Indonesian Journal on Geoscience Vol. 7 No. 2 August 2020: 215-224 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306 Risk Assessment of Groundwater Abstraction Vulnerability Using Spatial Analysis: Case Study at Salatiga Groundwater Basin, Indonesia Thomas Triadi Putranto, Tri Winarno, and Axel Prima Agita Susanta Department of Geological Engineering, Diponegoro University Jln. Prof. H. Soedharto,S.H., Tembalang - Semarang, Indonesia 50275 Corresponding author: [email protected] Manuscript received: April, 4, 2019; revised: September, 19, 2019; approved: January, 23, 2020; available online: July, 16, 2020 Abstract - Salatiga Groundwater Basin (SGB) is located in Java Island, Indonesia. Administratively, it covers Se- marang 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 moder- ate 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. Keywords: risk assessment, groundwater, vulnerability, abstraction, aquifer, Salatiga Groundwater Basin © IJOG - 2020. All right reserved How to cite this article: Putranto, T.T., Winarno, T., and Susanta, A.P.A., 2020. Risk Assessment of Groundwater Abstraction Vulner- ability Using Spatial Analysis: Case Study at Salatiga Groundwater Basin, Indonesia. Indo nesian Journal on Geoscience, 7 (2), p.215-224. DOI: 10.17014/ijog.7.2.215-224 Introduction (Witkowski et al., 2007). The risk assessment of groundwater vulnerability due to abstraction is The term of groundwater vulnerability was developed by aggregating some major vulner- introduced by a French hydrogeologist, Mar- ability parameters to one vulnerability index, gat, in 1968, and IJOGthe first vulnerability map involving various steps of selection, transform- was constructed in France by Albined in 1970. ing some parameters into raster data, rating, Since the early 1980s, more complex methods and weighting. In this research, groundwater of groundwater vulnerability assessment have vulnerability is mainly formulated as an intrin- been developed, and a considerable number of sic property of the aquifer system that relies on vulnerability maps of various scales and objec- the sensitivity of the system to human and/or tives have been produced throughout the world natural impact. Accredited by: RISTEKDIKTI (1st grade), valid August 2016 - April 2020 Indexed by: SCOPUS (Q3) 215 Indonesian Journal on Geoscience, Vol. 7 No. 2 August 2020: 215-224 Salatiga groundwater basin/SGB is located in locally spread in the south-west of SGB. Payung Java Island, Indonesia. It covers Semarang Re- Formation is of Pleistocene age. gency, Salatiga City, and Boyolali Regency (Figure Merbabu Volcanic Rock Formation (Qme) 1) with a total number of industry, both small and contains volcanic breccia, lava, tuff, and laharic big,was around 2.981 in 2011 and increased up to breccia. They entirely spread at the SGB area. 3.030 in 2013. In this basin, there were 249.081 Merbabu volcanic rocks are of Holocene age. inhabitants in 2011, and became 253.297 people in Sumbing Lava Formation (Qls) consists of lava 2013 (Badan Pusat Statistik Kabupaten Boyolali, flow, and the dome consists ofaugite-hornblende 2004; Badan Pusat Statistik Kabupaten Semarang, andesite. They spread at the south-west of SGB, 2004; Badan Pusat Statistik Kota Salatiga, 2014). i.e. belowMount Merbabu. This formation is Both industry and community use groundwater of the Holocene age. Basalt (Qba) is composed to fulfil their daily need. They withdraw ground- of volcanic rocks which are grey, dense, sco- water through deep wells in confined aquifers. The riaceous, and porphyritic. The alluvium (Qa) is number of deep wells that extract groundwater formed by river and lake deposits which consist will cause some environmental problems, such of pebble, cobble, sand, and silt with the thick- as lowering groundwater level, subsidence, and ness ranging of 1 - 3 m. seawater intrusion if the groundwater extraction is Based on the results of the study of groundwa- over its potency. Thus, there is a need to assess the ter potency by Dinas Pertambangan dan Energi adverse impacts of groundwater extraction at SGB. Provinsi Jawa Tengah (2005) as shown in Figure Based on the regional geological map of 3, SGB has moderately and low groundwater po- Magelang-Semarang and Salatiga (Sukardi and tency in both unconfined and confined aquifers, Budhitrisna, 1992; Thanden et al., 1996) SGB, respectively. The aquifers consist of young vol- from old to young, consists of some formationsas canic products of Mount Merbabu, and ground- shown in Figure 2. Payung Formation (Qp) con- water flows through fissures and interstices. The sists of lahar, claystone, breccia, and tuff. They medium potency has groundwater discharge from N 0 95 km Indonesia Java Sea N SGB Java Island 0 550 km Indian Ocean a 43000 440000 450000 460000 b N Grobogan Regency 0 4.5 9 km Bawen 9200000 Semarang Regency 9200000 Tuntang Bringin Bancak Rawapening Lake Sidorejo Pabelan Wonosegoro Sidomukti Tingkir 9190000 9190000 Salatiga City Argomulyo Suruh IJOGGetasan Tengaran Susukan Magelang Regency 9180000 Ampel Legend 9180000 Regency/City Boundary Boyolali Regency Sub-District Boundary River Elevation (masl) High: 3.061 Low: 91.5 43000 440000 450000 460000 c Figure 1. Locality map of studied area of SGB. 216 Risk Assessment of Groundwater Abstraction Vulnerability Using Spatial Analysis: Case Study at Salatiga Groundwater Basin, Indonesia (T. Putranto et al.) 43000 440000 450000 460000 N Grobogan Regency Bawen Qba 0 4.5 9 km 9200000 Semarang Regency 9200000 Tuntang Bringin Qp Bancak Rawapening Lake Sidorejo Pabelan Qa Wonosegoro Sidomukti Tingkir 9190000 9190000 Salatiga City Qmc Suruh Argomulyo Getasan Tengaran Susukan Qls 9180000 Legend 9180000 Ampel Regency/City Boundary Sub-District Boundary Magelang Regency Boyolali Regency Qa Qba Qls Qmc Qp Lineament Thrust Fault (Indicated) 43000 440000 450000 460000 Figure 2. Regional geological map of Salatiga Groundwater Basin. 440000 45000 460000 Bergas Pringapus Grobogan Regency Kedungjati Bringin SB-31 Bawen 9200000 SB-26 Bancak 9200000 SB-20 SB-16 SB-25 Ambarawa Tuntang SB-15 Wonosegoro Sidorejo SB-9 Pabelan Banyubiru SB-8 SB-11 9190000 N Sidomukti SB-1 Tingkir 9190000 Salatiga City SB-34 SB-35 SB-18 SB-32 Suruh SB-29 SB-30 0 5 km Argomulyo SB-7 SB-3 SB-33 Karanggede SB-5 SB-6 Legend Getasan SB-28 Lake SB-27 Deep Well SB-22 Susukan Pakis Tengaran Non Groundwater Basin 9180000 SB-21 Magelang Regency Groundwater Potency Low (<2 L/d) Ampel Boyolali Regency Sawangan Kaliwungu Medium (2-10 L/d) Musuk Selo Cepogo Recharge - Discharge Boundary IJOG440000 45000 Figure 3. Locality map of deep wells at the SGB. 2 to 10 l/sec, located in the centre to the north of Materials and Method SGB. While the low potency is distributed in the south of SGB. These areas have groundwater There were thirty-five deep wells located discharge of up to 2 l/sec. at SGB (Figure 3). Table 1 provides the total 217 Indonesian Journal on Geoscience, Vol. 7 No. 2 August 2020: 215-224 depth of wells, the aquifer thickness, and the vulnerability due to abstraction can be classified piezometric levels measuredon May 2015. (Table 3). Four wells did not identify piezometric level Figure 4 shows the cross-section of the due to the maintenance of the piezometer on the subsurface setting based on the borehole and wells. To assess the groundwater vulnerability, georesistivity measurement. aprevious study applied spatial analysis to There were two cross-sections, i.e. the cross- construct some thematic maps using Geographic section in A - A’ from the south to the north Information System/GIS (Ghayoumianet al., and B - B’ depicted from the west to the east. 2006; Baker et al., 2007; Jhaet al., 2007; Al- The lithology consists of topsoil, clay, and tuff, Quadah and Abu-Jaber, 2009; Chowdary et al. in some parts it comprises andesite, breccia, 2009; Cheniniet al., 2010). The groundwater sandstone, and claystone. vulnerability to abstraction in the groundwater To assess the risk of groundwater vul- basin involves the class and score of relevant nerability due to abstraction, the result of an overlying parameters (Putra and Indrawan, 2014) intrinsic groundwater vulnerability map was in raster GIS format (Table 2). overlaid on the land use map (Figure 5). The The highest score represents the highest land use map in the studied area is classed based vulnerability of the parameters due to ab- on its risk to groundwater abstraction (Table 4). straction.All parameters were transformed The highest score of land use represents the into raster images,then calculated by using highest groundwater used. The result of the risk araster calculator in the spatial analysis tool in of groundwater abstraction was classified in Table ArcGIS.