Aquifer potential investigation applying vertical electrical sounding in Bango sub-catchment area

The freshwater requirement in the Bango sub-catchment is rising along with land use change. The main source of freshwater that can be used is groundwater in the aquifer layer. The research on aquifer potential was conducted using the vertical electrical sounding (VES) method. Vertical electrical sounding is a geoelectric method to measure the resistivity of the rocks. This instrument is used to obtain subsurface information about aquifer depth. The measurements along fourteen measurement points that the length of each track used is a maximum of 4 km. The layer which is the potential to be an aquifer is a tuff layer with resistivity values for tuff layer at each measurement point varying from 20,2 to 78,16 Ωm. The results showed that the rock lithology is influenced by volcanic activity and there is a layer suspected of being a potential shallow aquifer. The potential shallow groundwater in the Bango sub-catchment is approx. 5,73 billion cubic meters.


Introduction
Water is one of the sources of power and energy on this earth.Water is one of the basic needs of living things, especially humans for survival [1].Based on its existence, water can be divided into 2 types, namely surface water and groundwater.In general, humans use groundwater both for direct needs such as drinking water, industrial water, and sanitation and for indirect needs such as irrigation, animal husbandry, hydroelectric power plants, and other purposes.In life, groundwater has a greater role because of its good water quality, relatively low investment costs, and easy utilization, which can be done in situ [2].
In recent decades there has been a change in land use in the Bango watershed where land previously used for agricultural activities has been converted into residential areas.The conversion of land into residential areas proves that there is population growth in the Bango watershed area.Of course, this will become a new issue related to the increasing need for clean water to meet the daily needs of the population.Population growth is followed by economic and industrial growth, such as the construction of factories and the planned Kawasan Ekonomi Khusus program that will be built in the Bango watershed area.Broadly speaking, the area that covers the Bango Sub-watershed requires a fairly high IOP Publishing doi:10.1088/1755-1315/1311/1/012038 2 supply of clean water.The filling of clean water comes from the groundwater that is present in the aquifer layer.The aquifer is a porous, permeable, and saturated layer, such as unconsolidated sand, which can flow and store groundwater.Therefore, the aquifer layer is an important source of water for wells and springs [3].
Aquifers can be located at various depths, ranging from shallow aquifers close to the ground surface to deep aquifers located at depths of hundreds to thousands of meters below the surface.Estimation of subsurface conditions, especially related to the presence of groundwater, is carried out using the technique of estimating specific resistance (resistivity) using the Vertical Electrical Sounding (VES) method [4].The detection of aquifers to determine the distribution and depth of groundwater is one of the efforts to fulfill clean water needs in the region.

Time and Place of Research
This study used a quantitative survey approach where measurements were taken in the Bango watershed area that crosses the administrative areas of Malang Regency and Malang City in August 2023.Astronomically, the Bango watershed is located at 07°45'52" S -07°59'40" S and 112°33'05" E -112°46'55" E. The measurement points were determined using the Systematic Grid Sampling method through the ArcGIS application with the coordinates of the measurement points shown in Figure 1.

Figure 1. Geoelectric Sounding Point Location Map
The geoelectric measurement points are spread across several different land uses including forests, agricultural land in the form of fields and paddy fields, and yards in residential areas.

Tools and materials
The materials needed in the research are as follows: 1) Bango watershed boundary map of Malang Regency, 2) Geological Map of Malang Sheet scale 1:100,000 produced by Geological Research and Development Center 1992, 3) Hydrogeological Map of East Java Province horizontal scale 1:125,000 and vertical 1:50,000 produced by the Directorate of Environmental Geology.The resistivity meter will be connected to the battery as an energy source and electrodes to conduct electric current into the earth.

Methods
The resistivity geoelectric method is one of the most widely used geophysical techniques to obtain information about subsurface conditions and potential groundwater aquifer layers [5], [6].The geoelectric method uses the principle of electric current flow where electric current flows through the rocks and is heavily influenced by the presence of groundwater and the salts contained therein [7].The geoelectric measurement technique carried out in this research uses VES (Vertical Electrical Sounding) with a Schlumberger configuration which can significantly reduce the time needed to carry out sounding and the costs are relatively small.
The first stage of geoelectric measurement is injecting electric current using 2 current electrodes (AB) and 2 potential electrodes (MN) arranged in a straight line with the arrangement of the potential electrode distance smaller than the current electrode distance [7].The reading of the resistivity meter at the sounding point is carried out each time the electrode displacement is changed arbitrarily, starting from a small electrode distance and then gradually increasing [8].The longer the AB electrode distance will cause the flow of electric current to penetrate deeper rock layers [9], [10].The geoelectric measurement data in the form of current (I) and voltage (V) is then processed to obtain the apparent resistivity value using the following equation: (1) where: ρa = apparent resistivity (ohm-m) K = geometry factor ΔV= potential difference (volts) I = current strength (ampere)  =  ( where: The next stage is the inversion process using the IPI2WIN application to reconstruct subsurface conditions based on measurement data.Inversion is often called curve fitting which is done by finding model parameters to produce a response that matches field observations [11].The results of the inversion or curve fitting process with IPI2WIN are in the form of resistivity values in ohmmeter units (ρ), thickness in meter units, and layer depth in meter units (d).Furthermore, the inversion results will be interpreted based on geological maps and resistivity values to determine the lithology of each layer.The range of resistivity values against rock types shown in Table 1, will help in the process of interpreting the resistivity log into a lithology log [12].Using the Rockwork 16 application, the amount of groundwater volume and visualization of the lithology cross-section can be determined.

Results and Discussion
There are 14 geoelectric measurement points with an interval of 4 km for each point spread over 2 points in Malang City and 12 points in Malang Regency.This research focuses on the potential of shallow groundwater so that geoelectric data collection is only carried out with a depth of 50-60m.Processing of geoelectric measurement data is carried out using IPI2win software to get the smallest error number (RMS error).Analysis based on differences in resistivity values resulted in 5 to 7 layers.Geological formations in the Bango watershed are dominated by Arjuno Welirang volcanic rock formations, Malang Tuff Formation, and Gendis and Buring volcanic formations.The range of resistivity values is very large ranging from 4,35 Ωm to 3889 Ωm.The low resistivity values are interpreted as clay and tuff layers.Rock layers of breccia to lava that are solid and hard are interpreted from high resistivity values.The next step is to interpret the geoelectric measurement data.The results of data interpretation at each measurement point are as Geoelectric points 7 and 8 identified rock layers consisting of topsoil, clay, and tuff layers with resistivity values ranging from 10,8 Ωm to 73,7 Ωm.The topsoil layer is located at a depth of 0 -1,56 meters, the next rock layer is dominated by inserted between clay and tuff layers.The clay layer is identified at depths of 1,5 -7,72 m; 14,2 -30 m; and 52,3 -60 meters.The tuff layer is identified at depths of 7,72 -14,2 m; and 30 -52,3 m.The A-A' profile cut in Figure 6 is planned to determine the rock lithology profile at the research site.By combining geoelectric points TGL 4, TGL 5, TGL 6, and TGL 14, which are located in the Arjuno Welirang volcanic rock formation, Malang Tuff Formation, and Gendis volcanic formation.Taking geoelectric points as cross-section profiles are based on the number of points in 1 (one) straight track and the location of points that can represent lithological differences.

Figure 5. Lithology profile of the A -A' section
Based on the cut profile A -A' of points TGL 04 and TGL 06, the surface layer is dominated by hard lava rock and breccia to volcanic breccia.Point TGL 14 which is on the opposite side is dominated by breccia to volcanic breccia on the surface, then clay and tuff.Point TGL 06 which is located in the lowland layer is dominated by clay and tuff layers.
Based on the hydrogeologic map, the Bango watershed area is an area with a diverse and widespread level of aquifer productivity.Areas included in the small productivity are in Tawangargo Village.Aquifers with local productivity are located in Donowarih Village and Sumbul Village where groundwater is generally not utilized due to small discharge and deep-water table.Boreholes are mostly found in the upstream and middle sections with a depth of more than 60 m.Meanwhile, dug wells are found in the downstream section with a depth of about 14 m.Of the rock types in the area, tuff is the best rock as an aquifer with a fairly large pore structure in the downstream part which is detected at a depth of between 3.83 and 57.05 m.In the upstream and middle parts, the aquifer layer is in the form of fractures between breccia rocks.While in the upstream and middle part of the aquifer layer in the form of fractures between volcanic breccia rocks and lava at a depth of more than 60 m.Then, the results of shallow groundwater volume in the Bango watershed area is 5.73 billion cubic meters.

Conclusion
In general, the rock lithology in the Bango watershed area is dominated by volcanic rocks consisting of clay, tuff, breccia, volcanic breccia, and lava.Where in the upstream part is dominated by breccia-lava rocks, the middle part is dominated by volcanic breccia rocks, in the downstream part is dominated by tuff rocks.Shallow aquifers are identified at the downstream area in the Malang Tuff formation with tuff as the aquifer layer at a depth of 3.83 -57.05 meters.Then in the upstream and middle areas of volcanic rock formations with aquifers in the form of fractures between volcanic breccia rocks and lava at a depth of more than 60 m.Bango sub-watershed has good groundwater potential with a shallow groundwater volume of about 5.73 billion cubic meters.The results of the study can be used as a reference for the utilization and conservation of groundwater in the Bango Subwatershed area.

9 Figure 6 .
Figure 6.3D visual of Bango watershed rock lithology

Table 2 .
Types of Rocks in Each Layer in Malang City

Table 3 .
Rock Type of Each Groundwater Layer in Malang District

Table 3
shows that the geoelectric points in Malang Regency are located in the geological formations of Arjuna Welirang Volcano (Qvaw), Middle Quaternary Volcano (Qp), and Malang Tuff (Qvtm).The resistivity values range from 5.45 Ωm to 3889 Ωm with rock layers consisting of topsoil, clay, tuff, breccia, volcanic breccia, and lava.