THE APPLICATION OF SUB INDEX AGGREGATION METHOD TO ASSESS THE HEALTH OF A WATER BODY

Abstract

The environmental health of inland water bodies can be described using water qualityparameters that describe the physical, chemical and biological status of the water. Each ofthese parameters reflect different characteristics of the water body and can be assessed bycomparing with threshold values detaikd in water quality guidelines such as the FreshWater and Marine Water Quality Guidelines. Assessing the status of a water body using asuite of water quality parameters is cumbersome and fraught with difficulty, as for a givenlocation some parameters may fall under Ihreshold values whilst others exceed guidelinevalues. To simplify and unify the interpretation based on individual water qualityparameters, these can be combined into a single dimensionless number by considering eachmeasured water quality concentration and its corresponding guideline value. This singlenumber is called the 'water quality index' (WQI) and is used to assess the overall status ofthe water body quantitatively and objectively.

Many methods have been derived over the years to determine the WQI for differentapplications. These methods are &roupedinto two categories; 'sub-index' and 'statistical'.As to-date, no guidelines have been specified on the selection of a methodology. However,it has been acknowledged that the method used to derive the WQI should retain valuableinformation from each parameter and retain its contribution to determining the overallhealth during the combining process.

This paper reviews methods available to determine WQls for a range of applications. Thepaper will describe the application of the SIA method to a set of water quality data alongthe Yarra River in Victoria, Australia. The results were useful in identifying the trend inWQvariation. The Sub Indices Aggregation (SIA) method could be used by Authorities forrapid assessment of water bodies to identify their suitability for different purposes based onthe quality of water. Application of the method can also assist with locating critical areasthat could urgently require rehabilitation