Determining groundwater facies and water quality index in Tanah Bumbu Regency/South Borneo Indonesia

. Most people in Tanah Bumbu Regency use water from dug wells for their daily activities. Kotabaru Regency is located in Pagatan Groundwater Basin, which is located in three administrative boundaries, namely Tanah Laut Regency, Tanah Bumbu Regency, The purposes of this study are to determine the type of groundwater facies and Water Quality Index (WQI). The methods were hydrogeological mapping and hydrogeochemical analysis of groundwater samples to evaluate groundwater quality. There were 170 groundwater samples mapped in the study area. The study area has differences in groundwater quality in both confined aquifer and unconfined aquifer. The unconfined aquifer has two types of groundwater facies, namely Ca-Mg-HCO 3 (Calcium Magnesium Bicarbonate) and Ca-Cl (Calcium Chloride). In comparison, the confined aquifer has two types, which are Ca-Mg-HCO 3 - (Calcium Magnesium Bicarbonate) and Na-HCO 3 - (Sodium Bicarbonate). Based on the calculation of the Water Quality Index (WQI) of unconfined aquifer samples show that 70% of groundwater samples have poor quality. Meanwhile, the confined aquifer samples indicate that 85% of groundwater samples have good to excellent, which is suitable for drinking water.


Introduction
Tanah Bumbu Regency is part of the Pagatan Groundwater Basin which is a cross-regency groundwater basin covering Tanah Laut Regency, Tanah Bumbu Regency and Kotabaru Regency [1].
The Pagatan groundwater basin in Tanah Bumbu Regency has an area of 5,067 km 2 (506,714 ha) or 13.50% of the total area of South Kalimantan Province which consists of ten sub-districts (Figure 1a). In 2019, the population of Tanah Bumbu Regency was 320,452 inhabitants [2]. Most people in Tanah Bumbu Regency withdrawn groundwater via dug wells for daily activities. Therefore, it is necessary to conduct a study regarding the quality of groundwater both groundwater from the unconfined aquifer and confined aquifer to determine the groundwater facies and Water Quality Index (WQI) as the goals of this research. Analysis of hydrogeochemistry and WQI is widely developed recently by some researchers to assess groundwater quality for defining the groundwater facies and a rating which are reflecting the composite influence of different water quality parameters on the overall quality of water [4][5][6][7][8].
Tanah Bumbu Regency is included in the Kotabaru [3] and the Banjarmasin regional geological map sheets [12] (Figure 1b). Based on the geological map, it can be seen that the geological conditions of the study area are dominated by sedimentary rock types. Lithostratigraphically, the type of rock in the study area from the oldest to the youngest, namely as follows: •  [13], the groundwater potency in Pagatan groundwater basin can be classified into two. Aquifer with the depth up to 30 m depth is only feasible for household activities with a maximum discharge of 100 m 3 /month/well. While aquifer with the depth of more than 30 m depth is categorized with a low potential is limited to a maximum of 540 m 3 /day/well. There are two types of aquifers in the study area, as follows: 1. Groundwater which flows in the lowland aquifer system Generally occupying in the western, the southern, the eastern, and locally in the coastal areas, in the flood plain and inter-hill/mountain plain. The rocks consist of loose material such as clay, sand and gravel; Groundwater flow takes place through a space between granular aquifer systems. The aquifer system generally shows the unconfined aquifer.

Karst Aquifer System
It is a typical morphological appearance of limestone hills which is characterized by the presence of fissures, fractures, and dissolution channels.

Methods
The activities in the field campaign were geological and hydrogeological mapping, respectively. Geomorphology and surface geological mapping was the activities in the geological mapping. Groundwater level measurement in the dug wells and groundwater sampling were the main activities in hydrogeological mapping. There were 170 samples taken to analyze pH and EC values. While 30 of them were tested in hydrogeochemical for both major cations (Ca 2 + , Mg 2 + , K + , and Na + ), and anions (HCO3 -, Cl -, and SO4 2-) content in the laboratory of Geological Agency in Bandung. A Piper diagram [14] was applied to define the groundwater facies. [6] developed a practical graphic procedure to separate relevant analytical data from comprehending the sources of the dissolved constituents in groundwater. This procedure was developed under the proclamation that most natural groundwater contains major cations and anions in chemical equilibrium. It is affected that the most abundant cations are two alkaline earth calcium (Ca 2+ ) and magnesium (Mg 2+ ) and one Alkali sodium (Na + ). The most major anions are one weak acid bicarbonate (HCO3 -) and two strong acids, sulfate (SO4 2-) and chloride (Cl -). Weighting groundwater quality based on the Water Quality Index (WQI) [11] can be approached to evaluate the quality of groundwater in its use as drinking water. Each parameter is determined according to its effect on health. The relative weight value (Wi) is obtained from Equation 1 as follows: Whereas Wi was the relative weight value given to each parameter, and n is the number of parameters. Next was calculating the quality rating scale (qi) for each predetermined parameter. The qi value was obtained from the comparison of the specified parameter value (Ci) to the specified standard value (Si), as shown in Eq. 2. The standard value for each parameter used was the maximum value of each parameter based on the Minister of Health Regulation [15] for the water drinking standard. The Sub Index (SI) value was calculated by multiplication between relative weight and the rating scale values. Then, the WQI was conducted by the total value of the Sub Index (SI). The range of WQI was shown in table 1. The parameters to assess WQI for the unconfined aquifer were color, turbidity, pH, and Fe 3+ . Furthermore, the WQI for confined aquifer were calculated based on color, turbidity, Fe 3+ , Na 2+ , and Total Dissolved Solids/TDS values.

Groundwater flow direction
Based on the hydrogeological mapping, in the unconfined aquifer groundwater flow towards the east or the Java Sea. The direction of the groundwater flow pattern follows the topographic, which tends to be lower to the east (Figure 2a). The groundwater level of an unconfined aquifer is at an elevation 0.7-49 m above sea level (masl) with a depth of the water table is from 0.1 to 7.7 m depth below ground surface. The intensive groundwater used via dug wells is depicted by the very dense presence of dug wells. Meanwhile, the pattern of groundwater flow of the confined aquifer has a pattern relatively similar to the flow direction of the unconfined aquifer, which flows to the east (Figure 2b), but different in the number of deep wells in the study area. Mainly, groundwater used both via dug, and deep wells are utilized for daily activities.

Hydrogeochemistry
There are thirty groundwater samples that are analyzed physical and chemical content in the Geological Agency laboratory in Bandung, as shown in Table 2. Twenty of them are selected from the confined aquifer samples (code: SB), while the rest are samples of the unconfined aquifer (code: SG). Based on table 2, there are five samples of the confined aquifer exceed the limit standard of turbidity from the Indonesian Ministry of Health, 5 NTU, while the unconfined aquifer samples are seven. Enormous numbers higher of turbidity values in the unconfined aquifer are due to the poor quality of dug wells construction in Tanah Bumbu Regency. Many dug wells are used the uncemented wall. Thus the rocks in the wall of dug well are mostly dissolved into the well. While the color parameter shows that groundwater samples from unconfined aquifers exceed the standard of water, 15 TCU, more than samples from unconfined aquifers, it reflects the contact intensity between water and rocks due to groundwater extraction through deep wells.
Meanwhile, the range of electrical conductivity of the unconfined aquifer is 20-405 µS/cm. Indeed, the EC value of the confined aquifer has a variety of 44-1,507 µS/cm. These values, both for the unconfined aquifer and the confined aquifer, are suitable for drinking water. Similar to the EC value, the TDS value of groundwater samples of the unconfined aquifer is 16-272 mg/L, which indicates below the standard limit, 500 mg/L. While the TDS value of groundwater samples from the confined aquifer show from 32 to 1,008 mg/L, which indicates two-fold higher than the standard limit. The pH values of groundwater samples of the unconfined aquifer and confined aquifer are 3.89-7.16 and 4.43-8.40, respectively. The low values of pH below the standard limit are caused by the dominance of peatlands, which are mostly found in acidic, thereby reducing the pH value of groundwater in the study area. Meanwhile, based on the total hardness of all groundwater samples indicate that water is classified as soft water with a maximum value of 327 mg/L. The total hardness values are below the standard limit for drinking water, 500 mg/L.
The major cation contents are Calcium (Ca 2+ ) and Sodium (Na + ), while Hydrogen carbonate (HCO3 -) and Chloride (Cl -) are the highest anions concentration in the groundwater samples both the unconfined aquifer and the confined aquifer. Water types of the confined aquifer are divided into two dominant groundwater facies, namely facies of Ca-Mg-HCO3 (Calcium Magnesium Bicarbonate) and facies of Na-HCO3 (Sodium Bicarbonate) as shown in Figure 3. Facies of Ca-Mg-HCO3 indicate that water is associated with ions Ca 2+ and Mg 2+ from the carbonate minerals in sandstone and limestone units as well as from clay minerals in alluvium deposits and claystones. While HCO3ion comes from surface water (meteoric water) where surface water experiences mixing with CO2 from the atmosphere, which then infiltrates into the soil. Facies of Na-HCO3 where Na 2+ ions are interpreted from rocks formed in the marine depositional conditions, rocks sediments and alluvium deposits.
Meanwhile, groundwater samples in the unconfined aquifers are divided into two dominant groundwater facies, namely facies of Ca-Mg-HCO3 (Calcium Magnesium Bicarbonate) and facies of Ca-Cl (Calcium Chloride) as shown in Figure 4. Water type of Ca-Mg-HCO3 in the unconfined aquifer is similar to water type from the confined aquifer as a result of contact between sedimentary rocks and unconsolidated materials (from the carbonate minerals in sandstone, limestone claystone, and alluvial deposit). Ca-Cl facies shows that the water comes from a mixture of groundwater associated with Ca 2+ and Mg 2+ ions from carbonate minerals in sandstone units, limestones, and clay minerals in alluvium deposits. Clions are derived from the infiltration of seawater into the aquifer and sedimentary rocks that have marine depositional conditions.

Water Quality Index (WQI)
Parameters that are applied to calculate WQI for the confined aquifer samples are color, turbidity, pH, Fe 3+ , Na + , and TDS, as shown in Table 3. Based on table 3 and equation 1-4, the WQI of the twenty groundwater samples of the confined aquifer results in five classes, as shown in Table 4. The calculation of the WQI obtains the minimum value of WQI, which is equal to 11.3 and a maximum value of 556 with an average value of 85.2. Good to excellent quality samples are up to 85% while the rest are poor to unfeasible for drinking water. Figure  5 shows the distribution of WQI groundwater samples of the confined aquifer. Good to excellent samples are mainly located in Quaternary alluvium aquifer while poor to unfeasible for drinking water samples consist of unconsolidated quartz sandstone intercalated with clay, limestone, and lignite.  Colour, turbidity, pH, and Fe 2+ are the parameters applied to calculate WQI for ten unconfined aquifer samples, as shown in Table 5. The WQI of the ten groundwater samples of the unconfined aquifer results in four classes, as shown in Table 5 with the range of WQI value is 7.36-1,398. There are four samples that have unfeasible for drinking water while another three samples have a very poor water quality index. The rest are good to excellent quality. Samples that have poor to unfeasible for drinking water are located along the coastline of the Java Sea. The aquifer consists of Quarternary alluvium. The lowest WQI, SG-114, is caused by the highest value of parameters turbidity, color, and Fe 3+ due to the dug well of SG-114 has uncasing on the well. Thus the unconsolidated material such as clay, and clayey sand will be dissolved with groundwater. Moreover, SG-114 also contains low pH, 5.5, as shown in Table 2 that is below the standard limit for drinking water. Groundwater samples for both unconfined aquifer and confined aquifer define the groundwater facies which indicate particular character and chemistry. Water types of the confined aquifer are divided into two dominant groundwater facies, namely Ca-Mg-HCO3 (Calcium Magnesium Bicarbonate), and Na-HCO3 (Sodium Bicarbonate). Facies of Ca-Mg-HCO3 indicate that water is associated with ions Ca 2+ and Mg 2+ from the carbonate minerals of the aquifer content in sandstone and limestone units as well as from clay minerals in alluvium deposits and claystones. While HCO3ion comes from surface water (meteoric water). Meanwhile, groundwater samples in the unconfined aquifers are divided into two dominant groundwater facies, namely Ca-Mg-HCO3 (Calcium Magnesium Bicarbonate) and Ca-Cl (Calcium Chloride). Facies of Ca-Mg-HCO3 is a similar characteristic to the confined aquifer while Ca-Cl facies shows that the water comes from a mixture of groundwater associated with carbonate minerals in sandstone units, limestones, and clay minerals in alluvium deposits. Moreover, Clions are derived from the infiltration of seawater into the aquifer.
The WQI of the twenty groundwater samples of the confined aquifer results in five classes from unfeasible for drinking water to excellent in quality. The good to excellent groundwater quality samples are mainly located in Quaternary alluvium aquifer. Meanwhile, the poor to unfeasible for drinking water samples consist of unconsolidated quartz sandstone intercalated with clay, limestone, and lignite. The 85% of the confined aquifer samples are good to excellent in quality, while only 30% of the unconfined aquifer samples are suitable for drinking water.