Hydrogeochemistry of the Umm er Radhuma Unconfined Aquifer, Western Desert Iraq-Saudi border

The Umm Er Radhuma unconfined aquifer Hydrogeochemistry in the Saudi – Iraq border desert was studied to identify the main hydrogeochemical processes and rock-water interaction. The measurements were done using standard APHA procedures. The results indicated that Na + and Cl - are the dominant ions in the groundwater. The average contribution of cations in the aquifer is Na + + K + ( 26 %), Ca 2+ (14 %), and Mg 2+ (10%) ; whereas anions contribution is Cl - (24 %) , SO 42-(20 %), and HCO 3-(6 %). The results reflect that the examined water is hard water of neutral to slightly alkaline, slightly brackish, with mineralized pH conditions. The main conclusion of chemical reactions indicated carbonates, evaporate dissolution, and clay minerals water interaction through ionic exchanging.


Materials and methods
Twenty-five groundwater samples were collected in October 2009 from Umm Raduma unconfined aquifer in the western desert near the Iraqi-Saudi Arabia borders (Figure 1(. The depth of groundwater wells ranges from 100-300 m, according to Al-Fatlawi, 2021 [16]. The measurements were done using the procedure of APHA, 1998 [17], including hydrogen number (pH), electrical conductivity (E.C.), and TDS using TDS-EC-pH and HANA, type HI 9811. All samples were analyzed for major cations (K + , Na + , Ca 2+ , and Mg 2+ ) and major anions (HCO 3   -, SO 4 2-, and Cl -). Sodium and K were analyzed using a flame photometer. Calcium, Mg, Cl, CO3, and HCO3 were determined using titration. Sulfate was determined using a spectrophotometer. The analytical accuracy was calculated according to Hem, 1985 [18] and Al-Hamadani, 2009 [19]. Accordingly, the accuracy of the results is accepted. The hydrochemical formula was computed as the average formula based on the Kurlolov formula, which was referred to in Ivanov et al., 1968 [20].

Results and discussion
Groundwater is the predominant source of water supply for domestic and other uses. Groundwater abstraction from boreholes for water supply has a long history. There is generally a high rate of abstraction without matching recharge. A general decline in rainfall in the area has led to the over-exploitation of groundwater, and the available groundwater resource is declining. Water levels in boreholes have fallen, and borehole yields have drastically declined.

Properties of Umm Raduma unconfined aquifer Groundwater
A slightly-brackish is the classification of groundwater according to Todd, 2007 [1] as revealed by the results, where the TDS varies from 1410 to 3300 ppm ( Table 1). The wide salinity range is due to the various lithology types. The range of E.C. varied from 1774 to 4970 µs/cm, revealing mineralized water excessively type [21], while it is slightly alkaline to neutral according to Matthess,1982 [22], and the pH ranges between 7.1 to 8.5. The Na + , with a mean of 55.5%, is the dominated cation, while Cl-is the most dominant anions and forms 47.3% on average of the total ions ( Figure 2).
The Na ranges from 6.2 to 16.5 epm, while chloride varies between 4.5 and 22.0 epm. Although groundwater in the Umm Raduma unconfined aquifer contains a wide ionic range, it tends to have a uniform water quality, which is with Na-Cl type of water as follows: The access quantity of sodium which is higher than the chloride molar value indicated that it doesn't come from halite and supports that the groundwater originated from a deep fault source to mix the Umm er Radhuma unconfined aquifer meteoric water.

The interaction of Umm Raduma Groundwater and the rocks of the aquifer
The data on groundwater quality give important clues to the geological history of the rocks and indications of groundwater recharges, discharges, movement, and storage.
The mathematical ratios between ions (Hydrochemical functions) are used to identify the groundwater properties and to conclude the interaction of Umm Raduma groundwater and the rocks of the aquifer [6 and 23].
The Na (epm) / Cl (epm) show (1.2) mean amount of rNa + /rClreflect the occurrence of other input of Na which is not as halite rocks but could be the partial leaching of terrestrial minerals ( Table 2). The average ratio of rCa 2+ /rMg 2+ (1.5) that is ten times greater than of seawater (actually, this ratio in seawater is 0.14) and five times less than of the rainwater (actually, this ratio in rainwater is 7.14) [24]. These results indicated the interaction of Umm er Radhuma Groundwater and the rocks of the aquifer by limestone and dolomite rocks leaching under the pH of rainwater (relatively acidic pH). In contrast, the leaching of the sulfate rocks in the Umm Raduma aquifer is shown by the high average value of rSO   Maya and Loucks,1995 [27] concluded that the ratio Ca/Mg equal to 1.0 indicates dissolution of dolomite, but a higher ratio reflects a more significant calcite contribution. In this research, the average ratio of Ca/Mg is 1.5. Moreover, the high Ca 2+ + Mg 2+ molar amounts (higher than 5.0 to lower than 21.0 epm) indicate Ca and Mg's contribution to groundwater due to the dissolution of calcite, dolomite and gypsum ( Figure 4).
The groundwater points are drawn with the Na + + K + and ½ total cations ( Figure 6) [24]. It refers to the weathering of silicate and halite rocks leaching.

The active chemical processes
The sources of heavy metals in soil are mainly natural, including geologic sources such as rock formations and soils [23]. The evaporation has affected the system of groundwater and salinity [6, 13 and 29]. The plot relation of Na/Cl versus EC. was applied to understand the active chemical processes in Umm Raduma unconfined aquifer. Mayback,1987 [27] stated that the molar ratio of Na/Cl is approximately close to 1.0, showing the dissolution of halite, while over 1.0 indicates the release of Na ion from weathering of silica. Accordingly, in this research, the Na/Cl ratio of 19 out of 25 samples is greater than 1.0, with an average of 1.2, which indicates halite or silica (clay minerals ) dissolution (Table 2 and Figure 7). 76% of the samples are higher than the 1:1 ratio showing the direct relation of Cl with Na that attributed exchange to the ions in clay minerals and silicate weathering (Figure 8) [18].
According to Gibbs [30] diagram, it is noticed that the weathering processes are the active chemical processes without dilution process of annual rainfall that recharges and feeds the Umm er Radhuma unconfined aquifer (Figure 9 A-B).

The groundwater origin and Ions sources
Because of the geological nature of the study area, it is characterized by distinguished hydrogeology through the presence of the Abu Jir, Schbicha, Athamin -Um Zulaim and Safawi-Samawa fault systems. The hydrogeology situation depends on the nature of the structural, geological, type of formations, and nature of the water-bearing rocks and cavities. The relationship between ions is used to determine the origin of the water. Sullin, 1946 [31] used the (rNa / rCl) to indicate the marine origin (rNa / rCl ≤ 1) or continental origin (rNa / rCl ≥ 1) of groundwater. The groundwater environment in geological formations leads to the variation in ion concentration ratios (dilution or concentration), so two classes could be distinguished for both origins depending on the appearance of hypothetical salts. It is possible to find the dilution or concentration ratios by comparing them with the concentration of seawater or continental water [32]. Accordingly, the results show that 20 well samples out of 25 are of continental origin and that the hypothetical salt is sodium sulfate ( Na 2 SO 4 ) since most of these groundwater samples are located in the recharge zone area ( Figure 10, Table 3). This finding follows the ratio of Na/(Na+Cl) that reflects marine origin groundwater suffering changes in its ions concentration after deposition as a result of mixing with rain water penetrating from the surface to the groundwater through the faults and fractures in the area in 72% wells, where the values of this ratio are more than 0.5.
To describe source ions and rocks (type of lithology) affected groundwater chemistry according to Hounslow, 1995[33]. The chemical functions are calculated (Table 3) and used. The results indicated that the Mg/ (Ca+Mg) is lower than 0.5, reflecting carbonate rocks' leaching as the main operation, but over 0.5 shows calcite precipitation and dolomite dissolution. As well as, the ratio of Ca/ (Ca+SO 4 ) is minor from 0.5, getting rid of Ca by ion exchange or calcite deposition. While the amounts of Mg/ (Ca+SO 4 ) are lower than 0.5 in the Umm er Radhuma groundwater aquifer, reflecting that the calcite and gypsum have more contribution than dolomite. The result of the (Ca+Mg)/SO 4 ratio indicates dedolomitization operation by about 56 % of the calculated ratios of this function. The ratio of (Ca+Mg)/SO 4 is within 0.8-1.2, reflecting dedolomitization. The TDS values reveal carbonate weathering that is higher than 500ppm, but if it is lower than 500 ppm, indicating weathering of silicate minerals. According to the ratio of Cl /∑anions, the rock weathering is the active operation in Umm Raduma unconfined aquifer, indicated by the lower ratios than 0.8. While the computed amounts of HCO 3 /∑anions ratio is lower than 0.8, reflecting the brine water of Abu Jir Fault deep aquifers.