A comprehensive water quality index based on analytical hierarchy process

Abstract

The popular water quality index (WQI) and the recently developed integrated water quality index (IWQI) were used to evaluate the potability of 32 groundwater samples collected from Nalagarh valley, Himachal Pradesh, India. But their analytical results were surprisingly contrasting. While WQI classified 14 of the samples as ‘excellent’ and 17 as ‘good’, IWQI put all of them under ‘unsuitable’ category. This led to the development of a new index, called ‘comprehensive water quality index (CWQI)’, on the basis of analytical hierarchy process (AHP). Unit weights and sub-index values of both relaxable (that have both acceptable and permissible limits) and non-relaxable (that have only one guideline value) parameters were considered in the calculation process. Two segments were calculated: PCWQI (positive CWQI) and NCWQI (negative CWQI) with their scores varying between 0 and 1 and 0 and (−0.840), respectively. Grading of water quality from drinking perspective was done essentially with NCWQI scores, such as ‘excellent’ (when NCWQI = 0), ‘good’ (NCWQI = 0 to −0.21), ‘marginal’ (NCWQI = −0.21 to −0.42), ‘poor’ (NCWQI = −0.42 to −0.63), ‘very poor’ (NCWQI = −0.63 to −0.84) and ‘unacceptable’ (NCWQI = <−0.84). In fact, whenNCWQI = 0, higher the PCWQI, better the water quality. When two water samples had the same NCWQI score, they were graded by their respective PCWQI score. When two water samples had the same magnitudes of PCWQI and NCWQI, they were graded equally. On the basis of this classification, groundwater in majority of the study area was classified under ‘good’ category for drinking purposes in absence of an alternative source of water. Compared to WQI and IWQI, CWQI proved itself to be a robust and comprehensive technique for assessing the groundwater quality. It was hence recommended to be used universally for groundwater quality assessment.