Hydrogeochemical evaluation of shallow alluvial aquifer of Wadi Marwani, western Saudi Arabia

1 Introduction

As groundwater is often the only reliable source of fresh water and it is the only renewable water resource in arid regions, this is due to sporadic rainfall; groundwater recharge is limited to infiltration during the intermittent flood in dry wadi channels (wadi is a dry stream bed with surface runoff). In such regions, groundwater is commonly the only water resource. Saudi Arabia lies within one of the driest areas in the world representing the eastern continuation of the Sahara Desert in North Africa. Water resources in such areas are mainly derived from groundwater and infrequent surface runoff, where water conservation projects are applied. In Saudi Arabia, groundwater is considered as a major source of fresh water that is used in different purposes. It is currently exploited from two main sources: (1) deep aquifer systems with nonrenewable groundwater in sedimentary rocks to the east of the Arabian Shield, and (2) shallow alluvial aquifers and fractured Precambrian basement rocks in the mountainous western and southwestern parts. The latter represents the most important renewable water resource in west Saudi Arabia, which is used for irrigation and domestic purposes, and for drinking in many areas. These shallow alluvial aquifers are mainly located in the major wadis draining the Arabian Shield area. The increasing exploitation due to farming frequently causes deterioration in water quality. Therefore, variations in natural and human activities reflect spatial variations in the hydrochemical parameters of the groundwater. The difference of dissolved ions concentration in groundwater are generally, governed by lithology, velocity and quantity of groundwater flow, nature of geochemical reactions, solubility of salts and human activities (Karanth, 1997; Bhatt and Salakani, 1996). Wadi Marwani is one of the most important wadis in the western part of Saudi Arabia to the northeast of Jeddah City. It is intensively inhabited during the last decenniums, leading to expansion of the residential and agricultural areas. An important high potential shallow alluvial aquifer underlies this wadi, where groundwater is pumped from several hand dug wells to use for different purposes. Suitable quantity and quality of groundwater become a more crucial alternative resource to meet the drastic increase in social and agricultural development and to avoid the expected deterioration of groundwater quality due to heavy abstraction for miscellaneous uses. Recently, several agricultural and settlement activities have been developed in this area. These activities pose a major threat to groundwater storage and quality through the impact of agricultural practices and domestic waste disposal on the shallow groundwater. The main objective of this study was the evaluation of natural and human-induced chemical processes in the shallow alluvial aquifer of Wadi Marwani. The paper intends to find an appropriate combination of methods for a qualitative and quantitative description of aqueous geochemical processes. Assessment of groundwater quality for irrigation and drinking purposes were also carried out. This kind of investigation, when carried out extensively, would enable planners and policymakers to evolve a strategy to solve similar problems elsewhere.

2 Study area

Wadi Marwani lies in the Rabigh region of the central western Saudi Arabia to the northeast of Jeddah (about 90 km) and is located approximately between longitudes 39°22′ and 39°55′E and latitudes 22°05′ and 22°19′N (Fig. 1). It is considered as the most important tributary of Wadi Khulais, which supplies Jeddah city with an appreciable amount of domestic water. Moreover, this area is an important agricultural area in the coastal region, which supplies Jeddah with its requirements of vegetables and fruits. The Wadi Marwani area is characterized by an elevation of about 350 m above mean sea level (amsl) in the mountainous part that is totally made up of Precambrian basement rocks, whereas the wadi floor is a plain with an elevation varying between 90 and 240 m amsl. The catchment area of Wadi Marwani is 2347 km², a total length of 70 km, maximum width of 2 km and minimum width of 150 m (Al-Nujaidi, 1978). Wadi Marwani flows through a stream of an irregular shape and direction and it is generally from east to west. The wadi floor is covered by alluvial deposits of variable grain sizes ranging from sand to gravel and boulder. The area is of arid to semi-arid climate, which is greatly modified by the Red Sea in the west and the Harrat Rahat in the east. Summer season is characterized by a humid hot climate, where maximum temperature ranges between 33 and 43 °C. Winter is characterized by mild climate with maximum temperature varies between 22 and 30 °C, generally with low relative humidity. The study area receives sporadic and scanty rainfall, which is extremely variable both spatially and temporally. Rainfall generally increases from west to east, where relatively high rates are usually recorded at the eastern boundaries of the catchment areas. Rains occur in fall, winter and spring, where the maximum total annual rainfall reached 135 and 155 mm as recorded in Khulais and Al-Kamil meteorological stations, respectively. Runoff occurs as flash floods associated with heavy short-duration storms. Most of this runoff water is lost by infiltration through highly permeable alluvial deposits filling the wadi channel, where possible recharge of the underlying shallow aquifer can occur. The available runoff records at Wadi Marwani gauge station during the period 1966–1976 shows that the maximum amount was 223.9 × 10⁶ m³ recorded in 1974/1975.

Close

Figure 1

Location map of the study area.

Export to PPT

3 Geologic setting

Wadi Marwani is located to the southeast of Rabigh where the entire area represents an important part of the southern Hijaz region in the western part of Saudi Arabia. The area occupies a sector of the Arabian Shield, which is composed of Precambrian igneous and metamorphic rocks (Fig. 2). The geologic setting of this region was described by many authors (Skiba and Gilboy, 1975; Al-Nujaidi, 1978; Skiba, 1980). The dominant rock unit in the study area is the alkali olivine basalt of the Rahat Group (Hammah Formation) ranging in age from Paleocene to Miocene with subordinate alkalic intermediate lava, some minor feldspathoidal lava and some minor pyroclastics. The upstream part of Wadi Marwani drains this dominant rock unit. Sericite–chlorite schist of the Samran Group (Late Proterozoic) represents the oldest rock unit encountered in the central part of Wadi Marwani. Major part of the wadi drains an area comprises different rock units of the Kamil Suite, which is made up of slightly metamorphosed hornblende–biotite tonalite and granodiorite with minor granitic and dioritic rocks. Moreover, some outcrops of post-tectonic unmetamorphosed plutonic rocks are encountered in the study area including granite, quartz diorite, quartz monzodiorite, syenite and gabbro. The floor of Wadi Marwani is filled with Quaternary alluvial deposits being the weathering products of the varying rock units drained by the wadi. This alluvial cover is mainly composed of medium to coarse grained gravel along the wadi channels and fine sands and clay in the flat area.

Close

Figure 2

Geologic map of the Rabigh area (Ramsay, 1986).

Export to PPT

4 Hydrogeologic setting

The western part of Saudi Arabia is considered as an important region from the hydrogeological point of view, where it has very important water resources, on which many development projects have been recently established. Abdulrazzak et al. (1992), Bazuhair and Wood (1996) and Al-Shaibani (2008) estimated groundwater recharge rates for some selected wadis in western Saudi Arabia using the chloride mass-balance method. Hydrogeological and geophysical studies of the Wadi Marwani area have been carried out by several investigators. Italoconsult (1967, 1976), Al-Nujaidi (1978) and Bazuhair et al. (1992) studied the hydrogeology of the water resources of Wadi Marwani–Khulais area using electrical geophysical methods. In the present study, field surveys were carried out to investigate geology and to measure groundwater levels in 16 hand dug wells scattered along Wadi Marwani. Groundwater exists under unconfined conditions in the alluvial deposits. Groundwater is exploited from a shallow alluvial unconfined aquifer where the depth to water table varies from 5 to 15.3 m below the ground surface. Shallow depth of groundwater and the occurrence of highly porous and permeable overlying alluvial deposits increase the aquifer vulnerability to contamination from human activities in this area. Therefore, recirculation of groundwater is common when the water is used for irrigation. Groundwater levels vary between 386 m in the upstream portion and 238 m near the downstream. The groundwater flows from east towards west with an average value of hydraulic gradient of 0.004. Al-Nujaidi (1978) revealed that the depth to basement rocks in Wadi Marwani varies from 25 to 100 m. This variation of the depth to basement rocks was interpreted as a result of faulting structures. The saturated thickness of the aquifer varies due to variation in depth to basement rocks as well as the lateral extent of the aquifer. The aquifer transmissivity varies between 144 and 5760 m²/day and its specific yield ranges between 0.001 and 0.006 (Al-Nujaidi, 1978).

5 Sampling and analytical procedures

Sixteen groundwater samples were collected from private supply wells at depths of 5.00–15.30 m for hydrochemical investigation (Fig. 3). All the samples were analyzed for major ions (calcium Ca²⁺, magnesium Mg²⁺, sodium Na⁺, potassium K⁺, bicarbonate ${\text{HCO}}_3^{-}$ , sulfate ${\text{SO}}_4^{2-}$ , and chloride Cl⁻) and some minor ions (nitrate ${\text{NO}}_3^{-}$ , nitrite ${\text{NO}}_2^{-}$ , fluoride F⁻ and silica SiO₂) using the standard methods (American Public Health Association, APHA, 1998). Electrical conductivity (EC) and hydrogen ion concentration (pH) were measured at the wellhead using portable EC- and pH-meter. Chemical analyses for major and some minor ions were performed at the Laboratory of Faculty of Earth Sciences, King Abdulaziz University, Saudi Arabia. Total dissolved solids (TDS) were measured by sample evaporation techniques. ${\text{HCO}}_3^{-}$ was determined by titration against standard HCl. Cl⁻ was estimated by titration against standard solution of AgNO₃. ${\text{SO}}_4^{2-}$ was determined gravimetrically. Ca²⁺ and Mg²⁺ were analyzed by compleximetric titration against standard EDTA solution. Na⁺ and K⁺ were analyzed by flame photometry. ${\text{NO}}_3^{-}$ , ${\text{NO}}_2^{-}$ , F⁻ and SiO₂ were determined colorimetrically using spectrophotometric technique (APHA, 1998). The analytical precision for the measurements of cations and anions is indicated by the ionic balance error, which was computed on the basis of ions expressed in milliequivalent per liter. The values were observed to be within a standard limit of ±5% (Domenico and Schwartz, 1998). All concentration values were expressed in milligram per liter (mg/l) unless otherwise indicated.

Close

Figure 3

Well location map of Wadi Marwani.

Export to PPT

6 Results and discussion

Hydrochemical characteristics of groundwater in the study area are summarized in Table 1. The pH of groundwater varies within small range (7.2–7.4), which elaborates a slight trend of alkaline chemical reaction within the groundwater system. The electrical conductivity (EC) varies from 490 to 2295 μS/cm. Table 1 shows that only one water sample is fresh water (<500 μS/cm), while most of the samples are marginal waters (500–1500 μS/cm) and only one sample shows brackish water type (1500–5000 μS/cm) in the study area. TDS values range between 372 mg/l in the upstream part of Wadi Marwani to 1568 mg/l in the downstream part and it appears that about 15 samples have TDS less than 760 mg/l while only one sample (no. 13) reached to 1568 mg/l. The average TDS for the study area is 630 mg/l. It is clear that there is a gradual increase in groundwater salinity from upstream to downstream which coincides with the groundwater flow direction. Extraordinary salinity values of groundwater sample no. 2 in the upstream part, sample no. 7 in the middle part, and sample no. 13 near the downstream may be attributed to the influence of residential and farming activities at these locations. This can be caused by leakage of domestic wastewater from improperly designed septic tanks, which are usually unlined, and by return flow from irrigation practices in the farm areas. These two influences are reflected in the high salinity values. Such a wide range of EC and TDS values indicates that hydrochemistry of groundwaters in Wadi Marwani is controlled by various hydrochemical processes such as anthropogenic pollution and water–rock interaction. As shown in Table 1, the concentrations of major dissolved constituents were also quite variable. Such wide ranges of ionic concentrations also indicate the involvement of several hydrochemical processes influencing the water quality.

6.2

6.2 Representation of hydrochemical data

To understand the spatial control of major ions concentration, the relationships between TDS (as a useful indicator of anthropogenic contamination) and major ions are illustrated in Fig. 4. For all major ions examined, concentrations tend to increase with increasing TDS, whereas an anomalous sample (no. 13) with the highest salinity value was observed. This positive correlation suggest the impact of agricultural activities on groundwater chemistry through leaching of readily soluble salts from the soil zone, where these salts are precipitated under the influence of high rate of evaporation. The leaching process leads usually to increase concentrations of most major ions.

Close

Figure 4

Relationships between TDS and the examined major and minor ions.

Export to PPT

TDS is positively correlated to K concentrations of groundwater indicating the influence of agricultural activities and wastewater leakage, where it is known that potassium ion originates from agricultural fertilizer and wastewater (Trauth and Xanthopoulos, 1997). On the other hand, the relationship between NO₃ and TDS show similar trend which suggested the leaching of nitrate fertilizers applied in the agricultural areas. Only one sample (no. 2) showed anomalously high nitrate concentration which is mainly attributed leakage of domestic wastewater from the residential area that is very close to this well. NO₃ concentration may be further affected by complex hydrochemical processes such as nitrification or denitrification (Arnade, 1999; Graniel et al., 1999; Rosen et al., 1999; Sliva and Williams, 2001). Fluoride concentration varies from 0.24 to 0.79 mg/l and show indistinct trend with TDS, where its spatial distribution is independent of salinity of groundwater. This suggested different source of fluoride in groundwater rather than anthropogenic sources. The possible source of fluoride in groundwater is the dissolution of silicate minerals composing the aquifer matrix.

General distribution of groundwaters of Wadi Marwani is shown on a Piper diagram (Fig. 5). On the basis of this diagram, it is clear that in all waters, alkali earth elements (Ca + Mg) are higher than alkali elements (Na + K). The abundance of the alkali earth elements is attributed to dissolution of Ca and Mg-rich silicate minerals in the aquifer matrix. The removal of Ca, through the precipitation of carbonate minerals, may cause a disturbance in chemical equilibrium, resulting in the dissolution of minerals containing calcium such as calcite and gypsum (Freeze and Cherry, 1979). These minerals are possibly precipitated from irrigation water under the influence of high rate of evaporation and evapotranspiration. In few samples (nos. 1, 3, and 4), weak acids (HCO₃) are higher than strong acids (Cl + SO₄). These samples are representing the upstream part of Wadi Marwani, whereas the rest of the samples are characterized by the dominance of strong acids over weak acids. This change of the anion dominance is consistent with groundwater flow from the upstream to downstream, where the anthropogenic activities are the controlling factor. Irrigation practices involve dissolution of the readily soluble salts of chloride and sulfate ions precipitated in the soil zone due to high rate of evaporation. Generally, water chemistry is related to the different rocks and their interaction with water as well as impact of human activities which tend to alter the groundwater quality.

Close

Figure 5

Piper diagram for the Wadi Marwani alluvial aquifer groundwaters.

Export to PPT

The difference between ionic composition lines of groundwaters on the Schoeller diagram (Fig. 6) indicates that there are three major water types originating during water–rock interaction as well as impact of agricultural activities. The first group has the lower salinity values, which represents the upstream portion of Wadi Marwani and includes three samples (1, 3, and 4), where these samples are characterizing the recharge water composition. The second group includes clusters of most of samples having relatively higher salinity than the previous group. In this group, change of the anion dominance from HCO₃ to Cl and SO₄ is pronounced with groundwater flow. The third group includes only one sample (no. 13) of the highest salinity and dominance of Cl and SO₄. This sample with anomalous high salinity represents an area of extensive agricultural development and irrigation practices.

Close

Figure 6

Schoeller diagram for the groundwaters of Wadi Marwani.

Export to PPT

6.7

6.7 Groundwater quality assessment for different purposes

The main objective following the hydrochemical investigations of groundwater is to determine its suitability for different uses.

6.7.1

6.7.1 Drinking purposes

According to the WHO (1993), no standards have been established for TDS, chloride, sodium, potassium, hardness, and sulfate and their effects on health. The only standards that have been established are based on taste. The nitrate standard, however, is based on health effects. This also applies to the fluoride standard, but depending on local climatological conditions, and the amount of water consumed, it is sometimes difficult to meet this standard. The chloride standard is primarily based on the undesirable taste effect at a concentration higher than about 250 mg/l. However, people may get used to drinking water with a chloride concentration higher than 250 mg/l, and a concentration up to 600 mg/l is considered safe. Regarding sodium, it is believed that there is a relationship between sodium levels in drinking water and the occurrence of high blood pressure, however, no firm evidence supports this. Therefore, the sodium standard is only based on negative taste effects. These occur at a concentration higher than 200 mg/l. No data are available on the possible health effects of the TDS content. However, TDS may strongly influence the taste of drinking water. Water with a TDS lower than 1000 mg/l is usually acceptable for consumers, although this may strongly depend on local conditions. Water with a low TDS tastes flat and is often considered to be tasteless. There are indications that extremely soft water adversely affects the mineral balance. The public acceptance of the hardness of the water strongly depends on the local conditions. The taste limit for calcium is, somewhere, between 100 and 300 mg/l and for magnesium it is probably lower. In some cases, a hardness of 500 mg/l is tolerated by consumers. Sulfate is one of the least toxic anions. However, the presence of a high concentration in the drinking water may lead to dehydration, stomach complaints, and possibly diarrhea. Therefore, authorities are advised to be alert in cases of occurrence of water with a sulfate concentration higher than 500 mg/l. In general, the adverse affect on the taste is said to be minimal at levels lower than 250 mg/l. The 50 mg/l standard for nitrate is based on health effects. The WHO standard for fluoride of 1.5 mg/l is also based on health effects. A higher concentration increases the chance of skeletal deformations (fluorosis).

Comparison of the concentration of chemical constituents of well water of Wadi Marwani with respect to the drinking water standards of WHO (1993) showed that sample 2 (upstream part) and no. 13 (downstream part) had NO₃ contents above the permissible limits. Generally, concentrations of the investigated ions in groundwater are below their limits, therefore, groundwater can be used safely for drinking. However, high NO₃ content in most groundwater samples is alarming for further increase due to increase of the anthropogenic activities in the study area. In addition, microbiological investigation of these groundwaters should be carried out periodically to ensure absence of harmful microorganisms.

6.7.2

6.7.2 Irrigation purposes

In order to identify the suitability of groundwaters for irrigation use, the Wilcox classification diagram (1955) in Fig. 7 has been used. This graph is based on the electrical conductivity (EC) and on the sodium adsorption ratio (SAR). This parameter is of particular importance because high Na contents in irrigation waters may increase soil hardness and reduce its permeability (Tijani, 1994). The use of saline waters in permeable rocks may also increase the salinity of groundwaters. In Fig. 7, only one sample (no. 13) was plotted in C4-S2 field indicating very high salinity hazard and medium sodium hazard. Irrigation using groundwater from this site should be assessed in terms of selection of salt-tolerant crops and good drainage especially on clay soils. The remaining samples were clustered in two fields (C2-S1 and C3-S1) indicating medium to high salinity hazard and low sodium hazard. These groundwaters can be used safely for irrigation on clay soil, however, selection of crops depending on salt tolerance should be carried out prior to cultivation. Boron content in groundwater should be periodically determined because of its toxic effect on plants over certain limits.

Close

Figure 7

Wilcox diagram showing salinity and sodium hazards of groundwaters of Wadi Marwani.

Export to PPT

7 Conclusions and recommendations

Hydrochemical studies of shallow groundwater from Wadi Marwani, central western Saudi Arabia indicated the presence of anthropogenic sources of contamination. The TDS concentration appears to be a useful indicator of this contamination and generally increases in agricultural and residential areas. The groundwater chemistry in Piper’s diagram indicated that Ca + Mg are the dominant cations in the hydrochemical facies, however, it changes widely from a HCO₃–Cl–(NO₃)–SO₄ type (in the upstream part) to a Cl–SO₄–HCO₃ type (downgradient), suggesting that increase in Cl and NO₃ concentrations is typical of anthropogenically contaminated groundwaters. However, the concentrations of all the major ions examined increase with the degree of anthropogenic contamination. The hydrochemistry of groundwaters encountered in the upstream part can be used as the background water quality criteria for the evaluation of groundwater quality in Wadi Marwani. Careful examination of the distribution of major ions showed that the contamination of groundwaters in this area is affected by direct input of various contaminants in relation to land-use type. Slow dissolution of the aquifer matrix had little contribution in addition of solutes to groundwater. Results of this study showed that appropriate control of the specific pollutant source(s) in relation to land-use characteristics, necessarily based on long-term monitoring of spatial variation of major ions, will be most effective for protection of Wadi Marwani groundwaters from anthropogenic contamination. The water quality information presented in this paper will be useful for sustainable management of this important groundwater resource. The following recommendations should be taken into consideration for proper management and sustainable development of the groundwater resource in the study area:

• Subsurface drip irrigation method should be applied and generalized in this area to achieve water conservation approach and to prevent aquifer depletion caused by overdraft of groundwater. This irrigation technique will help in decreasing the chance of precipitation of Ca and Mg carbonate salts in the soil zone.

• Estimation of water requirements for each crop should be performed to help in water conservation practices.

• Complete quality assessment of irrigation water should be carried out especially for boron content.

• Complete and detailed quality assessment program including detection of the possible contaminants, which are possibly leached from the soil of the agricultural areas. Nitrate, phosphate and pesticides are of concern in such program. In addition, microbiological examination of groundwater samples should be carried out to monitor possible contamination from the wastewater disposal system.

• Disposal of domestic wastewater from the residential areas in unlined septic tanks should be prohibited and replaced by properly designed and lined septic tanks.