Review of water quality index models as river quality evaluation tool: insight from various rivers in Indonesia

As industrial activities grow and increase, it is directly proportional to the wastewater produced, which is disposed of into the river. One of the calculation models commonly used for evaluating surface water quality is the water quality index (WQI). The method produces uncertainty in the conversion and is site specific dependent. The data collected for the last five years used a comparative method to assess the status classification of those rivers’ quality. In addition, this study also aims to classify the river quality status and evaluate the WQI model in determining the impact of industrial activities on Sumatra Island and Kalimantan Island, the five longest rivers in Indonesia. This paper uses the WQI to review the water quality condition of Indonesia’s five longest rivers, i.e., Kapuas River, Mahakam River, Barito River, Batanghari River, and Musi River, where the community heavily uses these rivers for their daily needs. This review study concluded that the WQI of a river could vary depending on the measurement data used in calculating the WQI value. The data describing the WQI of a river is a momentary WQI or periodic WQI. The study is expected to be used as a basis for developing a sustainable water pollution control strategy to maintain the grade status of a river according to its classification.


Introduction
The rapid development and growth of industry and population will be directly proportional to the quantity of wastewater generated [1].Generally, wastewater from domestic and industrial activities in Indonesia will be piped into rivers.Wastewater that flows into the river will then pollute rivers, portrayed by decreased river water quality [2].It will negatively impact the biota living in the river and humans that depend on it [3].Therefore, assessing the classification of surface water quality is critical.
The Water Quality Index (WQI) is a surface water condition/quality assessment model used to determine surface water quality based on its designation class.The WQI is a metric to simplify several values from several types of parameters into a single index that can illustrate the water quality of the surface water studied [4].In Indonesia, the evaluation of the surface water quality index is regulated in the Minister of Environment Decree No. 115 of 2003 about Guidelines for Determining Water Quality Status; two methods are allowed: the Pollution Index method and the STORET method [5].In addition to these two methods, several countries have developed the WQI method, namely India with the Overall Pollution Index (OPI) method, Malaysia with the INWQS method, and Canada with the CCME WQI (Canadian Council of Ministers of the Environment) method [6].The advantages of the water quality index method are that it can compare the quality of water from various water sources so that it can be used as a basis for making decisions on the use of water bodies, providing river water quality information for the government and the community that is easy to understand [7], and can be used as a basis and reference in making policies by authorities in a more objective manner [8].The WQI method will 1263 (2023) 012040 IOP Publishing doi:10.1088/1755-1315/1263/1/012040 2 evaluate surface water quality by comparing data from the actual laboratory results with the applicable water quality standards [9].Several factors influence the classification of the index, the study's objective, the sampling location, the hydrogeological factors that interfere with the surface water, and the parameters considered [10].Research evaluating surface water quality using the WQI in Indonesia has been conducted in the Sisikar River in West Semarang.The study results show that the WQI value of the Siangkar River is 1.55 -29.72, with a description of the river being included in the polluted class mild to very heavily polluted [11].
The Kapuas River, Mahakam River, Barito River, Batanghari River, and Musi River are the five longest rivers in Indonesia.Geographically, three of the five rivers (Kapuas River, Barito River, and Mahakam River) are on the island of Kalimantan.Based on previous research, it is known that apart from being a source of raw water, rivers in Kalimantan are also used for transportation, economic activity, and domestic needs, such as bathing, washing, and wastewater disposal [12].The Batanghari River in Jambi and the Musi River in Palembang are also used as raw water sources and receiving water bodies for industrial wastewater discharges.However, based on research by the Jambi City government, it is known that there has been significant pollution in the Batanghari River, with the results of the study showing that the Batanghari River no longer meets the criteria for Class I and Class II rivers [13] with sources of pollutants entering the river coming from mining activities and industry [14].Industrial and domestic activities around the Musi River basin also led to the entry of pollutant sources that can potentially reduce river water quality [15].Industries operated along the Musi River include rubber processing industries, wood processing industries, textile industries, oil and gas extraction, and also coal stockpiles.Most of these industries are questionable to have an adequate wastewater treatment process, so the wastewater potentially pollutes the Musi River.From the previous research, the five longest rivers in Indonesia have a vital role in meeting the needs of people's lives and regional economic activities.Therefore, river management is essential to maintain and improve the quality of these rivers.
This study aims to review the water quality of Indonesia's longest rivers, e.g., Kapuas River, Mahakam River, Barito River, Batanghari River, and Musi River, based on the WQI calculation.The method used is a comparative method with data compared using data from the last five years.The result of this study is expected to provide knowledge related to the quality status of rivers that receive pollutant load input so that it can be used as a basis for planning river pollution control strategies following the designation objectives of the river.

Methods
Data for WQI calculation were collected from the Kapuas River (length: 1143 km), Mahakam River (length: 920 km), Barito River (length: 909 km), Batanghari River (length: 800 km), and Musi River (length: 750 km), which are the five longest rivers in Indonesia [16][17].The water quality data collected are actual monitoring data from several studies from 2018 to 2023.The data will be calculated and reclassified based on the WQI method in Indonesia.The WQI uses two calculation methods: the Storet method and the Pollution Index method.The Storet method uses time series water quality and discharge periodic data.The concentration value for each water parameter is compared with the surface water quality standard values according to the classification.Then calculate the Storet score referred to Appendix I Environmental Ministry Decree no.115 of 2003.Meanwhile, the Pollution Index method uses single data on water quality and river water discharge.Then using the below equation to obtain the Pollution Index value (Pij).

WQI calculation based on the Storet method
Parameters BOD, COD, and TSS became the dominant parameters whose concentrations exceeded the quality standard in 5 rivers, followed by several other parameters such as heavy metals, nitrate, and phosphate [18]- [22], thus affecting the river water quality index.The higher the Storet score obtained, the lower the WQI value.Figure 1 shows that the WQI of the Kapuas River and Musi River are in the medium-heavily polluted category, the Mahakam River and Barito River are moderately polluted, and the Batanghari River is lightly-polluted.Even though the Kapuas River is the longest river in Indonesia which is located in West Kalimantan Province with a length of about 1,086 km with a watershed (DAS) which has an area of 98,249 km 2 [23], it does not make the Kapuas River the river with the lowest WQI compared to the other four rivers.This is due to heavy pollution in the Kapuas River due to life around the river (settlements) as the primary pollutant, industry, and oil palm plantations [24].Meanwhile, the Batanghari River, the 4th longest river in Indonesia with a length of 800 km [25], receives the main pollutant inputs from industry.Hence, BOD, COD, and TSS concentrations that enter are higher than domestic pollutants [2,25].

WQI calculation based on Pollution Index (PI) method
Some literature mentions that applying the water quality index is vital in determining the river water quality status.The water quality index plays a role in assessing water quality from a particular source time by time and the factors that influence the condition [26].WQI measurement with the Pollution Index Method is a method that is more widely used compared to the Storet Method.The result is due to the characteristics of the PI method, which can use single data observations to save time in assessing water quality [9].  Figure 2 shows the WQI results from previous studies that the Kapuas River, Barito River, and Batanghari River are in the moderately polluted category.Meanwhile, the Musi River is heavily polluted, and the Mahakam River is mildly polluted.The WQI results differ from the Storet method, which describes the Batanghari River as having a higher pollution level than the other four rivers.Meanwhile, for the WQI using PI method describe the Musi River as in the heaviest pollution level.This result is because the Storet method is sensitive in responding to the dynamics of the water quality index at each location with a few or many parameters.Unlike PI method, which only uses a single data, the water quality status only describes the water quality status for a moment.However, it can efficiently save time determining the situation [6,9].Research by Rahutami [15] showed that from 18 sampling points in the Musi River, the parameters of TSS, BOD, COD, and heavy metals (Pb and Cr) exceeded the quality standard the most.Sources of incoming pollution come from waste from community activities along the river, industry, and runoff from agricultural activities.Sources of incoming pollution come from waste from domestic community activities along the river, waste from industry, and runoff from agricultural activities.The river is a water source that acts as a raw source of drinking water, a bathing location, a washing location, a means of transportation, irrigation, a place to cultivate fish, and a place of recreation.In addition, the Mus River is often used as a liquid waste disposal site by industry.This behavior causes the river to be vulnerable to pollution.Some industries along the Musi River are the rubber processing industry, wood processing industry, fertilizers, ceramics, detergents, oil, gas, cold storage, electroplating, soft drinks, and cloth dyeing.Apart from that, barges and other ships have high mobility through the Musi River as cruise ship routes.Most industries do not have adequate Wastewater Treatment Plants (IPAL).The topography of the Palembang city area also shows that the people living on the banks of the Musi River are still dense [27], thus contributing to the emergence of domestic waste that enters the waters, impacting the Musi River IKA.

Comparison between Storet and PI method
The water quality index (WQI) is a multilevel assessment that describes the condition of water pollution.The WQI describes the combined effect of various water quality parameters (physics, chemistry, and biology).The results are used as a basis for information for public and legislative decision-makers to carry out appropriate and sustainable management in dealing with environmental problems, especially the problem of river conditions as a container for storing pollutant substances [28].In Indonesia, the WQI method used is the Storet method and the Pollution Index (PI).
The differences in the Storet and PI methods apart from the type of data are also seen from the process of assessment used.In PI, sub-index or subjective evaluation of parameters is not used in contrast to the Storet method.Biological parameters have a higher value than chemical and physical parameters, and chemical parameters have a higher value than physics.Therefore, the absence of biological parameters can impact the results of water quality status.In the Storet method, the more water quality parameters are measured, the more values that do not meet the quality standards will be monitored (based on the maximum, minimum, and average values obtained).This comparison can be seen in Figure 1 and Figure 2, which show the different WQI results between the Storet and PI methods from the Kapuas River, Mahakam River, Barito River, Batanghari River, and Musi River.
Storet and PI methods have their respective advantages and disadvantages.The Storet method excels in using a series of data that are repeated from the results of several sampling times so that the water quality status better describes the water quality conditions in a certain period [29].In contrast, the PI method is superior in time because it uses a single data [9].The Storet and PI methods have flexibility in the number and type of water quality parameters to determine water quality status.However, this flexibility will create heterogeneity in water quality parameters essential for determining a river's water quality index [30].To eliminate the possibility of partiality of these parameters in influencing WQI, based on the Minister of Environment and Forestry Regulation No. 27 of 2021 concerning the Environmental Quality Index, a mandatory parameter of river water is determined to calculate the value of the Water Quality Index.These parameters are the degree of acidity (pH); dissolved oxygen (DO); biological oxygen demand (BOD); chemical oxygen demand (COD); total suspended solids (TSS); nitrate (NO3-N); total phosphate (T-Phosphat); and fecal coliforms.By setting these mandatory parameters, it is expected to be able to standardize the types of pollutant parameters that enter the river, which are calculated in the WQI assessment [31].However, the Storet and PI methods should be compared comprehensively, namely with the same amount of data, and the WQI value of a river should be calculated.Given the advantages and disadvantages of each of these methods, it is hoped that the government, which currently uses the PI method in determining WQI, will also have an overview of water quality data periodically or in the hope that the WQI method can be developed to obtain an indexing method with a good level of sensitivity, describing conditions regularly and pay attention to parameter assessment.Such as the CCME WQI method from Canada, which, according to [6], is the most appropriate method for analyzing water quality in various countries, including Indonesia, both in surface water and groundwater, with a higher level of effectiveness and sensitivity compared to the Storet and PI methods and the use of the amount and type flexible parameters.
Furthermore, the factors that also affect the quality of river water are that the increase in pollution load is higher than efforts to reduce pollution load, especially related to reducing pollution load from household activities, a ratio of minimum-maximum water discharge during the dry season, and rainy season; industrial waste disposal permits are not yet effective because they have not considered the capacity to accommodate pollution loads; and efforts to control water pollution have not been practical or have not been able to reduce the pollution load to an adequate level [32].

Conclusions
This review study concluded that the WQI of a river could vary depending on the measurement data used in calculating the WQI value.The data describing the WQI of a river is a momentary WQI or periodic WQI.However, although the WQI results varied, only some of the five longest rivers in Indonesia were found to be in good condition or meet quality standards.This study offers insight into the comparison between instantaneous and periodic WQI based on the mandatory parameter types specified in the overall environmental index to better predict river water quality conditions as an effort to set targets for increasing WQI.
concentration below regulation standard Ci : Parameters concentration based on actual monitoring Pij : Pollution Index Value based on classification The water quality category is based on both methods as follows [5]:

Figure 1 .
Figure 1.Water quality index calculation based on the Storet method.

Figure 2 .
Figure 2. Water quality index calculation based on PI method.

Table 1 .
Scoring and water quality category between PI and Storet method.