Indonesian Journal on Geoscience Vol. 12 No. 3 December 2025: 343-365 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: h ps://ijog. ISSN 2355-9314, e-ISSN 2355-9306 Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in the Penguluran Basin. East Java. Indonesia Ferryati Masitoh1. Alfi Nur Rusydi2, and Didik Taryana1 Department of Geography. Universitas Negeri Malang. Jln. Semarang 5. Malang City. Indonesia Department of Information System. Universitas Brawijaya. Jln. Veteran. Malang City. Indonesia Corresponding Author: ferryati. fis@um. Manuscript received: January, 05, 2024. revised: April, 28, 2025. approved: September, 24, 2025. available online: October, 14, 2025 Abstract - Research of the hydrogeochemistry and groundwater quality of shallow groundwater in the Penguluran Basin. East Java. Indonesia, is still very limited. This study aims to identify hydrogeochemistry and groundwater quality in the shallow groundwater of the Penguluran Basin. Twelve water samples were taken from the residents' wells in July during the dry season. Groundwater samples were analyzed in the laboratory to determine the concentration of major ions. The major ions, including Mg2 . Na . K . Ca and anion CO32-. HCO3-. SO42-. Cl-. Laboratory results were analyzed using Piper Trilinear Diagram and Gibbs Diagram. Weathering type. Sodium Adsorption Ratio (SAR). Soluble Sodium Percentage (SSP). Residual Sodium Carbonate (RSC). Permeable Index (PI). Magnesium Hazard (MH). Chloro-Alkaline Indices (CAI). Corrosivity Ratio (CR), and Anthropogenic Impact (AI) using NO3-. The results showed that hydrogeochemical facies in the studied area were of Ca - Mg2 - HCO3- type. Groundwater cations were dominated by Ca , while anions were dominated by HCO3-. The concentration of cations were Ca > Mg2 > Na > K , while the anions were HCO3- > Cl- > SO42- > CO32-. Groundwater in Penguluran Basin was freshwater with silicate weathering type. Analysis of major groundwater ions for agricultural irrigation purposes showed that most groundwater samples were safe for agricultural irrigation. CAI-I and CAI-II had mostly negative values. Samples showing negative CAI values also showed silica weathering. CR values were mostly <1 indicated that naturally groundwater was safe from corrosive vulnerability for industry purposes. Nitrate levels in the groundwater showed that 58 % exceeded the allowable limit due to the high risk of anthropogenic impacts to groundwater. The research is expected to provide new information about groundwater in the Penguluran Basin. Keywords: hydrogeochemistry, groundwater quality, shallow groundwater. Penguluran Basin A IJOG - 2025 How to cite this article: Masitoh. Rusydi. , and Taryana. , 2025. Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in the Penguluran Basin. East Java. Indonesia. IndoAnesian Journal on GeoAscience, 12 . , p. DOI: 10. 17014/ijog. Introduction Background The Java Island has a large number of people population in Indonesia (BPS, 2. , so the groundwater is more needed than in other islands. The highest groundwater needs are in big cities and industrial cities (Marganingrum, 2018. Taufiq et al. , 2018. Bremard, 2. However, groundwater information had not yet been evenly distributed in various regions in Java, including Penguluran Basin. South Malang, that were in East Java Province. Indonesia. Until now, there are still limited research about groundwater conditions in the basin (Juwono et al. , 2. especially regarding the groundwater quality. The Penguluran Basin is primarily used for agricultural land, including forestry, sugarcane Indexed by: SCOPUS PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 plantations, paddy fields, mixed gardens, and agroforestry (Figure 1. Intensive agricultural areas like paddy fields are only found near the estuary, irrigated from the Penguluran River. Other agricultural lands utilize rainwater or dug wells for groundwater. For nonagricultural purposes, residents solely rely on shallow groundwater to meet their needs. Residents create traditional shallow dug wells close to their homes or agricultural lands (Figure 1. The groundwater is then pumped and distributed through a pipe network, and used by several households. Groundwater is the main source for fulfilling water needs in The Penguluran Basin. The increasing population in this area requires a large amount of water (Arifianto, 2016. BPS, 2. as well as varying geological conditions, which necessitate a comprehensive study, especially on its groundwater quality (Heru and Prasetia. Rositha et al. , 2. Groundwater quality encompasses hydrogeochemical facies and groundwater quality in Penguluran Basin (Poetra et al. , 2. This study aims to identify hydrogeochemical facies to assess the groundwater quality in Penguluran Basin. This research is expected to provide comprehensive information of groundwater quality in Penguluran Basin. This research provides novelty in the form of hydrogeochemical and shallow groundwater quality analyses in Penguluran Basin. To date, research in Penguluran Basin has been more related to disaster conditions and its lithological conditions (Arifianto, 2016. Yuwanto and Ridwan, 2017. Utama et al. , 2020. Heru and Prasetia, 2. Thus, this study offers a comprehensive overview of the hydrogeochemical conditions and water quality, particularly for its shallow groundwater. Literature Review Geological conditions greatly determine the groundwater quality (Sefie et al. , 2. The groundwater quality in the mountains was different compared to that in plain areas (Gibrilla et al. Hydrogeochemistry is an analysis of the interaction between groundwater and its environmental conditions, therefore it can distinguish the groundwater quality (Sefie et al. , 2015. Egbueri et al. , 2021. Yuan et al. , 2. The hydrogeochemistry of groundwater had been widely researched to determine the groundwater quality for a lot of purposes (Ghalib, 2017. Asadi et al. , 2. Research of groundwater hydrogeochemistry, plays an important role in the identification process of groundwater for agricultural irrigation (Ghalib, 2017. Vasilache et al. , 2. , groundwater pollution, and its risks to human health (Zhang et al. , 2. , and the identification of geological processes and structures that affect groundwater (Li et al. , 2015. Bouderbala, 2. Hydrochemical groundwater conditions are shown by several chemical groundwater parameters which included pH, electrical conductivity (EC), salinity, major anions, and cations (Ghalib, 2. The hydrochemistry of groundwater is influenced by many factors. The water movement in the water cycle also affects groundwater, which includes precipitation. Figure 1. Mixed garden near sugarcane plantation, and . Traditional shallow dug well in Sumberagung Village. PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. evaporation, cation exchange, mineral disassociation in geological structures, and water-pollutants mixed with human activities, agriculture, and various biological activities (Appelo and Postma. Hydrogeochemical facies were related to the leaching and rocks weathering process (Xu and Su, 2. Hydrogeochemichal facies were a classification of major chemical compositions presented in Piper. Durov. Kurlov. Gibbs, and Schoeller Diagram. Those diagram would show the hydrogeochemical group of groundwater facies based on the dominant element of the groundwater chemical major compositions (Jankowski, 2. Previous studies have extensively focused on hydrogeochemical facies, using various diagrams to classify groundwater quality such as Piper. Durov (Razi et al. , 2. and Gibbs (Poetra et al. Razi et al. , 2024. Suhendar et al. , 2. Piper diagrams are most commonly used to examine hydrogeochemical facies and to explain the interactions between water and rocks naturally (Razi et al. , 2. Hydrogeochemical research using radioisotopes has also been used in previous researches, for example in structural areas . ault and fold syste. (Suhendar et al. , 2. and karst (Setiawan et al. , 2. However, overall, the study did not explain water quality specifically such as Sodium Adsorption Ratio (SAR). Soluble Sodium Percentage (SSP). Residual Sodium Carbonate (RSC). Permeable Index (PI). Magnesium Hazard (MH). Chloro-Alkaline Indices (CAI). Corrosivity Ratio (CR), and Anthropogenic Impact (AI). All of these groundwater quality parameters will be appropriate for use in areas dominated by agriculture (Hwang et al. , 2. This study offers a comprehensive explanation of groundwater conditions, particularly focusing on hydrogeochemical facies and groundwater quality within The Penguluran Basin. Geological and Hydrogeological Setting Penguluran Basin is in Malang Regency. East Java. Indonesia. This area is about 16,802 km2, located at elevation of 0 Ae 715 m from the mean sea level. This region belongs to the southern mountainous zone, characterized by its Tertiary volcanic origins (Bemmelen, 1. The Penguluran Basin has six geological formations: Mandalika (Tom. Nampol (Tm. Wuni (Tm. Wonosari (Tmw. Tuff Mandalika (Tom. , and Swamps and Rivers Sediment (Qa. (Geological Agency of Indonesia, 2. (Figure . The Mandalika Formation (Tom. consists of volcanic geological materials including andesite, basaltic, trachyitic, dacitic lava, and propylitized andesitic breccia. These rocks have undergone extensive alterations (Yuwanto and Ridwan, 2017. Heru and Prasetia, 2. The Nampol Formation (Tm. is dominated by clastic sedimentary material and tuff sandstone. Its rocks consist of tuffaceous or calcareous sandstone, black claystone, and sandy marl. Iron oxides and gypsum sheets are sometimes also present. This formation provides good groundwater aquifer conditions (Sukadana and Indrastomo, 2. The Wuni Formation (Tm. consists of andesitebasalt breccia and lava, tuff breccia, lava breccia, and sandy tuff. The Wonosari Formation (Tmw. consist of limestone, calcareous sandstone, sandy marl, and claystone intercalations (Juwono et al. The Mandalika Tuff consists of andesiticrhyolitic-dacitic tuff, and pumiceous tuff breccia. The Mandalika Tuff (Tom. and Mandalika Formation underly the Wonosari Formation, while the swamps and river deposits units are located along the downstream to the Penguluran Basin estuaries (Poespowardoyo, 1. The swamp and river sediment (Qa. formation consists of gravel, sand, clay, and plant remains. The groundwater conditions of each geological formation were different due to the interaction between geological conditions and the surrounding environment. All geological formations were formed on the tertiary lithological layer, except swamp and river deposits which formed during the quaternary period. Based on The Indonesian Hydrogeological Maps. Penguluran aquifers had medium to high productivity, but limited for aquifers formed by cracks, gaps, fracturing, and ducts. The area is dominated by rocks of bedded reef limestone with varying degrees of karstification by hitchhiking over other geological formations. Groundwater potential was small and scarce in the PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 Figure 2. Penguluran geological map and groundwater sample distribution. aquifers of northern and central basins. Aquifers were dominated by andesite lava flows. Local aquifer which had moderate productivity was in a small percentage area of the Penguluran Basin (Poespowardoyo, 1. Naturally, groundwater quality can be influenced by the interaction between rocks and groundwater . , or the input of other substances, for instance due to biological activity and pollution (Gonzales Amaya et al. Yuan et al. , 2022. Sun and Liu, 2. The interaction process between groundwater begins with rock weathering. Rock weathering processes can occur physically, chemically, and biologically. Minerals contained within weathered rocks then undergo a dissolution process (Zamroni et , 2. In limestone, weathering leads to dissolution, which adds Ca. MgAA. SOCEAA. HCOCEA, and ClA ions into groundwater (Mujib et al. , 2024. Soro et al. , 2. The type of rock-forming mineral and its weathering conditions would de- termine the magnitude of the ions dissolved in the water (Gibrilla et al. , 2010. Hussien and Faiyad. Groundwater in the shallow aquifer basin was freshwater. Freshwater occurred due to the weathering process which had the characteristics of high HCO3- and low Cl- (Zhi et al. , 2. Groundwater with weathering process also had the characteristics of Na / HCO3- (<. Mg2 / Ca2 (<. , and Ca2 / Cl- (>0. Identification of the weathering type could be known through HCOCEA SOCEAA and Ca MgAA (Kaur et al. , 2. Geomorphological Setting Figure 3 shows the landform map of Penguluran Basin. The studied area has five landform units: River Valley (Ls-V). Denudational Hills . D-Ibl. Denudational Mountains . D-Ibl. Karst Solutional Plain . k-IIs. , and Karst Solutional Hills . k-IIs. The River Valley unit is influenced by the Penguluran River System, which has alluvial river deposits. The Denu- PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. Figure 3. Landform map of Penguluran Basin. dational Hill Unit . D-Ibl. and Denudational Mountain Unit . D-Ibl. are areas that experience intensive erosion and weathering. Geologically, these areas feature andesitic basaltic volcanic The Karst Solutional Units in the studied area consist of Karst Solutional Plain . k-IIs. and Karst Solutional Hills . k-IIs. Both of these areas have karst landscapes, but possess different topographical conditions. Methods and Materials Groundwater Sampling and Laboratory Analyses Groundwater sampling considered local hydrogeological conditions. The number of groundwater samples taken were twelve samples located around residential areas. Groundwater samples were tested in-situ and through laboratory analyses. Samples were taken in July 2022, in the dry season on a sunny day. In-situ groundwater samples were tested using a Multi-parameter Water Quality Checker for twelve traditional shallow dug wells. In-situ testing included type of water body . ell and sprin. Electrical Conductivity (EC), and Turbidity. Water samples were stored in glass bottles equipped with secure lids. The quantity of water samples was determined based on the distribution of geological formations (Figure . For wells with pumps, water was pumped out and its quality measured while flowing. For wells without pumps, water was collected using a bucket, and then its quality was measured. After in-situ measurements, the water was subsequently stored in sample bottles. Groundwater samples were also analyzed in the laboratory to obtain major chemical components that included cation PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 Mg2 . Na . K . Ca2 , and anion CO32- . HCO3-. SO42- . Cl-. Charge Balance Error (CBE) was used to analyze standard errors from laboratory analysis. The ideal CBE is close to 0 (Li et al. , 2. The CBE value used Equation 1. If the CBE value was A 5 %, then the results of laboratory analysis were acceptable/reliable (Poetra et al. , 2020. Tiwari et al. , 2. CBE values were calculated using cation and anion levels in meq/L. CBE values of each sample varied between -0. 1 and 0. The CBE value of all samples is 0. 12 which indicates that all samples taken are reliable (Table . Data Analysis Laboratory results of groundwater samples were analyzed based on major ions (Gibrilla et al. , 2. Water quality in this study also included water quality analysis for agricultural irrigation. Chloro-Alkaline Indices (CAI). Corrosivity Ratio (CR), and Anthropogenic Impact to Water quality for irrigation included Sodium Adsorption Ratio (SAR). Soluble Sodium Percentage (SSP). Residual Sodium Carbonate (RSC). Permeable Index (PI). Magnesium Hazard (MH) (Hwang et al. , 2. ACAI was commonly used by many studies to determine the degree of ion exchange reactions between groundwater and aquifer minerals (Schoeller, 1977. Li et al. Singh et al. , 2. CR was used to determine the groundwater susceptibility to corrosive risks based on its major ions (Rout and Setia. Ram et al. , 2. Anthropogenic impact to groundwater used proper nitrate levels for the identification of groundwater pollution (Re et al. Zhang et al. , 2. Analysis of groundwater chemistry data used several parameters . ee equation 2 . , and the Piper Trilinear Diagram and Gibbs Diagram. The Piper Trilinear Diagram was very well used for determination of hydrogeochemical groundwater facies (Li et al. , 2015. Zhi et al. , 2. and physicochemical analyzes (Vasilache et al. Gibbs Diagram was used to determine the rock weathering rates (Hwang et al. , 2. The composition of groundwater would provide information of the interaction between groundwater and rocks, aquifer conditions, rock weathering, and groundwater recharge quality (Wilopo et , 2. Table 1. Major Ion Concentration (K . Na . Mg2 . Ca2 . HCO3-. SO42-. Cl-) Location Kation . Anion . CBE HCO3 SO4 <0. < 0. < 0. Minimum Maksimum Mean St. Dev. Note: < below detection limit PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. Chloro-Alkaline Indices (CAI) were used to determine the degree of ion exchange reactions between groundwater and aquifer minerals (Schoeller, 1. The value of CAI was determined through Equations 2 and 3. The CAI index would be positive if there were Na and K ions exchanged by Mg2 and Ca2 . Positive value of CAI indicated a direct reaction of alkaline exchange, or Chloro-Alkali Equilibrium. The negative CAI index showed the exchange reaction in reverse order and was in indirect process. This condition was called Chloro-Alkali Disequilibrium (Hwang et al. Tiwari et al. , 2. Laboratory tests were also using Corrosivity Ratio (CR) that could be used to identify groundwater susceptibility to corrosion. CR was needed in this study, because groundwater was widely used for distribution to houses. CR will ensure that the water is safe enough if it passes through metal pipes. Determination of CR values used ion Cl-. SO42-. HCO3-, and CO32-. values could be calculated using Equation 2 in mg/l units for each parameter (Kouser et al. , 2. The research also analyzed the suitability of groundwater if it was used as agricultural irrigation The used parameters included Sodium Adsorption Ratio (SAR). Soluble Sodium Percentage (SSP). Residual Sodium Carbonate (RSC). Permeable Index (PI). Magnesium Hazard (MH) (Hwang et al. , 2017. Kouser et al. , 2022. Tiwari et al. , 2. SAR was used to analyze water for agricultural purposes. SAR was a relative measure of sodium to calcium and magnesium in groundwater (Hwang et al. , 2017. Sridharan and Senthil Nathan, 2. SAR values were classified into Excellent . Good . Permissible . Doubtful/Poor (>. , based on Equation 5. Soluble Sodium Percentage (SSP) was related to the percentage of Na that could react with the soil. Na exceeding 60 % would degrade the physical farmland condition, so it was not suitable if it was used for agricultural irrigation purpose (Eyankware et al. , 2020. Li et al. , 2. The SSP in groundwater could be known using Equation 6. SRC was more concerned with levels of CO32and HCO3- to Ca2 and Mg2 in groundwater. high RSC value would damage the soil property (Sridharan and Senthil Nathan, 2. SRC was divided into three classes which are Good (<1. Medium . 25 Ae 2. , and Unsuitable/Bad (>2. A high SRC indicated that groundwater could not be used for irrigation (Hwang et al. , 2. Determination of the SRC value used Equation 7. Permeability Index (PI) related to the suitability of groundwater for irrigation based on HCO3- . Ca2 . Mg2 and Na in the long-term used (Eyankware et al. , 2. PI used the Equation 8. PI value was classified into three classifications. Class I is Excellent (PI>75%). Class II is Good . % 50 %). Mg2 levels in groundwater would have an impact in increasing soil alkalinity. These conditions could reduce agricultural yields (Hwang et al. , 2. PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 Result and Analysis Groundwater Quality Groundwater samples were taken from traditional shallow dug wells owned by residents. The residents' wells have the average depth of less than 15 m. Table 2 shows the type, colour. EC. DO, and turbidity values of groundwater samples. Most of the groundwater colour was clear, but for Sample 10, the groundwater colour was brownish The smallest EC value was 132 AAS/cm for Sample 4, and the highest one was for Sample All samples had EC values below 1000 AA/cm, indicating very weakly mineralized water (Detay and Carpenter, 1. Low EC values occurred due to low water-rock interaction, both in the form of mineral dissolution process and evaporation rates (Xu and Su, 2. The interaction between rocks and groundwater could also be known by the salinity of groundwater (Table . The entire samples had low salinity, so it is categorized as fresh water. Granitic igneous rocks contribute to low mineral concentrations, thus reducing water salinity. Salinity will also increase if the aquifer contains clay (Salami and Akperi, 2. Sampling locations far from the sea also minimize the interaction of seawater and In general, the Na . Cl- concentration, and EC will increase if groundwater is located close to seawater (Hussien and Faiyad, 2. The water turbidity indicated the presence of material that scatters a certain amount of light in the water (Jankowski, 2. A low turbidity value indicated that the water was clear. Sample 6 was the lowest turbidity value, while Sample 10 was the highest turbidity of 331 NTU. Based on Indonesian Government Regulations, the safe turbidity value for drinking water is less than 5 NTU. The study showed that only 6 samples had the turbidity below the maximum allowable limit. Table 1 and Figure 4 present the concentration of major ions in the Penguluran Basin. The concentration of Ca2 had a range between 1. Ae 6. 71 meq/l. Sample 5 had the highest concentration of Ca2 , while Sample 4 had the lowest one. The concentration of Mg2 had the range of 0. 52 meq/l, with the highest level was for Sample 5, while the lowest one was for Sample 2. The lowest Na concentration was <0. 012 meq/l for Samples 4 and 6, while Na was the highest one for Sample 9. The lowest K concentration was <0. 112 meq/l, while the highest one was 1. meq/l for Sample 10. HCO3- concentration had the lowest value of 0. 944 meq/l for Sample 1 and the highest one was 5. 710 meq/l for Sample 5. The range of Cl- concentration was from 0. 112 up to 784 meq/l. The lowest Cl- concentration was Sample 2, while highest one was Sample 9. The concentration of SO42- was in the range of 0. - 1. 722 meq/l. The lowest SO42- concentration was Sample 2, while highest one was Sample 5. Figure 5 shows the flownet map of the researched area. The flownet map includes groundwater elevation information, displayed as Table 2. Type. Colour. EC, pH. DO, and Turbidity Values of Samples ID Samples Location Type Color EC (AAS/c. Salinity (%) Ringinkembar Well Clear Tegalrejo Well Clear Sekarbanyu Well Clear Argotirto Well Clear Harjokuncaran 1 Well Clear Harjokuncaran 2 Well Clear Sumberagung Well Clear Sitiarjo 1 Well Clear Sitiarjo 2 Well Clear Sitiarjo 3 Well Turbid Kedungbanteng 1 Well Clear Kedungbanteng 2 Well Clear PUBLISHED IN IJOG Turbidity (NTU) Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. Figure 4. Major ion concentration plotting graph. Figure 5. Flownet map. PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 equipotential lines. Equipotential lines consider the depth of the well water level and its topography (Korkmaz, 2. Flownets also provide flowline information, which indicates the direction of groundwater flow. The flowlines provide information on both the source and flow direction. A previous research has shown that flowlines can also be used to analyze the source and direction of groundwater pollution, heat transfer, and groundwater well vulnerability (Tyth et al. Based on Figure 4, the groundwater flow direction is towards the southeast, which has a lower topographical contour. Hydrogeochemical Facies Hydrogeochemical Facies could be determined by the value of major ions (Leyn et al. Ruiz-Pico et al. , 2. Table 3 shows the hydrogeochemical facies type of groundwater in the studied area. Dissolved cations included K . Na . Mg2 . Ca2 . Such cations would contribute alkalinity properties into groundwater compared to bicarbonate and carbonate (Jankowski, 2. Anions included CO3-. HCO3-. SO42-. Cl-. The ions composition of groundwater naturally occurred because of the dissolution process in the groundwater (Dogramaci et al. , 2. Hydrogeochemical facies were known through the Piper Trilinear Diagram (Hussien and Faiyad. Ruiz-Pico et al. , 2019. Wilopo et al. , 2. Piper Trilinear Diagrams were built using diagram software (Figure . Overall, hydrogeochemical facies included Ca-Mg-HCO3 for groundwater samples, except for Sample 1 in the studied site (Table . Sample 1 was mixed type which had the same formation with Sample 2 in The Nampol Formation. However. Sample 2 was Ca-Mg-HCO3 The HCO3- value for Sample 2 was higher than Sample 1. Nampol Formation contains calcareous sandstone (Sukadana and Indrastomo. The dissolving process of calcareous sandstone increased the level of HCO3- in groundwater (Jankowski, 2. The high Ca-Mg-HCO3 was formed due to the presence of bedded reef limestone with varying karstification in the studied area Figure 6. Piper trilinear diagram. Table 3. Hydrogeochemical Facies Location Hydrogeochemical Facies Kation Type Anion Type Ringinkembar Mixed type No Dominant Type No Dominant Type Tegalrejo Ca2 -Mg2 -HCO3- Calcium Type Bicarbonate Type Sekarbanyu Mixed type No Dominant Type Bicarbonate Type Argotirto Ca -Mg -HCO3 Calcium Type Bicarbonate Type Harjokuncaran 1 Ca -Mg -HCO3 Calcium Type Bicarbonate Type Harjokuncaran 2 Ca2 -Mg2 -HCO3- Calcium Type Bicarbonate Type Sumberagung Ca -Mg -HCO3 Calcium Type Bicarbonate Type Sitiarjo 1 Ca -Mg -HCO3 Calcium Type Bicarbonate Type Sitiarjo 2 Ca2 -Mg2 -HCO3- Calcium Type Bicarbonate Type Sitiarjo 3 Mixed type No Dominant Type Bicarbonate Type Kedungbanteng 1 Ca -Mg -HCO3 Calcium Type Bicarbonate Type Kedungbanteng 2 Ca2 -Mg2 -HCO3- Calcium Type Bicarbonate Type PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. (Poespowardoyo, 1. High concentrations of Ca-Mg-HCOCE can also occur in volcanic rocks, as shown in studies following the La Palma volcanic eruption and in the Malabar Tilu deposits (Jimynez et al. , 2024. Maria et al. , 2. Enrichment of Ca-MgAA typically occurs in shallow groundwater (Maria et al. , 2021. Salami and Akperi, 2. The process of water acidification due to interaction with rocks containing CaMgAA will also increase the levels of Ca-MgAA in groundwater (Jimynez-Valera et al. , 2. A previous research states that Ca weathering in silica-rich minerals originating from basaltic andesitic volcanic rocks will also increase Ca (Maria et al. , 2. Shallow groundwater in Merapi Volcano also has groundwater rich in Ca-MgAA (Hendrayana et al. , 2. This aligns with the conditions of the studied area, which is dominated by andesitic rocks, basaltic lava, and tuff (Bemmelen, 1. , as well as in its shallow Figure 7 shows the orders of cations (Figure 7. and anions (Figure 7. in the groundwater of Penguluran Basin. The figure shows that Ca2 is the dominant cation, while HCO3- is the domi- nant anion. The cation concentration was Ca2 > Mg2 > Na > K, while the anion was HCO3- > Cl- > SO42- . Cation levels are shown as Ca-Mg-(Na K) in the cation triangle, where about 75 % of the samples were dominated by the calcium type. The remaining percentage of those samples belonged to the no dominant type, specifically Samples 1, 3, and 10. Samples 1 and 3 were located in The Nampol Formation (Tm. , and Sample 10 was in The Wonosari Formation (Tmw. The highest Ca concentration was 6,707 meq/l for Sample 9, while the lowest was 1,118 meq/l for Sample 4. Sample 9 was found in the limestone formation. Sample 4 was in the tuff member of The Mandalika Formation (Tom. , which consists of tuff breccia sediment with low calcium levels. The highest sodium in The Penguluran Basin of 0. meq/l was for Sample 9 in Tmwl Formation. The lowest sodium was for Sample 4 in Tomt Formation. The highest potassium value of 1,476 meq/l was for Sample 10 in the Tmwl Formation. The lowest potassium value was for Sample 9 in the Tuff Formation. Based on the cation triangle, the groundwater is classified as alkaline water. Figure 7. Cation and . anion in groundwater of Penguluran Basin. PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 characterized by high NaA KA and low calcium content, along with low water hardness (Hwang et al. , 2. Anion levels are shown by HCO3- Cl-SO4 in the anion triangle. In the anion triangle, 91 % of groundwater samples were dominated by bicarbonate type, and 9 % were no dominant HCO3- in groundwater would impact to groundwater to be alkaline. This condition is consistent with the cation triangle tendency towards Alkalinity in natural groundwater was produced by the dissolution of carbon dioxide, bicarbonate, and carbonate (Jankowski, 2. Alkaline groundwater could be formed by fractions of CO2 gas in the atmosphere, atmospheric gases in the soil, and groundwater. Other sources could come from sulphate reduction and carbonate rock metamorphism (Weight, 2. , microbial respiration (Eslami et al. , 2. , and silica weathering in clays that released carbonate and bicarbonate ions (Ruiz-Pico et al. , 2. The lowest alkalinity in bicarbonate form of 0. meq/l was for Sample 1, while the highest one of 5,710 meq/l was for Sample 5 in Nampol Formation. High HCO3- indicated that groundwater was rich in dissolved CO2 (Yan et al. , 2. The value of chloride in groundwater was generally lower than in other major constituents. Sample 9 had the highest chloride value of 0. 784 meq/l, which was in Wonosari Formation. Sample 2 had the lowest chloride content of 0. 112 meq/l, which was in Nampol Formation consisting of tuffaceous The highest sulphate of 1,722 meq/l was for Sample 5, while the lowest sulphate of 005 meq/l was for Sample 2. Samples 5 and 2 were in Nampol Formation. However. Sample 5 is located closer to Mandalika Formation, which consists of andesitic-basaltic rocks (Heru and Prasetia, 2. Weathering Types Naturally, groundwater quality results from the interaction between chemical elements in rocks and groundwater. One method to identify this process is through analysis using a Gibbs Diagram. Figure 8 presents the Gibbs Diagram, which is utilized to identify the processes controlling the hydrogeochemistry of groundwater. The diagram also provides information on Total Dissolved Solids (TDS), which represents the amount of dissolved solids in groundwater (Gao et al. , 2. All groundwater samples had a TDS value between 142. 60 and 608. 90 mg/l. The Gibbs Diagram further shows that all groundwater samples fall within the weathering dominance group. This indicates that the groundwater chemistry is primarily influenced by rock weathering processes . ock dominanc. (Razi et al. , 2. Figure 9 is a weathering scatter diagram of rocks with a 1:1 dash line in scale (Kaur et Figure 8. Gibbs diagram. PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. breccia (Yuwanto and Ridwan, 2017. Heru and Prasetia, 2. , clastic sedimentary material and tuffaceous sandstone (Sukadana and Indrastomo. Mandalika Formation is the most widespread formation in the studied area. The formation contains silica varying from moderate to high (Heru and Prasetia, 2. This sufficient content proves the dominant silica weathering process in the studied area. Figure 9. Plot of HCO3 SO4 vs Ca Mg2 (Modified from (Kaur et al. , 2. , 2. This diagram is used to analyze the weathering process through the identification of HCOCEA SOCEAA versus Ca MgAA concentrations. The graph in Figure 9 shows two weathering groups: carbonate weathering . bove the red lin. and silica weathering . elow the red Groundwater samples falling into the carbonate weathering group are Samples 1 and Sample 1 is located in The Wuni Formation, which consists of basaltic, andesitic, and tuff However. Sample 1 is situated close to the red boundary line. Sample 1 is also adjacent to Sample 4, which was collected from the tuff member of Mandalika Formation. Both of these samples show relatively low ion concentrations. Low ion concentrations can occur in areas with less intensive weathering or shorter water-rock contact times. Sample 9 is located in Wonosari Formation (Tmw. , which consists of limestone, calcareous sandstone, sandy marl, and claystone intercalations (Juwono et al. , 2. This indicates that the groundwater is intensively dissolving calcium carbonate and magnesium carbonate minerals. Most of the groundwater samples fall within the silica weathering process. Samples belonging to this group include: 2, 3, 4, 5, 6, 7, 8, 10, 11. The high number of samples categorized under silica weathering aligns with the geological conditions in the studied area, which are dominated by old volcanic rock deposits and volcaniclastic sedimentary rocks (Bemmelen, 1. These rocks include andesite, basaltic, trachyitic, and dacitic lava, as well as propylitized andesitic Chloro-Alkaline Indices (CAI) Chloro-Alkaline Indices (CAI) I-II were calculated using Equations 2 and 3 (Schoeller. The calculation of CAI-I and II resulted that 83 % had negative values. Most samples reflected the same cation exchange (Figure . This study is similar to the research by Yan et al. which also had negative CAI-I and CAI-II values in the areas consisted of dolomite, gypsum, and calcite rocks. A negative CAI value described that there was an exchange of Ca2 and Mg2 ions from water with Na and K from rocks. CAI-I values was from -5,902 to -0. CAI-II had range values of -0. 221 to -0. Such conditions would increase Ca and Mg in groundwater (Yan et al. , 2. CAI-I and CAI-II were positive for about 17 % of the total sample. A positive CAI value illustrates that there was a directly exchange of Na and K ions from water with Ca2 and Mg2 from rocks (Singh et al. , 2. CAI-I had range 026 and 0. 486, while CAI-II was between 005 and 0. Samples 1 and 9 had CAI-I and Figure 10. CAI-I vs CAI-II. PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 CAI-II in positive values. Sample 9 had a lower K value and was dominated by Na . About 83 % of the samples showed silicate weathering, and had negative CAI-I and CAI-II values. A CAI value dominated by negative values indicated that the cation exchange determined the chemical composition of groundwater (Hartanto and Lubis. Singh et al. , 2. Corosivity Ratio Corrosivity Ratio (CR) is a vulnerability level indicator of groundwater to the corrosion risk (Rout and Setia, 2. Groundwater is used for various purposes. In water distribution systems for water supply, water with corrosive properties will easily damage pipes, especially metallic pipes, leading to leaks (Siddha and Sahu, 2. CR of <1 indicated that the groundwater was safe, while if it was >1, the groundwater was corrosive (Kaur et al. , 2. The CR value was calculated using Equation 2. About 92 % groundwater in the studied area was corrosive-safe, while that remaining was unsafe. The CR value indicated corrosive-safe groundwater was between 0. 469 (Figure . Hydrogeochemical facies in groundwater showed that all samples belonged to the bicarbonate anion type with low CR values. Based on Piper Trilinear Diagram, only Sample 1 was the non-dominant type which had unsafe CR value of 1,209. This indicates that for Sample 1, the groundwater is unsafe for piping networks that use metallic pipes (Hwang et al. , 2017. Siddha and Sahu, 2. Figure 11. Corrosivity ratio on each groundwater samples. Groundwater Suitability for Irrigation Purposes The identifications of groundwater suitability for agriculture in this study were SAR. SSP. RSC. PI, and MH. The five indices show the interaction between groundwater and soil. Such interactions could have an impact on soil and plants. SAR, SSP, and RSC considered Na in groundwater. High Na and Mg2 in groundwater would harm soil property. Sodium Absorption Ratio (SAR) SAR is a relative measure of sodium to calcium and magnesium in groundwater (Hwang et , 2017. Sridharan and Senthil Nathan, 2. SAR showed alkalinity hazard on groundwater, which could interfere to plant growth (Jalil et , 2. The SAR value was calculated based on Equation 5. The calculation results indicated that 50 % were the Poor class, 33 % were Good, and 17 % were Excellent if groundwater was used for agriculture (Table . Areas which had poor class of SAR values were located in the northern and southern sides of the basin. Figure 12 is a SAR vs EC data plot using US Salinity Laboratory (USSL) Diagram. The classification results showed that 25 % of the samples were C1S1, 50 % were C1S2, and 25 % were C1S3. Overall, the sample had low Na levels with varying salinity. In C1S3, groundwater conditions were salty, with low Na , but it could be used with certain Soluble Sodium Percentage (SSP) SSP (% N. was also used to determine the groundwater suitability in agricultural irrigation SSP is a sodium hazard as percentage of Na to other ions. If the SSP > 60 % resulted soil aggregate dispersion, it would harm to plants and soil (Jalil et al. , 2. SSP classification was based on Equation 6. The result of SSP Classification were 33 % Excellent, 42 % were Good, and 25 % were Fair (Table . Classification of SSPs on Wilcox Diagrams through SSP and EC plots used DIAGRAMMES software (Figure12. The SSP vs EC plot results in Figure 12, show that PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. Table 4. Groundwater Classifications by SAR. SSP. RSC. PI. Indicator Value Suitability % Samples Number of samples SAR >26 <20 >80 <1. >2. >75% 25 75% <25% <50% >50 Excellent Good Permissible Poor/Doubtful Excellent Good Fair Poor Safe/Good Medium/Marginal Unsuitable Excellent Good Unsuitable Suitable Unsuitable SSP (% N. RSC Figure 12. SAR and SSP (% N. most groundwater samples were in Excellent and Good classes. This indicated that the most samples were in safe condition if they were used for agricultural irrigation purposes. Residual Sodium Carbonate (RSC) RSC indicated bicarbonate hazard to groundwater (Jalil et al. , 2. RSC considered bicarbonate and carbonate levels against calcium and magnesium. Identification of the RSC was important, because the studied area contains limestone located near its surface. RSC determination used Equation 7, and was classified using Table 4. The calculation results showed that 45 % of the samples had negative RSC values. The value indicated that the amount of calcium and magnesium were greater than carbonate and bicarbonate. The RSC values in the studied area were classified into 92 % . afe/goo. and 8 % medium/marginal. Based on the class of RSC, generally groundwater was quite safe for irrigation purposes. PUBLISHED IN IJOG Indonesian Journal on Geoscience. Vol. 12 No. 3 December 2025: 343-365 Permeability Index (PI) PI indicated the effect of groundwater to soil PI was calculated based on exposure to sodium, calcium, magnesium, and bicarbonate in groundwater (Eyankware et al. , 2. A high PI value indicated good permeability, so it could supply groundwater, but it was also susceptible to pollution (Mansouri et al. , 2. Table 4 shows the PI values were classified into 83 % Good, and 17 % Unsuitable. The unsuitable PI were for Samples 5 and 7. Magnesium Hazard (MH) Mg2 levels increased if they occurred due to Na in irrigated soils. Increased magnesium in the soil caused soil sodication, which ruined the clay In these conditions, the relative value of hydraulic conductivity would decrease, because Mg2 in the soil behaved like Na (Hwang et al. If the MH value in groundwater was <50 %, then groundwater was safe to be used for agriculture purposes (Jalil et al. , 2. Overall groundwater samples in the basin had a low MH, so they were suitable for irrigation (Table . Anthropogenic Impact Hydrogeochemical conditions of aquifers were also influenced by human activity (Appelo and Postma, 1993. Hussein et al. , 2019. Sun and Liu, 2. Shallow aquifer had high susceptibility of anthropogenic activity (Ekere et al. , 2. The anthropogenic impact on groundwater could be observed through EC and potassium concentration (Ruiz-Pico et al. If EC and potassium levels were low, the anthropological impact on groundwater was also in small intensity. Ca2 Mg2 vs SO4 HCO3concentrations could also be used to determine the anthropogenic impact on groundwater (Yan et , 2. Figure 13 shows that the slope of the linear fitting equation for Ca MgAA vs SOCEAA HCOCEA is RA = 0. This value indicates that the alkaline nature of the groundwater is closely related to natural weathering. Groundwater samples related to anthropogenic impact are Samples 1 and 9. Consider- Figure 13. Ca2 Mg vs SO4 HCO3 (Modified from (Yan et al. , 2. ing the hydrogeochemical processes. Sample 1 falls into the mixed type category. In Figure 13. Sample 1 is close to the 1:1 line. Thus. Sample 1 shows the presence of anthropogenic impact factors, but still retains cation exchange properties. It is different from Sample 9, which falls into the anthropogenic impact category based on the Ca MgAA vs SOCEAA HCOCEA concentrations, and deviates significantly from the 1:1 line. Figure 13 also shows samples belonging to the cation exchange group, namely samples 2, 3, 4, 5, 6, 7, 8, 10, 11, and 12. However, overall, the samples belonging to the cation exchange group are close to the 1:1 line. This indicates a continuing possibility of anthropogenic impact influencing the concentrations of Ca MgAA and SOCEAA HCOCEA. Other studies mentioned that using nitrate was more depicted as the anthropogenic impact to groundwater (Re et al. , 2017. Adimalla et al. Li et al. , 2. Natural groundwater did not form nitrates in the water. Nitrate levels in groundwater were more influenced by factors such as geography, landuse, hydrological characteristics, and climate ( Li et al. , 2021. Xie et , 2. Indonesian Government Regulations provided guidelines that the maximum level of nitrates in the water was 10 mg/l. Figure 14 shows the nitrate levels when they are compared to government guidelines. The study resulted that 17 % of the samples were below the maximum allowable limit. About 25 % of the samples were close to the level, while the remaining of 58 % exceeded the government allowable limit. PUBLISHED IN IJOG Hydrogeochemistry and Groundwater Quality Assessment of Shallow Groundwater in The Penguluran Basin. East Java. Indonesia (F. Masitoh et al. Figure 14. Nitrate concentration in each groundwater sample. Settlements were randomly located and varied with other landuse such as forests, mixed gardens, and farms. Nitrate would increase if there was agricultural activity, and it causes agricultural pollution (Re et al. , 2017. Li et al. , 2. The location of wells which were close to human activities and animal husbandry area would also increase the nitrate levels (Li et al. , 2. Groundwater pollution caused by nitrate . roundwater nitrate pollutio. would be aggravated if the people population had poor sanitation (Bu et al. Adimalla et al. , 2. If it was associated with a maximum limit of nitrate concentration in groundwater which was 10 mg/l, then the groundwater in the Penguluran Basin had been polluted due to anthropogenic activity (Yan et al. , 2. Conclusions This study successfully characterized the hydrogeochemical features and assessed the groundwater quality of the shallow aquifer system in the Penguluran Basin. East Java. Indonesia. The groundwater is predominantly of the CaMg-HCOCE hydrogeochemical facies, dominated by Ca cations and HCOCEA anions. Most samples indicate freshwater characteristics formed by silicate weathering, exhibiting low corrosivity risk. Furthermore, the groundwater quality generally falls within good to excellent classes for irrigation However, anthropogenic impacts were evident, with more than half of the samples exceeding safe nitrate concentrations, highlighting a significant threat to groundwater sustainability. The study accomplished its goals by inventorying groundwater resources, analyzing important hydrochemical processes, determining the water suitability for irrigation, and identifying probable anthropogenic contamination sources. Despite this accomplishment, the study is restricted by the small number of sampling locations and the lack of temporal . monitoring, which may impair the robustness of groundwater quality assessments over time. Future research should include expanding sampling coverage both spatially and seasonally, using stable isotope techniques to better trace pollutant origins, and incorporating numerical hydrogeological modelling to predict groundwater quality changes under increasing anthropogenic pressures and climate variability. These developments are critical for developing sustainable groundwater management techniques in karstic and sedimentary aquifer systems. Acknowledgments The first author would like to thank the Universitas Negeri Malang through research funding. The author would also like to thank the laboratory assistants for helping in sample collection and laboratory analysis. A word of gratitude is also extended to anonymous reviewers for their very valuable suggestions. References