Due to the large number of water-quality criteria and the extent of their variability, the idea of creating a universal index of water quality has often seemed desirable. Unfortunately, the number of indices proposed approaches the number of variables themselves. All such indices are inherently subjective in terms of the weighting functions applied to different variables, and in terms of the set of variables included. To date, none of the indices has been accepted, and water quality continues to be assessed on a case by case basis.
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