A new approach for indexing groundwater heavy metal pollution

Abstract

A new approach has been proposed for indexing heavy metal pollution in groundwater based on highest desirable and maximum permissible concentrations. This new approach aims to address various shortcomings of the existing indexing systems. According to the new indexing system, heavy metal pollution status of any water sample may be expressed by a pair of positive index (PI) and negative index (NI). Positive index reflects heavy metal concentration in water exceeding highest desirable limit, while negative index corresponds to heavy metals within desirable limit. Water quality deteriorates with increasing PI and the pristinity increases with decreasing NI. A comprehensive PI and NI scale has been drawn for labeling the drinking worthiness with regards to heavy metal concentration. When all the constituent heavy metals are less than or equal to prescribed highest desirable limit, PI is assigned a value zero and NI can assume a value less than zero but not less than −1. When all the heavy metals are present below detection limit, the PI of the sample is still zero but NI will be −1. When concentrations of all heavy metals are above the highest desirable limit, its NI will be zero and PI will have some positive value. Heavy metal pollution indices of 100 groundwater samples in the Subarnarekha River Basin (SRB) collected during the pre and post-monsoon seasons have been computed using the proposed new indexing system. The drinking worthiness of all these samples has been discussed in the light of newly proposed indexing system. GIS interpolation maps of new indices, drawn for the pre- and post-monsoon SRB ground waters, clearly reflect the spatio-temporal water quality change pattern. The new indexing system is robust, flexible and may be computed using any water quality standard. It takes care of many of the apparent deficiencies of the existing indexing systems.