RESEARCH ARTICLE Predictive numerical modeling of groundwater drawdown impacts in Jakarta Iwan Nursyirwan, Muhammad Bisri, Lily Montarcih and Ery Suhartanto Civil Engineering Department, Faculty of Engineering, Brawijaya University, Indonesia Abstract An excessive groundwater usage is happening in Jakarta, Indonesia, due to the population growth and industrial development so that it experiences a signifcant groundwater drawdown which could enhance the risk of seawater intrusion and land subsidence. Existing conditions in 2018 show that seawater intrusion occurred at the Western and Central coastal area and land subsidence happen in the Northern and Central part. Tis research, a numerical simulation, is conducted by modeling such causality during the critical period, the next 20 years. Te result shows that for every groundwater drawdown of 10 m/year, it will cause intrusion 0.7 km/year in the Western and Central and 1.1 km/year in the Eastern area afer 2028. Te 10 m/ year groundwater drawdown also results in land subsidence of 5.7 cm/year in the Northern and 2.5 cm/year in Central Jakarta. Tis result is useful as an input for groundwater management policies and to prevent the environmental impacts occurred at other large coastal cities. Indonesian Journal of Geography Vol. 51 No. 3, December 2019 (231-241) DOI: http://dx.doi.org/10.22146/ijg.47438 © 2019 by the authors. Tis article is an open access article distributed under the terms and conditions of the Creative Commons Attribution(CC BY NC) licensehttps://creativecommons.org/licenses/by-nc/4.0 Received: 2019-07-28 Accepted: 2019-09-30 Keywords: Groundwater drawdown; Seawater intrusion; Land subsidence; Jakarta. Correspondent Email: nursyirwan.iwan@gmail.com 1. Introduction A signifcant groundwater level drawdown in large cities in Indonesia (Wirakusumah & Danaryanto, 2004; Taniguchi, Burnett, & Ness, 2008; Hosono, et al., 2011; Kagabu, et al., 2011, 2012, Taufq, et al., 2018a), has caused many subsurface issues: land subsidence (Taufq, 2010; Sarah, et al., 2018; Bandono, 1983; DGTL & Dinas Pertambangan DKI Jakarta, 1996; Ramdhan & Hutasoit, 2007), seawater intrusion (Hehanusa, 1979; Djijono, 2002; Setiawan, et al., 2017, Onodera, et al., 2009 ), groundwater pollution (Hosono, et al., 2009; Taufq, et al., 2019), the increase of groundwater temperature (Lubis, et al., 2013), groundwater mixing (Taufq, et al., 2018b), groundwater age rejuvenation (Kagabu, et al., 2012; Taufq, et al., 2018a) and others. Jakarta, the capital city of Indonesia, is one of the largest cities in Southeast Asia. A drastic groundwater drawdown is happening as the consequences of the water resource needs due to a high rate of population and industrial growths (Wirakusumah & Danaryanto, 2004; Delinom, 2008). Te groundwater drawdown in Jakarta area has afected the subsurface environment in negative ways, such as seawater intrusion and land subsidence. Te seawater intrusion under unconfned aquifers in Jakarta area had been identifed from the presence of brackish water which has a Cl/HCO 3 > 1 ratio (Setiawan, et al., 2017) and groundwater levels below sea level (Hehanusa, 1979). Soenarto & Widjaya (1985) also found that the intrusion afected not only unconfned aquifers but also confned aquifers. Djaeni, et al. (1986) also reported that the presence of brackish water from a depth of 0 to 100 m has spread up to 6 km from the coastline. On the other hand, land subsidence in Jakarta area was already known since 1926. At the beginning of the 1980s ,the land subsidence had been caused also by groundwater drawdown, which was analyzed by using leveling survey method, extensometer measurement, and GPS survey (Abidin, et al., 2007) as well as with analytical method and modeling (Marylin & Hutasoit, 2018). Geodetic observation conducted by Dinas Pertambangan DKI Jakarta with LPM-ITB (1999) showed that land subsidence reached 200 cm in the 1982-1999 period. Te rate of land subsidence in Jakarta in the period of 1997 - 2005 was 1-10 cm/ year and the greatest number was 15-20 cm/year in the northern and central Jakarta. Visually, land subsidence in Jakarta causes damage at the foundation of buildings and food inundation becomes widespread in several locations (Abidin et al., 2014). 2. e Methods Te analysis method in this study is a numerical groundwater fow model using sofware MODFLOW v2010 (A modular three-dimensional (3D) fnite- diference groundwater fow model and structured rectilinear grid operating system). Tis tool was applied to simulate the groundwater drawdown condition scenario and its impact on seawater intrusion and land subsidence. Subsurface hydrostratigraphic data and hydraulic parameters (Table 1) used in numerical simulations were obtained from previous studies (Kagabu, et al., 2012; Dirjen SDA dan PT Ganesha Piramida, 2014) and processed result from the Jakarta Groundwater Conservation Agency report, Geology Agency.