Water Quality Index by Arithmetic Mean
Water Quality Index by Arithmetic Mean – This could be used for any water body to measure its water quality status in any region of the globe.
Uses and Limitation
Water Quality Index by Arithmetic Mean is used for the evaluation of Dalmatian country water quality (surface water quality).
Probably the greatest obstacle to a broader index application in the world is the lack of consent for the selection of quality evaluation parameters
Categorization Table
Standards Required
Variables Selection
Table 1. List of parameters for divergent use of water.
Calculation of Water Quality Index by Arithmetic Mean:
The equation used for computation of WQI is:
\begin{eqnarray*}
WQI = \sum_{i=1}^{n} \ \frac{W_{i}q_{i}}{W_{i}}
\end{eqnarray*}
where, qi = subindex or quality rating for the ith parameter
Wi = unit weight for the ith parameter
The calculation of WQI involves 4 steps:
1st is the selection of parameters, in this study 14 hydro-chemical variables were selected out of 19 due to the lack of proposed permissible limit of drinking water by WHO (2011) and BIS (2012).
2nd is the computation of sub-index or quality rating (qi), the equation is expressed as (Brown et al. 1972):
\begin{eqnarray*}
q_i= \frac{V_{a}-V_{i}}{V_{s}-V_{i}}
\end{eqnarray*}
where, qi = subindex for the ith parameter
Va = actual value present of the ith parameter at a given sampling station.
Vi = ideal value for the ith parameter
Vs = standard value for the ith parameter
If quality rating = zero that means the complete absence of pollutants. While, quality rating 0<qi<100 implies that, the pollutants are above the standards (Ahmad 2014).
3rd step is the calculation of unit weight (Wi) for the ith parameter, which is inversely proportional to the standard value of that particular variable.
\begin{eqnarray*}
W_i= \frac{K}{S_{i}}
\end{eqnarray*}
where, Si = standard value for the ith parameter
k = proportionality constant, which can be calculated as:
\begin{eqnarray*}
K = \frac{1}{\sum{\frac{1}S_{i}}}
\end{eqnarray*}
4th step is the categorization of computed WQI values into five classes for water quality is given as: (Banerjee and Srivastava 2011) (Table 1).
Case Studies based on Water Quality Index by Arithmetic Mean
Narmada River is considered to be the holy river of the state Madhya Pradesh. A study was considered for the development of water quality index using eight parameters pH, Temperature, Total Dissolved Solids (TDS), Turbidity, Nitrate-Nitrogen (NO3-N), Phosphate (PO43−), Biological Oxygen Demand (BOD), Dissolved Oxygen (DO) measured at six different sites (S1–S6) along theriver Narmada. Three methods (Weighted Arithmetic Water Quality Index, National Sanitation Foundation Water Quality Indexand Canadian Council of Ministers of the Environment Water Quality Index) were used for calculation of water quality index. Thiswas observed that the water quality was found to be excellent to good in the season summer and winter and poor to unsuitable forhuman consumption in the season monsoon along the river Narmada. The fall in the quality of water in monsoon season was dueto poor sanitation, turbulent flow, soil erosion and high anthropogenic activities.
References
Gupta, N., Pandey, Pankaj and Hussain, Jakir (2017). Effect of physicochemical and biological parameters on the qualityof river water of Narmada, Madhya Pradesh, India. Water Science 31, 11–23.