Arab J Geosci DOI 10.1007/s12517-015-2065-3

ORIGINAL PAPER

Ionic ratios as tracers to assess seawater intrusion and to identify salinity sources in Jazan coastal aquifer, Saudi Arabia Fathy Abdalla 1,2

Received: 20 September 2014 / Accepted: 10 September 2015 # Saudi Society for Geosciences 2015

Abstract The impact of seawater intrusion in coastal aquifers is a major groundwater contamination issue worldwide. The shallow coastal aquifer in Jazan, southwestern part of the Red Sea coast of Saudi Arabia, is vulnerable to salinization by seawater intrusion due to overexploitation. The present study helps to understand the impact of seawater intrusion in Jazan aquifer and to assess and determine the most dominant hydrogeochemical processes controlling the groundwater salinity. For these purposes, 70 groundwater samples have been collected and chemically analyzed for major cations and anions. Based on the analytical results, the groundwater in the study area is generally brackish to saline and alkaline in nature. Chadda’s, Piper, and other bivariate diagrams that presented the geochemical facies of the groundwater showed Na-Cl as the dominant water type followed by Ca/Mg-Cl, thus indicating the influence of saltwater intrusion and inverse cation exchange reactions in the aquifer. Salinity values (EC) of groundwater are highly variable rising from 430 μS/cm in the boreholes located further inland to 13,000 μS/cm close to the coastline (avg 3246 μS/cm) with the majority above 2500 μS/cm indicating significant salinization of groundwater. The contribution of seawater in groundwater varies from less than 0.01 % in the inland areas to 17 % in areas close to the coastline. Moreover, a significant correlation observed between EC with Na+, Cl−, and SO42− ions suggests the same source and possible incursion of nearby saline water. The combination of

* Fathy Abdalla [email protected] 1

Geology Department, Faculty of Science, South Valley University, Qena, Egypt

2

Present address: Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia

ionic ratios and recorded EC values clearly justifies the view that the main mechanism for salinization is seawater intrusion, where the saline groundwater results from mixing with seawater. Three main strategies were proposed including monitoring and management to help in the long run to reduce and control the deterioration of groundwater quality in the study area. Keywords Seawater intrusion . Coastal aquifer . Salinization . Inverse cation exchange . Molar ratios . Jazan

Introduction The increasing demands for freshwater in coastal areas (8 of the 10 largest cities in the world are located at the coastline, Post 2005) and the anticipated impacts of climate change such as the accelerated rise of sea level and variations in precipitation may result in increases in the incidence of seawater intrusion (Werner 2010). Worldwide, seawater intrusion is a common problem in the coastal aquifers especially in arid or semiarid regions where groundwater is the only source of freshwater. Its potential contamination and toxicity is related to an overall concentration of dissolved solutes and not to specific constituents. It is caused when excessive pumping of freshwater reduces the water pressure and intensifies the lateral movement of seawater into freshwater aquifers. For example, in Europe, at least 100 areas have been affected by seawater intrusion (Scheidleger et al. 2004). and in the coasts of the USA, saltwater intrusion has been documented all along for more than 150 years (Barlow and Reichard 2010). Potential salinization problems of groundwater aquifers can be caused by different processes, such as natural saline groundwater (Falgas et al. 2009) and seawater intrusion. Seawater intrusion is considered the most common and widespread in coastal areas worldwide (Post 2005; Lee and Song 2007; Abdollahi-

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Nasab et al. 2010; Allow 2012; Naidu et al. 2012; Singaraja et al. 2013; Ahmed et al. 2013; Abdalla et al. 2014). Dissolution of geogenic salts especially halite (Milnes 2011) releases equal concentrations of Na+ and Cl− (Tijani 2008). and anthropogenic contamination results from domestic, agricultural, and industrial effluents (Jones et al. 1999; Custodio 2010). In many coastal areas, several salinization processes or multiple sources are superimposed. Early and proper recognition of the salinization source helps to prevent more deterioration in groundwater quality. Seawater intrusion in coastal aquifers is accompanied by several other processes like reverse ion exchange, mineral dissolution/precipitation, and sulfate reduction (Appelo and Postma 2005). The extent of saltwater intrusion into coastal aquifers is controlled by several parameters (Barlow and Reichard 2010; Saidi et al. 2013). These parameters are as follows: the total rate of aquifer discharge compared to freshwater recharge, distance separating wells from the coastline, hydraulic conductivity, groundwater depth, and the presence of confining units that may prevent saltwater from moving vertically toward or within the aquifer. The quality of groundwater deteriorates when groundwater is mixed with 2 % saltwater, hence increasing the saltwater portion to 4 % and resulting to significant groundwater use impairments that leads to further adverse environmental impacts. If saltwater fraction were to increase to 6 %, groundwater is almost unusable for human consumption and may only be used for some industrial purposes (Custodio 2002; Darnault and Godinez 2008). In the case of Jazan area, the shallow aquifer formed from alluvial deposits in the wadis and coastal plains of the Red Sea is prone to seawater intrusion due to long-term overexploitation (Abdalla et al. 2014; Mogren 2015). Over the past four decades, the intensive use of nonrenewable groundwater was unavoidable particularly for agricultural and domestic purposes; and for most of the rural areas, it is for drinking purposes that has put the aquifers under stress. Scattered studies conducted in this coastal region (Mogren et al. 2011; Hussein and Loni 2011; Batayneh et al. 2012; Abdalla et al. 2014; Mogren 2015) have reported signs of seawater impacts. Mogren (2015) studied the deterioration of groundwater quality in the coastal plain of Jazan area using vertical electrical sounding surveys. His results showed that the shallow groundwater aquifer is hydraulically in contact with the seawater where the resistivity of the saturated zone decreases toward the coastline, thus indicating the influence of seawater. However, a detailed study that discussed the impact of seawater intrusion in Jazan area has not been hitherto reported. Hence, it is the objective of this research to assess the extent and degree of progressive encroachment of seawater intrusion and its potential impacts unto the shallow coastal aquifer in Jazan area. The study seeks to identify the salinity sources in groundwater as well as the processes of controlling such variability. Finally, the study describes actions that need to be taken to control the deterioration of water quality. The results

of this study could provide essential information for the effective design of monitoring and management strategies or measures that could help in the long run to reduce contamination risk in the coastal environment.

Site description Geological and hydrogeological setting The study area includes the coastal areas of the southwest of Saudi Arabia, between latitude 16° 45′ N–17° 30′ N and longitude 42° 15′ E–43° 00′ E (Fig. 1). The region is typified by humid and hot weather conditions with average temperature ranging from 21 °C in winter to 40 °C in summer. The average annual precipitation in the vicinity of the site ranges from more than 500 mm in the eastern parts to less than about 200 mm in the western part near the Red Sea coast, reflecting the effect of topography. Rainfall occurs during the spring and summer months and the average evaporation rates exceed 2000 mm/year. Geologically, the study area spans the western margin of the Proterozoic Arabian Shield and the eastern margin of the Cenozoic Red Sea basin (Fig. 2): It includes a small area of Precambrian rocks present in the upstream parts, comprising metamorphosed volcanosedimentary successions intruded by granite and gabbro (Al-Bassam and Hussein 2008; Elawadi et al. 2012). The Cenozoic rocks are represented by the clastic sedimentary succession underlying the black basaltic sheet of lava flows. The Quaternary deposits covered about half of Jazan area in the wadi beds and the coastal plain. They consist of interbedded clay and sands, silts, cobbles, and gravels of wadi beds with variable thickness from one place to another. The coastal plain in Jazan area is situated in a geological depression extending from the sea to the foot of the Asir Mountains. The most prominent structural elements observed in the study area are faults and joints that are controlled by the Red Sea fault system (Basahel et al. 1983). The possible existence of a large structural basin bounded with NW- and NEtrending faults makes the study area a promising area for hosting groundwater (Elawadi et al. 2012). The presence of such faults contributes extensively in the seawater intrusion as the seawater invades through the crushed rocks in fault zones related to the Red Sea rifting (Mogren 2015). However, the presence of subsurface volcanic intrusions hinders groundwater exploration and drilling activities, and in some localities, these volcanic flows crop out at the surface and cover the groundwater-bearing formations. Hydrogeologically, groundwater in the study area is present within two geologic units: the alluvial deposits of the wadi systems and the clastic coarse members of the CretaceousTertiary sedimentary succession (Hussein and Bazuhair 1992). The shallow alluvial aquifers in Jazan are composed of

Arab J Geosci

Fig. 1 Location map of the study area showing groundwater sampling points

the Quaternary wadi deposits (heterogeneous unconsolidated cobbles, gravels, sand, and silt) that enhance horizontal seawater intrusion due to its lithological variations (Al-Bassam and Hussein 2008; Abdalla et al. 2014; Mogren 2015). Typically, a main wadi channel width varies from about 100 to 1000 m (Hussain and Ibrahim 1997). The thickness of alluvial deposits varies widely from a few meters overlying lava flows up to 100 m in the center of the wadis, e.g., at El

Mwasm site, it ranges between 48 and 60 m. The average thickness of the water-bearing unit ranges from about 3 m in the upstream part to about 40 m or more in the downstream. Transmissivity value varies between 540 and 5400 m2/day with an average of 2190 m2/day, which indicates that the aquifer has good storage and conductive properties that result in a wider cone of depression during pumping associated with the higher inland movement of the seawater fronts. The storativity

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Fig. 2 Simplified geological map of Saudi Arabia (after the Saudi Geological Survey, http://www.sgs.org.sa/english/geology/phanerozoic/pages/default.aspx)

coefficient ranges between 0.01 and 0.25 with an average of 0.13 (El Mwasm site, Al Trbag et al. 1997) increasing toward the west directions. The aquifer is fed by percolation through the wadi beds from flood spates and rain that falls directly above the elevated areas in the east and southeastern sides, or due to local surface water infiltrations. Recharge rate was found to account for between 10 and 45 % of total runoff (AlTurki 1995). More recharge may keep water table high to prevent further seawater intrusion into the aquifer. Discharge from the aquifer takes place through the uncontrolled pumping for different purposes and by evaporation.

Materials and methods A total of 70 groundwater samples from both shallow dug wells and boreholes tapping the Quaternary alluvial aquifer throughout the study area (Fig. 1) were collected. Most of these wells are used for domestic and agricultural purposes. These samples were collected in clean 1000-ml polyethylene plastic bottles and stored in a refrigerator at 4 °C. Laboratory work carried out included chemical analysis (Table 1) of the collected water samples following standard methods

suggested by APHA (2005). The chemical analyses were performed for the major cations (Ca2+, Mg2+, Na+, K+) using an atomic absorption spectrophotometer. The titration method was used for the determination of the anions HCO3−, CO3−2, and Cl− and the spectrophotometer was used for the determination of SO42−. Total hardness (TH), total alkalinity, pH, salinity, electrical conductivity (EC), and total dissolved solids (TDS) of the water samples were measured in the field. Data analyses were done using, e.g., AquaChem 3.7 for Windows. Correlation analysis was performed to study the relationships between the variables EC, pH, Na+, K+, Ca2+, Mg2+, SO42−, Cl−, rNa/rCl, rSO4/rCl, rCl/rHCO3, and rCa/r(HCO3 +SO4). The seawater fractions (fSeawater) in the groundwater samples were calculated using the Cl− concentration as a conservative tracer (Appelo and Postma 2005) as follows: f Seawater ¼

C Cl‐sample −C Cl‐fresh C Cl‐Sea −C Cl‐fresh

ð1Þ

where CCl-sample is the Cl− concentration of the sample, CCl-sea is the Cl− concentration of the Red Sea water (22.219 mg/l), and CCl-fresh is the Cl− concentration (40 mg/l sample no 50) of freshwater (sample has the lowest EC value; Slama 2010).

Arab J Geosci Table 1 Summary of the concentrations of the major hydrochemical parameters and ionic deltas ΔCi in Jazan area Red Sea SD

Average Range 7.58

Parameter

pH

–

0.28

8.10

6.80

TDS (mg/l) EC (μS/cm) Ca2+ (mg/l)

41,000 51,250 225.00

2746.0 430 2537.6 3246.0 86.6 123.8

10,4 13.0 554

344.0 430.0 22.2

Mg2+ (mg/l) Na+ (mg/l) K+ (mg/l) SO42− (mg/l) Cl− (mg/l) HCO3− (mg/l)

742.00 14.255 210.00 3078.00 22.22 146.00

45.5 343.1 6.2 276.6 769.2 96.3

43.4 352.8 7.99 343.40 703.20 218.40

210 1599 31 1558 3669 518.20

4.6 23.0 1.95 46.60 34.70 47.60

rNa/Cl (meq/l) rSO4/Cl (meq/l) rCl/HCO3 (meq/l) rSO4/HCO3 (meq/l) rNa/Ca (meq/l) rMg/Ca (meq/l)

0.989 0.102 262.01 0.037 – –

0.53 0.84 10.32 2.33 6.89 0.74

1.01 0.71 7.23 2.41 4.16 0.64

2.75 5.50 64.10 7.78 31.90 2.57

0.30 0.41 0.41 0.33 0.06 0.17

% Seawater ΔCa2+ (meq/l) ΔMg2+ (meq/l) ΔNa+ (meq/l) ΔSO42− (meq/l) ΔHCO3− (meq/l)

– – – – – –

3.46 4.49 4.14 52.95 6.90 3.03

3.01 4.50 1.30 −0.30 3.10 −0.10

17.00 27.70 17.30 18.40 31.60 6.10

0.01 −5.10 −9.40 −76.00 −21.60 −7.70

Major ion chemistry and chemical processes

The expected (theoretical) mix-concentration was calculated using the seawater fractions for a given species I (Zghibi et al. 2013) as follows: C i‐mix ¼ f sea *C i‐sea þ ð1−f sea Þ*C i‐fresh

ð2Þ

The ionic delta ΔCi (react-concentration) resulting from any chemical reaction occurring with mixing was calculated by subtracting the mix-concentration from the measured concentration (Fidelibus 2003) as follows: ΔC i ¼ C i‐sample −C i‐mix

saline groundwater in coastal aquifers (Fig. 3) show that groundwater mixing with seawater is mostly confined to the coastal areas, where saline groundwater is overlain by fresh groundwater. Excessive pumping from the fresh groundwater causes upconing of the water table around the well and the interface freshwater salt water to rise in response to the pressure reduction on the interface. The movement of water in coastal aquifers is generally toward the sea due to the positive hydraulic gradient set up by the balance between recharge inland and discharge toward the sea. However, excessive fresh groundwater pumping causes a modification of the natural flow systems (reversing the hydraulic gradients) and, thus, induces seawater intrusion.

ð3Þ

Results and discussion Conceptual model and groundwater flow system A coastal aquifer is defined as a water-bearing formation that is hydraulically connected to the sea. In Jazan coastal aquifer, groundwater occurs at shallow depths, where groundwater levels are varying from 10 to 30 m below the ground surface. The piezometric gradient varies from 0.005 in the upper parts of the wadi to 0.001 at the beginning of the coastal plain (Abdalla et al. 2014). The conceptual models of fresh and

Our study focused on the major ions Ca2+, Mg2+, Na+, K+, SO42−, Cl−, and HCO3− as well as the most important relationship such as Na and rCl/r(HCO3 +CO3) versus Cl−, rNa/Ca versus rCl/SO4, rCa/r(HCO3 +SO4) versus rNa/Cl, and (Ca+ Mg)-(Na+K) versus HCO3−(SO4 +Cl), since those could be regarded as indicators for an interaction between the groundwater and seawater in the coastal aquifers. It is easily recognized from the data presented in Table 1 that there is a wide range and high standard deviations for most of the parameters such as the EC, Na+, Cl−, and SO42− contents. Such wide ranges of concentrations suggest that multiple sources regulate the chemical composition of the groundwater in the study area, i.e., the groundwater has a mixed origin that is possibly an infiltration of pure meteoric water affected by seawater intrusion. pH value of the groundwater ranges from 6.8 to 8.1 (avg 7.6) reflecting its slightly alkaline nature. Salinity EC values vary from 430 to 13,000 (avg 3246 μS/cm) indicating an influence from seawater intrusion. Concentrations of the major cations Na+, K+, Ca2+, and Mg2+ range from 23 to 1599 (avg 365), 2 to 31 (avg 8.2), 22.2 to 555 (avg 124), and 4.6 to 216 (avg 50 mg/l), respectively, indicating Na+ as the dominant cation in groundwater. The increase of Ca2+ and Mg2+ concentrations with increasing salinity could be the indication of reverse ion exchange in the aquifer. Concentrations of the major anions Cl−, SO42−, and HCO−3 in groundwater range from 35 to 3669 (avg 750), 47 to 1558 (avg 367), and 48 to 519 (avg 218 mg/l), respectively, indicating Cl− as the dominant anion in groundwater. According to the ion concentration, Na+ and Cl− are highly positively associated representing the dominant ions in the majority of groundwater samples (close to the coastline) that may indicate the increased mixing effect of seawater (Bouwer 1978). The results of the chemical analyses also showed that the spatial distributions of the major constituents followed the general trend of the EC values.

Arab J Geosci Fig. 3 Simplified conceptual model of seawater intrusion

Groundwater salinization processes The hydrogeochemical processes occur when the groundwater moves toward equilibrium in major ion concentrations (Nwankwoala and Udom 2011). Salinity is considered as the most significant form of contamination in coastal aquifers. Major ion chemistry and the molar ratio of different ions were successfully used to identify the origin of salinity, especially the detection of seawater intrusion as opposed to other salinity sources in coastal aquifers. Seawater in general has a uniform chemistry due to a long residence time of major ionic constituents with the predominance of Cl− and Na+ ions possessing a molar ratio of 0.854. In contrast, fresh groundwater is characterized by highly variable chemical compositions (El Moujabber et al. 2006). To determine whether the salinity is derived solely from mixing with seawater, the concentrations of the various cations are presented versus Cl− concentrations. An increase in Cl− concentration in groundwater represents the most indicative of seawater intrusion as a proxy for salinity. In areas where seawater intrudes a fresh coastal aquifer, cation exchange reactions will influence the chemical composition of groundwater according to this form Ca(Mg)+ NaClay ↔2Na+ Ca(Mg)Clay. Water-rock interactions include cation exchange with clay material (Apello and Willemsen 1987). In coastal aquifers, the direct cation exchange process is indicative of flushing processing during rainy periods, and it is characterized by an r(HCO3 +SO4)> r(Ca+Mg). The reverse cation exchange is referring to the seawater influence, where there is an exchange between Na+ or K+ in groundwater with Ca2+ or Mg2+ in the aquifer matrix and is marked by r(Ca + Mg) > r(HCO 3 + SO 4 ) (Fisher and Mulican 1997). In the study area, the ratio r(Ca + Mg)/r(HCO3 + SO4) > 1 was observed for 98 % of the studied groundwater samples, indicating a reverse ion exchange process linked to seawater intrusion. For inland

parts of the aquifer, the ratio r(Ca+Mg)/r(HCO3 +SO4)<1 was observed only in 2 % of the studied samples, showing a simultaneous ion exchange, where there is an exchange between Ca2+ or Mg2+ in groundwater with Na+ or K+ in the aquifer matrix. The exchange of Ca2+ and Na+ between the aquifer matrix and intruding seawater is well displayed in the Chadha’s and Piper plots (Figs. 4 and 5). From the Chadha’s plot (Fig. 4), it is shown that majority of groundwater samples fall in the zone of seawater (Na-Cl water type), and these samples are found to be located near the coastline indicating the predominance of seawater influence into the aquifer. The rest of the samples fall in the zone of reverse ion exchange water (Ca/Mg-Cl water type) which is also seen as indicative of active seawater intrusion into the aquifer. Hydrogeochemical characteristics of groundwater based on Chadha’s plot show that saline groundwater conditions are possibly attributed mainly to the seawater encroachment, the reverse ion exchange reactions into the aquifer, and to very few extents due to insignificant groundwater recharge because of the low seasonal rainfall. The geochemical processes occurring along the flow path of the groundwater in Jazan coastal aquifer were evaluated using the Piper diagram (Piper 1944). The diagram (Fig. 5) reveals two main water types. A large portion of the studied groundwater samples (65 %) falls in the saline region (Na-Cl type water zone 1), and these samples are located in close proximity to the coastline, referring to the seawater influence of the aquifer. The rest of the samples (15 %) were Ca-Cl water type (zone 2) and 20 % were mixed Ca-Mg-Cl water type (zone 3). The Piper plot also shows that alkalis (Na+ +K+) exceed alkaline earths (Ca2+ +Mg2+) and strong acids (Cl− + SO42−) exceed the weak acid (HCO3−). The lines inside the diamond shape of the figure represent the simple mixing zone of freshwater-seawater (line A), and reverse ion exchange indicates active seawater intrusion (line B) and hydrochemical

Arab J Geosci 20

Ion exchange water zone (Na-HCO3 type)

Fig. 4 Hydrogeochemical processes based on Chadha’s plot of groundwater chemistry

Recharge water zone (Ca-HCO3 type)

0 -80

-60

-40

-20

0

20

40

60

(HCO3)-(SO4+Cl)

-20

Seawater zone (Na-Cl type)

-40

Reverse ion exchange water zone (Ca-Mg-Cl type)

-60 -80 -100 -120 -140

(Ca+Mg)-(Na+K)

evaluation along the flow path (line C). The mixed types of water revealed that groundwater in the study area is under a continuous evolving process. Furthermore, the figure shows that the (rSO4/rHCO3) ratio is greater than 1 for 85 % of the studied groundwater samples, indicating that these samples are predominantly influenced by seawater intrusion. Besides, (rMg/rCa) ratio less than 1 was observed in most of the samples (75 %) that confirms the mixture of freshwater with seawater. The relationship between log (rNa/rCa) and log (rCl/rSO4) has often been used to identify geochemical facies distribution of groundwater in the aquifer. This relationship shows that most dominant ions are sodium and chloride suggesting that these ions have probably a common origin, which was

confirmed from the observed significant correlation between Na+ and Cl−. When comparing this with the Piper diagram (Fig. 5), it is evident that the majority of the groundwater samples fall in the sodium and chloride zone that could be attributed to saltwater upconing that occurs in coastal aquifers due to overpumping. To determine whether the salinity is derived solely from mixing with seawater, the concentrations of the various cations are presented versus Cl− concentrations. For this reason, ionic ratios rNa/rCl, rSO4/rCl, and rCl/ r(HCO3 +CO3) were calculated in milliequivalents per liter to evaluate the extent of seawater effect on the freshwater aquifer in the study area (Lee and Song 2007; Singaraja et al. 2013). In the seawater, Cl− is the dominant ion and it is only available in small quantities in groundwater, while HCO−3 that is available in large quantities in groundwater occurs only in very small quantities in seawater. The rCl/ r(HCO3 +CO3) known as Simpson’s ratio is important as evidence for seawater intrusion into the freshwater aquifer (El Moujabber et al. 2006). The hydrochemical data showed that the collected groundwater samples have rCl/r(HCO3 +CO3) ratio ranging between 0.412 and 64.10. According to Simpson (reported by Todd 1959), the effect of seawater encroachment could be classified using the rCl/r(HCO3 +CO3) ratio into six classes (Table 2).

Table 2 Contamination by seawater based on rCl/r(HCO3 +CO3) ratio (Todd 1959)

Fig. 5 Geochemical facies distribution based on the Piper diagram

rCl/rHCO3 +CO3 Water class

Percent (no. of samples)

≤0.5 0.5−1.3 1.3−2.8 2.8−6.6 6.6−15.5 >15.5

14 (10) 22.9 (16) 8.6 (6) 12.9 (9) 24.2 (17) 17.1 (12)

Good water (not affected) Slightly contaminated Moderately contaminated Injuriously contaminated Highly contaminated Severely contaminated

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If one carefully examines (Table 2), one could notice that 59 samples (84.3 %) have rCl/r(HCO3 +CO3)>0.5 indicating simple mixing with seawater, while the rest (sample nos. 34, 36, 49, 50, 51, 52, 55, 56, 63, 67, and 69 located away from the coastline, see Fig. 1) are unaffected by seawater. Out of the 59 samples, 22 samples (31.5 %) have rCl/r(HCO3 +CO3) and the ratio ranges between 0.5 and 2.8 which indicates that groundwater is slightly to moderately affected by seawater. Nine samples (12.9 %) have rCl/r(HCO3 +CO3) with ratio ranging between 2.8 and 6.6 indicating injurious contamination with seawater. Finally, 17 samples (24.2 %) have rCl/ r(HCO3 +CO3); the ratio ranges between 6.6 and 15.5 indicating high contamination with seawater. The rest of the samples (12 samples, 17 %) have rCl/r(HCO3 +CO3)>15.5 that shows severe contamination with seawater. Similarly, the rCl/ r(HCO3 +CO3) ratio as a function of distance from the coast (Fig. 6) indicates that the boreholes close to the coastline have much higher rCl/r(HCO3 +CO3) molar ratios than the boreholes drilled away from the coastline. This also shows how the proximity from the coastline can affect groundwater quality by seawater intrusion. To confirm the effect of reverse ion exchange, chloroalkaline index CAI1 was calculated in milliequivalents per liter according to the relationship proposed by Schoeller (1965). CAI1=Cl-(Na+K)/Cl. If reverse ion exchange occurs in groundwater, CAI1 values are positive. The calculated CAI1 values showed that 50 samples (71.4 %) are positive indicating that reverse ion exchange is the dominant process, while the rest of the samples show negative values that undergo ion exchange. This shows that the interaction between the seawater and groundwater in the study area is playing a major role in the contamination of the aquifer by seawater intrusion. The abovementioned ionic ratios reflect the meteoric origin of groundwater in the study area that is affected by seawater intrusion. However, some wells (37, 38, and 40) have high ionic ratios in spite of their great distance from the coastline (Fig. 1) suggesting another source of salinization. These main sources might be due to the dissolution of more acidic silicate minerals in the alluvium deposits, dissolution of evaporite minerals, and cation exchange on clays. Moreover,

anthropogenic sources such as detergent factories and town effluent discharges into wadis could be also another source. In summary, the salinization process from seawater intrusion followed by cation exchange reactions linked to seawater intrusion mainly controls the changes in the chemical composition of groundwater. The rCl/r(HCO3 +CO3) showed a positive linear relationship (Fig. 7) with Cl− concentration indicating simple mixing with saline water. The rNa/rCl molar ratio has often been used to distinguish the source of the salinity in groundwater (Herczeg and Edmunds 1999; Garcia et al. 2001). The analyzed groundwater samples have rNa/rCl molar ratio ranging from 0.3 to 2.75 and theoretical seawater ratio of 0.86. Ratios less than the seawater ratio (0.86) indicate that fresh groundwater was contaminated with the saline water, where inverse cation exchange occurs and Na+ is taken by the exchanger. Most of the groundwater samples have rNa/rCl molar ratio less than the seawater ratio, which serves as evidence for marine influence on the water salinity. The bivariate diagram between Na+ and Cl− (Fig. 8) shows that the majority of the samples are falling lower than the theoretical seawater dilution or mixing freshwater-seawater line. This is confirmed also by the calculated values of ionic deltas (Table 1) where the majorities of the samples are depleted in Na+ and are plotted below the mixing line (Fig. 8). The calculated Na/Cl molar ratios showed that 42 samples (60 %) are less than the seawater ratio indicating contamination by seawater to some extent, while 9 samples (13 %) showed values close to the seawater value, thus indicating recent simple mixing with seawater (Mercado 1985). These wells are mostly located in the southern and western parts, in particular, close to the Red Sea coast. The rest of the samples (27 %) exceeded the seawater value, which might be due to anthropogenic sources like domestic wastewaters (Jones et al. 1999) or probably controlled by water-rock interaction (Gaofeng et al. 2010) that reflects meteoric water recharges. The combination of the high rNa/rCl molar ratios with the

100 R² = 0.6031 Severe affected

Cl/(HCO3+CO3)

50

Cl/(HCO3+CO3)

40 30 20

10 Injuriously/highly affected

Slightly/moderatelyaffected

1 Notaffected

10 0 0

10

20

30

40

50

60

Distance from coastline (Km)

Fig. 6 Plot of the ratio rCl/r(HCO3 +CO3) as a function of distance from the coastline

0.1 10

100

1000

10000

Cl (mg/l)

Fig. 7 Geochemical facies distribution based on the scatterplot of rCl/ r(HCO3 +CO3) versus Cl−

Arab J Geosci 1000

120

Cl (meq/l)

100

Cl (meq/l)

80 Cl-Excess

Cl enriched zone

10 Cl depleted zone

1

40

0.1 100

Na-Depleted

1000

10000

100000

EC (µS/Cm)

Fig. 10 Scatterplot of EC versus Cl− 0

0

20

40 Na (meq/l)

60

80

−

+

Fig. 8 Scatterplot of Na versus Cl

high salinity values serves as evidence that seawater intrusion could be the main cause of salinization at locations close to the coastline. The rNa/rCl versus rCa/r(HCO3 +SO4) plot (Fig. 9) indicates that a large proportion of the groundwater samples (59 %) fall in the saline region (Cl− excess) and (24 %) in the base exchange, referring to the seawater influence of the aquifer. The rest of the few locations (16 %) fall in the saline region (Ca2+ excess). The variation in the molar ratios of rSO4/rCl was calculated with respect to Cl− concentrations to assess the impact of seawater intrusion in the study area. The calculated ratios range between 0.12 (well no. 26 located nearer to the coastline) and 3.31 (well no. 65 located away from the coastline) reflecting a decrease in rSO4/rCl value with an increase in Cl− concentrations. The molar ratios (87.2 %) of the examined samples have rSO4/rCl<1 which reflects the dominance of Cl− over SO42− that corroborates the contamination of the aquifer by seawater intrusion. The relationship between EC versus Cl− concentration reveals a strong positive correlation for most samples (Fig. 10).

4

Ca/(HCO3+SO4)

3 Ca-Excess

2 Natural state

Samples located above the simple mixing line may result from pure mixing between Cl−-enriched freshwater with seawater. Correlations among the physicochemical parameters of the collected samples revealed positive correlation values (r values that range from 0.70 to 0.94) among EC, with the major components of seawater Na+, Cl−, and SO42−. Subsequently, the EC values were mostly influenced by ion concentrations that are dominant in seawater (Na+, Cl−, and SO42−). These signs justify the influences of seawater on groundwater salinity. Strong positive correlations (r=0.86 to 0.96) were observed between EC with Cl− (r=0.98) and Na+ (r=0.91) and a moderate positive correlation (r=0.46 to 0.74) with Cl/ HCO3 (r=0.72). Cl− concentration shows strong a positive correlation with Na+ (r=0.89) indicating the same source (Kim et al. 2003) and a moderate positive correlation with SO42− (r=0.58). Na+ showed a moderate positive correlation (r=0.55) with SO42−. The overall view of the above correlations indicates that the main contributors to the groundwater salinity are Cl−, Na+, and SO42− which prevailed in most of the samples as a result of seawater intrusion into the aquifer. Mixing rate and quantification of seawater intrusion The contribution of seawater (seawater fractions) as calculated from Eq. (2) varies from less than 0.01 % (well no. 50) in the inland areas to 17 % (well no. 41) near the shoreline (close to the coast). The highest value of fSeawater corresponds with the highest measured values of Cl− and EC (3369 mg/l and 13, 000 μS/cm, respectively). The calculated values of ionic deltas ΔCi (Table 1) show that the majority of the samples (%) are depleted in Na+ (negative values) and enriched in Ca2+ (positive values). Where under the conditions of saline water intrusion, Na+ in seawater replaces Ca2+ adsorbed onto the surface of clays and this results in a relative depletion of Na+ in groundwater.

1 Cl-Excess

Monitoring and management strategies

Base-Exchange

0 0

1

2 Na/Cl

Fig. 9 Scatterplot of rNa/rCl versus rCa/r(HCO3 +SO4)

3

In the light of the results of the study, three main strategies are proposed for monitoring and management to help in the long

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run to reduce the contamination risk and to control the deterioration of the aquifer by raising groundwater table. The first is water quality monitoring measures. Establishing a seawater intrusion monitoring well network and warning systems, especially in the western part of the area, is necessary to determine if more seawater intrudes into the aquifer. Automatic monitoring programs (especially water level, EC, and water temperature) are measured regularly inside the well in addition to chemical analysis especially the predominance of Na+ and Cl− that indicates saline water impact. The second proposed strategy is called Bengineering measures.^ The most recommended engineering measure is to construct subsurface barriers and dams (rockfill or concrete) which may be more effective in reducing the seawater intrusion risk. The area of Jazan and Tihama plains accounts for more than 60 % of the quantities of floods in the Kingdom as the total volume of water floods annually 1250 MCM (Abdalla et al. 2014). This water has not been fully utilized because of its direct drain-out into the Red Sea. Beneficiation from this water can be achieved, by constructing dams and trenches as recharging aquifers and storage facilities (Fig. 11). This also helps in settling Bedouins by enabling them to take up more stable farming activities. Jazan Dam was built in the Jazan Wadi at Malaki, 35 km from Jazan City over a catchment area about 1100 km2 and maximum discharge about 2100 m3/s. The wadi fillings consist mainly of thick alluvium or loose gravel, sand, and silt, which have pore spaces between the grains and easily allow water to infiltrate to the subsurface strata. Fractures also play a role as conduits for infiltration of the surface water into the subsurface strata. Moreover, the treated wastewater can be used to recharge the aquifer using injection wells along the shoreline to push the interface toward the sea. The third is the regulation and legislation measures. Reviewing the existing policies of water resources management is the most needed regulation and legislation measures to protect aquifers from exploitation and pollution. This includes abstraction limits, licensing of private well drilling, number of wells in the coastal area as well as getting the abstracting wells away from the coast (claimed protected zones). Fig. 11 Multifunctional dam— left: schematic sketch for the recharge dam modified after Japan International Cooperation Agency (2010). right: field photograph of Jazan Dam

Summary and conclusions In this study, an attempt was made to explain changes in groundwater composition as a result of seawater intrusion in Jazan area, southwestern Saudi Arabia. The study was carried out in a shallow phreatic aquifer, which is subject to overexploitation, to accommodate the water demands. The mixing of seawater with freshwater was analyzed and confirmed by using ionic deviations and Piper, Chadda’s, and bivariate diagrams as well as molar ionic ratios. These showed that the groundwater in the inland areas is fresher than that close to the coast, which justifies the influences of seawater intrusion along with cation exchange linked to seawater intrusion. Moreover, careful examination of max, min, median, and standard deviation for the different parameters, especially Na+, Ca2+, Cl−, and SO42− ions and EC values, is evident that seawater plays a very significant role in the quality/composition of the groundwater in the study area. Furthermore, seawater intrusion in the aquifer is confirmed from the higher values of EC and seawater fraction in the southern and western parts close to the Red Sea coast. In addition, the correlation analysis of the available data sets revealed that EC was largely influenced by ions that are dominant in seawater (Na+ and Cl−); hence, seawater intrusion was confirmed as the main cause of the site’s salinity. These findings are consistent with the findings of Mogren (2015) who reported groundwater quality deterioration in the coastal plain of Jazan area by seawater intrusion using vertical electrical sounding surveys. In conclusion, the groundwater in the study area originated from the mixing of infiltrated rainwater (fresh groundwater) and seawater. The zone where the freshwater could be found is located in the east and the northeast of the study area. In addition, the results draw attention toward the importance of establishing a seawater intrusion monitoring well network (EC monitoring and periodic chemical analysis) particularly near the coastal area to track salinization especially in the western and southern parts. Moreover, management decisions regarding reduced exploitation/controlled pumping and constructing multifunctional dams and trenches along with

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artificial recharging through surface water from pond, trench, and wells help to prevent or at least minimize upconing and lateral migration of saline groundwater. It is concluded that the processes that govern the salinity of groundwater as revealed by chemical and statistical analyses are mainly associated with seawater intrusion and ion exchange processes linked to seawater intrusion. The present study could be useful to help planners and decision-makers protect our limited groundwater resources for future generations. Acknowledgments The author is grateful to the journal editor and reviewers for their suggestions and comments. He also thanks Prof. Ali AlTurky, Professor of Soil Sciences, King Saud University, and Prof. Antar Abdellah, Professor of English Language Pedagogy, Taibah University, for their help. The author would like to extend their sincere appreciation to the Deanship of Scientific Research at king Saud University for its funding through Research group (RGP-1437-012).

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