Abstrak.

Banjaran adalah wilayah sub urban di daerah vulkanik Cekungan Bandung Selatan yang mengalami perkembangan antropogenik secara dinamis. Disisi lain, variasi kondisi geologi di daerah penelitian berpengaruh pada interaksi batuan dan airtanah. Berdasarkan pada latar belakang tersebut, penelitian ini bertujuan untuk mengetahui pengaruh geologi dan lingkungan terhadap kondisi hidrokimia airtanah di daerah vulkanik. Metode yang digunakan adalah diagram piper, diagram Gibbs, korelasi dan bivariat ion utama. Pengambilan sampel airtanah sebanyak 23 sampel yang terdiri dari 10 sumur gali, 10 sumur bor dan 3 mata air. Hasil penelitian menunjukkan bahwa proses interaksi airtanah dipengaruhi oleh pelapukan batuan. Proses hidrogeokimia ion-ion utama di daerah vulkanik air tanah dicirikan oleh pelapukan silika. Fasies airtanah adalah CaMgCl, CaMgHCO3, NaHCO3. Proses hidrogeokimia ion-ion utama di daerah vulkanik air tanah dicirikan oleh pelapukan silika. Daerah hulu airtanah dipengaruhi oleh interaksi airtanah-batuan, sedangkan daerah hilir airtanah dipengaruhi oleh faktor antropogenik. Airtanah dangkal dengan tutupan lahan pertanian dan pemukiman lebih rentan terhadap pencemaran oleh aktivitas antropogenik. Kontrol struktural yang tinggi memungkinkan air mengalir melalui rekahan dan memudahkan proses pengenceran air tanah, sehingga membantu proses interaksi airtanah-batuan.

Kata kunci: Antropogenik, hidrogeokimia, interaksi airtanah-batuan, pelapukan batuan.

1. Introduction

Groundwater is essential to meet domestic, agricultural, livestock, and industrial needs (Luo, at al., 2018). Natural and anthropogenic environmental factors influence groundwater quality (Voutsis, at al., 2015). The hydrochemistry of groundwater is controlled by the interaction of water and rock involving minerals; besides that, anthropogenic processes from agriculture, domestic, and industry also affect the chemical quality of groundwater (Li, at al., 2013). Among anthropogenic activities, agricultural activities and fertilization most often cause a decrease in groundwater quality (Stellato, at al., 2020). Agricultural activities and active fertilization will increase nutrient concentrations in groundwater. Ammonium is very dangerous for humans because they interact with hemoglobin which inhibits the flow of oxygen into cells (methemoglobinemia) (Pastén, at al., 2014), forming nitrosamines as carcinogenic compounds (Chen, at al., 221). The natural factor affecting the volcanic aguifer is the high mineral dissolution process, which causes groundwater mineralization to be quite dominant (Aboubaker, at al., 2013). Hydrogeochemical data can describe hydrogeochemical processes that control groundwater quality, such as mineral dissolutiondeposition, ion exchange, and evaporation (Sosi, at al., 2019). The Banjaran sub-urban Area in the South Bandung Basin is a volcanic area with lithological variations and high anthropogenic activities. Based on this background, this study aims to analyze the influence of geology and the environment on the hydrogeochemical conditions of groundwater in volcanic areas. The analytical method used is the piper diagram, Gibbs diagram, correlation, and bivariate major ion. Major ion bivariate analysis will make it easier to understand the hydrogeochemical process of groundwater (Singh, at al., 2020). Statistical analysis of correlation and grouping will help understand the relationship between the major ion and other anthropogenic factors (Shi, at al., 2018).

Figure 1. Research location in the Banjaran area

2. Study Area

This research was conducted in the southern part of Bandung Bansin, about 25 km south of Bandung City, West Java, by geographic location from 107°34' - 107°36' and - 7°04' - 7°02' (Figure 1). Banjaran area which includes Banjaran, Katapang and Pameungpeuk sub-districts located in Bandung Regency, West Java. This area has a morphology of undulating hills and lowlands; it has the potential to be developed for settlements.

3. Methodology

This research begins with a literature study followed by field investigations and laboratory analysis.

The study was conducted in the dry season. Groundwater data were collected from 10 dug wells, 10 drilled wells, and 3 springs. Physical properties of groundwater, including measurement of pH, conductivity, and temperature using a Horiba measuring instrument. All sampling is done at the research site. Laboratory analyzes were performed for major ions: Ca²⁺, Mg² Na⁺, K⁺, SO₄^2-, Cl^-, HCO₃^-, CO₃^-, NH₄N, Fe, Mn. The analysis was carried out in the groundwater laboratory of the BRIN Research Center for Geotechnology. Analysis of Na⁺ and K⁺ used atomic absorption flame photometry (AAS) method, analysis of Ca²⁺ and Mg²⁺ ions using the complexometric method. The HCO3 analysis uses the acidimetric method, SO42 analysis uses the turbidimetric method, Cl- analysis uses the argentometry method, and TDS analysis uses the gravimetric method.

Physical and chemical hydrogeochemistry results from pH, EC, Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄^2-, Cl^-, and HCO₃ were processed by bivariate, multivariate, statistical, and spatial analysis. Statistical analysis of hydrogeochemical correlation was carried out to determine the relationship between groundwater quality parameters (Mallick, at al., 2018). Geostatistical analysis and correlation coefficient using the Pearson method (Mohamed, at al., 2019). Hierarchical group analysis was conducted to analyze homogeneous data by grouping data with similar properties based on the Physico-chemical properties of groundwater (Sosi, at al., 2019). Hydrochemical modeling was carried out using Groundwater Workbench software (GWB.2021) (Paternoster, at al., 2021). The results of the hydrogeochemical characterization as the basis for building a conceptual model of the hydrogeological system.

4. Data

Geological objects that are spatially distributed are Malabar volcanic deposits (Qmt) and lake deposits (Qd) (Alzwar, at al., 1992). The distribution of groundwater resources is located in the surrounding of The Malabar volcano (Qmt), with an altitude ranging from 756 - 858 m. Springs are strategic resources widely used for the fulfillment of raw water. The youngest rock unit at the study site is a Holocene-old lake deposit (Qd), consisting of loose material the size

Figure 2. Geological map in the Banjaran area (modification from Geological Map of the Garut and Pameungpeuk Quadrangle, Java, 1992 )

Figure 3. The land use in the Banjaran sub urban area

Table 1. The summary of the physico-chemical properties in the research area

of clay, silt, sand, and gravel that is tuffaceous. Malabar volcanic deposits (Qmt) consist of basalt lava and andesite basalt. The basalt lava flow is grey-black and has a very fine aphanitic to the porphyritic glass structure. Phenocryst consists of plagioclase and pyroxene. Basal andesite is dark grey, has a fine aphanitic porphyry texture, and phenocrysts consisting of plagioclase and pyroxene embedded in an aphanitic bed mass (Figure 2).

The characteristics of volcanic areas are based on geological conditions, sedimentary structures, and biology (Bronto, 2006). The research area has medial and distal volcanic facies. The medial facies is located on the lower slope, and the distal facies is located on the plains of the lake deposits (Maria, at al., 2021).

The Banjaran area uses plantations, settlements, dry land agriculture, and rice fields (Figure 3). The point of sampling location for springs, especially in settlements, plantations, and rice fields. The land use pattern in the study area affects the hydrogeochemical conditions of the study area. Domestic, animal husbandry, and agricultural activities affect the concentration of the primary ion elements (Esteller, et al., 2016). In other areas, residues from agricultural activities are a source of pollution from ammonium (H. R. V., et al., 2011).

5. Results and Discussion

The hydrogeochemical spatial modeling shows the distribution pattern of hydrogeochemical variations. Physico-chemical properties from field data collection and chemical properties are taken from the laboratory (Table 1). In general, the major cations is in the range of \(Ca^{2+} > Mg^{2+} > Na^{+} > K^{+}\) and the major anions \(HCO_{3}^{-} > Cl^{-} > SO_{4}^{-}\). The concentration of \(Ca^{2+}\) in dug wells ranged from 2.87 - 52.48 mg/L, in drilling wells ranged from 5.01- 42.26 mg/L, and in springs ranged from 2.77-7.93 mg/L. Mg²⁺ content in dug wells ranged from 3.64-25.21mg/L, in drilling wells ranged from 1.20-21.09mg/L, and in springs, it ranged from 2.31-5.19 mg/L. The concentration of Na⁺ in dug wells ranged from 3.65-26.68mg/L, in drilling wells ranged from 1.31-43.18mg/L, and in springs ranged from 2.03 -7.69mg/L. The K⁺ content in dug wells ranged from 0.50 - 11.79 mg/L, in drilling wells ranged from 0.68-8.59 mg/L, and in springs, it ranged from 0.38 - 0.68 mg/L. Cl concentrations in dug wells ranged from 6.89 - 46.83 mg/L; in bore wells, they ranged from 5.26-53.76 mg/L; and in springs, they ranged from 6.77 - 22.04 mg/L. The SO₄ content in dug wells ranged from 4.12 - 49.52 mg/L. In drilling wells, it ranged from 4.64 - 8.34 mg/L, and in springs, it ranged from 5.10 - 10.59 mg/L. The concentration of HCO₃ in dug wells ranged from 12.18 - 216.09 mg/L, in drilling wells ranged from 13.08 - 258.25 mg/L, and in springs, it ranged from 9.45 - 21.35 mg/L. The concentration of Mn in dug wells ranged from 0.01 -1.45 mg/L. In drilling wells, it ranged from 0.01 - 0.81 mg/L, and in springs, it ranged from 0.01 - 0.01mg/L. The concentration of Fe in dug wells ranged from 0.01 - 4.53 mg/L; in drilling wells, it ranged from 0.01 -6.56 mg/L; in springs, it ranged from 0.01 - 0.05 mg/L. The NH₄N content in dug wells ranged from 0.06 -2.52 mg/L; in drilling wells, it ranged from 0.09 - 6.46 mg/L, and in springs, it ranged from 0.02 - 0.11 mg/L.

The concentration of the major ion in drilling wells is relatively higher than in dug wells and springs; this is influenced by the journey and time of groundwater transport. The spring has a relatively lower elemental concentration because it is close to the recharge site, so the groundwater has not been in contact for too long with rock minerals. Ca²⁺ and Mg²⁺ cations are alkaline earth metals, essential elements in igneous minerals (Dedzo, et al., 2017). The increase in concentration occurs naturally due to traveling through sedimentary rock layers through which groundwater passes (Gnanachandrasamy, et al., 2015). The concentration of Cl^- anion comes from the influence of hydrothermal activities around the springs. The concentration of HCO₃^- ions in groundwater sources indicates that groundwater comes from local meteoric (Hem, 1985).

The Pearson correlation on groundwater in the Banjaran area is (Table 2): pH value has a moderate correlation with EC, TDS, Na⁺, and HCO₃^-. Electrical conductivity (EC) strongly correlates with TDS, Na⁺, Ca²⁺, HCO₃^-, moderately correlated with pH, Ca²⁺, Cl^-, HCO₃^- dan NH₄N. TDS strongly correlates with EC, Na⁺, moderately correlated with pH, Ca²⁺, Cl^-, HCO₃^-, and NH₄N. Sodium (Na⁺) strongly correlates with EC, TDS, and NH₄N, with moderate correlation with pH, HCO₃^- and Cl-. Potassium (K⁺) correlates moderately with EC, Ca²⁺, HCO₃^-, Mn, and NH₄N. Calcium (Ca²⁺) is strongly correlated with EC, HCO₃^-, moderately correlated with TDS, Mg²⁺, K⁺, HCO₃^- and NH₄N.

Magnesium (Mg²⁺) strongly correlated with HCO₃ moderately correlated with EC, Ca²⁺, Fe, and Mn. Chloride (Cl^-) is moderately correlated with Na⁺ and Ca²⁺. Bicarbonate (HCO₃^-) is strongly correlated with EC, Ca²⁺, Mg²⁺, and NH₄N and moderately correlated with pH, TDS, Na⁺, K⁺, and Fe. Manganese (Mn) is moderately correlated with HCO₃^-, Mg²⁺, and Fe, and Iron (Fe) is moderately correlated with HCO₃^-, Mg²⁺ and Mn. Ammonium (NH₄N) strongly correlates with Na⁺, HCO₃^-, and moderately correlated with EC, TDS, K⁺, Mn, and Fe. High values of TDS, EC, Fe, SO₄^2-, Cl^- and NH₄N are seen in distal zones. Based on field investigations and laboratory analysis, the presence of SO₄^2-, Cl^- and NH₄N comes from environmental pollution from nature and the influence of human activities such as fertilizers, households, and agriculture.

Groundwater in each region has different characteristics (Esteller, et al., 2016). a Piper diagram analysis was carried out to determine the characteristics of the groundwater (Güler, et al., 2002).

Table 2. Pearson correlation on groundwater in the Banjaran area

220 Jurnal Teknik Sipil

Based on the piper diagram analysis, it is known that the groundwater facies are CaMgCl, CaHCO₃ and NaHCO₃ (Figure 4). The CaMgCl facies is a spring on the slopes of a lower volcano that Mg²⁺ cations have enriched in the interaction process of groundwater and rock.

Calcium-magnesium concentrations result from weathering of Ca²⁺-rich silica minerals from basaltic andesite volcanic rocks. The dominance of Cl concentration comes from hydrothermal lateralization in the upstream mountains and anthropogenic activities. The CaHCO₃ facies group is groundwater influenced by local meteoric water, which indicates that groundwater circulation is not too far away. The dominant Ca²⁺ concentration is due to interactions with volcanic rocks. The concentration of HCO₃ ions indicates that groundwater has mixed with shallow groundwater as local recharge. The NaHCO₃ facies is groundwater with substantial anthropogenic influence in the distal facies area. The source of Na⁺ cations and HCO₃ anions comes from weathering volcanic deposits compositions of Na-feldspar, K-feldspar, pyroxene, and olivine. The HCO₃ ion concentration indicates that groundwater from local recharge has a meteoric origin.

Figure 4. Piper diagram in Banjaran Area

The interaction of rock and water is generally seen in dissolving groundwater and rock, including chemical and physical reactions (Morán, et al., 2016). The interaction of rock and water can be evaluated by plotting \((Ca^{2+} + Mg^{2+})\) and \((HCO_3^- + SO_4^{2-})\). The results of bivariate \((Ca^{2+} + Mg^{2+})\) and \((HCO_3^- + SO_4^{2-})\)showed the hydrogeochemical process of major ions in groundwater volcanic regions is characterized by silica weathering (Figure 5). The Ca²⁺ and Mg² cations are alkaline earth metals and essential elements in igneous minerals. Concentration increasingly occurs naturally due to traveling through sedimentary rock layers through which groundwater passes.

Hydrogeochemical processes in groundwater control the mechanism of groundwater originating from rainwater, rock weathering, crystallization, or evaporation (Okiongbo

Figure 5. Bivariate (\(Ca^{2+} + Mg^{2+}\)) and (\(HCO_3^- + SO_4^{2-}\)).

and Douglas, 2015). One of the methods used is the Gibbs Diagram which can distinguish between the interaction effects of rock weathering, evaporation, and the impact of rainwater on the chemical processes of groundwater (Xiong, et al., 2020).

In the Gibbs diagram of groundwater based on the ratio of anion TDS to Cl^- / (Cl^- + HCO₃^-), the study area shows that groundwater is dominantly influenced by rock weathering. In contrast, in some places, it is affected by rainwater (Figure 6a). Based on the comparison of TDS anion with \((Na^++K^+)\) / \((Na^++K^++Ca^{2^+})\), it shows that groundwater is dominantly influenced by weathering of rocks and, in some places, influenced by rainwater (Figure 6b). Groundwater affected by rock weathering is located in Qmt formation in the proximal and medial facies. Groundwater affected by rainwater is located in lake deposits (Qd) in the distal facies.

TDS shows the concentration of ions dissolved in water; the greater the value is directly proportional to

Figure 6. The gibbs diagram

Figure 7. Bivariate TDS and EC in the Banjaran urban area

Figure 8. Bivariate Cl^- and NH₄N

Figure 9. Bivariate SO₄2- and (Na+K+)

the dissolved ion (Effendi, 2013). TDS and EC come from environmental conditions such as natural conditions and the influence of human activities such as households, industry, and agriculture (Rusydi, 2018). The relationship between the major ion and TDS shows the hydrogeological processes in the aquifer system (Wu, et al., 2015). The value of TDS indicates the travel time of groundwater (Kehew, 2001). Areas with low TDS have a shorter contact time with rocks, so they can be indicated as a recharge area (Maria, et al., 2021).

In the bivariate analysis of TDS and EC, there were 2 groups of groundwater. Group A is groundwater with a low value of TDS and EC. This group is located in a proximal area with short groundwater interaction times and is close to the recharge area. Group A is distributed in Malabar volcanic deposits (Qmt) and has a high fracture control that allows the groundwater dilution process, thus helping the rock-water interaction. Group B is a high value of TDS and EC; this group is located in a medial-distal area with long groundwater interaction times and many interaction processes with rocks, indicating the discharge area. The positive correlation between the EC and TDS shows groundwater's hydrogeological processes and travel time; it can be indicated as a recharge area.

Anthropogenic influence groundwater on hydrogeochemistry is seen from the aspect of Cl., \(SO_4^2\)and NH₄N concentrations (Figure 8). Agricultural activities will produce fertilizer residues, increasing the ion value for Cl^-, NH₄N, SO₄^2-, and (Na⁺ +K⁺) (Pawar and Shaikh, 1995). Various anthropogenic activities such as plantation agricultural fertilizers, livestock, and domestic waste will affect groundwater quality (Deka, et al., 2015). The bivariate analysis of Cl^- and NH₄N showed there were 3 groups of groundwater. Group A is groundwater with a high value of Cl but a low value of NH₄N, located in a proximal area with short groundwater interaction times. The high concentration comes from volcanic activity. Group B is groundwater with a low value of Cl and NH4N, indicating the recently occurring interaction process between groundwater and rock. This area can also be called the recharge area. The land use pattern in the upstream area is dominated by agriculture and, plantations, animal husbandry, while the downstream area is densely populated, affecting groundwater quality. Group C has a low value of Cl but a high-value NH₄N. The high NH₄N concentration comes from anthropogenic activity, agricultural residues, and livestock. High NH4N concentrations occur in the rainy season due to mineral dissolution and accumulation in the distal area with lake deposits (Qd).

Anthropogenic effects can be evaluated by plotting Cl and NH₄N. The bivariate Cl and NH₄N showed three groups (Figure 8). Group A is groundwater with high Cl and NH₄N values, located in a medial area. Group B is groundwater with low Cl and NH₄N values, located in a proximal area, and Group C is groundwater with low Cl value but high NH₄N values, located in a distal area. Hydrothermal and water-rock interactions influence high Cl values, and high NH₄N values are influenced by anthropogenic activities such as livestock, manure, agricultural fertilizers, and domestic waste.

Bivariate SO₄^2- and (Na⁺ + K⁺) showed three groups (Figure 9). Group A is groundwater with high (Na⁺ + K⁺) but low SO₄^2- values, distributed in the medial facies of Malabar volcanic deposits (Qmt). Group B is groundwater with low SO₄^2- and (Na⁺ +K⁺) values, distributed in the proximal Malabar volcanic deposits (Qmt). Group C is groundwater with low (Na⁺ +K⁺) but high SO₄^2- values, distributed in the distal facies lake deposits (Qd) (Figure 9). High (Na⁺ +K⁺) values are influenced by rock water interactions. High SO₄^2- agricultural activities influence values. Groundwater upstream area dominated influenced by the groundwater-rock interaction, while anthropogenic

factors influence groundwater downstream area. Shallow groundwater with land covering agricultural and settlements is more susceptible to contamination by anthropogenic activities. Prevailing geological structure conditions such as rock fractures allow water to flow and facilitate the dilution of groundwater, thus helping the process of rock-water interaction.

6. Conclusion

Based on the results of the analysis, the following conclusions were obtained ;

• 1. The hydrogeochemical process of major ions in groundwater volcanic regions is characterized by weathering of silica. The highest weathering effect of silica minerals in the distal facies results in the enrichment of Ca2+, Mg2+, and HCO³ ions.
• 2. Groundwater has a high enough HCO³ ratio which shows rock weathering as a significant factor, weathering minerals rich in calcium and magnesium.
• 3. Groundwater upstream area dominated influenced by the groundwater-rock interaction, while anthropogenic factors influence groundwater downstream area.
• 4. Shallow groundwater with land covering agricultural and settlements is more susceptible to contamination by anthropogenic activities.
• 5. High structural control allows water to flow through the fracture and facilitates the groundwater dilution process, thus helping the process of rock-water interaction.

7. Acknowledgments

The author would like to thanks the Saintek Scholarship Program of the Ministry of Research and Technology, which has funded the education program. Thanks, are also given to the Head of the Research Center for Geotechnology, Indonesian Institute of Science, Dr. Eko Yulianto, who has given permission for providing laboratory facilities to carry out the water sample analysis. Mr. Dady Sukmayadi, and Mr. Wahyu Purwoko have helped a lot in laboratory analysis.

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