Water Quality Status of Omang Pond in Gunungsewu Karst Area, Gunungkidul, Indonesia

. The Gunungsewu Karst Area has a unique hydrological system with limited surface water resources, especially in dry seasons. Karst ponds, thereby, play an essential role in meeting the local water needs. Omang is a karst pond used intensively for bathing and washing, which is thought to decrease water quality. The research objectives were (1) to determine the characteristics of the pond, (2) to analyze the pond water quality, and (3) to assess the water quality status. Water quality analysis was conducted at several points selected by purposive random sampling. Temperature, pH, DO, and TDS were measured directly in the field, while nitrate, phosphate, turbidity, BOD, and fecal coliform were tested in the laboratory. Also, the National Sanitation Foundation-Water Quality Index (NSF-WQI) was employed to assess the water quality status. Results showed that the pond has a shallow depth, making the bottom sediment prone to mixing, and a volume of 4,706.09 m 3 . Per Governor Regulation Number 20 of 2008, DO, BOD, nitrate, phosphate, and pH exceeded their respective upper limits for class II purposes. In contrast, temperature, turbidity, TDS, and fecal coliform met the recommended standards. Based on the NSF-WQI values, the status of the pond water quality was moderately polluted.


Introduction
Water resources are a fundamental element in meeting the daily needs of humans that are unevenly distributed across geographies.Physical characteristics such as rainfall, geology, topography, and land cover shape the hydrological condition of a region.Karst landscape is among the areas with unique hydrological properties.According to [1], karst is defined as a terrain with unique control over water movement and distribution because the constituent rocks dissolve easily and have welldeveloped secondary porosity.For these reasons, water falling on the surface quickly seeps through underground cracks and passages and accumulates in karst aquifers or underground rivers.According to [2], the surface hydrological system of a karst area centers on water-submerged dolines (i.e., doline ponds); thus, it can be said that a karst area is characterized by the formation of dolines, karst hills, and underground river systems.
Ponds are one of the water resources in a karst landscape.This landform gives considerable support to nearby populations in fulfilling their water needs [3].It makes a significant contribution to the overall water supply in the karst area, especially in dry seasons when harvested rainwater has run out.Residents mainly rely on perennial ponds, which bear water and never dry up throughout the year, for domestic purposes (e.g., bathing and clothes washing) and livestock farming (e.g., drinking and bathing).These activities produce and release waste directly into the pond system, potentially decreasing water quality.There are three environmental problems in ponds: reduced water storage, rapid water loss, and water quality decline [4].Reduced water storage refers to the diminished volume of water in ponds, which can increase the concentration of pollutants in water.
Omang Pond is one of the perennial ponds in the Gunungsewu Karst Area, Gunungkidul, Indonesia.It is located in Planjang, Saptosari District.Like many other perennial ponds, it is a reliable water supply to be used notably during dry seasons.Therefore, this research was conducted to (1) describe the characteristics of the pond, (2) analyze the pond water quality, and (3) assess the water quality status to determine the level of pollution.Studying pollution levels will provide essential information for maintaining the function of the pond as a usable water resource and for evaluating pond water management.

Method
In the context of environmental management, water quality parameters must be composited into a single water quality index (WQI) to indicate the status [5].The water quality status of a pond is shaped by the characteristics of its surroundings.In this study, the identified characteristics are the pond's water volume, land use in the catchment area, and pond utilization by the residents.Research data were collected by direct measurements in the field in December 2022, including soundings to measure the depth of the pond, water quality sampling, land use data validation, and interviews with selected residents to gather detailed information on pond utilization.
Depth soundings were conducted with an echosounder using a systematic random sampling method that divided the pond into grids, as seen in Figure 1.Water samples were collected once from four locations (Table 1), determined using a purposive sampling technique according to closeness to pollutant sources (see the yellow dots in Figure 1).Water was sampled at 0.5 depth from the water surface, which is assumed to be the center of the pond's water mass [6].To explain the water quality, the parameters observed were temperature, pH, DO, TDS, turbidity, nitrate, phosphate, BOD, and fecal coliform.The measured parameter values were compared against the class II water quality standards laid by the provincial government of Daerah Istimewa Yogyakarta in Governor Regulation Number 20 of 2008.Class II refers to non-potable water, which fits the current utilization of the pond for purposes other than drinking water supplies.The middle of the pond Point 3 Near the part of the pond that is used most intensively for washing and near a drainage channel Point 4 Near a water inlet and the part of the pond used for washing Source: Field observation, 2022 Water quality status was assessed using the NSF-WQI (National Sanitation Foundation-Water Quality Index), which is the most widely used index for water quality analysis in various countries and as a benchmark for addressing water pollution [7].NSF-WQI incorporates nine basic water quality parameters: temperature, pH, DO, TDS, turbidity, nitrate, phosphate, BOD, and fecal coliform, with a weighting system [8].NSF-WQI is the sum of the multiplication of the weight (Wi) and the sub-index (Qi) for the i-th parameter, as expressed in the formula below: The weight and sub-index values were computed with an online NSF-WQI calculator (at http://www.waterresearch.net/watrqualindex/index.htm).Table 2 shows the weighting value used for each parameter.Source: [8] The NSF-WQI classifies the water quality status into five categories, as presented in Table 3. 3 Results and Discussion

Pond Characteristics
The pond characteristics were determined by the volume of water, land use in the catchment area, and pond utilization by nearby populations.Omang Pond has a closed system with no outlets and inlets.In a closed body, the water input comes from rainwater, runoff, and surface water.However, because there are no flowing rivers around the pond, the water availability largely depends on rainfall.The pond has natural bottom sediments and is hemmed in concrete embankments, creating an area of 17,094 m 2 .Figure 2 shows the bathymetry map obtained from depth soundings, with a 3-dimensional visualization in Figure 3. Based on the calculation results, the pond had a water volume of 4,706.09m 3 , an average depth (Z) of 0.3 m, a maximum depth of 2.7m, and a relative depth (Zr) of 0.02%.Due to less stable stratification, the sediment is highly susceptible to mixing.Further, with a Zr of less than 0.02%, the water layer tends to be homogenous.The catchment area was delineated by considering the highest contour (topography) around the pond.This was used as the farthest limit of surface runoff that drains into the pond.Delineation results showed a catchment area of 215,704 m 2 .Google Earth imagery and ArcGIS base map interpretations found four types of land use in the area: multiple-species plantations, dryland farms, settlements, and ponds (Figure 4).The percentage of each land use area is shown in Table 4.  Based on the interpretation results and data validation in the field, the catchment area was dominantly used for multiple-species plantations (68%), followed by settlements (27%), dryland farms (6%), and the pond (8%).The multiple-species plantations were mainly comprised of woody plants, including banyan trees, Albizia chinensis, teaks, and coconut trees.The dryland farms were primarily cultivated with cassava, corn, groundnut, and other crops that can grow with little water (drought-tolerant) and in thin soil.Therefore, to achieve maximum yields, farmers increase soil fertility by applying amendments like urea and manure.
Interviews and direct observations revealed that the residents used the pond water for bathing, washing clothes, fishing, and drinking water for livestock (Figure 5).In addition, these activities took place directly in the pond water area.This situation stems from the uneven distribution of pipe water to every household in some neighborhood units (Rukun Tetangga, RT) in nearby sub-villages (Padukuhan): RT 02 in Tritis and RT 04 in Sumber.Both are located on the hillside, which may be the main challenge in installing the water transmission system by the stateowned water supply company, PDAM.The pond water is more intensively used in dry seasons when the storage tanks for harvested rainwater have dried up.In areas outside the reach of the pipeline installation, the residents would buy clean water for drinking and cooking and use the pond water for other purposes like clothes washing and bathing to save on water bills.

Water Quality
Temperature, pH, and TDS were gauged directly in the field, while nitrate, phosphate, BOD, and fecal coliform were tested in the laboratory.[9] stated that water temperature is regulated by sunlight intensity, heat exchange between the water and the surrounding air, geographical elevation, and vegetation canopy on the edge of a water body.Figure 6 shows that the water temperature at four sampling points ranged between 29.8°C and 31.2°C.The temperature at point 2 was higher than at the other points because it is located in the middle of the pond, which is not covered by the tree canopy and thus receives direct sunlight.
Dissolved oxygen (DO) is an essential macronutrient for the survival and metabolism of aquatic organisms [10].DO was calculated in the field immediately after water sampling to prevent any deviations due to temperature change.DO was found at 6.9, 6.8, 6.5, and 6.7 mg/L at the four sampling points (Figure 7), meeting the recommended standard.Per Governor Regulation No. 20 of 2008, DO should be at least 5 mg/L for class II water.Based on this parameter, the pond water quality is suitable for the survival of aquatic organisms.Thus, these four points, which have many fish populations, are also used for fishing.
TDS is a chemical parameter that is related to and controlled by other parameters like turbidity, hardness, alkalinity, and conductivity [11].TDS was gauged with a water checker.As shown in Figure 8, the four sampling locations had varying TDS levels: 310, 156, 321, and 286 mg/L, respectively.The highest TDS was detected at point 3, close to a drainage channel through which dissolved substances in its surroundings are discharged together with surface runoff into the pond.Based on the governor's regulation, the measurement results are below the maximum permissible concentration of TDS for class II purposes, 1000 mg/L.Turbidity indicates the amount of suspended solids in water, including colloids and fine particles [12].Figure 9 shows different turbidities at the four sample points: 42.6, 39.9, 58.8, and 48.6 NTUs.The highest turbidity was found at point 3, which is not only near a drainage channel but is also used most intensively for washing.Accordingly, this part of the pond receives more surface runoff and waste input than the other points.This turbidity corresponds to the high TDS at point 3.The governor regulation, however, does not include any standard or maximum allowable level for turbidity in class II water.High biological oxygen demand (BOD) demonstrates the high presence of nutrients and organic matter in the water, both in the form of dissolved solids and particulates [13].Based on the laboratory analysis, the BOD levels at the four sample points were 4.83; 3.62; 3.23; and 4.44 mg/L (Figure 10), which do not meet the safe limit per the governor regulation for class II water, i.e., 3 mg/L.Primary sources of nutrients and organic matter responsible for the high BOD levels at the pond are runoffs from multiple-species plantations and agricultural fields that transport leaf litter, dead plant material, and fertilizer residues to the pond.
Nitrate is the by-product of various human activities in the catchment area, especially livestock and crop farming [14].As seen in Figure 11, nitrate was detected at 32.3, 36.9, 35.7, and 39.7 mg/L, exceeding the maximum allowable concentration.The governor's regulation recommends that class II water should not contain nitrate higher than 10 mg/L.High nitrate levels in the pond can be attributed to the intensive agricultural activities in the catchment area.The highest concentration was identified at point 4, near a water inlet that drains surface runoff carrying fertilizer residues from multiple-species plantations and dryland farms into the pond.
Phosphate is most commonly present in wastewater in the form of organic phosphate from sewage and food scraps, orthophosphate from agricultural fertilizers, and polyphosphate from the use of synthetic detergents [15].Phosphate is released by the decomposition of organic and inorganic matter (industrial effluents and detergents), suggesting a directly proportional correlation between phosphate concentration and the amount of detergent in the water.Laboratory analysis found different phosphate levels at the four sample locations: 0.63, 0.22, 0.55, and 0.39 mg/L (Figure 12), exceeding the maximum permissible concentration laid by the governor regulation, 0.2 mg/L.Moreover, the highest phosphate level, 0.63 mg/L, was spotted at point 1, close to the part of the pond that is most intensively used for washing clothes with detergents.High phosphate concentrations suggest clothes washing and bathing very close to or in the pond water.
The degree of acidity (pH) is a physical parameter of water quality considered in determining and planning suitable water treatments.Freshwater ponds or lakes typically have a pH of 6.5-7.5.Hazardous materials discharged into the water can change the hydrogen ions (pH) to either more acidic or alkaline depending on the type and chemical content [16].Direct measurements in the field showed pH values of 9.6, 10, 9.9, and 10 (Figure 13), which are higher than the upper limit for class II water, i.e., > 9.This alkaline water can be attributed to the use of detergents.Liquid waste containing detergents can raise water pH and disturb the life of aquatic organisms [16].It may also result from the direct contact between the pond water and the natural bottom sediment, i.e., highly dissolved limestone.Limestone solution can increase the pH of the water.Fecal coliform is a biological parameter of water quality.Even though this type of bacteria does not directly cause diseases, its presence in the pond water indicates poor sanitation.Primary sources of fecal coliforms are human and animal feces [17].Results showed that the four water samples contained 75, 460, 210, and 460 MPN/100 ml of fecal coliform (Figure 14), which are within the safe limit for class II water according to Governor Regulation No. 20 of 2008, i.e., < 1,000 MPN/100 ml.Interviews with residents around the pond revealed that livestock bathing was no longer allowed in the pond; thus, no input from animal feces from this source.

Water Quality Status
Water quality status was determined using the National Sanitation Foundation-Water Quality Index (NSF-WQI) based on nine parameters: temperature, pH, DO, BOD, TDS, turbidity, phosphate, nitrate, and fecal coliform.NSF-WQI represents the overall and actual water quality of Omang Pond, making it easier to understand, monitor, and compare water quality conditions regularly (Table 5).
Calculation results showed that the NSF-WQI values of 56.1 at point 1 (medium or moderately polluted), 49.8 at point 2 (bad), 57.5 at point 3 (medium), and 49.5 at point 4 (bad).With a mean value of 53.22, the Omang Pond shows indications of moderate pollution.This can be linked to several parameter values that exceed their respective upper limits, including DO, BOD, nitrate, phosphate, and pH.

Figure 1 . 2 .
Figure 1.Grid sampling for depth sounding (bathymetry) and water sampling points for water quality assessment in Omang PondTable 2. Weights of the parameters used in the National Sanitation Foundation-Water Quality Index (NSF-WQI) calculation for Omang Pond Parameter Weight DO (Dissolved Oxygen) 0.17 Fecal Coliform 0.16 pH 0.11 BOD (Biochemical Oxygen Demand) 0.11 Temperature 0.10 Phosphate 0.10 Nitrate 0.10 Turbidity 0.08 TDS (Total Dissolved Solids) 0.07

Figure 4 .
Figure 4. Land use map of the catchment area of Omang Pond Source: Data analysis, 2023

Figure 5 .
Figure 5. Part of the Omang Pond that is commonly used for washing clothes Source: Personal documentation, 2022

Table 3 .
Water quality status classification according to the NSF-WQI method

Table 4 .
Land use area and percentage in the catchment area of Omang Pond

Table 5 .
Water quality index calculation using the NSF-WQI methodOmang Pond is a body of water with a closed system and, thus, no outlets.Consequently, surface runoff, together with waste and other contaminants from different land use forms in the catchment area, accumulate in the pond.With a maximum depth of only 2.7 m, the bottom sediment is prone to mixing upon disturbances, affecting the amount of solids in the water.The catchment area is dominated by multiple-species plantations that produce waste in the form of fertilizer residues and leaf litter.Other sources of pollution are bathing and clothes washing in several parts of the pond area.Per Governor Regulation Number 20 of 2008, DO, BOD, nitrate, phosphate, and pH levels have exceeded their maximum allowable presence in class II water.Higher concentrations of pollutants have been found at sample points near part of the pond that is frequently used for washing and near the water inlet.Based on the NSF-WQI assessment, the water quality status of the pond is moderately polluted.