Middle East Journal of Applied Sciences ISSN 2077-4613

Volume : 08 | Issue :04 |Oct.-Dec.| 2018 Pages: 1112-1130

Corresponding Author: Yousra H. Kotp, Hydrogeochemistry Dept., Desert Research Center, ElMataryia Cairo, Egypt, 11753. E-mail: yoso20002000@yahoo.com

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Impact of the geothermal activities on physico-chemical characteristics of the groundwater in Nubian sandstone aquifer in Farafra Oases, Western Desert, Egypt

Yousra H. Kotp, Mohamed E.A. Ali, Muhammad Gomaah

Hydrogeochemistry Dept., Desert Research Center, ElMataryia Cairo, Egypt, 11753

Received: 25 August 2018 / Accepted: 28 Oct. 2018 / Publication date: 01 Nov. 2018 ABSTRACT

Farafra Oasis is one of the largest depressions, excavated in the limestone plateau of the Western Desert of Egypt. It lies in the Western Desert between latitudes 26o36`, 27o12` and longitudes 26o36`, 28o00`. The Nubian Sandstone Aquifer is the main aquifer in Farafra Oasis which is composed of thick sequence of coarse, clastic sediments of sandstone and sandy clay with shale and clay beds. The drilled wells in Farafra are partially penetrating the aquifer due to its large thickness which varies between 2525 to 2675 m, and the total depth of wells ranges from 350 to 1200 m. The total dissolved solids (TDS) of groundwater samples in abstracted of the Nubian Aquifer ranges between 78 and 185 ppm reflecting preserved fresh meteoric water. Most of the groundwater samples are of Ca-HCO3 type, while some samples are of MgCa-HCO3. Only two samples are of Ca-Cl type. The high content of HCO3 is more likely related to groundwater heated by geothermal fluids rich in CO2 through deep faults. The interaction between the geothermal solutions and Nubian Sandstone Aquifer is mainly constrained by local and partial equilibrium. The main geochemical reaction which account for ions is silicate weathering, where the geothermal fluids rich in CO2 interact with the Nubian Sandstone and facilitate such reactions. Different geochemical geothermometers have been used to infer the geothermal temperatures which ranged between 262 and 1184°C. Keywords: Geothermal, Nubian Sandstone Aquifer, Hydrogeochemistry, Farafra Oasis, Groundwater

Introduction

Egypt and many countries in the world suffer from shortage of water supply which is associated along with population growth and climate change. To tackle the problem, the government will have to adopt new policies to manage the groundwater resources. It is very urgent for the Egyptian government to apply new technologies in order to desalt saline water and to recycle waste water. Understanding the geochemical processes which affect water quality is very important in managing groundwater resources. The factors which determine the quality of groundwater are; dissolution and precipitation of minerals, geological structure, and mineralogical composition of the watersheds and aquifers. All these factors combined lead to different water compositions (Murray 1996; Rosen and Jones1998). The Nubian Sandstone aquifer (NSA) underlies large areas (about 1,800,000 Km2) of Egypt, Libya, Sudan, and Chad. It is filled with a thick succession of sandstone and shales which belong from Paleozoic to Mesozoic age and extends to a depth of 3.6 Km below the ground surface (Trac 1984, Ebraheem et al., 2002). The Nubian Aquifer occupies almost 850,000 Km2of the Western Desert of Egypt, including the New Valley at Kharga, Dakhla, Farafra and Bahariya (Karanjac 1995). Groundwater is preserved under unsteady conditions over hundred thousands of years which deemed to be unrenewable fossil water (Ebraheem 2002 and Gossel et al., 2004). In the 1960s, the Egyptian government has started the New Valley project to cultivate the Western Desert. Since then, the levels of groundwater started to decline rapidly due to unplanned extraction of groundwater.

In the geothermal studies, the use of geochemistry to evaluate the geothermal reservoir temperature is frequently referred as chemical geothermometery. The chemical composition of most of the geothermal solutions is mainly controlled by partial and local equilibria between the geothermal fluids and the host rock. These chemical equilibria, in most cases, are temperature-sensitive, i.e. the chemical composition of the fluid is a function of the temperature in the geothermal system

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(Fridriksson and Ármannsson 2007), which allows to infer the subsurface temperature of the geothermal reservoir. In the geothermal systems, the overall fluid-rock equilibrium is partially attained (Giggenbach, 1981) and the chemical composition of the sampled fluids is the result of combined factors which include the initial composition of fluid, the kinetics of primary mineral dissolution and secondary mineral deposition, in addition to vapor loss, dilution and mixing with fluids of different origin. By knowing the full equilibrium, the temperature of the geothermal fluid can be estimated.

1. The Study area

Farafra Oasis occupies an area of 86,200 Km2representing one of the largest morphotectonic

depressions, excavated in the limestone plateau of the Western Desert of Egypt. It lies in the middle of the Western Desert between latitudes 26o36`, 27o12` and longitudes 26o36`, 28o00` (Fig. 1), forming an irregular triangle, where the apex points aretowards Dakhla Oasis. The area is characterized by arid climate where the recorded mean annual rainfall is 3 mm, temperature ranges between 20oC in winter and 38oC in summer. The e9vaporation is high, reaching 5.6 mm/day in December and 19.8 mm/day in June. However a hyper-arid climate prevails in the investigated area, the presence of playa deposits reflect wet paleoclimate (Elsheikh 2015).

Fig. 1: Wells location map of El Farafra Oasis

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2.1 Geology and Hydrogeology

Detailed investigations for the subsurface and surface geology of El Farafra Oasis have been done by many authors (Zaghloul 1983; and Salem 2002). According to these studies, the area has a sedimentary rocks ranging from Upper Cretaceous to Quaternary (Fig 2). The Upper Cretaceous succession includes Dakhla, Khoman Chalk, El Hafhuf and Wadi Hennis Formations. The recently drilled wells gave hydrologic and geologic information on the Nubian aquifer. The succession overlies the Nubian Sandstone reaches 220 m thickness as Dakhla shale, Farafra limestone, and dolomitic limestone. On the other hand, the Nubian succession includes sand layers separated alternatively by shale layers (Fig. 3). The Paleocene rocks include Esna shale (123m thick) and Tarawan Chalk Formation (73m thick). Eocene rocks are exposed in north, east and west Farafra Oasis as fossiliferrous limestone with minor shale intercalations with thickness of 65 m. The post Miocene rocks are represented by Minqar et-Tahan Formation which is formed of sandstone of lacustrine origin with siltstone topped by lacustrine limestone. The Quaternary sediments include playa and aeolian deposits. The aeolian sand is scattered as sand dunes, while the playa deposits are developed in the depression as fine sand, silt,and clay mixed with halite and gypsum (Salem 2002).

Structurally, there are four N-E folds; Ain Dalla and Farafra anticlines which alternate with El Ghard and El Quss Abu Said and synclines (Fig 4). There are also two faults bordering El Quss Abu Said and other small faults the NE-SW, NW-SE and N-S directions which act as conduits between the Nubian aquifer and the upper chalky limestone aquifer (Zaghloul et al., 1983).

According to Barakat and Abdel Hamid, (1974) and Ebraheem et al. (2002), the Nubian sandstone aquifer is composed of thick coarse clastic sediments of sandstone and sandy clay interbedded with shale and clay beds (Fig 5). The sandstone beds represent the aquifer horizons. The drilled wells in El Farafra are partially penetrating the aquifer due to its large thickness which varies between 2525 to 2675 m, and the total depth of wells ranges from 350 to 1200 m.

Fig. 2: Geological map of El Farafra area (compiled after CONCO, 1987).

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Fig. 3: Subsurface hydrogeology of El Farafra Oasis(after Salem 2002)

Fig. 4: Structural map ofEl Farafra Oasis (Zaghloul et al., 1983).

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Fig. 5: Hydrogeological cross section B-B' (Elsheikh 2015).

Materials and Methods Groundwater Sampling and Laboratory Analyses

One field trip was conducted in July 2018 for collecting groundwater samples and conducting various field measurements. The chemical analyses for groundwater samples have been carried out at the laboratory of the Desert Research Center (DRC), based in Cairo, Egypt. The collection, preservation and analyses of the samples followed the standard methods of the American Society of Testing and Materials (American Public Health Association APHA) 1995, 1998). The hydrogen ion concentration (pH), Temperature and electrical conductivity (EC) have been measured in the field using portable pH and EC meters (Hanna Instruments, Ann Arbor, Michigan, USA), where EC was expressed in micro-Siemens per centimeter at 25 °C. EC and pH were measured in the field immediately after sampling. Before each measurement, the pH and EC meters were calibrated with reference solutions. The groundwater samples were collected in 1000-ml-capacity polyethylene bottles. The collected samples have been preserved and analyzed at the laboratory for major ions (Ca2+, Mg2+, Na+ , K+, HCO3

−, CO3−, SO4

2−, Cl− ) using standard methods suggested by Rainwater and Thatcher (1960); Fishman and Friedman (1985), and American Public Health Association (APHA) 1995. Alkali metal ions sodium (Na+) and potassium (K+) have been determined by flame photometer (Jenway PFP 7). Total hardness (TH), CaCO3, and calcium (Ca2+) have been determined titrimetrically using the standard EDTA procedures. Carbonate (CO3) and bicarbonate (HCO3

−) have been analyzed by volumetric methods. Magnesium (Mg2+) has been calculated from TH and Ca2+ contents. Chloride was estimated by AgNO3 titration. The turbid metric method was used for the analysis of sulfate (Guler and Thyne 2004). Total dissolved solids (TDSs) were computed by multiplying the EC (ds m−1) by a factor of 0.64(Subramani et al. 2005). The results of the chemical data have been expressed in milligram per liter (mg/l) or part per million (ppm). Analytical precision for the measurements of cations and anions, obtained from ionic balance error (IBE), was computed in terms of ions expressed in milliequivalent per liter. The observed IBE values do not exceed the limit of ±5 % (Domenico and Schwartz 1990). Data of the present work were statistically analyzed by using computer program of Statistix version 9 (Analytical software, 2008).

The temperature of the geothermal reservoir can be estimated using the chemical concentration of certain constituents which are known as geothermometers. It can be based on the

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concentration of one reactive constituent (univariant) or based on ratios of reactive components. The most common used univariant geothermometer is the silica geothermometer which is based on the equilibrium between quartz and the geothermal solution. In addition, the concentrations of CO2, H2, and H2S are widely used as univariant gas thermometers. The ratio between two major constituents can be used as ratio geothermometr. The most common used cation ratio is Na/K geothermometer. It is typically assumed that Na/K geothermometer is governed by the equilibria between the geothermal fluid and the sodium and potassium feldspars. The controlling reaction is described as follow: NaAlSi3O8 + K+ = KAlSi3O8 + Na+ (1) (Albite) (Potash feldspar)

Fournier and Truesdell (1973) proposed the Na/K/Ca geothermometer which uses both Na/K and Na/Ca0.5 to predict the reservoir temperature. Giggenbach (1988) proposed a ternary diagram representing the use of the Na/K and K/Mg0.5 ratio geothermometers. The ternary diagram method gives an estimation of the reservoir temperature and indicates the maturity of the geothermal fluid by including the results of the fast equilibrating K/Mg0.5 geothermometer and the slow Na/K geothermometer.

Results and Discussion

Hydrogeochemistry

Based on the chemical analyses data and temperatures for the groundwater samples of the Nubian aquifer (Table 1&2). The salinity is very low ranging between 78 to 185 ppm, however the high content of iron makes it unsafe for drinking where iron is over the assigned limit (> 0.3) for almost all groundwater samples in the study area. All groundwater samples show negative values for the oxidation reduction potential (ORP) which suggests reducing conditions and probable loss of oxygen during the convection of geothermal fluids and the reduction of H2S and the concentration of sulfur ranges from 4.8 to 119.04mg/l. Most of the groundwater samples are of Ca-HCO3 type (Fig. 6), while some samples are of MgCa-HCO3. Only two samples are of Ca-Cl type. The high content of HCO3 with some sulfate and chloride is more likely related to groundwater heated by geothermal fluids rich in CO2 through deep faults. On the other hand, the source of Ca2+ is probably due to the incongruent dissolution of Anorthite while interacting with the geothermal fluids: CaAl2Si2O8 + 2H++ 6H2O 2H4SiO4 + 2Al(OH)3 + Ca++ (2)

(Anorthite) (Gibbsite)

The sources of sodium and potassium are most likelyAlbite and potash feldspars weathering. It is obvious fromthe relationship between HCO3

-/Na+ against Ca2+/Na+(Fig.7)that all groundwater samples are plotted inside the field of silicate weathering. The reactions are presented as follow: NaAl Si3O6+7H2O +H+ 3H4SiO4 + Al(OH)3 + Na (3)

(Albite)(Gibbsite)

Al Si3O6+7H2O +H+ 3H4SiO4 + Al(OH)3 + K+ (3)

(Potassium Feldspars)

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Table 1: Chemical analyses data for the groundwater samples in ppm. Sample No.

TDS

(mg/l) EC

(µs/cm Eh pH ToC Na+

(mg/l) K+

(mg/l) Ca2+ (mg/l)

Mg2+ (mg/l)

CO3 (mg/l)

HCO3- (mg/l)

SO4 (mg/l)

Cl- (mg/l)

SiO2 (mg/l)

S (mg/l)

1 98 148 -5 6.4 40 11 2 20 7 0 67.1 19 26 43 N.D 2 161 251.9 -5 6.4 38 10 2 36 5 0 91.5 19 27 20 42.8 3 145 226.4 -12 6.4 39 8 3 28 10 0 85.4 20 21 28 N.D 4 135 211.4 -10 7.4 45 8 2 20 15 0 73.2 18 36 26 N.D 5 121 189.8 10 6.4 33.8 6 7 12 15 0 61 20 24 20 119.0 6 75 116.7 43 6.3 39 6 2 12 5 0 48.8 14 15 25 42.8 7 128 200.1 -5 6.5 36 6 7 16 12 0 61 20 23 21 21.1 8 138 215.8 -25 6.6 36.3 4 9 24 5 0 79.3 19 19 25 N.D 9 99 154.6 8.3 6.4 41 5 2 12 10 0 61 18 16 21 21.1

10 78 121.9 21.8 6.4 41 6 2 16 5 0 36.6 18 16 24 86.4 11 152 237.6 -20 6.4 34.7 6 8 20 12 0 91.5 19 24 18 4.8 12 150 234.3 -30 6.4 38.1 6 6 32 5 0 79.3 25 20 20 64.6 13 136 212.9 -29 6.5 38.4 6 9 16 15 0 85.4 20 20 20 21.1 14 144 288.6 -20 6.5 41 6 6 24 7 0 79.3 20 20 20 4.8 15 185 227.1 -30 6.5 39.8 8 6 32 10 0 85.4 40 27 21 48.3 16 145 247.3 -30.7 6.3 35.8 6 7 28 5 0 79.3 21 20 22 N. D 17 158 146.8 -35 6.4 45 9 6 36 15 0 91.5 27 48 18 48.3 18 94 187.5 -6.5 6.3 35.8 7 2 26 5 0 48.8 20 25 23 N.D 19 120 162.3 27.5 6.3 41 9 5 25 10 0 48.8 22 40 19 21.1 20 104 223.2 -5 6.2 36 6 2 18 8 0 30.5 19 29 22 26.5 21 143 245.7 -20 6.5 38.5 6 8 20 12 0 73.2 20 25 18 N.D 22 157 220.4 -27.6 6.5 36 7 10 24 12 0 91.5 24 22 20 4.8 23 141 227.8 -35 6.7 35.4 6 8 19 12 0 73.2 21 19 28 32 24 146 202.8 -40 6.4 40.7 5 8 20 12 0 85.4 19 21 21 N.D 25 130 218.8 -30.2 6.4 39 6 2 28 7 0 79.3 18 17 22 53.7 26 140 219.7 -29.5 6.4 40.5 7 6 24 7 0 73.2 22 23 23 59.2 27 141 170.9 -30 6.4 42 6 2 32 5 0 91.5 20 16 21 N.D 28 109 225 -33.1 7 2 7 2 24 5 0 61 18 18 25 N.D

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Table 2: Concentrations of minor and trace constituents in mg/l in the study area, (2018) Sample

No. Ag Al B Ba Cd Co Cr Cu Fe Mn P Pb Si Sr V Zn

1 N.D 0.45 N.D 5.55 N.D N.D N.D N.D 6.15 0.23 0.03 0.01 20.01 0.48 0.1 N.D 2 N.D N.D N.D 2.16 N.D N.D N.D N.D 7.80 0.4 0.06 0.03 9.5 0.09 N.D N.D 3 N.D N.D N.D 2.30 N.D N.D N.D N.D 5.98 0.386 0.11 N.D 13.27 0.05 N.D N.D 4 N.D N.D N.D 1.61 N.D N.D 0.01 N.D 6.11 0.235 0.00 N.D 12.27 0.06 N.D N.D 5 N.D N.D N.D 2.79 0.14 N.D 0.01 N.D 2.90 0.3 0.88 0.01 9.14 0.079 N.D N.D 6 N.D 0.13 N.D 2.18 N.D N.D N.D N.D 6.24 0.24 0.15 N.D 11.71 0.05 N.D N.D 7 N.D N.D N.D 2.90 N.D N.D N.D N.D 2.97 0.27 0.38 N.D 9.77 0.10 N.D 0.01 8 N.D N.D N.D 2.85 N.D N.D N.D N.D 1.60 0.24 0.07 N.D 11.52 0.08 N.D N.D 9 N.D 0.07 N.D 2.22 N.D N.D N.D N.D 6.42 0.24 0.03 0.64 9.82 0.05 N.D N.D

10 N.D N.D N.D 2.07 N.D N.D N.D N.D 6.46 0.23 0.01 N.D 11.11 0.05 0.01 N.D 11 N.D N.D N.D 2.68 N.D N.D N.D N.D 2.34 0.25 0.24 0.01 8.58 0.08 N.D N.D 12 N.D N.D N.D 2.74 N.D N.D N.D N.D 3.10 0.27 0.13 0.12 9.23 0.06 0.01 N.D 13 N.D N.D N.D 3.16 N.D N.D N.D N.D N.D 0.28 0.70 N.D 9.4 0.08 N.D N.D 14 N.D N.D N.D 2.70 N.D N.D N.D N.D 3.90 0.30 0.18 N.D 9.52 0.06 0.01 N.D 15 N.D N.D N.D 2.99 N.D N.D N.D N.D 3.26 0.29 0.17 N.D 9.73 0.06 N.D N.D 16 N.D 0.04 N.D 2.76 N.D N.D 0.01 N.D 2.96 0.29 0.16 N.D 10.28 0.06 N.D N.D 17 N.D N.D N.D 2.65 N.D N.D N.D N.D 4.24 0.35 0.15 0.03 8.35 0.06 N.D N.D 18 N.D N.D N.D 1.94 N.D N.D N.D N.D 5.85 0.22 0.01 N.D 10.94 0.06 N.D N.D 19 N.D N.D N.D 2.87 N.D N.D N.D N.D 4.64 0.36 0.09 N.D 8.83 0.06 N.D N.D 20 N.D N.D N.D 2.52 N.D N.D N.D N.D 5.51 0.23 0.00 N.D 10.12 0.06 N.D N.D 21 N.D N.D N.D 2.75 0.24 N.D N.D N.D 3.72 0.35 0.26 N.D 8.28 0.07 0.01 N.D 22 N.D N.D N.D 2.48 N.D N.D N.D N.D 4.31 0.32 0.09 N.D 9.13 0.05 N.D N.D 23 N.D N.D N.D 0.67 N.D N.D N.D 0.04 0.70 0.05 0.20 0.01 12.9 0.02 N.D N.D 24 N.D N.D N.D 2.62 N.D N.D N.D N.D 2.45 0.23 0.07 N.D 9.95 0.08 0.01 N.D 25 N.D 0.07 N.D 2.59 N.D N.D N.D N.D 5.91 0.23 0.11 0.68 10.28 0.06 N.D N.D 26 N.D N.D N.D 2.45 N.D N.D 0.01 N.D 3.90 0.03 0.33 0.01 10.59 0.05 0.01 N.D 27 N.D N.D N.D 2.52 N.D N.D N.D N.D 5.76 0.23 0.02 N.D 9.95 0.05 N.D N.D 28 N.D N.D N.D 2.05 N.D N.D 0.01 N.D 4.99 0.22 0.05 0.01 11.56 0.06 N.D N.D

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Fig. 6: Piper`s plot for groundwater samples showing that, the most of the samples are of Ca-HCO3

type

Fig. 7: Ca2+ VS HCO3 plot showing that all samples are influenced by silicate weathering.

Geochemical Geothermometers

Na/K geothermometer

The most common used geothermometer is the Na/K geothermometer which is based on the ionic exchange,lack of equilibrium between solutes and hydrothermal alteration minerals present in the system, and the enrichment of some cations. For example at equilibrium, ratios of sodium (Na+)

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and potassium (K+) in a solution are controlled by a temperature dependent exchange reaction in equation (5) by Fournier and Truesdell (1973): NaAlSi3O8 + K+ = KAlSi3O8 + Na+ (5)

The approximate equilibrium constant could be written as follow,:

log (Na+)/(K+)=1390/T-1.75 (6)

or: log (Na+)/(K+)=l190/T-1.35 (7)

The relationship between the logarithm of the Na/K molar ratio and the reciprocal of aquifer temperature is well illustrated in Fig. 8.The result of the estimated temperatures from the Na/K for the Nubian Sandstone aquifer is shown in Table 3 where the inferred temperature ranges from 274 to 803 oC.

2.0 2.2 2.4 2.6 2.8 3.0-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

log(

Na/

K)

(1/T)102+

Fig. 8: Logarithm of the Na/K molar ratio as a function of the reciprocal of aquifer temperature

Mg/(K)2 Geothermometer

Giggenbach et al. (1983) suggested a Mg-K geothermometer which is represented as follow and Fig. 9: log Mg/(K)2= 1610/T- 3.95 (8) It its observed from Table 3 that the estimated temperatures inferred by the Mg-K geothermometer for the groundwater samples range between 31 and 64 oC.

Na-K-Mg ternary diagram

The advantage of this method depends on combining the results of the fast equilibrating K/Mg0.5 geothermometer and the slow Na/K geothermometer. This method gives an estimation of the reservoir temperature in addition to the “maturity” of the geothermal solution. It is observed from Fig. 10 that all of the groundwater samples are immature and plot close to the Mg corner, so their Na-K temperatures have lower reliability (Giggenbach, 1988).

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Table 3: The estimated temperatures of the Nubian sandstone aquifer a according to different methods

Sample No.

Chalcedony conductive

Quartz conductive

Na-K-Ca

Na-K-Ca Mg corr

Na/K Fournier

1979

Na/K Truesdell

1976

Na/K Giggenbach

1988

Na/K Arnorsson

1983

K/Mg Giggenbach

1986

1 64 95 30 30 274 262 285 265 37

2 32 64 20 20 285 277 294 278 41 3 46 78 32 32 364 394 366 384 42 4 42 74 27 27 311 313 318 312 31 5 31 63 68 27 586 809 553 734 55 6 40 73 33 33 348 368 351 361 41 7 33 65 62 41 586 809 553 734 57 8 40 72 58 58 803 1421 721 1183 73 9 33 66 32 32 374 408 374 398 35

10 38 70 29 29 348 368 351 361 41 11 28 60 62 49 623 895 582 802 60 12 31 63 45 45 547 725 521 666 64 13 32 64 70 33 658 983 610 869 61 14 34 66 58 50 547 725 521 666 56 15 32 64 50 50 547 725 521 666 60 16 33 65 47 47 484 598 468 561 56 17 35 67 51 51 586 809 553 734 68 18 27 59 46 46 460 555 449 525 52 19 38 70 22 22 327 337 333 334 41 20 29 61 48 48 427 496 420 474 53 21 35 67 26 26 348 368 351 361 36

22 27 59 62 49 623 895 582 802 60

23 30 63 66 54 643 945 598 840 65 24 44 76 63 47 623 895 582 802 60 25 34 66 60 50 678 1037 626 910 60 26 35 67 20 20 348 368 351 361 37

27 36 69 51 51 512 653 492 607 60

28 34 66 18 18 348 368 351 361 41 29 40 72 23 23 327 337 333 334 41

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2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

(1/T)102+

log(M

g/K

2 )

Fig. 9: Logarithm of Mg/(K) 2 vs. aquifer temperature

Fig. 10: Na-K-Mg ternary diagram showing the temperature and “maturity” of groundwater samples.

All samples lie in the immature waters field.

Groundwater

samples

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Ca/(K) 2 Geothermometer

The Ca/(K) 2 ratio has been considered as a potential geothermometer by Fournier and Truesdell ( 1973 ) who associate it with the Na- K thermometer in the well-known Na-K-Ca geothermometer. From their equations, the empirical relationship:

log Ca/(K) 2 = 2920/T- 3.02 (9)

Fig. 11. Shows that with increasing of aquifer temperature log Ca/(K) 2 increase which is confirmed in Table 3.

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(1/T)102+

Log

(Ca/

K2 )

Fig. 11: Logarithm of Ca/(K) 2 vs. aquifer temperature.

Silica Geothermometer

The increased solubility of quartz and its polymorphs at elevated temperatures has been used as an indicator of geothermal temperatures (Truesdell and Hulston, 1980; Fournier and Potter, 1982). The two silica based geothermometers most stable at lower temperatures are quartz adiabatic and conductive cooling. Equilibrium with quartz has been found to control the silica concentration in systems above about 180 to 190°C, whereas chalcedony is the controlling phase at lower temperatures (Arnason, 1976). Figs. 12 and 13 shows that all the groundwater samples are in equilibrium with chalcedony with equilibrium temperatures between 27 and 74°C . The thermometric equations are respectively written as:

log(H4SiO4)=-1306/T+0.38 (10) and

log(H4SiO4)=-1306/T+0.38 (11)

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Fig. 12: K-Mg and Quartz (Conductive) Diagram illustrating that all the groundwater samples are in

equilibrium with chalcedony with equilibrium temperatures between 27 and 64°C.

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.00.8

1.0

1.2

1.4

Log

Si

(1/T)102+

Fig. 13: Logarithm of H4SiO4 vs. aquifer temperature.

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Based on the iso-temperature and iso-silica contour maps (Figs. 14&15) of the Nubian sandstone aquifer, the temperature and silica increase in the same direction with increasing measured groundwater temperature (at ground surface). The higher silica concentrations and water temperature values are strictly confined to the areas affected by fault plains. This is due to hydraulic connection with underlying magma of high temperature and silica concentrations. These maps are nearly identical where the areas of high temperature and high silica content are the same.

Fig. 14: Distribution of temperature in groundwater in Farafra Depression

Fig. 15: Distribution of silica in groundwater in Farafra Depression

Saturation indices

The saturation indices (Log Q/K) was calculated for minerals deemed to be relevant like calcite (CaCO3), aragonite(CaCO3), dolomite (MgCa(CO3)2), anhydrite (CaSO4), gypsum (CaSO42H2O), hematite (Fe2O3), goethite (FeOOH), siderite (FeCO3), fluorite (CaF2) and halite

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(NaCl) were calculated to verify precipitation and dissolution of these minerals. The selected minerals were based on the major ions in groundwater from the study area. Figure 16 is a diagram showing SI values for these selected minerals. In general, most all the analyzed samples are undersaturated (SI less than-1) with respect to calcite and dolomite, aragonite, anhydrite, gypsum and halite. Hence, if these minerals are present, their dissolution will play a rolein determining the groundwater composition, especially in systems with limited groundwater flow (Alfarrah et al., 2017). In general, all the analyzed samples have saturation indices between 2 and 8with respect to goethite and between 8 and 20 with respect to hematite, where groundwaters are oversaturated (SI<1), such that there is a tendency for precipitation of these minerals. For siderite, and barite most of the analysed samples have saturation indices close to saturation (SI mostly close to 0) and groundwater are at equilibrium (SI = 0) with respect to this mineral.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30-10

-5

0

5

10

15

20

Sat

ura

tion

In

dex

(SI)

Sample Number

Calcite Aragonite Dolomite Siderite Gypsum Anhydrite Bairite Fluorite

Hematite Goethite

Fig. 16: Calculated saturation indices of groundwater samples with respect to selected minerals Chemical indicators of corrosion and water aggressivity

Natural geochemical processes play an important role in groundwater quality (Coetsiers and Walraevens 2006). Devadas et al. (2007) reported that the hydrogeochemical characteristics of the area should be considered when evaluating the corrosivity of the groundwater. Alkalinity, pH and chloride are primary water quality parameters affecting metal corrosion (Tang et al. 2006). The ranges of mean values of pHs, Langelier index (LSI), Ryznar index (RSI) and aggressive index (AI) for the all groundwater samples are presented in Table 4. The values of Langelier, Ryznar and aggressive indices are shown in Fig. 17. Negative LSI represents corrosion (Withers, 2005), and positive values predict scale formation from water. On the other hand, water is considered to be corrosive when the RSI exceeds 6.0, and scale forming when this index is less than 6.0 (Pa´tzay et al. 1998). LSI values are negative for some wells in and range from -0.23 to 1.31. Therefore, the groundwater from the study area is little scale or corrosion. RSI values range from 4.7 in to 6.7 also indicating little to very little dissolution of scale and corrosion.

Table 4: Statistical analysis of the chemical indicators of corrosion and water aggressivity Statistical parameter Langelier index (LSI) Ryznar index(RSI) Aggressive index(AI)

Max 1.3101 6.77 12.24 Min -0.23 4.75 10.76 Mean 0.27 5.88 11.32

CV % 11.77 8.76 2.83 SD -0.32 0.51 0.32

Note, SD standard deviation, %CV percentage coefficient of variation

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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300

5

10

15

20

25

30

35

40

-1

0

1

2

Ryz

nar

an

d A

ggre

ssiv

e in

dec

is a

nd

So 4

(mg/

l)

Langeller Index

Well number

SO4, mg/l

Ryznar Index Agressive Index Langeller Index

Fig. 17: Values of Langelier index (LSI), Ryznar index (RSI) and aggressive index (AI) for28

groundwater samples from the study area.

Conclusion

The salinity of the groundwater samples in the study area is very low, where sit ranged between 78 to 185 ppm. Most of the groundwater samples are of Ca-HCO3 type, while some samples are of MgCa-HCO3. Only two samples are of Ca-Cl type. The high content of HCO3 is more likely attributed to groundwater heated by geothermal fluids rich in CO2 through deep faults.

The chemical composition of the groundwater samples revealed that the interaction between the geothermal solutions and the Nubian Sandstone Aquifer is mainly controlled by local and partial equilibria. The main geochemical reaction which account for ions is silicate weathering, where the geothermal fluids rich in CO2 interact with the Nubian Sandstone and facilitate such reactions. Different geochemical geothermometers have been used to infer the geothermal temperatures which showed wide range between 27 to 1184oC. El Farafra area is considered a good potential for harnessing the geothermal energy.

References

Alfarrah, N., G. Berhane and A. Hweesh, 2017. Hydrochemical characteristics and flow of the Nubian Aquifer System in Tazerbo Wellfield, SE Libya. Environ Earth Sci., 76: 356. https://doi.org/10.1007/s12665-017-6683-9.

American Public Health Association (APHA), 1995. American Water Works Association, Water Pollution Control Federation, Standard methods for the examination of water and wastewater. Washington: American Public Health Association.

American Public Health Association (APHA), 1998. Standard methods for the examination of water and wastewater, 20th edn. Washington, DC, 46p. The Open Geology Journal, 8 (Suppl 1: M3), 34e53.

Arnason, B., 1976. Ground Water Systems in Iceland Traced by Deuterium, Societas Scientiarum Islandica, 42, 236.

Barakat, M.G. and M.L. Abdel Hamid, 1974. Subsurface geology of Farafra Oasis, Western Desert Egypt. J Geol 17(2):97–110

Middle East J. Appl. Sci., 8(4): 1112-1130, 2018 ISSN 2077-4613

1129

Coetsiers, M. and K. Walraevens, 2006. Chemical characterization of the Neogene Aquifer, Belgium. Hydrogeol J 14:1556–1568.

Devadas, D.J., N.S. Rao, B.T. Rao, K.V.S. Rao and A. Subrahmanyam, 2007. Hydrogeochemistry of the Sarada river basin, Visakhapatnam district, Andhra Pradesh, India. Environ Geol 52(7):1331–1342.

Domenico, P.A. and F.W. Schwartz, 1990. Physical and chemical hydrogeology (p. 824). New York: Wiley.

Ebraheem, A.M., S. Riad, P. Wycisk and A.M. Seifelnasr, 2002. Simulation of impact of present and future groundwater extraction from the non-replenished Nubian Sandstone aquifer in southeast Egypt. J Environ Geol., 43:188–196.

Elsheikh, A.E., 2015. Mitigation of groundwater level deterioration of the Nubian Sandstone aquifer in Farafra Oasis, Western Desert, Egypt. Environ Earth Sci., 74: 2351. https://doi.org/10.1007/s12665-015-4236-7

Fishman, M.J. and L.C. Friedman, 1985. Methods for determination of inorganic substances in water and fluvial sediments, U.S. Geological Survei Book 5, Chapter A1. Open File Report 84:85–495 Denver Colorado U.S.A. for hydrogen isotope analysis. Analytical Chemistry, 63, 910–912.

Foumier, R.O., 1977. Chemical geothermometers and mixing models for geothermal systems. Geothermics, 5: 41-50.

Fournier, R.O., 1989. Lectures on geochemical interpretation of hydrothermal waters. UNU-GTP, Iceland, report 10: p 73.

Fournier, R.O. and R.W. Potter, 1982. An equation calculating the solubility of quartz in water from 25 ° to 900°C and at pressures up to 10000 bars. Geochim. Cosmochim. Acta, 46:1969-1974.

Fournier, R.O. and A.H. Truesdell, 1973. An empirical Na-K-Ca geothermometer for natural waters. Geo- chim. Cosmochim. Acta, 37: 1255-1275.

Fridriksson, T. and H. Ármannsson, 2007. Application of geochemistry in geothermal resource assessments, Resource Assessment and Environmental Management.

Giggenbach, W., 1988. Geothermal solute equilibria- Derivation ofNa-K-Mg Ca geoindicators. Geochim. Cosmochim. Acta, 52: 2749-2765.

Giggenbach, W.F., 1984. Mass transfer in hydrothermal alteration systems. Geochim. Cosmochim. Acta Ire, 2693-2711.

Giggenbach, W.F., R. Gonfiantini, B.L. Jangi and A.H. Truesdell, 1983. Isotopic and chemical composition of Parbati Valley geothermal discharges, Northwest Himalaya, India. Geothermics l2, 199-222.

Giggenbach, W.P., 1981. Geothermal mineral equilibria, Geochim. Cosmochim. Acta 45, 393-410. Gossel. W., A.M. Ebraheem and P. Wycisk, 2004. A very large scale GIS-base groundwater flow

model for the Nubian Sandstone aquifer in Easertn Sahara (Egypt, northern Sudan and eastern Libya) Hydrogeo. J., 12:698–713.

Guler, C. and G.D. Thyne, 2004. Hydrologic and geologic factors controlling surface and groundwater chemistry in Indian Wells-Owens Valley area, southeastern California, USA. Journal of Hydrology, 285, 177–198.

Helgeson, H.C., J. Delany, H.W. Nesbitt and D.K. Bird, 1978. Summary and critique of the thermodynamic properties of rock forming minerals. Am. J. Sci., 278A: 1-229.

Karanjac, J., 1995. Regional Programme for the Development & Utilization of the Nubian Sandstone Aquifer. CEDARE and IFAD, Cairo, Egypt: 7–20 September 1995 and Atlanta, USA: 21–24 September 1995.

Michard, G., 1983. Recueil de donn6es thermodyna- miques concernant les 6quilibres eaux-min6raux dans les r6servoirs g6othermaux. Rapp. Comm. Eur., Brus- sels, EUR 8590 FR.

Michard, G., 1982. R61e des anions mobiles dans le trans- port des 616 ments par les solutions hydrothermales. C.R. Acad. Sci., Paris, S6r. II, 295: 451-454.

Michard, G., 1987. Controls of the chemical composition of geothermal waters. In." H.C. Helgeson (Editor), Chemical Transport in Metasomatic Processes. NATO (North Atlantic Treaty Org.), ASI (Adv. Sci. Inst.), Ser. C, 218: 323-353.

Middle East J. Appl. Sci., 8(4): 1112-1130, 2018 ISSN 2077-4613

1130

Michard, G., D. Grimaud, F. d'Amore and R. Fanceui, 1989. Influence of mobile ion concentrations on the chemical composition of geothermal waters - Example of hot springs from Piemonte (Italy). Geothermics, 18: 729-741.

Murray, K.S., 1996. Hydrology and geochemistry of thermal waters in the Upper Napa Valley, California. Ground Water 34:1115–1124.

Nordstrm, D.K., J.W. Ball, R.J. Donahoe and D. Whittemore, 1989. Groundwater chemistry and water- rock interaction at Stripa. Geochim. Cosmochim. Acta, 53: 1727-1740.

Pa´tzay, G., G. Sta´hl, H. Ka´rma´n and E. Ka´lma´n, 1998. Modeling of scale formation and corrosion from geotermal water. Electrochim Acta 43(1–2):137–147

Plummer, L.N., F.B. Jones and A.H. Truesdell, 1984. WATEQ: A computer program for calculating chemical equilibrium of natural waters. U.S. Geol. Survey Water Resources Investigations, U.S.A., 76-13.

Powell, T. and W. Cumming, 2010. Thirty-Fifth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 1-3, 2010 SGP-TR-188

Rainwater, F. H. and L.L. Thatcher, 1960. Methods for collection and analysis of water samples. U.S. Geological survey water supply, paper 1454. Washington: USGS.

Rosen, M. and S. Jones, 1998. Controls on the chemical composition of ground water from alluvial aquifers in the Wanaka and Wakatipu basins, CentralOtago, NewZealand. Hydrogeol J6:264–281

Salem, A.A., 2002. Hydrogeological studies on the Nubian sandstone aquifer in Bahariya and Farafra depressions, Western Desert Egypt, Ph. D. Thesis, Geol. Dep. Fac. of Sci. Ain Shams Univ., Egypt, p 157

Shikazono, N., 1978. Possible cation buffering in chlo- ride-rich geothermal waters. Chem. Geol., 23: 239-254.

Subramani, T., L. Elango and S.R. Damodarasamy, 2005. Groundwater quality and its suitability for drinking and agricultural use in Chithar River Basin, Tamil Nadu India. Environmental Geology, 47, 1099–1110.

Tang, Z., S. Hong, W. Xiao and J. Taylor, 2006. Characteristics of iron corrosion scales established under blending of ground, surface, and saline waters and their impacts on iron release in the pipe distribution system. Corros Sci 48:322–342.

Trac, N.Q., 1984. Nubian sandstone aquifer modelling and groundwater resources planning for new valley. Technica Report PP/1981-1983/2/7.3/03

Truesdell, A.H., 1975. Geochemical exploration. Proc. 2nd U.N. Symp. on Development and Use of Geothermal Resources, San Francisco, Calif., 1: 1 i- 1 xxii.

Zaghloul, E.A., 1983. Geology of Abu Minqar-Farafra-Ain Dalla strech, Western Desert, Egypt, Ph. D. Thesis, Geology Dep., Fac. of Sci., Cairo Univ., Egypt. p 127.