-In Semarang City, groundwater has been exploited as a natural resource since 1841. The groundwater exploited in deep wells is concentrated in confined aquifers. The previous hydrogeological model was developed in one unit of aquifer and refined then by using several hydrostratigraphical units following a regional hydrogeological map without any further analysis. At present, there is a lack of precise hydrogeological model which integrates geological and hydrogeological data, in particular for multiple aquifers in Semarang. Thus, the aim of this paper is to develop a hydrogeological model for the multiple aquifers in Semarang using an integrated data approach. Groundwater samples in the confined aquifers have been analyzed to define the water type and its lateral distribution. Two hydrogeological cross sections were then created based on several borelog data to define a hydrostratigraphical unit (HSU). The HSU result indicates the hydrogeological model of Semarang consists of two aquifers, three aquitards, and one aquiclude. Aquifer 1 is unconfined, while Aquifer 2 is confined. Aquifer 2 is classified into three groups (2a, 2b, and 2c) based on analyses of major ion content and hydrostratigraphical cross sections.

The high demand of clean water supply and groundwater usages in Yogyakarta City, Indonesia has caused the lowering of groundwater table of about 3 m in average since 1984. This condition is caused by the increase number of groundwater pumping in this city relate to the rising of the urban population, economics condition, education and cultural activities, change of land use and increase number of hotels on this famous tourism city in Indonesia. Therefore, it became necessary to assess the optimum yield from the groundwater system beneath this city which can still preserve the recent level of shallow groundwater. This is important because most of the people in this city depends their daily water supply from shallow dug well. In order to assess the safe yield, a groundwater modeling is conducted. The data used on this modeling was taken from previous research and also primary data collected during this research such as; aquifer geometry, aquifer characteristics, shallow groundwater level, water usage/pumping wells discharge, river discharge and surface water level, and climate data. Prediction of the impact of groundwater pumping was made by increasing the water usage/pumping wells discharge on several scenarios. The result of this modeling shows that the optimum yield of the aquifer beneath Yogyakarta City area is about 125,000 m3/day. Keywords: Optimum yield, groundwater usage, groundwater table, pumping, urban population, shallow dug well.

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Salatiga Groundwater Basin (SGB) is located in Java Island, Indonesia. Administratively, it covers Semarang Regency, Salatiga City, and Boyolali Regency. Industry and community use groundwater to fulfil their daily need. Increasing number of deep wells that extract groundwater will cause some environmental problems, such as lowering groundwater level and subsidence at SGB. Thus, there is a need to assess the adverse impacts of groundwater abstraction. Risk assessment of groundwater vulnerability due to abstraction is the goal of this study. The research method was taking account of weighting of geological parameters, such as response characteristics of the aquifers, characteristics of aquifer storage, aquifer thickness, piezometric depth, and distance from the shoreline to conduct the groundwater vulnerability mapping. It was then overlaid on a map of regional spatial plan to develop the map of vulnerability risk due to abstraction. The groundwater vulnerability due to abstraction is categorized in the medium level. After being overlaid by the land use map, the risk of groundwater vulnerability due to abstraction is classified into three kinds, which are low, medium, and high. Regions with a low class can be neglected. Areas with moderate risk require an exhaustive review of technical requirements of the use of borewell. Areas with high-risk need a comprehensive consideration to use artesian wells by monitoring wells with drill licenses, tightening the permit to add new production wells, and conducting periodic review of groundwater monitoring.