Groundwater investigation in the Batang Integrated Industrial Park, Central Java, Indonesia

Batang Integrated Industrial Park (BIIP), Central Java, Indonesia, needs the support of water sources, one of which is groundwater. In addition, industrial waste also can contaminate the groundwater. Therefore, assessing groundwater potential and quality is vital for an environmental baseline to manage sustainable groundwater resources. This research aims to identify the subsurface geology, especially the aquifer system and physicochemicals of groundwater, unprecedented in BIIP. The aquifer system determined the research method through geological surface mapping and geoelectrical surveys. Physicochemical properties, including temperature, pH, total dissolved solids (TDS), and electrical conductivity (EC), were directly measured in wells and springs in the industrial areas. The result shows that the BIIP is dominated by sand, silt, and clay, with sand as the primary aquifer located in-depth 1.35 to 47 meters, concentrated mainly in the western region and towards the east, decreasing as an unconfined aquifer. Groundwater flows from the south toward the Java Sea in the north. Groundwater has a temperature of around 27.3–30.9°C, pH 5.26–7.47, TDS 49–568 mg/L, and EC 100 – 613 μs/cm. It indicates that the quality and quantity of groundwater in the BIIP are quite suitable for use as raw water.


Introduction
Batang Integrated Industrial Park is an integrated industrial center with various supporting infrastructure facilities.Located in Ketanggan, Gringsing District, Batang Regency, Central Java, with an area of ±4.300 hectares [1].Water demand in industrial estates is a requirement for production activities, industrial workers, and supporting industrial needs [2].The water demand of the estates can be estimated, without analyzing each independent company, by calculating the size of the estate and the upper/lower water demand estimate for an estate in Indonesia.The current industrial water demand is estimated between 18-67 (m 3 /ha/day) [3].Due to the demand for industrial water, it is possible to use groundwater as a source.
Groundwater management is necessary to ensure groundwater sustainability [4].Therefore, it is required to understand the groundwater potential and quality as essential information to develop groundwater resources in the industrial park to guarantee their sustainability.Groundwater is typically held within a geological formation called an aquifer, which becomes saturated with water and can store and transmit water [5].This research aims to identify the subsurface geological, especially the aquifer system and physicochemicals of groundwater.The geological surface mapping and geoelectrical survey were conducted to determine an aquifer system.The aquifer system is interpreted from the presence of low resistivity values [6], [7], [8].
A previous researcher with a similar topic investigated groundwater in Tegal District, Central Java, Indonesia, using the geoelectrical method and slug test [7].The results led to the discovery the Tegal district in the lowlands is made up of clay, silt, sand, breccia, and lava deposits.The research area lacked groundwater resources and has potential for seawater intrusion.Another research is groundwater mapping in Palopo City, South Sulawesi, Indonesia, using the geoelectrical Schlumberger configuration method [8].The result showed that sand as a primary aquifer has groundwater potential, encompassing 11.5% of the research area.The groundwater potential assessment was conducted in the Faculty of Engineering of the University of Jember, Indonesia, using the geoelectrical Wenner configuration method [9].The result led to groundwater potential at the Faculty of Engineering of the University of Jember, classified as moderate productive, and distribution is located at a depth of 25.3-39.7 m, which indicated that the soil layer at this location contains plain alluvium deposits and coarse to medium grain gravel and sand, with the addition of clay.
Based on the success of previous researchers, identifying the subsurface geological, especially the aquifer system and physicochemicals of groundwater in Batang Integrated Industrial Park, has not been done before, will be very beneficial for the future development of the industrial area, considering that the Batang Integrated Industrial Park is still in the phase-1construction process until 2024.It has two more phases that have not started yet.This research can be utilized to estimate groundwater exploration locations for industrial raw water supply and prevent groundwater contamination from industrial waste because the aquifer type, aquifer thickness, and groundwater flow direction are already known.

Materials and Methods
The steps performed in the Groundwater Investigation in The Batang Integrated Industrial Park are divine in two as follows:

Identify The Aquifer System
The aquifer system was identified based on the geophysical survey and well-log data.Geoelectric is a useful geophysical exploration method that provides valuable subsurface information [10] and estimates the potential of groundwater level.Geoelectric survey measures resistivity by injecting current into the ground through two current electrodes and measuring the resulting voltage difference at two potential electrodes [11].The geoelectric method can measure the difference in resistivity of rock layers.Groundwater-bearing layers have low resistivity, whereas rock layers have high resistivity with little water content or dry [6], [7].The geoelectric method is considered successful in discovering aquifer layers at certain depths and estimates the potential of groundwater level [10].The electrode patterns used in geoelectric surveying are the Wenner (Figure 1) [10], [12].

Identify The Physicochemical Properties and Groundwater Table
Maps of the water table for an unconfined aquifer and the potentiometric surface of a confined aquifer are basic tools of hydrogeologic interpretation.The data used to construct water tables and potentiometric surface maps are water level elevations measured in the wells [15].This research gathers data on 13 dug wells owned by residents, 24 rivers, and two springs around the Integrated Industrial Park (Figure 4) during March and April, then performs physicochemical examinations of the water wells and interviews the owner regarding year-round water availability.
The physicochemical examination of the water wells includes color, odor, pH, temperature, electrical conductivity (EC), and total dissolved solids (TDS) using a water test kit from Hanna [16].The hydraulic head at various locations was obtained by subtracting the depth static groundwater level (SWL) from the elevation (E), which is derived from the Demnas Maps to the mean sea level (Figure 3) [17].The wells data were plotted into a GIS map to obtain the groundwater flow pattern.

Aquifer System
Based on the regional geological map, Batang Integrated Industrial Park is divided into two areas.In the North part, near the Java Sea, the area consists of the rock formation of Alluvium (Qa) and Alluvium Fan (Qf), which consists of pebbles, sands, silt, and clays from rivers and beaches, mainly volcanic debris that dissected.These rock formations formed during the Holocene age, forming coastal plains and river edge plains composed of clay and sand with a thickness of 50 meters or more.The sand deposits are mainly formed by delta sedimentation, reaching a thickness of 80 meters or more.Meanwhile, along the river edges, riverbed bars are created with a thickness of 1-3 meters.The boulders consist of andesite, limestone, and a small amount of sandstone [18].
The South part, near the residential area, consists of the rock formation of-Pleistocene Sediments (QTs), which is an interbedded volcanic breccia, coarse tuff, conglomerate, fine tuff, and tuffaceous claystone.Component of breccia is andesite, dacite and basalt.Coarse tuff contains hornblende and silicified wood.In some places, fine tuff with intercalations of calcareous sandstone containing mollusca and coral; claystone having small foraminifers and lenses of limestone.Locally, it is from lahar deposits.In this unit, crossbedding and boudinage are found.The unit's thickness is between 900 and 1000 m thick, including the Damar Fm (QTd) [18], as shown in Figure 5.The geoelectric survey was conducted at the same points as the drilling log presented in Figure 8 and Figure 9.The correlation between the geological maps, geoelectric data, and drilling log results can be a reference for identifying the subsurface aquifer system [20], [21].The geophysical inversion of the measured apparent resistivity is the basis for the quantitative interpretation of VES curves [21].Batang Integrated Industrial Park consists of 3 to 4 layers based on the resistivity value.In Figures 8, 9, and 10, Vertical Electric Sounding data show that the research area has a true resistivity value of 0.0981-2004 Ωm, and sand (aquifer) has a true resistivity value of 20-100 Ωm.The results are summarized in Table 1, which shows the relationship between resistivity values, depth, and lithology type.

Groundwater Physicochemical Properties and Groundwater Table
The groundwater level/table is generally not horizontal but tends to follow the topography of the surface above it [15], [22], [23].Groundwater level data in Batang Integrated Industrial Park obtained from observation of dug wells has an average depth of -1 to -3 meters from the ground surface elevation.The entire data of the water level investigation are presented in Table 2.The groundwater has a temperature of 27.3 o C -30.9 o C, pH 5.26 -7.47, TDS 49 mg/L -568 mg/L, and EC 100 μs/cm -613 μs/cm.Therefore, groundwater is recommended for raw water because the TDS and EC were quite low (TDS < 1.000 and EC < 700) [24], [25].The groundwater flow pattern is obtained based on the groundwater table and then plotted on a GIS map.The groundwater flow pattern is always perpendicular to the contour of the groundwater level and flows from higher to lower contour [26], [27].Figure 11 is the groundwater flow pattern map showing the direction of groundwater movement from the south towards the Java Sea in the north.

Conclusions
Batang Integrated Industrial Park is dominated by clay, sand, and breccia, with sand as the primary aquifer located 1.35 m to 47 m below the surface.The sand layer distributes in the western area, and towards the East, the thickness decreases.The groundwater level depth is around 0.56 m to 13.56 from the surface flowing to the Java Sea in the north.The groundwater has a temperature of around 27.3 o C -30.9 o C, pH 5.26 -7.47, TDS 49 mg/L -568 mg/L, and EC 100 μs/cm -613 μs/cm.The highest value of TDS and EC is shown near the Java Sea in the northern part.This information can be used to determine the placement of upstream and downstream monitoring wells along the groundwater flow for groundwater contamination observation in the industrial area.According to the groundwater physicochemical properties, of groundwater is from one type of aquifer and is suitable for use as raw water.Suppose Batang Integrated Industrial Park wants to use the groundwater to support industrial activities.In that case, it is recommended to build groundwater production wells in the western part due to the thickness of the aquifer.

Figure 4 .
Figure 4. Distribution of data location in the research area

Figure 5 .
Figure 5. Regional geological map of Batang Integrated Industrial Park [18]Subsurface research results from the drill log are presented in Figure6and Figure 7[19], showing the subsurface soil/rock to 30 m depth.The drill log results identified five material layers: clayey silt, silty clay, silty sand, gravelly sand, and breccia.

Figure 11 .
Figure 11.Groundwater flow pattern in the research area.

Table 1 .
Interpretation based on the geoelectrical survey

Table 2 .
Water level from observation in the research area