Al-Ahsa, often referred to as the largest and the oldest Oasis in the Arabian
Peninsula, is located in the Eastern Region of Saudi Arabia about 150 km South
of the port of Dammam and 320 km South-East of the capital, Riyadh. Its geographical
location is between 49° 10′ and 49° 55' eastern longitude and 25°
05' and 25° 40′ Northern latitude (Al-Taher, 1999)
which is around 130-160 m above sea level (Al-Barrak, 1993).
It embraces an L-shaped area of 320 km2 with vertical stroke lying
in a due North-South direction and the province capital, Hofuf lying in the
corner of the L. The entire cultivated area, which used to be over 20,000 ha,
is not continuous at present, being interrupted around the towns of Hofuf and
Al-Mubaraz in the South-Western corner of the Oasis. The overall area is considered
as twin Oasis with an Oasis in the North and the other in the South.
Natural water springs were widespread along the Western edge of the Oasis from
South of Al-Hofuf to North of Al-Mutirifi village and divided under four main
groups (Fig. 1). Three of these groups are located within
the jurisdiction of Al-Hassa Irrigation and Drainage Authority (HIDA) and the
fourth group is situated out of this organization. These groups are over three
main sectors i.e., Eastern, Middle and Northern sectors. Ain Umm Sabah was classified
under third group and is located South of Al-Mutirifi village. Previously, Al-Hassa
Oasis was irrigated by limited groundwater resources represented by a hundred
water wells and natural springs.
Al-Kuwaiti and Ahmed (2003) stated that the total water
discharge of free flowing springs was 10 m3 sec-1 according
to BRGM (1977) and 15.2 m3 sec-1 as recorded by HIDA in
cooperation with Waste Water Authority and the Directorate of Agriculture and
Water. Leichtweiss-Institute reported that the discharge of all free flowing
water springs was 10.3 m3 sec-1 (Anonymous,
Water salinity of Ain Umm Sabah, one of the main natural springs, was 1536
mg L-1 in 1979 (Anonymous, 1984), 1540 and
1560 mg L-1 by Leichtweiss-Institute in 1978 and 1979, respectively,
1750 mg L-1 (top of Ghawar Structure) to 2200 mg L-1 (East
of Shedgam Plateau) in 1978 and 1690 mg L-1 by Hussain
and Sadiq (1991) and 2579 mg L-1 by Al-Hawas
(2002). The temporal changes in water quality especially the total water
salinity may be the result of intensive pumping to meet water requirements for
various uses (Blaszyk and Gorski, 1981; Appelo
and Postma, 1994).
Metal composition of groundwater depends on the aquifer geology, rock-water interface, recharge from precipitation, deep percolation of drainage water, seepage from irrigation fields into the shallow aquifers, intrusion of seawater due to over exploitation of groundwater resources to meet growing water needs for different uses and nutrient losses from irrigated fields into the groundwater. The main objective of this study was to determine elemental chemistry of groundwater in Al-Ahsa Oasis where the main water supply sources were the free flowing springs (now replaced by wells) and the shallow groundwater aquifers such as Neogene and Al-Khobar.
Hydrogeology of the study area: The geology and hydrogeology of the
aquifers in the Eastern Province of Saudi Arabia were described by Naimi
(1965). The ground water potential of the area has been investigated in
great details during the last two decades by Italconsult (1969),
BRGM (1977) BRGD (1976, 1977),
GDC (1980) and Water Atlas of Kingdom
of Saudi Arabia (1985) and Ministry of Agriculture and
The Dammam Formation, which mainly consists of carbonate rocks with inter-bedded shales and marls, is bounded at the base by the chalky limestones of the Rus Formation and at the top by the Eocene-Neogene unconformity. The Formation is subdivided into five members: the Midra Shale, the Saila Shale, the Alveolina Limestone, the Khobar and the Alat Member (Fig. 2). The last two members consist of an upper limestone unit and a lower unit and they are the most economically exploitable aquifers in the Dammam Formation.
The Midra and Saila Shale members are considered a single lithological unit in view of the great lithological similarity between them. They constitute the basal level of the Dammam Formation and consists of blue to blue gray marl or shales and the limestones. Their combine thickness over the whole study area ranges from zero on the top of the Ghawar anticline west of Hofuf, to 20-25 m in the coastal belt area, the average thickness being 15-20 m. These two members, together with the underlying Rus |Formation, form the hydraulic separation between the Dammam and Umm Er Radhuma Formations.
The Alveolina Limestone member overlies the Midra and Saila Shales and is bounded at the top by the Khobar member. The thickness of this member ranged from Zero to about 20 m. The Khobar member overlies the Alveolina Limstone member. It is bounded at the top by the marls of the Alat Member and it consists mainly of skeletal-detrital limestone, dolomitic limestone and a basal marl unit. The limestone and dolomitic limestone of the Khobar Member form the most productive reservoir in the Dammam Formation. The thickness of the Khobar limestone member ranges from a minimum of zero meter to a maximum of about 60 m.
Previous investigations by Italconsult (1969) and BRGM
(1977) have shown that the transmissivity of the Khobar aquifer is extremely
variable, ranging from a minimum of 3.0x 0-6 m2 sec-1
to a maximum of 3.0x10-1 m2 sec-1. Storage
coefficient (10-3 to 10-5) are low, indicating a confined
Climate of saudi arabia: The Arabian Peninsula is located in an arid
belt extending from Northern Africa through Arabian Peninsula, Iran and Mongolia.
According to Lin (1984), the yearly potential evaporation
(Hofuf-3359 mm) is much greater than the yearly mean rainfall (Hofuf-69.6 mm).
High evaporative conditions and inadequate irrigation supplies determine the
hydrology, land development and vegetation of the area.
High temperature during Summer is the most significant climatic factors Saudi
Arabia. An extreme maximum air-temperature of 51.3°C was recorded at Hofuf
in June 1983. However, in general, the maximum daily air-temperature often exceeds
45°C and the relative humidity is also very low in Summer. The diurnal variation
of the air-temperature is strikingly high and causes the apparent diurnal variations
of relative humidity. Though the overall air-temperature variation have been
observed from -2.6 to 51.3°C but night frosts are rare. Mean annual rainfall
is about 70.3 mm (Al-Kuwaiti and Ahmed, 2003).
||Generalized Litho-stratigraphic Sequence of Tertiary in Eastern
Saudi Arabia [After Italconsultant (1969)]
Water resources: Main water sources in Al-Ahsa Oasis were the free flowing
springs (now replaced by wells) and the different groundwater aquifers namely
Neogene, Al-Khobar and Umm-Radhuma. Presently, most of the free flowing springs
are not operating naturally due to depletion in groundwater level as a result
of intensive groundwater pumping by the shallow and deep wells in order to meet
increasing water demand for agricultural expansion and urbanization. The significance
of the two major water supply sources is summarized below.
Free flowing natural springs: Free flowing springs (now replaced by
wells) are the basis for the existence of Al-Hassa Oasis. Early records on number,
location, water quality, water temperature, purity and discharge date back to
1941 and 1951 (Vidal, 1951). Early investigations carried
out on free flowing springs for the design of Hassa Irrigation and Drainage
Authority (HIDA) showed that only 32 springs were productive and useable for
the establishment of new irrigation project. The new irrigation project consisted
of irrigation channels (major and minor) for the supply of irrigation water
for agricultural activities and the drainage network for the disposal of drainage
water to the two main evaporation lakes on the edges of Oasis covering the South
and East part of the Oasis. The main springs are situated on a line connecting
Hofuf, Mubarraz, Al-Mutarifi on the Western Border of Oasis (Fig.
1). The 32 main springs were:
|Group of Springs (Al-Mutarifi)
|Group of Springs (East of Hofuf)
|Ain-Nasser (Al-Qrain Village)
Wakuti (1964) conducted first detailed study on spring
discharge while doing feasibility study for the entire Oasis. The detail is
After the completion of HIDA project in 1971, only 32 main springs, considered
most productive and active, were used for water supply. The evident decrease
in spring discharge after 1980 is not only due to the project implementation
but also due to the increase in private well drilling which is now thought to
be in excess of 2000 wells tapping the Neogene aquifer alone. The once free
flowing springs are now dried and replaced by wells.
Groundwater aquifers: Important aquifers in the Eastern Region of Saudi
Arabia are Wasia, Umm-Er-Radhuma, Al-Khobar, Alat and Neogene. The water quality
varies within each aquifer as well as between aquifers. It generally increases
in the direction of the hydraulic gradient ranging between 2000-6000 mg L-1
(Wasia) and 1000-3500 mg L-1 (Neogene) as reported by Ministry
of Agriculture and Water (1992).
Chemistry of groundwater
Water salinity (EC dS m-1): Al-Zarah (2008)
surveyed the groundwater of Al-Ahsa Oasis and observed that mean EC (expressed
as dS m-1) ranged from 1.23-5.05 (Table 1). The
groundwater is classified as C2S1 to C4S3, i.e., medium salinity and high sodium
to very high salinity and high sodium waters.
|| Chemical composition of groundwater of Al-Ahsa Oasis
|Data Source: Al-Zarah (2008)
|| Contour map of groundwater salinity of Al-Ahsa Oasis. Al-Zarah
The contour map of groundwater salinity (Al-Zarah, 2008)
showed that the groundwater salinity varies from 4400 mg L-1 (total
dissolved solids, TDS) along the Gulf coast on the eastern side to around 1800
mg L-1 on the western side of Al-Ahsa oasis (Fig. 3).
The high TDS on the eastern side could be due to: (1) High pumping rate deteriorated
the groundwater quality by extracting a mixture of freshwater and saline water
from the aquifer, because freshwater layer is on the top of saline water and
(2) Possibility of seawater intrusion into the adjoining aquifer through natural
drainage to equilibrate the depleting zone. Overall, the TDS of groundwater
decreased from East to West direction, but improved in the middle (Western part)
and the South-East corner of Al-Ahsa Oasis. The improvement in total salinity
(TDS) of groundwater could be attributed to shallow well depth and the minimum
influence of seawater intrusion. The groundwater is dominant by Na ion followed
by Mg, Ca and K in descending order. Whereas, the anion concentration order
is Cl>SO4>HCO3 (Table 1). Overall,
Na and Cl ions dominate the total salinity of the groundwater.
Total hardness: Mean hardness ranged from 460-1420 (mg L-1)
in the groundwater of AlAhsa Oasis at different locations (Table
1). Overall, the groundwater is unsuitable for drinking and other purposes
according to WHO (1984) drinking water quality standards
and household uses unless hardness is either totally removed or minimized to
the recommended safe limits. The concentration of CaCO3 dissolved
in water by its degree of hardness is presented below:
Nitrate (NO3) concentration: Mean nitrate concentration (mg
L-1) ranged from 5.3-78.6 in the groundwater of Al-Ahsa Oasis (Table
1). The maximum permissible limit of nitrate concentration in water for
various purposes especially for drinking is 50 mg L-1 according to
WHO (1984). Overall, most of the groundwater samples contain
low level of nitrate which is within permissible limits and will not create
health hazards upon consumption.
Fluoride (F) concentration: Mean fluoride concentration (mg L-1)
ranged from 1.0-2.7 in the groundwater of Al-Ahsa Oasis (Table
1). The WHO (1984) guidelines suggested that in warm
climate regions, the fluoride concentration in drinking water should remain
below 1 mg L-1, while in cooler climate it could go up to 1.2 mg
L-1. The guidelines value (permissible upper limit) for fluoride
was set at 1.5 mg L-1. However, the F concentration in groundwater
of Al-Ahsa Oasis is within permissible limit according to WHO
(1984). The Saturation Indice (SI) value of fluorite mineral is negative
in the groundwater of Al-Ahsa Oasis at four different locations.
Saturation indices: Groundwater chemistry is influenced by various dissolution
and precipitation reactions occurring in the soil during rock-water interaction.
Because the concentration of many ions is influenced by many environmental factors,
especially the position and the solubility of rock strata (water-rock interaction)
(Lin and Clemency, 1980; Ronge and Claesson,
Saturation Indices (SI) were calculated for groundwater samples from Al-Ahsa
Oasis using the speciation code WATEQ4 (Ball and Nordstrom,
1992) and the PHREEQC model developed by Parkhurst (1995).
Mean saturation indices of different minerals are given in Table
2. The groundwater is under-saturated (negative SI) with respect to certain
minerals (for example: calcite, dolomite, gypsum, anhydrite, halite, pyrite,
fluorite and aragonite) and oversaturated (positive SI) with respect to some
other minerals (For example: goethite, Siderite and hematite). Actually, the
SI is a measure of the thermodynamics state of a solution relative to the equilibrium
with a specified solid-phase mineral. However, the minerals with positive SI
will precipitate and adversely affect the aquifer properties.
|| Mean Saturation Indices (SI) of different minerals of groundwater
of Al-Ahsa Oasis, Eastern region Saudi Arabia
|Source: Al-Zarah (2008)
Similarly, the minerals with negative SI will dissolve aquifer rock during
groundwater flow which will increase its porosity and permeability.
Chemistry of spring water: Many researchers have investigated the chemistry
of free flowing spring waters (Anonymous, 1979; Hussain
and Sadiq, 1991; Al-Naeem, 2008). The total salinity
(EC dS m-1) of spring waters ranged between 2.1-2.6 (1975), 2.02-3.35
(1991) and 2.90-4.24 (2002) between 1975 and 2002. The data showed a significant
change in the total water salinity over a period of 25 years or more (Table
3). This temporal variability in the total water salinity could be attributed
to (1) over exploitation of the groundwater which deteriorated the water quality
and (2) possibility of seawater intrusion from the Southeast side in order to
fulfill the depleting aquifer from Al-Oqair region.
Hussain and Sadiq (1991) classified all the spring
waters as C3S1 to C4S2, i.e., high salinity and low sodium to very high salinity
and medium sodium waters according to Ayers and Westcot (1985)
water classification scheme for crop irrigation. The TDS of spring waters ranged
between 1293 and 2144 mg L-1. The TDS range suggests that the springs
were drawing waters from aquifers with variable salinity. Sodium was the most
abundant cation in all water samples followed by Ca, Mg, Sr and K in descending
order. The concentration of all trace and heavy metals was below 0.1 mg L-1.
Thermodynamics calculations revealed that a significant fraction of Ca and Mg
in spring waters was associated with SO4 and HCO3. However,
most of the Na and Cl were found in free form. Thermodynamics calculations showed
that Ca and Mg would preferentially precipitate as their CO3 followed
Chemistry of umm-khurisan spring: Al-Mohandis (1991)
studied the geochemistry of Umm-Khurisan spring located near Jamal Al-Qarah,
Al-Ahsa Oasis. He found high concentrations of total hardness, total dissolved
solids, Cl, NO3 and F and the high values of the electrical conductivity
of the spring water are above the safe limits recommended by the WHO
(1984) standards with respect to potability. The concentrations of other
minor constituents are around these limits. The Umm-Khurisan water is a saline
water with medium sodium hazard and should be used only on soils of moderate
to good permeability. The Umm-Khurisan spring water is of meteoric origin of
deep percolating water (Table 4).
|| Chemical analyses of Umm-Khurisan spring water
|WHO: World Health Organization, NASNAE: National Academy of
Science and National Academy of Engineering
The concentration of all the cations/anions in the spring water are within
the maximum permissible limits for use in agriculture except Cl, NO3,
pH and TDS which are higher than the permissible limits. The high concentration
of these cations/anions can create soil salinity, Cl ion toxicity in plants
(citrus). Also the high NO3 contents defines this water as unsuitable for drinking
|| Elemental concentration (mg L-1) of Umm-Khurisan
|WHO: World Health Organization, NASNAE: National Academy of
Science and National Academy of Engineering
The concentration of trace elements and heavy metals is presented in Table
5. The concentration of all the trace elements and heavy metals is within
the permissible limits according to WHO (1984) except Pb,
NO3 and F which are higher than the recommended limits of WHO
(1984) whereas the Selenium and Arsenic are equal or less than the established
standards of WHO (1984). The relatively high nitrate concentration
could be due to the application of nitrogen fertilizer to crops around the spring
or to the biological activity of nitrogen fixing bacteria. The level of fluoride
must not exceed the optimum level, i.e., 0.5-1 mg L-1 in drinking
water, because it is harmful to children and develops F related problems.
Micro-elements in spring waters: Hussain and Sadiq
(1991) reported that the concentration of trace elements and heavy metals
in the spring waters is very low and is within permissible limits according
to WHO (1984) standards for drinking water and other purposes
Ain Umm sabah spring: Recently, Al-Naeem (2008)
reported the trace elements and heavy metal composition of Ain Umm-Sabah natural
spring (Table 7). Presently, the spring is not flowing naturally
and a shallow pump is installed at the site of spring for pumping water to the
main water reservoir. The data indicate that the concentration of trace elements
and heavy metals is very low as compared to the permissible limits of WHO
(1984) for drinking and other uses. Also, the concentration of trace and
heavy metal ions was within safe limits for drinking water when compared to
the established standards of European Countries Guidelines for drinking water
and other uses.
|| Concentration of micro-elements
|ND: Not detectable (beyond detection limit of mg L-1)
Ain al-khadoud spring: Fathi and Al-Khahtani (2009)
studied the water quality of Al-Khadoud spring and concluded that after receiving
water from the outlets either treated sewage water or of re-use drainage water,
the spring water had an obvious increase in electrical conductivity, COD, total
alkalinity, nitrates, phosphorus, chloride and potassium. These features indicated
pollution with organic wastes, increased salinity and deteriorated oxygenated
state. Based on this, it can be concluded that all these factors could affect
both the soil and plants cultivated in the area of Al-Hassa. The physio-chemical
characteristics of spring water are shown in Table 8.
Classification of groundwater for irrigation: The groundwater was classified
on the basis of chemical composition of water samples from Al-Ahsa Oasis for
irrigation by following the guidelines according to Ayers
and Westcot (1985). The groundwater falls in the category of C3S1 to C4S3,
i.e., high salinity and low sodium to very high salinity and medium sodium waters
Infiltration rate of soils: The groundwater of Al-Ahsa Oasis was classified
for its effect on the infiltration rate of soils after irrigation (Fig.
5). The data show that the Al-Ahsa groundwater is moderate to highly saline
with low to medium sodicity hazards after irrigation. Generally, high salinity
of irrigation helps the flocculation process of soil particles which improves
the soil structure thus resulting in high rate of water infiltration and enhance
aeration to plant roots for optimal growth.
||Mean metal concentration (mg L-1) of water of Ain
Umm Sabah spring in Al-Hassa, Eastern Province as compared to the WHO
|**EEC (European Countries Guidelines)
|| The physico-chemical characteristics of Al-Khadoud spring
water during the investigation period
|| Irrigation water classification according to USDA
On the other hand, irrigation water with low salinity results in deflocculation
of soil structure due to hydrolysis process and deteriorates the soil structure.
This will considerably affect the soil productivity. Therefore, It was noticed
that the groundwater will not affect the infiltration rate of soils after irrigation
according to the FAO Guidelines (Ayers and Westcot, 1985).
Because, the groundwater is of moderate salinity with medium sodicity hazards
and falls in the category of no-reduction zone with respect to infiltration