Groundwater vulnerability zonation using Aplis and Foster method in The Ponorogo-Ngawi groundwater basin

In 2021, Ngawi district became the largest rice producer in East Java. Groundwater is the main water source used for irrigation purposes. Lack of management for developing necessary irrigation wells has resulted in uncontrolled groundwater use, potentially reducing groundwater quantity and quality. This study aims to analyze groundwater vulnerability zones. An assessment was conducted using the Aplis and Foster methods, and their parameter classes can be customized to match the conditions of the research area. The Aplis method considers five parameters: altitude (A), slope (P), lithology (L), infiltration (I), and soil (S). The Foster method considers four parameters: aquifer response characteristics (RA), aquifer storage characteristics (DS), aquifer thickness (s), and groundwater depth (h). The vulnerability values obtained using the Aplis method ranged from 30 to 131 and were divided into four classes: low, moderate, high, and very high. The Vulnerability values obtained using the Foster method ranged from 10 to 15 for the low and moderate classes. A non-technical approach through the strict application of permits and restrictions on groundwater usage is a basis for formulating policies related to groundwater management in the research area.


Introduction
The Ponorogo-Ngawi Groundwater Basin spans seven districts in two provinces, Central Java and East Java, with an area of 3,902 km².The groundwater potential in the basin was estimated to be 1,547 million m³/year for unconfined aquifers and 66 million m³/year for confined aquifers.One of the districts that utilize the groundwater potential of the Ngawi-Ponorogo Groundwater Basin is Ngawi District.Ngawi District will be the largest rice producer in the East Java Province in 2021.The rice fields in the Ngawi District cover an area of 50,715 ha, which is 39.13% of the district's total area.The rice production in the district is approximately 906,817 tons, with a productivity of 6.39 tons/ha [1].
Groundwater is a water source used for irrigation purposes.According to data from the BBWS Bengawan Solo in 2022, there are 111 groundwater irrigation wells (JIAT) and numerous deep wells built by farmers, known as "sumur pantek."The lack of regulations and management for developing irrigation wells that are not yet available has led to uncontrolled groundwater utilization.
A rising demand for groundwater accompanies the increase in agricultural productivity.Investigative activities must be conducted through the surface or subsurface of the soil to discover aquifer layers, thickness, and depth [2] to explore groundwater's potential.Using groundwater without proper planning, management, monitoring, and protection can have consequences for the quality and quantity of 1311 (2024) 012025 IOP Publishing doi:10.1088/1755-1315/1311/1/012025 2 groundwater and the surrounding environment.A large number of deep wells can lead to decreased groundwater quantity.The ability of an aquifer system to be impacted by excessive water withdrawal is referred to as aquifer vulnerability.The vulnerability concept was described based on the vadose zone's capacity to preserve groundwater quality [3][4].Vulnerable aquifers can impact the amount and quality of groundwater.This study analyzes groundwater vulnerability zones to protect groundwater availability in the study area.The method used in this study involves processing primary and secondary data using a Geographic Information System (GIS) to generate several vulnerability parameters.Vulnerability zones are produced by overlaying multiple parameters obtained through a Geographic Information System (GIS) based on the vulnerability formulas of the Aplis and Foster methods.Evaluating groundwater quality vulnerability is particularly important for conservation and management [5].

Material and Method
The method used in this study involved the collection of primary and secondary data.Primary data, specifically geological and hydrological data, were obtained through field observations.Secondary data were acquired from sources published by government institutions.The secondary data included the RBI map from DEMNAS, soil-type maps from the Public Works Agency of Ngawi District [6], and data on pump test wells, borehole logs, and climatology from BBWS Bengawan Solo [7].The Advent of Geographic Information System (GIS) has facilitated the assessment of groundwater vulnerability through mapping [8][9] [10].The Aplis method is a geographic information-system-based approach that considers five main parameters: altitude (A), slope (P), lithology (L), infiltration (I), and soil (S) [11][12][13] [14].Each parameter was assigned a weight within a certain range.The weight of each parameter ranges from 1 to 10.A weight of 1 indicates a small influence on vulnerability, whereas a weight of 10 indicates a greater influence [12][13] [14].The vulnerability value was derived from the analysis of each parameter using Equation 1V = (A+P+3L+I+S)/0.9 V is vulnerability value, A is altitude, P is slope, L is lithology, I is infiltration, and S is soil type.The vulnerability class classification in the Aplis method was divided into five classes [11][12][13] [14] with vulnerability values modified according to the conditions of the research area (Table 1).
Foster's method calls for a single indicator of sensitivity to groundwater level drop, saline/seawater intrusion, and subsidence [15] [16].The parameters considered in the investigation of aquifer vulnerability due to groundwater pumping are aquifer response characteristics (RA), aquifer storage characteristics (DS), aquifer thickness (s), and groundwater depth (h) [15][16] [17].The parameters and weight values classes were grouped, as shown in Table 2.The high score from values indicated that the parameter significantly influenced the vulnerability level [15] [17].
The vulnerability value was obtained by summing the weights of each parameter by mapping the data using GIS.The vulnerability values are classified into four classes [15][17] (Table 3).

Vulnerability with Aplis Method
Table 4 presents the findings of the vulnerability analysis using the Aplis approach.The assignment of scores from 1 to 10 refers to the classifications [12][13] [14], where a score of 10 indicates parameters with the most significant influence on groundwater vulnerability.The weight assignment interpretation is conducted by applying an approach to each parameter's type, characteristics, and attributes in relation to the scale of their influence on vulnerability occurrence.Based on Figure 1, this research area is located at an elevation of 43 to 3.103 meters above sea level (masl).The highest elevation is situated on the southern side of the research area, bordered by Mount Lawu.The low elevations consist of plains stretching west to east, dominating the research area.Therefore, the lower the elevation, the smaller the influence on the vulnerability level.Based on Figure 3, the results of geological mapping, the research area consists of seven rock formations: lava rock, claystone, limestone, tuff, volcanic breccia, sandstone, and alluvium.The volcanic breccia and alluvium formations dominate the area.Based on the characteristics and properties of rocks, alluvium is a sedimentary rock formed from sand, gravel, and mud, thus having a high infiltration capability.The soil type map in Figure 5 showed that the research area consisted of alluvial, non-calcareous, Mediteran, regosol, and grumusol soils.The research area is dominated by grumusol.Regosols are a type of soil that significantly influences vulnerability due to their loose soil structure, making water movement easier and possessing good water retention capabilities.The zoning of vulnerability classes in the research area was obtained by analyzing overlaying maps for each parameter with SIG, as shown in Figure 6, using Equation 1.As depicted in Figure 6, the research area had vulnerability values between 27 and 135.The research area had four vulnerability classes: low, moderate, high, and very high.Areas with high vulnerability predominantly characterize the region's western part, whereas areas with very high vulnerability dominate the eastern part.

Vulnerability Analysis Using Foster Method
Table 5 presents the vulnerability analysis based on the foster method.The assignment of scores refers to the classifications [15][17], where a score of 5 indicates parameters with the most significant influence on groundwater vulnerability.Assigning score values based on the classified categories of processed parameters using GIS in the form of a map The results of the pumping test analysis in the research area indicated that the transmissivity values ranged from 100 to 455 m²/day, indicating a relatively good ability of the area to transmit water.The storativity values in the research area range from 0.43 to 5.1, indicating a relatively high type of aquifer that has a better ability to store water.
Based on Figure 7, the aquifer response characteristic values in the research area range from 54.7 to 484.6 m²/day.The parameter weights had values between 2 and 3, indicating that the aquifer had a relatively rapid response to land subsidence caused by pumping.

Vulnerability analysis
Different vulnerability classes were obtained using these two methods based on the vulnerability analysis.A summary of the analysis results for each method is presented in Table 6 Base map source : BIG Base map source : BIG

Figure 1 .
Figure 1.Altitude Parameter Map.The slopes in the research area had varying values ranging from 0.1 to 54.52%, based on Figure 2. The research area encompassed flat, moderate, steep, and very steep areas.Flat to moderate slope areas dominated most of the research area.Hence, the flatter the area, its influence on the vulnerability level is smaller.

Figure 3 .
Figure 3. Lithology Parameter Map.Infiltration is the process of water moving from the ground surface into the soil through pores.Infiltration calculations were based on the texture and type of surface soil.The surface soil types in the research area consisted of sandy claystone, sandy silt, sandy soil, gravelly sand, and loose gravelly sand.Based on Figure4, The research area is predominantly sandy claystone.The smaller the rock porosity, the smaller its influence on the vulnerability.Infiltration is the process of water moving from the ground surface into the soil through pores.

Figure 7 .
Figure 7. Aquifer Respon Characteristic Parameter Map.Based on rainfall data, the research area has recharge values ranging from 390 to 787 mm/year.The pattern of groundwater recharge zones decreased towards the northeast.Aquifer storage characteristics describe the volume of water that can be stored in an aquifer with groundwater recharge values.The aquifer storage values in the research area ranged from 0.00082 to 0.01 years/mm.Based on Figure 8, the dominant aquifer storage values were between 0.001 and 0.01 years/mm.

Figure 8 .
Figure 8. Aquifer Storage Characteristic Parameter Map.The thickness of the aquifer was based on borelog data.Based on Figure 9, aquifer thickness ranged from 13 to 120 m in the research area.The dominant aquifer thickness was between 50 and 100 m.A thicker aquifer generally indicates a lower vulnerability.

Figure 9 .
Figure 9. Aquifer Thickness Parameter Map.Based on hydrogeological mapping, the groundwater level in the research area ranges from 0.28 to 19.75 m.Groundwater levels between 1 and 10 m dominated most of the area shown in Figure10.Shallow groundwater levels are more vulnerable to subsidence owing to pumping.

Table 1 .
Classification of Vulnerability Classes.

Table 2 .
Classification of Vulnerability Parameters.

Table 3 .
Classification of Vulnerability Classes.

Table 5 .
Rating of Foster Method Parameter