Groundwater Quality Index:
Water quality criteria aim, among other things, at protecting the aquatic environment. With our present state of knowledge, it is impossible to set criteria that will guarantee absolute protection of the ecosystem; therefore, these criteria will remain a subject of public discussion, dominated by a struggle between economic interests and environmental considerations (Van der Gaag et al., 1991). Ground water, as a renewable natural resources, plays an important role in water management all over the world. Resources of exploitable ground water are very large in comparison to surface water sources, and, in general, ground water is less polluted than surface water. In many areas ground water is the sole source of raw water for households. Underground water constitutes a vast and almost ubiquitous resource for satisfying water requirements of all kinds. Fresh water, lakes hold 120 000 km3, while the estimated amount of ground water, to a depth of half a mile into the crust of the earth, holds about 4 million km3 of fresh water. An additional 14 million km3 of water occurs at depths between half mile and two miles (UN, 1975).
In Egypt, ground water is considered the third water source for irrigation and other human uses after the river Nile, and irrigation canals and drains. Thus it is considered an important source to irrigate some agricultural areas in the Delta region, and some cultivated lands not accessible to Nile water. On the other hand, in many parts of the Nile Delta area ground water is widely used for drinking and other domestic purposes (Fahim et al., 1995). The wide range of contamination sources is one of many factors contributing to the complexity of a ground water quality assessment. It is important to know the geochemistry of the chemical-soil ground water interactions in order to assess the fate and impact of a chemical discharge into the ground water (Miller, 1985). The kinds and degrees of chemical contamination of ground water depend on the geology and mineral composition of the soil through which the water flows before reaching the aquifers (Zuane, 1990). Many research studies were carried out on the quality of ground water in Egypt at different localities (Fahim et al., 1995; Farouk et al., 1995; Ghandour, 1988; Diab, 1982), but little attention was focused on ground water resources in Dakhla Oasis (Egyptian West Desert) as a remote area. Abu Al-Izz (1971) stated that the water source of the Dakhla Oasis wells is the Nubian sandstone which is saturated with water originating from equitorial rainfall.
The main objectives of this study are to evaluate the Groundwater Quality Index in this area and to study the influence of nature and content of the soil on water quality. The monitoring of ground water quality is necessary in order to detect pollution and to prevent use of contaminated ground water for public water supply.
Uses and Limitation:
This Groundwater Quality Index method is used to detect the groundwater quality and to prevent the use of contaminated ground water for public water supply.
Categorization Table
The AWQI = 0 when all pollutants are absent, and the AWQI = 100 when all pollutants reach their permissible limits. Values of AWQI exceeding 100 indicate that the water sample may suffer from serious pollution problems.
Standards Required
Any national or international water quality standards could be used in this method.
Variables Selection
Nine water quality parameters (NO3–, PO43-, Cl−, TDS, BOD, Cd, Cr, Ni, and Pb) were considered according to their importance as water quality assessment indicators.
Calculation of Groundwater Quality Index:
The overall Groundwater Quality Index can be calculated as follows:
\begin{eqnarray*}
WQI =\sum_{i=1}^{n} \ {q_{i}}
\end{eqnarray*}
where: n = number of parameter, qi = quality rating for the i th water quality parameter. The quality rating can be obtained by the following relation:
\begin{eqnarray*}
qi=100* \frac{1}{n}
\end{eqnarray*}
where Vi = observed value of the i th parameter at a given sampling site and Si = water quality standard for i. The permissible or critical pollution index value is 100. The average water quality index (AWQI) for n parameters can be calculated using the following equation:
\begin{eqnarray*}
AWQI=\frac{\sum_{i=1}^{n} Q_i}{n}\
\end{eqnarray*}