*Hydrogeology*

The hydraulic conductivities measured at Liwa Crescent and Madinat Zayed ranged from 10–100 m/d. They were marked as a peak in Abu Dhabi. This property can be attributed to the sands that are unconsolidated and homogeneous. In the north of Liwa, water is of low salinity. It was classified as old water as it arrived at the aquifer long ago. Most of the groundwater levels are shown at the agricultural zones of Liwa with high nitrate concentrations [34]. The farm soils of Liwa are Torripsamments with no specific profile [35]. The agricultural activity in this area depends on desalinated water, and hence the native soils are less saline. The Liwa region can be considered an essential food production zone in the United Arab Emirates. Its water reserves are abundant for agriculture. The aquifer beneath this region has become vulnerable to pollution, attributed to several factors. The annual precipitation is confined to the winter season accounting for 100 mm/year [36]. The groundwater recharge is just 4%, and this area is devoid of surface water resources. The Liwa region is being degraded due to excess salinity. Its aquifer lodges increased levels of chromium derived from natural resources.

The Liwa aquifer can be categorized as lens-shaped and with a thickness of 121 mm. The average transmissivity was observed as 300 m2/day [37]. The groundwater movement was observed in the north and south, especially in low-lying areas like sabkha physiographic regions, dunes, and sand salt flats [38]. Barchan dune complexes border the Liwa oasis at the south. There is a gradual incline passing to Oman's Hajjar massif mountain. The oil and gas-related activities injected brine into the groundwater zone. Ummer, Radhuma, Dammam, and Miocene are important aquifers underlying the Liwa region. Rus and Lower Fars were designated as confining units of this area [39]. Limestone dominates Ummer Radhuma and Dammam. Figure 1 shows the hydrogeological cross section of the Liwa region. *Water* **2022**, *14*, x FOR PEER REVIEW 4 of 20

> Miocene aquifer is dominated by sandstone. There is minimal silt and clay at the upper zones of the soil, resulting in high transmissivity and porosity. The Liwa area is hy-

> The groundwater samples were collected from the Liwa region of the UAE. Forty samples were collected from the study area, and their hydrogeochemical parameters were analyzed. The water samples were analyzed using the American Public Health Association (APHA) standards [45]. The Inductively Coupled Plasma—optical emission spectroscopy (ICP-OES, Avio 200, Perkin Elmer, Waltham, MA, USA) was used to quantify the heavy metals present in groundwater samples. The correlation coefficient and PCA were determined to know the correlation between the elements in the sample. After analyzing the database, the spatial distribution of quality parameters was obtained using ArcGIS 10.8 software to create spatial and layered maps. Semi-variograms were prepared and an appropriate model selected based on the nugget analysis to establish spatial dependence. Polyethylene bottles of 1 L capacity were used to collect the samples. In order to minimize the risk of contamination, the plastic bottles were rinsed with distilled water prior to being filled with sampled water. The samples were preserved with 65% nitric acid (HNO3) for a pH of 2, and bottles were kept cool at 4 °C. ICP-OES system was used to study the heavy and trace elements (As, Cr, Al, Mn, Ni, Cu, Pb, Zn, Cr, and Cu). Potassium (K), Calcium (Ca), Nitrate (NO3), and Sulfate (SO4) were analyzed using Ion Chromatography (ICS 5000+, Thermo Fisher, Waltham, MA, USA). Bicarbonate was determined by titration. Analyses were conducted in duplicate to minimize manual and instrumental er-

> resource. Under the Liwa lens, it was observed that 38 bcm (billion cubic meters) of groundwater with TDS greater than 15,000 mg/L is present [40]. Since this aquifer is limited with its connectivity with adjoining aquifer systems, most of the solutes observed might be of atmospheric origins. The recharge of this shallow aquifer is negligible. The flow will be confined to the upper aquifer during intense precipitation especially in unsaturated zones [41]. Unmitigated irrigation-related activities are dumping nitrates into the aquifers. The freshwater lens is being depleted in the Liwa area due to several agricultural activities per se. Liwa oasis provided an agricultural base for the semi-nomadic Bani Yas community. This led to settlements on a large scale in Abu Dhabi [42]. The groundwater studies using geostatistics of the study area were attempted previously and concluded that the anthropogenic and natural processes affected the quality of the ground-

**Figure 1.** Hydrogeological cross-section of the northern Liwa area. **Figure 1.** Hydrogeological cross-section of the northern Liwa area.

water [43,44].

rors.

**3. Materials and Method**  *3.1. Sampling and Analysis* 

Miocene aquifer is dominated by sandstone. There is minimal silt and clay at the upper zones of the soil, resulting in high transmissivity and porosity. The Liwa area is hydrologically unique in the western region, making it a highly productive groundwater resource. Under the Liwa lens, it was observed that 38 bcm (billion cubic meters) of groundwater with TDS greater than 15,000 mg/L is present [40]. Since this aquifer is limited with its connectivity with adjoining aquifer systems, most of the solutes observed might be of atmospheric origins. The recharge of this shallow aquifer is negligible. The flow will be confined to the upper aquifer during intense precipitation especially in unsaturated zones [41]. Unmitigated irrigation-related activities are dumping nitrates into the aquifers. The freshwater lens is being depleted in the Liwa area due to several agricultural activities per se. Liwa oasis provided an agricultural base for the semi-nomadic Bani Yas community. This led to settlements on a large scale in Abu Dhabi [42]. The groundwater studies using geostatistics of the study area were attempted previously and concluded that the anthropogenic and natural processes affected the quality of the groundwater [43,44].
