Universal Water Quality Index
A new index called the Universal Water Quality Index (UWQI) was developed to provide a simpler method for describing the quality of the surface water used for drinking water supply. UWQI has advantages over pre-existing indices by reflecting appropriateness of water for specific use – drinking water supply rather than general supply and has been developed by studying the supranational standard. Previous indices were mostly developed in order to assess stream-water quality for general recreational uses. In addition, they were based on the national standards of any particular country and this limited their application to within the country of origin.
According to EC legislation (75/440/EEC), water quality of surface waters intended for the abstraction of drinking water is classified into three groups. For each class the treatment level required to transform surface water into drinking water is different and can be summarised as:
- Class I: Simple physical treatment and disinfection, e.g. rapid filtration and disinfection
- Class II: Normal physical treatment, chemical treatment and disinfection, e.g. pre-chlorination, coagulation, flocculation, decantation, filtration, disinfection (final chlorination)
- Class III: Intensive physical and chemical treatment, extended treatment and disinfection, e.g. chlorination to break-point, coagulation, flocculation, decantation, filtration, adsorption (activated carbon), disinfection (ozone, final chlorination) (EC, 1991).
Uses and Limitation
It provides a simple representation of the extensive and complex variables (physical, biological and chemical) that govern the overall quality of surface water that is intended for potable use (drinking water supply).
Categorization Table
The index value between 0 to less than 25 represents poor quality, 25 to less than 50 marginal quality, 50 to 75 fair quality, 75 to less than 95 good quality and above excellent quality.
Table 1. Universal Water Quality Index categorization scheme
Standards Required
The UWQI was developed on the basis of the following water quality standards:
- ‘The quality required of surface water intended for the abstraction of drinking water in the Member States 75/440/EEC’ set by the Council of the European Communities (EC, 1991)
- ‘The classification of inland waters according to quality – Turkish water pollution control regulation – WPCR’ (Official Gazette, 1988)
- Other reported scientific information
Variables Selection
Based on expert opinions and international experiences, 12 water quality parameters including cadmium, cyanide, mercury, selenium, arsenic, fluoride, nitrate, dissolved oxygen, BOD, phosphorus, pH and total coliform were considered as the significant indicator parameters of UWQI to assess the quality of surface water sources.
Calculation of Universal Water Quality Index:
The aggregation function is represented as:
\begin{eqnarray*}
UWQI = \sum_{i=1}^{p} \ {W_{i}I_{i}}
\end{eqnarray*}
where:
Wi = weight for ith parameter
Ii = sub-index for ith parameter (symbolises with ´y` of Table 2)
The temporary weights ranged from 1 to 4 on a basic scale of importance. On this scale 1, 2, 3 and 4 denote respectively little, 0average, great and very great importance. Each weight was then divided by the sum of all weights to arrive at the final weight factor (see Table 3).
Case Studies based on Universal Water Quality Index
Case study 1
The proposed water quality classification scheme was applied to assess the water quality status at Tahtali Reservoir, which is the main drinking water reservoir for the City of Izmir in Turkey. Tahtali Basin is located in the western part of the country. The climate of the region is typically Mediterranean: hot and dry in summers and temperate and rainy in winters. The land area of the Tahtali Basin is approximately 550 km2, with forest and agriculture as the primary land uses. The region has long been subjected to rapid increases in population since the 1990s and 60 000 people live in the catchment area as of 2000. The existing situation in that region is that this water is the main source for many decades for Izmir City where approximately three million inhabitants live. This conclusion increases the priority to protect the Basin water with respect to pollution and over-abstraction (Alpaslan et al., 2002).
The water quality data of the unique observation site in the reservoir were considered for estimation of UWQI. The subset of the data included 12 parameters for index calculation. Descriptive statistics of these parameters are presented in
In the second step sub-index values were calculated using mathematical expressions that were to assign each parameter a value of between 0 and 100. To calculate the sub-indices, the characteristic value was accepted as 90th percentile of data set for each parameter. In the last step a weighted sum method was employed to obtain overall index values using weights.
The index value was calculated as 94.5 which almost fell under the ‘excellent water’ class (index value between 95 and 100. See Fig. 2). On the other hand, sub-index values were the lowest for the parameters total phosphorus, BOD5, nitrate-nitrogen and also total coliform. Low values for nitrate-nitrogen and total phosphorus can be explained by the presence of agricultural activities in the basin where 18% of the total area is agricultural land. In addition, low BOD5 and total coliform sub-indices were the indicators of negative effects of the domestic discharges caused by urbanisation. As has been mentioned above, the region has long been subjected to rapid increases in population since the 1990s and 60 000 people live in the catchment area as of 2000. In spite of all the preventative measures taken by the local authorities, it is clear that domestic discharges and agricultural activities are major threats to reservoir water quality.
Case study 2
Another study aimed to demonstrate efficiency of documented index method “universal water quality index-UWQI” to evaluate surface water quality and investigate seasonal and temporal changes, in the case of Gediz River Basin Turkey. UWQI expressed results relative to levels according to criteria specified in European legislation (75-440 EEC). The method produced a unitless number ranging from 1 to 100 and a higher number was indicator of better water quality. Water quality is classified into five classes and index scores between 95-100 represent excellent and lower than 24 represent poor quality. In the study, dissolved oxygen-DO, pH, mercury-Hg, cadmium-Cd, total phosphorus-TP, biochemical oxygen demand- BOD and nitrate nitrogen-NO3-N have been chosen as index determinants. Samples analyzed for these variables were collected from five stations on monthly basis along two years. Based on UWQI classification scheme, water quality at sampling stations had scores below 40 and assigned to “marginal” which is between fair and poor quality class. On the other hand, sub-indices of water quality determinants showed seasonal differences for some parameters. Cd concentrations were higher in “high flow” and lower values were observed in “low flow” periods. This was explained by negative impact of urban runoff on water quality. On the other hand, DO concentrations were higher in “high flow” period. Under “low flow” conditions water quality at upstream stations (where the industrial density is low) was comparably better than downstream part. The study showed that index approach can be efficient tool to: a) evaluate water quality, b) investigate spatial and seasonal variations and finally, c) extract required information from complex data sets that is understandable by non-technical people.
References
BOYACIOĞLU, Hülya and GÜNDOĞDU, Vildan (2013). Efficiency of water quality index approach as an evaluation tool. ECOL CHEM ENG S. 20(2):247-255.