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Environ Eng Res > Volume 26(2); 2020 > Article
Putra, Hadian, Alam, Yuskar, Yaacob, Datta, and Harnum: Geochemistry of groundwater and saltwater intrusion in a coastal region of an island in Malacca Strait, Indonesia

Abstract

Most people in Bengkalis island use groundwater as their primary resource, but brackish water discovered in the wells located near the shore as well as in the distance. A hydrogeology study through geochemistry and isotope aspect conducted for the first time to identify the effect of saltwater intrusion (SWI) on groundwater. Na > Mg > K > Ca were the most dominant cation sequence in groundwater, while Cl > HCO3 > SO4 was the most dominant for anion. NaCl is the main type of groundwater in shallow aquifer and NaCl & NaHCO3 both became the most dominant type of groundwater in the deeper aquifer system. Most of the groundwater samples from dug wells indicating the weak acid exceeding strong acid. Meanwhile, the groundwater samples from bore wells dominated by alkalis exceeding alkaline earth. SWI analysis shows 36.4% of groundwater samples from dug wells and 40% of bore water samples experiencing the salinization process. From isotope analysis, the groundwater samples found to be originated from the higher elevation or distance source. Meanwhile, there are lighter 2H values in the samples that are indicating the groundwater flown in the deeper part of the aquifer.

1. Introduction

The people in developing countries often had dependency to groundwater resource and thus it becomes an essential and valuable water resource [1], the number of societies facing water shortages is increasing as the level of the groundwater table decreases as a consequence of excessive usage and the level of pollution of surface and groundwater resources increases due to several anthropogenic activities [2]. For this reason, there have been many studies on water quality in various parts of the world. As a vital source of drinking water and irrigation, groundwater quality should be monitor and protect from the contaminant [3]. Groundwater in the coastal area is commonly susceptible to an environmental problem such as saltwater intrusion [4, 5] and upconing [6]. The salinity of water resources is a significant problem that the world population suffers. It has been studied intensively during the past decades, particularly in coastal aquifers [7]. The complexity and non-linearity of saltwater intrusion in the coastal area become a challenge [8], several reasons could explain the salinity of groundwater such as a consequence of leaching of evaporates in waterlogged areas, weathering of minerals, the movement of saline groundwater to the shallower aquifer as a result of over-pumping [9], change and variation of climate condition and rising of sea-level [10]. At this time, the coastal area facing rapid development. In several areas, the used of coastal land changed into the industrial area, which results in more problems not only for the groundwater but also seawater, especially in the area where the industrial waste has not managed improperly. Those could lead to groundwater quality problems, and even worse, the surrounding residents cannot use the resources in daily activity due to health issues [11].
Pumping test is a method to determine hydraulic properties [12], which could be used to identify the movement of the contaminant, the possibility of saltwater intrusion, and the aquifer capability as the primary source of water [13]. In addition, the electrical resistivity tomography (ERT) method could become a supporting data to model the geometry of aquifer [14] and locating contaminant [15]. This method could also detect the pathway of pollution [16] and the boundary of freshwater and saltwater from the difference of electrical current [17].
Groundwater conditions in coastal areas in Indonesia had been affected by the human, land used activity, and groundwork [18]. Seasonal differences affected the discharge volume and nutrient variations of groundwater [19]. The salinity of groundwater occurred in major cities and found to intruded the groundwater wells, both shallow and deep wells [20]. It is occurred not only on the wells that closed to the shoreline [21] but also in further sites [22]. One of the major provinces in Indonesia, Riau, has also faced saltwater intrusion and upconing problems, particularly in the coastal areas and in the islands. The study conducted in Bengkalis island (Fig. 1), located in the Malacca Strait on the east coast of Riau mainland. Initial groundwater monitoring had detected the salty water in freshwater wells as well as in the river [23]. The northern part of the study area occupied by people and there are several beaches for tourism spots, meanwhile in the land in the southern part of the study area occupied by the harbors, and populated residential area.
Stratigraphy condition of the study area (Fig. 1) based on the regional geological map from [24] was composed of Older Superficial Deposit (Qp) and Young Superficial Deposit (Qh). Older Superficial Deposits consist of clays, silts, clayey gravels, vegetation rafts, and Young Superficial Deposit consists of clays, silts, clean gravel, vegetation rafts, peat swamps [25].

2. Methodology

Fieldwork activities consist of measuring the existing wells to obtain several data such as groundwater level, in-situ groundwater quality, and groundwater geochemistry (from water samples). Groundwater well was measured randomly both from shallow (dug) and deep wells across the island. In total, there are 22 dug wells and five bore wells (deep well) located near the coast and in the middle of the study area (Fig. 2). Five bore wells chosen by the availability of well’s information (Table 1) as well as geological information of the well. The connection between dug well and bore well is the similarity of geological conditions surrounding the well.
Well’s depth, electrical conductivity (EC), and total dissolved solids (TDS) were among the in-situ parameters that measured directly in the field. Water samples collected using a plastic bottle sealed with an inner cap, the bottle must be full with water so that no water bubbles inside, and preserved in the container to keep the water temperature in stable condition. The samples delivered to the Water Analytic Laboratory of National Nuclear Energy Agency (BATAN) to analyze the major ion such as Na, K, Ca, Mg, Cl, HCO3, and SO4. Stiff and piper diagram then plotted to determine the groundwater type.
Saltwater intrusion analysis conducted using several methods commonly used in the previous study [26] to identified groundwater status:
  • a. The enrichment of Ca calculated using the formula from [27] and [28] based on the ratio of Ca and Mg concentration (in mg/L). High ratios (> 1) could be the indication of saltwater intrusion [27]

    (1)
    Ca Enrichment=(CaMg)
  • b. [29] proposed the Simpson Ratio based on the Cl, HCO3, and CO3 concentration (in mg/L).

    (2)
    Simpson Ratio=ClHCO3+CO3
The ratio describes the contamination level and divided into five classes as shown in Table 2.
  • c. The ratio of sodium chloride (in mg/L) based on [27] to shows the impact on the groundwater. Saltwater intrusion impact would be shown by ratios less than 0.86, while anthropogenic sources of contamination would be demonstrated by ratio > 1. For the ratio value between 0.86 – 1, the mean value (0.93) become the limit to determine the contamination process. If it is less than 0.93, the contaminant source seems to be SWI. Meanwhile, if it is more than 0.93, the anthropogenic activity would be the cause of contamination.

    (3)
    Sodium Chloride Ratio=NaCl
  • d. Calculating Base Exchange Indices (BEX) proposed by [30] to distinguish if an aquifer in the salinization or freshening process.

    (4)
    BEX=Na+K+Mg-1.0716Cl (inmeqL)
Positive BEX indicates the freshening process, negative BEX indicates the salinization process, and zero value indicates no base exchange.
Stable Isotopes (δ2H, δ2O) were analyzed using LGR Laser DLT-100. The result of stable isotopes plotted into the graph to identify the origin of the water. Interaction of groundwater-seawater traced using δ2H and δ2O isotope analysis and hydrochemical methods. Both isotopes used as a marker in identifying the source of groundwater in the aquifer, the possibility could be coming from rain, deeper or shallower groundwater, seawater, fossil water, or magmatic water ([21]; [31]). The isotope analysis in Riau province studied by [32] was the first to define the Local Meteoric Water Line (LMWL) found that the reference value for LMWL in this area is δ2H = 7.6 δ18O + 10.5 (r2 = 0.921).

3. Result and Discussion

Hydrogeochemical analysis shows several major ions that have a significant role in the groundwater. The dominant ions found were sodium, chloride, bicarbonate, and sulfate. Carbonate analyzed, but the reagent shows no reaction indicated the absence of that ion. Table 3 shows the value for each major ion in groundwater samples.
The sequence of major ion could define the primary ions in a groundwater sample [33]. From groundwater samples that located near the coast, the cation sequences found as Na > Mg > K > Ca, Na > Ca > Mg > K, Na > Ca > K > Mg and Mg > Na > K > Ca, while Na > Ca > Mg > K, Na > Mg > Ca > K and Na > Mg > K > Ca sequences found in the samples located in the middle of the island. In general, the most dominant cation sequence is Na > Mg > K > Ca found in 11 samples.
The anion sequences found as Cl > HCO3 > SO4, Cl > SO4 > HCO3, HCO3 > Cl > SO4 dan SO4 > Cl > HCO3 for water samples that located close to the coast, while Cl > SO4 > HCO3 and HCO3 > Cl > SO4 sequences found in the samples located in the middle of island. Cl > HCO3 > SO4 is the most dominant anion sequence in the study area, found in 10 samples.

3.1. Groundwater Facies

The concentration of each major ion plotted into Stiff diagram (Fig. 2) and Piper diagram (Fig. 3) to identify the type and facies of groundwater in the study area.
Stiff diagram shows the water type based on dominant cation and anion in groundwater samples. Several water types had found, such as sodium chloride (NaCl), sodium bicarbonate (NaHCO3), magnesium chloride (MgCl), magnesium sodium bicarbonate (Mg, Na-K, HCO3) and sodium sulfate (NaSO4). Sodium seems to be the governing cation to determined water type from groundwater samples, and for anion, chloride became dominant ions that are ruling in the determination of water types. Fig. 3 shows the distribution of dug wells groundwater samples and the distribution of deep wells groundwater samples using Piper diagram to identify the facies and evolution of groundwater in the study area.
There are several groundwater facies determined from the cation triangle (left side), such as magnesium type, sodium or potassium type, and no dominant type (shown by red triangle). Sodium or potassium type was the dominant type of groundwater from the cation plot. From the anion triangle (right side), there are sulfate type groundwater, chloride type, bicarbonate type, and no dominant type (also shown by red triangle). Bicarbonate type is the most dominant type of groundwater from anion plot. The diamond diagram of Piper plot displays several groundwater facies such as calcium sulfate waters, sodium chloride waters, sodium bicarbonate waters, and calcium bicarbonate waters. The most dominant water facies is sodium bicarbonate waters found in 17 wells. In general, the shallow groundwater of the study area falls into zone 3 and zone 4. Zone 3 indicating weak acid exceeding strong acid and zone 4 indicating strong acid exceeding weak acid.
Groundwater samples in deep wells shown to be dominated by sodium or potassium type in the cation triangle. Meanwhile, both chloride type and bicarbonate type dominated in anion triangle. In combination, the piper diamond graph indicating the domination of both sodium chloride waters and sodium bicarbonates waters. Deep wells groundwater samples fall into zone 2, which points out alkalis exceeding alkaline earth.

3.2. Saltwater Intrusion Analysis

Several analyses had done to determine the status of groundwater toward the saltwater intrusion phenomenon (Table 4).

3.2.1. The enrichment of Ca

36.4% of shallow groundwater samples found to be affected by the saltwater intrusion having a ratio greater than 1. It comes from BKS 2, BKS 8, BKS 9, BKS 10, BKS 12, BKS 14, BKS 15, and BKS 19 station, while the rest of samples indicating normal condition by having a ratio less than 1. Meanwhile, the deep wells sample not yet reporting the saltwater intrusion problem.

3.2.2. Simpson ratio

From the calculation of chloride and bicarbonate ratio, 45.5% of shallow groundwater samples are fall into good condition category, 40.9% are in slightly contaminated, and 13.6% are in injuriously contaminated. Meanwhile, the deep groundwater samples found to be in good condition in BKS B2, slightly contaminated in BKS B1, moderately contaminated in BKS B5, and BKS B3, BKS B4 classified into injuriously contaminated state.

3.2.3. Sodium chloride ratio

Na/Cl ratio indicating the source of the contaminant in groundwater. 63.6% of shallow groundwater samples contaminated by saltwater intrusion (SWI), while the other 36.4% contaminated by anthropogenic sources. Deep well groundwater samples show saltwater intrusion contamination in BKS B3, BKS B4 and BKS B5, and anthropogenic contamination in BKS B1, BKS B2.

3.2.4. Base exchange indices (BEX)

Salinization or freshening process in groundwater could identify using BEX value. The positive value indicating the freshening process shown by 77.3% shallow groundwater samples, and the other 22.7% shows the salinization process, which indicating from negative BEX value. BKS B1, BKS B2, BKS B5 shows freshening process in deep water well, while BKS B3 and BKS B4 indicated the salinization process.

3.2.5. Saltwater intrusion zone

The calculation methods show differences based on the result’s similarity and the relationship between the techniques. Even though in several wells indicate the disagreement of the result compares to one method with another, in general, all these four methods are suitable to determine the saltwater intrusion problem in groundwater samples. Saltwater intrusion process found in 36.4% shallow groundwater samples and 40% of deep groundwater samples. The saltwater intrusion zone map (Fig. 4) shows the conditions of aquifer processing in the northern part and the southern part of the study area.
Most of the process was freshening of the aquifer and some indications of contamination from anthropogenic activity. However, several wells found to experience the saltwater intrusion process. The zone of intrusion likely to be upconing condition. The SWI impacted wells located in the middle of the freshening process, and the cause of this phenomenon could be excessive use of water.

3.3. Isotope Groundwater

Stable isotope δ2H and δ2O determined from 5 groundwater samples. They then plotted with Global Meteoric Water Line (GMWL) shown by a firm orange line and with Local Meteoric Water Line (LMWL) of Riau Province from [32] demonstrated by dash orange line.
The groundwater samples found to be in line with GMWL and LMWL, the water samples might be originated from the higher elevation or distance source. The more SWI process in the wells (Table 4) indicating the mixing of groundwater and seawater that intruded the wells, and it is an undergoing process. Meanwhile, BKS B4 and BKS B5 shown lighter 2H value compare to the other samples that are indicating the groundwater flown in the deeper part of the aquifer.

4. Conclusions

In conclusion, the result of this research found several water types in the study area determined from Stiff diagram, such as sodium chloride (NaCl), sodium bicarbonate (NaHCO3), magnesium chloride (MgCl), magnesium sodium bicarbonate (Mg, Na-K, HCO3) and sodium sulfate (NaSO4). Also, Piper diagram shows the occurrence of several groundwater facies such as calcium sulfate waters, sodium chloride waters, sodium bicarbonate waters, and calcium bicarbonate waters. The identification of the saltwater intrusion problem using four different calculations shows a similar process in each groundwater well. Most of the groundwater aquifer experienced the freshening process. However, several wells suggested the saltwater intrusion impact and the mode of intrusion likely to be an upconing process. Isotope groundwater data in some wells give the information of water origin. The groundwater found originated either from the higher elevation or distance source, and there was an indication of deeper groundwater flow in the aquifer as well. The further study of isotope groundwater needs to perform to analyze groundwater-seawater interaction.

Supplementary Information

Acknowledgment

The author shows gratitude to the Indonesia Ministry of Research and Higher Education & Universitas Islam Riau as the funding provider for this research activity. Grant number 285/KONTRAK/LPPM/4-2019.

Notes

Author Contributions

D.B.E.P (M.Sc) and Y.Y (M.Eng) conceived the study. D.B.E.P (M.Sc), M.S.D.H (Ph.D.), B.Y.C.S.S.S.A (Ph.D.) and W.Z.W.Y (Ph.D.) designed the research methods. D.B.E.P (M.Sc), W.P.D.H (B.Eng) and Y.Y (Ph.D.) data acquisition, M.S.D.H (Ph.D.), W.Z.W.Y (Ph.D.) and B.D (Ph.D.) provided statistical input; D.B.E.P (M.Sc), W.P.D.H (B.Eng) and B.Y.C.S.S.A (Ph.D.) data analysis, interpretation and wrote the manuscript. W.Z.W.Y (Ph.D.) and B.D (Ph.D.) provided grammatical revision to the manuscript. D.B.E.P (M.Sc) and W.P.D.H (B.Eng) provided manuscript revision.

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Fig. 1
Geological map of Bengkalis Island and well location map in the study area.
/upload/thumbnails/eer-2020-006f1.gif
Fig. 2
Stiff diagram plot for groundwater in the study area shows the groundwater type in each location.
/upload/thumbnails/eer-2020-006f2.gif
Fig. 3
Piper plot of shallow groundwater and deep groundwater samples.
/upload/thumbnails/eer-2020-006f3.gif
Fig. 4
The saltwater intrusion zone map in the study area shows the pattern of the saltwater intrusion zone.
/upload/thumbnails/eer-2020-006f4.gif
Fig. 5
Plot of groundwater isotope with global meteoric water line and local meteoric water line [32].
/upload/thumbnails/eer-2020-006f5.gif
Table. 1
Information on Groundwater Well in the Study Area
Well ID Depth (m) Average depth of water table (m) Distance from the coast (m)
BKS 1 3.3 2.1 2,250
BKS 2 1.2 0.8 740
BKS 3 2.1 0.8 1,430
BKS 4 2.9 0.9 1,720
BKS 5 4.4 4.2 2,990
BKS 6 2.7 2.2 8,860
BKS 7 3.8 1 1,390
BKS 8 1.9 0.4 1,760
BKS 9 3.2 1.1 1,150
BKS 10 3 3 1,110
BKS 11 1.6 1.2 1,210
BKS 12 2.6 0.1 1,360
BKS 13 2.5 0.6 3,180
BKS 14 3 1.1 1,300
BKS 15 1.4 1.2 4,670
BKS 16 3.5 1.5 6,820
BKS 17 2.5 1.9 5,160
BKS 18 1.9 0.4 1,280
BKS 19 1.9 0.4 1,430
BKS 20 1.2 0.5 2,820
BKS 21 3 1 5,090
BKS 22 2 1 5,460
BKS B1 115 60 2,650
BKS B2 60 40 1,580
BKS B3 54 40 2,740
BKS B4 57 35 2,940
BKS B5 81 60 2,300
Table 2
Groundwater Classes Based on Simpson Ratio
Ratio Quality
< 0.5 Good quality
0.5–1.3 Slightly contaminated
1.3–2.8 Moderately contaminated
2.8–6.6 Injuriously contaminated
6.6–15.5 Highly contaminated
Table. 3
The Concentration of Each Major Ion in Groundwater Samples
Well ID Parameters (Miligram/Liter)

Na K Ca Mg Cl HCO3 SO4 CO3
BKS 1 611.80 51.03 10.51 85.46 695.04 579.21 37.73 0.00
BKS 2 129.85 8.34 76.80 5.50 150.73 41.49 335.82 0.00
BKS 3 148.08 16.76 14.02 24.62 301.46 73.07 61.48 0.00
BKS 4 105.65 6.49 15.45 22.83 263.83 41.70 25.56 0.00
BKS 5 1,046.73 67.38 2.98 67.42 1,063.78 1,160.08 38.40 0.00
BKS 6 968.03 58.87 10.21 37.39 850.48 1,067.60 48.76 0.00
BKS 7 489.54 20.95 1.05 20.63 317.75 607.29 37.98 0.00
BKS B1 446.88 21.70 0.75 20.46 360.33 603.57 42.06 0.00
BKS B2 66.37 11.29 5.21 12.95 70.55 154.40 15.89 0.00
BKS 8 1,041.46 28.21 79.24 69.69 1,858.65 29.72 156.88 0.00
BKS 9 32.83 9.00 12.42 2.75 25.00 122.02 12.00 0.00
BKS 10 33.74 6.62 13.65 1.88 58.11 0.00 20.17 0.00
BKS 11 16.31 3.25 6.56 9.15 15.20 0.00 56.36 0.00
BKS 12 18.20 4.19 13.61 8.20 13.86 0.00 94.88 0.00
BKS 13 330.79 32.86 2.38 53.79 304.91 478.89 46.88 0.00
BKS 14 47.04 8.58 17.17 12.05 62.21 0.00 134.88 0.00
BKS 15 5.22 0.46 0.50 0.50 7.19 0.00 4.96 0.00
BKS 16 4.34 1.10 0.40 0.83 6.85 0.00 3.36 0.00
BKS 17 139.25 21.24 1.41 39.85 113.61 288.16 41.74 0.00
BKS 18 78.43 6.07 11.93 14.22 193.87 0.00 46.75 0.00
BKS 19 908.07 43.99 106.82 68.44 1,962.23 0.00 268.18 0.00
BKS 20 3,987.91 155.48 158.48 545.75 7,271.78 0.00 836.64 0.00
BKS 21 47.09 9.13 3.83 25.22 62.89 64.61 60.31 0.00
BKS 22 306.37 19.95 11.58 41.30 332.47 398.80 53.07 0.00
BKS B3 654.08 60.27 61.88 162.71 1,463.96 284.24 50.93 0.00
BKS B4 679.69 71.92 55.61 154.72 1,489.74 285.48 40.75 0.00
BKS B5 550.89 46.34 8.05 46.62 892.26 478.07 8.51 0.00
Maximum 3,987.91 155.48 158.48 545.75 7,271.78 1,160.08 836.64 0.00
Minimum 4.34 0.46 0.40 0.50 6.85 0.00 3.36 0.00
Average 477.58 29.31 26.02 57.59 748.47 250.31 95.59 0.00
Table. 4
Saltwater Intrusion Analysis for Each Groundwater Well
WELL ID Ca Enrichment Remark Simpson Ratio Remark Sodium Chloride Ratio Remark BEX Remark Overall Remark
BKS 1 0.122 Normal 1.199 Slightly 0.880 SWI 13.861 Freshening More Freshening Process
BKS 2 13.963 SWI 3.632 Injuriously 0.861 SWI 1.740 Freshening More SWI Process
BKS 3 0.569 Normal 4.125 Injuriously 0.491 SWI −0.251 Salinization More SWI Process
BKS 4 0.676 Normal 6.326 Injuriously 0.400 SWI −1.365 Salinization More SWI Process
BKS 5 0.044 Normal 0.916 Slightly 0.983 Anthropogenic 20.524 Freshening More Freshening Process
BKS 6 0.273 Normal 0.796 Slightly 1.138 Anthropogenic 20.884 Freshening More Freshening Process
BKS 7 0.050 Normal 0.523 Slightly 1.540 Anthropogenic 13.886 Freshening More Freshening Process
BKS 8 1.137 SWI 62.538 Highly 0.560 SWI −4.641 Salinization More SWI Process
BKS 9 4.516 SWI 0.204 Good 1.313 Anthropogenic 1.126 Freshening More Freshening Process
BKS 10 7.260 SWI 0.000 Good 0.580 SWI 0.028 Freshening More SWI Process
BKS 11 0.716 Normal 0.000 Slightly 1.073 Anthropogenic 1.084 Freshening More Freshening Process
BKS 12 1.659 SWI 0.000 Good 1.313 Anthropogenic 1.153 Freshening More Freshening Process
BKS 13 0.044 Normal 0.636 Slightly 1.084 Anthropogenic 10.404 Freshening More Freshening Process
BKS 14 1.424 SWI 0.000 Good 0.756 SWI 1.369 Freshening More SWI Process
BKS 15 1.000 SWI 0.000 Good 0.726 SWI 0.061 Freshening More SWI Process
BKS 16 0.481 Normal 0.000 Good 0.633 SWI 0.077 Freshening More Freshening Process
BKS 17 0.035 Normal 0.394 Good 1.225 Anthropogenic 6.433 Freshening More Freshening Process
BKS 18 0.838 Normal 0.000 Good 0.404 SWI −1.145 Salinization More Freshening Process
BKS 19 1.560 SWI 0.000 Slightly 0.462 SWI −13.280 Salinization More SWI Process
BKS 20 0.290 Normal 0.000 Slightly 0.548 SWI 1.700 Freshening More Freshening Process
BKS 21 0.151 Normal 0.973 Slightly 0.748 SWI 2.449 Freshening More Freshening Process
BKS 22 0.280 Normal 0.833 Good 0.921 SWI 7.147 Freshening More Freshening Process
BKS B1 0.036 Normal 0.596 Slightly 1.240 Anthropogenic 10.744 Freshening More Freshening Process
BKS B2 0.402 Normal 0.456 Good 0.940 Anthropogenic 2.100 Freshening More Freshening Process
BKS B3 0.380 Normal 5.150 Injuriously 0.446 SWI −1.038 Salinization More SWI Process
BKS B4 0.359 Normal 5.218 Injuriously 0.456 SWI −1.066 Salinization More SWI Process
BKS B5 0.172 Normal 1.866 Moderately 0.617 SWI 1.910 Freshening More Freshening Process
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