Groundwater potential zones identification using geoelectrical resistivity sounding and GIS in upper Metro sub-catchment, Malang, Indonesia

Recently, Upper Metro Sub-Catchment and its surroundings have witnessed intensive land investments. The lack of surface water and the human activities of this region increase the demand for groundwater. Several water boreholes were installed to meet the large water needs; however, these boreholes no longer cover the high demand for water due to the increase in population, increase in irrigated areas, and wear and tear on pumps. Therefore, in order to better define the catchment area and find new sites that meet the water quality and quantity requirements, a geoelectrical survey using the electrical resistivity method was carried out in the upper Metro sub-catchment potentials. Using the Schlumberger array, a total of 19 vertical electrical soundings were conducted along 2 profiles. The recorded data were interpreted quantitatively and qualitatively through the use of apparent resistivity maps, geoelectrical pseudo-section analysis, and the established geoelectric sections. In this study, we identified sandstone tuff as an aquifer layer at a depth of 20 – 100 meters. So it can be known that there are 3 zones of groundwater potential consisting of poor, medium, and high.


Introduction
Water is an essential need for all living things, including humans.To maintain the continuity of daily life such as drinking water sources, agriculture, and meet the needs of industrial processes, the availability of an adequate supply of clean water is very important.In general, fertile areas are often characterized by high groundwater discharge, while less fertile or dry areas have limitations in groundwater discharge [1].
Transformations in human life patterns, technological advances, population growth, and socioeconomic dynamics are elements that are closely tied to the times.All these transformations affect land use change, where an increase in population drives the demand for housing, infrastructure, and other facilities [2].In addition, economic growth and industrialization also encouraged the construction of factories, industrial sectors, and trade centers.All these changes affect land use change, which in turn requires the supply of water resources, the majority of which is obtained from groundwater.
The research area in the upper Metro sub-catchment in Malang Regency has the potential to deal with these issues.The Metro Sub-catchment area covers the Kawi Mountain plateau and some parts of Malang Regency.The upper part of the Metro sub-catchment is a transitional area that has experienced land use change over time.Areas that were previously used as agriculture or forests are now transformed into urban, residential, or industrial zones.As a result, the infiltration capacity of the soil and its ability to retain water is reduced.This leads to a decrease in subsurface water supply [3].
To solve the issue of groundwater availability in the upper Metro Sub-catchment, an identification survey using the Geoelectric method is required [4].This approach is effective for identifying the presence of aquifers, evaluating water storage capacity, assessing groundwater conditions, as well as supporting water resources management efforts.

Material A. Study Location
The location of this research is in the Metro Sub-catchment area.Metro Sub-catchment is geographically located at 112°22'00" -112°37'00" E and 7°56'40 -8°10'10" S with an area of 167.835 km 2 .This study covers 15 villages with the distribution of points as follows.

C. Hydrogeologic Conditions
The hydrogeologic map used in this study is sheet X Kediri (Java) with a layer of medium productive aquifer and high productive aquifer [1].

Figure 3. Hydrogeologic Map of The Research Site
Measurement points TGL 1 -TGL 16 and TGL 18 are located in medium-productive aquifer areas with wide distribution and the depth of the unconfined aquifer is generally deep.Measurement point TGL 17 is located in a highly productive aquifer area with a wide distribution and the depth of the water table is very diverse.Meanwhile, measurement point TGL 19 is in a productive aquifer area with a wide distribution and groundwater table or groundwater pisometric height near or above the ground surface.

Tools
The tools used in this study include a set of measuring devices geoelectric resistivity meter, 2 stainless electrodes, accu, 4 cable reels with a length of 200 m each, distance roll, hammer, GPS, and Handy Talky.In this research, it began with data collection.The required data consists of primary data and secondary data.Primary data collection involved a geoelectric data survey and a groundwater depth survey at various points in the Metro Sub-watershed location.Secondary data was obtained from administrative maps, geological maps, and hydrogeological maps of the Metro Sub-watershed research area.After obtaining both primary and secondary data, these data underwent a data acquisition process or data collection and preparation process to make them ready for further processing [11], [12].From the acquisition data, there was a phase to process the geoelectric data.The processed geoelectric data results were then interpreted to determine the lithology of rock types.Once the lithology of rock types was known, the location of the aquifer distribution could be identified.This can be obtained from the volume of the aquifer potential and the map of groundwater potential zones in the research area.

Equation A. Apparent Resistivity.
The measured apparent resistivity is the combined resistivity of several soil layers which are considered as one homogeneous layer.This apparent resistivity is formulated by [3].

B. Groundwater Passability Test (Transmissivity).
The coefficient of permeability (Coeficient of Permeability/Hydraulic Conductivity) is the ability to conduct water in rock cavities without changing the properties of water.Using the following equation [3].

Typical Resistivity Analysis and Data Interpretation
After conducting geoelectric measurements in the Metro Sub-catchment area using the Schlumberger method, the next step is to input the data obtained from the measurements into the IPI2WIN and Rockwork V16 applications.The goal is to obtain information on the distribution of specific resistivity values.After the specific resistivity data and layer depth have been obtained, the next step is to interpret the specific resistivity data based on the available rock lithology table.The results of the analysis of specific resistivity and data interpretation can be seen in the table below.From the cross-section B-B' picture, it is known that TGL 5 and TGL 6 are located at an elevation of 639 -699 m which is dominated by tuff rock layers.Meanwhile, TGL 7 and TGL 8 which are located at an elevation of 543 -606 m are dominated by a layer of sandy tuff rock.Based on the results of the analysis of rock types and their distribution, it was found that the rock layer has the potential to be an aquifer layer.Of all the rocks identified, sandy tuff is the most optimal rock layer to store and distribute groundwater, due to its large porous structure.The analysis shows the potential volume value of the aquifer is 1,645,000,000 m 3 .

Figure 1 .
Figure 1.Research location map Of the 15 villages, 19 geoelectric points were scattered to represent the condition of the rock layers in the Metro Sub-catchment area.Data collection is carried out in plantations, rice fields, and roadsides with each 200 m span.The following is a description of each geoelectric measurement point.

Figure 5
Figure 5 is intended to determine the rock lithology profile at the research location in one pass.Cross-section A-A' consists of a combination of measurement points TGL 1, TGL 4, TGL 5, TGL 6, TGL 12, TGL 13, TGL 14 TGL 15 and TGL 16.While cross-section B-B' consists of a combination of measurement points TGL 5, TGL 6, TGL 7, and TGL 8.The cut line can provide a lithological profile and rock distribution.

Table 1 .
Study Location

Table 2 .
Specific Resistivity Analysis Results and Interpretation Data