WQI is an effective water quality assessment tool (Ochuko et al., 2014; Amuah et al., 2021). The idea of indexing water quality with a numerical value is dependent on biological, chemical, and physical variables considered in the study. It (NSF-WQI) was developed in the 1960s by Horton (1965). Over the years, several refinements have been made in the quest to develop a WQI that is globally accepted (Lumb et al., 2011). The Scottish Research Development Department (SRDD) created the SRDD-WQI in 1973. It is based on Brown's model and is used for determining the quality of rivers (Uddin et al., 2021). The SRDD-WQI has subsequent variants such as the House and the Dalmatian Indices. The Environmental Quality Index model was later created by Steinhart et al. (1982) to assess the quality of the Great Lakes ecosystems. A similarly significant breakthrough was the British Columbia Water Health Index (BCWQI) by the British Columbia Ministry of Environment, Lands and Parks in the mid-1990s. It was developed for the assessment of various water systems across the province in Canada (Saffran et al., 2001). Models like the Liou, Malaysian, and Almeida Indices have been created in recent years (Uddin et al., 2021). Various nations and/or agencies have used more than 35 WQI models to assess surface water quality across the world (Ewaid et al., 2020; Ustaolu et al., 2020; Miyittah et al., 2020). In most regions of the world, WQI models have been used (Uddin et al., 2021). Lumb et al. (2011) further showed that the difference in the various WQIs depends on the statistical integration and interpretation of parametric values which make them subjective. Though WQI models have been used to assess different sources of water, 82% of applications have been to understand the qualities of rivers, it has also been employed even in groundwater quality (Ramakrishnaiah et al., 2009; Khan and Jhariya, 2017; Jha et al., 2020) and packaged water quality (Amuah et al., 2021).
2.1 Procedure for computing WPI
Over the years, several refinements have been made in the quest to develop a WQI that is globally accepted (Lumb et al., 2011). Lumb et al. (2011) further showed that the difference in the various WQIs is depends on the statistical integration and interpretation of parametric values which make them subjective. Khan and Jhariya (2017) used 8 water quality parameters (chloride, nitrate, pH, alkalinity, magnesium, fluoride, hardness, and calcium) to evaluate the WQI using groundwater from the Raipur city, India using two systematic approaches; (1) determined the weightage factor of each parameter based on its significance using the formula:
$${W}_{i}= \frac{wi}{{\sum }_{i=1}^{n}wi}$$
1
From Eq. 1, Wi = relative weight, wi = weight of each parameter and n = number of parameters
$$qi=\left[\frac{\text{C}\text{i} – \text{C}\text{i}0}{\text{S}\text{i} – \text{C}\text{i}0}\right]\text{x} 100$$
2
Ci = measured level (mg/L) of each chemical parameter in each water sample, Ci0 = ideal value of the parameter in “pure” water, and Si = standard value.
The WQI was determined by initially calculating the sub-index of each parameter using the formula:
$$\text{S}\text{I} = \text{w}\text{i} \text{x} \text{q}\text{i}$$
3
SI is the sub-index of the ith parameter and Qi represents the rating based on the level of the ith parameter
WQI was derived by summing the sub-index of all the 8 parameters using Eq. 4:
$$\text{W}\text{Q}\text{I} = \sum \text{S}\text{I}\text{i}$$
4
Using the formulae, the study concluded that the quality of water in certain areas were unsuitable for drinking purposes due to levels of Mg2+, NO3−, and Ca2+ in above-threshold levels suggested by the Bureau of Indian Standards (BIS).
Singh and Kamal (2015) also used a different approach to understand the WQI of groundwater in Goa, India. Although the study used the same pattern adopted by Khan and Jhariya (2017), the approach was different as Singh and Kamal (2015) stated that the weightage average for the water quality parameters was expected to be contrariwise to the corresponding standards by the Bureau of Indian Standards for respective parameters. Hence the weightage of each parameter was calculated as:
Wi = unit weight for the ith parameter, Si = standard for the ith and i = 1, 2, 3, 4 …. ith, and K = constant of proportionality.
The quality rating (qi) was calculated using the formula:
Vi = the measured value of the ith parameter in the groundwater sample under consideration and Si = is the standard or permissible limit for the ith parameter.
Finally, the WQI was computed using the formula:
$$\text{W}\text{Q}\text{I} = \sum (qi \text{x} wi)/\sum wi$$
7
Where WQI = Water Quality Index, qi = quality rating, and wi = is the unit weight of the ith parameter.
Table 1
Generally used WQI rating
WQI | Rating |
---|
0–25 | Very good |
26–50 | Good |
51–75 | Moderate |
> 75 | Poor |