ORIGINAL PAPER

Integrated geophysical and hydrochemical investigationsfor seawater intrusion: a case study in southwestern Saudi Arabia

Hussain Alfaifi1 & Ali Kahal1 & Abdulaziz Albassam1& Elkhedr Ibrahim1,2

& Kamal Abdelrahman1,3& Faisal Zaidi1 &

Saad Alhumidan1

Received: 12 September 2018 /Accepted: 16 May 2019 /Published online: 3 June 2019# Saudi Society for Geosciences 2019

AbstractThe objective of the present study was to assess the deterioration in groundwater quality due to saline water intrusion in the Ad-Darb region of southwestern Saudi Arabia. An integrated approach using geochemical and geophysical methods was applied toassess the extent of saline water intrusion. Geochemical methods involved the determination of main groundwater facies presentin the region using Piper and extended Durov plots. The geophysical methods used in the study included seismic refraction (SR),seismic refraction tomography (SRT), vertical electrical soundings (VES), and electrical resistivity tomography (ERT). Two SRand SRT surveys, three ERT surveys, and nine VES surveys were carried out. Piper plot shows that the water mainly belongs tothe SO4-Cl type of anionic facies. The Durov plot indicates base ion exchange, linear mixing, and saline water intrusion as themain factors influencing the groundwater chemistry of the area. The TDS increases towards the coast, with values reaching ashigh as 9000 mg/l. These results confirm the interpretation of the vertical electrical sounding (VES) and electrical resistivitytomography (ERT) sections which indicates the presence of saline water intrusion, with the thickness of the intruded zoneincreasing towards the sea. The geoelectric resistivity results indicate four geoelectric resistivity layers. The average thicknessof upper layer is 2.5 m and the resistivity ranges from 22.5 to 280 Ωm. The P wave velocity of this layer varies from 488 to 787m/s. The second geoelectric resistivity zone consists of relatively disconnected layers with 10 to 33m thickness and 52 to 120Ωmresistivity. The third zone is a low resistivity zone, increasing in thickness westwards towards the sea and with resistivities of 0.3to 19.5 Ωm. This is the main zone (consisting of unconsolidated Quaternary sediments) affected by saline water intrusion. Thiszone is intercalated with muddy marine intercalations, as indicated by its low resistivity (as low as 0.3 Ωm) The second and thirdgeoelectric resistivity layer is represented by a single seismic zone with P wave seismic velocities ranging from 1231 to 1524m/s,indicating the presence of water. The resistivity of the lowest layer ranges from 56 to 822 Ωm and corresponds to the basement.This bedrock layer has seismic velocities varying between 2315 and 3164 m/s.

Keywords Seawater intrusion . Coastal aquifer . Geophysical investigation . Southwest Saudi Arabia

Introduction

Rapid economic growth coupled with urban development andintensive agricultural activities have led to decline in ground-water levels and deterioration of groundwater quality.Deterioration of groundwater quality due to saline water in-trusion is a common phenomenon in coastal aquifers mainlydue to the increasing groundwater abstraction to meet the do-mestic, agricultural, and industrial water demand. Seawaterintrusion may extend laterally and vertically due to excessivepumping of groundwater that is recharged by rainfall and/orfloods, (Urish and Frohlich 1990; Frohlich et al. 1994). Theextent of seawater intrusion is affected by the hydrogeology ofthe freshwater aquifer, rainfall levels, the hydraulic gradient,the rate of pumping and recharge.

Editorial handling: Mansour A. Al-Garni

* Hussain Alfaifihualfaifi@KSU.EDU.SA

1 Geology and Geophysics Department College of Science, King SaudUniversity, Riyadh, Kingdom of Saudi Arabia

2 Geology Department Faculty of Science, Mansoura University,Mansoura, Egypt

3 Seismology Department, National Research Institute of Astronomy& Geophysics, Cairo, Egypt

Arabian Journal of Geosciences (2019) 12: 372https://doi.org/10.1007/s12517-019-4540-8

Geophysical techniques such as vertical electric sound-ing (VES), electrical resistivity tomography (ERT), andseismic refraction tomography (SRT) have been frequent-ly used to monitor seawater intrusion in coastal plains,(Bauer et al. 2006; Sherif et al. 2006; Satriani et al.2012; Vafidis et al. 2014; Hamdan et al. 2015).Geophysical techniques in combination with hydrogeo-chemical analysis have also been studied to evaluate theimpact of saline water intrusion in coastal areas by nu-merous workers (Samsudin et al. 2007). Geostatisticaltechniques have also been used for studying the ground-water quality in unconfined aquifers in coastal areas (ElAlfy et al. 2015; Zghibi et al. 2014).

The Jazan-Tihama coastal plain (Fig.1) in southwesternSaudi Arabia has several populated communities which relyheavily on groundwater available in the shallow unconfinedcoastal aquifers, (MoWE 2014). The excessive abstraction ofgroundwater might be responsible for saline water intrusion inthe region which needs to be thoroughly investigated and isthe main objective of the present study.

The study area has been investigated by numerous workersusing geophysical methods to evaluate the saltwater intrusionand its effect in the coastal areas (Albassam and Hussein 2008; El Alfy et al. 2015).

The present study focuses on the integrated use ofhydrochemical and geophysical investigation to assess the na-ture and extent of saline water intrusion. The hydrochemicalinvestigations include the hydrochemical facies analysis usingPiper’s plot and understanding the main hydrogeological pro-cesses using the Durov plot. The obtained results are integrat-ed with the results of the geophysical investigations to under-stand the extent of saltwater intrusion in the shallow aquifersin the Ad-Darb area along the Red Sea coast of Jazan provincein southwestern Saudi Arabia.

Geology and hydrology

The investigated area is located in the Tihama coastal plain.Geologically, the area comprises of sand, sandy gravel, and

Fig. 1 Location map for the study area

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alluvial deposits of the middle to late Quaternary periodforming the main aquifer unit (Fig. 2). The elevation in theregion ranges from 916 m amsl in the northeastern corner(Asir highlands) to 0 m amsl along the Red Sea coast.Figure 3 shows the DEM and the major streams in the area.The groundwater in the region occurs at shallow depths under

unconfined conditions. Groundwater levels were obtainedfrom 20 dug wells (MoWE 2015). The groundwater depthranges from 3 to 21 m below ground level. The piezometriclevel ranges from 144 to 2.78 m amsl with groundwaterflowing in the southwest direction towards the Red Sea coast,following the regional topography (Fig. 4).

Fig. 2 The geologic map of the study area with the major streams and the location of the resistivity surveys

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Hydrochemistry of the study area

General hydrochemistry

Hydrochemical data from 21 groundwater samples (pH, EC,TDS, Ca, Na, Mg, K, HCO3, Cl, SO4, and NO3) wereassessed. The statistical analysis of these parameters is givenin Table 1.

The pH values range from 7.5 to 8.6 (average 8), indicatingthat the water is mildly alkaline. The EC and TDS values showa wide range of values. The EC values range from 554 to13,180 μs/cm. The TDS concentration ranges from 328 to9094 mg/l. The average TDS values as shown in Fig. 5 areapproximately 2496 mg/l and the groundwater falls in thebrackish category (Freeze and Cherry 1979). High TDSvalues occur along the coast and extend in the center of thestudy area probably indicating saline water mixing. Na and Clare the most dominant ions having average values of 512.20mg/l and 1025.62 mg/l respectively.

The overall cationic abundance is Na > Ca > Mg > K andthe anionic concentrations are Cl > SO4 > HCO3. NO3 is acommon nitrogenous compound arising from the natural pro-cess of the nitrogen cycle, since nitrogen is the most abundantelement found in the earth’s atmosphere and can react withrainwater to enter the hydrologic cycle. Anthropogenic activ-ities, however, especially the application of fertilizers contain-ing nitrogen, have greatly increased the nitrate concentration,particularly in groundwater (Spalding and Exner 1993). NO3

is a minor constituent of groundwater and the levels are oftenbelow 10 mg/l (WHO 2011). Values above 10 mg/l are indic-ative of anthropogenic influence on the groundwater quality.In this study area, the NO3 concentration in the groundwater isvery high, with an average of 127 mg/l. This may be related tothe extensive agricultural activity facilitated by the presence ofshallow groundwater and fertile alluvium found in the studyarea (Fig. 5). Sorghum, millets, maize, sesame, tomatoes, egg-plants, and melon are the main crops and vegetables grown inthe area.

Fig. 3 DEM of the study area with the major streams and the location of the groundwater samples and piezometers

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Piper classification

The hydrochemical facies of the study area (Fig. 6) was clas-sified using the Piper classification (Piper 1944).

The Piper diagram is helpful in comparing the ionic com-position of water samples and deciphering the mainhydrochemical facies existing in a given area; however, ithas limitations in the sense that spatial comparison of the

different samples cannot be done. On the cationic triangle,the samples fall within the Bno dominant^ category and theNa+K dominant category. On the anionic triangle, the samplesfall within the Bno dominant^ category and Cl dominant cat-egory. It can be seen on the anionic triangle that samplesshowing high concentrations of Cl also show high values ofTDS.

On the diamond of the Piper plot, the groundwater in theinvestigated area can be classified into four basichydrochemical facies. One sample each belongs to the Ca-Mg-HCO3 type and Na-K-HCO3 type. The Ca-Mg-HCO3

type is indicative of meteoric recharge and the presence of asample in this category, indicating the absence of rainfall re-charge in the area, or its presence has been masked by mixingwith more evolved types of groundwater characterized by thedominance of Cl or SO4. Thirteen groundwater samples be-long to the Ca-Mg-SO4-Cl type and six to the Na-K-Cl type ofgroundwater facies. Cl and SO4 are the main ions which in-fluence the TDS of the groundwater.

Durov plot

The expanded Durov diagram (Lloyd and Heathcote 1985)better explains the geochemical processes that govern thegroundwater chemistry in a given area. Durov plot helps not

Fig. 4 Map showing the piezometric contours of the study area. The piezometric values are in meters above mean sea level

Table 1 Statistics of the different parameters of the analyzed samples.TDS and ionic concentrations are in mg/l

Parameters Min Max Mean Standard deviation

pH 7.5 8.6 7.99 0.33

EC (μS/cm) 554 13,180 4403.33 3602.67

TDS 328.1 9094.2 2978.97 2495.54

Ca 28.9 1226 293.93 307.72

Mg 7.4 314 111.81 91.98

Na 43.1 1945 512.20 515.20

K 0.35 45.8 5.87 9.75

HCO3 146.4 524.6 317.49 100.26

Cl 44.9 4392 1025.62 1148.47

SO4 17.9 1808 561.38 455.83

NO3 0.47 535 90.74 127.21

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only in the plotting of the hydrochemical data but also indefining the main hydrochemical processes, which dominatethe groundwater chemistry (Albassam et al. 1997). TheDurovPwin software (Albassam and Khalil 2012) was usedto prepare the Durov plot (Fig. 7).

The samples fall within seven facies on the Durov plot witheach facies characterized by a particular set of hydrochemicalreactions. One sample falls in field 2 and belongs to the Ca-Mg-HCO3 facies. One sample falls in field 3 characterized byNa-HCO3 facies. Fields 2 and 3 are dominated by the presenceof Na, indicating ion exchange phenomenon. One sample fallsin field 4, which belongs to the Ca-SO4 facies. Gypsum dis-solution especially from the sabkha deposits in the region mayhave resulted in the formation of this type of facies (Zaidi et al.2017). Six samples belong to field 5. Samples in field 5 belongto the mixed groundwater facies, which may be a result of themixing of groundwater from two or more than two facies.Three samples fall within field 6 and belong to the Na-SO4

facies. Water belonging to this facies may be a result of ionexchange and pyrite oxidation in the alluvial aquifers of thestudy area. Five samples fall within field 8, which is charac-terized by the Mg-(Na)-(Ca)-Cl type of groundwater faciesand may be a result of the mixing of fresh and saline water.Reverse ion exchange map also results in the formation of thistype of groundwater facies. Four samples fall in field 9 and are

characterized by the Na-Cl type of groundwater. Samples fall-ing closer to the coast may be influenced by saline waterintrusion whereas samples farther away from the coast maybe influenced by halite dissolution. Based on the Durov clas-sification, it can be concluded that ion exchange, linearmixing, saline water intrusion, and reverse ion exchange arethe predominant hydrochemical reactions controlling thegroundwater chemistry. Table 2 shows the number of samplesfalling within each hydrochemical facies on the Durov plot.

Geophysical investigation

Seismic refraction survey

In the investigated area, the seismic refraction (SR) survey wasconducted on two profiles using a Geode seismic recorder with24 channels. The location and orientation of these two profilesare shown in Fig. 1. Each SR profile has a length of 115 m withgeophone spacing of 5 m. The resonance frequency of the usedgeophones was 4.5 Hz. The energy source used in the surveywas a sledgehammer striking a steel plate. In this method, Pwave measurements are usually carried out using five shotsper spread: one in the middle, two forward, and two reverseshots. Two off-end forward and reverse shots were set at

Fig. 5 TDS distribution map. TDS values are in mg/l

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distances of 25 m, which was sufficient for a complete coverageof the refractor. The processing and interpretation of seismicrefraction P wave data was conducted using the SeisImager/2D refraction data analysis software. This employs threemethods for refraction data analysis, including time-term inver-sion, the reciprocal method, and tomography. First arrivals wereidentified using the Pickwinmodule (version 4.0.1.5). Then, thetravel time curves were used in the calculation of the modelvelocities and the depths to the interfaces, using the Plotrefamodule (version 2.9.1.6). The method starts with a user-selected initial velocity model and iteratively traces rays throughthe model with the goal of minimizing the RMS error betweenthe observed and calculated travel times. It was difficult to iden-tify first arrivals due to noisy data; however, this also indicatesthe unconsolidated nature of the surveyed area.

The interpretation of seismic refraction for profile 1 (Fig.8a) shows three subsurface layers. The P wave velocities arein the range of 488, 1231, and 2315 m/s, representing uncon-solidated, saturated alluvial, and weathered or fractured

basement respectively (Sumanovac and Weisser 2001;Stampolidis et al. 2005).

The depth of the saturated layer is about 3 m below theground surface, while the depth of the weathered/fracturedbasement rocks ranges from 20 to 35 m below the groundsurface. Profile 2 illustrates three layers with P wave velocitiesof 787, 1524, and 3164 m/s for the 1st, 2nd, and 3rd layersrespectively (Fig. 8b). These velocities represent unconsoli-dated alluvial, saturated alluvial, and weathered or fracturedlimestone rocks. The depth of the saturated layer varies from 3to 6 m below the ground surface; while the weathered orfractured basement is about 20 m below the ground surface.

Seismic refraction tomography survey

Seismic refraction tomography (SRT) is the velocity gradienttechnique usually considered appropriate to resolve 2D sub-surface velocity structure based on first-arrival travel times(Zhu and McMechan 1989; Stefani 1995 and White 2007)

Fig. 6 Piper plot showing the main hydrochemical facies

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with a denser source-receiver spacing. SRT depends on aniterative solution of the linearized segments and permits de-tection of continuous velocity differences and shapes of thesub-horizontal boundaries. A shooting process to build thelinear system achieves Ray tracing between two points. Thevelocity zone is well defined based on triangular cells of con-stant velocity gradient, leading to the analytical estimation ofray paths. The SRT technique determines the seismic velocitydistribution from the first arrivals. It is necessary to obtain anumber of seismic records by placing the seismic source atdifferent locations along a seismic line; confirming that SRT is

more suitable for areas with strong lateral velocity gradients(e.g., seawater intrusion in coastal strips).

The SRT technique was conducted along the same refrac-tion profiles in the study area using a Geode seismic recorderwith 24 channels. Each SRT profile length was 115 m withgeophone spacing of 5 m. The resonance frequency of thegeophones was 4.5 Hz. The energy source used in the surveywas a sledgehammer striking a steel plate. In this method, a Pwave was generated at all geophones, meaning there are 24shot points distributed along the geophone spread for a com-plete coverage of the refractor. The SeisImager software wasutilized to identify first breaks and inversion.

The interpreted velocity model for profile 1 in the area (Fig.9a) reveals three subsurface layers where the P wave velocitiesare in the range of 488, 1231, and 2315 m/s representingunconsolidated alluvial, saturated alluvial, and weathered orfractured basement rock, respectively. The top of the saturatedlayer is about 3 m below the ground surface while the depth ofthe weathered or fractured basement is approximately 20 to 35m below the ground surface.

Electrical resistivity survey

Electrical resistivity surveys are widely used in environmentaland hydrogeological investigations, where the resistivity ofsoils and rocks vary based on lithology, porosity, water

Fig. 7 Durov plot for the study area

Table 2 The hydrochemical facies associated with the 9 numberedfields in Durov plot and the number of samples falling in each field

Field number Predominant facies Number of samples

1 Ca-HCO3 0

2 Mg-HCO3 1

3 Na-HCO3 1

4 Ca-SO4 1

5 No clear facies 6

6 Na-SO4 3

7 Ca-Cl 0

8 Mg-(Na)-(Ca)-Cl 5

9 Na-Cl 4

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content, and dissolved salts. In this context, several configu-rations may be employed (Brookes and Kearey 1988; Zohdyet al. 1974). Dipole-dipole and Schlumberger arrays are themost appropriate techniques to detect the depth, the thickness,and the boundary of the aquifer ( Asfahani 2006; Bello andMakinde 2007; Omosuyi et al. 2007), to recognize the salineand fresh water interference, to evaluate the quality of ground-water (Arshad et al. 2007), and to detect the saltwater intrusionin coastal aquifers (Benkabbour and Toto 2004 and Sung-Hoand Jin-Yong 2007).

In the present study, a geoelectrical resistivity survey usingvertical electrical soundings (VES) and 2D electric resistivitytomography (ERT) was conducted in the investigated area. Inthis respect, nine VES stations and three 2D resistivity tomog-raphy profiles were undertaken (Fig. 1). This survey was con-ducted with the aim of understanding the extent and depth ofsaltwater intrusion into the shallow groundwater aquifer alongthe coastal zone of the study area.

Vertical electric sounding survey

TheVES resistivity survey includes nineVES stationsmeasuredalong the investigated area (Fig. 1) using an IRIS SYSCAL R2

system. The Schlumberger configuration was used with currentelectrodes separated by a maximum of 1000 m. The acquiredVES data were interpreted in terms of resistivities and thick-nesses of various subsurface layers using IX1D computer soft-ware (Interpex, USA). Using the iteration process, the IX1Dsoftware creates a resistivity model that fit the field data withthe least error. The output of true resistivities and the thicknessesof the subsurface geoelectric layers are listed in Table 3.

These are then used along with the elevation of the VESstations to construct three true geoelectric resistivity cross sec-tions (Fig. 10) showing the presence of four geoelectric resis-tivity zones; the topmost unsaturated zone, followed by abrackish to fresh water-bearing zone, a saline water-bearingzone (SWBZ), and a relatively highly resistive zone at thebottom. The topmost zone consists of thin layers with an av-erage thickness of about 2.5 m and resistivities ranging from22.5 to 280 Ωm. The lateral variations in the resistivity of thiszone could be related to variations in lithology, moisture con-tent, clay content, and salinity (Choudhury et al. 2001). Thiszone is underlain by a relatively disconnected geoelectric re-sistivity zone with a thickness of 10 to 33 m and resistivitiesranging from 52 to 120 Ωm. This zone could be interpreted asgravely sand saturated with brackish to fresh water.

Fig. 8 Seismic refractioninterpreted ground model

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The second zone of brackish to fresh water is followed by asaline water-bearing zone (SWBZ) in all the sections with the

thickness varying from 11.7 m under VES-8 to 120 m underVES-2, and resistivity values varying from 3.5 to 19.5 Ωm

Fig. 9 Seismic refraction tomography section

Table 3 The interpreted values ofthe true resistivities andthicknesses of the subsurfacegeoelectric layers

No. Layer-1 Layer-2 Layer-3 Layer-4 Layer-5

Rho-1 H-1 Rho-2 H-2 Rho-3 H-3 Rho-4 H-4 Rho-5

1 2.0 2.6 5.0 39.8 60 – – –

2 64.4 0.7 16.8 2.5 55.0 10.1 8.3 66.3 826.5

3 64.2 0.7 18.2 3.6 52.4 18.3 3.6 37.9 358.5

4 265.5 0.8 96.4 11.7 13 55 74

5 187.7 1.2 120.8 33 19.5 48.0 822.2

6 29.2 2.0 3.6 54.2 56.4 – – –

7 64.2 0.7 16.1 4.9 6.6 20 87 –

8 181.9 0.7 22.5 3.2 6.3 11.7 214.2 –

9 280.5 2.4 156.0 30 70.2 101.2 452.8

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Fig. 10 Geoelectric resistivity cross sections illustrate the geolectric resistivity zones with their resistivity ranges and thicknesses. The locations of theVES stations are shown in Fig. 1

Fig. 11 Resistivity map of the water-bearing zone shows the interpretedsaline water-intruded zone (a) of Quaternary heterogeneous unconsoli-dated deposits intercalated withmarinemuddy sediments and fresh water-

bearing zone (b) of the interpreted poorly sorted gravelly sand and clayeysilt with huge boulders outwash from Asir high mountainous

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(Fig. 10). This zone could be interpreted as containing uncon-solidated Quaternary coastal and alluvial sediments saturatedwith saline water. It is clear that this zone of low resistivity isvisible in all sections (Fig. 10) and extends inland to a distanceof about 20 km from the seacoast. This low resistivity zone isunderlain by a relatively highly resistive zone (between 56 and822 Ωm), indicating the presence of hard rocks with varyingdegrees of weathering. Based on the VES results, an iso-resistivity map was constructed indicating the areal extensionof the shallow groundwater-bearing zone (Fig. 11) in the Ad-Darb area.

The distribution of the resistivity in the constructed iso-resistivity map indicates that saline water from the Red Seaintrudes into the coastal zone to a distance of about 20 km.This saline water-bearing zone is interpreted as linked toQuaternary heterogeneous unconsolidated deposits intercalat-ed with marine muddy sediments. The extension of this zonein the eastern side of the investigated with high resistivity wasinterpreted as a fresh water-bearing zone of poorly sortedgravelly sand and clayey silt with huge boulders washed outfrom the Asir Mountains.

2D Electrical resistivity tomography survey

In this study, three 2D ERT resistivity profiles were measuredalong the investigated area in a direction perpendicular to theRed Sea coast (Fig. 1) using the IRIS SYSCAL Pro-72 elec-trode system. The Dipole-dipole array was used. The inter-electrode spacing was 5 m with a maximum length of 350m. The RES2DINV software was used to create an invertedresistivity model. The maximum investigation depth of 75 mbelow the surface was reached (Fig. 12).

In all profiles, the low resistivity zone of seawater intrusionis clearly visible, with resistivities ranging from 0.3 to 20 Ωmand thickness decreasing eastward. The ERT-3 section mea-sured in the eastern side of the investigated area, showing anabrupt change in the resistivity values suggestive of a faultaffecting the bedrock and the overlying sedimentary succes-sion (Fig. 12). Given that the Red Sea fault system is the mostprominent structural element in the investigated area (Elawadiet al. 2012), it is likely that these faults could contribute in theseawater intrusion as the seawater invades through the crushedrocks in the fault zones (Mogren 2015).

Fig. 12 2D resistivity tomography sections measured in the investigated area. The locations of the profiles are shown in Fig. 1

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Discussion

The maximum TDS value of the groundwater reaches up to9000 mg/l. On the TDS distribution map (Fig. 4), a ground-water zone having high TDS values extends from the coast toa distance of about 15 km inland. The interpretation of theVES sections (Fig. 9) and 2D ERT section (Fig. 11) confirmsthe presence of saline water intrusion with the increasingthickness of this intruded zone towards the sea. The low re-sistivity values are a result of saline water intrusion, which isreflected by the presence of high TDS values for the ground-water in the same area. These findings are in accordance withearlier studies in the region confirming the presence of salinewater intrusion (Albassam and Hussein 2008; Batayneh et al.2012 and El Alfy et al. 2015).

Geologically, the zone represented by high TDS valuesconsists of Quaternary alluvium and terrace deposits compris-ing mostly of sand and silt. Numerous scholars (Andersenet al. 1988; Abarca et al. 2006; Khalil and Monterio 2009)have reported instances of saline water intrusion due to thehigh transmissivity of coastal aquifers. The thickness of thecoastal aquifers in the present study, along with its high hy-draulic conductivity, has facilitated the inward movement ofthe saline water, as has been confirmed by the geophysicalsurveys.

The resistivity results indicate a low resistivity zone thatis interpreted as the heterogeneous unconsolidated cobbles,gravel, sand, and silt of the Quaternary wadi deposits thatfacilitate the seawater intrusion. The thickness of this zoneincreases from about 12 m in the upstream side to about 70m towards the coastal and downstream sides. The resistivityof this zone, from 3.5 to 19.5 Ωm, indicates a saline water-bearing zone with dissolved solids. Along this zone, theresistivity values reach 0.3 Ωm in some places indicatingintercalations of muddy marine sediments that, according toAbou Auf and Elshater (1992), characterize the shelf andnearshore areas of the southwest of Saudi Arabia. This zoneextends in the eastern side of the study area with a relativelyhighly resistive layer (156 Ωm, see Fig. 9) of poorly sortedgravel, sand, and clayey silt with huge boulders washed outfrom the Asir Mountains that bound the studied site on itseastern side. The geoelectric investigation confirmed thatthe thickness of this seawater-intruded zone decreases east-wards, in line with the hydrogeological study of Abdalla(2016). This saline water zone is overlain by a relativelydisconnected geoelectric resistivity zone with a thicknessof 10 to 33 m and resistivities ranging from 52 to 120 Ωm,respectively. This zone could be interpreted as gravely sandsaturated with brackish to fresh water. These twogeoelectric resistivity zones are reflected in the seismic sec-tions as one seismic zone with P wave velocities of 1231 and1524 m/s that represent water-bearing alluvial and coastalsediments (Stampolidis et al. 2005).

The low resistivity zone discussed above is underlain by arelatively highly resistive zone that shows a wide range ofresistivities (from 56 to 822 Ωm) that could be correlated withthe bedrock of variable resistivity and the variable degree ofweathering. This weathered bedrock zone appears in the SRand SRT sections with higher P wave velocities (from 2315 to3164 m/s), and the lateral variation in the seismic velocitiescould be related to the variable degree of weathering.

Conclusion

Seismic refraction and geoelectric resistivity surveys were in-tegrated with hydrogeochemical investigation in this study inorder to recognize and identify the extent to which seawaterhas invaded the shallow groundwater aquifer along the coastalstrip of Ad-Darb area in southwestern Saudi Arabia.

Groundwater facies analysis shows that groundwater dom-inantly belongs to the SO4-Cl type of facies. Durov plot indi-cates base ion exchange, linear mixing, and saline water in-trusion as the main factors affecting the groundwater chemis-try of the area.

The geoelectric resistivity results indicate four geoelectricresistivity layers consisting of an upper unsaturated zone,followed by brackish to fresh water-bearing layer, salinewater-bearing layer, and the bottom layer consisting ofweathered/fractured basement rocks. The seismic velocitiesfor each of these layers are in accordance with the resistivityvalues.

The interpreted and concluded saltwater intrusion could berelated to the long-term overexploitation and/or the lithologiccharacteristics of the coastal sediments facilitating the seawa-ter intrusion in the study area. It is recommended to put regu-lations for drilling new borewells in the region and regulatethe amount of pumping in the existing wells to minimize theimpact of salt water intrusion in the coastal zones.

Funding information This study was financially supported by theDeanship of Scientific Research at King Saud University through re-search group no RGP-1437-041.

References

Abarca E, Vázquez-Suñé E, Carrera J, Capino B, Gámez D, Batlle F(2006) Optimal design of measures to correct seawater intrusion.Water Resour Res 42(9)

Abdalla F (2016) Ionic ratios as tracers to assess seawater intrusion and toidentify salinity sources in Jazan coastal aquifer, Saudi Arabia. A JGeosci 9(40). https://doi.org/10.1007/s12517-015-2065-3

Abou Auf M, Elshater A (1992) Sedimentology and mineralogy of Jizanshelf sediments Red Sea, Saudi Arabia. J KAU Mar Sci 3:39–54

Albassam AM, Hussein MT (2008) Combined geo-electrical and hydro-chemical methods to detect salt-water intrusion. Manag EnvironQual 19(2):179–193

Arab J Geosci (2019) 12: 372 Page 13 of 14 372

Albassam AM, Khalil AR (2012) DurovPwin: A new version to plot theexpanded Durov diagram for hydro-chemical data analysis. ComputGeosci 42:1–6

Albassam AM, Awad HS, Al-Alawi JA (1997) Durov plot: a computerprogram for processing and plotting hydrochemical data.Groundwater 35(2):362–367

Andersen PF, Mercer JW, White HO (1988) Numerical modeling of salt-water intrusion at Hallandale, Florida. Groundwater 26(5):619–630

Arshad M, Cheema JM, Ahmed S (2007) Determination of lithology andgroundwater quality using electrical resistivity survey. Int J AgricBiol 9:143–146

Asfahani J (2006) Geoelectrical investigation for characterizing thehydrogeological conditions in semi-arid region in Khanasser valley,Syria. J Arid Environ 68:31–52

Batayneh A, Elawadi E, Mogren S, Ibrahim E, Qaisy S (2012)Groundwater quality of the shallow alluvial aquifer of Wadi Jazan(southwest Saudi Arabia) and its suitability for domestic and irriga-tion purpose. Sci Res Essays 7(3):352–364

Bauer P, Supper R, Zimmermann S, Kinzelbach W (2006) Geoelectricalimaging of groundwater salinization in the Okavango Delta,Botswana. J Appl Geophys 60(2):126–114

Bello AA, Makinde V (2007) Delineation of the aquifer in theSouthwestern part of the Nupe Basin, Kwara State. Nigeria J AmSci 3:36–44

Benkabbour B, Toto EA (2004) Fakir Y (2004) Using DC resistivitymethod to characterize the geometry and the salinity of thePlioquaternary consolidated coastal aquifer of the Mamora plain.Morocco Environ Geol 45:518–526

Brookes M, Kearey P (1988) An introduction to geophysical exploration.English Language Book society/ Blackwell Scientific Publication, P.296.

Choudhury K, Saha DK, Chakraborty P (2001) Geophysical study forsaline water intrusion in a coastal alluvial terrain. J Appl Geophys46:189–200

El Alfy M, Lashin A, Al-Arifi N, Al-Bassam A (2015) Groundwatercharacteristics and pollution assessment using integratedhydrochemical investigations GIS and multivariate geostatisticaltechniques in arid areas. Water Resour Manag 29(15):5593–5612

Elawadi E, Mogren S, Ibrahim E, Batayneh A, Albassam A (2012)Utilizing potential field data to support delineation of groundwateraquifers in the southern Red Sea coast, Saudi Arabia. J Geophys Eng9(3):327–335

Freeze RA, Cherry J A (1979) Groundwater, 604 pp.Frohlich RK, Urish DW, Fuller J, Reilly MO (1994) Use of geoelectrical

method in groundwater pollution surveys in a coastal environment. JAppl Geophys 32:139–154

Hamdan H, Candansayar E, Demirci I, Economou N, Andronikidis N,Arslan H, Soupios P, Vafidis A (2015) Imaging the saline/freshwater interface at Bafra, Turkey using joint inversion of seismicrefraction and ERT data. Eighth Congress of the BalkanGeophysical Society, 05 October 2015, DOI. https://doi.org/10.3997/2214-4609.201414146

Khalil MA, Monterio FAS (2009) Influence of degree of saturation in theelectric resistivity-hydraulic conductivity relationship. SurvGeophys, 2009;https://doi.org/10.1007/s10712-009-9072-4

Lloyd JW, Heathcote JAA (1985) Natural inorganic hydrochemistry inrelation to groundwater. Clarendon Press, Oxford

Mogren S (2015) Saltwater intrusion in Jizan coastal zone, southwestSaudi Arabia. Inferred from geoelectric resistivity survey. Int JGeosci 2(6):286–297

MoWE (2014) National water strategy, Kingdom of Saudi Arabia.Unpublished Report.

MoWE (2015) Detailed water resources assessment of western coastalplain of Saudi Arabia. Unpublished report

Omosuyi GO, AdeyemoA, AdegokeAO (2007) Investigation of ground-water prospect using electromagnetic and geoelectric sounding atAfunbiowo, near Akure. Niger Pac J Sci Technol 8:172–182

Piper AM (1944) A graphic procedure in geochemical interpretation ofwater analyses. Trans Am Geophys Union 25:914–923

Samsudin AR, Haryono A, Hamzah U, Rafek AG (2007) Salinity map-ping of coastal groundwater aquifers using hydro geochemical andgeophysical methods: a case study from north Kelantan, Malaysia.Environ Geol 55(8):1737–1743

Satriani A, Loperte A, Imbrenda V, Lapenna V (2012) Geoelectricalsurveys for characterization of the coastal saltwater intrusion inMetapontum Forest Reserve (Southern Italy). Int J Geophys238478:8. https://doi.org/10.1155/2012/238478

Sherif M, El Mahmoudi A, Garamoon H, Kacimov A (2006)Geoelectrical and hydrogeochemical studies for delineating seawa-ter intrusion in the outlet of Wadi Ham, UAE. Environ Geol 49(4):536–555

Spalding RF, Exner ME (1993) Occurrence of nitrate in groundwater-areview. J Environ Qual 22(3):392–402

Stampolidis A, Tsourlos P, Soupios P, Th M, Tsokas GN, Vargemezis G,Vafidis A (2005) Integrated geophysical investigation around thebrackish spring of Rina, kalimnos Isl., SW Greece. J BalkanGeophys Soc 8(3):63–73

Stefani JP (1995) Turning-ray tomography. Geophysics 60:1917–1929Sumanovac F, Weisser M (2001) Evaluation of resistivity and seismic

methods for hydrogeological mapping in karst terrains. J ApplGeophys 47:13–28

Sung-Ho S, Jin-Yong L (2007) Namsik P (2007) Use of vertical electricalsoundings to delineate seawater intrusion in a coastal area ofByunsan. Korea Environ Geol 52:1207–1219

Urish DW, Frohlich RK (1990) Surface electrical resistivity in coastalgroundwater exploration. Geoexploration 26(4):267–289

Vafidis A, Soupios P, Economou N, Hamdan Andronikidis HN,Kritikakis G, Panagopoulos G, Manoutsoglou E, Steiakakis M,Candansayar E, Schafmeister M (2014) Seawater intrusion imagingat Tybaki, Crete, using geophysical data and joint inversion of elec-trical and seismic data. Near Surf Geosci 32:107–114

White DJ (2007) Two-Dimensional seismic refraction tomography.Geophys J Int 97(2):223–245

WHO (2011) Guidelines for drinking-water quality, fourth edition. ISBN:9789241548151. pp. 564

Zaidi FK, Al-Bassam AM, Kassem OM, Alfaifi HJ, Alhumidan SM(2017) Factors influencing the major ion chemistry in the Tihamacoastal plain of southern Saudi Arabia: evidences fromhydrochemical facies analyses and ionic relationships. EnvironEarth Sci 76(14):472

Zghibi A, Mezougui A, Zouhri L, Tarhouni J (2014) Understandinggroundwater chemistry using multivariate statistics techniques tothe study of contamination in the Korba unconfined aquifer systemof Cap-Bon (North-east of Tunisia). J Afr Earth Sci 89:1–15

Zhu X, McMechan GA (1989) Estimation of two-dimensional seismiccompressional wave velocity distribution by interactive tomograph-ic imaging. Int J Image Sys Technol 1:13–17

Zohdy A, Eaton G, MabeyD (1974) Application of surface geophysics togroundwater investigations. U.S.G.S., Book 2, Chapter D.

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