View of Water quality and distribution of Selorejo Reservoir using the Pollution Index, Oregon-WQI, and NSF-WQI methods

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Jurnal Teknik Pengairan: Journal of Water Resources Engineering, 2023, 14(1) pp. 64-75 https://jurnalpengairan.ub.ac.id/ | p-ISSN : 2086-1761 | e-ISSN : 2477-6068

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Water quality and distribution of Selorejo Reservoir using the Pollution Index, Oregon-WQI, and NSF-WQI methods

Ken Diah Mangar Nastiti^1*), Rini Wahyu Sayekti¹, Emma Yuliani¹

1Department of Water Resources Engineering, Faculty of Engineering, Brawijaya University, Malang 65145, Indonesia

Article info: Research Article

DOI:

10.21776/ub.pengairan.2023.014.01.06

Keywords:

Oregon-WQI; pollution index; pollution load; reservoirs; water quality

Article history:

Received: 15-12-2022 Accepted: 22-05-2023

*)Correspondence e-mail:

[email protected]

Creative Commons License

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

Abstract

Selorejo Reservoir is used for flood control activities, hydropower, agricultural irrigation, fisheries, and tourism. In 2021, the water quality of Selorejo Reservoir worsened, which was indicated by fish mortality and an increase of water lettuce plants; the decline in water quality of Selorejo Reservoir may be due to agricultural and farming waste that was carried by surface runoff and entered the reservoir. This study was conducted to determine the status of water quality in Selorejo Reservoir using the Pollution Index, Oregon-WQI, CCME-WQI, and NFS-WQI methods in the dry season based on the parameters of pH, BOD, COD, DO, TSS, Nitrates, Ammonia, Phosphate, and fecal coliform. Based on the Pollution Index method, the water quality was in the "Slightly Polluted" condition. The Oregon-WQI method indicated the

"Severely Polluted" condition. The CCME-WQI method indicated the "Good" condition. The NSF-WQI method indicated the "Fair"

condition. The trophic status of Selorejo Reservoir was found to be eutrophic, with high levels of phosphorus. Calculation of the pollution carrying capacity based on the Total-P of the dry season in 2021 showed that the upstream, middle, and downstream parts of the reservoir are respectively able to accommodate 71.77 mg/m³, 80.14 mg/m³, and 77.90 mg/m³.

Cite this as : Nastiti, Ken Diah Mangar, Sayekti, Rini Wahyu, Yuliani, Emma. (2023). Water Quality and Distribution of Selorejo Reservoir using the Pollution Index, Oregon-WQI, NSF-WQI methods. Jurnal Teknik Pengairan: Journal of Water Resources Engineering, 14(1), page. 64-75. https://doi.org/10.21776/ub.pengairan.2023.014.01.06.

1. Introduction

Water is a component that is very much necessary for the continuity of life for living creatures.

Law No. 17 of the Year 2019 on Water Resources states that water comprises all forms found above or below the soil surface, including rainwater, groundwater, surface water, and inland seawater [1]. Water left over from human activities allows for the reduction of water quality, or water pollution, to occur [2]. Reduction of water quality can potentially cause various problems for living organisms. Therefore, to check and control water to remain in a safe and non-hazardous condition, it becomes necessary to conduct water quality observations.

Reservoirs function as sources of water for humans and other organisms. Reservoirs are ponds that are created by dams that block off rivers. Rivers have a great potential to receive inputs of waste that originate from agricultural areas and residential settlements and may contain phosphorus, which is then carried by rainwater runoff into channels and brought to the reservoir.

An excessive phosphorus content may increase the growth of phytoplankton, which may cause algae blooms and affect the water quality in the reservoir. Therefore, for this research, water quality tests are conducted to maximize the utilization of Selorejo Reservoir.

Regarding its trophic status, for the dry season and the wet season, Selorejo Reservoir experienced eutrophication; in terms of percentages, the reservoir was 4.17% mesotrophic, 83.3%

eutrophic, and 12.5% hypereutrophic, for which the most dominant status of eutrophic is taken [3].

In July 2021, Selorejo Reservoir experienced a reduction in water quality conditions, which caused many fish to die suddenly in the reservoir and the reservoir's surface to be filled with water lettuce

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65 Nastiti, Sayekti, Yuliani: Water and Quality Distribution in Selorejo Reservoir

plants. It may be that the reduction of water quality in the Selorejo Reservoir was caused by waste from agriculture and farming that then entered the reservoir along with rainwater and surface runoff [4]. In addition, fertilizers left behind from agricultural areas surrounding the Selorejo Reservoir may contribute significantly to nutrients in the reservoir water, which may cause eutrophication [5].

Considering that the water quality of reservoirs can change at any time or may be said to be fluctuating, and based on the problem that occurs in Selorejo Reservoir, it becomes necessary to conduct a review to find out the water quality status of Selorejo Reservoir at present. This research used four methods, which are Pollution Index (Indeks Pencemaran, IP), a method often used in Indonesia; Oregon-WQI, a method that is used to analyze a set of established water quality variables and to result in a score that can illustrate water quality generally; CCME-WQI, a method for evaluating changes in water quality in a certain location from time to time and to compare overall indices among locations that use the same variables and quality standards; and NFS-WQI, a method possessing several advantages, one of them being the capability to show water quality representatively through the use of nine parameters [6]. What differentiates this research from prior research is that this research only involves the analysis of the dry season to find out the pollutants that originate from land upstream of the reservoir. The utilized parameters are physical (pH, TSS), chemical (BOD, COD, DO, Nitrates, Ammonia, Phosphates), and biological (fecal coliform) parameters that are evaluated each month for the period of the last ten years (2012-2021) to find out the fluctuations in the water quality of Selorejo Reservoir.

After analyzing the water quality status, the next step is calculating the carrying capacity of the pollution load to find out the capability of the reservoir to contain pollutants by using the total-P parameter for 2021. Suppose the water quality status of the reservoir is not in a polluted state. In that case, it is expected that the water quality status of the reservoir can be maintained. In contrast, if the water quality status is polluted, it is expected that the water quality status can be improved.

This research is expected to be used as a consideration or a reference to conduct further research on the water quality status and distribution of Selorejo Reservoir.

2. Materials and Methods

The sub-section of Materials and Methods describes obtaining the research results within a given time and at the specified research location [7]. All measurements reported in the Results and Discussion sub-section reference the utilized method for obtaining the results [8].

All observations reported in the Results and Discussion sub-section are to result from procedures or steps that can be reproduced and have been clearly outlined in the Materials and Methods sub-section. Therefore, the measurements reported in the Results and Discussion sub- section need to be ensured to represent trustable results [9].

2.1. Materials

2.1.1. Research Location

The selected location for this research is the Selorejo Reservoir, which is located in Selorejo Hamlet, Pandansari Village, Ngantang Sub-District, Malang Regency. The coordinate location of Selorejo Reservoir is at 7° 51' 55" South Latitude and 112° 21' 40" East Longitude at a height of ± 618 m above sea level. Selorejo Reservoir receives water from the Konto River, Kwayangan River, and Pinjal River. This reservoir, which PJT I manage, has a surface area of approximately 2.96 million m². The sampling points for water quality are located upstream, with sampling conducted at depths of 0.3 m and 5 m, in the middle at depths of 0.3 m, 5 m, and 10 m, and downstream at depths of 0.3 m, 5 m, and 10 m. The study location of Selorejo Reservoir is displayed in Figure 1.

2.1.2. Research Data

The execution of this research requires data that are obtained from two institutions, the Department of Public Works and Natural Resources of Malang Regency and PJT I. The following are the required data:

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1. Rainfall Data

This research used monthly rainfall data for ten years, covering 2012-2021. Rainfall data were obtained from the Malang Regency Department of Public Works and Natural Resources.

Rainfall data was taken from five rain stations, as the following:

- Pujon Rain Station (7° 50’ 20.92” SL and 112° 28’ 1.76” EL) - Ngantang Rain Station (7° 5’ 15.26” SL and 112° 22’ 13.08” EL) - Sekar Rain Station (7° 53’ 59.53” SL and 112° 22’ 5.09” EL) - Kedungrejo Rain Station (7° 51’ 31.11” SL and 112° 25’ 57.28” EL) - Jombok Rain Station (7° 49’ 25.5” SL and 112° 22’ 11.9” EL)

Figure 1. Location Map of Selorejo Reservoir 2. Water Quality Parameter Data

This research used water quality data from the dry season for ten years, from 2012-2021. The utilized methods were Pollution Index, Oregon-WQI, CCME-WQI, and NFS-WQI, requiring the water quality parameters of pH, BOD, COD, DO, TSS, nitrates, ammonia, phosphates, and fecal coliform. The utilized water quality parameters comprise secondary data, and they were obtained from samples taken at the following points:

- Upstream Point at depths of 0.3 m and 5 m - Middle Point at depths of 0.3 m, 5 m, and 10 m - Downstream Point at depths of 0.3 m, 5 m, and 10 m

3. Data on the volume (V), reservoir surface area (A), and outflow discharge (Qo) of Selorejo Reservoir for 2021.

4. DEM-NAS Data of Malang Regency, which was used to map out the distribution of water quality of Selorejo Reservoir with ArcMap 10.8.

2.2. Methods

2.2.1. Seasonal Division

Hydrologic analysis was performed to determine the division of seasons; for this research, analysis was carried out on rainfall data from the Ngantang, Pujon, Sekar, Kedungrejo, and Jombok rainfall stations with the double mass curve method to test for the consistency of data. In addition,

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the Mann-Whitney test was performed to check whether two classes of unpaired data originate from the same community [10], and stationary testing was performed with the F-test and T-test.

Suppose an area has several rain stations that are spread out; the rainfall value can be obtained by calculating its mean [11]. The mean regional rainfall was analyzed using the arithmetic mean method. Determining the division of seasons was based on the method of Schmidt and Ferguson by evaluating the total monthly rainfall. If the rainfall in a month has a value of 100 mm, the month is considered wet, and conversely, if the value is less than 100 mm, the month is considered dry [12].

2.2.2. Analysis of Water Quality Status

The analysis used methods of Pollution Index, Oregon-WQI, CCME-WQI, and NFS-WQI, with the following equations:

1. Pollution Index Method

The Pollution Index is designed to measure the water quality of certain bodies of water and to implement measures to improve the water quality if it becomes degraded by polluting substances [13]. This method compares data on water quality parameters with the Class 2 water quality standard. The utilized parameters for this method were pH, BOD, COD, DO, TSS, nitrates, ammonia, phosphates, and fecal coliform, based on the following equation:

𝑃𝐼_𝑗= √⁽

𝐶𝑖

𝐿𝑖𝑗)𝑀²+(^𝐶𝑖

𝐿𝑖𝑗)𝑅²

2 (1)

PIj is the Pollution Index for j, Ci represents each parameter's analyzed concentration, and Lij represents the concentration requirement according to the water quality standard for j. Then, (Ci/Lij)M represents the largest Ci/Lij value, and (Ci/Lij)R represents the average Ci/Lij value.

2. Oregon-WQI Method

The Oregon-WQI method is designed to analyze a set of certain water quality variables and result in a ranking that can represent water quality in general [14]. The utilized parameters for this method were pH, BOD, DO, TSS, nitrates, ammonia, phosphates, and fecal coliform. Each parameter was evaluated using the parameter sub-index of Oregon-WQI before being evaluated with the following equation:

𝑂𝑊𝑄𝐼 = √_∑ ^𝑛¹

𝑆𝐼2 𝑛𝑖=1

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In the above equation, Si represents the sub-index of Oregon-WQI for each parameter, and n represents the overall sum of parameters.

3. CCME-WQI Method

This method assists in evaluating the change in water quality from time to time in a certain location and compares the overall indicators of each location using the same variables and water quality standards [15]. This method also compares existing data on water quality parameters with the Class 2 water quality standard. The utilized parameters for this method were pH, BOD, COD, DO, TSS, nitrates, ammonia, phosphates, and fecal coliform, with the following equation:

𝐶𝐶𝑀𝐸 − 𝑊𝑄𝐼 = 100 − [^{√𝐹1+𝐹2+𝐹3}_1.732 ] (3) In the above equation, F1 represents the percentage of variables that do not fulfill the quality standard, F2 represents the percentage of parameters that do not fulfill the quality standard, and F3 represents the sum of tests that failed to meet the quality standard.

4. NSF-WQI Method

This method multiplies the sub-index value of parameters with the usage of the curve NSF- WQI by the parameter weight. The parameter weight utilized a modified weight according to the number of used parameters. The utilized parameters for this method were pH, BOD, DO, TSS, nitrates, phosphates, and fecal coliform, with the following equation:

𝑁𝑆𝐹 − 𝑊𝑄𝐼 = ∑^𝑛_𝑖=1𝑊𝑖. 𝑞𝑖 (4)

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In the above equation, Wi represents each parameter's weight, and qi represents each parameter's sub-index.

2.2.3. Distribution Mapping with the Inverse Distance Weighting (IDW) Method

Mapping was performed with the ArcMap 10.8 application. It began with evaluating the percentage of reservoir pollution to find out the most dominant water quality status of Selorejo Reservoir for each method. Then, in the ArcMap 10.8 application, in ArcToolbox, spatial analyst tools were selected, followed by interpolation and IDW. Finally, for the input of xyz data, x and y represent the coordinates of the upstream, middle, and downstream sampling points, while z represents the pollution percentage by the methods of Pollution Index, CCME-WQI, Oregon-WQI, and NSF-WQI.

2.2.4. Analysis of Pollution Load Carrying Capacity for Selorejo Reservoir

To find out the pollution load of the reservoir, it is first necessary to analyze the trophic status of the reservoir. It is followed by calculating the average reservoir depth (Z) and rate of change of reservoir water (ρ). Next, the pollution load carrying capacity was calculated based on Total-P data for the dry season 2021, referring to Environmental Ministry Regulation No. 28 of 2009.

[Pa]STD = [Pa]i + [Pa]DAS + [Pa] (5) [Pa]d = [Pa]STD − [Pa]i − [P]DAS (6) [Pa]STD represents the maximum value of parameter Pa based on the water quality standard in mg/m³ units. [Pa]I represent the content of parameter Pa by the results of reservoir measurement in mg/m³ units. [Pa]DAS represents the value of load allocation Pa for the watershed or catchment area in mg/m³ units. Finally, [Pa]d represents the location of load Pa as remnants of activities in the reservoir in mg/m³ units.

3. Results and Discussion 3.1. Results and Discussion

After testing the monthly rainfall data for the period of 10 years (2012-2021) from the rain stations of Ngantang, Pujon, Sekar, Kedungrejo, and Jombok based on the double mass curve method and performing the Mann-Whitney test and stationary tests (F-test and T-test), the mean regional rainfall was calculated by the arithmetic mean method and seasons were divided based on the method of Schmidt and Ferguson. The results are displayed in Table 1.

Table 1. Seasonal Division

Year

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Des

2012 565.80 465.60 349.00 172.20 63.40 16.40 0.20 0.00 0.60 69.60 261.20 407.00

2013 914.60 508.20 353.00 413.80 203.60 245.60 123.60 0.40 0.00 87.40 344.60 640.20

2014 636.80 630.60 306.60 193.00 86.60 23.80 45.20 11.00 0.00 0.00 250.40 594.60

2015 174.60 416.40 450.20 303.20 129.20 42.40 0.40 0.00 4.60 1.00 133.20 422.60

2016 394.60 567.20 475.20 160.40 208.40 245.60 118.60 64.00 132.20 225.20 308.60 347.00 2017 571.40 339.00 451.60 300.20 224.40 95.20 44.80 6.80 24.20 91.20 411.60 389.60

2018 869.20 596.80 335.40 141.20 69.00 46.00 2.00 2.00 9.20 16.40 213.80 349.00

2019 593.20 477.40 482.20 326.00 75.20 2.00 2.00 2.00 3.00 32.20 132.00 345.20

2020 452.20 849.80 538.00 348.60 242.60 64.60 24.40 26.60 31.00 170.60 367.00 696.00 2021 1026.40 1041.60 401.00 274.20 48.40 241.60 17.00 52.60 110.60 92.80 758.00 362.20 Total 6198.80 5892.60 4142.20 2632.80 1350.80 1023.20 378.20 165.40 315.40 786.40 3180.40 4553.40 Mean 619.88 589.26 414.22 263.28 135.08 102.32 37.82 16.54 31.54 78.64 318.04 455.34

Remarks Wet Wet Wet Wet Wet Wet Dry Dry Dry Dry Wet Wet

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In Table 1, by the analysis based on the method of Schmidt and Ferguson for the period of 10 years from 2012-2021, it was found that the dry months with mean values less than 100 ml/month occurred for four months, and the wet months with mean values greater than 100 ml/month occurred for eight months. Therefore, this research used data from the dry months or those with mean values less than 100 ml/month for July, August, September, and October.

3.2. Analysis of Water Quality Status 3.2.1. Testing of Water Quality Data

Before conducting tests to determine water quality, it is necessary to perform homogeneity testing of the water quality data from each sampling point and depth of Selorejo Reservoir, as the upstream sampling point with depths I (0.3 m) and II (4 m), middle sampling point at depths I (0.3 m), II (5 m), and III (10 m), and downstream sampling point with depths I (0.3 m), II (5 m), and III (10 m). Homogeneity testing was performed by using the F-test. The testing results on the water quality data for Selorejo Reservoir for the dry months in the years 2012-2021 showed a homogenous condition.

3.2.2. Determining the Water Quality Status

Determination of the water quality status for Selorejo Reservoir was conducted using four methods: Pollution Index, Oregon-WQI, CCME-WQI, and NSF-WQI. The analysis results for determining the water quality status for Selorejo Reservoir for the dry seasons of 2012-2021 are shown in Table 2.

Table 2. Summary of Water Quality Percentages for Selorejo Reservoir from 2012-2021

Location Depth

IP (%) Oregon-WQI (%) CCME-WQI (%) NSF-WQI (%)

Good Lightly Polluted

Moderately Polluted

Heavily

Polluted Excellent Good Fair Marginal Poor Good Fair Polluted

Upstream

0.3 m 3 98 20 80 0 82 18 0 0 43 57 0

5 m 0 100 10 90 0 61 34 3 3 17 83 0

Middle

0.3 m 5 95 30 70 0 88 13 0 0 50 50 0

5 m 8 93 13 88 0 63 28 10 0 15 80 5

10 m 3 98 10 90 3 50 43 3 3 10 88 3

Downstream

0.3 m 13 88 25 75 5 88 8 0 0 50 50 0

5 m 3 98 8 93 0 58 40 3 0 13 88 0

10 m 5 95 5 95 0 48 50 3 0 13 85 3

Table 2 shows that for the percentage values of water quality status, based on the Pollution Index method, the most dominant water quality status was indicated to be Lightly Polluted, with a percentage of 93-100%. For the Oregon-WQI method, the most dominant water quality status was indicated to be Heavily Polluted, with a percentage of 70-95%. For the CCME-WQI method, the most dominant water quality status was indicated to be Good, with a percentage of 48-88%.

Finally, for the NSF-WQI method, the most dominant water quality status was indicated to be Fair, with a percentage of 50-88%.

In this research, no comparisons were performed between methods because each method has different procedures. For example, the method for determining water quality status that the Government of Indonesia recommends is Pollution Index, but the selection of the Oregon-WQI, CCME-WQI, and NSF-WQI methods had the objective of being able to affirm the determination results from the Pollution Index. As such, many different results occurred in the analyses for determining water quality status because each method has different classification methods, procedures, and required parameters.

The determining of water quality status based on the Pollution Index method according to Minister of the Environment Decree No. 115 of the Year 2003 is by calculating the value of the

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Pollution Index and then comparing it with the water quality status according to the established quality standard, for which this research used the Class II quality standard. The calculation procedure for the Pollution Index method is that a new value is sought if a parameter value is not in line with the quality standard. Further, the calculation of parameters with quality standards that have ranges is different from those that do not possess ranges. In this research, the results of determining the water quality status with the Pollution Index method for 2012-2021 showed the category of Lightly Polluted, which was mostly influenced by the parameters of TSS, BOD, and DO.

The CCME-WQI method also utilizes a water quality standard in determining the water quality status. CCME-WQI combines three elements: Scope or total parameters that do not meet the water quality standard, Frequency or total occurrences of targets that do not meet the quality standard, and Amplitude or how far off the target was not achieved. This research showed a water quality status of Fair to Good. In reflecting the water quality status, the index value of CCME-WQI ranges from 0 (Very Poor) to 100 (Excellent), divided into the five classes of Excellent (95-100), Good (80-94), Fair (65-79), Marginal (45-64), Poor (0-44). As such, it may be regarded that the established water quality status of Selorejo Reservoir in this research is in the condition of Lightly Polluted. The parameters that had the most influence were BOD and DO.

The two methods of Pollution Index and CCME-WQI utilized nine parameters, while the NSF- WQI method used seven parameters: pH, TSS, BOD, DO, NO3, PO4, and fecal coliform. In contrast to the other two methods, the NSF-WQI method does not compare with a quality standard;

instead, the weight value of each parameter is multiplied by the sub-index value of the test parameter, which is obtained from a curve. The weighting for each parameter involved performing proportional modifications for the seven parameters above. This research found that the water quality status of Selorejo Reservoir using the NSF-WQI method was Fair or Lightly Polluted.

Like the NSF-WQI method, Oregon-WQI does not utilize a water quality standard for comparison. However, for this research, the Oregon-WQI method resulted in a different indication from the three other methods: Heavily Polluted. This difference of indication in water quality status is tied to several factors. One of them is the difference in the number of utilized parameters: the Oregon-WQI method uses eight parameters, 7 of which are the same as those of NSF-WQI with the addition of the NH3 parameter. Although it differs by only one parameter from NSF-WQI, the Oregon-WQI method has a sub-index calculation that is quite complex for each parameter compared to NSF-WQI. Another factor may also be the effect from the ranges of index values possessed by Oregon-WQI, as Excellent (90-100), Good (85-89), Lightly Polluted (80-84), Moderately Polluted (60-79), and Heavily Polluted (10-59). The range for the Heavily Polluted classification is greater than for other classifications, for which this aspect may become a factor why most of the calculations for Oregon-WQI in this research resulted in the classification of Heavily Polluted. Considering the parameter value, DO has a small value, and thus when calculated with the sub-index for Oregon-WQI, the value is also small and affects the score reduction.

3.2.3. Mapping of Water Quality Distribution of Selorejo Reservoir with the IDW Method

Mapping was performed using the Inverse Distance Weighted (IDW) method in ArcMap 10.8 and based on the data from Table 2. The mapping of results from each analysis with the Pollution Index, Oregon-WQI, CCME-WQI, and NFS-WQI methods for each depth is displayed in Figures 2-5.

In Figure 2, by the results of water quality distribution mapping based on the Pollution Index method, a depth I had water quality distributions of "Lightly Polluted" for the upstream sampling point (97.5%), middle sampling point (95%), and downstream sampling point (87.5%). Depth II had water quality distributions of "Lightly Polluted" for the upstream sampling point (100%), middle sampling point (92.5%), and downstream sampling point (97.5%). Finally, depth III had water quality distributions of "Lightly Polluted" for the middle sampling point (97.5%) and downstream sampling point (95%). The parameters that affected pollution most greatly for this method were TSS, BOD, and DO. In addition, it illustrates the influence of wastewater and erosion in the upstream part.

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Depth I (0.3 m) Depth II (5 m) Depth III (10 m)

Figure 2. Mapping of Water Quality Distribution with the Pollution Index Method

Depth I (0,3 m) Depth II (5 m) Depth III (10 m)

Figure 3. Mapping of Water Quality Distribution with the Oregon-WQI Method

In Figure 3, by the results of water quality distribution mapping based on the Oregon-WQI method, a depth I had water quality distributions of "Heavily Polluted" for the upstream sampling point (80%), middle sampling point (70%), and downstream sampling point (75%). Depth II had water quality distributions of "Heavily Polluted" for the upstream sampling point (90%), middle sampling point (87.5%), and downstream sampling point (92.5%). Finally, depth III had water quality distributions of "Heavily Polluted" for the middle sampling point (90%) and downstream sampling point (95%). The parameter that affected pollution most greatly for this method was DO because it had a small value for the sub-index calculation of Oregon-WQI. Aside from the factor of sub-index calculation, this may illustrate that there was an influence from wastewater.

Depth I (0,3 m) Depth II (5 m) Depth III (10 m)

Figure 4. Mapping of Water Quality Distribution with the CCME-WQI Method

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In Figure 4, by the results of water quality distribution mapping based on the CCME-WQI method, a depth I had water quality distributions of "Good" for the upstream sampling point (81.75%), middle sampling point (87.5%), and downstream sampling point (87.5%). Depth II had water quality distributions of "Good" for the upstream sampling point (60.75%), middle sampling point (62.5%), and downstream sampling point (57.5%). Depth III had water quality distributions of "Good" for the middle sampling point (50%) and downstream sampling point (47.5%). The parameters that affected pollution most greatly for this method were BOD and DO. It illustrates that there was an influence from wastewater that was carried from the catchment area.

Depth I (0.3 m) Depth II (5 m) Depth III (10 m)

Figure 5. Mapping of Water Quality Distribution with the NSF-WQI Method

In Figure 5, by the results of water quality distribution mapping based on the NSF-WQI method, a depth I had water quality distributions of "Fair" for the upstream sampling point (56.75%), middle sampling point (50%), and downstream sampling point (50%). Depth II had water quality distributions of "Fair" for the upstream sampling point (83.25%), middle sampling point (80%), and downstream sampling point (87.5%). Depth III had water quality distributions of "Fair" for the middle sampling point (87.5%) and downstream sampling point (85%). The parameter that affected pollution most greatly for this method was BOD. The reading of BOD for the sub-index curve of NSF-WQI fluctuated; this may illustrate the influence of wastewater carried from the catchment area.

Domestic waste, agricultural waste, and farming waste that is carried to the reservoir caused the present reduction in water quality of the Selorejo Reservoir [4].

3.3. Analysis of Carrying Capacity of Pollution Load for Selorejo Reservoir 3.3.1. Testing of the Total-P Parameter

Homogeneity testing of Total-P was conducted to find out the uniformity of the data.

Homogeneity testing was performed with the F-test method. The utilized data were from the dry months in 2021. The results of the analysis are displayed in Table 3.

Table 3. Summary of F-Test of Total-P for 2021 Homogeneity

Testing of Total-P

Sampling Points of Selorejo Reservoir

Depth I Depth II Depth III

(0.3 m) (5 m) (10 m)

F count 0.494 0.067 0.00000663

F table 19.380 234.00 234.00

Results Homogenous Homogenous Homogenous

From Table 3, it can be seen that the results of homogeneity testing of the Total-P parameter for the dry months or the months of July, August, September, and October of 2021 showed the results

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of Homogenous for each sampling point at each depth, and thus could be used in the analysis of the trophic status.

3.3.2. Analysis of Trophic Status Determination of Selorejo Reservoir during the Dry Months of 2021

Determination of the trophic status of the reservoir was based on Ministry of the Environment Regulation No. 28 of the Year 2009. After conducting the analysis, results were obtained, as seen in Table 4.

Table 4. Summary of Trophic Status for Selorejo Reservoir Based on Total-P for the Dry Season of 2021

Location Depth Trophic Status Based on Total-P

Oligotrophic Mesotrophic Eutrophic Hypereutrophic Selorejo Reservoir,

Upstream

0.3 m - 33% 67% -

5 m - - 100% -

Selorejo Reservoir, Middle

0.3 m - 25% 75% -

5 m - - 100% -

10 m - 33% 67% -

Selorejo Reservoir, Downstream

0.3 m - - 100% -

5 m - - 100% -

10 m 25% - 75% -

From Table 4, based on the summary results, it may be regarded that Selorejo Reservoir was indicated to experience eutrophication, with the eutrophic status of its water. This condition indicated that Selorejo Reservoir contains nutrients to a high level and illustrates that the water became polluted due to increased phosphorus content. To find out the maximum limit of the reservoir in accommodating phosphorus content, a calculation of the carrying capacity for the pollution load was performed.

3.3.3. Analysis of Carrying Capacity for the Pollution Load of Selorejo Reservoir for 2021

Limiting the entry of phosphorus content into the water needs to be conducted to avoid occurrences of blooming. Analysis of the carrying capacity for the pollution load was performed to indicate the maximum amount of phosphorus content that Selorejo Reservoir may accommodate.

Table 5: Carrying Capacity for the Pollution Load of Selorejo Reservoir, 2021 No. Monitoring Points of

Selorejo Reservoir

Mean Total-P Content, 2021

Carrying Capacity of Pollution Load Based on Total-P

(mg/m³) (kg/P/dry season) (mg/m³)

1. Upstream 42.475 762,712.17 71.77

2. Middle 35.025 861,490.19 80.14

3. Downstream 35.83 837,513.87 77.90

From Table 5, the analysis results indicated that the total-P content of Selorejo Reservoir was still below the maximum limit for reservoir pollution. The upstream part of the reservoir had the greatest amount of phosphorus content because the upstream part of the reservoir receives a great inflow of phosphorus. However, the analysis results still indicated that the condition was safe, as it did not exceed the maximum limit of the pollution load for the reservoir.

Considering the analysis results of evaluating the reservoir water quality status with the four methods as well as the carrying capacity for the pollution load of the reservoir, which is still considered safe, it may be regarded that the methods that showed results in line with the pollution load carrying capacity evaluation were the methods that resulted in the Lightly Polluted status, which were Pollution Index, CCME-WQI, and NSF-WQI.

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The Oregon-WQI method had the result of Heavily Polluted, which is contradictory to the other three methods and the evaluation of the carrying capacity for the pollution load and is thus considered less appropriate for representing the condition of Selorejo Reservoir.

4. Conclusion

The analysis of the water quality status of Selorejo Reservoir that was conducted for ten years (2012-2021) using four methods led to different results. The determination results cannot be compared because they were obtained from different procedures. From the results of the analysis, the findings are that the water quality status of Selorejo Reservoir with the Pollution Index method is "Lightly Polluted", with the Oregon-WQI method is "Heavily Polluted", with the CCME-WQI method is "Good", and with the NSF-WQI method is "Fair".

From the evaluation of the carrying capacity for the pollution load of the reservoir at the upstream, middle, and downstream parts, it is known that the total-P content in the reservoir is still below the maximum limit for the pollution load of the reservoir. Therefore, it indicates that the reservoir is in a lightly polluted condition and still within safe limits. Furthermore, the analysis results for the pollution load carrying capacity show that the reservoir is in the "Lightly Polluted"

condition as the load is below the maximum limit for the reservoir, and the data on existing conditions relatively fulfill the Class II water quality standard. As such, it can be concluded that the water quality condition of Selorejo Reservoir is of the Lightly Polluted status.

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