**Abstract**

Water quality index (WQI) provides a single number that expresses the overall water quality, at a certain location and time, based on several water quality parameters. The objective of WQI is to turn complex water quality data into information that is understandable and usable by the public. A number of indices have been developed to summarize water quality data in an easily expressible and easily understood format. The WQI is basically a mathematical means of calculating a single value from multiple test results. This chapter discusses, in detail, the application of a water quality index for the assessment of water quality to different several water sources in Nigeria.

**Keywords:** Water Quality Index, Water Quality Indicators, Surface Water, Underground Water, Environmental Health

#### **1. Introduction**

Clean, safe and adequate freshwater is of utmost importance to human existence and the survival of all living components in the ecosystem. Water quality issues are complex and diverse, deserving urgent global attention and action [1]. The decline in water quality has become a global issue of concern because of its inherent ability to cause major alterations to the hydrological cycle. The past decade has seen remarkable impact of man on the environment due to unprecedented increase in population and rapid rate of urbanization as well as the intensification and expansion in agricultural practices. This has led to progressive and continual degradation of resources especially surface water. Polluted water is an important vehicle for the spread of diseases. In developing countries about 1.8 million people, mostly children, die every year as a result of water-borne diseases [2]. According to Bullard, [3] inferred that impaired surface water quality always result in an unhealthy socio-economic environment.

The characteristics of water are defined by its composition and are commonly referred to as water quality. Water quality is generally defined as "the chemical, physical and biological characteristic of water usually in respect to its suitability for a designated use" [4]. The assessment of water quality, usually carried out by determining its physico-chemical and biological properties or parameters against a set of standards, is used to determine whether the water is suitable for consumption or safe for the environment. Water quality assessment can be defined "as the evaluation of physical, chemical and biological state of the water in relation with the natural state, anthropogenic effects and future uses" [5]. The water quality parameters are then used as a reference to a set of standards based on the intended usage of the water broadly classified into industrial/domestic use, human consumption (portability) and restoration (in the environment/ecosystem, generally for health of human/aquatic life). Water quality standards are used to protect different designated uses of water. The standards of each one of these designated uses are very different from each others. For example, the water used for drinking requires a higher standard compared to the standard used for agricultural and industrial use (water for domestic purposes should therefore be free from toxic substances and organisms in order to prevent waterborne diseases).
