Salinity and frequency distribution of major ions as indicators of groundwater Paleo-recharge rates through the Nubian sandstone aquifer in east el-Owienat area, western desert, Egypt

Abstract EN

This paper deals with frequency distribution of salinity, water hardness and major ions in the groundwater of the Nubian sandstone aquifer East El Owienat area.

The stable isotopes are also used to estimate the polio-climatic conditions at the time of groundwater recharge (air temperature 3.17° C, relative humidity 90% and yearly precipitation, 1350 mm./ y).

Normal and log normal distributions of chloride concentrations in groundwater, rainfall rates, and initial concentration of chloride ions in rain water could be used as guides for determination of the average recharge rates of groundwater.

The study reveals that there are three water salinity classes within a general fresh water zone, the first is the ultra-fresh water class that occupies the southwestern parts of the study area, the second is the fresh water class which occupies the area from its southeastern to northwestern localities and the third is the fairly fresh water class which dominates the central and western Localities.

The hydro chemical processes affecting water salinity are leaching and dissolution of the terrestrial and marine salts of the Nubian sandstone matrix.

Other processes include the contribution of CO2, action exchange and residence time in the sediments.

These reactions play an important role in the chemistry of groundwater's resulting in temporary hardness salts rather than permanent hardness.

Most of the groundwater samples of the Nubian sandstone were super-saturated with carbonate and phosphate minerals.

This confirms the role of marine deposits that were overlying the Nubian sandstone before its weathering and formation of the southern depressions.

Most of the groundwater samples of the Nubian sandstone were also super-saturated with iron and silicate minerals.

This confirms the impact of continental deposits transported after weathering of basement and sedimentary rocks.

The present hydrogeological setting of the aquifer in this area indicates that it behaves as an unconfined aquifer.

However, our hydro geochemical findings confirm that the aquifer previously developed as a closed system.

This may suggest that the confining cap rock (silt and clay beds) observed in northern localities Dakhla, Kharga and Darb El- Arbaein) had extended and covered the area of East El-Oweinat and was subjected later on to weathering and removal especially during Pleistocene time.

The present study indicates that groundwater of East El-Owienat area has a mixed origin, i.e.

possibly pure meteoric water that was affected, to a less extent, by traces of the originally deposited seawater.

The frequency distribution of chloride ion content in groundwater revealed that the airborne chloride was concentrated by high evapotranspiration and also by evaporates dissolution and mixing.

Considering these results it was possible to estimate average polio-recharge rates of 15% of the yearly precipitation (200mm./ y).

These results are also confirmed by the determination of average recharge rate within the area from an empirical formula that takes recharge as a proportion of precipitation which gave the same value of average recharge rate 16.3% of an annual precipitation of 220 mm.

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