The WQI indicates the quality of water in terms of the index number, represents the overall water quality for any intended use. It is defined as a rating reflecting the composite effect of various water quality parameters. Taking the WHO and IQS standards as an example, the water quality indexes of twelve important physical and chemical parameters of river to ten different locations were calculated over a 12-month period over the past year. Physico-chemical analysis of water, consisting of the following parameters; Temperature, pH, turbidity, total dissolved solid (TDS), electrical conductivity (EC), total alkalinity, total hardness, chloride, phosphate, calcium, magnesium, sodium and potassium.

The calculation of WQI was performed using the Arithmetic index technique in the following steps 9;

1-Calculation of the unit weight of the parameter,

2-Calculation of the secondary index of quality rating (qn) for each one of the water quality parameters, and

3-Calculation of the Water Quality Index (WQI).

Calculation of Unit Weight (Wn)

The unit weight (Wn) for each water quality parameter is inversely Proportional to the recommended standard of the corresponding parameters 10;

Wn = K/Vs (1)

Where:

Wn = unit weight for n-th parameter

K = Proportionality constant

Vs = Standard allowed value for nth parameter.

Calculation of Quality Rating (qn)

The quality ranking or sub-index (qn) corresponding to the nth parameter is a number reflecting the relative value of this parameter in the polluted water with appreciate to its standard permissible value. The cost of qn is calculated using the following expressions 11;

qn = 100 (Vn-Vi) /(Vs-Vi) (2)

where:

QN = Quality rating for the n-th parameter of water quality.

Vn = The detected value of the n-th parameter at this sampling station.

Vs = Standard value for n-th parameters

Vi = The ideal value of the n-th parameter in clean water.

In Equation (2), Vi values for all parameters are considered to be equal to zero, except for the ph value equal to 7.0. For PH, the ideal value is 7.0 (for natural water), and the permissible fee is 8.5

(for contaminated water). Therefore, the use of the following ratio is calculated to assess the ph quality:

qpH = 100 (VpH-7.0)/(8.5-7.0) (3)

where :

VpH = Observed ph value during the entire study period.

Calculation of WQI

The WQI is calculated according to the following equation 12.

WQI = qn Wn / ?Wn (4)

The appendix presents the results of statistics of physico-Chemical

Table (1): Statistics of the constituents in the Shatt al-Arab Raw Water

Table (2): Statistics of the components of treated water