Bhargava’s Index

Classification and zoning of rivers have administrative and other uses and form an integral part of water pollution control programs. A river may be classified into various grades indicating the beneficial use(s) to which it may be put. The grades are based on the permissible limits of relevant pollution parameters (water quality variables) or standards set by the various authorities. Depending on the quality of water in variousstretches of a river, the river can be zoned according to each stretch’ssuitability for the beneficial use(s).

The assigning of a class to a river reach is not simple. Only in rare cases are all the relevant variables found to be within the permissible limits of a given class. In other words, in most cases, the river water would be classified into one grade with regard to one set of variables and into another class with regard to another variable or set of variables. For example, a river at one point may have an average: Biochemical Oxygen Demand (BOD) = 6.0mglitre- l, Dissolved Oxygen (DO) = 3.7 mglitre- 1, coliform MPN = 4300 100 ml- 1 and chloride = 45 mg litre- 1

According to the US Atomic Energy Commission standards (USAEC, 1974) for classifying raw water sources for domestic supply, Group I (Excellent, requiring only disinfection), Group II (Good, requiringusual treatment such as filtration and disinfection), Group III(Poor, requiring special or auxiliary treatment and disinfection) would, respectively, require BOD = 0.75-1.5 mg litre- 1, 1.5-2.5 mg litre – 1, more than 2.5 mg litre – 1 ; DO = 4.0-7.5 mg litre – 1, 4.0-6.5 mg litre – 1,4.0 mg litre- 1 ; coliform MPN = 50-100 100 ml- 1,50-5000 100 ml- 1, more than 5000 100 ml- 1; and chloride = 50 mg litre- 1 or less, 50-250 mg litre- 1, more than 250 mglitre-1. Therefore, a river may be placed in Group I with respect to chloride, in Group II with respect to coliform MPN, in Group III with respect toBOD and in no group at all with respect to DO. Hence, one can classify the river either in a lower class, thus underestimating the river’s utility potential, or in a higher class, in which case the safety or value of the higher class may be questionable. Otherwise, the classification would arbitrarily be made on the basis of judgement, importance of each variable to a use, reliability of monitoring, etc. To avoid use in a questionable grade, one would tend to place the river in the lower grade, which may not result in the full exploitation of the river.

In view of the above, it would be appropriate to base river classification on the ranges of an index representing the integrated effect of the concentrations and importance values of the relevant variables for a use. Important requirements of such a Water Quality Index (WQI) are as follows:

1.  It should be sensitive to changes in the relevant variables’ value(s).
2. It should provide greater change in the WQI by a change in thevalue of a variable of greater relevance to the beneficial use; that is,the variable’s weight representing the relevance of the variable to agiven beneficial use should be incorporated in the WQI.
3. It should attain a very low value (perhaps zero), when a relevant single variable reaches a certain value beyond which the water quality for a given use is considered unsuitable.
4. It should remain unchanged when a relevant variable reaches a value beyond which further change does not affect water use.
5. Variation in the index should reflect the different levels of importance of a single variable for the use(s).

It is proposed to develop WQIs for the various grades of water for the various beneficial uses, and to apply the index to rationally classify the Ganga river of the Indo-gangetic plain.