The Origin and Quality of the Groundwater of the Rawadanau Basin in Serang Banten, Indonesia

Rawadanau is a tropical mountain swamp located in Serang, Banten, Indonesia. Rawadanau groundwater is the main source of supply for the Cilegon Banten industrial area. Knowing the origin and quality of the groundwater can help preserve existing resources. This paper aims to clarify the origin of Rawadanau groundwater with new evidence of stable isotopes ( 18 O and 2 H) and hydrochemical data. Field measurements included pH, temperature, EC, HCO 3 ˉ, while the analyses of cations, anions and stable isotopes were carried out in the laboratory. The existing water supplies include springs, excavated wells, and river water. Cluster hierarchy analysis based on water chemistry and stable isotopes can be grouped into two clusters, cluster K (K1 and K2) and cluster L. Data δ 18 O and δ 2 H springs in Rawadanau indicate that they are of meteoric origin and that there has been evaporation from several springs. Water comes from meteoric water with a stable isotope content of δ 18 O between -6.39 to -4.82 ‰ and δ 2 H between -41.35 to -31.30 ‰, which are controlled by two main mechanisms, namely rock dissolution and evaporation dominance. Aquifers are composed of volcanic rocks with a dominant composition of porphyritic andesite, basaltic andesite, and andesite. According to the multivariate statistical analyses results other than pH and SO 42- , all parameters (cations and anions) showed a significant correlation. There are four hydrochemical facies of groundwater, namely Ca-HCO 3 , Ca-Mg-HCO 3 , Mg-Ca-HCO 3 , and Na-Cl.


Introduction
Rawadanau is a typical tropical mountain swamp surrounded by volcanoes and located 91 metres above sea level (asl). It becomes a marsh during the dry season and a lake in the rainy season. This investigation of Rawadanau groundwater is to understand its hydrochemical origin and isotope characteristics and thus help explain actual groundwater supply conditions in the study area. Groundwater is the most important freshwater resource at Rawadanau and at present, provides almost all of the water needs in the industrial area Cilegon. Rawadanau Sub-basin has an area of 375 km 2 (KEMEN-ESDM, 2017), which is the primary source of water fulfilment for the Cilegon industrial area. Groundwater reverse surface water is available almost everywhere in the area, and is used for drinking, irrigation, industries, and municipalities (Hussain and Abed, 2019).
As groundwater moves through rock, its chemical composition normally changes. In general, the longer groundwater remains in contact with aquifer rocks, the greater the amount of material it will take into the solution (Chebotarev, 1955). Rawadanau also has geothermal potential, with several identified hot spring locations. More than 100 companies in the industrial area of Cilegon depend on the Rawadanau water reserves, which are collected by damming the Cidanau River (Hartanto et al., 2019). Existing water comes from rainwater, river water, springs flowing through several river branches before entering the Cidanau River.
The groundwater, including surface water and hot and cold springs, all indirectly come from precipitation. This proves that the groundwater is meteoric water, although some gradually infiltrates the ground and interacts with the rocks in its path. This injected water continues its journey to the saturation zone to become part of the aquifer. It is interesting to study Rawadanau Basin's hydrogeological condition because it is composed of quaternary vol-Rudarsko-geološko-naftni zbornik i autori (The Mining-Geology-Petroleum Engineering Bulletin and the authors) ©, 2021, pp. 11-24, DOI: 10.17794/rgn.2021.2.2 canic products. Its lithology contributes to an aquifer layer, which is relatively influential in terms of quality. This quality is influenced by environmental and anthropogenic factors such as the geological context, climate, precipitation, the interaction between groundwater and aquifers, and human activities (Houria et al., 2020).
Stable isotopes can be used as tracers in the groundwater environment to understand water origin in a hydrological cycle (Taniguchi et al., 2000). Stable isotopes do not decay over geological time. The stable isotopes of oxygen and hydrogen in water are 16 O, 17 O, 18 O, and 1 H and 2 H, respectively, whose values are expressed in δ (Tweed et al., 2019). Isotopes provide vital records that occur in thermal and nonthermal groundwater so they can play an essential role in hydrogeological investigations (Oyuntsetseg et al., 2015). Hydrochemical data and isotopes can be an effective tools to solve various hydrology and hydrogeology problems (Chen et al., 2011;Clark and Fritz, 2013;Liu et al., 2015). The origin, evolution, and formation of hot springs can be explained using hydrochemical and trace stable isotopes. Hydrochemistry was analyzed to determine the quality, type, dominant hydrogeochemical processes, stable isotopes of oxygen ( 18 O), and hydrogen ( 2 H) and thus clarify the origin of the groundwater.
This study aims to determine the groundwater origin of Rawadanau basin using new evidence covering the main elements of water chemistry and stable tracer isotopes ( 2 H and 18 O) from springs, dug wells, and surface water. The data was processed using statistics for grouping and classification of sources, using some frequently used models.

Methods
The research was carried out based on the results of field observations and laboratory data in the tropics. The stable isotope approach is used to understand the origin of groundwater. Meanwhile, to determine water quality, the chemical analyses of water were carried out, analysed, and modelled using SPSS statistical software. Figure 1 shows a workflow methodology of this research.

. Location
Rawadanau is located on the west side of Java Island at a morphology between 100 to 1350 metres above sea level (asl) (see Figure 2), located in the Serang province of Banten Indonesia. The research area is located at coordinates between 06° 07` 10" -06° 19` 10" S and 105° 51` 30" -106° 03` 00" E. The Cidanau River is the main river in the research area and flows to the Sunda Strait.
Based on meteorological data between 1996 and 2014, the average annual rainfall is 2806 mm (Hartanto et al., 2019). Rainy days occur throughout the year, and above ten days/month occur between November and May, when the intensity of monthly rainfall is above 200 mm, except from May to October. Minimum rainy days, occurring less than five days occur in August and September (see Figure 3), with average monthly rainfall intensity of 61.2 mm/month and 56.6 mm/month, respectively. The average dry peak occurs from August to September (intensity 50 mm/month), while the peak rainy season is from De-  Rudarsko-geološko-naftni zbornik i autori (The Mining-Geology-Petroleum Engineering Bulletin and the authors) ©, 2021, pp. 11-24, DOI: 10.17794/rgn.2021.2.2 cember to February with an intensity above 300 mm/ month (see Figure 4), respectively.
The southern side of the research area is Mount Parakasak (990 metres asl), Mount Karang (1739 metres asl), Mount Aseupan (1081 metres asl), the north side is Mount Gede (741 metres asl), and the east side is Mount Kupak (361 metres asl).

Geological and hydrogeological characteristics
Rawadanau Sub-basin covers approximately 375 km 2 , with the constituent rocks being the products of Mount Garung and Karang-Pulosari Volcano and alluvium (Alam et al., 2014;Mulyadi, 1985;Suryadarma and Fauzi, 1991). Two fault systems control Rawadanau, namely the Garung and Wangun system. Both of them have the same straight line pattern, namely NW-SE with rocks consisting of porphyritic andesite, basaltic andesite, and andesite formed since the Pleistocene era (Suryadarma and Fauzi, 1991). The discovery of hot springs containing solfatara around Rawadanau, which are the slopes of Mount Karang-Pulosari and Mount Garung, shows the remnants of volcanic activity and geothermal manifestations (Mulyadi, 1985).
The study area is comprised of Rawadanau deposits (Qr), consisting of gravel, clay sand, mud, and pumice crust. Colluvium sediment (Qk) forms from ruins, talus, and the dumping of volcanic ruins. Coral volcanic rocks (Qvk), composed of volcanic breccia, lava, tuff, and lava, are inseparable. Young volcanic rock deposits (Qhv) are composed of volcanic breccia, lava, tuff, lava flows, and other volcanic eruptions (Mount Aseupan, Mount Parakasak). Upper Banten tuff (Qvtb) consists of tuff, pumice tuff, sandy tuff (at the top) and crystal tuff, rocky pumice tuff, glass tuff, and clay tuff insert at the bottom. Young Lake Volcanic Rock Deposits (Qvd) are composed of lava flows composed of andesite or basalt flapping, volcanic breccia, and tuff. Lower Banten tuff (Qptb) is composed of breccias, agglomerates, pumice tuffs, lapilli tuffs, and sandy tuffs. Old Lake Volcanic Rocks (Qpd) are a form of lava flow composed of andesite or raised basalt, volcanic breccia, and tuff.
Based on hydrogeology (see Figure 5), the study area aquifers are divided into cleft flow aquifers and intergrain spaces, aquifers through fractures, channels, cavities, and small productive aquifers and rare groundwater areas. Discharge areas include active, productive aquifers (discharge more than 5 litres/second), medium productive aquifers (discharge less than 5 litres/second), and rare groundwater (Suryaman, 1999).

Water sampling and analysis
Water sampling was carried out at the peak of the dry season in September 2019. A set of 28 samples were taken, from wells, cold spring, rivers and hot springs. The 28 sample locations include seven rivers, five wells and 16 springs. Several water parameters were measured directly (in situ measure) using Water Checker Toac portable devices, including pH, temperature, and electrical conductivity (EC). All samples were put in 500 ml polyethylene bottles and analyzed for major ions (anions and cations) based on APHA 2005 at the Water Laboratory of the Research Center for Geotechnology -Indonesian Institute of Sciences (LIPI). Titrations analysis (Ca 2+ , Mg 2+ , HCO 3ˉ, and Cl -), flame photometry (Na + and K + ), and spectrophotometry (for analysis of SO 4 2-, and SiO 2 ) were also undertaken. HCO 3ˉ analysis was carried out directly on the same day of collection, titrated using / with 0.01 or 0.1 M HCl against methyl orange and Bromocresol green indicators (APHA, 2005). The resulting analysis of the physical and chemical analyses are presented in Table 1. Minimum raw elements were analyzed to assess drinking water suitability (Appelo and Postma, 2005). The accuracy of the chemical analysis of water was verified by calculating the electrical balance (EB) where the error is generally less than about 5% with the equation: (1) Where ΣCations is total cations in meq/l, ΣAnions is total anions in meq/l. An error above 5% indicates that analytical procedures should be examined (Appelo and Postma, 2005). For isotope analysis ( 18 O and 2 H), the sample was put in a 100 ml polyethylene bottle analyzed at the Hydrochemical Laboratory of the Center for Groundwater and Environmental Geology, the Geological Agency of Indonesia using the Picarro L-2130-i analyzer. The results of the stable isotope analysis ( 18 O and 2 H) are presented in Table 2.
The TDS value is considered an essential value in determining water use and describing inorganic salts' presence and correlates with EC (Logeshkumaran et al., 2015;Rusydi, 2018). In this study, the TDS value is not measured directly, because this parameter is correlated with EC, usually expressed by a simple equation (Clark and Fritz, 2013;Rusydi, 2018): The interpretation of water types based on major ions is plotted in two basic triangles as the percentage of cations and milliequivalent anions of water chemistry using the Piper Diagram (Piper, 1944). Groundwater's hydrochemical characteristics, and correlation methods were used for the hydrochemical properties and the piper diagram (Mirčovski et al., 2018). The Gibbs Diagram was used to better understand the processes that occur during groundwater flowing and mechanisms for hydrochemical formation, (Chintalapudi et al., 2016;Gibbs, 1970;Marandi and Shand, 2018). To process statistical data SPSS V26 was used to process statistical data.

Statistical analysis
Statistical analysis was performed on the data from the analysis of water chemistry and stable isotopes. The first step is to perform clustering based on the stable isotopes δ 2 H and δ 18 O (see Figure 6). Some of the water's chemical properties are related to each other, as indicated by the value of r (Pearson's correlation coefficient) with a value above 0.6 (Marsal, 1987). From the clustering results, the descriptive statistics and the correlation of each element were analyzed. The statistical analysis of all physical and chemical parameters, including minimum, maximum, mean, standard deviation, are presented in Table 3.
The main water elements can be easily determined using a box plot diagram (Boateng et  ). An increase in the main water element concentration is illustrated in the box plot diagram (see Figure 7). The box plot shows the upper and lower quartiles; the maximum and minimum values are shown as lines. The abundance of the main elements of anions in water is HCO 3ˉ > Clˉ > SO 4 2-> CO 3ˉ and the cations is Ca 2+ > Na + > Mg 2+ > K + , respectively.

Hydrochemical characteristics
The hydrochemical analysis results, univariate of each cluster are shown in Table 1 and Table 3. The primary ions value a charge balance error in water, from -4.96 to -2.21, while the mean error is -4.05. The error level shows that the analysis data meets the requirements, i.e., less than 5% (Appelo and Postma, 2005), to be used for further analysis.
The groundwater pH study area generally ranged from 5.7 to 7.7. Sub-cluster K1 had an average pH of 6.5, Sub-cluster K2 had an average pH of 6.7, and Cluster L had an average pH of 6.9, respectively. Water temperature ranged from 23.5 to 51.8°C (mean 32.1°C), sub-cluster K1 average temperature of 34.0°C, sub-cluster K2 averages 33.2°C, and cluster L averages 28.3°C, respectively. Hot springs were found in sub-clusters K2 and K1. At Batukuwung spring (near location 29S), the water temperature reached 65°C (Hochstein and Sudarman, 2008). More than 37.8°C can classify as hot springs (Alfaro and Wallace, 1994). Even if the water temperature is more than the average surface temperature in general or more than 36.7°C, it can be said to be hot springs (Pentecost et al., 2003).
The electrical conductivity (EC) Sub-cluster K1 ranged from 75 to 2410 μS/cm (mean 845.8 μS/cm), Sub-cluster K2 from 46 to 2850 μS/cm (mean 834.4 μS/ cm), Cluster L was between 73 and 612 μS/cm (mean EC 73 μS / cm), respectively. This indicates that the groundwater has low salinity properties. An increase in the TDS value followed by an increase in the EC value was caused by the dissolving process that occurs when water flows due to evaporation, ion exchange resulting in an increasing concentration (Delinom, 2009;Liu et al., 2015;Tóth, 1999). For sub-cluster K1 and sub-cluster K2, respectively, the mean EC values of 845.5 μS/cm and 834.4 μS/cm, respectively, indicate that the existing groundwater contains many rock dissolved minerals. TDS value is considered an important value in determining water use. Total dissolved solids concentration (TDS) ranged from 46.0 to 1852.5 mg/L, TDS sub-cluster K1 ranged from 75.4 to 1566.5 mg/L (mean 549.8 mg/L), sub-cluster K2 ranged from 29.9 to 1852.5 mg/L (mean 542.4 mg/L) and cluster L between 47.5 and 397.8 mg/L (mean 187.6 mg/L), respectively. An abundance of the main element cation (see Figure  7 and Table 3) is Ca 2+ > Na + > Mg 2+ > K + . In general, content of the sequential cations were Ca 2+ (between 8.0 and 116.8 mg/L) > Na + (1.1 to 212.3 mg/L) > Mg 2+ (2.5 to 86.7 mg/L) > K + (0.6 to 102.4 mg/L), respectively. The Ca 2+ content of the K1 sub-cluster ranged from 16.4 to 102.8 mg/L (mean 42.5 mg/L), the K2 sub-cluster ranged from 8.0 to 116.8 mg/L (mean 36.8 mg/L) and cluster L ranged from 11.4 to 62.2 mg/L (mean 30.3 mg/L). The high Ca 2+ content is caused by enrichment due to the dissolution of Ca-plagioclase minerals (Equation 3, 4 and 5) (Alam et al., 2014;Appelo and Postma, 2005). The content of Mg 2+ sub-cluster K1 ranged from 4.6 to 86.7 mg/L (mean 27.9 mg/L), sub-cluster K2 ranged from 2.5 to 78.6 mg/L (mean 22.1 mg/L) and cluster L ranged from 3.7 to 46.8 mg/L (mean 11.6 mg/L), respectively. Rocks composed of tuff caused high Mg 2+ content. The groundwater passed through aquifers composed of tuffs and andesitic rocks. The formation of secondary clay minerals resulting from the dissolution of silicate minerals such as albite produces clay minerals such as kaolinite, which can enrich minerals in groundwater system as shown in the following equations (Equation 3, 4, 5, 6 and 7 The Na + content of the K1 sub-cluster ranged from 2.1 to 86.7 mg/L (mean 28.9 mg/L), the K2 sub-cluster ranged from 1.1 to 212.2 mg/L (mean 43.7 mg/L) and cluster L ranged from 1.7 to 38.7 mg/L (mean 13.3 mg/L), respectively. Some locations where very high Na + content (31S and 33S) was found are thought to be formation water. The K + content K1 sub-cluster ranged from 2.9 to 56.6 mg/L (mean 18.5 mg/L), the K2 subcluster ranged from 1.1 to 102.4 mg/L (mean 21.2 mg/L) and the L cluster ranged from 0.6 to 18.1 mg/L (mean 5.0 mg/L), respectively. The presence of K + in water results from dissolving igneous rock minerals (Equation

5) (Appelo and Postma, 2005).
The presence of the main elements of anions in water (see Figure 7 and Table 3) is HCO 3ˉ > Clˉ > SO 4 2-> CO 3ˉ. HCO 3ˉ (between 34.3 and 624.7 mg/L) > Clˉ (between 6.9 and 771.0 mg/L) > SO 4 2-(between 2.1 and 14.90 mg/L) > CO 3ˉ (between 0.0 and 58.9 mg/L), respectively. The HCO 3ˉ content -sub-cluster K1 ranged   Carbonate (CO 3ˉ) was generally only detected at the K2 sub-cluster between 0.0 and 58.9 mg/L (mean 16.0 mg/L), and in the K1 sub-cluster, only one location (26S) had a value of 7.8 mg/L and one location in the L cluster (28R) of 4.2 mg/L. The chloride content of the K1 sub-cluster was 8.3 to 298.0 mg/L (mean 84.4 mg/L), the K2 sub-cluster was between 6.9 to 771.0 mg/L (mean 134.2 mg/L), and the L cluster between 8.3 to 38.6 mg/L (24.3 mg/L), respectively. Likewise, the high Na + content, followed by the high Clˉ content in several locations, is probably sourced from formation water flowing at hot springs (13S, 31S, and 33S).
The sulfate (SO 4 2-) content of the K1 sub-cluster ranged from 2.2 to 3.6 mg/L (mean 2.5 mg/L), the K2 sub-cluster ranged from 2.1 to 14.2 mg/L (mean 5.4 mg/L) and the L cluster ranged from 2.3 to 14.9 mg/L (mean 5.9 mg/L), respectively. The low SO 4 2content proves that the study area's water source has not been contaminated by anthropogenic factors or mineral dissolution, rich in sulfates. Low-sulfate groundwater is pristine freshwater (Appelo and Postma, 2005).
In September, during the peak dry season, silica (SiO 2 ) in groundwater varied from 34.24 mg/L to 157.07 mg/L. SiO 2 content was found in all clusters, SiO 2 in the sub-cluster K1 was between 52.6 to 128.4 mg/L (mean 103.0 mg/L), in the K2 sub-cluster between 37.6 to 157.1 mg/L (mean 88.9 mg/L), and in cluster L between 34.2 to 85.6 mg/L (mean 54.1 mg/L), respectively. The presence of silica indicates the dissolution of rock constituent minerals, which are usually closely related to geothermal source (Giggenbach, 1981).
The results plotted on a piper diagram yield two dominant water types: Ca-Mg-HCO 3 -Cl and Ca-Mg-HCO 3 . Two locations (31S and 33S) were showing the Na-Cl type (see Figure 8). The sodium chloride types of water (31S and 33S) are likely formed due to groundwater interaction with magmatic rock with minerals containing sodium and chlorine (Mirčovski et al., 2018). Calciummagnesium bicarbonate facies reflects the geology and climate of the study area. Besides, the research area is composed of volcanic rock products, so the groundwater interacts with mineral compounds in the aquifer between the groundwater and the rocks it passes through (Alam et al., 2014). Ion exchange occurs between Na + + K + ions in water and soil Ca 2+ and Mg 2+ during the long water's residence time in the rock (Equation 3, 4, 5, 6 and 7). Several locations show a high Clcontent; this indicates that water has travelled relatively far from its source, thus experiencing enrichment of mineral chloride due to mineral dissolution and ancient salt. In general, the longer the groundwater remains in contact with the aquifer rocks, the greater the material it will take into solution, the chemical evolution of groundwater tends toward seawater's chemical composition (Chebotarev, 1955).

Correlation analysis
Several chemical properties of water are related to each other (Marsal, 1987). Pearson's correlation coefficient is used to measure and find the strength of any linear relationship between two numerical variables. The results of the correlation matrix between 17 chemical water parameters are calculated and presented in Table 4. Pearson's correlation coefficient values range between +1 and -1, where values close to 0.00 do not indicate a linear relationship (Tabachnick and Fidell, 2013). An r value of +1.00 or -1.00 indicates one of perfect predictability when the other is known. Based on Pearson's correlation, the positive correlation is significant and high (r > 0.7). Table 4, a positive, robust, and consistent correlation is shown by temperature and EC with almost all variables from the correlation test result. In addition, a powerful correlation was shown by sodium with potassium, calcium, magnesium and chloride (r = 0.80 -0.98), between potassium and calcium, magnesium, chloride (r = 0.82 -0.94), between calcium and magnesium, bicarbonate, chloride (r = 0.81 -0.94), between magnesium and bicarbonate (r = 0.86), respectively. The powerful and positive correlation between Ca 2+ , Mg 2+ , and Na + is probably a manifestation of the ion exchange process that has taken place during the dissolution of silicate minerals (Equation 3, 4, 5, 6 and 7) (Adams et al., 2001;Zhang et al., 2018). Strong correlation between sodium and carbonate (r = 0.69), potassium with carbonate (r = 0.60), magnesium with chloride (r = 0.78), carbonate with chloride (r = 0.62). The content of silica  (SiO 2 ) has a strong correlation with major ions (r = 0.60 -0.79); this indicates that the dissolution of the minerals that make up the rock produces silica minerals, which are usually closely related to geothermal (Giggenbach, 1981). The correlation between SiO 2 and temperature can be seen in Figure 9. Figure 9 shows an existing correlation between temperature (T) and silica (SiO 2 ); an increase in temperature will also offset the more effective SiO 2 content. This SiO 2 can be used as a reliable geothermometer guide in geothermal investigations (Zhang et al., 2019). The graph shows that springs have a temperature between 41.3 to 51.8°C (7S, 11S, 12S, 13S, 29S, 31S, 33S) with SiO 2 concentrations ranging from 119.4 to 157.1 mg/L. The increase in concentration mainly comes from dissolving rocks containing SiO 2 during groundwater circulation in the rock.

Water stable isotopes
A graph representing the relationship between δ 18 O and δ 2 H regarding the global meteoric water line is presented in Figure 10. The graph shows that the K1 subcluster and K2 sub-cluster are distributed adjacent to the global meteoric water line (GMWL) as described by Craig (1961). The local water meteoric line in the study area refers to Jakarta's closest model (Lubis et al., 2008); the equation used is y = 8x + 10. Cluster L under GMWL shows that it has been heavily influenced by evaporation so that enrichment occurs. Cluster L is heavier than sub-clusters K1 and K2, and this indicates that the groundwater from sub-cluster K1 and sub-cluster K2 has interacted a lot with rocks on its way.
Cluster analysis is based on the dependence of several water quality parameters. This study was carried out with an approach based on the stable isotope content (δ 18 O and δ 2 H) in water (see Figure 6). The use of stable isotopes (δ 18 O and δ 2 H) was considered because they are both durable and do not decay over a geological period (Clark and Fritz, 2013;Liu et al., 2015;Tweed et al., 2019). The dendrogram graph based on the stable isotopes δ 2 H and δ 18 O (see Figure 6) show two groups, namely cluster K 20 samples and L cluster 8 samples. Cluster K includes sub-cluster K1; there are 9 locations (6S, 7S, 8S, 10S, 11S, 12S, 13S, 20S, and 26S). The lightest isotope content of Sub-Cluster K1, maximum δ 18 O and δ 2 H (-6.1 ‰ and -39.6 ‰), minimum (-6.4 ‰ and -41.3 ‰), mean (-6.3 ‰ and -40.5 ‰), respectively. The sample from the springs tends to be lighter so that the isotope content is depleted.

Hydrochemical characteristics
The hydrochemical analysis result showed that groundwater pH ranged from 5.7 to 7.7; the water temperature was between 23.5 and 51.8°C. A water temperature of more than 36.7°C indicates hot springs (Pentecost et al., 2003). There was a water temperature of up to 51.8°C, which is more than 36.7°C, and silica (SiO 2 ) is found as an indication of a geothermal source (Pentecost et al., 2003;Zhang et al., 2019). Throughout the geothermal system, the processes that occur show that the dissolution of the minerals that make up the rock produces silica, which is the most stable mineral under a given set of conditions (Giggenbach, 1981). The hot springs (13S, 31S, and 33S) contain high levels of Na + and Cl -, possibly from formation water that comes out as springs or as seawater trapped in aquifers (Villegas et al., 2018).
Hydrochemistry is influenced by the interaction between water and the medium in which it passes, in this case, rock, changes in water's chemical characteristics depending on the type of rock and the water composition. Water is predominantly bicarbonate (HCO 3 -), and the main cation is Ca 2+ . According to the piper diagram (see Figure 8) of water types are generally Ca-HCO 3 and Mg-HCO 3 . Water facies like this show that groundwater is still relatively pristine, originating from rainwater. The study area is primarily composed of volcanic products, mainly tuff, porphyritic andesite, basaltic andesite, and andesite, rich in plagioclase. The groundwater flow occurs mineral dissolution, resulting in mineral enrichment in aquifers' groundwater (Alam et al., 2014). According to the piper diagram (see Figure 8), the SO 4 2content is relatively low, not more than 14.9 mg/L, proving that groundwater is freshwater that is still original (Appelo and Postma, 2005). The SO 4 2is related to pesticides or fertilizers, besides dissolution minerals as gypsum and seawater related (Villegas et al., 2018).

Groundwater isotope characteristics
The study area's local meteoric water line refers to the nearest meteoric, namely GMWL Jakarta (Lubis et al., 2008), the equation used is y = 8x + 10. Plotting the relationship between δ 2 H and δ 18 O (see Figure 10) has at least two patterns: a) coincide with the GMWL, which is occupied by the K-1 and K-2 sub-clusters; b) the pattern that tends to move away from the GMWL is occupied by the L sub-cluster. The K-1 and K2 sub-clusters almost coincide with the GMWL, indicating that the groundwater is affected by local climate and local topography (Lee et al., 1999).
Although the type of water is relatively the same, dominated by springs, the isotope content of the sub-Cluster K2 is generally heavier than the sub-Cluster K1. The isotope content (δ 2 H and δ 18 O) of the K1 sub-Cluster is more depleted and lighter than others (K2, L1, and L2), indicating that these samples are characterized by water with a longer residence time in the aquifer. Furthermore, the same kind of water is from river water; the sub-Cluster K2 contents isotope is lighter than the subcluster L.

Groundwater interactions
Studying the mechanism of groundwater evolution, Gibbs (1970) provided a simple solution by making a semi-logarithmic graph of the relationship between TDS versus the ratio of Na + / (Na + + Ca 2+ ) and Clˉ / (Clˉ + HCO 3ˉ) . Three mechanisms form the graph: dominance of precipitation, dominance of rock dissolution processes, and evaporation dominance (Gibbs, 1970;Srinivasamoorthy et al., 2014).
The results of plotting data on the Gibb chart (see Figure 11 and Figure 12) through the TDS ratio of Na + / (Na + + Ca 2+ ) and Clˉ / (Clˉ + HCO 3ˉ) the ratio of are generally in the area of rock dissolution dominance and evaporation dominance (Gibbs, 1970;Marandi and Shand, 2018;Srinivasamoorthy et al., 2014). That indicates that there is an area of evaporation from several springs. Also, samples from river water, wells, and cold springs are heavily influenced by dissolution and weathering of the minerals that make up volcanic rocks from tuff and andesite, generally containing feldspar (Alam et al., 2014;Appelo and Postma, 2005).

Conclusions
Hydrochemical analysis has been performed to determine the quality, type, and dominant hydrochemical processes in the study area. The study examined groundwater origin in Rawadanau using a stable isotope approach (δ 2 H and δ 18 O) and hydrochemical analyses. Groundwater quality was assessed by physical properties (pH, EC, temperature) and water chemistry (cations and anions). The pH was found to be between 5.6 to 7.7, except for the nearby hot springs, which are more acidic (pH 5.7 -5.9). There are two sub-clusters of the grouping using stable isotopes: cluster K and cluster L. The isotope content (δ 2 H and δ 18 O) of cluster K is lighter than the L cluster. According to stable isotope data (δ 2 H and δ 18 O), groundwater that fills Rawadanau comes from meteoric water.
Meanwhile, the evaporation dominance and rock dissolution dominance factors have changed the groundwater, as shown in the Gibbs diagram. The effect of evaporation and interaction with a rock when groundwater flows cause depleted isotopes and various water types. The type of groundwater in Rawadanau is influenced by mineral dissolution and mineral content volcanic rocks, and this is in line with the evolutionary sequence of Chebotarev. The rocks are comprised of porphyritic andesite, basaltic andesite, and andesite. The chemical compositions are mainly affected by volcanic dissolution. The enrichment of weathered igneous rock products at peak dry season produces four types of groundwater. The Ca-HCO 3 type dominates river water and cold springs. Meanwhile, hot springs have three types of water: Mg-Ca-HCO 3 -Cl, Ca-Mg-HCO 3 , and Na-Cl.