ENVIRONMENTAL BACKGROUND

Damascus Oasis plays an important economical role for Damascus city through agricultural, industrial and urban activities. It has an area about 1200 km² and the mean elevation of the Oasis is about 650 m (a.s.l.). It is located in the southern part of the Syrian Arab Republic (Fig.1). The climate is Mediterranean type, which is characterized by rainy and cold winters and hot dry summers [15]. The mean annual rainfall is between 221 mm/y as measured at the Mazzeh meteoric station and 136 mm/y as measured at Damascus airport meteoric station [16].

FIG. 1. Geological map of study area [17].

The Damascus basin has a convex structure that is bounded by faults with a NE-SW orientation.

Quaternary sediments of the Damascus depression near the Rabweh area grades, from pebble, gravel and conglomerates, fine sand and silty soil in the centre of the Oasis to more recent Quaternary sediments consisting of loamy and clayey lacustrine deposits near Oteibeh Lake. In the south and southeastern part of the Oasis there are basaltic lava flows of Neogene and Quaternary age (Fig.1) [17].

The Barada and Awaj are the main rivers in the hydrographic network in Damascus Oasis. The Barada River originates in the Anti-Lebanon Mountain with an elevation of 1500 m (a.s.l.) and terminates in Oteibeh Lake (597 m) during the flood period only. Since the 1970s, Oteibeh Lake has been dried and the Barada River does not reach the Lake. The Barada River is fed by the Barada spring, which has an average discharge of 3.12 m³/s and by the Cretaceous karstic Figeh spring, which has an average discharge 7.7 m³/s. The Awaj River originates from many springs in the eastern part of Haramoun mountain, (elevation 2814 m (a.s.l.)) and terminates in Hyjaneh Lake, again only during flood period.

In Damascus Oasis, the main aquifer is a Quaternary formation 400 m thick. This aquifer consists of several sub-aquifers (multi layers) that differ in lithological characteristics. Sediments consist of gravel, conglomerates and sand. In some places the aquifer becomes semi-confined by clay lenses.

The water type is typically calcium bicarbonate or magnesium bicarbonate [18, 2]. The piezometric levels in the Oasis change according to the alternation of flood and dry periods. Recently, distinctive changes in piezometric levels have been occurred. These changes are caused by extreme dry years (1999 to 2001). In the north of Oasis, the water table has risen as a result of return flow by irrigation water supplied by treated sewage water. Groundwater recharge sources in Damascus basin are from the surface water (Barada and Awaj rivers), irrigation channels and the direct infiltration from precipitation. Several wells were drilled in Damascus Oasis, approximately more than 20 000 wells,

which are used for agricultural purposes. Over exploitation beyond the natural replenishment had caused a continual drop in piezometric levels and formation of deep hydrological depression associated with increasing in salinity especially in the eastern part of the Oasis (Oteibeh and H.Awamid). At the beginning of the 90 s, groundwater depth in the Oasis was between 6–10 m depth in the western part and 20–50 m depth in the eastern part. Recently, the groundwater depth decreases to 20–30 m depth in the western part and 60–80 m depth in the eastern part of the Oasis. Groundwater flow direction is from west to east in paralleled with the flow direction of Barada River.

In January 2001, heavy metal concentrations in the Barada River were measured by the Ministry of irrigation and found to be below the permitted concentrations limit, which indicates there is no pollution in surface water, even though the concentrations increased from upstream to the downstream.

Falouh [1] studied heavy metal concentrations in the Barada River stream sediments, where it was concluded that the surface water is highly vulnerable to direct pollution, in particular, in industrial areas. While groundwater pollution by heavy metals is less than that in surface water and in stream sediments, high concentrations of Cd and Pb were found in stream sediments in the industrial areas nonetheless.

Comparing results of Falouh [1] as described earlier with the results of this study, it is found that the trace elements in groundwater, show low concentrations and are below the permitted limit for drinking water, however, concentrations were higher in some samples compared to the Falouh study. It is speculated that after three extremely dry years between 1999 and 2001, during which time the groundwater levels have decreased by about 20 m, the risk of groundwater pollution by heavy metals is less because the unsaturated zone, which plays a protective role, has become thicker.

3. METHODOLOGY

Twenty six drinking wells were selected in the Oasis for sampling (Fig.2). Water samples were collected from these wells during November and October, 2001. The wells are located on the both sides of the Barada River, between Damascus city and Oteibeh Lake. Physical parameters, including pH, temperature, electrical conductivity (EC), total alkalinity (ALK) and dissolved oxygen (DO) were measured in-situ. Sample location (latitude and longitude) was determined using a portable GPS instrument (Table 1). Major element analyses were performed, using Chromatograph (Dionex 120) and stable isotopes (¹⁸O, ²H) of water samples were measured using mass spectrometer (delta plus) in the laboratory of Geology department (SAEC) (Table 2). Trace element concentrations in groundwater were measured using the anodic stripping voltametric method [19] with a 693 VA processor, Metrohom (Table 3). Three soil profiles (KA, KB and KS) in Kabbas area was drilled to a depth of 550, 375 and 220 cm using a hand-auger. The water table was located at 20 m depth. A total of sixty seven soil samples were taken at 10 cm intervals. The location of the soil profiles is near one of the most polluted areas in Damascus Oasis where the leather industries (tannery) are located (Fig. 2).

Water content measurements, defined as the ratio of the mass of water to the total mass (solid and liquid) after heating the soil sample to 105ºC [20], grain size distribution [21] and mineralogical analysis using X ray diffraction were performed on each soil sample. Soil samples digested prior to analyses Cd, Cr and As using Atomic Absorption with three different methods; furnace with a detection limit were analyzed of 10 ppb, flame with detection limit of 5 ppm, and hydrolyte methods with detection limit of 10 ppb. Pb, Zn and Cu were analyzed using Energy Dispersive X ray Florescence (EDXRF) (Chemistry Dept. Syrian Atomic Energy Commission (SAEC). The detection limits were 14 ppm, 33 ppm for Pb and Cu, respectively [22]. Total Organic Carbon (TOC) content in the soil was analyzed using “Apollo 9000” instrument and stable isotope in the soil was analyzed using Gas Chromatograph Mass Spectrometer (GC–IRMS) were performed in Environmental Engineering Research Centre in Belfast, UK.

4. RESULTS AND DISCUSSION