Groundwater and Solute Budget (A Case Study from Sabkha Matti, Saudi Arabia)

hydrology

Article Groundwater and Solute Budget (A Case Study from Sabkha Matti, Saudi Arabia)

Waleed Saeed 1,* , Orfan Shouakar-Stash 1,2,3 , Warren Wood 4, Beth Parker 3 and André Unger 1

1 Department of Earth and Environmental Science, University of Waterloo, Waterloo, ON N2L 3G1, Canada; [email protected] (O.S.-S.); [email protected] (A.U.) 2 Isotope Tracer Technologies Inc., Waterloo, ON N2V 1Z5, Canada 3 School of Engineering, University of Guelph, Guelph, ON N1G 2W2, Canada; [email protected] 4 Department of Earth and Environmental Sciences, Michigan State University, 288 Farm Lane, East Lansing, MI 48824, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-519-500-9078

Received: 11 November 2020; Accepted: 30 November 2020; Published: 3 December 2020

Abstract: Sabkha Matti is the largest inland sabkha (2950 km2) in the Arabian Peninsula. The drainage area supporting this sabkha is >250,000 km2 and is the discharge point for part of the ten thousand meter thick regional groundwater systems ranging in age from Precambrian through Miocene in the Rub’ al Khali structural basin. A hydrologic budget was constructed for this sabkha, where water fluxes were calculated on the basis of hydraulic gradient and conductivities measured in both shallow and deep wells. The evaporation rates from the surface of the sabkha were estimated from the published data and indicate that almost all the annual rainfall is lost by surface evaporation. The water flux multiplied by its solute concentration showed that nearly all the solutes in the sabkha were derived by upward leakage from the underlying regional aquifers rather than the weathering of the aquifer framework, from precipitation, or from other sources. Steady-state estimates within a rectilinear control volume of the sabkha indicate that about 1 m3/year of water enters by lateral groundwater flow, 2 m3/year of water exits by lateral groundwater flow, 20 m3/year enters by upward leakage, 780 m3/year enters by recharge from rainfall, and 780 m3/year is lost by evaporation. The proposed conceptual model of the hydrology for sabkha Matti is assumed to apply to the rest of the inland sabkhas of the Arabia Peninsula and to many ancient environments of deposition observed in the geologic record.

Keywords: Rub’ al Khali; water budget; sabkha; solute budget; groundwater recharge

1. Introduction Sabkha is an Arabic term that is widely used for a salt ﬂat. Evaporation greater than precipitation and a source of solutes are the main requirements for the occurrence of this phenomenon. Sabkhat (plural of Sabkha) are characterized by ﬂat landscapes, shallow groundwater levels (usually less than one meter), and high water salinities. In the Arabian Peninsula, there are two types of sabkhat: coastal and inland. Coastal sabkhat are marginal marine areas [1,2]. In contrast, inland sabkhat are found in basins, away from the coast, and typically surrounded by sand dunes [3]. The formation of both coastal and inland sabkhat are explained by the evaporation of continental groundwaters through the surface, which produces brines and subsequently causes the precipitation of evaporite minerals [1]. Thus, the inland sabkha systems are important in water resource assessments owing to the fact that they represent discharge points of local and regional groundwater systems. In certain large, closed basins, such as the Caspian Sea in Eurasia, the Great Salt Lake in North America, Lake Eyre in Australia,

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LakeHydrology Titicaca 2020, 7,in x FOR South PEER America, REVIEW and Lake Chad in Africa, surface water rather than groundwater2 of 12 dominates the solute input. The Rub’ al Khali (RAK) structural basin lies below the largest sand desert in the world. The The Rub’ al Khali (RAK) structural basin lies below the largest sand desert in the world. structural basin covered an area of approximately 560,000 km2 and is bound by the Central Arabian The structural basin covered an area of approximately 560,000 km2 and is bound by the Central Arabian Arch in the north, the Oman Thrust in the east, the Hadhramaut-Dhofar Arch in the south, and the Arch in the north, the Oman Thrust in the east, the Hadhramaut-Dhofar Arch in the south, and the Arabian Shield in the west (Figure 1a). Sabkha Matti (SM) is a flat salt covered portion of the sabkha Arabian Shield in the west (Figure1a). Sabkha Matti (SM) is a flat salt covered portion of the sabkha Matti drainage basin (>250,000 km2) that is located in the Ar Rub al Kali desert and is underlain by Matti drainage basin (>250,000 km2) that is located in the Ar Rub al Kali desert and is underlain by the larger Ar Rub al Kali structural basin (Figure 1a,b). SM extends about 150 km south from the the larger Ar Rub al Kali structural basin (Figure1a,b). SM extends about 150 km south from the western Abu Dhabi coastline and across the border between the United Arab Emirates and Saudi western Abu Dhabi coastline and across the border between the United Arab Emirates and Saudi Arabia [4]. At the coast of Abu Dhabi, SM is characterized by a narrow strip of supratidal carbonate Arabia [4]. At the coast of Abu Dhabi, SM is characterized by a narrow strip of supratidal carbonate sands and evaporites that form a coastal sabkha. Southward, it grades into an area of inland sands and evaporites that form a coastal sabkha. Southward, it grades into an area of inland siliciclastic siliciclastic sediments. The conceptual model for the origin of solutes in the coastal sabkha is the sediments. The conceptual model for the origin of solutes in the coastal sabkha is the ascending brine ascending brine model given by [1]. That is, the origin and evolution of solutes and mineral model given by [1]. That is, the origin and evolution of solutes and mineral precipitation involves: precipitation involves: (1) the upward leakage of brines from the regional aquifers that underlay the (1) the upward leakage of brines from the regional aquifers that underlay the sabkha, (2) deflation sabkha, (2) deflation that removes sand down to the level of the water table, and (3) extensive that removes sand down to the level of the water table, and (3) extensive evaporation that causes the evaporation that causes the formation of a halite crust and the accumulation of anhydrite and gypsum formation of a halite crust and the accumulation of anhydrite and gypsum in the shallow subsurface. in the shallow subsurface. The question addressed in the current study is whether this ascending The question addressed in the current study is whether this ascending brine conceptual model is viable brine conceptual model is viable for the sabkha Matti inland sabkha. Thus, the aim of the current for the sabkha Matti inland sabkha. Thus, the aim of the current study is to quantitively estimate study is to quantitively estimate the water and solute fluxes to SM, which would ultimately lead to a the water and solute fluxes to SM, which would ultimately lead to a better overall understanding of better overall understanding of the regional hydrology in the Rub’ al Khali structural basin and to the regional hydrology in the Rub’ al Khali structural basin and to investigate the role of SM in the investigate the role of SM in the regional hydrogeological system as a potential surface discharge regional hydrogeological system as a potential surface discharge point. It is assumed that the proposed point. It is assumed that the proposed conceptual model of the hydrology for sabkha Matti is conceptual model of the hydrology for sabkha Matti is applicable to the rest of the inland sabkhas of applicable to the rest of the inland sabkhas of the Arabia Peninsula and to many ancient environments the Arabia Peninsula and to many ancient environments of deposition observed in the geologic record. of deposition observed in the geologic record.

Figure 1. Cont.

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Figure 1. (a) Geologic sketch map of the Arabia Peninsula showing the location and boundaries of theFigure Rub’ 1. al(a Khali) Geologic structural sketch basin map (modifiedof the Arabia after Peninsul [5]); (ba) mapshowing showing the location the location and boundaries of sabkha Matti,of the samplingRub’ al Khali locations, structural surface basin elevation, (modified water after level [5]); contours, (b) map and showing flow direction the location in the of sabkha. sabkha Matti, sampling locations, surface elevation, water level contours, and flow direction in the sabkha. 2. Geological Setting 2. GeologicalThe Matti Setting sabkha is a wide, north-south trending, salt-covered area that extends about 150 km southThe from Matti the westernsabkha is Abu a wide, Dhabi north-south coastline and trending across, the salt-covered border between area that the extends United Arab about Emirates 150 km 2 andsouth Saudi from Arabia the western (Figure Abu1). It Dhabi covers coastline a portion and of theacro sabkhass the border Matti (SM) between drainage the United basin ( Arab>250,000 Emirates km ) thatand isSaudi located Arabia in the (Figure Ar Rub 1). al KaliIt covers desert a andportion is underlain of the sabkha by the Matti larger (SM) Ar Rub drainage al Kali structuralbasin (>250,000 basin (Figurekm2) that1). is The located sabkha in partthe Ar of theRub larger al Kali drainage desert and system is underlain is bound by in the southlarger byAr the Rub sand al Kali dune structural system ofbasin the Rub’(Figure al Khali1). The desert. sabkha The part western of the and larger eastern drainage boundaries system of is the bound sabkha in arethe formedsouth by by the the sand N-S lineardune system margins of of the Neogene-age Rub’ al Khali outcrops, desert. The known wester asn Ras and Musherib eastern boundaries and Ras Mugherij, of the sabkha respectively are formed [6]. Aby generalized the N-S linear W-E margins geologic of cross Neogene-age section of theoutc SMrops, is shownknown in as Figure Ras Musherib2. Beneath and the Ras sabkha Mugherij, lies a thickrespectively (6000–8000 [6]. A m) generalized Paleozoic-Cenozoic W-E geologic sedimentary cross section sequence of the of SM carbonates, is shown in clastics, Figure and 2. Beneath evaporites the thatsabkha accumulated lies a thick on (6000–8000 the stable Arabianm) Paleozoic-Cenozoic platform [7]. Above sedimentary this sequence sequence is a of clastic carbonates, deposit clastics, of Late Oligocene-Mioceneand evaporites that age,accumulated known as on the the Hadrukh stable Arabian Formation. platform Following [7]. Above the deposition this sequence of the is Hadrukh, a clastic thedeposit area of now Late occupied Oligocene-Miocene by the Arabian age, Gulfknown area as wasthe Hadrukh subject to Formation. lagoonal-evaporitic Following depositionthe deposition [8]. Inof thisthe Hadrukh, setting, marl, the clay,area dolomite,now occupied dolomitic by the limestone, Arabian Gulf anhydrite, area was and subject nodular to gypsumlagoonal-evaporitic of the Dam Formationdeposition were[8]. In deposited this setting, [9]. In marl, Mid-Miocene, clay, dolomite, the area dolomitic was subject limestone, to gentle anhydrite, tilting and rapidand nodular erosion ofgypsum the Arabian of the Dam shield Formation basement were as a deposited result of the [9]. Miocene-Pliocene In Mid-Miocene, the uplift area (Zagros was subject Orogeny) to gentle [10]. Thistilting setting and rapid favored erosion the deposition of the Arabian of continental shield basement clastic sedimentsas a result of of the the Shuweihat Miocene-Pliocene Formation uplift in fluvial,(Zagros aeolian, Orogeny) and [10]. sabkha Thisenvironments setting favored [11 the]. Since deposition the Late of Miocene-Pliocene,continental clastic thesediments Arabian of Gulf the foredeepShuweihat basin Formation has developed in fluvial, in aeolian, front of and the sa risingbkha Zagros environments Mountains [11]. in Since response the Late to a Miocene- regional compressionalPliocene, the Arabian event between Gulf foredeep the Arabian basin and has Eurasian developed plates in front [12]. of In the the rising Late Quaternary, Zagros Mountains the region in ofresponse SM was to the a regional site of drainagecompressional systems event which between derived the fluvialArabian sediments and Eurasian from plates rising [12]. mountains In the Late in westernQuaternary, Saudi the Arabia region andof SM Oman was the and site deposited of drainage them systems into the which Matti derived drainage fluvial basin sediments [6]. Limited from reworkingrising mountains and desiccation in western to theseSaudi fluvial Arabia sediments and Oman occurred and deposited in the Mid-Holocene them into the in response Matti drainage to rises inbasin the [6]. groundwater Limited reworking table in response and desiccation to the ~140 to thes m risee fluvial in the sediments Arabian Gulfoccurred [1]. Subsequent in the Mid-Holocene deflation removedin response sands to rises down in to the the groundwater level of the rising table water in resp table,onse causing to the ~140 the development m rise in the ofArabian evaporite Gulf facies [1]. withinSubsequent the fluvial deflation sands removed [4,13]. It sands is proposed down thatto the this level fluvial of sequence, the rising within water which table, the causing evaporite the development of evaporite facies within the fluvial sands [4,13]. It is proposed that this fluvial

Hydrology 2020, 7, 94 4 of 12 Hydrology 2020, 7, x FOR PEER REVIEW 4 of 12 sequence,minerals havewithin developed, which the be evaporite given the minerals name sabkha have Matti developed, Formation be given (SMF) the because name of sabkha its extensive Matti Formationdevelopment. (SMF) The because SMF has of anits extensive average thickness developmen of aboutt. The 5 SMF m, laterally has an average continues thickness by 50–80 of kmabout E-W, 5 m,and laterally extends continues inland N-S by from 50–80 the km Arabian E-W, coastand extends for nearly inland 150 km. N-S The from water the tableArabian within coast the for sabkha nearly is 150less km. than The 1 m water below table the landwithin surface, the sabkha which is allows less than the 1 evaporation m below the of land groundwater surface, which to occur allows directly the evaporationfrom the water of groundwater table. to occur directly from the water table.

FigureFigure 2. 2. GeneralizedGeneralized geologic geologic cross cross section section of of the the sabkha sabkha Matti Matti from from Ras Ras Musherib Musherib in in the the west west to to Ras Ras MugherijMugherij in in the the east, east, as as shown shown in in Figure Figure 11..

3.3. Materials Materials and and Methods Methods FigureFigure 11 showsshows thethe locationlocation ofof thethe drilleddrilled holesholes inin thethe studystudy area.area. WithinWithin thethe borderborder ofof SaudiSaudi Arabia,Arabia, holes forfor 102102 shallow shallow piezometers piezometers (< (<1010 m) m) were were drilled drilled between between 1 and 1 and 2 m 2 below m below the water the water table tableby using by ausing Geoprobe, a Geoprobe, Model 6620DT,Model 6620DT, and a direct and push a direct machine. push Nine machine. piezometers Nine piezometers of intermediate of intermediatedepth (>10 m depth and < (>1025 m) m and were <25 drilled m) were by adrilled rotary by drill a rotary rig (B53, drill manufactured rig (B53, manufactured by Mobile by Drill Mobile Intl) mounted on a 4 4 truck. All the shallow and intermediate piezometers were constructed from a Drill Intl) mounted× on a 4 × 4 truck. All the shallow and intermediate piezometers were constructed from50 mm-diameter a 50 mm-diameter schedule schedule 40 PVC 40 pipe, PVC slottedpipe, slo bytted a hacksaw by a hacksaw on the on bottom the bottom 1.5 m 1.5 and m coveredand covered with withfilter filter sock, sock, with with threaded threaded joints joints containing containing couplings. couplings. The holesThe holes were were backfilled backfilled with with the loose the loose sand sandcollapsed collapsed around around the piezometers. the piezometers. Within theWithin border the of Abuborder Dhabi, of Abu water Dhabi, samples water were samples collected were from collectedeleven pre-existing from eleven monitoring pre-existing wells. monitoring Details of wells. the well Details construction of the well data construction for these wells data are for reported these wellsin [1, 14are,15 reported]. Water samplesin [1,14,15]. were Water collected samples using were a dedicated collected plastic using bailer.a dedicated Three plastic deep wells bailer. (> 100Three m) deepwere drilledwells (>100 with m) rotary were drills drilled into with the Dam rotary and dr Hadrukhills into the Formations. Dam and These Hadrukh wells Formations. were finished These with wells18 5/8 were inch-diameter finished with steel 18 casings 5/8 inch-diameter through the steel overburden casings through sediments the to overburden the surface sediments and were to left the as surfaceopen bedrock and were borehole left as completions open bedrock from borehole the bottom completions of the casing from to thethe remainderbottom of ofthe the casing depth. to Each the remainderof the three of deep the depth. wells were Each provided of the three with deep a cap, we shut-olls wereff valve, provided and with pressure a cap, gauge. shut-off All valve, three deepand pressurewells were gauge. under All flowing three artesiandeep wells conditions, were under which flowing allowed artesian the collection conditions, of water which samples allowed at the the collectionsurface. For of water this study, samples five at well the surface. cluster systemsFor this werestudy, drilled five well along cluster a research systems profile, were drilled as shown along in a research profile, as shown in Figure 3. Each of the cluster wells was completed at different depths,

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Hydrology 2020, 7, x FOR PEER REVIEW 5 of 12 Figure3. Each of the cluster wells was completed at di fferent depths, and groundwater samples were collectedand groundwater from each samples aquifer individuallywere collected to from investigate each aquifer any chemical individually variations to investigate between any the aquifers.chemical Analysesvariations of between all the water the aquifers. samples Analyses for the electrical of all the conductivity water samples (EC), for pH, the temperature,electrical conductivity and dissolved (EC), oxygenpH, temperature, content were and carried dissolved out inoxygen the field content using were a portable carried HQ40d out in the meter field (Hach using Ltd., a portable London, HQ40d UK). Fieldmeter measurements (Hach Ltd., London, of alkalinity Canada). were Field made measurements using a Hach field of alkalinity titration kitwere and made expressed using asa Hach the mg field/L µ oftitration equivalent kit HCOand expressed3. All the water as the samples mg/L wereof equivalent filtered through HCO3. 0.45All them filterswater and samples then acidified were filtered with double-distilledthrough 0.45 μm nitric filters acid and for then cation acidified analysis with and preserveddouble-distilled unacidified nitric foracid anion for cation analysis. analysis Analyses and ofpreserved major ions unacidified were performed for anion by theanalysis. ALS laboratory Analyses group,of major Waterloo, ions were Canada. performed The analyses by the were ALS carriedlaboratory out usinggroup, ion Waterloo, chromatography Canada. in The accordance analyses with were the carried protocol out for using analytical ion chromatography methods used by in theaccordance Environmental with the Protection protocol Act for EPAanalytical 300.1 [16meth]. Theods analyticalused by the reproducibility Environmental and Protection precision forAct each EPA ion300.1 analysis [16]. The were analytical better than reproducibility 5%. and precision for each ion analysis were better than 5%.

FigureFigure 3. 3.Transect Transect M1–M2 M1–M2 showing showing multi-level multi-level observation observation wells wells that that were were installed installed in in the the sabkha sabkha MattiMatti area. area. Trace Trace of of line line of of section section is is shown shown in in Figure Figure1. 1.

4.4. Results Results 4.1. Hydrologic Budget 4.1. Hydrologic Budget A water budget for the SM was developed in order to understand the processes that control the A water budget for the SM was developed in order to understand the processes that control the hydrology of the sabkha and to quantify the individual components. The regional-scale system is hydrology of the sabkha and to quantify the individual components. The regional-scale system is assumed to operate under a steady-state flow condition, which means that all of the inflows to the assumed to operate under a steady-state flow condition, which means that all of the inflows to the sabkha must equal all of the outflows. The steady-state assumption is based on field observations, sabkha must equal all of the outflows. The steady-state assumption is based on field observations, which confirm that the hydraulic heads throughout the sabkha area fluctuate very little over long time which confirm that the hydraulic heads throughout the sabkha area fluctuate very little over long scales (>3 years). The budget was developed around a representative control volume parallel to the time scales (>3 years). The budget was developed around a representative control volume parallel to groundwater flow direction of the sabkha that is 20 km long, 1 m wide, and 5 m deep (see Figure4). the groundwater flow direction of the sabkha that is 20 km long, 1 m wide, and 5 m deep (see Figure 4).

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FigureFigure 4. 4. WaterWater volume volume and mass balance in the sabkha Matti. The inflow components to the SM include: (1) direct precipitation on the aquifer, (2) shallow The inflow components to the SM include: (1) direct precipitation on the aquifer, (2) shallow groundwater entering the SM laterally from the mainland, and (3) upward leakage from the underlying groundwater entering the SM laterally from the mainland, and (3) upward leakage from the regional aquifers. No substantial surface runoff has occurred to the sabkha through the duration of underlying regional aquifers. No substantial surface runoff has occurred to the sabkha through the this study. However, field observations demonstrate that there must have been some recent overland duration of this study. However, field observations demonstrate that there must have been some flow in the last 100 years, as many of the surface features have a fluvial not aeolian expression and recent overland flow in the last 100 years, as many of the surface features have a fluvial not aeolian little of the sabkha is covered with dunes/sand sheets. For simplicity’s sake and due to the absence of expression and little of the sabkha is covered with dunes/sand sheets. For simplicity’s sake and due data, the surface runoff to and from the control volume is assumed to be negligible. Water may leave to the absence of data, the surface runoff to and from the control volume is assumed to be negligible. the sabkha by discharge laterally to the Arabian Gulf and by direct evaporation. Water may leave the sabkha by discharge laterally to the Arabian Gulf and by direct evaporation. 4.1.1. Lateral Groundwater Fluxes 4.1.1. Lateral Groundwater Fluxes B Darcy’s law (Equation (1)) was used to estimate the lateral fluxes of groundwater into and out B Darcy’s law (Equation (1)) was used to estimate the lateral fluxes of groundwater into and out of the SM. of the SM. q = K (dh/dl), (1) − q = −K (dh/dl), (1) where q is the flux in dimensions of length per time (L/T), K is the hydraulic conductivity of the porous wheremedium q is in the dimensions flux in dimensions of length per of timelength (L per/T), ti andme dh(L/T),/dl isK the is the gradient hydraulic in the conductivity hydraulic head of the in porousthe direction medium of flowin dimensions in dimensions of length of length per pertime length (L/T), (dimensionless). and dh/dl is the Ingradient the SMF, in thethe gradienthydraulic is headtoward in thethe coastline direction and of theflow water in dimensions table has a typicalof length gradient per length of approximately (dimensionless). 2 m in In 10 the km (FigureSMF, the1). gradientTo obtain is horizontal toward the hydraulic coastline conductivity and the water values table (Khash), a slugtypical tests gradient were performed of approximately and evaluated 2 m in on10 kmeight (Figure short-screened 1). To obtain observation horizontal wells hydraulic using the conductivity method of Hvorslev values (K [17h),]. Theslug lithologytests were in performed these wells andranges evaluated from marly on eight sands short-screened to clean sand facies,observatio whichn makeswells using them goodthe method representative of Hvorslev samples [17]. of The the lithologyfluvial and in sabkha these wells nature ranges of the SMF.from Themarly slug sand tests results to clean give sand an arithmetic facies, which mean makes Kh value them of aboutgood representative1.4 m/d (Table S1,samples Supporting of the Information). fluvial and sabkha From the nature gradient of the and SMF. Kh values, The slug the lateraltest results groundwater give an arithmeticflux in the mean SMF isK abouth value 0.2 of mmabout/d. 1.4 m/d (Table S1, Supporting Information). From the gradient and Kh values, the lateral groundwater flux in the SMF is about 0.2 mm/d. 4.1.2. Upward Leakage 4.1.2.The Upward artesian Leakage hydraulic heads in the regional Hadrukh and Dam Formations, and upward hydraulic gradientsThe artesian across the hydraulic Shuweihat heads and SMin the Formations regional (Table Hadrukh S2, Supporting and Dam Information),Formations, areand consistent upward hydraulicwith the fact gradients that regional across groundwater the Shuweihat is moving and SM slowly Formations upward (Table into the S2, base Supporting of the sabkha. Information), From the arehydrostatic consistent data, with an the average fact that upward regional hydraulic groundwa gradientter is moving of 0.14 wasslowly calculated upward frominto the three base separate of the sabkha.pairs of From multilevel the hydrostatic wells. The data, hydraulic an average conductivity upward in hydraulic this case isgradient the vertical of 0.14 hydraulic was calculated conductivity from three separate pairs of multilevel wells. The hydraulic conductivity in this case is the vertical hydraulic conductivity (Kv) of the least conductive layer underlying the sabkha. Measuring Kv in situ

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(Kv) of the least conductive layer underlying the sabkha. Measuring Kv in situ is usually difficult, in that it must be measured either on cores or as a secondary parameter on a test for horizontal conductivity. However, it has been verified based on field data that a harmonic weighting of the horizontal conductivities can yield a reasonable estimate of the net vertical conductivity [14]. Slug tests were carried out at three observation wells, where the screens were installed within the aquitard underlying the sabkha. The obtained K values from the three-slug test are 3 10 4, 4 10 5, h × − × − and 9 10 6 m/d. Field observation suggests that this variation in the K values is mainly due to × − h the high heterogeneity in the aquitard layer, which is composed of a mixture of silty sands and well-compacted mudstones. These data reveal that there might be a significant level of uncertainty in the estimated Kv for the aquitard. With this in mind, a harmonic weighting of the Kh value was 5 calculated to be about 2 10 m/d, which is assumed to represent the net Kv value for the confining × − layer underlying the sabkha. This value corresponds to clayey sandstones [18], consistent with the geological logs. Using Darcy’s law, the mean upward leakage calculated with the above values of head 6 gradient and Kv is approximately 3 10 m/d or about 1 mm/year. This estimate of the upward flux × − is a calculation of water flux multiplied by the solute concentration of the upwelling brines, providing a mass flux. This estimate of the upward flux is compared with the chemistry of the upwelling brines in the below section, and the results demonstrate that the estimates are consistent with the solute mass balance. These calculations over the 5000 years given for the age of the sabkha Matti Fm. demonstrate that the estimates are consistent with the origin of solute mass in the aquifer system. A relatively similar vertical water flux of approximately 4 mm/y was reported from the coastal sabkha of Abu Dhabi, United Arab Emirates [14].

4.1.3. Evaporation and Recharge The actual evaporation from the surface of the SMF was estimated to be 39 13 mm/y, based on a ± remote sensing technique [19]. The assumption that the SM is operating under a steady-state condition requires that the flux out by evaporation must equal the flux in by rainfall. To evaluate this assumption, the mean annual rainfall was taken over a fourteen-year period from three meteorological stations in the vicinity of the SM (Figure1). The data range from 44.5 mm at the Al Ruwais station in the further northeast to 42.2 mm at the Al Gheweifat station in the northwest to 29 mm at the Al Jazeera station southeast of the SM [20]. This gives a mean annual rainfall of about 39 mm/year. In order to determine the amount of rainwater that infiltrated to the groundwater table, water tables were monitored in three designated wells during the summer, where no rainfall occurred, and one measurement was taken after a 13.3 mm rain event in March of 2017 (Table S3, Supporting Information). After the rain event, the rises in water levels were measured to be 35, 30, and 30 mm. By multiplying these values with the porosity of 0.38, the ratio of infiltrated water to the rain event suggests that between 86%–100% of the rainfall was accounted for at the water table. The highest recharge ratio to the water table was recorded in a well that was drilled adjacent to a solution collapse feature, which is very common in the area as a result of salt crust dissolution. Similar observations were reported from the coastal sabkha of Abu Dhabi, where more than 80% of the rainfall infiltrates to the water table [14]. The outcomes of the evaporation and recharge data provide further evidence of the steady-state nature of the sabkha system, where the water lost by evaporation is balanced by the mean annual recharge.

4.2. Solute Budget The concentrations of major ionic constituents are plotted in the triangular plots, using the Geochemist’s Workbench software (Figure5). The triangular diagram is used to infer hydro-geochemical facies by showing the relative concentrations of diﬀerent ions in the analyzed water samples [21]. What the data on the triangular plots show is that, regardless of the aquifer, all the water samples are of Na-Cl water types and have similar ionic compositions. This similarity in the water types and ionic compositions among all groundwater samples suggests that the waters may have the same or similar origins. Hydrology 2020, 7, 94 8 of 12 Hydrology 2020, 7, x FOR PEER REVIEW 8 of 12

FigureFigure 5.5. TriangularTriangular plotsplots ofof ((aa)) majormajor cationscations andand ((bb)) majormajor anionsanions preparedprepared fromfrom concentrationsconcentrations inin equivalentequivalent unitsunits ofof waterwater samplessamples collectedcollected fromfrom didifferentﬀerent aquifers aquifers in in the the sabkha sabkha Matti Matti area. area.

AA solutesolute budget was similarly developed developed around around a a volumetric volumetric cross cross section section of of the the sabkha sabkha that that is is20 20 km km long, long, 1 1m m wide, wide, and and 5 5m m deep. deep. The The major major ioni ionicc constituents constituents and and strontium strontium isotope isotope ratios ratios in inwater water samples samples collected collected from from the the Hadrukh, Hadrukh, Dam, Dam, and and Shuweihat Shuweihat formations resemble thosethose onesones thatthat werewere obtainedobtained fromfrom thethe near-surfacenear-surface sabkhasabkha brinebrine [[4].4]. TheseThese datadata alongalong withwith hydrostatichydrostatic headhead measurementsmeasurements suggestsuggest aa majormajor sourcesource forfor thethe solutessolutes inin thethe SMSM toto bebe associatedassociated withwith ascendingascending brinebrine fromfrom thethe underlyingunderlying regionalregional aquifers.aquifers. Yet, solutessolutes maymay alsoalso enterenter thethe SMSM withwith thethe surface-watersurface-water transport,transport, fromfrom atmosphericatmospheric precipitation,precipitation, bebe formedformed byby thethe reactionreaction ofof thethe waterwater withwith thethe aquiferaquifer solids,solids, enterenter the the system system by by an an eolian eolian process, process, or or form form by by reaction reaction with with the the gasses. gasses. Solute Solute may may leave leave the systemthe system by being by being transported transported with thewith groundwater the groundwate or surfacer or surface flow, leave flow, by leave diffusion, by diffusion, leave by leave mineral by precipitation,mineral precipitation, leave by leave an eolian by an process, eolian process, or leave or as leave a gas as phase. a gas Solute phase. concentrations Solute concentrations are almost are certainlyalmost certainly transient, transient, as will be as shown will be below shown by theirbelow continuous by their continuous input and relativeinput and lack relative of output. lack of output.The mean dissolved solids concentration results from the different geological formations at the SM areThe shown mean indissolved Table1. Thesolids components concentration of the results solute from budget the different can be calculated geological from formations multiplying at the theSM meanare shown dissolved in Table solids 1. The concentrations, components represented of the solute by budget the mean can chloridebe calculated concentrations, from multiplying by the waterthe mean budget dissolved [14]. By solids using concentrations, the lateral water-inflow represented value by of the about mean 445 chloride million concentrations, L/year and a Cl by input the concentrationwater budget [14]. value By of using 160 gthe/L, lateral the lateral water-inflow inflow of value solutes of about from 445 the million upgradient L/year of and the sabkhaa Cl input is estimatedconcentration to be value about of 70 160 kg/year. g/L, Similarly,the lateral a inflow lateral waterof solutes outflow from value the ofupgradient approximately of the 1800 sabkha L/year is multipliedestimated to by be an about outflow 70concentration kg/year. Similarly, of Cl ofa 200lateral g/L water results outflow in an estimated value of 350 approximately kg/year of solute 1800 outflow.L/year multiplied Likewise, by the an upwardoutflow concentration leakage value of of Cl around of 200 g/L 22,170 results L/year in an multiplied estimated by350 a kg/year mean Clof concentrationsolute outflow. of Likewise, 55 g/L results the upward in 1250 leakage kg/year value of upward of around leakage 22,170 of L/year solutes. multiplied It is assumed by a mean that the Cl concentrationconcentration increasesof 55 g/L over results time in by 1250 a combination kg/year of upward of evaporation, leakage mineralof solutes. solution It is assumed by recharge, that and the density-drivenconcentration increases free convection over time that by is circulatinga combination the solutes.of evaporation, It is also mineral assumed solution that some by saltrecharge, is leaving and thedensity-driven system through free the convection land surface that by is deflation.circulating A compilationthe solutes. ofIt theis also component assumed values that ofsome the watersalt is andleaving solute the budget system is through summarized the land in Table surface2. by deflation. A compilation of the component values of the water and solute budget is summarized in Table 2.

Table 1. Solute chemistry (mg/L) of the groundwater samples collected from different aquifers in the sabkha Matti area.

Aquifer Ca K Mg Na Cl SO4 Sr Hadrukh 4800 1300 2600 35,000 69,500 2850 85 Hadrukh 4500 1300 2500 34,000 67,600 2950 85 Dam 2160 581 1370 27,100 43,909 3914 46 Shuweihat 2840 522 1150 31,200 54,927 3350 75 Shuweihat 2800 541 1090 29,500 45,567 3083 54 Sabkha (n = 119) 10,894 3438 7854 72,406 170,654 1739 254

Hydrology 2020, 7, 94 9 of 12

Table 1. Solute chemistry (mg/L) of the groundwater samples collected from diﬀerent aquifers in the sabkha Matti area.

Table 2. Components of the water and solute budgets calculated in this study, based on the control volume of the sabkha that is 20 km long, 1 m wide, and 5 m deep.

Budget Component Flux Volumetric Water Flux (m3/year) Total Solute (kg/year) Lateral ﬂux in 1 70 Lateral ﬂux out 2 350 Upward leakage 20 1250 Evaporation 780 0 Rainfall 780–670 <1 Increase in storage 0 970

5. Discussion The lateral groundwater flux in the SMF is calculated to be 0.2 mm/d. This flux value can be converted to a seepage velocity by dividing the flux by the effective porosity. Based on grain-size distribution analyses [22], several laboratory analyses of the sand from the SMF yield a consistent effective porosity of approximately 0.38. Given this value of porosity, the lateral seepage velocity through the SMF is estimated to be about 0.5 mm/d. The lateral flux out of the SMF was calculated downgradient of the sabkha. In this area, the hydraulic gradient is about 7 m in 10 km (0.0007). Accordingly, the lateral groundwater flux out of SM is calculated to be about 1 mm/d. For the purposes of comparing the magnitude of the different components of the hydrologic budget, the fluxes are multiplied by the width (1 m) and thickness (5 m) of the sabkha to obtain lateral volumetric fluxes. The fluxes in and out of the sabkha obtain lateral volumetric flux values of about 1 m3/year and 2 m3/year, respectively (Table2). The average residence time for the lateral groundwater flux to be transmitted from the proximal end to the distal end at the discharge zone is calculated by dividing one half the length of the sabkha (150 km) by the seepage velocity. Accordingly, the average residence time of groundwater is approximately 416,000 years. Given that the age of the SM is approximately 8000 to 5000 years old, it is concluded that lateral influx of groundwater has not contributed significantly to the current volume of water and solutes in the sabkha. The calculated upward flux is multiplied by the width (1 m) and length (20,000 m) of the section to obtain a total upward volumetric flux of about 20 m3/year. This value is significantly larger than the horizontal flux, of 1 m3/year. These calculations suggest that the upward leakage from the underlying regional aquifers provides an important source of groundwater entering the sabkha. The pore volume of brine that might fill the SMF over its life span was calculated by multiplying the mean upward flux by the porosity and the age of the sabkha. Based on this, an approximately three pore volumes should have filled the 5-m-thick section of the SMF over its saturated life span of 5000 years. Hypothetically, the ratios of the average concentrations of conservative solutes from the underlying regional aquifers to those in the sabkha should be equal to the number of pore volumes that have entered the sabkha. Such ratios are shown in Figure6. Field observations suggest a relatively conservative nature for magnesium and potassium, as no significant precipitations of both minerals were observed throughout the study area. Accordingly, K and Mg were assumed to be the most conservative ions in this geological setting, and their ratios indicate that about three pore volumes have entered the sabkha. This result reveals that the solute and Hydrology 2020, 7, 94 10 of 12 water budget estimates of three pore volumes are in complete agreement. However, minerals that have significantly precipitated on the sabkha surface show lower ratios. This includes sodium and chloride, which precipitate as halite, calcium, and sulfate precipitate as anhydrite. Thus, the solute budget is totally in line with the hydrologic budget and the age of the SM. Moreover, the actual evaporation from the surface of the SMF of 39 mm/y is multiplied by the section width of 1 m and the section length of 20,000 m to give a volumetric flux out of about 780 m3/year. As the system is assumed to operate under a steady-state condition, the water lost by evaporation is balanced by the mean annual recharge that is calculated to be between 670 and 780 m3/year. These data reveal that evaporation and recharge on the sabkha are the dominant components, whereas the lateral and upwelling groundwater components are minor. Note, however, that the solute mass is dominated by upwelling water. The water mass balanceHydrology in 2020 the, SM 7, x isFOR illustrated PEER REVIEW in Figure 4. 10 of 12

Figure 6. Ratios of the major element concentrations for the upwelling regional brine relative to the Figure 6. Ratios of the major element concentrations for the upwelling regional brine relative to the brine in the sabkha Matti. brine in the sabkha Matti. 6. Conclusions 6. Conclusions A hydrologic budget of the sabkha Matti was constructed as an approach to better understand the regionalA groundwaterhydrologic budget systems of ofthe the sabkha large drainageMatti was area constructed>250,000 as km an2 in approach the Rub’ to al better Khali structuralunderstand 2 basin.the regional The data groundwater reveal that the systems hydrologic of the budget large fordrainage the sabkha area Matti>250,000 system km is in operating the Rub’ under al Khali a steady-statestructural basin. condition, The data where reveal all of that the the inflows hydrolog to theic sabkhabudget equalfor the all sabkha of the Matti outflows. system Steady-state is operating estimatesunder a within steady-state a rectilinear condition, control where volume all of of the the infl sabkhaows to indicate the sabkha that aboutequal 1al ml of3/year the outflows. of waterenters Steady- 3 bystate lateral estimates groundwater within flow, a rectilinear 2 m3/year control of water volume exits byof the lateral sabkha groundwater indicate that flow, about 20 m 13 /myear/year enters of water by 3 3 upwardenters leakage,by lateral 780 groundwater m3/year enters flow, by 2 recharge m /year fromof water rainfall, exits andby lateral 780 m 3groundwater/year is lost by flow, evaporation. 20 m /year 3 3 Therefore,enters by a upward quantification leakage, of 780 the m components/year enters to by the recharge waterbudget from rainfall, suggests and that 780 evaporation m /year is lost and by rainfallevaporation. are the dominantTherefore, components, a quantification whereas of thethe lateralcomponents and upwelling to the water groundwater budget componentssuggests that areevaporation minor. In contrast, and rainfall the analysis are the of thedominant solute budget components, shows that whereas the dominant the lateral source and of solutesupwelling is upwardgroundwater leakage components from the underlying are minor. regional In contrast, aquifers, the consistentanalysis of withthe solute the ascending budget shows brine modelthat the proposeddominant by source [1,3,4,14 of]. solutes For instance, is upward the calculationleakage from of the the underlying solute budget regional shows aquifers, that about consistent 95% of the with solutesthe ascending are derived brine from model ascending proposed continental by [1,3,4,14]. brines, For whereas instance, only the minor calculation amounts of are the derived solute budget from rainfallshows and that from about groundwater 95% of the entering solutes fromare derived up-gradient from areas.ascending Therefore, continental this study brines, shows whereas that theonly waterminor and amounts solutes inare the derived sabkha from Matti rainfall are from and diff erentfrom sources.groundwater These entering data also fr showom up-gradient that about three areas. poreTherefore, volumes this of brinestudy may shows have that leaked the water upward and intosolute thes in control the sabkha volume Matti of the are sabkha from different since the sources. time it wasThese first data formed also show about that 5000 about years three ago. pore Over volumes time, of the brine solute may concentrations have leaked upward have increased into the control by a combinationvolume of ofthe evaporation, sabkha since mineral the time solution it was byfirst recharge, formed andabout density-driven 5000 years ago. free-convection Over time, the that solute is circulatingconcentrations the solutes have within increased the sabkha.by a combination The proposed of evaporation, conceptual model mineral of the solution hydrology by recharge, for sabkha and density-driven free-convection that is circulating the solutes within the sabkha. The proposed conceptual model of the hydrology for sabkha Matti is assumed to apply to the rest of the inland sabkhas of the Arabia Peninsula and to many ancient environments of deposition observed in the geologic record.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Table S1: Well information and estimated horizontal hydraulic conductivities for the sabkha Matti Formation, obtained from slug tests. Table S2: Water level measurements from the multi-level wells at the sabkha Matti, confirming the potential of upward leakage from the regional Hadrukh and Dam Formations into the sabkha. Water levels are in meters above sea level. Table S3: Water level measurement data to determine the ratio of infiltrated rainwater to the sabkha Matti Formation.

Author Contributions: Conceptualization, W.S., O.S.-S., and W.W.; methodology, W.S., O.S.-S., validation, W.S., O.S.-S., A.U., W.W., B.P.; formal analysis, W.S., O.S.-S.; investigation, W.S.; resources, W.S., O.S.-S., A.U., W.W., B.P.; data curation, W.S.; writing—original draft preparation, W.S.; writing—review and editing, O.S.-S., A.U.,

Hydrology 2020, 7, 94 11 of 12

Matti is assumed to apply to the rest of the inland sabkhas of the Arabia Peninsula and to many ancient environments of deposition observed in the geologic record.

Supplementary Materials: The following are available online at http://www.mdpi.com/2306-5338/7/4/94/s1: Table S1: Well information and estimated horizontal hydraulic conductivities for the sabkha Matti Formation, obtained from slug tests. Table S2: Water level measurements from the multi-level wells at the sabkha Matti, confirming the potential of upward leakage from the regional Hadrukh and Dam Formations into the sabkha. Water levels are in meters above sea level. Table S3: Water level measurement data to determine the ratio of infiltrated rainwater to the sabkha Matti Formation. Author Contributions: Conceptualization, W.S., O.S.-S., and W.W.; methodology, W.S., O.S.-S., validation, W.S., O.S.-S., A.U., W.W., B.P.; formal analysis, W.S., O.S.-S.; investigation, W.S.; resources, W.S., O.S.-S., A.U., W.W., B.P.; data curation, W.S.; writing—original draft preparation, W.S.; writing—review and editing, O.S.-S., A.U., W.W., B.P.; visualization, W.S.; O.S.-S. and W.W.; supervision, O.S.-S., B.P., A.U.; project administration, W.S.; funding acquisition, W.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the SAUDI ARAMCO OIL COMPANY. Acknowledgments: The author would also like to thank the Environmental Agency Abu Dhabi (EAD) for granting access to the field sites in the United Arab Emirates and for the information derived from its website, which was used in the preparation of this work. Conflicts of Interest: The authors declare no conflict of interest.

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