Abstract
Groundwater samples (54) collected from different geological units (alluvium, Tertiary, ophiolite, and Hawasina) located in the transboundary groundwater basin in north Oman at the United Arab Emirates (UAE) borders were analyzed for general hydrochemistry and water isotopes, and subsets thereof were analyzed for 14C and 3H and 87Sr/86Sr. The chemical composition, percentage of modern carbon (pmc), δ2H, δ18O, and 87Sr/86Sr of the groundwater in the study area progressively change from the recharge zone in the elevated area of the North Oman Mountains (NOM) to the flat plains at the UAE borders. While the water-rock interaction is the dominant process controlling the groundwater chemistry, evaporation and groundwater mixing affect the hydrochemistry at the UAE borders. Therefore, groundwater evolves from carbonate-dominant in the NOM into sodium chloride-dominant close to the UAE borders. It is also evident that groundwater lateral recharge from the ophiolites into the alluvium retains the chemical affinity of the ophiolites. Groundwater dating (high pmc), homogeneous 87Sr/86Sr ratios, and enriched δ2H and δ18O demonstrate the presence of modern recharge in the shallow zones of the ophiolites and alluvium. However, deep zones and areas at the UAE border contain older groundwater form during cooler and wetter climatic conditions as supported by the depleted δ2H and δ18O and lower 87Sr/86Sr ratios and pmc. Furthermore, the data clearly showed that modern groundwater mixes with older groundwater along the flow path from the NOM into the UAE border. Modern recharge occurs as lateral recharge from NOM and direct recharge in the plain area. The current findings support future development of aflaj system along NOM slopes and shallow wells in the plain areas.
1. Introduction
Transboundary groundwater basins, which cross countries’ borders, are normally of regional scale and thus represent important shared water resources. Proper management of these resources is fundamental for sustainability and development [1–3]. Groundwater in the border regions separating the Sultanate of Oman and the United Arab Emirates (UAE), near the Al Buraimi and Al Ain areas, represents an important resource for sustainable agricultural and urban development [4]. While North Oman Mountains (NOMs), located in the eastern part of this area, have been delineated as the principal recharge source [5], there is no prior hydrochemical and isotopic analyses in this region validating such argument. The hydrochemical study is an important component to understand the availability and nature of groundwater through identifying moisture sources and different geochemical processes that control the quality of water [6]. Hydrochemistry investigation may identify natural processes such as dissociations of salts in water [7, 8], redox processes [9], water-rock interaction, and mixing processes that strongly influence groundwater chemistry. Besides, environmental and some radioactive isotopes have become an important tool in identifying different patterns of precipitation, recognizing mixing processes, tracing the flow path, and dating groundwater. The current paper utilizes hydrochemistry and isotopes to understand recharge process and identify the processes controlling the groundwater chemistry in the transboundary aquifer of Al Buraimi-Al Ain areas.
2. Study Area and Hydrogeological Characteristics
The study area lies between 24°22 N to 24°38 N latitude and 55°44 E to 56°14 E longitude and covers around 1600 km2 in the Al Buraimi northwest of Oman. It is bounded by border with United Arab Emirates (UAE) on the west and by the ridge of North Oman Mountain (NOM) on the east (Figure 1). The study area with an annual average rainfall of 82 mm (from 30 mm to 178 mm) is characterized by very low humidity and precipitation. Most of the rainfall is between December and April. In summer months, the daytime temperature may exceed 45°C. The long-term annual potential evapotranspiration is about 2700 mm [10–12].
The geologic units of the study area are divided into three principal zones that include the Semail nappes (ophiolite), the Hawasina nappes, and the post-nappe strata units (Aruma Group and Tertiary bedrock and Quaternary alluvium) (Figure 1). The term “ophiolite” refers collectively to igneous rock that crops out in the study area with various dark, colored, crystalline, and microcrystalline characteristics. The Hawasina nappe exposures mainly occur as broken hills in the eastern piedmont zone. The post-nappe strata consist of the Cretaceous Aruma Group and Tertiary bedrock that were deposited in a foredeep basin downfolded along the frontal margin of the nappes. Folding associated with mountain building in the Late Tertiary turned over the nappes and Tertiary strata into their present structural configurations. Afterwards, erosive processes associated with flowing water led to the deposition of alluvium throughout the piedmont and alluvial fan zones to the west of the mountains. In terms of regional hydrogeological significance, surface waters running off the mountainous portion of the study area recharge the piedmont and alluvial fan zones. Groundwater recharged into the fractures of the ophiolite during rainfall and runoff events gradually drain laterally and vertically downhill through the fractures towards the plains of the lower basin. Groundwater flow from the ophiolite is generally to the west, and it is intercepted by aflaj and a few wells at the mountain front and continues to the alluvial aquifers underlying the upper basin. Compared to other geological units, the hydrological properties of the Hawasina nappes and Tertiary limestones are poor in terms of groundwater storage and transmission. However, they may provide significant groundwater supply through fractures. The alluvium in the piedmont and mountain front fan zones is the most important aquifer in the study area and is composed of an unconsolidated mixture of chert, limestone, and dolomite. The alluvium generally has good hydraulic conductivity (8 to 43 m/d) and water yields (0.1 to 31 l/s), which vary only with the coarseness of the alluvium and overall saturated thickness [10, 11, 13–17].
3. Material and Methods
Several field trips were carried out to the study area between the years 2014 and 2015 that span different seasons. A total of 54 locations (Figure 1) were sampled during these field trips including hand-dug wells, boreholes, monitoring wells, precipitation, and aflaj (refers to an open channel that drains water from the slope of the mountain and transports water via gravity into demand areas). All samples’ locations were recorded using a global positioning system (GPS). Different parameters including temperature, electrical conductivity (EC), pH, and total dissolved solid (TDS) were measured immediately in the field using Aquaread Aquameter with multiple electrodes. Sampling procedure varies according to the location. For instance, the boreholes’ samples were collected directly from the pumping wells after a few hours of continuous pumping. The hand-dug wells were bailed using a Solinst stainless steel bailer after being purged with the aid of a purging pump. The monitoring wells were similarly sampled after purging while samples from aflaj and precipitation were collected directly. The hand-dug wells tap the shallow water system, and they comprise private wells mostly for irrigation purposes. The boreholes however tap deeper horizons and screened different intervals which may constitute inevitable groundwater mixing. Three samples (for cations, anions, and isotopes) at each location were collected in 500 ml plastic bottles for analyses. The bottles collected for cation analysis were acidified using 2-3 drops of diluted (5%) nitric acid. The bottles were transported in iced container and preserved in a refrigerator at a temperature of 5°C. All the samples were immediately analyzed for the major ions and alkalinity in the Central Analytical and Applied Research Unit in Sultan Qaboos University (CAARU-SQU) using ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). The stable isotopes deuterium (2H) and oxygen-18 (18O) were analyzed in the Hydrogeology Laboratory Unit in Sultan Qaboos University (SQU) using Triple Isotopic Water Analyzer (TIWA-DLT-EP) from Los Gatos Research (LGR). Selected samples were analyzed in the Environmental Isotopes Laboratory of the University of Waterloo for carbon isotopes (13C and 14C), tritium (3H) (11 samples), and strontium isotopes (87Sr and 88Sr) (17 samples).
4. Results and Discussion
4.1. Chemical Composition and Groundwater Classification
Chemical analyses of the collected samples from different aquifers are tabulated in Table 1. The hydrochemical characterization of the groundwater in the study area, aided by the Piper diagram (Figure 2), reveals three main groups: Mg-HCO3, Mg-Cl-SO4, and Na-Cl dominant groups. Except of one sample that may suggest a deeper source, all of the aflaj samples belong to the Mg-HCO3, circulate within the ophiolite rocks, and are located along the slope of the NOM. The dominance of the carbonates indicates exchange with the atmospheric CO2 and circulation of modern water at shallower depths. The Mg on the other hand is the weathering product of the ophiolites at shallower depth where the pH (around or less than 9) favors the dissolution of brucite which is a magnesium hydroxide. In addition, about half of the alluvium groundwater also falls within the Mg-HCO3. This group of alluvium groundwater represents recent recharge from direct infiltration through streambeds. The dominance of Mg in these samples is attributed to the presence of ophiolitic fragments in the alluvium.
Contrary to the Mg-HCO3 group, the Mg-Cl-SO4 and Na-Cl groups may represent older groundwater (carbon-14 dating is discussed later). The deep boreholes penetrating the ophiolites (sample numbers 52 and 53) and the Tertiary (sample number 50) have shown the Mg-Cl-SO4 and Na-Cl dominance, respectively. The hydrochemical dominance in ophiolite groundwater changes with depth from Mg-HCO3 to Mg-Cl-SO4 (Figure 2). While Cl is the dominant anion in both ophiolites and Tertiary deep groundwater, Na is the dominant cation in the former and Mg is the dominant cation in the latter. This cationic variation is attributed to the mineralogical composition of the hosting rock. Nonetheless, a couple of odd samples has shown a reversed dominance likely attributed to cationic exchange. Such cation variation supports the role of water-rock interaction and suggests that time circulation of the Mg-Cl-SO4 and Na-Cl groundwater is larger compared to the Mg-HCO3 groundwater type. The other half of the alluvium groundwater samples, mostly from relative deeper depths, belong to the Mg-Cl-SO4 and Na-Cl groups, and similar to the deep borehole samples, their chemistry indicates no modern recharge, dissolution of evaporites and silicate minerals. Therefore, groundwater recharge processes seem to influence the hydrochemistry in the study area. Recharge takes place by two mechanisms and from two zones: higher elevated mountains and wadi systems such as Wadi Safwan. Recharge in the elevated high mountains is a direct recharge from rainfall into fractures, whereas recharge in the plain area of wadi systems is a direct percolation through streambed into the aquifer. Along its flow path from the NOM into the plains before the UAE borders, the groundwater hydrochemistry changes in the study area from carbonate-dominant into chloride/sulphate-dominant. The dissolution of halite, gypsum, and anhydrite present in the Hawasina and the Tertiary rocks may explain the dominance of the chloride and sulphate. Moreover, additional atmospheric source of Cl in such an arid area is possible where Cl from aerosols first accumulates into the soil profile and then leaches to the groundwater [18].
In general, the groundwater chemistry evolves with increasing depth and proximity to the NOM as indicated in the Piper diagram (Figure 2) signifying the role of water-rock interaction. Such role has been recognized in several arid areas across the world, for instance, in Chile [19], UAE [20] adjacent to the study area, and in Tunisia [21]. Hydrochemical investigations of groundwater in Oman (e.g., [22–28]) have made a consensus that water-rock interaction is the primary factor controlling the groundwater chemistry with a secondary contribution from evaporation. The NOM marks the region of fresh water and groundwater salinity increases away from NOM with increasing mineral dissolution. However, due to geological diversity of NOM, the anion/cation suit of groundwater varies.
Although nitrate is reported in most of the samples, it exceeds the limits for the Omani Standards (1998) in two samples number 49 and number 54 which were collected from farm numbers 1 and 5 (Tables 1 and 2). Thus, nitrate elevated concentration is due to agricultural activities and animal manure. The magnesium vs. calcium plot (Figure 3) shows that the Mg concentration exceeds the Ca concentration except for two samples collected from the alluvium aquifer. The highest ratio is observed in groundwater samples collected from the ophiolite aquifer and those collected from wells penetrating both alluvium and ophiolite (Mg/Ca ratio higher than 3). While the lowest ratio (less than 1) is observed in the groundwater samples collected from only the alluvium aquifer, the magnesium is likely to be driven from ophiolite weathering as well as from the dolomite in the Tertiary sediments. The limestone dissolution in the Tertiary sediments seems less effective; as in general, groundwater affected by limestone dissolution will reflect an average Mg/Ca ratio of 1 : 2 to 1 : 1.5 [29]. The excess of Ca + Mg over HCO3 (Figure 4) indicates an excess dissolution of silicates over the carbonates, namely, the dolomite. If the main source was carbonate mineral dissolution, the Ca + Mg : HCO3 should have shown a near to 1 value. The excess of the Ca + Mg indicates a source lacking HCO3, and the available lithology will suggest the silicates found in the ophiolites.
The plot of different samples in the Gibbs diagram (Figure 5), a plot of TDS versus weight ratio of Na/(Na + Ca), suggests that the chemical composition of most of the groundwater samples collected from the different aquifers is controlled by weathering rather than evaporation except for a few samples that show evaporation dominance. Evaporation can be atmospheric which takes place during precipitation or may occur during runoff as direct evaporation of surface water. The atmospheric evaporation effect is reflected in the isotopic signature of groundwater (will be discussed in the following section) and is seen in the highlands. The surface water evaporation is observed in the plain areas, particularly in the Safwan area. The surface water evaporation in the highlands is unlikely due to the steeper slope and thus the residence time of water on the surface is short which allows no direct surface water evaporation. Thus, only the effect of atmospheric evaporation could be seen. However, in the plain areas where the runoff along the wadis get ample time on the surface, direct evaporation is significant, and therefore, the TDS of the groundwater progressively increases. Further discussion of the evaporation effects is elaborated in the next section.
4.2. Isotopic Characteristics
4.2.1. Stable Isotopes: δ2H and δ18O
The stable isotopes δ2H and δ18O data analyzed during this study (Table 1) are presented in Figure 6 with reference to the global meteoric water line (GMWL) after Craig [30]. The δ18O ranges from −2.917 (EW-9, sample number 39) to +0.918 (Khatwa falaj, sample no. 18) while that of δ2H ranges from −4.327 (farm 2 well 1, sample number 50) to +7.739 (P-18, sample number 15). The regression line for all groundwater and aflaj data yields a slope of 5.3 indicating an evaporation effect. The most isotopic enriched group of groundwater samples is the aflaj discharging from the ophiolite aquifer (shown as orange diamonds in the Figure 6). This group of groundwater samples is located at the slopes of the highlands, and they receive direct recharge from precipitation and thus reflects the effect of atmospheric evaporation. Secondary isotopic enrichment due to evaporation from aflaj channels is minimum as samples were collected directly from the mother well (first water discharge point). The short time circulation of groundwater within this shallow system does not allow for mixing with deeper older water that is likely depleted in its isotopic signature. Comparing the isotopic composition of the fossil stalagmites deposited during the early to middle Holocene with the modern stalagmites in Oman has revealed that the fossil stalagmites are much more isotopically depleted [31]. Reconstructed groundwater values for these time intervals indicate that groundwater was predominately recharged by monsoon precipitation from the Indian Ocean remarkably characterized by depleted isotopic signature. Hence, the most isotopically depleted samples in the study area, which are the Tertiary and a few alluvium samples close to the UAE border, are likely to form during the early to middle Holocene time when the climatic conditions were different from todays. The groundwater sample collected from farm 2 well 1 (sample number 50) penetrating the Tertiary rocks shows the lowest percentage of modern carbon (pmc) indicating older ages (pmc is discussed in the next section). In addition, two groundwater samples from the alluvium located closer to the UAE border (E4, sample number 34, and MF1, sample number 43) show low pmc. The general trend of the isotopic composition when different aquifers are considered is that it gets more depleted in the direction of the groundwater flow from the elevated mountains of the ophiolites in the east to the plain areas of the alluvium in the west crossing the piedmonts of the Hawasina and Tertiary. This phenomenon can be attributed to mixing between the current isotopically heavy current precipitation and depleted older groundwater already present in the system. Therefore, the groundwater becomes progressively depleted as one goes away from the recharge zone at the NOM (Figure 7). It is also suggested that climatic conditions in the early to middle Holocene were cooler and wetter than todays and the evaporation was less.
4.2.2. Strontium Isotopes
Strontium in groundwater is mainly released from hosting rocks because of the water-rock interaction. Aquifers with high amount of potassium-bearing minerals contain more Rb and therefore more 87Sr isotopes than carbonate aquifers. However, the concentration of strontium may change due to evaporation or mixing of different water’s sources. Furthermore, the radioactivity of the strontium increases with increasing groundwater residence time in the aquifer. However, the ratio of 87Sr/86Sr does not change with latitude or altitude. The correlation between the ratio of 87Sr/86Sr and 1/Sr is a useful tool to identify the source rocks. Two mixing groundwater sources will show a strong positive correlation, whereas a poor correlation indicates that there are more than two groundwater sources [27].
The Sr concentration in the groundwater samples (17 samples) collected during this study varies from 0.02 to 2.17 mg/l, whereas the 87Sr/86Sr ratio remains unchanged except for 2 samples (E4, sample number 34, and farm 2 well 1, sample number 50). For the 15 samples, the 87Sr/86Sr ratio is either 0.7085, 0.7086, or 0.7087 while a slightly lower ratio is reported for E4 (0.7082) and farm 2 well 1 (0.7078). These two samples are those which have shown extra depleted stable isotopes and low pmc, thus suggesting older groundwater form under different climatic conditions. The homogeneity of the 87Sr/86Sr ratio, except for the two samples E4 (sample number 34) and farm 2 well 1 (sample number 50), suggests a unified Sr isotopes source, likely the ophiolites. Such source can either be the main ophiolite rock or the fragments present in the alluvium. The 87Sr/86Sr homogeneity of the groundwater in all aquifers also suggests that these aquifers are hydraulically connected regardless of their lithology and hydraulic property variations.
Contrary to the homogenous 87Sr/86Sr isotopic ratio, the Sr concentration shows considerable variation. The highest Sr concentration is observed in the sample from well EW-9 (sample number 39), whereas the lowest concentration characterizes the groundwater from Mehasen farm (Table 2). The concentration of Sr in the groundwater samples collected from the ophiolite aquifer in the study area ranges between 0.02 and 0.65 mg/l with an 87Sr/86Sr ratio about 0.7085–0.7087 (Figure 8). For samples collected from the Tertiary aquifer, the Sr concentration varies from 0.06 to 1.54 mg/l. In groundwater samples from ophiolite aquifer, the 87Sr/86Sr ratio is higher than the values reported by Weyhenmeyer et al. [32]. This is attributed to the absence of the carbonate rocks of the Hajar Super Group (HSG) in the study area, which according to Semhi et al. [27] and Weyhenmeyer et al. [32] marks the main source of recharge. In the study area, only the ophiolites are present, which when interact with the groundwater will release more Sr as opposed to the HSG. There is a little variation of the 87Sr/86Sr ratio, for all aquifers as opposed to Sr concentration, which exhibit a large variation. Large variation of Sr concentration may indicate the effect of groundwater residence time. This is well demonstrated by reasonable correlation between the Sr concentration and δ18O ‰ (Figure 9) of groundwater. Similar to δ18O ‰ that gets depleted with the flow direction, the Sr concentration increases which signifies the increasing groundwater residence time.
4.2.3. Carbon 14 and Tritium
In parallel to tracing, isotopes can also be used to estimate the age of groundwater. During infiltration of groundwater through the soil, carbon 14 is introduced to the groundwater from CO2 produced in the soil and from the atmosphere. The dissolved inorganic carbon decays with time. The term percentage of modern carbon “pmc” is used to express the content of modern carbon in the sample. The higher pmc, the younger will be the groundwater sample. The highest age, which can be determined using this method, cannot exceed 40000 years. If the water is older than 40000-year, the result will be almost 0% pmc [33]. Data of carbon 14 in the samples of the present study show that most of the groundwater in the ophiolite have high pmc (Table 3). The water collected from the ophiolite aquifer have a depth less than 32 m and the area itself is elevated and sloped. One sample shows 1% modern carbon, which is “farm 2 well 1” sample located close to the UAE borders. The well “farm 2 well 1” penetrates the Tertiary, and its actual depth is reported as exceeding 100 m (the real depth is unknown). Relatively lower pmc values are also reported in 2 groundwater samples collected from the alluvium: E-4 (sample number 34) and MF1 (sample number 43) (42.51 and 54.55, respectively). Both samples are also located close to the UAE boarder, i.e., the lower reaches of the groundwater flow path. Also, sample number 54 (farm 5) shows a 64 pmc which may also suggest a relatively older groundwater.
Tritium was also analyzed during this study to complement carbon-14 dating. The results of the tritium analysis during this study do not tell much. The tritium results (direct method that has a precision of 6 T.U.) indicate for all the analyzed samples a concentration less than 6 T.U. (Table 3). In the view of the banned nuclear activities, tritium is no longer introduced to the groundwater systems as during the 1950s, and therefore, its efficacy as an indirect measurement of groundwater age is questioned [34].
5. Discussion Summary
In the view of the findings described in the above sections, a hydrogeological/hydrogeochemical cross section is shown in Figure 10 about the hydrochemistry of the area. The cross section illustrates the evolution of the groundwater chemistry from the recharge zone in NOM dominated by ophiolites to the discharge zone in the plain area at the UAE borders dominated by alluvium cropping at the surface. Induced recharge in the shallow zones of the ophiolite and alluvium increases Mg and HCO3 concentration, enriches δ2H and δ18O, provides the source for the strontium isotopes, and introduces fresh recent groundwater (high pmc) to the system. Flowing away from the NOM, the groundwater hydrochemistry gradually changes to dissolve more Cl, Na, and SO4 ions, gets depleted in δ2H and δ18O, maintains the same strontium isotopes but increases Sr concentration, and decreases pmc which means becomes older. Similar lateral changes in groundwater are also observed vertically as deeper groundwater contains more Cl, Na, and SO4 ions, is depleted in δ2H and δ18O, and has low pmc. The current qualitative recharge assessment in the study area constrains the estimated groundwater recharge in the same region conducted by Izady et al. [4]. The present study added the hydrochemical aspect and identified processes controlling water chemistry. The findings of the three studies encourage further developments in the region along the slope of the NOM and in the shallow zone of the plain area. However, this development should not exceed the estimated sustainable recharge values. Izady et al. [4] suggested 18.09 Mm3/year as long-term regional groundwater recharge and 5.67 Mm3/year as long-term lateral groundwater flux from the ophiolites in NOM into the alluvial zone. Exceeding these values will cause hydraulic imbalance that will encourage upconing of deeper saline groundwater. Conducting a proper solute transport modeling is recommended to identify possible zones of upward groundwater flow to prevent salinization.
6. Conclusion
The hydrochemical characterization and isotopic investigation of groundwater in the study area have revealed different water types (Mg-HCO3, Mg-Cl-SO4, and Na-Cl). The variation in the chemical composition of the groundwater is attributed to recharge processes, and the mineralogical composition of the hosting rocks as the mineral dissolution was found to be the main factor affecting the chemistry of the groundwater. Lateral groundwater recharge from the ophiolite induces an increase of CO2 from atmospheric circulation along with Mg concentration from the dissolution of silicates. Increasing Cl, SO4, and Na in the downstream is due to lack of recharge and the dissolution of evaporites. Radiogenic isotopes of carbon-14 indicate modern groundwater in the shallow zones of the ophiolites and the alluvium and old water in the deeper horizons. Such modern recharge and its mechanism are indicated from the stable isotopes of deuterium and O-18 that shows evaporation effects on the shallow groundwater and a completely different signature in the deep groundwater formed under different climatic conditions. The analysis of strontium isotopes reveals considerable homogeneity in the groundwater, which demonstrates a single source of Sr isotopes and hydraulic connectivity of the aquifers. The future development of groundwater resources in the investigated area can be planned in the view of the current findings. Aflaj systems along the slope of the NOM and the downstream shallow alluvium aquifer can provide additional water supply. Such future exploitation must be supported by confirmed presence of modern groundwater evident from application of recent techniques (e.g., helium/tritium).
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
We would like to thank the Sultan Qaboos University for providing financial support under the project # CL/SQU-UAEU/14/04.