Dr. Darharta Dahrin, Dadi Abdurrahman ST., Mt.

Geophysical Engineering Program, Bandung Institute of Technology

*Correspondence: darharta@gmail.com or dahrin@gf.itb.ac.id

Abstract

Cisarua has a morphology of hills with the area of ridges and valleys. Generally, in this area, the groundwater flow is quite large and the variation of the groundwater mass is highly dynamic which is highly dependent on the variation of rainfall and topographic slope. Hydrological studies based on gravity and geoelectric methods are expected to provide a framework for obtaining a groundwater volume distribution model. In order to determine the variation of the groundwater volume in a given time, we can use gravity and geoelectric data. The results show the groundwater level of approximately 5-10 m, with a thickness of between 10-20 m. Estimated maximum groundwater volume can be estimated based on unstressed aquifer volume to approximately 10.8 x 10⁶ m³. With average depth of groundwater level is 7.5 m with the same area and porosity then the volume of water that can be stored at that time is 5.4 x10⁶ m³. Keywords : Hydrogeology, gravity, geoelctric, groundwater

Introduction

The research about groundwater in the northern part of Bandung Basin which has a morphology of hills with the area of ridges and valleys. Generally, in this area, the groundwater flow is quite large and the variation of the groundwater mass is highly dynamic which is highly dependent on the variation of rainfall and topographic slope. Hydrological studies based on gravity and geoelectric methods are expected to provide a framework for obtaining a groundwater volume distribution model.

In order to determine the variation of the groundwater volume in a given time, we can use gravity and geoelectric data measured in time variations. Time-lapse microgravity data is strongly associated with or indispensable for monitoring program of variation/changes in groundwater mass volume such as in the Bandung Basin.

The decrease in groundwater volume can result in the loss of the citizens among those already occurring in Bandung are increasingly difficult to obtain the volume of groundwater for household needs and the need for water for vegetable plantations, especially during the dry season.

Methodology

Gravity and geoelectric measurements were conducted from August to October 2017, which coincided with the beginning of the rainy season. Geoelectric data is used to estimate the depth of the groundwater layer based on the interpretation of resistivity model. The gravity data were used to estimate the groundwater distribution based on the density contrast model. Rainfall data is used to estimate the volume of water flowing on the surface and infiltrated into the soil.

The results already obtained

The research area is Tugumukti Village, Cisarua District, West Bandung Regency, is the slopes of Mount Tangkuban Parahu in Northwestern Bandung as in Figure 1 (a). Several modeling results show an approximate depth of groundwater, in Figure can be estimated the groundwater level of approximately 5-10 m, with a thickness of between 10-20 m.

Estimated maximum groundwater volume can be estimated based on unstressed aquifer volume (on a soil layer) based on area (2.4 million m²) multiplied by aquifer thickness (15 m) multiplied by porosity (30%). Estimated maximum groundwater volume to approximately 10.8 x 10⁶ m³. However, a functioning aquifer can withstand the maximum amount of water at rainfall is the volume of an aqueous unsaturated upper aquifer. At the time of measurement at the beginning of the rainy season (Fig. 1 (c)) as shown in the cross-sectional section of the geoelectric type the zone is a zone above ground water level (5-10 m). If the average depth of groundwater level is 7.5 m with the same area and porosity then the volume of water that can be stored at that time is 5.4 x10⁶ m³. The amount of recharge groundwater after rain

Conclusion

Based on the geoelectric resistivity model we can estimate the aquifer thickness above the volcanic grain and into the groundwater table, reinforced by gravity data. Based on the area of the ridge and the porosity of the land, we can estimate the storage capacity (recharge) of groundwater when it occurs in the zone above the groundwater level (water-unsaturated zone). The amount of groundwater that is stored (recharge) can be observed using gravity time.

Figure 1. (a) Topographic map of the research area, (b) sub-Basin 1 (Bandung Basin), sub-Basin 10 (red box), (c) Comparative discharge on sub-basin 10 and sub-basin 1 in Citarum Upstream.

Figure 2. (a) Gravity response (black-line observation and blue-line calculation), (b) resistivity model based on geoelectric measurement and (c) mass-type distribution model based on gravity method.