4.2. Hydrochemical Characteristics and Health Risks
Developing new policies for groundwater management rely greatly on detailed knowledge of the quality conditions of the aquifers to assure the safe services they provide. Descriptive statistics (minimum, maximum, mean, standard deviation, and counts) of the aquifers (Table 1
) proved important insights for the aquifers and their pollutants. Total hardness values ranged from 92 to 4737 mg L−1
proved very hard with average of 607 mg L−1
due to elevated values of calcium and magnesium cations together with carbonate, bicarbonate, chloride, and sulfate anions [32
] as well as for the material and texture of pathway of groundwater, as it passes through calcareous layers and limestone of the Khuff Fm and collects carbonate as well as bicarbonate ions, which later accumulate in water resources. Water with a hardness of above 200 mg L−1
showed recent evidences for cardiovascular disorders in addition to the scale formation in the distribution network, system fouling, and increased boiling point [33
]. Levels higher than about1000 mg L−1
for TDS [35
] make the water unpalatable and unpleasant to users due to extreme scaling in heaters, water pipes, household appliances, and boilers.
Nitrate originates from many sources such as agricultural activities especially fertilizers, animal wastes, plant remains, industrial, and sewage disposal. In this study, its level ranged from 2.6 to 2781 mg L−1
which suggest the influence of agricultural activities and sewage disposal. Health risks of elevated NO3−
content exceeding the maximum World Health Organization (WHO)’s allowable concentration of 50 are methaemoglobinaemia, blue baby syndrome, hypertension, diabetes, thyroid disease, stomach cancer, abortion, and altered immune function [36
].Two zones showed the highest value of nitrate, wherein they are located in the city center, where it is densely populated and in densely cultivated areas. Fe2+
ranged from 0.03 to 8.53 mg L−1
with its largest average recorded in the Quaternary aquifer resulted from the widespread occurrence of ironstone and kaolinite-rich claystone lenses in the sediments.
Ammonia indicated very low concentrations reach of 0.17 to 5.66 mg L−1
, mostly owing to adsorption of clay particles and to the action of bacteria on oxidizing ammonia to nitrate and nitrite [39
]. Ammonia in drinking water is not of immediate health relevance. The pH value for all the water samples is within the natural waters range of 6.5–8.5 pH for drinking and reached a maximum of 8.79 in W. Dawasir and 8.8 in Lower Wajid. Phosphate reached the maximum limit in drinking water of 0.5 mg L−1
] only in the Lower Wajid samples formed mostly from fertilizer use and anthropogenic practices or from the minerals in parent rock [41
The level of bicarbonate ranged from 54 to 335 mg L−1 that lies within the WHO’s standard range of 500 mg L−1 with largest average of 177.5 marked the Quaternary. The level of soluble carbon dioxide, temperature, pH, cations, and some soluble salts control the carbonates concentration in natural waters.
Fluoride obeyed the guideline of 1.5 mg L−1
] except in the Quaternary samples, where it reached a maximum of 2.98 mg L−1
originated from rock dissolution of fluorine-rich minerals as well as from geothermal sources or from runoff, infiltration of fertilizers, industrial wastes, and manure treatment system [43
]. In arid areas with consumption of large quantities of water, lower concentrations should be appropriate (<1 mg L−1
]. Long-term exposure to fluoride in drinking water at concentrations above about 1.5 mg L−1
can result in dental fluorosis, while values above 4 mg L−1
can result in skeletal fluorosis and above about 10 mg L−1
, crippling fluorosis can result [46
]. More than 200 million people worldwide suffer the effects of chronic endemic fluorosis, mostly in the developing countries, that are thought to be drinking water with fluoride in excess of the WHO guideline value [44
The Quaternary aquifer was the most affected by TDS (4659 mg L−1), total hardness (1519 mg L−1), NO3− (483 mg L−1), SiO2 (28 mg L−1), Fe2+ (2.8 mg L−1), F (0.8 mg L−1), and HCO3− (177 mg L−1). Khuff-Kumdah showed largest means of dissolved oxygen—O2 (4.4 mg L−1) and NH4 (2.4 mg L−1), confirming its connection to the surface entered through direct absorption from the atmosphere, by rapid movement, or as a waste product of photosynthesis. Khuff-Kumdah clarified the lowest average temperature (32.5 °C) that supports easier oxygen dissolution in cooler than in warmer water. Upper Wajid clarified the largest average in NO2 (0.3 mg L−1). Lower Wajid proved largest in averages for pH (7.7), Eh (−75), and PO4 (0.5 mg L−1). Gradual upward increase was noticed for the total hardness, TDS, and Fe contents. Total hardness mean values clarified upward increase from 559 to 1519 mg L−1 with lowered values in KhuffFm (629 mg L−1). The salinity (average values of TDS) upward change from 1518 to 4660 mg L−1 that implies larger contribution from marine deposits of shale in Wajid sandstone, carbonates of Khuff, and the return flow of the saline irrigation water in the Quaternary aquifer. Fe2+ average values also increased from 0.4 to 2.8 mg L−1 mostly related to the presence of iron-rich intercalations.
L. Wajid showed inferior groundwater quality compared to U. Wajid attributed to compositional differences. Well logs indicated the dominance of clean sandstone with high effective porosity (up to 25%), low shale content (<10%), and high water production (>75%) in the upper section while the middle and lower zones attain high shale content and hence lower porosity degrading the aquifer properties [25
Correlation analysis indicated significant (>50%) inter-relationships among the total hardness, salinity (TDS), and the iron contents in a turbid water that is rich with NH4
strongly positively correlated with total hardness (0.90, 0.66), TDS (0.80, 0.85), Fe (0.98, 0.51), and turbidity (0.84, 0.50), respectively, and seem to be related to their elevated contents. The occurrence of these nitrogenous compounds in deep aquifers indicates that the aquifers are vertically interconnected and is connected to the surface mostly through fault systems. NH4 sourced from fertilizers and manures was also strongly related to the elevated contents of fluoride (r
= 0.66) and silicate (r
= 0.90) and decreases in fresh water rich with bicarbonate (r
= −0.61) recharged from irrigation.
4.3. Emergence and Spatial Distribution of Pollutants
To date, this assessment has consisted primarily of physical and chemical measurements, and comparing data against world guidelines to provide an assessment of condition, as part of the water quality monitoring, evaluation and reporting program. Based on the percentage of samples exceeding the guidelines [39
] (Table 3
), the study area showed emergence in total hardness (98.8%), TDS (65.7%), Fe2+
− (32%), pH (5.5%), turbidity (2.85%), F (1.2%), and NH4 (0.72%), in decreasing order. All the samples were hard exceeding the level of 200 mg L−1
except the Khuff Fm that clarified 91.4% violating the guideline. Depending on the interaction of other factors, such as pH and alkalinity, water with hardness above approximately 200 mg L−1
may cause scale deposition in the treatment works, distribution system and pipework and tanks within buildings.
Violation of the international standards clarified largest emergence of the pH (33.3%) for the Lower Wajid, Fe2+
(68.18%) and NO3−
(44.7%) for the Upper Wajid, total hardness (100%), TDS (85.7%), Fluoride (18.8%), turbidity (16.6%), and NH4
(2.3%) for the Quaternary aquifer. TDS showed upward emergence with percentages of 59.7, 63.44, 70.2, and 85.7, from the Lower Wajid to the Quaternary. Largest Fe2+
emergence was recorded for the Upper Wajid (68.18%), followed by the Quaternary (62.5%), Khuff (50%), and the Lower Wajid (44.4%). The low emergence of Lower Wajid in Fe2+
is mostly related to the extremely low organic carbon content that favors a reducing environment [47
As a consequence of the dominating agricultural activity (including excess application of inorganic nitrogenous fertilizers and manures), from wastewater disposal and from oxidation of nitrogenous waste products in human and animal excreta, including septic tanks, and to protect against methaemoglobinaemia in bottle-fed infants (short-term exposure), a level of 50is recommended by Reference [35
]. Nitrate (NO3−
) clarified largest emergence in the Upper Wajid (33.7%), followed by the Lower Wajid and Quaternary (33.3%), and the Khuff Fm (14.7%). The pH emerged at 33.3% and 3.41% for the Lower and Upper Wajid, respectively.
Turbid water is a consequence of inert clay or chalk particles or the precipitation of non-soluble reduced iron and other oxides when water is pumped from anaerobic waters and is more likely to include attached microorganisms in shallow aquifers that are a threat to health. To ensure effectiveness of disinfection, turbidity, expressed as nephelometric turbidity units (NTU), should be no more than 1 NTU, and preferably much lower [35
]. Turbidity disobeying the unity dominated the Quaternary (16.6%), 6.6% of the Lower Wajid, and 1.78% for the Upper Wajid samples.
Fluoride obeyed the guidelines and emerged only at 18.18% for the Quaternary aquifer mostly concentrated from phosphatic fertilizers and dissolution of evaporative salts deposited in the arid zone during evaporation where alkalinity is greater than hardness [48
]. The WHO guideline value for fluoride in drinking water is 1.5 mg L−1
. Above 1.5 mg L−1
mottling of teeth may occur to an objectionable degree. Concentrations between 3 and 6 may cause skeletal fluorosis. Continued consumption of water with fluoride levels in excess of 10 mg L−1
can result in crippling fluorosis.
Ammonia disobeyed the odor threshold of 1.5 mg L−1
concentration at alkaline pH [35
] for 2.3% of the Quaternary aquifer and 0.56% of the Upper Wajid samples. Nitrite (NO2−
) did not present any significant concentrations and obeyed the 3 mg L−1
limit indicating an oxidizing environment and interconnections among aquifers. No health-based guideline value is recommended for temperature, phosphates, silicates, and dissolved oxygen by Reference [35
]. However, high water temperature enhances the growth of microorganisms and may increase problems related to taste, odor, color, and corrosion. Silicates and phosphates are corrosion inhibitors and they can complex dissolved iron (in the iron (II) state) and prevent its precipitation as visibly obvious red “rust”. These compounds may act by masking the effects of corrosion rather than by preventing it. Orthophosphate and other phosphates are effective in suppressing dissolution of lead. Depletion of dissolved oxygen in water supplies can encourage the microbial reduction of nitrate to nitrite and sulfate to sulfide, enhances the ferrous iron content, and very high levels of dissolved oxygen may exacerbate corrosion of metal pipes.
Spatial distribution of pollutants is shown on Figure 5
where sampling points are present in the three-dimensional space. Plumes of elevated values are marked brown to red tints. TDS and total hardness plumes occur in the north western area of the four-water bearing formations. Nitrate (NO3−
) was largest in the north in the Lower and Upper Wajid, and in the eastern downstream area for the Khuff and the Quaternary aquifers. Plumes of the other pollutants varied in their occurrence places within the aquifers.
4.4. Mechanisms Controlling Hydrochemistry
In order to highlight the major mechanisms controlling groundwater chemistry and the dominated hydrogeochemical facies of the study area, the Gibbs diagram [28
], as exhibited between TDS vs. Na+
) and Cl−
), was used and clarified the dominance of the water–rock interaction and evaporation processes (Figure 6
), primarily controlled by the chemical composition of recharge waters, water–aquifer matrix interaction, and groundwater residence time [49
]. Vertical distribution of groundwater quality within the multi-layered aquifer showed pronounced zonation, in particular for the salinization. Water–rock interaction processes dominate at large depths (e.g., Lower and Upper Wajid) and the evaporation dominates at shallower depths (e.g., Quaternary) underpinned by the upward increase of the recharge inputs from waters influenced by intensive evaporation. Khuff-Kumdah aquifer clarified the combined dominance of the water–rock interaction and the evaporation processes.
The excess of Na+
and hence salinization result from the exchange of Ca2+
abundant in fresh groundwater with the Na+
on the surface of clay minerals, which results in an increase in Na+
concentration on the expense of the decrease of the Ca2+
]. In addition, silicate weathering of Albite to Kaolinite in the aquifers could also contribute Na+
to groundwater [52
]. Dissolutions of minerals such as gypsum, anhydrite, aragonite, calcite, and dolomite are the potential ion sources to groundwater in the mineralization process [53
In dry climatic conditions, during the long dry seasons typical for hyper-arid desert environments, areas with sparse vegetation in bare soil, shallow water table, and coarse unsaturated zone material are prone to substantial groundwater evaporation [54
].The potential evaporation become extremely high and the recharge become practically nil after short-time precipitation events. Evaporation decreases exponentially with groundwater depth, approaching a constant value of about 0.02 mm per year for water table depths below 500 m [55
4.5. Factor Analysis (FA)
To effectively utilize water resources in these arid environments, it is important to understand the spatial variability of the hydrochemical properties and the factors influencing their variations where field-derived insights are critically needed. The spatial distribution of the variables in the spaces defined by F1 and F2 is shown in Figure 7
FA distinguished six main factors that explain for 74.34% of the total groundwater quality variation (Table 4
). The shared variance (communalities) of the factors, which defined relevant to the structure was large (>0.67) for most of the variables and below this value for F, Fe2+
, T, DO, and NO2−
. F1 accounting for 44.23% of the total variance had strong and positive loads of salinization indicated by the TDS (0.98), total hardness (0.95), nitrate NO3−
(0.84), turbidity (0.78), ammonia (0.67), moderately loaded by F (0.47), and Fe2+
(0.31). F2 (10.06%) was strongly positively affected by Eh (0.70) and negatively loaded by pH (0.63), HCO3−
(0.55), and temperature T °C (0.34). Loads of PO42−
(0.65) was strong on F3 (7%). F4 accounted for 6.24% and was weakly loaded by the dissolved oxygen (0.13). F5 accounted for 4.63% and was led by strong positive loads of SiO2
(0.54). Negative weak loads of NO2−
(0.10) leaded F6 (2.24%). The first factor discloses the major contribution from salinization and excessive application of nitrogenous fertilizer that might be one important source of NO3−
-N released by decomposition of organic matter, in addition to water–rock interaction marked by the dissemination of Fe and F. The second factor is governed by the redox dynamics and the recharge from surface mostly rainwater rich of bicarbonate. F3 reveals the extensive application of K and phosphate-rich fertilizers. F4 indicated the effect of oxygen enrichment mostly through progression of oxygen saturation during abstraction and recovery phases [55
] supported by the presence of intense network of faults and joints of the sandstone. Infiltrating oxic surface water can cause an enormous widening of the oxic zone towards the abstracting well. Silicate dissolution leads F5 while F6 clarified application of nitrogenous fertilizer as a source of the NO2−
to the groundwater pollution.