DESIGNING A WATER QUALITY INDEX FOR KESBEWA LAKE

Abstract

Water Quality Index (WQI) is a raung reflecting the composite influence on overall quality of anumber of individual quality parameters in a selected water body. The objective of this researchproject is to establish a comprehensive Water Quality Index (WQI) for the Kesbewa Lake byusing physical, chemical and microbiological water quality parameters, to identify the pollutionlevels. These results could be USI.;~( to maintain the quality of water, and conserve flora andfauna, and also to advise the individuals, organizations and funding bodies associated with thelake.

The Water Quality Index is defined as, WQI =I WiQi where, WQI = Water Quality Index, anumber between 0 - 100, Qi == Quality ofthe ith parameter, Wi = The weighting factor of the ilbparameter, a number between 1 an 1'), such that, I Wi = 1 for n number of parameters, Wi = Xi IY, Xi = Points given by reference material data for ilb parameter, Y = The total points for nnumber of parameters.

The raw analytical results for each parameter, having different units of measurements, aretransformed into unit less Qwalues by using the respective function of quality value of eachparameter in the Q-value graphs. These Q- value graphs are plotted with respect to the eachmeasured parameter value and their relative Q-values assigned by a points system.

Based on the World Health Organization (WHO) guidelines, points system for the Q-values for aparameter is assigned and Central Environment Authority (CEA) standards were used, where theWHO guidelines are not available. The highest Q-value 100 is assigned for the best value of aparticular parameter that falls within the above guideline values. Separate ratings for Qualityvalues were given for drinking and bathing, irrigation water, and for fish and aquatic life. Thesethree Quality values were averaged to one value, which gives the Q-value for the measuredvalue of that particular parameter ill the Q-value graph. By using the functions of these graphs,the Q-value (Qi) for any measured value of ith parameter can be obtained.

The weighting factor, (Wi) was determined by considering a large number of reference materialdata obtained from different water quality indices, in which the relative contribution of aparameter for the overall water quality has been weighted in different manner, according to thedifferent point of views of scientists. The Q-value (Qi) is then multiplied by the weighting factor(Wi) and resulted values of all n number of parameters are summed to yield the total value of theWQI.

Twenty samples were collected from ten different locations of the lake over a period of eightmonths. According to the analyzed results, the prepared WQI for the Kesbewa lake gets ageneral ratings of 49.74, which falls, in the region of Bad in the water quality index. Incomparing the average values of measured parameters with WHO and Sri Lankan Standards, itcan be concluded that the water of Kesbewa Lake is relatively polluted and water quality shouldbe improved for aquatic life, irrigation and drinking and bathing purposes