1. Introduction

Water, a prime natural resource and precious national asset, forms the chief constituent of ecosystem. Water sources may be mainly in the form of rivers, lakes, glaciers, rain water, ground water etc. Besides the need of water for drinking, water resources play a vital role in various sectors of economy such as agriculture, livestock production, forestry, industrial activities, hydropower generation, fisheries and other creative activities. The availability and quality of water either surface or ground, have been deteriorated due to some important factors like increasing population, industrialization, urbanization etc.

Water quality of any specific area or specific source can be assessed using physical, chemical and biological parameters. The values of these parameters are harmful for human health if they occurred more than defined limits ^{[1, 2, 3, 4]}. Therefore, the suitability of water sources for human consumption has been described in terms of Water quality index (WQI), which is one of the most effective ways to describe the quality of water. WQI utilizes the water quality data and helps in the modification of the policies, which are formulated by various environmental monitoring agencies. It has been realized that the use of individual water quality variable in order to describe the water quality for common public is not easily understandable ^{[5, 6]}. That's why, WQI has the capability to reduce the bulk of the information into a single value to express the data in a simplified and logical form ^[7]. It takes information from a number of sources and combines them to develop an overall status of a water system [8-25]^[8]. They increase the understanding ability of highlighted water quality issues by the policy makers as well as for the general public as users of the water resources ^[26]. The present study reviews some of the important water quality indices used in water quality assessment and provides their mathematical structure, set of parameters and calculations along with their merits and demerits, which are being used worldwide.

2. Water Quality Index

Initially, WQI was developed by Horton (1965) ^[27] in United States by selecting 10 most commonly used water quality variables like dissolved oxygen (DO), pH, coliforms, specific conductance, alkalinity and chloride etc. and has been widely applied and accepted in European, African and Asian countries. The assigned weight reflected significance of a parameter for a particular use and has considerable impact on the index. Furthermore, a new WQI similar to Horton’s index has also been developed by the group of Brown in 1970 ^[28], which was based on weights to individual parameter. Recently, many modifications have been considered for WQI concept through various scientists and experts ^{[29, 30]}.

A general WQI approach ^[31] is based on the most common factors, which are described in the following three steps:

1. Parameter Selection: This is carried out by judgment of professional experts, agencies or government institutions that is determined in the legislative area. The selection of the variables from the 5 classes namely oxygen level, eutrophication, health aspects, physical characteristics and dissolved substances, which have the considerable impact on water quality, are recommended ^[32].

2. Determination of Quality Function (curve) for Each Parameter Considered as the Sub-Index: Sub-indices transform to non-dimensional scale values from the variables of its different units (ppm, saturation percentage, counts/volume etc.).

3. Sub-Indices Aggregation with Mathematical Expression: This is frequently utilized through arithmetic or geometric averages.

However, a huge number of water quality indices viz. Weight Arithmetic Water Quality Index (WAWQI), National Sanitation Foundation Water Quality Index (NSFWQI), Canadian Council of Ministers of the Environment Water Quality Index (CCMEWQI), Oregon Water Quality Index (OWQI) etc. have been formulated by several national and international organizations. These WQI have been applied for evaluation of water quality in a particular area ^{[33, 34]}. Moreover, these indices are often based on the varying number and types of water quality parameters as compared with respective standards of a particular region. Water quality indices are accredited to demonstrate annual cycles, spatial and temporal variations in water quality and trends in water quality even at low concentrations in an efficient and timely manner. On the basis of reviewed literature, available indices have many variations and limitations based on number of water quality variables used and not accepted worldwide ^[35]. Hence, it needs worldwide acceptability with varying number of water quality variables. Various WQI determination methods have been described herein.

A usual water quality index method was developed by paying great rigor in selecting parameters, developing a common scale and assigning weights. The attempt was supported by the National Sanitation Foundation (NSF) and therefore as NSFWQI in order to calculate WQI of various water bodies critically polluted. The proposed method for comparing the water quality of various water sources is based upon nine water quality parameters such as temperature, pH, turbidity, fecal coliform, dissolved oxygen, biochemical oxygen demand, total phosphates, nitrates and total solids ^{[28, 36]}. The water quality data are recorded and transferred to a weighting curve chart, where a numerical value of Qi is obtained. The mathematical expression for NSF WQI is given by

Where,

= sub-index for ith water quality parameter;

= weight associated with ith water quality parameter;

= number of water quality parameters.

For this NSFWQI method, the ratings of water quality have been defined by using following Table 1:

Table 1. Water Quality Rating as per different Water Quality Index methods

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CCME WQI provides a consistent method, which was formulated by Canadian jurisdictions to convey the water quality information for both management and the public. Moreover, a committee established under the Canadian Council of Ministers of the Environment (CCME) has developed WQI, which can be applied by many water agencies in various countries with slight modification ^{[37, 38, 39]}. This method has been developed to evaluate surface water for protection of aquatic life in accordance to specific guidelines. The parameters related with various measurements may vary from one station to the other and sampling protocol requires atleast four parameters, sampled atleast four times ^{[40, 41]}. The calculation of index scores in CCME WQI method can be obtained by using the following relation:

Where,

Scope (F₁) = Number of variables, whose objectives are not met.

F₁= [No. of failed variables /Total no. of variables]*100

Frequency (F₂) = Number of times by which the objectives are not met.

F₂ = [No. of failed tests/Total no. of tests]*100

Amplitude (F₃) = Amount by which the objectives are not met.

(a) excursion_i = [Failed test value_i /Objective_j ]-1

(b) normalized sum of excursions (nse) = excursions_i /No of tests

(c) F₃ = [nse/0.01nse+0.01]

Therefore, five categories have been suggested to categorize the water qualities which are summarized in Table 1.

OWQI creates a score to evaluate the general water quality of Oregon’s stream and the application of this method to other geographic regions, which combines eight water quality variables into a single number. The parameters covered in this method are temperature, dissolved oxygen (DO), biochemical oxygen demand (BOD), pH, ammonia and nitrate nitrogen, total phosphorus, total solids and fecal coliform ^{[32, 42]}. The original OWQI was designed after the NSFWQI where the Delphi method was used for variable selection. It expresses water quality status and trends for the legislatively mandated water quality status assessment. The index is free from the arbitration in weighting the parameters and employs the concept of harmonic averaging. The mathematical expression of this WQI method is given by

Where,

= number of subindices

= subindex of ith parameter

Furthermore, the rating scale of this OWQI has also been categorized in various classes, which are given under Table 1 ^[43].

Weighted arithmetic water quality index method classified the water quality according to the degree of purity by using the most commonly measured water quality variables. The method has been widely used by the various scientists ^{[44, 45, 46, 47]} and the calculation of WQI was made ^[48] by using the following equation:

The quality rating scale (Qi) for each parameter is calculated by using this expression:

Where,

is estimated concentration of ith parameter in the analysed water

is the ideal value of this parameter in pure water

= 0 (except pH =7.0 and DO = 14.6 mg/l)

is recommended standard value of ith parameter

The unit weight (Wi) for each water quality parameter is calculated by using the following formula:

Where,

= proportionality constant and can also be calculated by using the following equation:

The rating of water quality according to this WQI is given in Table 2.

Table 2. Water Quality Rating as per Weight Arithmetic Water Quality Index Method

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3. Merits and Demerits of Selected Water Quality Index Methods

A comparison of all these water quality indices is also performed under the study considering their merits and demerits. Table 3 explains about the merits and demerits of WQI methods.

4. Conclusions

After the study of different water quality indices, it may be inferred that the aim of WQI is to give a single value to water quality of a source alongwith reducing higher number of parameters into a simple expression resulting into easy interpretation of water quality monitoring data. Moreover, this is an effort to review the important indices used in water quality vulnerability assessment and also provides information about indices composition and mathematical forms. These indices utilize various physico-chemical and biological parameters and have been resulted as an outcome of efforts and research and development carried out by different government agencies and experts in this area globally. In spite of all the efforts and different discussed indices being used globally, no index has so far been universally accepted and search for more useful and universal water quality index is still going on, so that water agencies, users and water managers in different countries may use and adopted it with little modifications.

Table 3. Merits and Demerits of Selected Water Quality Indices

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Acknowledgement

The authors are thankful to USERC administration to provide necessary infrastructure to complete this work. The authors are also thankful to DAV (PG) College Dehradun, India for necessary help.

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