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Geoelectrical survey and groundwater chemical analysis in Sumowono Groundwater Basin

Sumowono groundwater basin is an inter-regency groundwater basin located in Central Java Province. The administrative areas are Semarang Regency, Temanggung Regency, and Kendal Regency. Hilly regions dominate the study area. This study aims to identify the subsurface lithology and analysis of the water type of groundwater samples. The methods were geoelectrical survey using Schlumberger array and physical and chemical of groundwater samples analyses from dug wells and springs. The results of the geoelectrical survey conduct five types of lithology with the resistivity value from 0 to more than 190 Ωm. They are claystone, tuff, tuffaceous sandstone, sandstone, and breccia. The chemical properties of groundwater samples indicate that the pH has a range from 6.4 to 7.27 with electrical conductivity (EC) from 71.6 to 511 µS/cm. Moreover, the Total Dissolved Solids (TDS) start from 45.8 to 327 mg/L. The water types are Mg2+-Ca2+-HCO3 −Cl-, Ca2+-Mg2+-HCO3 --Cl-, Ca2+-Mg2+-HCO3 -, Ca2+-HCO3 --Cl-, Ca2+-Mg2+-HCO3 --SO4 2-, and Ca2+-HCO3 --SO4 2-.


Introduction
The Sumowono Groundwater Basin is located in Central Java Province. Administratively, the Sumowono groundwater basin is in Semarang Regency, Kendal Regency, and Temanggung Regency. Specifically, it is at 396658-426897 East Longitude and 9197102-9215729 South Latitude (Figure 1). With geomorphological characteristics, it is dominated by hilly areas with an average rainfall of 1,383 mm/year [1].
A Geoelectric survey is a geophysical method using electric currents to determine the condition of the underground layer. This survey is usually define the subsurface condition by measuring the resistivity value of the rock [2]. It is also used to determine aquifer geometry and distribution [3]. The aquifer geometry includes aquifer thickness and aquifer depth [4]. Researchers in the world mostly conduct this survey because it is advantageous and efficient. The application of this survey is very appropriate to describe the subsurface condition because the porosity and permeability properties of rocks are very suitable to be combined with rock resistivity [5]. Rock resistivity that differs from the measurement results is usually due to the mineral content in the rock. Then the resistivity values are grouped and combined with the geological conditions in the study area. This measurement can also determine the potential for groundwater in aquifers [6]. Geoelectric investigations collect a lot of  Chemical analysis of groundwater is usually carried out to determine the quality of groundwater such as the content of cations (Ca 2+ , Mg 2+ , Na + +K + ) and anions (Cl -, SO4 2-, HCO3 -) as well as the pH, TDS, and EC values. The major cations and anions can be plotted on a stiff diagram showing changes in water composition. The quality and hydrochemistry of groundwater can change at any time, depending on the interaction of groundwater with rocks that store groundwater, which ultimately affects the water type [7]. The pH value, electrical conductivity/EC, and TDS can be used to determine the quality of groundwater. The pH of the water represents the relative amounts of free hydrogen (H + ) and hydroxyl ions (OH -). Moreover, the pH value is influenced by the soil where the water flows. Also, the factors of water sources, rainfall, and contaminants significantly affect the pH value. pH in water analysis is used as an indicator of relative acidity. Value> 9.5 indicates alkaline, while value <3 indicates acidity [8]. The normal pH range for drinking water is from 6.5 to 8.5. In comparison, the EC value estimates the total amount of dissolved solids in water which is usually controlled by lithology, salt solubility, basic chemical properties, and human activity [9]. Changes in groundwater chemistry are strongly influenced by the dissolution and deposition of minerals in rocks that become aquifers [10].
The purpose of this study is to determine the condition of the subsurface condition by measurement the resistivity value using the geoelectrical survey. Moreover, it has another objective to assess groundwater quality and water type.

Geology and hydrogeology setting
The Sumowono groundwater basin is located at the topography of 260-1,010 masl with the dominance of lithology in the form of volcanic rocks. The rock stratigraphy in Sumowono from old to young deposits, according to [11], consists of andesite intrusion (Tma) and basalt intrusion (Tmb). It then  Based on the Regional Hydrogeological Map of the Pekalongan Sheet [12] and the Semarang Sheet [13], the Sumowono groundwater basin has two types in which aquifer flow fissures, fractures, and channels and its productivity from medium to high. The second aquifer flow in which fissures or porous with low productivity and region without groundwater abstraction (figure 3).

Methods
A hydrogeological mapping was carried out to present data on subsurface conditions and analysis of groundwater quality in June 2020. Hydrogeological mapping was conducted geoelectrical measurements of 10 points (Figure 4) using the Putranto resistivity equipment. The Schlumberger array method was applied with a measurement path length of 400 m and a potential point of 25 m in the resistivity measurement. The geoelectrical survey employed Vertical Electrical Sounding (VES) to obtain the rock resistivity by injecting currents into the ground [4]. With a path length of 400 m, it is estimated that vertical depth data is 130-150 m deep or one-third of the maximum electrode current [14]. From the data of the geoelectrical measurements in the field, a matching curve analysis was carried out to obtain the actual thickness, depth, and resistivity values of the rock, because the resistivity value measurement in the field campaign was an apparent resistivity value. Furthermore, the results of the matching curve analysis were correlated in 3 dimensions to determine the distribution of rocks.
Groundwater samples were taken from 9 dug wells and 1 spring for analyzing their physical and chemical properties, including pH, TDS, and EC using a portable multi-meter multiline 3510 IDS. Indeed, the chemical analysis of the main cations and anions of groundwater was analyzed in the Envilab geochemical laboratory. The pH, TDS, and EC values were analyzed spatially using Surfer 11 and ArcGIS 10.4 software. Visualization of groundwater hydrochemical values is plotted in a stiff diagram using Aquachem software.

Geoelectrical survey
The results of the analysis of 10 geoelectric measurement points in the Sumowono Groundwater Basin, there are several types of lithology obtained based on the rock resistivity value in the [2] as shown in Table 1 below. The soil which is deposited on the top has varied from 2.14 to 462. 29. The lowest resistivity is claystone, with a resistivity up to 5 Ωm. The volcanic product has a ranges from 10 Ωm to more than 190 Ωm. The highest resistivity indicated compact rocks such as breccia. Tuff and tuffaceous Breccia is found in GL 4 with a thickness of hundreds of meters in the east. Sandstone is identified extend dominantly in the south with the thickness of hundreds of meters as shown in GL 1. Tuffaceous sandstone spreads mainly thicker in the northeast, more than 100 m, and becomes thinner to the center up to 45 m which is across GL 6 to GL 8. At the same time, tuff is dominated in the west in GL 9 to GL 10. The thickness is from tens of meters up to 45 m. Claystone is locally deposited in the south across GL 2 to 3 with a thickness from 10 to 53 m.  Figure 1 shows the groundwater sampling location with the results of the hydrogeochemical analysis of 10 groundwater samples in the Sumowono Groundwater Basin is presented in Table 2 below. The result showed that the highest EC value, 377 µS/cm ( Figure 6) was in the Muncar, Gemawang (SG-05) area, and the lowest EC value, 71.6 µS/cm, was in springs (MA-06) located in Rowoseneng Village, Kandangan. Overall, the resulting EC value indicates a safe condition for use, marked with an EC value below 1,000 µS/cm ( Figure 6). The TDS value of the Sumowono Groundwater Basin area as a whole sample has a value that is included in the secured category because it is less than 500 mg/L. The highest TDS value is 241 mg/L in the Muncar, Gemawang area and the lowest TDS value of 45.8 mg/L locates in Rowoseneng Village Kandangan with an average of 152.9 mg/L ( Figure 7). Overall, the pH value of the Sumowono Groundwater Basin is neutral in the range of 6.4-7.27 [15].

Conclusion
As a result of the geoelectrical survey, there are five lithologies, namely claystone, tuff, tuffaceous sandstone, sandstone, and breccia. Claystone has the lowest up to 5 Ωm. The volcanic product has ranged from 10 Ωm to more than 190 Ωm. The highest resistivity indicated compact rocks such as breccia. Tuff and tuffaceous sandstone have a range from 5 to 30 Ωm. At the same time, sandstone has a resistivity between 30-180 Ωm. Breccia is found in the east. Sandstone is identified to extend dominantly in the south. Tuffaceous sandstone spreads mainly thicker in the northeast and becomes thinner to the center. At the same time, tuff is dominated in the west, and claystone is locally deposited in the south.