslides. Given the significant number of landslide disasters, it is deemed crucial to conduct mapping and develop a landslide susceptibility map for the research area. The research utilizes direct data, including lithological information, geological structure, and geomorphology, as well as secondary data comprising landslide occurrence, topography, DEM, and land cover data in the research area. The method used in this research is the frequency ratio (FR) method. It is constructed based on the relationship between the location of the landslide incident and the factors that control the occurrence of landslides. The FR method is included in the bivariate statistical method, which is used to determine the relationship between two variables. Based on the geomorphological analysis, it was found that several landforms developed in the research area, namely River Body (F1), Alluvial Plain (F2), Structural Hills (S1), and Residual Hill (D1). The stratigraphy of the research area consists of six rock units, listed from oldest to youngest as follows: Kaligesing Pyroclastic Breccia unit, Kaligesing Andesite Lava unit, Kaligesing Epiclastic Breccia unit, Old Sumbing Laharic Breccia unit, Young Sumbing Tuf-lapilli unit, and Alluvial Deposits. The geological structures that develop in the research area are right strike-slip faults and left strike-slip faults, which have a west-east line. Based on the results of the analysis using the Frequency Ratio method, which uses parameters in the form of slope, lithology, structure distance, river distance, land use, and landslide occurrence points, the zoning in the research area is divided into 4, namely the Very Low Landslide Susceptibility Zone with a Landslide Susceptibility value. Index (LSI) 22.48 – 40.75; Low Landslide Susceptibility Zone with LSI value of 40.75 – 46.72; Medium Landslide Susceptibility Zone with LSI value 46.72 – 52.87; and High Landslide Susceptibility Zone with an LSI value of 52.87 – 67.27. After validating the landslide susceptibility map using the Area Under Curve (AUC) method, the results obtained were 70.95% and included in the good category, so the map was suitable for use.

Sediments are known to accumulate pollutants from terrestrial and coastal waters, and can be used as an indicator to monitor metal pollution in the biosphere and the effects of anthropogenic events in the environment. Eight samples of reef surface sediments were collected from the north and east coasts of Panjang Island.  Panjang Island, located to the north of Banten Bay, was chosen for this study because of its proximity to many industrial areas. Nevertheless, the island supports a natural ecosystem, including coral reefs, seagrass, and mangroves. The samples were analyzed for the grainsize, component analysis, and Pb concentrationusing AAS flame method. The result indicated Pb concentration in the grain-supported surface sediment in Panjang Island is varied. The detected Pb concentrations were 0-28.68 mg/kg in dry weight. The study indicated that the different concentration of Pb value is mainly controlled by the different of sediment grainsize. The highest Pb accumulation occurs in the areas with very fine sediment grainsize, which have better adsorption capabilities for heavy metals. This study also suggests that the medium sand fraction may accumulate different Pb values. The result indicated that, according to the classification of ANZECC/ARMCANZ, all sediment samples in the study area is in the low risk of toxicant level. Additionally, the SQG-Q (Sediment Quality Guidelines) quotient analysis indicates that Pb concentration in the study area posed a low-moderate impact to adverse biological effect.