الفهرس 
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Abstract
The present thesis is mainly dealing with the hydrogeochemistry of Toshka area; it lies on the western side of Lake Nasser where it is delineated by latitudes 22° 15` and 23° 6`N and longitudes 31° 18` and 32° 00` E. It should be noted that groundwater in the investigated area is used mainly for agricultural purposes. The collected data and the obtained results were edited and represented within five chapters under the following titles:-CHAPTER I: Introduction. CHAPTER II: Geomorphology, Geology and Hydrogeology.
-CHAPTER III: Hydrogeochemical aspects.
-CHAPTER IV: Hydrogeochemical modeling and statistical analysis
-CHAPTER V: Evaluation of groundwater quality for different purposes.
1-Introduction:
This chapter contains general outlines, location of the study area, aim and scope of the present work and previous related studies.
2- Geomorphology, Geology and Hydrogeology:
The area under consideration can be geomorpholgically subdivided into the following units:-
a- Aswan High Dam Lake.
b- The Nile Valley.
c- Wadi Kurkur pediplain
d- Tushka depression.
e- West Dungul plain
Geologically, the sedimentary succession are described in the following from older to younger
1- Abu Simbel Formation (Upper Jurassic-Lower Cretaceous).
2- Lake Nasser Formation and Sabaya Formation (Lower Cretaceous).
3- Kiseiba Formation (Upper Cretaceous).
4- Kurkur Formation (Paleocene).
5- Dungul Formation (Lower Eocene).
6- Fluvial and Alluvial deposits, Lake deposits and Eolian deposits (Quaternary).
Hydrogeologically, the water resources in the study area include two systems namely:
a- Surface water system
The surface water system includes both the High Dam Lake and El Sheikh Zayed canal.
b- Groundwater system
The Nubian sandstone facies constitute the main rock unit in the subsurface and they are composed of course to medium sandstone with intercalations of siltstone, clay and occasional conglomerate that rest unconformably on the Precambrian basement rocks at different depths. The groundwater exists under unconfined and semiconfind conditions. The thickness of the Nubian sandstone varies between 170 -320m, generally this thickness almostly increases toward southeastern directions, i.e. towards Aswan High Dam Lake, and decreases in the northwestern and northeastern.
The depth to water table in this aquifer is ranging between 20 and 147m. Recharge of the Nubian aquifer is from three sources: a) Seepage of the Nile water, Lake Nasser. b) Regional groundwater influx from areas with modern groundwater recharge and c) Local infiltration through the precipitation during wet periods in past.
the main groundwater flow direction is detected from the direction from Lake Nasser at the southeast towards the middle part of the study aquifer surrounding Toshka depression. The main trends of groundwater flow lines indicate that the Nubian aquifer is recharged at present from Lake Nasser.
III- Hydrogeochemical aspects
In the study area, majority of groundwater samples of the Nubian sandstone aquifer (81%) lie in the fresh zone, while the brackish water representing by 19 % of the total samples. The total dissolved solids range from 230 (well, 26) to 1875 mg/l (well, 54). The water salinity