Analysis of Groundwater Vulnerability to contamination in Wonosari Basin and Gunungsewu Karst Transition Zone, Yogyakarta, Indonesia

. The Wonosari Basin and the Gunungsewu Karst Transition Zone have enormous groundwater resources but are hydrogeologically vulnerable to contamination. To achieve sustainable groundwater protection and management, it is imperative to study groundwater vulnerability. Th e research aimed to determine the spatial distribution of groundwater vulnerability in both areas. The COP method, which integrates the concentration of flow (C-Factor), overlaying layer (O-Factor), and precipitation (P-Factor), was used to assess the groundwater vulnerability. Based on the analysis, the groundwater was lowly to very highly vulnerable to contamination. The moderate vulnerability was dominant in both areas. The v * ery low vulnerability was concentrated in the basin, while the low vulnerability was distributed both in the basin (40.43%) and the transition zone (32.79%). High (12.53%) and very high vulnerabilities (5.48%) were found in the Gunungsewu Karst Transition Zone, especially in swallow hole recharge area (< 1 km 2 ). The karst transition zone was more highly vulnerable to contamination than Wonosari Basin, a non -karst area. Various known contaminants are by-products of human activities, including nitrates from chemical fertilizers and domestic wastes. To prevent contamination, local governments should actively address the potential sources of contaminants while considering current groundwater vulnerabilities, particularly in the high and very high vulnerability zones.


Introduction
Water is the major component of all living matter and is abundant on earth.One of its sources, groundwater, makes up more than 97% of the world's freshwater [1], and about half of the global population depends on it for daily water usage [2].It primarily includes supplying drinking water, which is the most common use of groundwater because it has relatively good qua lity compared with other types of water sources.For the same reason, the Gunungkidul population also relies on groundwater abundance to meet their daily water needs.Gunungkidul is an administrative regency in the Southern Mountains of Java Island, Indonesia.This chain of mountains consists of three subzones with distinctive geomorphological features: Baturagung is a structural hill, Wonosari is a basin (hence, referred to as the Wonosari Basin), and Gunungsewu is made up of karst landforms [3].The area in-between Wonosari Basin and Gunungsewu Subzone is called the Gunungsewu Karst Transition Zone.It is distinguished from the two subzones because of differences in morphological variations [4].
Wonosari Basin has the largest population in the mountain range.Productive aquifers and enormous groundwater potential [5] substantially contribute to fulfilling the water needs of its inhabitants.This extractable resource can also be found in the Gunungsewu Karst Transition Zone, as evident in the transmissivity test results at 125 drilled wells in Wonosari Basin and its surroundings in 2016 [6].Even though both areas have high-yielding aquifers, the groundwater is vulnerable to Contamination, which is attributed to the hydrogeological makeup.Wonosari Basin and Gunungsewu Karst Transition Zone a re mainly composed of limestones.Limestones belong to the carbonate rocks group that is composed of more than 50% carbonate minerals.This rock can experience karstification due to contact with rainwater that acts as a solvent.In addition to the soluble nature of limestones, high rainfall is also the driver of karstification; the more a carbonate-based landform receives rainwater, the more it karstifies [7].Karstification causes fractures to take shape in a limestone formation, creating a way for contaminants to enter and reach groundwater, such as swallow holes.There are 15 swallow holes in the Wonosari Basin and the Gunungsewu Karst Transition Zone [8].A swallow hole indicates the presence of conduit flow, that is, a system where groundwater flows through cracks or cave fissures and forms underground flows [9].This system is mainly composed of karst passages that allow water to flow quickly in the form of underground rivers.In other words, swallow holes can accelerate the entry of contaminants into groundwater, making it more vulnerable to contamination.
As a result of these hydrogeological features, the groundwater is, to some extent, inherently vulnerable to contamination and needs sustainable protection and management practices.An example includes studying the degree, distribution, and contributing factors of vulnerability to provide a reference for making policies relevant to preventing groundwater contamination.Prevention is imperative because impurities that have entered and contaminated groundwater are difficult to remove.Therefore, the research a imed to determine the spatial distribution of groundwater vulnerability parameters and levels in the Wonosari Basin and the Gunungsewu Karst Transition Zone.
Previous research with the same method [10] was conducted in the Bribin watershed, a karst area.The current study applied the method to two areas with different characteristics: Wonosari Basin with a diffuse infiltration system and Gunungsewu Karst Transition Zone with a conduit infiltration system.Also, it was centered around and designed to compare their intrinsic groundwater vulnerabilities, where only hydrogeological factors were influential.The analysis did not include specific factors such as land use and contaminant sources.Therefore, further research with different methods and validation techniques and more diverse parameters is encouraged to contribute to a comprehensive understanding of karstic areas.

Research area
The research was conducted in nine administrative districts in Gunungkidul Regency, Yogyakarta, where Wonosari Basin and Gunungsewu Karst Transition Zone are located.The districts were Wonosari, Paliyan, Playen, Semanu, Karangmojo, Ngawen, Nglipar, Semin, and Ponjong.Wonosari Basin and Gunungsewu Karst Transition Zone located in Gunungkidul Regency, Indonesia.The basin has a bowl-like shape with flat to undulating topographies.It is bordered by the Gunungsewu Karst Subzone to the south, Panggung Masif Mountains to the east, and the mountainous Baturagung Subzone to the north.The map of the research area is shown in Figure 1.The study area has a tropical climate with high rainfall, with an average of 2321 mm/year according to the rain data in the last ten years (2012-2022) [8].Local water resources include rivers and groundwater.There are six perennial rivers: Oyo, Mungger, Kerco, Kedungdowo, Gowang, and Jirak.Kedungdowo, Gowang, and Jirak flow into the karst hills of Gunungsewu and form several lakes [11], such as Sureng Lake.Based on the Groundwater Database Compilation Report by the Mining and Energy Office [12], the dug wells in the Gunungkidul Regency are 6 m to 23 m deep, and the drilled wells can be 30 m to 150 m deep.Measurements of groundwater level by Febriarta (2011) found groundwater tables at 3-30 m.

Data acquisition methods
The research used primary data (obtained through field surveys) and secondary data (obtained from relevant agencies).The primary data included soil texture and thickness and the location of swallow holes (ponors) and sinking streams.The secondary data were Digital Elevation Model (DEM), Sentinel-2A image, drill data, and rainfall in 2012-2022.

Assessment of groundwater vulnerability to contamination
Groundwater vulnerability to contamination was assessed using the modified COP method [10], which is a modification of the COP method developed by COST Action 620.The modification is based on the ka rst characteristics, or variables, in Gunungsewu that are different from those in Europe [10].The swallow hole's recharge area replaced distance to the swallow hole to accommodate the concentrated and diffuse flows in the ponor's recharge area, and other variables like slopes and vegetation were integrated into the COP factors for their roles in inhibiting or accelerating flow rates [10].Similar to the COP method, the modified one considers three parameters: concentration of flow (C-Factor), overlaying layer (O-Factor), and precipitation (P-Fa ctor).The three COP parameters have different variables.The C-Factor comprises surface features, distances to swallow holes and sinking streams, and slope and vegetation.O-Factor represents the protection value that the overlaying conditions provide based on soil and lithology conditions.P-Factor indicates the quantity and temporal distribution of precipitation.Each of these variables has a score and is subdivided into several classes with distinct values, as summarized in Figures 2, 3, and 4.   C-Factor was divided into two scenarios: Scenario 1 for the swallow hole's recharge area and Scenario 2 for other areas [13].The analysis included two types of swallow holes: ponors and sinking streams.A sinking stream is a stream emptying into the underground through a swallow hole.Scenario 1 took into account the swallow hole's recharge area and slope and vegetation, while Scenario 2 included surface features and slopeand vegetation.However, the scores assigned to slope and vegetation in both scenarios represented different responses to runoff.For instance, terrain with a steep slope and low vegetation density in the swallow hole's recharge area was given a high score (1) because this condition makes surface water flow into the swallow hole quickly.In other areas, gently sloping terrain with high vegetation density was given a high score (1) because infiltration can occur diffusely in these physical conditions.

Results and discussion
4.1 Groundwater vulnerability variables

Concentration pf flow
The concentration of flow (C-Factor) demonstrates the complexity of recharge through unsaturated zones and potential runoff that seeps into overlaying layers [15].Therefore, C-Factor should be considered if the infiltration is concentrated at swallow holes (ponors), but it is negligible if the recharge water infiltrates diffusely without significant contributions from concentrated flows [13].
The C-Factor map of Scenario 1 was an overlay of two variables, i.e., swallow hole's recharge area (dh) and slope and vegetation (sv), while that of Scenario 2 was an overlay of surface features (sf) and slope and vegetation (sv).C-Factor mapsdescribe the reduction of the groundwater protection level based on the C-Factor.The concentration of flow indicates whether (or not) surface runoff infiltrates directly into groundwater through swallow holes like ponors or sinking streams, which can increase (or decrease) groundwater vulnerability [16].Thus, from this factor, it can be inferred that swallow holes greatly affect groundwater vulnerability to contamination.
Based on the C-Factor Map presented in Figure 5, the reduction of the groundwater protection level varied from very low (score=0.8-1),low (0.6-0.8), moderate (0.4-0.6), high (0.2-0.4), and very high (0-0.2).More than half of the Wonosari Basin area (67.72%) and the Gunungsewu Karst Transition Zone (52.19%) had moderate reduction (0.4-0.6).A very high reduction was spread in a larger area in the latter (6.07%) than in the former (1.79%) due to the presence of swallow holes in the transition zone that reduces protections to groundwater.Also, the travel time for surface water (also potential contaminants) to reach groundwater was shorter, indicating a higher potential for groundwater contamination.Table 1 shows the areal distribution (in km2 and %) of the reduction of groundwater protections based on the C-Factor in the two study areas.
In this case, the slope and vegetation density's influence on groundwater vulnerability was negligible because of the relatively homogenous terrain and high vegetation density in the study areas.The C-Factor Map shows that the high and very high reduction of groundwater protection coincided with a narrow swallow hole's recharge area, while the moderate one was in a broad swallow hole's recharge area.The nar ow recharge area creates a shorter travel time and thus accelerates the entry of runoffs into the groundwater through the swallow hole; hence, low groundwater protection.Low and very low reduction of protections was evenly spread outside the swallow hole's recharge area.This distribution is due to the impermeable-permeable layer in the surface features, contributing to higher protection.Overall, the Gunungsewu Ka rst Transition Zone has more areas with a very high reduction of groundwater protection than the Wonosari Basin. Figure 5 shows that the protection value was distributed following the pattern of rock formation and soil series.Moderate protection (2-4) was distributed in the Bulurejo and Sendangsari soil series; the former has sandy loam-textured soils with a thickness of >1 m, while the latter has silt loam soils thinner than the former (0.5-1 m).High protection (4-8) is in the Wonosari Formation and Alluvium and Oyo Formation, consisting of limestone and are permeable.Very high protection values (8)(9)(10)(11)(12)(13)(14)(15) were in Kepek and Semilir Formations, which are water-impermeable.The former consists of marl, and the latter is composed of tuff, making it difficult for pollutant-carrying water to enter the aquifer system.The laboratory analysis showed that sandy loam soils were composed of 60.66% sand, 27.14% silt, and 12.20% clay, and silt loam soils had 21.90% sand, 56.56% silt, and 21.54% clay.Sandy loam contains more sand but less silt than silt loam.Sand has a coarse texture, loose structure, and large micropores; thus, water can drain quickly.Therefore, even though sandy loam soils in the study area are more than 1 m thick, surface water can easily flow into the underground system because of the soil's nature.Silt has a small grain size, 0.002-0.05[19], larger than clay.This characteristic allows water to drain quickly and enter the aquifer system, especially in layers with a thickness of 0.5-1 m like the silt loam soils in the study area.In addition to soil texture, the limestone that makes up the lithology is permeable, creating a high possibility of surface water (and potential contaminants) infiltrating into the aquifer and thus relatively low protection.

Overlaying layer
Overlaying layer (O-Factor) refers to the protection of the aquifer above an unsaturated zone.It factors in the physical properties and thickness of layers in unsaturated zones [17] to determine its contaminant attenuation capacity, namely texture, soil thickness, lithology, unsaturated zone thickness, and degree of aquifer confinement [18].To put it simply, the O-Factor is a function of soil variables (Os, i.e., soil texture and thickness) and lithology variables (Ol, i.e., rock type, impermeability, and thickness.
Soil texture and thickness data were collected from units of soil series, and the study area consisted of 24 soil series.Based on sampling and laboratory analysis, clay was the dominant soil texture, as evidenced by the hard and dense soil observed in the field.Clay has a very small grain size, which is <0.002 mm, and micropores that are larger than macropores, allowing it to retain water for a very long time [19].As a result, surface water is much more likely to run off than seep into the ground into an aquifer system.The soil thickness measured in the field was 0.5-1 m and >1 m.A thinner soil means lower protection against Contamination because the distance from the soil surface to groundwater is shorter, and surface water can enter an aquifer system more quickly.
The rock types were interpreted from the geoelectric data in the Final Report of the Detailed Exploration Survey for Geological Environment of Groundwater in Gunungkidul Regency in 2020 and the Drill Data for the Wonosari Basin area in the Appendix [12].Based on these data, the lithology consists of marl and limestone from the Kepek Formation, limestone from the Wonsoari Formation, Alluvium, Oyo Formation, and tuff from the Semilir Formation.Because tuff can be aligned with volcanic breccias, it was classified as cemented or non-fissured conglomerates and breccias (score=100) in the COP classification.Based on this E3S Web of Conferences 468, 08002 (2023) https://doi.org/10.1051/e3sconf/202346808002ICST UGM 2023 lithology, the impermeability characteristics of each rock were obtained.Marl and tuff are impermeable, whereas limestone is permeable [12].As for the thickness of the lithology, it was determined from the depth to the water table measured during the field survey in 2011 [11].This technique was selected because the O-Factor represents the unsaturated zone, i.e., layers above the water table.
Soil (Os) and Lithology (Ol) variables were overlaid, creating the O-Factor Map presented in Figure 6.The map provides information on three protection values in the study area: moderate, high, and very high.Table 2 shows the area (km2 and %) of the protection value (O-Factor) in the Wonosari Basin and the Gunungsewu Karst Transition Zone.Both Figure 6 and Table 2 indicate that more areas in the Wonosari Basin had very high protection values than the Gunungsewu Karst Transition Zone.

Precipitation
Precipitation (P-Factor) includes total annual rainfal, frequency, duration, and intensity of extreme events that significantly influence the type and amount of infiltration as a determinant of groundwater vulnerability to contamination.High rainfall and large infiltration result in high groundwater recharge and rapid percolation through unsaturated zones, leading to fast transport of contaminants.However, at the same time, high rainfall induces intensive dilution rates and thus a shorter duration of contamination [13].The P-Factor was determined from two variables: precipitation quantity (PQ) and its temporal distribution (PI) [17].
Based on the rainfall data at 18 observation stations in the last ten years (2012-2022), the study area receives high rainfall, averaging 2321 mm/year.Precipitation quantity (PQ) of more than 1600 mm/year was calculated from the average annual rainfall in wet years at each station.Surface contaminants are mainly carried by precipitation (or runoff) into groundwater, wh ch means that high PQ increases the transfer of pollutants into groundwater through infiltration.However, large volumes of rainwater can also dilute contaminants.Therefore, rain intensity that is too large or too smal was given a high score in the groundwater vulnerability assessment.In addition to quantity, the temporal distribution of precipitation is also a factor.Based on the duration, the rainfall intensity exceeded 10 mm/day and might reach more than 20 mm/day.
The sum of the precipitation quantity (PQ) score and the temporal distribution (PI) score yields a P score that describes how much groundwater protection is reduced.A low P score represents a high reduction of protection, meaning that the groundwater is highly vulnerable to contamination.The protection reduction (P-Fa ctor) is presented in a map in Figure 7.
Based on the analysis, there were three levels of reduction: moderate, high, and very high.The areas km2 and %) of the reduction of groundwater protection (P-Factor) in the two study areas are presented in Table 3. Very high reductions of protection (score= 0.4-0.The reduction of protection was very high due to high precipitation quantity (2000-2400 mm/year) and high temporal distribution (>20 mm/day).High rainfal can cause a rapid flow of transport, creating a high potential for rainwater to transform into runoffs instead https://doi.org/10.1051/e3sconf/202346808002ICST UGM 2023 of seeping into the ground.On the contrary, very low rainfall creates small and slow transport flows into the soil.

Groundwater vulnerability to contamination
Groundwater vulnerability to contamination in the research areas was obtained by overlaying three COP parameters, i.e., concentration of flow (C-Factor), overlaying layer (O-Factor), and precipitation (P-Factor).Figure 8 shows the distribution of groundwater vulnerability to contamination.There are five classes of groundwater vulnerability: very low, low, moderate, high, and very high.
The groundwater vulnerability was generally moderate (covering more than half of the study area, 176.87 km2).The second-largest distribution was the low groundwater vulnerability (139.47 km2).Very low and very high groundwater vulnerabilities had a much smaller area.Details of the vulnerability levels and their areal distribution (km2 and %) in the Wonosari Basin and Gunungsewu Karst Transition Zone are presentedin Table 4.More areas in the Wonosari Basin (10.48 km2) had very low groundwater vulnerability than in the Gunungsewu Karst Transition Zone (0.07 km 2).This level of vulnerability was concentrated in the west, which is composed of marl and self-impermeable lithology due to the presence of joints [6].The majority of groundwater in the study areas had a low vulnerability to contamination, with a wider distribution in the Wonosari Basin (123.30km2) than in the Gunungsewu Karst Transition Zone (0.07 km 2).Low groundwater vulnerability was concentrated in the western and northern parts of the basin (near Baturagung hills) and the western area of the transition zone (in Paliyan District).Based on the distribution pattern, low and very low groundwater vulnerabilities are influenced by the impermeable layer on the surface features variable.Impermeable layers or lithologies prevent runoffand the contaminants they carry from infiltrating into the soil and entering groundwater, thus indicating high groundwater protection and very low vulnerability to contamination.
Moderate groundwater vulnerability had the la rgest area in the Wonosari Basin (152.67 km 2) and the Gunungsewu Karst Transition Zone (24.20 km2).It was distributed in nearly all of the study areas, especialy at the center.Furthermore, high groundwater vulnerability was found in a larger area (17.25 km2 in the Wonosari Basin and 6.18 km2 in the Gunungsewu Ka rst Transition Zone) than its very low and very high counterparts.A very high vulnerability to contamination was concentrated in the transition zone (2.70 km2), especially in the swallow hole's recharge zone with a narrow area of < 1 km 2. This finding corresponds to previous research [14] [20], where high and very high groundwater vulnerabilities have been attributed to the swallow hole's recharge area.In addition, this level of vulnerability was found in a smaller distribution in the Wonosari Basin (1.27 km2).
In terms of geomorphology, the high and very high groundwater vulnerabilities were mostly distributed in the south, i.e., Gunungsewu Karst Transition Zone.Assessment of groundwater vulnerability to contamination using the modified COP method showed that the karst transition zone had a higher vulnerability than the non-karst area (Wonosari Basin).This result is in line with previous research [17], where high and very high groundwater vulnerabilities were identified in Sierra d Li'bar, a karstic landform (swallow holes and sinking streams), while a moderate one was found in Torremolinos with only a few karst morphologies.
The Gunungsewu Karst Transition Zone has karst features such as swallow holes, including ponors and sinking streams that share a similar definition.Swallow holes or ponors are commonly found beside or at the lowest point of a closed depression where water on the surface can flow into the subsurface system [21].Sinking streams are a type of surface flow that disappears into the subsurface system [22].In an allogeneic system (swallow hole's recharge area), recharge from rainwater (and the potential contaminants it carries) can enter the aquifer system through swallow holes (sinking streams and ponors).
C-Factor (concentration of flow), O-Factor (overlaying layer), and P-Factor (precipitation) contribute to groundwater vulnerability to contamination from a hydrogeological perspective.
5) were dominant in the Wonosari Basin (115.18 km2) and the Gunungsewu Karst Transition Zone (3.69 km 2).Areas with moderate reduction (score= 0.7) covered 82.26 km2 of the Wonosari Basin area and 19.93 km2 of the Gunungsewu Karst Transition Zone.Areas with high reduction (score= 0.6) were found in 67.54 km2 of the Wonosari Basin area and 5.70 km2 of the Gunungsewu Karst Transition Zone.

Table 1 .
Distribution (km 2 and %) of the reduction of groundwater protection (C-Factor) in Wonosari Basin and Gunungsewu Karst Transition Zone.

Table 2 .
Distribution (km2 and %) of the protection values (O-Factor) in Wonosari Basin and Gunungsewu Karst Transition Zone.

Table 3 .
Distribution (km 2 and %) of the reduction of groundwater protection (P-Factor) in Wonosari Basin and Gunungsewu Karst Transition Zone.

Table 4 .
Distribution (km2 and %) of the Groundwater Vulnerabilities to Contamination in Wonosari Basin and Gunungsewu Karst Transition Zone.