Hydrochemistry and environmental isotopes of spring water and their relation to structure and lithology identified with remote sensing methods in Wadi Araba, Egypt

Springs located at the historical sites of Wadi Araba (Eastern Desert of Egypt) and emerging from the escarpments of the Northern and Southern Galala Plateaus were investigated. A combination of methods, including hydrochemistry, stable and radioisotope composition, and structural analyses based on satellite data, provided information about the structure of the subsurface and the derived groundwater flow paths. Satellite images reveal karst features within the northern plateau, e.g. conical landforms. Karstic caves were documented along both escarpments. Chemical analysis of floodwater from Wadi Araba indicates higher concentrations of terrestrial salts compared to floodwaters from central and southern parts of the desert. δ18O and δ2H signatures in spring waters resemble those of floodwater and fall on the global meteoric water line, confirming their fast infiltration with minor influence of evaporation. The aquifer feeding the springs of the Northern Galala Plateau has low retention and the springs dry out quickly, even after heavy rainfall. Contrastingly, 3H activities in springs emerging from the Southern Galala Plateau refer to much slower subsurface passage. With respect to 3H content (3.8 TU) in recent flood waters, the spring water at Southern Galala Plateau contains about 40% recently recharged groundwater. However, its largest spring—the St. Antony spring—discharges water with a radiocarbon age of about 15,000 years. In combination with this spring’s constant and high discharge over a period of several months, that age estimate suggests a large reservoir with moderate to high retention.


Introduction
In the Eastern Desert of Egypt, as in most arid regions worldwide, groundwater is the only noteworthy source for water supply. Since ancient times, the establishment of settlements and agricultural areas has relied on that source, particularly on perennial discharging springs. That situation is observable in Wadi Araba, a 110-km long and about 20-km wide arcuate valley, which is flanked by the Northern and Southern Galala Plateaus, and finally opens into the Gulf of Suez in the northwestern part of the Red Sea (Fig. 1). In the Wadi Araba Valley, groundwater is abstracted from springs and wells scattered over a wide area (Fig. 2). The springs predominantly originate from the fractured aquifer forming the Galala Plateaus .
The northern part, which extends up to the Suez Governate, contains several thermal springs. Previous studies investigated their stable isotope composition, mineralization, the source of the enhanced temperatures and energy potential (Sturchio et al. 1996;Abouzied et al. 2020). Nassim (1990) applied aerial images to analyse lineaments that focus the surface water drainage, mapped springs in the region, and classified groundwater aquifers in Wadi Araba. Following local knowledge, a number of springs has been documented along the northern flank of Wadi Araba, usually flowing in small ponds used by small communities.
Particularly in the Southern Galala Plateau region, productive springs have been used since ancient times to provide water for settlements such as the two >1,500-year-old monasteries-St. Anthony's Monastery (SAM), located at the southern flank of Wadi Araba, and St. Paul's Monastery (SPM), located some 20 km further SE (Fig. 1). Within the area of SAM several springs emerge along an 800-m-long lineament and are intended as emergency source of water supply for 350 residents and for irrigation purposes. Unlike SAM, in the area of SPM a single spring emerges, intended to serve the same purpose.
Except for a few quality data of the main springs of St. Anthony and St. Paul (Nassim 1990), neither recharge mechanisms nor discharge dynamics of the springs that emerge in Wadi Araba and in these historical sites have been adequately investigated yet. According to the authors' own measurements, the largest of the SAM springs yields an average discharge of 87 m 3 /day and is flowing all year round, while many of the small springs along the northern rim of the valley fall dry. Thus, this study aims at mapping and characterizing the groundwater springs in Wadi Araba, with a special emphasis on the springs at SAM and SPM. The evaluation of satellite image data can contribute to: (1) the detection of fault and fracture zones influencing the groundwater flow and fluid permeability of rocks, (2) Episodic floods formed erosional fluvial landforms including large dry beds and a meandering bed in the central part of Wadi Araba deriving the youngest fault pattern cutting the most recent sediments as preferable infiltration pathways, (3) mapping of structural features and lithologic units which play a role in storing and transporting groundwater, and (4) monitoring of morpho-dynamic processes influencing the infiltration of surface water such as sediment flow, accumulation and erosion.
The chemical analyses of water samples from spring locations detected in the area clarify which kind of aquifer is discharging to these springs and what kind of water/rock interactions take place in the subsurface. The stable isotope signatures of 18 O and 2 H contribute to the understanding of (1) climate conditions at which the water was recharged and (2) whether their isotope signatures indicate contributions of groundwater from the Nubian Sandstone aquifer, which springs elsewhere in the region. The radio isotope analyses of 3 H and 14 C were used to calculate the mean resident time of groundwater in the aquifer, which also allows insight into whether or not the emerging groundwaters are at least partially recharged recently by occasional rainfall or flash floods.

Climate
The climate in the Eastern Desert of Egypt is arid and generally characterized by hot summers and moderate winters. Sporadic and short precipitation events take place in winter months (between October and April) and their spatial extension and intensity are extremely variable, e.g. the event on 27 October 2016 produced more than 100 mm of precipitation and caused a destructive flash flood in the town of Ras Ghareb (Saber et al. 2020), while north (Suez) and central (El Gouna) (Fig. 1), less than 20 mm of precipitation occurred. The orographic influence of the Galala Plateaus on cloud distribution in Wadi Araba may lead to rain-bearing cloud concentration above the Northern and Southern Galala Plateaus. Observations in Suez, which has the closest climate station to Wadi Araba, show erratic precipitation events with usually low amounts, less than 6 mm/day, while only a few events produced more than 10 mm/day (Fig. 3).
The climate data presented in Fig. 4 were obtained from El Gouna climate station, situated at the Red Sea coastal plain (27.43 E, 33.66 N). The monthly averages and standard deviations are calculated using the data measured in the period between March 2014 and January 2019 with a time interval of 10 min. The range between average ± standard deviation represents 68% of the data.
The highest temperatures are recorded between June and August (>30°C) and the lowest in December/January (<20°C). The relative humidity with an annual average of 47% shows a weak seasonal variation with a minimum between May and August (average 45%) and a maximum between October and December (average of 50%). Evaporation data from the region were reported by Awad et al. (1996) with a maximum annual mean value in June with 10.11 mm/day and a minimum in December with 6.11 mm/day.

Geomorphology
The geomorphological units of the landscape that are important for recharge and precipitation runoff can be differentiated.
The Northern Galala Plateau shows typical conical landforms related to weathering of limestones and dolomites during more humid conditions in the past. The uncovered, flat, exposed fractured carbonate rocks show good properties for recharge.
The Southern Galala Plateau is block-wise intersected by distinct visible escarpments related to fault zones (Fig. 1). This area shows traces of an intense erosion such as deeply incised valleys. Sand and gravel sediments are accumulated in small depressions. While the northern escarpment is forming an almost continuous line, the southern border of the Wadi Araba basin is characterized by dissected front lines, intersected by larger N-S-oriented valleys. Along the northern escarpments, block gliding and rockfall towards the southern direction are a common feature. This morphology enables recharge in the hollows and fissures left behind and the steep slopes promote more surface runoff. The drainage network indicates a relatively flat bedding of the rocks and is probably the result of parallel tectonic structures.
The interaction between fluvial and tectonic processes is evident as the development of channels is influenced by subsurface structures and geotectonic movements related to plate tectonic rifting activity in the Gulf of Suez area and northward movements of the African Plate.

Geology and tectonic
Wadi Araba is a structurally controlled tectonic basin (Fig. 5). The major force of the structural development in this area was   As a result of the regional compressional and extensional forces, along the Gulf of Suez (El-Naby et al. 2009), a complex structural system of numerous horsts and grabens with variable relief and dimensions developed (El-Naby et al. 2010). Over the entire region, the three major tectono-sedimentary features, i.e. Wadi Araba and the flanking Northern and Southern Galala Plateaus, are dissected by a complex pattern of NW-SE, N-S, and E-W directed faults. Both Plateaus consist of clastic sedimentary rocks of Late Paleozoic to Early Cretaceous age, and carbonatic strata of Cretaceous to Paleogene age (Kuss et al. 2000;El-Sadek 2009). The trend of the deep fault pattern, which affects the basement, the overlying Mesozoic, and recent sediments, oscillates around the E-W direction (El-Sadek 1998), and these fault patterns are assumed to be linked at depth with the NW-SE faults in northern Egypt (Abdel-Fattah et al. 2013). At depth, in the study area, two almost parallel basic to ultrabasic dykes exist, which divide it into two parts: an eastern shallow part and a western deep one (El-Sadek 2009). Moreover, the study area represents a series of horsts (domes or anticlines) and grabens (troughs, basins or synclines). A meridional cutting geological profile is based on a cross section provided by Höntzsch et al. (2011) and preceding works (Moustafa and El-Rey 1993;Schütz 1994;Moustafa and Khalil 1995;Scheibner et al. 2001; Fig. 7).

Hydrogeology
Along the Gulf of Suez, groundwater emerges in the form of springs and artesian wells from the Nubian Sandstone aquifer; this was recharged in the late Pleistocene (Gat and Isaar 1974;Sturchio et al. 1996). Abouzied et al. (2020) reported that flash flood events contribute to the recharge of groundwater in the Eastern Desert along the Gulf of Suez; however, these assumptions are yet to be proven by calibrated infiltration models or age determination.   In the study area, four major aquifers exist: (1) Quaternary aquifer, (2) Upper Cretaceous aquifer, (3) Lower Cretaceous aquifer and (4) the Carboniferous aquifer (Nassim 1990;Fig. 6). The Quaternary aquifer occurs along the major dry Wadi and gets recharged during the rare occasional rainfall events. Three wells with yield between 5 and 23 m 3/ day and one spring in the east of the study area derive water from that aquifer (Nassim 1990 ; Fig. 2).
The Upper Cretaceous aquifer (the formations of Wata, Raha, Matulla) hosts groundwater with total dissolved solids (TDS) of 1,200-8,000 ppm (Nassim 1990). The most productive aquifer is the Lower Cretaceous, which is known as the Nubian Sandstone, mainly recharged during pluvial times (Abdel Moneim 2005). This aquifer is composed of coarse sandstone and sands and has confined conditions in the central area of the case study. The groundwater in this aquifer is brackish with salinities of 1,000-2,600 ppm (Nassim 1990). The Carboniferous aquifer is mainly composed of sand and sandstone interlaced with clay layers that create confining conditions in the Zafarana area in the east. One disappeared spring, Abu Darag (Fig. 2), can be associated to that aquifer and discharged water with TDS of 2300 ppm.
Local nomads living in the area reported 30 groundwater emergences, of which 17 are springs and 13 locations are shallow hand dug wells or drilled wells that reach 86-1,200 m depth (Nassim 1990). Out of the 17 springs, 13 discharge from the Upper Cretaceous aquifer with average rates of 2.5 m 3 /day during summer and 5 m 3 /day during winter time, respectively (Nassim 1990).
The springs emerge at different elevations and from different stratigraphic units indicating that groundwater occurrence is not only influenced by lithologic properties, but also by the position and hydraulic properties of fractures and fault zones. Due to the still active neotectonic activities, faults and fractures have developed recently and play an important role in conduit development. In addition, karstification of the fractured limestones of the Galala led to the formation of cavities and a stochastic drainage system in the plateaus, hampering the exact determination of spring catchments.

Materials and methods
The interdisciplinary approach used in the scope of this study comprises hydrological field investigations, evaluation of remote sensing data, and geological, geomorphological and topographic data.

Water samples and analyses
Two rainwater samples were collected in the area of Hurghada in 2016 (R4) and 2018 (R5) to analyse major ion composition in rainfall. Both were supplemented by data from Askar (2014) and Hadidi (2016) for precipitation (R1, R2, R3). In the period between 2016 and 2019 water samples from 9 springs in the area of Wadi Araba were collected for chemical composition, stable isotopes and 3 H analysis (Table 1; Fig. 2). In addition to these samples, a floodwater sample was collected on 18 February 2019, 3 days after a rainfall event (≈3 mm event at El Gouna weather station), directly from the area of SAM, to analyse the 3 H content and stable isotopes in the provoking rainfall. The main spring at SAM was sampled for 14 C analysis to determine the groundwater residence time.
The field parameters temperature (T), pH, and electrical conductivity (EC) were measured at each location using WTW portable devices. Alkalinity was determined using gran titration. To analyse anions and cations in flood water and groundwater, samples were filtered in the field using a 0.45-μm filter and filled into 50-ml bottles. The cation samples were acidified by adding 0.2 ml of HNO 3 and all samples were treated with 0.2 ml of C 4 H 8 N 2 S, respectively. Major anions (NO 3 , SO 4 , Cl, Br) were analysed using a Metrohm 881 Compact Ion Chromatograph pro Anion-MCS. Major cations (Ca, Mg, Na, K) were analysed using an Agilent 715 ICP-OES. All analyses were performed in a laboratory of the Technical University of Berlin at the El Gouna Campus, Egypt.
Samples for stable isotopes of water were filtered into 2-ml bottles, finally sealed to prevent evaporation according to IAEA (2009) and later measured with a PICARRO L1102-i isotope analyser at Museum of Natural History in Berlin (Germany). The L1102-i is based on the WS-CRDS (wavelength-scanned cavity ring down spectroscopy) technique (Gupta et al. 2009).
The stable isotope ratios are expressed in the conventional delta notation (δ 18 O, δD) in permil (‰) versus Vienna Standard Mean Ocean Water (VSMOW). For each sample, six replicate injections were performed and arithmetic average and standard deviations (1 sigma) calculated. The reproducibility of replicate measurements is generally better than 0.1‰ for oxygen and 0.5‰ for hydrogen.
To determine the 3 H content in the water, 1,000 ml could be sampled in the southern escarpment, from springs S5-S9 only (Fig. 2). Tritium analyses were carried out by applying the electrolytic enrichment using Liquid Scintillation Counting (LSC) in the Hydroisotop Laboratory, Schweitenkirchen (Germany) with a detection limit of 0.5 TU. Values are reported in tritium units (TU) where 1 TU equals an activity of 0.119 Bq/L (IAEA 1992). The tritium half-life time is (4,500 ± 8) days (≈12.3 years; Lucas and Unterweger 2000).
To estimate the groundwater age of the recent water, 3 H input function of the sampled water must be reconstructed for several periods in the past using the tritium values for the rainfall in the country (if available) or neighbouring regions (IAEA 1990;Geyh et al. 1995).
Following Małoszewski and Zuber (1982), the mathematical form to describe the input-output concentration of the tracer under steady state flow condition is given by Eq. (1): where C out (t) is the tritium output, C in (t-τ) is the tritium input, h(τ) is the system response function, λ is the radioactive decay constant, t is the time scale and τ is the transit time.
To calculate residence time distributions, the hydrological system has to be conceptualized, which might either follow the piston flow, the completely mixed reservoir or dispersion approach (Małoszewski and Zuber 1982). Assuming that groundwater is being recharged in the catchment area and gets mixed with groundwater in the aquifer, the exponential model was applied to calculate the mean residence time (Eq. 2) where τ 0 is the turnover time of the system. τ 0 is identical to the mean transit time in the uniformly mixed reservoir (Yurtsever 1983). The system response function described by Yurtsever (1983) represents the distribution of the transient time of the tracer input-output function.
14 C analysis allows the estimation of groundwater age between 1,000 and 40,000 years Fontes 1980, 1983;Wassenaar et al. 1991) depending on its activity in the total dissolved inorganic carbon (DIC). One sample (1 L) was collected from the main spring S5 at St. Anthony to determine the groundwater age using the 14 C technique. Emphasis was placed on the minimization of gas exchange between emerging the groundwater and atmosphere. The almost neutral pH (7.82) ( Table 1) was later enhanced to a value of 12 by adding NaOH. Later, CO 2 from the sample is separated by adding phosphoric acid. The 14 C activity was subsequently measured using an accelerator mass spectrometry (AMS) at the Hydroisotop Laboratory, Schweitenkirchen (Germany).
The activity ratio of radiocarbon is given in percent modern carbon (pMC) as described in Eq. (3): where A SN represents the normalized specific activity of the sample; A ON represents the normalized specific activity of oxalic acid (Stuvier and Polach 1977;Mook and van der Pflicht 1999). Groundwater ages (t) are calculated from the general law of radioactive decay and are reported as conventional radiocarbon age in years before present, i.e. before the year 1950 (Geyh 2000;Eq. 4).
A 0 is the initial 14 C activity in the recharge, A is the measured activity in the sample, and λ C is the Cambridge decay constant (λ C = 1/8267 year −1 ) which is based on the physical half-life of (5730 ± 30) years. For uncorrected water ages, A/A 0 is equal to A SN /A ON . To correct the 14 C value, 13 C (δ 13 C-DIC) was measured using an isotope ratio mass spectrometer (IRMS) and expressed as permil (‰) referenced to the Vienna Pee Dee Belemnite (VPDB) standard: 1 sigma = ± 0.3‰. The sample was analysed in Hydroisotop Laboratory, Schweitenkirchen (Germany).
All determined analyses are presented in Table 1. Since only one floodwater sample could be collected, additional flood water analyses from the El Quseir area (F1) and from El Gouna and Hurghada area (F2-4) were added from literature (Awad et al. 1996).

Hydraulic head measurements in spring S5
Between Nov 2018 and Feb 2019, the discharge and temperature of the major spring S5 in St. Andrews was permanently (30 min interval) recorded applying a temperature and watertable recorder at the artificial tunnel feeding S5.

Remote sensing and GIS-analysis
For the present study, optical, radar and digital elevation data were integrated into the GIS data bank. The satellite data were digitally processed using image processing (ENVI, SNAP) and ArcGIS software. The chosen image processing and RGB-combinations were focused on the enhancement of geologic, tectonic and surface water information. Information on the soil/sediment properties is very important in the scope of these studies as they have an effect on the infiltration capacity of surface water and water storage.
With its 14 spectral bands from the visible to the thermal infrared wavelength region, and its spatial resolution of 15-90 m, ASTER images were included in these investigations, especially the thermal bands. The RGB images created with the bands 10, 12 and 13 were converted from raster into polygon-shapefiles. The lowest values were extracted to get information on potential soil humidity lowering the reflectance in the thermal spectrum. The thermal bands of Landsat 8 were included as well.
Special attention was directed at precise mapping of traces of the tectonic pattern visible on satellite imageries, predominantly on areas with distinct expressed linear features (tonal linear anomalies, geomorphologic linear features, etc.). Lineament analysis helps to reveal traces of the tectonic structure. The term lineament is a neutral term for all linear, rectilinear or curvi-linear elements. Lineaments are often expressed as scarps, linear valleys, narrow depressions, linear zones of abundant watering, drainage network, and geologic anomalies. Tonal linear anomalies such as the linear arrangement of pixels depicting the same colour/grey tone were visually mapped as linear features as well as lineaments.  (Sonntag et al., 1978) Flood sample Rain samples (Eissa et al, 2013) Sinai recent recharged groundwater (Gat and Issar, 1974) Northern Galala samples Ain Sokhna (Sturchio et al., 1996) Southern Galala samples

St. Anthony spring S5
Nubian sandstone in Eastern Desert (Sultan et al., 2007) Nubian Sandstone-Western Desert (Schneider, 1986) Deep groundwater Eastern Desert (own data) Average stable isotopes values in rainfall (IAEA, 2020) S1, S2, S3 Ain Sokhna spring (Sturchio et al., 1996) b Fig. 8 a 18 O-2 H values in springs of Wadi Araba and their relation to other samples from Eastern Desert (Gat and Issar 1974;Sultan et al. 2007) and Western Desert (Schneider 1986). The Paleo Meteoric Water Line (PMWL) is constructed according to Sonntag et al. (1978). b Relationship between d-excess and δ 2 H in springs of Wadi Araba and their relation to other samples from Eastern Desert, Sinai Peninsula Structural features and lithologic units are better visible when merging RGB image products with filter-tool enhanced images. Low-pass and high-pass filters and directional variations were used for the detection of subtle surface structures. Merging the "morphologic" image products derived from "morphologic convolution" image processing in ENVI software with RGB imageries, the structural/tectonic evaluation feasibilities were improved. Surface-water input and resulting infiltration might be influenced by the fault and fracture pattern in the subsurface leading to a relatively higher permeability and hydraulic conductivity.

Stable isotope content in flood water and groundwater
The floodwater sample F1 lies on the local meteoric water line (LMWL) and the global meteoric water line (GWML; Craig 1961; Fig. 7), indicating no fractionation of the rainfall's signature due to evaporation. Comparing this sample with rainfall samples from the Sinai Peninsula (El Sayed 2006;Eissa et al. 2013) shows that rainfall in Wadi Araba is isotopically heavier than the Sinai rainfall. That might be related to the air mass movement from S to N (NOAA HYSPLIT model using GDAS meteorological data), indicating a rainout of heavier isotopes along the path and depleted signatures in Sinai.
After Dansgaard (1964), the deuterium excess value (d) can be determined with d = δ 2 H -8δ 18 O. This parameter is strongly dependent on the relative humidity and kinetic effect during evaporation (Merlivat and Jouzel 1979). A d-excess of 8.2‰, as calculated for F1, indicates a relative humidity of about 80-85% (Gonfiantini 1986; Pfahl and Sodemann 2013), preventing significant evaporation prior to or during runoff.
Similar to the floodwater sample F1, groundwater samples (S1-S9) from the springs (with an average value of δ 18 O = −6.55‰, δ 2 H = −40.9‰) lie on the GMWL too. That fact confirms a fast infiltration process where evaporation does not play a role (Fig. 7a), usually provided by fast infiltration into highly permeable sediments or open (karst) conduits, which swallow precipitation immediately.
Groundwater from S6 and S7 show the most depleted δ 18 O and δ 2 H ratios, revealing either cooler recharge conditions or a higher elevated catchment area. Contrastingly, S4 which is located on the northern flank of Wadi Araba, might be recharged at a lower morphologic position compared to the other springs. Isotope values from S1, S2, and S3 resemble those of the famous Ain Sokhna, which discharges in Suez from the NSAS and is assumed to have been recharged during the late Pleistocene (Sturchio et al. 1996). That similarity may point to the same feeding aquifer and will be discussed in section 'Concentrations of selected major ions and trace elements'.
In contrast to the spring waters, all groundwater from the deep wells in the area of Hurghada, having an average signature of δ 18 O = -5.8‰ and δ 2 H = −48‰, plot well below both the LMWL and GMWL, indicating a systematic evaporation effect. Since the trend line formed by the deep groundwater samples (Fig. 8a) starts from the GMWL close to the isotope ratio of S7, the source of recharge might be similar to S7, but the recharge process is much slower, providing room for evaporation. In that case, the trend line is an evaporation line. Alternatively, the recharge occurs similarly fast as for S7 but mixes later in the subsurface with a second, highly evaporated fluid. In that case, the trend line represents a mixing line. In any case, these deep groundwater samples are more enriched than the samples originating from the Nubian Sandstone in the Western Desert, proven to have been recharged in the Pleistocene era (Sturchio et al. 2004).
Plotting the d-excess of the water against δ 2 H (Fig. 8b), two groups of groundwater appear: (1) the deep groundwater with d-excess of <3 and (2) the springs from the Northern and Southern Galala Plateaus with higher d-excess values of >5.

Concentrations of selected major ions and trace elements
Precipitation along the Gulf of Suez is dominated by Cl, SO 4 , Na and Ca ions, indicating either the uptake of sea spray or the wash out of halite and sulphate from the atmosphere (Fig. 9a). According to the isotope results, the enhanced mineralization of sampled floodwater (F1) from Wadi Araba, which is 5-15 times higher in salinity than floodwaters from Hurghada and El Quseir, respectively (Fig. 9b), results from interaction of precipitation with the bedrock formations, which collect and transport the runoff. The variability of the Na/Cl ratios in the floodwater in the area of Hurghada (0.82-2.34) indicates different origins, while a value of 0.83 resembles the ratio in seawater, the Na/Cl of 2.34 refers to a different source of Na, probably from weathered plagioclase-rich rock formations (Siebert et al. 2014(Siebert et al. , 2019. As these formations do not exist in Wadi Araba, the Na/Cl ratio of the floodwater (F1) 0.97 refers to dissolution of halite, which is abundant in the region and has Na/Cl ratio of 1. The enhanced Ca, Mg and SO 4 concentrations in floodwater F1 compared to rainwater are the result of dissolution of gypsum, limestones and dolostones, which are exposed in the Southern Galala Plateau, and the release of these ions during the weathering process. Detectable fluorine (F) concentrations in the Hurghada area indicate release of that element from apatite and fluorite, which occur in the igneous metamorphic rocks of the Red Sea Hills. Since these rocks are missing in Wadi Araba, F is missing in floodwater from Wadi Araba.
To bring spring waters into context to earlier studies, analyses from Ain Sokhna spring (Sturchio et al. 1996) and Ain El-Boweirat (Nassim 1990 ; Fig. 2, currently dry) were included in Fig. 9c, where spring water composition is given.
Groundwater in the southern Galala seem to have lower fluorine concentrations than groundwater from the northern rim.
While Ca and SO 4 content in spring waters is lower than in floodwater, Cl, Na and Mg concentrations in spring waters are  Fig. 9 Schoeller-plots of the hydrochemical analysis of a rainfall samples, b floodwater samples and c groundwater samples. Ain Sokhna sample is derived from Sturchio et al. (1996) and El-Boweirat sample (Central area of Wadi Araba) from Nassim (1990) larger than those of the floodwater F1. That issue refers to the dissolution of aerially imported gypsum by floodwater, while groundwaters dissolve halite and Mg-minerals, which may exist in the local aquifer. Calculation of molar Ca/Mg ratio shows that groundwater in springs from northern Galala (S1-S4) receive slightly more Mg from aquifer rocks than those from the southern Galala (S5-S9). While the former show low ratios (0.83-0.98) those of the latter range from 1.25 to 1.7, indicating different aquifers of origin. Floodwater ratios are much higher (6.02-6.52), which is the result of additional gypsum, which does not originate locally. Considering SO 4 /Cl equivalent ratios, differences are again observable between both spring regions. Groundwater in the northern rim shows ratios of 0.24-0.49, while those in the southern Galala springs are higher (0.56-0.87). Separation is also given comparing Mg/Cl equivalent ratios, which are high in the north (0.41-0.49) and low in the south (0.18-0.41), which indicate a larger content of dolomite in the aquifer feeding the northern springs compared to that supplying the southern group of springs. This is supported by saturation indices (calculated applying PHREEQC Interactive, version 3.3.9 (USGS 2020)).
Groundwaters in the north are oversaturated in respect to dolomite, while those in the south are undersaturated.
Concerning the absolute salinity expressed as TDS, all groundwaters are brackish (1,500-2,600 mg/L) and not suitable as potable water. The modified Piper diagram (the position of Cl and HCO 3 + CO 3 is switched for a better representation, Fig. 10) shows the hydrochemical characteristics of the spring water.
Groundwater samples S5 and S8 seem to be not influenced by salinities which increase in S9 at St. Paul and continue to increase in S7 and S6. Ain Sokhna is most saline and has a different composition than all other groundwater samples in the northern group, referring to a different aquifer of origin. The dissolution of calcite and dolomite is lowest in groundwaters from S6 and S7, while the alkaline concentrations are highest. Field observations indicate the spring water seems to be partly recharged from the thick rubble overburden, which serves as storage. Though springs at S6 and S7 emerge close to S5 and S8, they emerge from different aquifers. While S5 and S8 originate from fractures and fissures in limestone, S6 and S7 obviously flow out of the shallow unconsolidated material. Hence, the hydrochemistry between both types of springs differ as stable isotope do.

Springs with detected 3 H
The tritium content detected in the groundwater samples in the area of SAM and SPM are equally low. The detectable concentrations belong to samples S6, S7 and are in a range between 1.1 and 1.5 TU (Table 1).
There are no records for 3 H content in precipitation at the Red Sea Coast.  (Table 1) enables an estimation of the mean residence time of the groundwater which contains 3 H.
Equation (2) was applied to calculate the tritium content curves for different time intervals 0, 5, 10, 15, 20, 25, 30 years for the 3 H (average weighted mean value) in the Bet Dagan station . The curves were calculated using a simple calculation model which was developed initially by Yurtsever (1983 S6, S7 and S9 in the sampling year 2019 after three periods of 5, 10 and 15 years. The calculated residence times which correspond to the measured contents are 15 years for springs S6 and S9, and 10 years for spring S7. Depending on the concentrations of 3 H in rainwater, the recent proportion of the water in S6, S7 and S9 could be estimated between as 29% and 39% in the reservoirs that feed these springs. 14 C in the main spring S5 of SAM In sample S5, which came from the permanent spring at SAM, the activity of 14 C and the content of 13 C in total DIC were determined as 3.86 pMC and − 7.8‰, respectively. Depending on the radioactive decay, the age can be determined (Eq. 5; Clark and Fritz 1999).
where t is the half time of 14 C (5730 ± 30 years), a 0 is the initial activity of 14 C, a t is the measured activity in the sampled water and q is the dilution factor. Schneider (1986) estimated the initial activity a 0 as 85 pMC when estimating groundwater age from the Nubian Sandstone in the Western Desert of Egypt. This value was adopted for the calculations made here, and leads to an age estimation of 25,690 years which likely represents an overestimate (Buckau et al. 2000).
The concentrations of ions in S5 (Table 1) show that the ionic ratio Ca:Na is 0.56, confirming a contribution of calcite minerals from the aquifer, hence the correction model should consider the dissolved carbonate in the aquifer and consequently the δ 13 C. Pearson (1965) and Pearson and Hanshaw (1970) defined the dilution factor for the δ 13 C correction model (Eq. 6) as: where δ 13 C DIC is the measured 13 C in groundwater, δ 13 C soil is equal to δ 13 C of the soil CO 2 (usually close to −23‰), and δ 13 C carb is equal to δ 13 C of the calcite being dissolved (usually close to 0‰) (Clark and Fritz 1999). Calculating q results in a dilution factor of 0.3 and t is consequently 15,856 years.

Processing of satellite images
The structural/geologic evaluation of optical satellite images and of radar data allows a reasonably precise mapping of larger lineaments, many of which are probably fault zones (Fig. 11). The evaluation of Landsat 8 and Sentinel 2 based filter images (Fig. 10a), which enhances surface topography, reveals prominent linear and parallel features. In parallel, equidistant linear features become visible on the satellite images processed with directional filters in ENVI in the areas covered by the youngest sediments. These parallel lineaments might reflect the stress pattern of the actual geodynamic activity. The most prominent lineaments are oriented in the NW-SE and NNW-SSE directions. The orientation and occurrence of these prominent linear features on the satellite images are obviously related to the subsurface tectonic pattern, Legend c d Fig. 11 (continued) characterized by block faulting (Nassim 1990). Larger fault and fracture zones might have an influence on groundwater flow as indicated in Fig. 11c, showing the example of the area of SAM. Fracture zones are most important for infiltration, migration, and storage of groundwater. The transmissive properties of the dominant rocks in this area are dependent on the presence of fracture zones to provide porosity and permeability. Whenever surface-water infiltrates after precipitation, it is likely that the infiltrated water flow is concentrated along the more permeable fracture zones. In the case of the St. Anthony area, several larger fault zones are intersecting, striking in SW-NE, NW-SE, N-S and NNW-SSE directions. The springs are aligned along the SW-NE-oriented fault zone bordering the Wadi Araba basin in the south. The lowest and flattest areas are more susceptible to higher surface water flow. During extreme rainfall events in the flatter areass, the run-off concentration is slower and infiltration in sediments and structures is possible. Wadis were developed along weak zones representing fractures or faults; therefore, groundwater recharge may take place in the flat part and the wadis of the Galala Plateaus.
Hydraulic head measurements at the main spring S5 of SAM Over the period of observation, the hydraulic head in spring S5 oscillated between 1.14 and 1.15 cm. Because of the exchange of air in the cave where the spring occurs, the temperature differences in the measurements reflect the differences of air temperature (Fig. 12).

Discussion
The recent opening of the Gulf of Suez has extended N-S and perpendicular striking fractures in the area of SAM which is evident on the geological map (Fig.5) and on the satellite image (Fig. 11c). The springs S1, S2, S3 and S4 at the Northern Galala Plateau are not permanently flowing and the recharge probably takes place mainly in karst features (Fig. 11b) after rain events. The temperature of the water in the northern samples reflects the mean air temperature (Fig. 4) and is different from the spring water at the Southern Galala Plateau. The residence time seems to be relative short and the ion content is similar to that of the floodwater. The flow of these springs decreases slowly after a stormwater event, and during sampling the water was almost stagnant. Apparently, no reservoirs of the limestone beds can store water for long periods in the system, resulting in a continuous discharge from the springs.
The satellite images and field inspection at the springs of Southern Galala (S5, S6, S7, S8, S9) show evidence of block sliding at the South Galala escarpment (Fig. 11c) map (Klitzsch et al. 1987;Fig. 5) together with the geological section of Höntzsch et al. (2011; Fig. 7) confirm that the Southern Galala springs are situated at the ESE-WNW striking Wadi Araba Fault, at the boundary of pre-Campanian marls and Campanian carbonates (Scheibner et al. 2003). Groundwater from the Nubian Sandstone can rise up here because the fault at this site forms a barrier for the groundwater flow. In this context, the age (15,000 years) of S5 water and the absence of 3 H in the S8 sample may refer to a partial origin from the Nubian Sandstone aquifer. The field observations in the area of the SAM and SPM confirm the existence of caves and the cavernous limestone in the escarpment. These caves seem to play the most important role in the regulation of the constant flow of the springs S5 and S8 in St. Anthony. Rainwater from different event intensities infiltrates directly in fractures and fissures at the rim of Galala escarpments. This infiltration process seems to influence the water in springs S6 and S7 where 3 H was detected.
The main storage medium of the water for all springs at the southern Galala Plateau must be the karstified limestone of the Maastrichtian age positioned above the thick marls of Campanian age. The water accumulates in huge caves from where the water flows in very tight fractures to the springs. The variation of hydraulic head in the reservoir caverns is very low; thus, the discharge of the springs is relatively constant, as proven by various measurements during the field work. The age of the spring water S5 proves this hypothesis.
Accordingly, two spring groups can be identified on the springs line: S5 and S8 (SW group) in the west, and S6 and S7 (SE group) east of the others. The stable isotope contents are also different in the two groups and the water from the SW group is slightly lighter than that from the SE group. The chemical composition of the SW group is quite different from that of the SE group. The electrical conductivity of the water from the SE group is much higher than that of the SW group but it is the sodium chloride content which makes the difference. In the floodwater from recent rainfall, the sodium chloride content is almost the same as in the SE springs. As no evaporites are shown in the column profiles from Scheibner et al. (2003), and the NaCl content in the water from the SW group is almost half, the sodium chloride content of the SE springs has its origin in recent rainwater. Both springs are located lower at the escarpment than the springs of the SW group and rock rubble lies above and around them. The sulphate content can be explained in the same way. The rainfall seems to discharge the higher salty water at the foot of the escarpment. The easternmost spring (Agathon, S7) occurs at a depth of some meters in an excavation and proves the mixture of young and old (fossil) water. As springs S6 and S7 are located in areas with higher soil moisture (Fig. 13), it is possible to find further springs which might be located in the piedmont planes and alluvial fans that might contain relatively higher soil moisture (Fig. 13b).

Conclusions
In the Galala Plateaus, and including the Wadi Araba, periodical heavy rainfall occurs every 3-5 years, recharging the local and regional aquifers. Unconsolidated wadi sediments in, as well as fissured flat areas on top of the plateaus serve as collectors for parts of these heavy rainfall events.
The analysed floodwater in Wadi Araba indicates no fractionation of the rainfall signature due to evaporation and is isotopically heavier than rainfall from the Sinai Peninsula. This water expresses influence from the halite deposits common in the area. Floodwater from Hurghada area is more influenced by weathered plagioclase-rich rock formations in the Red Sea Hills.
The periodic springs at the Northern Galala Plateau are fed by a densely fractured (or karstified) aquifer without retention potential, which is observable from spring water temperatures and chemical compositions that are similar to that of rainfall on one hand and the fast increase and drop of discharge briefly after rainfall starts and ends, respectively. The springs emerge where lineaments are intersected, indicating a structurally controlled flow-paths regime in the background.
Contrasting the extremely variable discharge of springs at the northern plateau, St. Anthony spring, which discharges on the edge of the Southern Galala Plateau, shows no measurable fluctuations, referring to a large reservoir and an aquifer with large retention. The locations of the springs on the Southern Galala escarpment are characterised with high moisture content. It is very likely that further springs can be found at other high-moisture locations like S8 and S7 which are partially recharged from the flood events.
The area of SPM shows a high density of crossed horizontal and vertical lineaments which seems to play a role in transporting recent recharge to the spring S9. Processing of the satellite images shows that not only structures that are important for pathways of groundwater can be extracted, but also possible near-surface groundwater in the wadis that lead to Wadi Araba, which would have to be explored more closely in the future. made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.