Ground Water Reservoirs  
South Sinai    
 

 

Locating ground water in the St. Catherine area using geophysical methods

 

Abstract

An attempt was made to locate ground water in the arid High Mountain Range of southern Sinai. A combination of classical geological methods, together with geophysical techniques were found to be particularly useful for the locality studied. It was found that water flow was mainly through the joints of the crystalline basement and that under favorable circumstances, fair ground-water supplies can be found in alluvial valley fill. It was concluded, that recharge of the valley fill is derived mainly from the joint system of the bedrock and only partially from flooding.

 

 

Introduction

The southern part of the Sinai Peninsula is an arid mountainous area with insufficient precipitation to maintain permanent, open bodies of water and with no extensive aquifers which can be exploited. This area supports a small population of Bedouin who have always suffered from the paucity of available water. An attempt was made therefore to locate small ground-water reservoirs which were known to occur within the alluvial fill of some of the wadis (dry river valleys) in the area, as well as in the joints of the bedrock.

 

The geology and geomorphology of the area

The area studied is a very rugged and deeply dissected pre-Cambrian basement complex consisting of acid Plutonic and volcanic bodies, which are intruded by numerous (predominantly acidic) dykes. Mountain peaks in the area exceed 2,600 m. and the relief is of the order of 1,400 m.

 

High altitude aerial photography and photographs taken by satellites revealed long, straight fracture zones criss-crossing the area. Drainage appears to be controlled by (and subsequently emphasizes) this large scale fracture system. The topography is also controlled by the variation in susceptibility to weathering of different types of dyke rocks. Resistant (positive) dykes (which can consist of K-feldspar-quartz porphyries or of diabase) tend to form long ridges and less resistant (negative) dykes (which consist exclusively of diabase) tend to form elongated depressions. Jointing is most pronounced in the volcanic rocks and in the positive dykes. Weathering of the negative dykes results in the production of much clay-size material, which under appropriate microclimatic conditions; tend to fill joints and fractures in the unweathered rock, making such dykes practically impermeable.

 

The alluvial fill in the wadis consist of gravel- to boulder-size rocks mixed with fine clastics. The permeability of this material varies greatly from place to place ranging from impermeable to permeable, where it forms the major aquifier, e.g., at the confluence of several wadis on in major alluvial fans.

 

Climate

The area is very arid, with an average annual precipitation of less than50 mm. on the highest peaks. This falls either as snow 9which may last for a few weeks0 or as heavy showers. A heavy storm, during which as much as 30 mm. may precipitated, can cause flooding which may last for several days. Most of the water precipitated during such a storm either evaporates or reaches the sea as surface runoff and only a small fraction perlocates into the ground. Snow, on the other hand, melts slowly and most of the derived water perlocates into the ground.

 

Ground water occurence

Ground water occurs in the pore spaces of the alluvial fill and in joints in the bedrock. Surface runoff flowing down the wadis moistens only the upper part of the wadi fill (down to a depth of about 10 metres) and rarely reaches the saturated zone. The joints and fractures in the bedrock appear to be the major conduit of ground water and a substantial part of the reservoir. This was deduced from the fact that during hand-boring, it was noted that wadi alluvium was dry as a rule and as soon as the top of the bedrock was reached, the water rose into the well. This implies that it is the joints in the bedrock which are the major conduit of ground water and that the alluvium may at times act as a confining layer. Recharge into the joint system takes place in areas where the bedrock is exposed.

 

The methods used by the Bedouins to locate ground water included digging in the areas where the surface water remained for long periods after the rain, where particular species of vegetation grew, and on the upstream side of the negative dykes, which they regarded as canals which carry the water down the mountain slopes and into the wadis. Most wells were dug in the alluvium, although in some places, where indications of water were unusually favorable wells were dug in the bedrock. Wells could only be dug down to bedrock with the techniques available to the Bedouins. This meant that only limited amounts of water could be withdrawn as virtually no drawdown is possible. As water levels dropped during dry years, such wells were deepened.

 

The search for new ground water supplies

This search was carried out using conventional geological methods to locate areas with potential, and geophysical techniques to confirm wether or not these areas contained ground water, these were not the target of the search because:

1)      It is very difficult to locate a joint beneath about 30. m. of alluvial fill.

2)      The Bedouins do not have the means to dig through solid bedrock, as would have to be done to reach the saturated zone.

 

 

 

 

Water in Wadi Feran
Although many industrial, commercial, agricultural and tourism projects have been initiated and accomplished along the northern and southern coasts of Sinai, the Wadi Feran basin remains underdeveloped. The lack of water resources necessary for initiating any development plans is the main reason behind the delay in developing this area.  The Wadi Feran basin is located in the southwestern part of the Sinai Peninsula, and covers an area of about 1,851.5 sq. km.  It forms part of a rugged mountain range of igneous and metamorphic rocks, which dominates South Sinai region; thus the basin is predominantly composed of Precambrian granitic (old and young), metamorphic, volcanic and sedimentary rocks. Wadi Feran basin is dissected by a number of wadis, which drain towards the Gulf of Suez in the west, e.g., Feran, El-Akhdar, El-Sheikh, and Solaf. Climatically, the region is typically arid; precipitation is scarce and random, and temperatures are high during most of the year. The little rain that does occur falls in winter, when temperatures are relatively low. The distribution of the scant precipitation appears to be mainly controlled by topography. Thus, the low coastal areas seldom get more than 15 mm of rain per year. The higher regions, such as Jebel  Katharina in the east, receive approximately 50 mm/year of precipitation, partly as snow. During exceptional years, torrential rains can raise the annual precipitation total to more than 100 mm.

 

El-Baz and Himida stated that the wadi fills constitute the best potential of low-cost groundwater resources in the southern Sinai mountain region. In some cases dikes serve as natural dams that hold water upstream in the valley fill such as in the case of Wadi Feran. (Gaber et al) demonstrated that two paleolakes formed along the Wadi Feran basin after damming the torrential rainfall, during humid periods, by the last invading acidic dikes. Such paleolakes were loaded with fine sediments, which were later deposited as well-stratified thin beds and formed the current outcropping lacustrine deposits. The same authors suggested that wherever the lacustrine sediments cropped out the underlying wadi deposits most probably stored water.

 

 

 

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