Hydrochemistry of Umm Er Radhuma Groundwater

Umm Er Radhuma Aquifer (UER) is the most important groundwater aquifer in Saudi Arabia, extending over 1.6 M·km 2 in the eastern part of the Arabian Peninsula, forming the main aquifer throughout the Rub’ Al-Khali (RAK). Groundwater salinity increased from west to east, reaching more than 27,000 mg/L near the border with the United Arab Emirates, where a Na + -Ca 2+ Cl − 24 SO − water type dominated. Gibbs diagrams indicated that the dissolu-tion/precipitation of carbonates and evaporation/precipitation of minerals, especially anhydrite, gypsum, and halite account for the solutes and salinity in groundwater. Most of the samples plot above the 1:1 line of (Ca 2+ + Mg 2+ ) against 3 HCO − , indicating other sources of Ca 2+ and Mg 2+ in the groundwater along with dolomite and calcium carbonate minerals. Phreeqc model indicated that the main clay minerals are kaolinite and gibbsite which had major effect on the cation exchange process as indicated by the Chloro-Alkaline index (CAI), where most of groundwater samples had values greater than zero which indicated the occurrence of reverse ion exchange between the groundwater and its host aquifer. The water type Na + -Ca 2+ Cl − 24 SO − dominated in the eastern part of the aquifer as the anaerobic conditions prevailed and the reduction of sulphate took place.


Introduction
Groundwater chemistry is a function of many chemical and physical factors such as the mineral composition, lithology of the aquifer, oxidation reduction reac-Journal of Geoscience and Environment Protection tions, natural or artificial recharge and discharge, weathering, exchange reactions (Elango & Kannan, 2007). Understanding the hydrochemistry of groundwater is of great importance, especially in such arid areas like Saudi Arabia where groundwater provides the major water supply.
The Kingdom of Saudi Arabia is commonly regarded as a dry country due to low precipitation and high evaporation rate. The mean annual rainfall across the Arabian Peninsula is less than 200 mm, and the temperature reaches more than 50˚C in summers. Though no perennial surface water in the kingdom, groundwater is the most important natural water resource which provides the kingdom with water supply for domestic and agricultural purposes. Among the most important groundwater reservoirs in Saudi Arabia is Umm Er Radhuma aquifer (UER) as many of Arab Gulf states depend on it for different purposes. It is extending over 1.6 M·km 2 in the eastern part of the Arabian Peninsula, forming the main aquifer throughout the Rub' al Khali (RAK) Desert where it is located in the southern areas of Saudi Arabia (Figure 1). RAK is considered the world's largest continuous sand desert, covering an area of 522,340 km 2 .

Geology and Hydrogeology
The Late Paleocene to Early Eocene sediments of Umm Er Radhuma formation the Arabian Peninsula as far as Jordan, Iraq, Oman and Yemen (Ziegler, 2001).
Calcareous shales deposited at the base of the formation followed by thick succession of carbonate rocks. The sedimentation took place on a wide carbonate shelf, which was distorted partly by tectonic movements during the movement of the Arabian plate into a series of deeps and shallows ( Figure 2 , 2017). The transmissivity ranges between 7.2 × 10 −5 m 2 /s from 5.2 × 10 −1 m 2 /s, and the conductivity varies between 2.9 × 10 −7 m/s and 8.8 × 10 −3 m/s, with an average of 1.6 × 10 −5 m/s. The wide range of the values is caused by the varying degree of karstification. The storage coefficient varies between 3 × 10 −4 and 3 × 10 −3 while the specific yield ranges from Sy = 1% to 7% (MEWA, 2017).

Groundwater Sampling and Laboratory Analyses
The data in this paper was carried out by the Ministry of Environment Water and Agriculture in association with the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) as an internal report project, titled "Detailed Groundwater Resources Studies in The Rub' Al Khali Desert", in addition to a historic data compiled from BRGM (1976). 44 groundwater samples have been collected by the GIZ during field trips from November 2010 to December 2011. Three samples were taken at each well for TDS, major cations, and major anions. Forty four Groundwater samples have been collected in 100 ml polyethylene screw-cap bottles which were rinsed three times with the water to be sampled. All samples were filtered using 0.45 μm filter and preserved by cooling. Additionally, nitric acid (HNO 3 70%) has been added to samples dedicated for cation analysis for further preservation. Field measurements included pH, EC, T, dissolved oxygen, bicarbonate and hydrogen sulfide using a flow-through-cell and multi-parameter pocket meters (Multi 340i, WTW, Germany). Moreover, groundwater samples were collected for trace elements in 250 ml polyethylene bottles. Samples were then shipped in cooling boxes to the hydrochemical laboratory in Darmstadt Technical University, Germany. The following standard analytical techniques were applied to investigate the hydrochemical composition of groundwater Phreeqc is a computer program written in the C programming language which has been designed to conduct a broad variety of aqueous geochemical calculations. Phreeqc has capabilities for 1) calculation of speciation and saturation-index, 2) reaction-path and advective-transport calculations involving specified irreversible reactions, mixing of solutions, mineral and gas equilibria, surface-complexation reactions, and ion-exchange reactions, and 3) inverse modeling, which identifies the mole transfers that account for composition changes between waters a long flow path (Parkhurst, 1995).

Results and Discussion
According to the statistical summary of the hydrochemical data of the groundwater samples of Umm Er Radhuma aquifer (Table 1 Figure 4).

Water-Rock Interaction and Sources of Solutes
Gibbs diagrams are considered an effective tool to identify the main geochemical processes which control the hydrogeochemical characteristics of groundwater in aquifers such as atmospheric precipitation, rock weathering, evaporation, and minerals precipitation. The diagrams describe the weight ratios of Na + /(Na + + Ca 2+ ) and Cl − /(Cl − + 3 HCO − ) against TDS as shown in Figure 5, where most of the samples plot between the rock weathering and the evaporation zones. Therefore, the dissolution/precipitation of carbonates and evaporation/precipitation of minerals, especially anhydrite, gypsum, and halite are the major geochemical reactions which account for the solutes in groundwater.
The dissolution of dolomite is well illustrated in Figure 6, where most of the cores of the dolomite rhombs undergo dissolution, which release equally charged amounts of (Ca 2+ + Mg 2+ ) and 3 HCO − , respectively, according to Equations (2) and (         against (Na + − Cl − ) has been applied. Based on Figure 14, most of groundwater samples plot close to 1:1 line, indicating that cation exchange is affecting the hydrochemical composition of the groundwater. Cation exchange can also be quantified by the Chloro-Alkaline indices (CAI) suggested by Schoeller (1977), to indicate ion exchange between the groundwater and its host environment during residence or travel. The value of these indices can be positive or negative.
If the value is positive then it explains that the exchange of Na + and K + ions are from water with Mg 2+ and Ca 2+ ions of the rocks. This indicates a direct base M. Gomaah Journal of Geoscience and Environment Protection  The most plausible source of Na + and Cl − is halite dissolution which releases equal molar amounts of Na + and Cl − into groundwater (Equation (6)

Conclusion
Groundwater salinity increased from west to east, reaching a TDS of more than 27,000 mg/L near the border with the United Arab Emirates with a dominating Na + -Ca 2+ Cl − - SO − water type dominated as the anaerobic conditions prevailed and the reduction of sulphate took place.