Dynamic weighting system for water quality index

Water quality standards are developed world over by National and International agencies for pollution control decision-making. Use-based water quality classification criteria and Water Quality Indices (WQIs) also play important role in assessment of water resources for their suitability with reference various uses. Formulation of value function curves and weights assigned to parameters in WQIs are often defined by local water experts and hence WQIs are known to inherit subjectivity. Assignment of weights a priori to various water quality parameters results in misclassification of water quality by WQI. A method of dynamic weighting has been developed in the present work to assign weights to water quality parameters with due consideration to their pollution effect at a particular site. Application of a methodology to Overall Index of Pollution (OIP) provides water quality classification of Indian rivers as "Polluted", "Acceptable", and "Slightly Polluted", which is comparable with the reported CPCB classification as well as with the statistical index CCME-WQI. The methodology developed is general and can be applied to any subjective index. This is exemplified by dynamically weighting the parameters in NSF-WQI for Red and Waikato rivers. Dynamic weighting system provides a true representation to comprehend water quality classification and to achieve River Quality Objectives.