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CiliwungRiver water quality and its vicinity have been continuously degraded with the increasing population. The objective of this research is to understand the association between river and groundwater, and the water quality profile. Field measurements ware taken from 65 stations from Bogor to Jakarta. Water level, temperature, pH, and TDS were measured to build the water flow map and hydrochemical profile. Small-scale geoelectrical survey was conducted at five locations to capture the aquifer's geometry. We identified three types of stream relationships between river and groundwater: effluent from Bogor to Katulampa (Segment 1), perched at the University of Indonesia (UI) area (Segment 2), and influent from UI to Muara (Segment 3), with low gradient from \0.1 to 0.3. The temperature profile of river and groundwater shows similar pattern as well as TDS profile. All similarities support close connection of river and groundwater. The increasing TDS towards downstream shows increasing enrichment and contamination. The erratic pattern of pH indicates chemical instability due to high contamination. This study highlights the benefit of understanding the hydrodynamic relationship between river water and groundwater. Such interaction triggers water quality exchange between both water bodies. Therefore, a similar study should also be done on other riverbanks in Indonesia to protect water quality.

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Gravity data analysis and interpretation are based, among others, on their spatial variation represented by horizontal and vertical gradients. The gradient or derivative of a gravity field can be calculated either in the spatial domain or the wave-number domain. Historically, the second vertical derivative (SVD) of gravity data can be used to delineate the boundaries of anomalous sources. This paper addresses inappropriate use of the SVD of gravity data, with reference to current practices in Indonesia. The SVD's relative magnitude along a profile is widely used to define whether a density contrast and its dipping orientation correspond to a normal or reverse fault, which may be geologically incorrect. Furthermore, the SVD is calculated by approximation using the horizontal derivative, which may be erroneous especially with poorly distributed data and anomalous 3D sources. We exemplify our analysis with synthetic data and propose a more appropriate spectral-based analysis using field data.

Most earthquake studies focus mainly on offshore subduction zones that often produce high-magnitude earthquakes. However, onshore active fault earthquakes also common to cause significant devastation, due to their proximity to human activity and their relatively shallow depth. Most of active faults in Java show a west-east (W-E) trending direction. However, the devastating Opak Fault ruptured in a NE-SW trend. Recent studies indicate that some shallow seismic clusters in Java cannot be associated with any known active fault traces. These seismic activities reported occurring in southern Garut, Cipamingkis (Sukabumi), Halimun-Salak Mountain (Bogor), and Grindulu in Pacitan. These seismic clusters show similar direction to the Opak Fault that ruptured an Mw6.4 earthquake in 2006 with thousands of fatalities. Here, we describe a brief review of the NE-SW trending active fault in the southern Garut area. Earthquake is a natural phenomenon with a repetitive mechanism. Therefore, we must be aware of the future hazard that has been demonstrated by recent seismic activities in this area.

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