Faculty
Of Engineering Ain Shams University, Cairo Egypt
e-mail: msoliman1@ hotmail.com
Abstract: In this study, the prevailing aquifers in Wadi Sudr are assessed for the rock type, the source of recharge and the water quality. The collected meteorological data were analyzed for the evaluation of the rainfall and runoff behavior as storm events.The changes of water quality in each of the investigated aquifers indicated prominent variation in the clastic aquifers (wadi fill and deltaic) rather than in the carbonate aquifer (limestone and dolomite). The direct response of groundwater quality of some aquifers and the non-response of the others may indicate different sources of recharge or a kind of separation among those aquifers. The water quality changes due to rainfall and runoff are represented by salinity dilution, sodium and chloride ion low concentrations, and calcium sulfate salt increasing. The changes in water quality are associated with immediate changes in water levels. The water quality of the deltaic aquifer is considerably affected by the sea water intrusion, where the rainfall effect upon the ion concentrations is of less importance.
Keywords: catchment(wadi),
rainfall, runoff, ground water, environment, water quality, springs, limestone,
aquifers
This study is focused on the changes of groundwater quality in relation to rainfall and runoff on both temporal and spatial bases. In Wadi Sudr, the groundwater is contained of three different aquifers; namely Fissured carbonate rocks (upstream), Wadi fill (midstream) and Deltaic deposits (downstream). The response of each aquifer for the rainfall and runoff events is evaluated. The study area (680 km2) lies between latitude 29° 30’ and 29° 52’ and longitude 32° 04’ and 33° 15’. This wadi debouches into the Gulf of Suez as shown in Fig. 1. Agriculture and tourism activities in the study area depend on groundwater resources, either directly or through treatment. South Sinai is categorized as arid region. The average annual rainfall is 15 mm, while records up to 14.6 mm were recorded in one storm (February 1999). To achieve the aim of this study, twenty-two water points were sampled for different time periods covering the rainy seasons between April 1998 to January 2000 (Fig. 1). The total numbers of collected and analyzed water samples during the present work are more than 100 samples.
Three main rock types representing three main geologic terrains are distinguished as aquifers in the area. These aquifers are present as follows (Fig. 1): -
2.1 The carbonate aquifer
It occupies an area of about 436 km2. It is mainly covering the upstream portion of Wadi Sudr. It extended from Wadi Meliha and Ain Sudr down to the middle of the wadi channel. This aquifer consists mainly of fractured limestone and dolomite. Such part of Wadi Sudr is mainly related to the Upper Cretaceous- Lower Miocene age. The groundwater in the fractured limestone flows naturally through perennial springs e.g. Sudr, Abu Ragem and Dersa springs.
It occupies the downstream part of Wadi Sudr. It consists mainly of alluvium deposits. It belongs mainly to the Miocene age. They are composed of poorly sorted calcareous gravel and sand interbeds of maximum 4 m thick. In this aquifer type, the hand-dug wells are existing e.g. Umm Garef, Abu Garad and El Arab wells.
It occupies the delta of the wadi, which covers an area of about 100 km2. This deltaic aquifer is related to the Quaternary age in general and the Pleistocene in particular. The delta of the wadi can be considered as destination of both surface runoff and sub-surface water flow routes. For the inventory and classification of the selected water points, Table 1 summarizes the basic data of such water points.
Table 1 aquifer type and water level for selected water points
|
Aquifer
Type |
Av.
Elevation (A.M.S.L. (m)) |
Depth to Water (m) |
||||
|
9/98 |
2/99 |
3/99 |
4/99 |
2/2000 |
||
|
Carbonate Aquifer |
400 |
1.0 |
0.00 |
0.2 |
0.21 |
0.20 |
|
Wadi Fill Aquifer |
180 |
1.6 |
0.78 |
1.00 |
0.5 |
0.5 |
|
Deltaic Aquifer |
15 |
9 |
7.8 |
12 |
11 |
- |
The groundwater quality was studied in relation to the climatic changes and storm events. In the carbonate aquifer the total dissolved solids (TDS) ranges between 5759 ppm (El Deasa spring) and 2303 ppm (Sudr spring). The variation in TDS values from site to site is a matter of different factors e.g. depth to water, flow velocity and distance from upstream. The higher salinity is attributed to evaporation, leaching effect and path length of water movement from the upstream. Moreover, it can be noticed that, in the carbonate aquifer, the increase of total dissolved solids value is mainly associated with an increase in sodium, chloride and magnesium ion contents. Due to the dominance of some major ions in the studied water points, the following ion ratios are calculated:
Na/Cl ranges between 1.121 (El Dersa spring) and 0.949 (El Deasa spring) with an average of 1.00
SO4/Cl ranges between 0.859 (El Dersa spring) and 0.333 (El Deasa spring) with an average of 0.4992
Ca/Mg ranges between 1.04 (El Dersa spring) and 0.61 (El Deasa spring) with an average of 0.9028.
From the values of the ion ratios, it can be noticed that there is a gradual decrease in all reported values from El Dersa to El Deasa springs, i.e. down gradient.
Since the limestone rocks are of marine origin, the ion ratios can be compared by that of the seawater, where Na/Cl ratio = 0.85, So4/Cl = 0.15and Ca/Mg = 0.1. The higher value of So4/Cl and Ca/Mg of the carbonate groundwater is attributed to the abundance of gypsum deposits.
The wells that represent the Wadi Fill aquifer are located in the midstream portion show lower salinity values if compared with the salinity of the water in the upstream springs (limestone aquifer). These values vary between 1170 ppm and 1630 ppm. This indicates that the water of the carbonate aquifer does not replenish the water of the wadi fill aquifer. Consequently, the wadi fill aquifer is directly recharged from the local precipitation as well as from the running water in the main wadi channel. In other words, one can conclude that the aquifers of the limestone in the upstream and the wadi fill in the midstream are not hydraulically connected.
In the wadi fill aquifer, it was noticed that the total dissolved solids (TDS) increase toward the downstream of the wadi. This is due to the dissolution of the rocks and the effect of leaching.
For the groundwater in the wadi fill aquifer, the water type is sodium chloride (NaCl) or sodium sulfate (Na2So4). However, it can be mentioned that the quality of the wadi fill water is highly influenced by the following factors:
(1) The quality of runoff and direct precipitation water.
(2) The evaporation at shallow depths
(3) The rate of water replenishment (infiltration rate).
The calculation of the ion ratio shows the following results:
Na/Cl ranges between 1.08 (El Arab well) and 1.01 (Umm Garef well) with an average of 1.05
SO4/Cl ranges between 1.36 (El Arab well) and 0.982 (Abu Garad well) with an average of 1.15
Ca/Mg ranges between 2.47 (Umm Garef well) and 1.55 (Abu Garad well) with an average of 1.89
The condition of the deltaic aquifer (serial no. 9 to 22) is quite different where the water chemistry depends considerably on different factors e.g. the drilled depth below sea level, the rate of discharge, the distance from the shore line and the distance from the main wadi channel. However the total salinity in the delta area varies between 2148 ppm and 8960 ppm.
Obviously, the highest value of water salinity in the delta area are represented by well no.11 which is located away from the wadi course, well no.6 which is tapping the aquifer at about 25 m below sea level and well no.13 which lies very close to the shore line. This means that seawater intrusion is the most important factor for water salinization in both well no.13 and well no.6 and well no.11. The sea water intrusion phenomenon appears in these wells due to the over pumping and the increased of well depth below sea level.
For the ground water in the delta aquifer, the dominant water type is Sodium Chloride or Sodium Sulfate, which may indicate seawater intrusion phenomenon or evaporate leaching. The great variation in water salinity at this part of the study area is accompanied by high contents of sodium, magnesium and chloride ions. In addition, a considerable amount of sulfate ions are encountered in most of the water points in the delta area. The sulfate -chloride ratio (SO4/Cl), however, ranges between 0.27 to 0.68. The sodium chloride ratio ranges between 0.72 to 0.9, which is comparable with that of the seawater (0.85).
The plotting of the chemical data on the so called Trilinear diagram (Piper 1948), has revealed different positions for the investigated aquifers which indicates different sources of recharge and, hence, different sources of mineralization. This genetic diagram can help in clarifying the aquifer relationships, where a connection among those aquifers is not evidenced from the water quality point of view. However, from this diagram the groundwater in the delta of Wadi Sudr is quite influenced by the sea water quality, while the groundwater of the carbonate aquifer and the wadi fill aquifer are comparable with that of the rainfall and runoff water.
In order to elucidate the response of rainfall and runoff upon groundwater quality, a number of field and laboratory measurements have been taken. The variation of TDS, Na, SO4, Cl, and water depth among the sampling periods have been depicted in graphs for selected water points representing the mid catchment area (Fig. 2,3). The effect of the flood (Feb 1999) upon the water quality for each of the three studied aquifers was also assessed in order to evaluate the relationship between the main source of recharge and the dominant quality in relation to the elapsed time.
From these figures, the following is concluded:
(1) In the limestone aquifer (upstream springs) the behavior of water quality and water levels as well in Ain Sudr indicates a kind of consistency of TDS and ion contents before the flood time, while after the flood the concentration of such constituents indicates higher values. In Ain Abu Ragem, the flood has a reverse effect on the water quality if compared with Ain Sudr, where the TDS and ion contents have been lowered considerably just after the flood in Feb.99 and went up again with time.
From these features, it is obvious, that the effect of runoff on the water quality is not equal among the different investigated sites. In case of direct connection with the surface runoff (e.g. Ain Abu Ragem) the flood water acts as a dilutante, while in the case of subsurface flow through fissures (e.g. Ain Sudr) the flood water brings more sulfate and chloride salts to the outlets. It's a matter of interest that the TDS and ion contents of the limestone springs returns back quickly to its original concentration after a limited time of the flood. However, such feature characterizes, generally, the fractured media where the water moves through fissures and joints. In other words, the effect of the flash flood upon the water quality in the fractured media is more or less promptly.
(2) In the Wadi-fill aquifer, which receives runoff water directly, the behavior of water quality with the flood and elapsed time is quite different. Actually the change in water quality due to flood reflects the velocity of surface and subsurface water flow through the main channel of Wadi Sudr. Furthermore, the rate of water infiltration, which differs from site to site, plays a considerable role in the rate of water quality changes. El Arab well in the downstream of Wadi Sudr shows these features. The depth to water in El Arab well raised up rapidly after the storm event in Feb. 99, meanwhile a considerable decrease in the ion content is noticed in the same time. Changes of the chemical type from NaCl before the flood to Na2SO4 after the flood is quiet noticeable. It also noticed that the location of El Arab well receive a surface water flow from one of the tributaries (second order) running from North to South. Therefore, we can consider that El Arab well is located at the intersection between the main trunk of Wadi Sudr and the second order tributaries. However, from the field observations, one can notice that El Arab well lies at the end of a sub-catchment area dissecting the foot slope of Gebel El Raha, where sand accumulation and mud soil are dominating.
(3) For the delatic aquifer, the matter is quiet different since the role of the runoff water as a diluttant differs considerably in relation with the distances between the water points and the Wadi channel in the delta. The closer the water points to the main course of the Wadi, the greater the effect of groundwater dilution. This means that the deltaic aquifer (porous medium) does not behave equally in response to the runoff water.
From the field observations and chemical analyses of the investigated water the following are concluded:
(1) The surface runoff changes considerably the water quality in both the wadi fill and deltaic aquifers. No slight changes have been recorded in the limestone aquifer before and after the flood, which indicates that this aquifer is not considerably recharged from the local precipitation.
(2) In the carbonate aquifer, after the flood of 07/02/1999, it was noticed that the quantity of groundwater increase while its quality does not change. On the other hand, in the wadi fill aquifer, the quantity of groundwater increases and its quality changes.
(3) The groundwater of the limestone aquifer is not directly connected with the wadi fill aquifer.
(4) Surface runoff and direct precipitation are the main source of recharge for the wadi fill aquifer.
(5) The groundwater in the deltaic deposits is considerably affected by two main sources; the runoff through the main wadi channel and the seawater intrusion from the Suez Gulf.
(6) The chemical water type of the different water points along Wadi Sudr is Sodium Chloride in the limestone aquifer, Sodium Chloride or Sodium Sulfate in both the wadi fill and the delta aquifers.
References
Nahla A. Monem” Hydrology of Wadi Sudr” M. Sc. C.E. Faculty of Engineering , Ain Shams University Cairo. Egypt(2000).
Soliman M.M. et al “ Environmental Hydrogeology” Lewis Publishing Co, New York, !998.
Fig. 1 Location map showing the main aquifers and water points
Fig. 2 Changes of TDS with time
Changes of water depth with time
Fig. 3 Behaviour of water depth and chemistry of El Arab well during Flood