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the world (PAWR 1986). Meteorological data from the. Thumrait weather station, located at 80km north of the Salalah Airport, was selected for this study because ...
Environ Earth Sci DOI 10.1007/s12665-011-1331-2

ORIGINAL ARTICLE

Evaluation of groundwater dynamics and quality in the Najd aquifers located in the Sultanate of Oman K. Al-Mashaikhi • S. Oswald • S. Attinger G. Bu¨chel • K. Kno¨ller • G. Strauch



Received: 25 August 2010 / Accepted: 23 August 2011  Springer-Verlag 2011

Abstract The Najd, Oman, is located in one of the most arid environments in the world. The groundwater in this region is occurring in four different aquifers A to D of the Hadhramaut Group consisting mainly of different types of limestone and dolomite. The quality of the groundwater is dominated by the major ions sodium, calcium, magnesium, sulphate, and chloride, but the hydrochemical character is varying among the four aquifers. Mineralization within the separate aquifers increases along the groundwater flow direction from south to north-northeast up to high saline sodium-chloride water in aquifer D in the northeast area of the Najd. Environmental isotope analyses of hydrogen and oxygen were conducted to monitor the groundwater dynamics and to evaluate the recharge conditions of

groundwater into the Najd aquifers. Results suggest an earlier recharge into these aquifers as well as ongoing recharge takes place in the region down to present day. Mixing of modern and submodern waters was detected by water isotopes in aquifer D in the mountain chain (Jabal) area and along the northern side of the mountain range. In addition, d2H and d18O variations suggest that aquifers A, B, and C are assumed to be connected by faults and fractures, and interaction between the aquifers may occur. Low tritium concentrations support the mixing assumption in the recharge area. The knowledge about the groundwater development is an important factor for the sustainable use of water resources in the Dhofar region. Keywords Environmental isotopes  Groundwater  Najd aquifer  Oman  Recharge  Water quality

Electronic supplementary material The online version of this article (doi:10.1007/s12665-011-1331-2) contains supplementary material, which is available to authorized users.

Introduction K. Al-Mashaikhi  S. Attinger  G. Strauch (&) Helmholtz Centre for Environmental Research, UFZ, Permoserstrasse 15, 04318 Leipzig, Germany e-mail: [email protected] K. Al-Mashaikhi Ministry of Regional Municipalities and Water Resources (MRMWR), Salalah, Sultanate of Oman S. Oswald University of Potsdam, Institue of Earth Sciences, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany S. Attinger  G. Bu¨chel Friedrich Schiller University Jena, Institute of Geosciences, Burgweg 11, 07749 Jena, Germany K. Kno¨ller Helmholtz Centre for Environmental Research, UFZ, Theodor-Lieser-Str. 4, 06120 Halle/Saale, Germany

The study area is located in the Dhofar Governorate in southern Oman. Locally, this region is known as Najd and covers an area of approximately 88,000 km2 (Fig. 1). The landscape is dominated by heterogeneous terrain with major valleys, small hills, and rolling sand dunes on the northern edge. Vegetation here is sparse, mainly consisting of desert shrubs, but more substantial vegetation can be found in the vicinity adjacent to the Dhofar Mountain Range in the south. The Najd region contains 5 towns and 30 villages with a collective population of approximately 21,000 inhabitants, and has been classified as one of the most arid zones in the world (PAWR 1986). Meteorological data from the Thumrait weather station, located at 80 km north of the Salalah Airport, was selected for this study because of

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Fig. 1 Simplified map of the study area (above), and Geological map of Dhofar Governorate (Map Source: MRMWR) with the cross sections A–A0 and B–B0

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the availability of an extensive database regarding historical weather patterns including air temperature, rainfall amounts, relative humidity, and evaporation (Supplementary material Table 1). The average yearly air temperature varies between 6.1 and 44.6C, with a grand mean of 26.2C. Rainfall amounts are generally low and irregular, with a yearly average of 31.2 mm year-1, but extreme values of 144.6, 227, 131.5, and 76 mm year-1 have occurred in 1983, 1989, 1992, and 2007, respectively. The climate in this region also lends itself to high potential evaporation rates and yearly averages of 161.4 mm year-1 typically occur. Another weather station, Qeroon Hairiti, located 50 km from the Thumrait station on the southern end of the research area has average rainfall amounts of 252 mm. The main Najd groundwater basin is geologically restricted within the Hadhramaut Group, but then separates into four distinct aquifers labelled A through D from top to Table 1 Geological units and hydrogeological characteristics of the study area (derived after GRC 2007)

bottom, respectively. The names (A–D) appear to have first been introduced by Hydrotechnica (1985), and these names were subsequently carried over by Mott MacDonald International (1991, 1994), PAWR (1986), and MWR (2000). The aquifer A occupies structures in Rus, Dammam formation and other layers. The B, C, and D aquifers are formed in Umm-Er-Radhuma (UER) formation and they are mostly confined (see Fig. 1; Table 1). This aquifer groundwater flow direction is to the north/northeast and creates artesian flow with high pressure near the international border with Saudi Arabia. Prior to 2004, exploration wells for these aquifers were very limited and only covered small portions of the aquifers. Therefore, previous research studies have primarily focused on surface waters, precipitation, and limited groundwater samples (PAWR 1986; Macumber et al. 1995). As a consequence of it, comprehensive sampling regimes were clearly needed to characterize the aquifers.

Thickness range, Age

Group

Formation

Aquifer

Average (m);

Lithological description

[n=data]

Interbedded reddish to yellowish siltstone

Marsawdad

Faris

Oligocene to Miocene

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ unconformity ~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~

and grey silty limestone Reddish conglomerates, siltstones and

Shisr

20

Grey to white micritic limestone, and

Montasar

places of brecciated limestone.

Dawqah ~~Pre-Neogene

Brecciated and lacrustrine limestone. A ~~Pre-Neogene Unconformity ~~

Unconformity ~~

Tertiary

3 - 109

Massive and thin bedded nodular limestone

30

with marl, yellow to orange shale with marl

132

and limestone.

9 – 229 Eocene

Rus

113

Hadhramaut

143 27 – 272 B

102 120 199 – 360

Paleocene

Cenozoic

Damman

Upper UER

Lower UER

limestone.

C&D

Shammar Shale

Breccia, chalky dolomite, marl and laminated gypsum. Grey to brown dolomitic limestone, very weak white and brown biomicrite and bluegrey shale. Brown granulated and fossiliferous limestone at base. Moderately weak olive, sparry limestone

297

interbedded with brown fossiliferous

11

limestone.

?

Grey brown, dark grey limestone and blue shale. Green mudstone interbeds.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ unconformity ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mesozoic

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Previous studies have attempted to describe the general hydrochemical parameters and provided estimation for the groundwater recharge rates in the Najd region (MRMWR 2005). Based on environmental isotopic sampling of the artesian groundwater from the UER formation by Clark and Fritz (1997), Clark et al. (1987) concluded that it originated under a more humid environment between 4,000 and 30,000 years ago, but could be also recharged recently by monsoon and cyclonic events (Macumber et al. 1995). In 2005, a study conducted at the University of Neuchaˆtel in Switzerland carried out chemical and isotope analyses of two natural springs and ten boreholes from aquifer C (GRC 2005). These analyses were performed by GRC which concluded that the groundwater in the C aquifer was more than 3,000 years old. The goal of this study was to review the general hydrogeochemistry of the Najd aquifers including new wells drilled by the Ministry of Regional Municipality and Water Resources (MRMWR) after 2005 and to elucidate dynamic trends in the groundwater from aquifers A through D. It also focused on the chemical behaviour and isotope distribution within the groundwater to determine how major ion concentrations and environmental isotopes vary in this system. This is the first study to extensively characterize the entire Najd system. Furthermore, the current study provides valuable hydrochemical and isotope data that can be used for future work on these formations.

Geological and hydrogeological review of the Najd area Structures and geological characteristics The Najd area is affected by two major faults that form a Graben structure. The characteristic geology in this region has been described previously by Watson Hawksley (1983); MacDonald (1991) and MPM (1992a, b), but mapping was not done presumably because of inadequate data at the time of the previous studies. Recent data obtained during MRMWR drilling project in Hanfeet Area indicated that the western fault of this structure passes through the western part of the Hanfeet Well Field (GRC 2007). The absolute direction of the fault is not clear, but it is known to pass through the middle of this Well Field. The eastern fault probably begins on the northern side of the mountains (recharge area), at about 30 km east of Thumrait, and runs in a north/northeastwardly direction towards Qitbit. These two major faults delineate a Graben structure, and uplift occurs on both sides of the thrust (Graben) faults. Several other faults are also known to occur in the area, but they are only expected to have local effects. The layers of Neogen and Tertiary formations dip from south to north in the eastern part of the study site and northeast in the

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western portion of the region near the international border with Yemen. Average hydrological gradients are assumed to be 0.002 (PAWR 1986). The area includes recent Alluvium and two stratigraphic units called the Fars Group and Hadhramaut Group, respectively, (MacDonald 1991; MPM 1992a, b) and have been highlighted on the geological map of the Dhofar Governorate (Fig. 1, lower map). The Alluvium mean thickness is about 4 m and can be ignored for the purposes of groundwater exploration. A limited portion of the northern Fars Group was reported by GRC (2005) to be in the range of zero to 78 m with an average of 29 m. Compiling old data with those from recent projects, the thicknesses of the Fars Group was thus considered in a range from 5 to 78 m with an average thickness of 20 m. Although the thickness of this group increases from central of Najd towards the north (above WGS84 UTM northing coordinate 2055539), this area does not represent a realistic target for groundwater retrieval from the Najd formation. The more favourable and important Hadhramaut Group originated in the Tertiary period, as is common for large areas of southern and eastern Arabia. The thickness of the Hadhramaut Group formations has been discussed in previous studies by PAWR (1986); GRC (2007) and other unpublished reports. While compiling old data records with the recent ones distributed in all Najd in order to find out a reasonable thickness of these formations, an average thickness of about 399 m was estimated and reported in Table 1. The Hadramout Group consists of chalky, crystalline and dolomitic limestone, shale, marl, siltstone, conglomerates, and gypsum. This group comprises the formations of Dammam (Eocene to possible lower-Eocene), Rus (lowerEocene), and Umm Er Radhuma (UER, lower-Eocene to Paleocene). Numerous marine fossils from lower-Eocene and Paleocene ages have been discovered in the UER formation, and this formation can be divided into either two (upper and lower UER) or three (upper, middle, and lower UER) units. In either case, the base of the upper UER formation is generally considered to coincide with an Eocene–Paleocene boundary. However, the base of the lower UER formation (Paleocene age) can be detected by the appearance of Shammer Shale. For better understanding, two hydrogeological cross sections have been created in the study area taking directions from south to north (A–A0 ) and from west to east (B–B0 ) (see Fig. 1, lower map). These cross sections extend through distance more than 230 km and pass through small occurrences of the Alluvium and the Fars Group. Because of their thickness of \5 m along the study area, they only appear in the geological map. The cross section A–A0 was constructed from boreholes located in aquifer D and C (Fig. 2). This section shows the dipping of all layers

Environ Earth Sci South

North

ELEVATION

m.a.sl 600 550 500 450 400 350 300 250 200 150 100 50 0 -50 -100 -150 -200 -250 -300 -350

DEP-7 DEP-5

DEP-3

W.Bharna

RBK-D

Bin Kh

WWD-25

WWD-34

D Water Level Piezometric Surface

Fars Group Dammam

D

D

Aquifer A

D

Rus

C Aquifer B

C

Fault

U.UER

C Aquifer C & D

C or D : Borehole in which aquifer 0 20 DISTANCE (km)

40

C L.UER

Expected Base of L.UER

60

80

100

120

140

160

180

200

220

240

West

East

ELEVATION

m.a.sl

DEP-14A DEP-9A

DEP-15

UbarFr

H.R.C

Ranada1

DEP-18

( WSW-3 )

400 350 300

Water Level Piezometric Surface

B

C

250 200

Rus

150

Aquifer A

100

C

50

Uper UER

C

D

Aquifer B

0 -50 -100 -150 -200

D

Legend Fars Group Alluvium Dammam Formation Rus Formation U.UER Formation L.UER Formation Shammar Shale

L. UER

C Aquifer C & D

D Expected Base of L.UER

Fault

B, C or D: Borehole in which aquifer

0 20 DISTANCE (km)

40

60

80

100

120

140

160

180

200

220

240

260

280

Fig. 2 Schematic cross sections along the study area of the Najd region: a South-North hydro-geological cross section A–A0 , b West-East hydrogeological cross section B–B0 through middle of Najd

northward and the appearance of Fars Group north of borehole W.Bharna. Moreover, the piezometric water level was detected above ground level surface (artesian) between boreholes W.Bharna and Bin Khawtar. The second cross section (B–B) is conducted through the middle of the Najd using boreholes located in aquifers B, C, and D. It starts from the west near the border of the Republic of Yemen to borehole Ranada 1 in the east north of Murmool, following borehole WSW-3 at Murmool Rahab Farm. This borehole is located 50 km south of borehole Ranada 1 and has been drilled through the base of L. UER into the Shammar Shale formation (below UER; see Table 1).

Hydrogeology of the Najd region In the Hadhramaut Group, four aquifers from A through D have been defined in the study area and are related to a karstic environment, particularly the aquifers C and D. Despite three of them are located in the UER formation (B, C, and D) and the upper aquifer A is restricted to the Damman and Rus formation, their hydraulic character is variable and connections exist (GRC 2007). These variations such as saturation thickness zones, discharge, transmissivity, and storage coefficient are summarized in Table 2.

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Environ Earth Sci Table 2 A summary of hydraulic characteristics from the Najd aquifers (saturation zone) (derived after GRC 2007)

Aquifer

Thickness (m)

Discharge (L s-1)

Transmissivity T (m2 day-1)

Storagecoefficient

Formation

28

485

3.2 9 10-6

Above UER (Rus & Dammam…act)

680

1 9 10-6

Upper UER

556

1.5 9 10-4

Upper of L. UER (or Middle)



Lower UER

A Count

45

Min

1

0.75

Max

112

201

Mean

23

14

38

32

1

0.4

B Count Min Max

43

140

Mean

14

22

C Count

91

89

Min

1

0.6

Max

69

138

Mean

30

50

D Count

21

21

Min

1

2

Max

133

43

Mean

69

22

The aquifer A has been suggested as unconfined to semiconfined. The quality of the groundwater is affected by the upper zone of the Rus formation where gypsum horizons are present (PAWR 1986). This groundwater is found largely within the Dammam and Rus formations, but may contain add-ons from the Fars Formation and alluvial aquifers. Yields are highly variable and reach 201 L s-1 on the north-eastern side of the study area. Aquifer A has the potential to produce good amount of groundwater within EC \4,000 lS cm-1, particularly in the area located between boreholes WWD-24 and WWD-27, at Bin Khawtar, Helat Ar Raka, near borehole Twirish, Hanfeet around borehole O.R., and Shisr around borehole M.Masin. Located in the upper portion of the UER (U.UER), aquifer B has an average thickness of 102 m (Table 1). It is confined and extends to the western and central Najd. Flow direction of groundwater is to the north/northeast and becomes artesian near the border with the Kingdom of Saudi Arabia. The aquifer B discharges between \1 L s-1 in the northern region dominated by sand dunes to 140 L s-1 along the eastern edge of Al Mazyunah City. Average rates are, however, within the 22 L s-1 range (see Table 2). The aquifer C is concentrated in the middle (or upperlower) portion of the UER (upper L. UER) formation and almost covers the entire Najd area (see Table 1). The upper 100 m of the L. UER formation is the actual location for

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13

this aquifer water strike starting at depth of 7–8 m in the western part of Najd and reaching a maximum depth of 70 m in the eastern regions of the study area near Qitbit (MWR 2000). The microfossils Sakesaria Dukhani seems to be one of the important parameters as an index fossil to identify the aquifer C contact. Aquifer C is confined and partially artesian. The pressure zone increases from the south to the north/northeast, generally along the direction of waterflow. Moreover, aquifer C occupies the highest priority in the Najd aquifer classification for agricultural purposes due to its high yield reaching 138 L s-1 and good distribution (Table 2). The aquifer D is fully confined and located below aquifer C in the lower UER formation at depth ranges between 50 and 100 m below Aquifer C and extends to the base of the L. UER formation. Distribution of the D aquifer covers the entire Najd range and could possibly be part of a much larger aquifer. Artesian pressures in this aquifer also increase along the direction of waterflow. The base of the D aquifer is defined by the presence of Shammer Shale (Table 1).

Sampling regimes and analytical methods The sampling locations were selected with the intention of characterizing the groundwater chemistry in the majority of

Environ Earth Sci

Fig. 3 Study area and distribution of the sampling locations from all aquifers depicted with the aquifer profiles (green, blue, black) and the major fault lines (Graben structure; red)

the Najd formations and to gain a representative overview of chemical and isotopic dynamics in this groundwater system (Fig. 3). Special consideration during sampling was given to wells that were built by the MRMWR because they are reliable and designed according to modern day standards. In addition to the MRMWR wells, five thrust-design wells and three springs are included in the sampling regime. During the first campaign, conducted during April through May 2008, samples were collected from sites covering the A through D aquifers (N = 65), three springs, and one monsoon rainfall event. Two methods for sampling were used: (1) 28 groundwater samples out of the 69 total were collected by bailer, and (2) the remaining samples (N = 41) were extracted from the aquifer by direct pumping or collection of spring discharge (Supplementary material Table 2). The rainfall sample was collected by a precipitation sampler during the monsoon season of June–August 2008. Hydrological parameters such as electrical conductivity (EC), pH, temperature, Eh, alkalinity, and CO2 were determined onsite during the sampling. Samples collected for hydrochemical analyses (N = 69) were filtered prior to collection in the HDPE bottles, labelled, and stored at 4C until

analytical procedures. Major ion analyses were conducted at the MRMWR laboratory in Muscat and all isotopic analyses were performed at Isotope Laboratory of the UFZ-Department Isotope Hydrology in Halle/Saale, Germany. Major ion determination was conducted by ICP and IC at a detection limit of 1 mg L-1 for every ion except fluoride (limit of 0.1 mg L-1). Stable isotopes of water (2H, 18O) were analysed for every site that was sampled (N = 69). Samples used for isotope analysis were collected directly from the source with no addition or filtering prior to testing. Isotopic ratios for 2H/H and 18O/16O were measured using the equilibration technique (Gehre and Strauch 2003) coupled to a isotope ratio mass spectrometer (IRMS) XL-plus (ThermoElectron) with an accuracy of 1% for d2H and 0.1% for d18O. Standard d-notation is used for reporting, so that    d ¼ Rsample =Rstandard  1 1000ð&Þ ð1Þ with R equal to changes in heavy and light isotopes compared with Vienna Standard Mean Ocean Water (V-SMOW) for both d18O and d2H. The 3H analyses were performed by Liquid Scintillation Counter (LSC) with a detection limit of 0.2 TU (tritium unit).

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Environ Earth Sci Table 3 Mineralization and hydrological parameters of groundwater from the different aquifers within the ranges analysed Aquifer

TDS (mg L-1)

EC (lS cm-1)

A

580–6,116

909–2,971

6.9–8.7

B

638–4,517

981–4,340

7.7–11.3

9

C D

635–5,850 535–10,747

1,065–8,230 871–14,910

7.1–12.6 7.5–9.3

33 19

pH

Total samples 7

Results and discussion Hydrochemistry of groundwater Water type characteristics

Fig. 4 Piper diagram from all groundwater samples from the Najd area

Groundwater chemistry varies among the different aquifers notwithstanding that three of them are located within the same formation (UER). Due to the geological characteristics of this formation, it was difficult to determine a simple linear vector for groundwater-flow direction. However, in general, water in the western part of Najd is less mineralized than water from the eastern part of this region. Mineralization increases as one moves towards the north/ northeast along the direction of groundwater flow. Supplementary material Table 2 summarizes the results from hydrochemical analyses. The total dissolved solids (TDS) of the 68 samples range between 535 and 10,747 mg L-1 with general average of 1,743 mg L-1. The lowest and highest TDS values were recorded in aquifer D at HAD-49 with 535 mg L-1 and WWD-37 with 10,747 mg L-1, respectively (Table 1). In average, the concentrations of dissolved ions reach about 2,000 mg L-1 in the aquifers A and B and are less

concentrated with 1,550 and 1,700 mg L-1 in the aquifers C and D with peaks up to 10,700 mg L-1 (Table 3). The pH ranges between 6.9 and 9.8 with two exceptions of 11.3 for well DEP16-A (aquifer B) and 12.6 for well DEP-4 (aquifer C). The relationship between the electric conductivity (EC; lS cm-1) and the total dissolved solids (mg L-1) was determined to be linear, as is generally known to be true [TDS = 0.725 EC (Ho¨lting 1992)], with the equation TDS = 0.748 EC - 160.22 (R2 = 0.90) describing the relationship between these two parameters. A mean distribution of the predominant major ions of the groundwater from the different Najd aquifers shows Table 4. For purposes of water type calculation, ion contents \5% were ignored. In general, representative dominant ions in the Najd aquifer follow the pattern of SO42- (24%) [ Na? (22%) [ Cl- (21%) [ Ca2? (16%) [ Mg2? (12%), with the remaining 5% consisting of subdominant ions (Table 4).

Table 4 Average of major ions concentration and their percentage in each aquifer Aquifer

Ca2?

Na?

Mg2?

K?

Sr2?

HCO3-

Cl-

SO4-2

NO3-

F-

Count

-1

Average concentration of major ions (meq. L ) A

12.06

11.93

6.66

0.55

0.16

2.12

11.60

17.09

0.32

0.19

7

B

15.22

9.50

9.69

0.63

0.29

1.34

6.98

25.15

0.03

0.28

9

C

6.67

14.51

6.91

0.69

0.52

1.59

14.21

9.29

0.03

0.42

33

D

4.95

17.04

5.74

0.53

0.44

2.31

17.33

5.81

0.02

0.44

19

Aquifer

2?

Ca

?

Na

Mg

2?

?

K

Abundance of major ions (meq. %) and water types A 19.2 19.0 10.6 0.9

2?

Sr

-

HCO3

Cl

-

-2

SO4

-

NO3

F

-

Water type

0.2

3.4

18.5

27.3

0.5

0.3

Ca–Na–SO4

B

22.0

13.8

14.0

0.9

0.4

1.9

10.1

36.4

0.0

0.4

Ca–(Mg)–SO4

C

12.2

26.5

12.6

1.3

0.9

2.9

25.9

16.9

0.1

0.8

Na–(Mg)–Cl–SO4

D

9.1

31.2

10.5

1.0

0.8

4.2

31.8

10.6

0.0

0.8

Na–Cl–SO4

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The piper plot (Fig. 4) provided shows the relationship among the major ions and reiterates that the majority of the groundwater chemistry consists of Na?, Ca2?, Mg2?, SO42-, and Cl-. Consequently, the water types of the Najd groundwater are characterized as earth alkaline–alkaline sulphate type occurring in the aquifers A and B. In the aquifers C and D, an alkaline-earth alkaline sulphatechloride type is dominant with increasing chloride content in aquifer D. Relative ion concentrations reached greater than 70% for sulphate, chloride, and sodium and were between 20 and 60% for calcium and magnesium. Sulphate and calcium were the most abundant ions in the aquifers A and B; sodium and chloride were dominant in the deeper C and D aquifers. The presence of magnesium depends on the dissolution of dolomite-limestone or ion exchange calcite or aragonite (see below: Saturation index), and bicarbonate was the most abundant in the spring waters of aquifer A and near the recharge area in the Jabal. A clustering of the hydrochemical signature was detected not only in the aquifers C and D, but also in B. This process seems to shift the water type in aquifer D to a sodium-chloride-dominated system in northern part of aquifer D. This obvious mixing and/or separation behaviour is clearly reflected in the log-plot of major ion ratios (Fig. 5). For example, the log-plot of ionic ratios of (Ca2? ? Mg2?)/(Na? ? K?) versus (Cl-/SO4-2) shows an increasing dominance of chloride and sodium along the C and D aquifer flow. Because of the presence of gypsum in the aquifer component and the dissolution of gypsum common in the upper UER formation, sulphate-dominant groundwater reduces consequently the ionic ratio (Cl-/SO4-2) in aquifer B and partially in C. The increase of groundwater salinity and dominance of chloride and sodium as observed at well WWD-37 in the northeast of the study area tend to support the observation of gypsum dissolution in the different parts of the UER formation component. Increasing

concentrations of fluoride and strontium ions were also observed from the A to D aquifers. These ions were recorded at the highest concentrations in aquifer C and the lowest concentration in aquifer A. Fossilized remains occur more frequently in the aquifer component of C than in other aquifers. The elevated content of limestone present here could also explain the higher strontium ion concentrations in this location (Hem 1989). In particular, aragonite could be another source of strontium when altered to calcite (Sunagawa et al. 2007). The increased fluoride ion concentrations in the C and D aquifers may be due to weathering of the aquifer component, ion exchange capacity, or the presence of fossilized remains within the formation. Potassium concentrations generally increased with aquifer depth, but the highest concentrations were observed in the C aquifer. Saturation index (SI) The limestone dominance in the aquifer components of B to D influences the groundwater quality due to mineral dissolution and precipitation. The calculation of the saturation index helps to understand such basic processes within the aquifers. Values of the saturation index of less than zero indicate under saturation, zero indicates saturation, and values greater than zero point to saturated ratios with regard to typical salt compounds (Supplementary material Table 3). Using the PHREEQC program package (Parkhurst 1995) SI values were calculated for all water samples. When the correlation of TDS (mg L-1) is plotted against SI = log (IAP/Ks) for calcite, dolomite, gypsum, and halite, the outcome reveals that calcite and dolomite are generally oversaturated within the aquifer complex (Fig. 6). However, groundwater and spring waters are under saturated with respect to evaporate minerals such as gypsum and halite. These waters also tend to precipitate carbonate minerals such as dolomite, calcite, and aragonite. This observation is indicative of non-significant dissolution

4

100.000

Cl-Brines

2

WWD -37

10.000

Sea water

log (IAPKs) = SI

Log (Cl/SO4 ) in meq

Na dominance

1.000

Aquifer A 0.100

Aquifer B Aquifer C

0.010

Aquifer D

-2 -4 -6

SO 4 dominance

Sea water 0.001 0.1

0

1.0

10.0

Log (Ca+Mg)/(Na+K) in meq

Fig. 5 Earth’s alkaline–alkaline ratio relative to the salt forming Cl/ SO4 ratio in the Najd aquifers

SI (Calcite)

SI (Gypsum)

SI (Dolomite)

SI (Halite)

-8 0

2000

4000

6000

8000

10000

12000

TDS (mg.L-1 )

Fig. 6 Saturation Index (SI) versus Total dissolved solids (TDS) of carbonates and evaporates in the Najd groundwater

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Environ Earth Sci

of calcite and indicates that Ca2? addition would increase the calcite precipitation. Hydrochemical evaluation of the groundwater The ion percentages (Table 4) of dominant elements in each aquifer show clearly distinguished patterns between calcium-sulphate and sodium-chloride characters. These variables also highlight a general trend, where sulphate levels are highest at shallow depths (aquifer A and B) and sodium and chloride concentrations accumulate with increasing distance downward from the surface (deeper aquifers C–D). Magnesium concentrations are similar for the A and D aquifers with values around 10.5%, although the concentrations vary with depth in the B and D aquifers. The maximum of sulphate detected percentages was observed in aquifer B. The bicarbonate of geological formations is of relatively constant percentage recorded, the lowest value of 1.9% in the B aquifer and the highest of 4.2% in aquifer D. The hydrochemical evaluation of the water quality of the four aquifers is highly relevant for the sustainable use of the water resources of the Najd region. The aquifer A is a locally important source of water in central and northern parts of the Najd area. This aquifer is the main source of water for many local farms in the Helat Ar Raka, Shisr and Hanfeet areas. Indeed, acceptable mineralization values for agriculture (1,500–3,000 lS cm-1) have been observed near WWD-24 on the north eastern edge of the region. The highest rates of discharge were discovered during drilling of well WWD-36, located in the northern corner of the study area. However, this area is also known to contain highly mineralized water (EC at 15,000 lS cm-1). Additionally, high mineralization was also discovered along the western edge of the region, approximately 120 km parallel to the border with Saudi Arabia. Values here exceeded 9,000 lS cm-1. As is consistent with mineralization values in this range, these two areas of the aquifer A do not represent potable sources. Several studies conducted in the Najd region by MacDonald (1991, 1994) and the Ministry of Water Resources (MWR 2000) before 2005 have suggested that aquifer B could provide a significant portion of the water required for the entire area. The latest project carried out by the MRMWR, however, has confirmed that the B aquifer could provide substantial quantum of water for western and central Najd. The Upper UER formation spreads north from DEP-17 (UTM WGS84 coordinate 1978362N), with the western edge near WWD-9 and the international border of Yemen, and the eastern edge beyond Helat Ar Raka close to DEP-18. Thicknesses in aquifer B, nonetheless, are largely diminished northeast of Al Mazyunah in the middle and western

123

regions of this area where thicknesses less than one metre were observed in boreholes for wells DEP-14, DEP-18, WWD-5, WWD-13, WWD-17, and WWD-19. Since the principle interrogation of groundwater in this region during the 1980s, it was assumed that aquifer C was the most important water resource. However, further characterization in 2004 revealed that the distribution of this aquifer was located further north than that initially believed and is located north of UTM WGS84 coordinate (1959600N). South of this coordinate, near the Jabal, aquifer C is largely dry and its distribution along the southern edge of the Najd is currently unknown (GRC 2005). Comparing all aquifers, the D aquifer is the most widely distributed as it starts on the northern side of the Jabal range and covers the entire region. It is also suggested that this portion of the L. UER formation possesses the highest alluvial thickness, but lower transmissivity (see Table 2) than the A, B, or C aquifer. The D aquifer also produces at a lower discharge rate than aquifer C (MWR 2000). Thus, the results from hydrochemical analyses and a survey of the current information of study area show that the suitability of groundwater for agricultural irrigation can be ranked as follows: A [ B [ C [ D. Environmental isotopes in groundwater of the Najd aquifers Variation of the water isotopes Water isotopic signatures were taken during this survey to evaluate recharge sources and elucidate the relationship between the different aquifers and groundwater flow direction. The water samples of 65 wells and 3 springs, in addition to rain water, were analysed for hydrogen and oxygen stable isotopes (Tables 5, 6). Comparable to the hydrochemistry of the groundwater, the mean values of d2H and d18O reflect a grouping with a tendency of decrease with depth from A to C; however, isotope values in the D aquifer increase slightly. Remarkably, both aquifers A (±18.1 and ±2.5% for d2H and d18O, respectively), and D (±12.8 and ±1.56% for d2H and d18O, respectively), were highly variable with respect to changes in their isotope signature. Table 5 The average concentration of isotopes in each aquifer Aquifer

d18O (% VSMOW)

d2H (% VSMOW)

A

-3.80 ± 2.50

-24.00 ± 18.05

B

-4.80 ± 0.68

-32.22 ± 5.33

9

C

-5.15 ± 0.76

-34.67 ± 6.45

33

D

-4.29 ± 1.56

-27.73 ± 12.83

19

Total samples 7

Environ Earth Sci Table 6 Variations of groundwater samples isotopes d2H, d18O and 3 H from different Najd aquifers 18

Well name

Aquifer

Sample date

d O (% VSMOW)

Gobib

Rainfall

June– August 2008

?0.01

M.Masin

A

29–04–08

-4.97

-33.1

-0.94

-3.3

SHTH

A

24–05–08

0.38

6.4

WWD-20

A

11–05–08

-4.60 -6.27

d18O (% VSMOW)

d2H (% VSMOW)

3

WWD-34

C

17–04–08

-5.98

-42.0

n.m.

WWD-35

C

17–04–08

-5.16

-35.7

n.m.

WWD-36

C

21–04–08

-6.28

-47.3

0.25

0.6

WWD-39

C

17–04–08

-6.32

-45.5

0.25

n.m.

WWD-8

C

4–05–08

-4.07

-28.4

n.m.

1.2

H.R.C

D

29–04–08

-5.60

-38.2

0.25

0.25

332/014

D

28–04–08

-2.62

-14.6

0.25

0.25

AQSR

D

10–05–08

-3.86

-24.2

n.m.

D D

13–05–08 15–05–08

-5.25 -3.71

-36.6 -20.8

0.25 0.25

D

15–05–08

-4.99

-29.8

0.25

n.m.

24–05–08 29–04–08

Sample date

?5.5

A A

Aquifer

H (T.U.)

NKR Twirish

Well name

2

d H (% VSMOW)

-29.7 -39.9

Table 6 continued

3

H (T.U.)

WWD-24

A

21–04–08

-4.65

-30.5

0.25

WWD-26

A

21–04–08

-5.53

-37.9

0.25

DEP-10 DEP-3

DEP-15

B

21–05–08

-4.51

-28.6

n.m.

DEP-5

DEP-17 DEP-16A

B B

21–05–08 4–05–08

-4.49 -3.71

-28.8 -24.5

0.25 0.25

DEP-6

D

28–04–08

-1.57

-6.3

0.25

DEP-7

D

15–05–08

-4.79

-28.1

0.25

1.2

DEP-8

D

13–05–08

-3.76

-22.9

n.m.

n.m.

DEP-11A

D

3–05–08

-3.83

-24.5

n.m.

D

4–05–08

-4.75

-33.2

0.25 1.3

DEP-9 WWD-05

B B

3–05–08 13–04–08

-4.41 -4.98

-29.1 -33.5

WWD-13

B

13–04–08

-5.70

-40.0

0.25

DEP-14A

WWD-17

B

13–04–08

-5.42

-36.6

n.m.

DWS-15

D

24–04–08

-4.49

-27.7

0.25

HAD-49

D

21–04–08

-6.13

-42.4

0.25

D

11–05–08

-0.06

5.1

0.25

WWD-19

B

11–05–08

-5.65

-39.1

WWD-9

B

4–05–08

-4.31

-29.9

n.m.

QISB

001/014

C

28-04–08

-4.62

-29.7

0.25

TOSNAT

D

14–05–08

-3.75

-22.6

n.m.

D

20–04–08

-6.15

-41.3

0.25

W.Baharn1

C

17–05–08

-4.67

-29.1

n.m.

WSW-3

Helat R

C

29–04–08

-4.83

-31.6

n.m.

WWD-06

D

13–04–08

-5.13

-35.0

n.m.

n.m.

WWD-14

D

13–04–08

-5.13

-36.9

0.25

D

21–04–08

-5.93

-47.0

0.25

Bashithn

C

21–04–08

-5.59

-39.7

DEP-18

C

17–05–08

-4.74

-28.8

n.m.

WWD-37

DEP-2

C

16–05–08

-4.12

-26.4

n.m.

n.m. not measured

DEP-4

C

16–05–08

-4.43

-29.1

n.m.

DEP-14 DEP-16

C C

4–05–08 4–05–08

-4.50 -3.71

-30.4 -22.1

n.m. 0.25

DEP-19

C

21–04–08

-5.96

-41.2

n.m.

DEP-9A

C

3–05–08

-3.73

-23.7

0.25

Goshaam-1

C

21–04–08

-5.47

-35.9

n.m.

M.Sp

C

6–05–08

-5.38

-33.9

n.m.

Bin Kh

C

16–04–08

-5.67

-38.4

n.m.

Qitbit

C

21–04–08

-6.44

-45.0

n.m.

Ranada1

C

20–04–08

-5.34

-34.7

n.m.

S.TMMI

C

3–05–08

-4.50

-29.2

n.m.

UBAR Fm

C

29–04–08

-4.81

-30.7

n.m.

WWD-02

C

13–04–08

-4.40

-28.0

n.m.

WWD-07

C

13–04–08

-5.02

-33.9

n.m.

WWD-11

C

13–04–08

-4.82

-32.9

n.m.

WWD-12

C

13–04–08

-5.27

-36.9

0.25

WWD-16

C

13–04–08

-4.87

-33.8

n.m.

WWD-21

C

11–05–08

-5.20

WWD-22

C

17–04–08

-6.02

-34.8 -40.5

n.m. n.m.

WWD-25

C

16–04–08

-5.99

-41.5

n.m.

WWD-27

C

17–04–08

-6.20

-42.9

0.25

WWD-32

C

21–04–08

-5.70

-40.3

0.25

Tritium was measured on groundwater from 33 wells and springs distributed along the aquifers (Supplementary material Table 2). The data range between 0.25 TU which refers the detection limit and up to 1.3 TU in DWS-15 (aquifer D). Only 4 out of 33 water samples have 3H values greater than 0.25 TU belonging to DWS-15 (aquifer D), DEP-9 (B), M.Masin, and spring SHTH (both A). 18

O and 2H relationship

The relationship between the stable water isotopes of H and O is plotted in Fig. 7, and the Global Meteoric Water Line (GMWL) d2H = 8 d18O ? 10 (IAEA 1981; IAEA/WMO 2006) has been plotted on the graph as a reference. Isotopic values of well and spring water samples were shifted from the GMWL and indicate that the groundwater within the Najd aquifers differs from the GMWL by slightly lower d2H and d18O. The relationship, depicted as the Najd Groundwater Line (NGL) in Fig. 7, between hydrogen and oxygen isotopes in these aquifers can be described by the equation d2H = 7.82 d18O ? 5.6. Also

123

Environ Earth Sci 20 Aquifer A

GMWL 18 H=8 O + 10

2

H [‰ VSMOW]

2

10

Aquifer B

0

Aquifer C

-10

Aquifer D

Najd 2 18 δ H = 7.82 δ O + 5.6 NOMWL 2 18 δ H = 5.1 δ O + 8 SOMWL 2 18 δ H = 7.2 δ O - 1.1

Gogib Rainfall

-20 -30 -40 -50 -60 -8

-7

-6

-5

-4

-3

18

O

-2

-1

0

1

2

[‰ VSMOW]

Fig. 7 d2H-d18O diagram of groundwater, spring water, and precipitation from the Dhofar region and the different Najd aquifers (Najd Groundwater Line [NGL]: black line bold); further: Northern Oman Meteoric Water Line (NOMWL: dotted), Southern Meteoric Water Line (SOMWL: dashed-dotted), Global Meteoric Water Line (GMWL) are depicted for reference

depicted on the graph are the Southern Oman meteoric water line (SOMWL, d2H = 7.2 d18O - 1.1) and the Northern Oman meteoric water line (NOMWL, d2H = 5.1 d18O ? 8.0). The NOMWL represents a reference value for water that is influenced by precipitation from the Northwesterlies (Macumber et al. 1997), whereas tropical cycles from the south have the strongest influence on the SOMWL (Macumber et al. 1995). Similarities between the hydrogen and oxygen relationships in the Najd aquifer and the SOMWL reveal that southern precipitation sources and rainfall in the Jabal range have contributed significantly to the water constituting aquifers A through D,. Three samples collected during this survey show an enrichment of 18 O and 2H, one spring (SHTH, ?0.38% in d18O, ?6.4% in d2H) from aquifer A, one well (QISB, -0.06, ?5.1%) from aquifer D, and the rainfall sample Gobib (?0.01, ?5.5%). All of these sites are located in the monsoon recharge area. The majority of samples from the groundwater including the Mudy-Spring (aquifer C) is grouped in a range \-3.7% for d18O and -20.8% for d2H which indicates decreasing monsoon recharge by infiltration processes. Waters heavily enriched in 18O and less in 2H in comparison with the GMWL could indicate that complex water–rock exchange processes are influencing the groundwater signature as that known for sedimentary rocks such as brines and solutions in contact with clay layers (IAEA 1981). However, there were no indications of sedimentary rock interactions from any of the samples collected during this survey. The isotopically enriched spring (SHTH) and well (QISB) affected by monsoon rainfall along the southern edge of the research area indicate that infiltration processes are occurring through fractures or dissolution channels of the limestone matrix located in the groundwater recharge area. Previous studies have

123

suggested this as a common phenomenon in geological formations from this region (Tetra Tech 1978). The groundwater from aquifers B, C, and most of D have d2H values below -20% and d18O values of -3.7%. The similar slope in d2H and d18O compared with GMWL indicates that the water here is not affected by evaporation during infiltration. Moreover, the 2H/18O relationship of groundwater from this formation suggests that atmospheric circulation systems from the Indian Ocean would be a highly likely source of precipitation during recharge (Macumber et al. 1995). The depletion of 2H and 18O in the groundwater pool also suggest that colder and wetter climatic conditions were present during the initial infiltration of groundwater into this area. These environmental changes could have also led to higher recharge and infiltration process rates. Clark and Fritz (1997) and Clark et al. (1987) support this hypothesis and they concluded that the water in the UER formation was mostly comprised of paleogroundwater in an area close to the recharge basin. However, mixing process in the recharge area close to the Jabal and further into the northern interior of Najd can be suggested from data collected herein. Tritium (3H, half-life time 12.43 years) is widely used as a radioisotope tracer to identify the modern recharge. Because the tritium input function is coupled to the H-bomb peak of 1960s, groundwater with high tritium concentrations [30 TU indicates a recharge during and after the 1960 s. Recent precipitation and recharged groundwater in the northern hemisphere has tritium concentrations up to 9 TU (IAEA/ WMO 2006). The groundwater containing 3H values \1 TU is often classified as submodern (Clark and Fritz 1997). The tritium analyses of samples from the Najd groundwater show that most of groundwater of all aquifers belongs to this range \1 TU. However, a mixture of submodern and fossil water could also mask input of modern recharge. Concerning the tritium values, only few groundwater samples with [0.5 TU point to mixing of modern recharge with the submodern groundwater. Overall, at this time, it is difficult to identify individual recharge sources for each aquifer in this area. Moreover, from the isotopic studies it has been determined that the groundwater is closely related to the four aquifers contained in the Najd region and seems to have a common development. Two hypotheses could be proposed from the water isotopic signatures: (i) groundwater within the aquifers is being mixed by hydraulic processes, and (ii) the water contained here initially infiltrated the aquifers under more humid conditions. 2H and 18O depletion signatures within all four aquifers would more strongly support the latter hypothesis. Only two springs (SHTH, NKR) from the A aquifer and two wells (QISB, DEP-6) from the D aquifer did not support the assumption that these aquifers were filled via a paleo-recharge process.

Environ Earth Sci

Groundwater dynamics within the Najd aquifers referring to hydrochemistry and water isotopes Groundwater flow dynamics in the upper aquifer A based on the 2H and 18O variations After the study, the general consensus view of aquifer A being representative of water from the entire Najd region can only be held for very limited areas within the aquifer. Considering isotopic signatures of water, d2H, d18O, and 3 H, from this aquifer, it is not likely to have arisen from modern recharge sources alone. Consequently, the recharge of the A aquifer cannot be completed by monsoon and direct rainfall events. It is further concluded that yet uncharacterized recharge sources from below aquifer A, i.e., formations of Dammam and Rus, contribute to the water pool in this aquifer. The soil coverage is very shallow in the Najd regions, often \4 m, underlain by hard limestone of the Dammam and Rus formation, and the permeability of this base layer is low. Subsequently, aquifer A seems to be confined in most of the Najd area even though according to some reports it is called as semiconfined to confined aquifer (i.e., PAWR 1986). Two observations emphasize the different hydraulic behaviour of aquifer A, and its recharge sources are taken into account for future considerations. First, the spring SHTH close to aquifer A and located in the recharge area contains water with an isotopic signature in 2H/18O and elevated tritium (see Table 6) of modern precipitation. Additionally, water from borehole M.Masin located in the centre of the desert (Shisr Village, aquifer A) has an isotopic signature in 2H/18O with less elevated tritium, similar to water from the UbarFm (aquifer C) well nearby. Changes in groundwater quality and flow related to geological fault intrusion It was previously discovered that two major and several minor fault lines form a Graben structure that runs through the centre of the Najd region (Watson Hawksley 1983; MMI 1991; Roger et al. 1992). These faults have a strong influence on the groundwater flow dynamics in the related aquifers. Chemical shifts were observed in aquifers B, C, and D along areas close to the fault lines. Water from the fault periphery also showed a slightly altered isotopic signature (see Fig. 8). However, the absolute direction of the eastern major fault boundary is still under investigation and there are only few data at this time. From water output, groundwater chemistry, and isotopic data, the boundary could be placed somewhere east of wells DEP-5 and DEP-3 (see Fig. 3). Despite the uncertainty of the fault directions within the Najd region, the hydrochemical and isotope data presented here can be used to assess the behaviour and interaction of groundwater in the

different aquifers. Seven groundwater profiles were presented from aquifers B, C, and D, belonging to the Um-ErRadhuma formation, selected with different proximities to the Graben structure (see Figs. 3, 8): east, west, north, and within the Graben. The aquifer A is excluded from these profiles because of a lack of boreholes for sampling. The following is a detailed view of those profiles. Profile B (see Fig. 3) was chosen along a south to north transect through aquifer B (Fig. 8a, b). The wells used for this profiling were DEP-16A, WWD-17, and WWD-13 located within the sand dunes. The groundwater quality along this transect is associated with the highest concentrations of sulphate and calcium, and low concentrations of sodium and chloride. The increased sulphate and calcium ions could have originated from dissolution of the gypsum layer present in the upper UER. Low concentrations of chloride, sulphate, and bicarbonate as marked in DEP-16A are usually indicative of surface water (rivers, wadi downflow) (Hem 1989) by vertical recharge. Indeed, the signature of DEP-16A points to portions of 2H and 18O enriched recharge. However, the tritium content of DEP-16A is too low as expecting an influence of ‘‘young’’ water. As well groundwater from borehole WWD-13 along the profile B is tritium free. South of the profile B, a higher tritium content of 1.2 TU was analysed in borehole DEP-9—located 50 km east of Al Mazyunah in Wadi Ayoon. The groundwater from aquifer B in this region—assumed from the profile—does not indicate recent vertical recharge northward and seems to have little contact with brine waters or halite layers which are abundant more in the north-east direction. However, the watershed of Wadi Aydam could influence the groundwater due to recharge by flood events. This can also be verified by the well DWS-15—built in aquifer D and located upstream DEP-9 (aquifer B)—with a tritium content of 1.3 TU. Flood and rain fall events along wadis seem to influence directly the recharge into aquifer systems independently of their position (Odeh et al. 2009; Matter et al. 2005). The C1 and D1 profiles (see Fig. 3) run parallel to the Saudi Arabian border, passing through wells WWD-7, WWD-12, and WWD-36 for aquifer C (Fig. 8c, d), and WWD-6, WWD-14, and WWD-37 for the D aquifer (Fig. 8e, f). In both profiles, sodium and chloride increases along the direction of groundwater flow may indicate mixing of sodium-chloride-bearing brines into the groundwater along the north-east direction, because limestone is the most dominant rock-type in this part of the aquifer and halite formations are extremely low here. The mixing hypothesis is supported by the existence of sabkhas in the Rub al Khali in the northeast edge of the Najd ranging to farther NE direction. Brines from these geomorphic features in the Rub al Khali are highly concentrated in Na?, Mg2?, Ca2?, Cl-, SO42- (Alsaaran 2008). The sabkhas are considered as the result of uprising

123

Environ Earth Sci Isotopes in aquifer B 0 -2

-20

-4

-30

-6 -8

H-2 O-18

-50

-10

Isotopes in aquifer C

-20

-4

-30

-6

-40

-8

H-2 O-18

-10

-50

O-18

-4 -6

-30 -40

-8

-50

-10

WWD-06

g

WWD-14

d2H (o/oo)

-2

O-18

-4

-20 -30

-6

-40

-8 -10

-50

RANADA

i

DEP-19

-10

-2

-20

-4 -6

-30 H-2 O-18

-8 -10

-50

WWD-11

WWD-16

Fig. 8 Profiles developed here for the hydrochemical and isotope development of the groundwater in aquifers B, C and D. The profiles follow the main directions of the groundwater flow: in aquifer B (a, b B) south to north, in aquifer C (c, d C1; g, h C2) and D (e, f D1)

WWD-13

Chemistry in aquifer C 80

Na

70 60

Ca

50 40 30 20

Mg HCO3 Cl SO4

10 0

WWD-12

WWD-36

Chemistry in aquifer D 180 160

Na

140

Ca

120

Mg

100

HCO3

80

Cl

60

SO4

40 20 0

WWD-14

WWD-37

Chemistry in aquifer C 30

Na

25

Ca Mg

20

HCO3

15

Cl SO4

10 5 0

j 0

WWD-17

90

RANADA

Isotopes in aquifer C

DEP-16

0

WWD-39

0

-40

10

h 0

H-2

SO4

20

WWD-06

Isotopes in aquifer C

-10

Cl

30

WWD-37

0

d2H (o/oo)

d18O (o/oo)

-2

d18O (o/oo)

d2H (o/oo)

-20

HCO3

f 0

H-2

Mg

40

WWD-07

Isotopes in aquifer D -10

Ca

50

WWD-36

0

123

d18O (o/oo)

-2

d18O (o/oo)

d2H (o/oo)

-10

e

d

0

WWD-12

Na

DEP-16A

0

WWD-07

Chemistry in aquifer B 60

WWD-13

Concentration (meq L-1)

c

WWD-17

Concentration (meq L-1)

DEP-16A

Concentration (meq L-1)

-40

d18O (o/oo)

d2H (o/oo)

-10

Concentration (meq L-1)

b

0

Concentration (meq L-1)

a

DEP-19

WWD-39

Chemistry in aquifer C 20

Na Ca

15

Mg HCO3 Cl

10

SO4

5 0

DEP-16

WWD-11

WWD-16

from south west to north east (see also Fig. 3). Profiles along the faults of the Graben structure in aquifer C (i, j C3), and profiles within the Graben structure in aquifer C northwards (k, l C4) and D northwards of the recharge area (m, n D2) (see Fig. 3)

Environ Earth Sci

0 -2

O-18

-20

-4

-30

-6

-40

-8

-50

d18O (o/oo)

d2H (o/oo)

H-2

-10

-10 DEP -4 DEP -2 001/014 001/290 001/280 MWR Bin WWD-36 Kh

m Isotopes in aquifer D (SE-NW, recharge area) -10

-2

-20

-4

-30

-6 H-2

-40

-8

O-18

-50

-10 DEP -7

DEP -5

DEP -3

332/014

Chemistry in aquifer C (northward) 90 80 70 60 50 40 30 20 10 0

HAD-49

Na Ca Mg HCO3 Cl SO4

DEP -4

n

0

d18O (o/oo)

d2H (o/oo)

0

Concentration (meq L-1)

l

Isotopes in aquifer C (northward) 0

Concentration (meq L-1)

k

DEP -2

001/014

001/290

001/280

MWR Bin WWD-36 Kh

Chemistry in aquifer D (SE-NW, recharge area) Na

20

Ca Mg

15

HCO3 Cl

10

SO4

5 0 DEP -7

DEP -5

DEP -3

332/014

HAD-49

Fig. 8 continued

paleo-groundwater from wetter periods older than 6,000 years BP and precipitation (Clark et al. 1987; Wood et al. 2002). The presence of marine fossils and brines justifies the mixing hypothesis along the flow path. The ‘‘tritium free’’ groundwater with values at the detection limit of 0.25 TU analysed in WWD-7 and WWD-12 confirm the exclusion of any recent water compartments along the north-east flow direction. Further profiling was done along both sides of the Graben structure to further characterize the C aquifer using wells Ranada 1, DEP-19, and WWD-39 in the east (profile C2, Fig. 8g, h), and wells DEP-16, WWD-11, and WWD-16 in the west (profile C3, Fig. 8i, j). The hydrochemistry inside the Graben structure tends to be heterogeneous because of an increased impact of the fault lines on groundwater flow and influence of the brine character observed in profiles C1 and D1. The following aquifer C from the recharge area north towards WWD-39, 2H and 18 O steadily decreases from the W.Bharnal well (001/280). Considering all C profiles as a whole, we conclude that groundwater is particularly impacted by the fault structure as well as the fossil groundwater located along the northeast edge of the study area. The Graben structure also crosses into profile D2 from east to west. Overall, groundwater from within the Graben structure is low mineralized and remarkably enriched in 2H and 18O. From this observation, it can be concluded that groundwater recharge in the mountainous southern part of aquifer D occurs by mixing of recent precipitation by monsoon and

cyclonic events (Macumber et al. 1995) with more ancient groundwater from beneath. The groundwater from the west of the Graben structure, however, is significantly depleted in 2H and 18O, particularly indicated around well HAD-49, and recharge from another area or at another time period must be assumed as well as structurally influenced flow system. In addition to isotopes, it is also clear from the major ion concentrations in water located within Graben structure that the fault plays an important role in determining groundwater chemistry. For example, ions such as Na?, Cl-, SO42-, Ca2?, and Mg2? generally increase northwards in all profiles that run east and west of the Graben structure, where 2H and 18 O are also depleted along the groundwater flow direction. The behaviour of these ions in aquifer C matches the profile of aquifer D, and the groundwater samples taken from wells WWD-36 (aquifer C) and WWD-37 (aquifer D) (Fig. 8d–f) clearly have similar chemistry and isotopic signature. In addition to the chemistry, groundwater mixing is assumed between aquifer B and C (Fig. 8a, b, i, j). From these profiles, it is suggested that the northern boundary of the Graben should be extended to the north, into the Dokah area that lies close to UTM WGS84 2080773N towards the direction of aquifer C. As already mentioned, sufficient boreholes were not available to profile aquifer A. However, an increase of major ion concentrations was observed along the groundwater flow direction. Specifically with depth, sodium and chloride increase and sulphate and calcium decrease

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(aquifer A–D). These observations taken together suggest that ion exchange or sulphate-reducing processes are occurring in the deepest parts of the aquifers. The B aquifer seems to be an outlier as far as ion concentrations are concerned, and hydrochemistry suggests that the aquifer matrix here most probably consists of limestone, gypsum, and mixed layers of them, as well as sodium-chloride. Considering the Ca2?/Mg2? ratios, the majority of water samples in aquifers C and D (ranging from the mountain foothills toward the Najd) tend to have a meq-ratio less than one, indicating that dolomitic dissolution and calcite precipitation occur here. The ratios support the assumption of a groundwater flow towards north-east and that groundwater of the deeper aquifers C and D interact with the aquifer component. Overall, groundwater in aquifers A, B and those samples from aquifer D located in or close to the recharge area (mountain range) is influenced by gypsum dissolution processes in these aquifers.

Conclusions The hydrochemistry and water isotopic signature data are presented here in a multi-parameter context to characterize the groundwater dynamics for the entire Najd region and for all aquifers from A to D belonging to the Hadhramaut and Fars Group. The quality of the groundwater along all aquifers is characterized as follows: SO42- [ Na? [ Cl- [ Ca2? [ Mg2?, but the hydrochemistry is varying between the four aquifers. Therefore, ions such as sulphate and calcium comprise larger relative percentages in aquifers A and B, and sodium and chloride dominate in aquifers C and D. The mineralization of groundwater increases along its flow direction from south to north-northeast up to high saline sodium-chloride groundwater in aquifer D in the northeast area of the Najd. The groundwater dynamics in the Najd region is controlled by geological and hydrogeological characteristics in the region, and local faults like the Graben structure divert the flow of groundwater in some areas. The stable isotopes 2H and 18O and the radioactive tritium have been shown to be valuable markers for investigations aimed at determining groundwater age and type. The recharge processes in the aquifers of the UER formation B, C, and D seemed to have occurred during colder and wetter climatic conditions, as indicated by the relationship of 2H and 18O between recent precipitation sampled during a monsoon event and water from the subterranean formation. Furthermore, significant recharging of aquifer must occur from the formation below. Mixing processes between sub-modern groundwater and modern water from the recharge basin are detected by water isotope

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data and hydrochemical analyses. Fault lines and the Graben structure clearly affect the flow and chemical composition of groundwater in these aquifers. The direct subsurface flow and recharge through local faults lines from a deeper aquifer could be possible. At least, two hypotheses about the groundwater dynamics could be proposed from the hydrochemistry and the water isotopic signatures: (1) groundwater within the aquifers is being mixed by hydraulic processes, and (2) the water contained here initially infiltrated the aquifers under more humid conditions. Acknowledgments We would like to express our sincere gratitude to his Excellency Abdullah bin Salim Al Ruwas, minister of Regional Municipality and Water Resources, and his undersecretary Eng. Ali Al Abri for their support and encouragement. Our thanks also go to the Departments of Groundwater and Surface Water, Water Resources in Dhofar, Laboratory of Water Quality, Manpower Center, Salalah Airport, GIS, DG of Environment and Climate Affairs in Dhofar, and DG of Water in Dhofar. We thank the staff of the UFZ-Departments Isotope Hydrology and Hydrogeology, Dr. Anke Hildebrandt for cooperation, and Dr. Andreas Musolff for his help with software analyses. Khalid Al-Mashaikhi thanks Abdullah Bawain, Mansoor Amer Jeed, Abdulaziz Al-Mushaikhi, Salim Alesh, Ghassan AlTamimi, and Ahmed Al Mawali for their support during sample collection. And last, we thank Brandon E. L. Morris for critical English suggestions.

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