Hydrogeology of the small island: a case study of Pisang island, West Coast Regency, Lampung province

Pisang Island has an area of 1.5 km2. The issue on small islands pertains to the quality and quantity of water. This is related to an increasing population and growing water demand. This research aims to address the sustainability of water resources. Therefore, a hydrogeological concept is required to assess and predict potential water resources to maintain the quality and quantity of water. The methodology used in this research includes geological mapping, measurement of rock resistivity, hydrometeorological analysis, and water quality testing. The research indicates that the lithologies of Pisang Island calcareous gravelly sand, calcareous sandstone, and porous limestone. The aquifer system on Pisang Island is unconfined. The aquifer was spread throughout the island with varying thicknesses. The lithology of calcareous gravelly sand is found in the island’s northern part, with a thickness ranging from 15 to 90 m. Calcareous sandstone is located in fold slopes of 3 to 5 m thickness. Porous limestone is distributed across the island, with a 30–170 m thickness. Pisang Island has an annual rainfall of 3,101.93 mm/year, evapotranspiration of 2,017.91 mm/year, and infiltration of 511.23 mm/year. The groundwater flow pattern moves from high elevations in hilly areas to low elevations in coastal areas.


Introduction
Pisang Island is categorized as a small island with an approximate area of 1.5 km² [1,2,3].Geographically, the western and southern parts of Pisang island directly border the Indian Ocean, while the northern and eastern parts border the island of Sumatra directly.Astronomically, Pisang island is located at 5' & 15.000" S and 103' 50' 45.138" E. Administratively, Pisang island belongs to the West Pesisir Regency, Lampung Province.The island's land is primarily used for residential areas and clove plantations.
Based on [4,5], the population of Pisang Island in 2021 was 1,626 people.This number has increased by 3% annually, based on data from 2019 and 2020.[6] assessed that Pisang Island has the potential to become a tourist destination for beach walking, sunbathing, and fishing.Considering the suitability index for tourism, the increasing population, and the tourism potential of Pisang Island, it is clear that there is a growing demand for water in the area in the future.Based on observations made by researchers, it was found that wells on the island were filled with water during the rainy season.However, some wells dry up during the dry season, and the water in some wells is brackish and salty [7,8,9,10].Hydrogeological research has been conducted on Pisang Island to better understand its water resources and usage in the context of groundwater conservation [11,12,13,14,15,16]

Material and Method
To achieve these research objectives, we collected data and information regarding the geology and hydrogeology of Pisang Island.This information was then used to create a conceptual hydrogeological model.The following research methods were employed.

Geology Mapping
Two methods are used in geological mapping: surface geological mapping and the geoelectric-resistivity method.Surface geological mapping was conducted by observing the outcrops on Pisang Island.Surface geological observations were conducted in February 2023 by establishing 33 observation stations distributed across Pisang Island.The rock outcrops encountered were tested using an HCl solution to determine if they contained calcium carbonate (CaCO3).Layering measurements were also performed to determine the stratigraphy of the Pisang island rocks.The mapping results were then presented in geological maps and cross-sections using GIS.
The geoelectric-resistivity method is a geophysical method used to measure subsurface rocks' resistivity or electrical resistance, including those related to porosity and subsurface fluids [14,17].The assessment was performed by measuring the potential difference from electrical current injection into the ground.This measurement involves inserting two electrodes to send an electrical current and another two electrodes to measure the potential difference on the surface resulting from the current injection.The measurements were performed with varying distances between the electrodes, and deeper layers were detected with increasing electrode spacing.Researchers obtained data from the Balai Wilayah Sungai Mesuji Sekampung, Ministry of Public Works and Housing, Republic of Indonesia, for this method.There were 18 measurement points distributed on Pisang Island, which were divided into five geoelectric lines for interpreting subsurface geology.The resistivity data collected from these measurements were processed and analyzed using IP2Win and Progress software.The results include various resistivity values of the rocks and their respective depths.The results obtained from the geoelectric-resistivity method were combined with the results of surface geological mapping to predict the lithological thickness.

Hydrogeology Mapping
Hydrogeological mapping was conducted by collecting data and information on the distribution of groundwater on Pisang Island [21].The water balance was obtained by calculating the precipitation, evapotranspiration, runoff, and infiltration [8,9,17,22].Groundwater recharge is the amount of water from the surface that infiltrates the soil.
The rainfall data obtained is the rainfall data for 10 years (2013-2022) at three rain gauge stations in West Pesisir District, which the researcher obtained from the Meteorology, Climatology, and Geophysics Agency of Pesawaran (BMKG).Water balance calculation aims to predict evaporation, surface runoff, and infiltration into the ground.The groundwater recharge was calculated using the following formula [23]: Groundwater quality refers to the properties and dissolved substances present in the water.It is expressed through various parameters, such as taste, pH, electrical conductivity (EC), and total dissolved solids (TDS).The classification of groundwater quality based on these parameters is as follows: a. pH pH is a measure of the acidity or alkalinity of water.In coastal areas, the presence of CO2 gas in seawater affects the water's pH.This is because the reaction between CO2 and water results in carbonic acid (H2CO3) formation, increasing the concentration of hydrogen ions (H + ) in the solution, making it more acidic.

Geology
Surface geological observations were conducted at 30 points across Pisang Island.b.Calcareous Sandstone.This rock consists of sand and carbonates.When fresh rock fragments were tested with HCl, the carbonate in the rock reacted, producing bubbles and leaving behind the sand.Calcareous sandstone is found in the hilly sloping areas of the island.c.Calcareous Gravelly Sand.It is a coastal deposit with a lithology comprising sand and gravel that exhibits carbonate characteristics.Calcareous gravelly sand has been found in the residential and coastal areas of the island.The results of surface geological mapping were compared with rock resistivity and served as the basis for the researchers in interpreting subsurface geology.Balai Wilayah Sungai Mesuji Sekampung, Ministry of Public Works and Housing, Republic of Indonesia in 2021, carried out the geoelectric measurements.There were a total of 18 measurement points distributed on Pisang Island, which were divided into five geoelectric lines for interpreting subsurface geology.The aquifer system in the study area is classified as a free aquifer, where porous limestone, calcareous sandstone, and calcareous gravelly sand are considered aquifers.The resistivity results at the geoelectric measurement points, as shown in Figure 3, were used to interpret the subsurface geology.
The research area consists of porous limestone, calcareous sandstone, and gravelly sand, which interact with groundwater and saline water.Figure 3 displays the interpretation of subsurface geology based on resistivity analysis.The thickness of the calcareous gravelly sand was between 15 and 90 m, the thickness of the calcareous sandstone was between 5 and 15 m, and the thickness of the porous limestone was between 10 and 180 m.  ).In conducting hydrogeological mapping, researchers collected 25 sampling points on Pisang Island [8].The tests involved measuring the elevation and depth of the groundwater table and evaluating groundwater quality based on parameters such as pH, total dissolved solids (TDS), and electrical conductivity (EC).Measurements were taken using a meter, with the groundwater table's depth measured from the ground's surface.To determine the elevation of the groundwater table, the elevation of the ground surface was subtracted from the depth of the groundwater table.The purpose of measuring elevation and groundwater depth is to understand and predict the direction of groundwater flow.Groundwater quality testing was carried out directly in the field using a water test kit.This included testing for parameters such as pH, TDS, and EC to determine whether the groundwater exhibited uniform characteristics or differed at various sampling points [17].This information is important as it can serve as an initial basis for predicting the types of aquifers present in the research area.However, for a more comprehensive understanding of the area's hydrogeology, further approaches, such as drilling and geochemical testing of groundwater, may be necessary to obtain more valid results.Figure 4 is a regional hydrogeological map of Pisang Island, illustrating the aquifer productivity on the island, which falls within the zone of scarce groundwater resources.Pisang Island comprises limestone geology, carbonate sandstone, and carbonate gravel sand, as observed in the field geology.Figure 5 represents cross-section A-B depicting its hydrogeological conditions.Based on the groundwater testing results for pH, TDS, and EC, the aquifer system on Pisang Island is considered a free-flowing aquifer.The uniform characteristics of the groundwater in the area support this.Based on the analysis of elevation data, the groundwater table values in the research area range from 4 to 19.7 meters.In coastal and settlement areas, the groundwater table values are predominantly in the range of 4 to 5 meters.The groundwater flow pattern in the research area is from the hilly regions towards the coastal and settlement areas, as indicated in Figure 6.Groundwater flows from the hilly regions located in the central part of the island towards the coastal areas.
The depth of the water table in the research area varies from 0.8 to 8.5 m, as shown in Figure 7.The dominant depth of the water table in the research area is between 4 to 5.5 m.The groundwater table is found at a depth greater than 4 m in coastal and settlement areas.
Groundwater quality observations were conducted at 25 well points and one river.The parameters used to determine the groundwater quality are as follows: a. TDS (Total Dissolved Solids) is one of the parameters used to assess water quality.The higher the TDS value, the higher the saltier the water tastes.The TDS range in the study area was 121 -4,690 ppm.Based on Figure 8, groundwater with TDS values below 1,000 ppm has a fresh taste and is dominant in residential areas.TDS values between 1,000 and 10,000 ppm have a brackish taste.b.Electrical Conductivity (EC) indicates the ability of water to conduct electricity.In the study area, 22 wells had EC values below 900 μS/cm, indicating fresh-tasting water.The two wells had a brackish taste, with EC values of 932 and 1,009 μS/cm.One well-tasted salty with an EC value of 9,372 μS/cm.Figure 9 shows that residential areas' dominant EC values ranged from 600 to 800.c.The pH of the groundwater indicates the acidic, neutral, or alkaline nature of the water, which measures the concentration of hydrogen ions.The pH values obtained for the 25 observation wells and one river ranged from 7 to 9, as shown in Figure 10.Based on the TDS, EC, and pH test results, the groundwater exhibits characteristics that can be considered consistent.This can be interpreted as the groundwater being present in the same aquifer.However, it is different from the groundwater in well 22, where the author suspects the possibility of intrusion.This conceptual hydrogeology diagram provides valuable insights into the characteristics of the groundwater system in Pisang Island, which is essential for effective water resource management and sustainable water utilization.

Conclusion
This study focuses solely on the geological, hydrogeological, groundwater quality, and water balance conditions of Pisang Island.The aquifer system in Pisang Island is a free aquifer system composed of sedimentary rocks, namely calcareous gravelly sand, calcareous sandstone, and porous limestone, distributed throughout the island.The aquifer thickness varied from 5 to 180 m.The elevation of the groundwater table ranges from 2 to 19.77 meters, with the groundwater flow pattern from the highest elevation in the hilly areas to the lowest elevation in the coastal and residential regions.The depth of the water table in the residential areas was 4-6 m.Based on groundwater's physical and chemical characteristics and the resistivity results, indications of seawater intrusion in the northern part of Pisang Island have been found.Further research on groundwater geochemistry is needed to obtain more detailed information about the groundwater conditions and aquifer environment in the research area to analyze the causes of the salinity found in Pisang Island.

Figure 1 .
Figure 1.Pisang Island Geology Map Based on Figure 1, the terrestrial stratigraphy of Pisang Island consists of the following rock types: a. Porous Limestone is a sedimentary rock formed from calcium carbonate deposits derived from the remains of marine organisms such as shells and coral reefs.It undergoes compaction, and becomes harder, but remains porous.Porous limestone is distributed throughout the island.

Figure 2 .
Figure 2. Geology Cross Section A-BIn addition, rock layering was observed, indicating the position of the rocks[10].Figures1 and 2provide a comprehensive depiction of the surface geology distribution on Pisang Island, coupled with detailed geological cross-sections.The verification of rock composition, specifically to confirm the presence of limestone or carbonate characteristics, was accomplished through a rigorous methodology involving the application of a 1 M hydrochloric acid (HCl) solution.The results of surface geological mapping were compared with rock resistivity and served as the basis for the researchers in interpreting subsurface geology.Balai Wilayah Sungai Mesuji Sekampung, Ministry of Public Works and Housing, Republic of Indonesia in 2021, carried out the geoelectric measurements.There were a total of 18 measurement points distributed on Pisang Island, which were divided into five geoelectric lines for interpreting subsurface geology.The aquifer system in the study area is classified as a free aquifer, where porous limestone, calcareous sandstone, and calcareous gravelly sand are considered aquifers.The resistivity results at the geoelectric measurement points, as shown in Figure3, were used to interpret the subsurface geology.The research area consists of porous limestone, calcareous sandstone, and gravelly sand, which interact with groundwater and saline water.Figure3displays the interpretation of subsurface geology based on resistivity analysis.The thickness of the calcareous gravelly sand was between 15 and 90 m, the thickness of the calcareous sandstone was between 5 and 15 m, and the thickness of the porous limestone was between 10 and 180 m.

Figure 7 .
Figure 7. Depth of Groundwater Level

Figure 11 .
Figure 11.Hydrogeology Conceptual Pisang Island3.3Hydrogeology Conceptual ModelBased on geological and hydrogeological analyses, the hydrogeological concept of the research area is shown in Figure11.The research area consists of sedimentary rocks, such as limestone, calcareous sandstone, and gravelly sand.The aquifer system on Pisang Island is confined.The aquifer thickness in the study area ranged from 5 to 180 m.

Table 1 .
[18,19,20]y Rock Value[18,19,20] , and pH to assess the uniformity of groundwater characteristics.Water quality measurements and testing based on TDS, EC, and pH were carried out in February 2023.Numerous tests were conducted on 25 groundwater samples from wells on Pisang Island.

Table 3 .
[29]26,27,28]n of Water based on Electric Conductivity[25,26,27,28]. TDS) refers to the total amount of dissolved solids in water, including minerals, salts, metals, and organic matter.TDS was measured in parts per million (ppm) or milligrams per liter (mg/L).Several factors, including geological conditions and human activities on the surface influence TDS values.TDS is an important parameter that affects water quality because it can influence the taste of water.Groundwater with high TDS values may be slightly saline.TDS is related to EC because conductivity is affected by the concentration of mineral salts[29].