The Influence of Seawater on the Chemical Composition of ... - BioOne

Journal of Coastal Research

28

1

64–75

West Palm Beach, Florida

January 2012

The Influence of Seawater on the Chemical Composition of Groundwater in a Small Island: The Example of Manukan Island, East Malaysia Ahmad Zaharin Aris{, Sarva Mangala Praveena{, and Mohd Harun Abdullah{ { Department of Environmental Sciences Faculty of Environmental Studies Universiti Putra Malaysia 43400 UPM Serdang, Selangor, Malaysia [email protected]

www.cerf-jcr.org

{ Environmental Science Programme School of Science and Technology Universiti Malaysia Sabah 88999 Kota Kinabalu, Sabah, Malaysia

ABSTRACT ARIS, A.Z.; PRAVEENA, S.M., and Abdullah M.H., 2012. The influence of seawater on the chemical composition of groundwater in a small island: the example of Manukan Island, East Malaysia. Journal of Coastal Research, 28(1), 64– 75. West Palm Beach (Florida), ISSN 0749-0208.

www.JCRonline.org

Manukan Island in Sabah, Malaysia, is characterized as a small, unique island where groundwater is a major source for domestic water and other water-related activities. Hydrochemical studies were carried out in the island with the objective of identifying the influence of seawater on the chemical composition of groundwater in Manukan Island via ionic ratios and saturation states. From the calculated ionic ratios, the chemical composition of groundwater in the study area in general is influenced by seawater intrusion. The Na/Cl ratios ranged from 0.10–2.70, implying that the fresh groundwater in Manukan Island was affected by the seawater signature. Values close to ratio of seawater indicate a recent intrusion of seawater into the aquifer. Saturation index values show that the cation exchange process is found to control the concentration of calcium, magnesium, and sodium in the groundwater by precipitation of carbonate minerals as an extended effect from the mixing of seawater and fresh groundwater from its aquifer. The findings show that even though the Manukan Island aquifer is surrounded by seawater and is vulnerable to seawater intrusion attributed to its physical characteristics, it is also heavily affected by human activity related to groundwater abstraction. The data clearly indicate that seawater intrusion is the main contributor to salinity enrichment in the study area.

ADDITIONAL INDEX WORDS:

Groundwater, seawater, hydrochemistry, ionic ratios, saturation states, small island.

INTRODUCTION Manukan Island is a fast-growing tourist attraction with some good stretches of beaches on the southern coastline, which are a major attraction of Kota Kinabalu, Sabah, as one of Malaysia’s heritage reserves. With the steep rise in the number of tourists, especially over a span of 8 years from 1997 (125,000 tourists) to 2004 (175,000 tourists), demand for groundwater resources has increased tremendously to meet domestic needs; because of the limited freshwater sources, groundwater is the only freshwater resource available on the island. This has resulted in an enormous increase in groundwater extraction (pumping from shallow island’s aquifer), leading to contamination of the wells in the island. With this current practice, incursion of seawater into the island aquifers, especially in the low-lying area of the island, is a natural occurrence and expected to have significance consequences on groundwater quality (Aris et al., 2009). The chemical characteristics of groundwater in island environments are strongly influenced by the inflow of marine compounds such as NaCl and also Ca, Mg, and SO4 (Custodio, 1991). This marine contamination suffered by coastal aquifers DOI: 10.2112/JCOASTRES-D-10-00020.1 received 6 February 2010; accepted in revision 25 May 2010. Published Pre-print online 3 August 2010. ’ Coastal Education & Research Foundation 2012

or groundwater close to the sea is the result of two processes: seawater intrusion and infiltration of atmospheric precipitation (rainfall, etc.). These contamination processes by salinization have a special impact on small islands, depending on their relief, climate, and aquifer lithology. Any environmental hazard such as water quality declination cannot be prevented or controlled altogether but can be mitigated by taking appropriate measures such as proper water use planning, management, and development. Hence, this study concerns the groundwater in Manukan Island, with descriptions of the impacts of seawater on the chemical composition of fresh groundwater via ionic states and saturation states of the freshwater–seawater mixing process in a closed system such as Manukan Island. Increased knowledge of groundwater chemical composition in tropic regions, especially the small island environment, could lead to improved understanding of hydrochemical systems in such areas. This can contribute to effective management and utilization of the groundwater resource by clarifying relations among the associated parameters.

MATERIALS AND METHODS Geography and Climate The island is located just offshore from Kota Kinabalu in Sabah, east Malaysia, on the island of Borneo (Figure 1);

Chemical Composition of Groundwater in a Small Island

Figure 1.

65

Schematic map showing the geographical locality of Manukan Island and sampling locations with insert showing the locality of Malaysia.

Manukan Island is one of the islands under Tunku Abdul Rahman Parks. It covers an area of 206, 000 m2; Manukan Island (5u579–5u589N and 115u599–116u019E) is the park’s second largest island after Gaya Island. It is surrounded by other magnificent small islands namely Sapi, Mamutik, and Sulug, and features the most developed tourist facilities, which include 20 units of chalets, a clubhouse, a few restaurants, and a diving center. Recreational facilities include a swimming pool, a football field, and squash and tennis courts. Infrastructural facilities include water support, electricity, desalination plant, and sewerage system. Almost 80% of the area is covered by dense vegetation on the high relief side, while the rest, 20% of the area, is developed for tourism activities that are located on the low-lying area of the island where the topography of the area is relatively hilly with maximum elevations of approximately 60 m on the western part of the island and a gradually decreasing elevation toward the eastern

coast of the island. The best beach is on the eastern tip of the island. Offshore of Manukan are coral reefs, which are ideal for snorkeling, diving, and swimming. The island is crescent shaped, 1.5 km long and 3 km wide in the middle. The island is enacted under the government’s Parks Enactment 1978, and the management of the island is under the supervision of The Sabah Parks Trustees. The area experiences a warm and humid climate with annual rainfall range between 2000 and 2500 mm, and large amounts of the precipitation reaches the groundwater, although there is also the possibility of recharge via groundwater movement from the hilly area. The monthly average rainfall distribution from 1995 to 2007 is shown in Figure 2. The yearly temperature ranges from 21uC to 32uC with humidity between 80%– 90%. The climate is affected by the northeast and southwest monsoons, tropical winds that alternate during the course of the year. The northeast monsoon blows from November to

Journal of Coastal Research, Vol. 28, No. 1, 2012

66

Figure 2.

Aris, Praveena, and Abdullah

Average monthly rainfall distribution for the study area from 1995 to 2007.

March, the southwest monsoon from May to September, and the periods between the monsoons are usually marked by heavy rainfall.

Geology and Hydrogeology Geologically, Manukan Island is underlain by interbedded sandstone and sedimentary rocks classified as the Crocker Range rock formation of the western coast of Sabah. Toward the end of the Middle Miocene, which happened about 1 million years ago, changes of sea level occurred, resulting in portions of the mainland being cut off by the sea and the formations seen today were deposited (Basir, Sanudin, and Tating, 1991). The lower sequence of the island consists of thick yellowish brown shale and dark grey sandstone interbedded with black-gray thin sandstone; the weathered sandstones are yellowish in colour, whereas the shales are brown and dark (Abdullah, Musta, and Tan, 1997). Exposed sandstone is a feature of the coasts of this island, forming cliffs and deep crevasses along the shore. The sedimentary rock of Manukan Island dips toward the low relief area (east–northeast), with angles of 15u–45u. The fold forms a slight symmetrical syncline in the low relief, and small-scale normal faults and joint sets can be observed in several locations around the island (Abdullah, Musta, and Tan, 1997). In the lowland area, the thickness may reach 12 m with equal amounts of carbonate, sands, and finer materials (Abdullah, 2001; Figure 3). An early study on the morphology of the island’s aquifer conducted by Abdullah, Kassim, and Hanapi (2002) found that the thickness of the overlying rocks is approximately 11 m at the southern area, 5.7 m at the northern part, and 12 m at the middle part from the ground surface to bedrock. In general, the soil profiles of the low-lying areas are thinner compared with those of the hilly area, as reported by Abdullah, Musta, and Tan (1997) (Figure 4). The changes in sea level that occurred during the Quaternary caused the formation of limestone terraces in coastal areas where Manukan Island is formed of carbonate rocks that originated from coral deposits and were overlain by Quarternary alluvium (Basir, Sanudin, and Tating, 1991). The alluviums are loose, not cemented, and act as sufficient water storage, which entirely depends on their thickness. A study by Abdullah et al.

(2002) indicates that the medium of the aquifer consists of fine to coarse sand mixed with some fine gravel. On the lowland, the sandstone has about the same thickness as shale and carbonate deposits. Small aquifers may occur in the sandstone alluvium regions that often occur at sites near the coast. Presently, Manukan Island relies on the shallow aquifer for its groundwater supply. Dug wells are used for extracting groundwater from its sandy aquifer with a pumping rate of 72 m3/d. All wells are subject to continuous pumping rates starting over a decade ago. The wells have a diameter of 150 cm and depths between 1.5 to 6.0 m from ground surface level. The wells were operated and controlled from a pump house and managed by the resort operators. The water from the wells is pumped automatically using a water pump that was installed with a water level meter. The water from the wells is pumped into a storage compartment located near to the pump house before it is drained to the main water tanks situated on the hill. Modeling and monitoring of the groundwater resources demonstrated that the usage of water to cater to the demands on the island were between 10,000 to 22,000 L/mo.

Sampling and Analyses Groundwater samples from the island were collected from March 2006 to January 2007. A total of 162 groundwater samples were obtained during the sampling period from nine existing wells located on the low-lying area of the island; these wells were treated as one sampling site. Based on the Eurachem guide, a sampling point is defined as the place where sampling occurs within the sampling location and a sampling location as the place where sampling occurs within the sampling target (Ramsey and Ellison, 2007). Polyethylene bottles soaked in 1 : 10 HNO3 acid wash and prerinsed with distilled water were used to store groundwater samples based on the methods described in APHA (1995). The analysis of water samples was carried out to assess pH, temperature, electrical conductivity (EC), salinity, total dissolved solids (TDS), and ions, namely sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), bicarbonates (HCO3), chloride (Cl), and sulfate (SO4). The water pH and temperature were measured using WTW pH 315i, WTW Cond. 315i



Figure 3.

67

Three-dimensional elevation and the aquifer cross section Y-X of Manukan Island.

measured EC, salinity, and TDS using Oaklab 13. Field measurements were done immediately following sample collection to acquire representative values of groundwater quality. The samples were transported in an ice-filled cooler box and kept refrigerated at ,4uC prior to analysis in the laboratory. Water samples collected were filtered through 0.45mm membrane filter paper (Millipore) using a glass filtration unit and acidified with concentrated HNO3 to a pH of ,2 for cation determination. Cl and HCO3 were analyzed using argentometric (AgNO3) and titration methods (HCl), respectively. Sulfate was detected using a HACH (DR/2040) meter. The major cations (Na, K, Ca, and Mg) were determined using flame (air–acetylene burner) atomic absorption spectrometry (Zeeman Atomic Absorption Spectrophotometer Z-5000, Hitachi, Japan). Overall, the procedures of analyses adopted in this study were based on the methods described in APHA (1995). The impacts of seawater on the chemical composition in fresh groundwater via ionic states and saturation states in fresh groundwater are identified and investigated. Ionic ratios are widely used to assess the impact of seawater on aquifer chemistry using Cl as the groundwater chemical constituent with the ionic concentrations given in milliequivalents per liter (meq/L). Additionally, geochemical analyses using PHREEQC were employed to calculate the saturation states of groundwater resulting from the freshwater–seawater mixing process occurring in Manukan Island.

RESULTS

Figure 4. The soils profiles of the lowland and the hilly area of Manukan Island.

The overall data obtained in this study are shown in Table 1. The pH values are between 6.59 and 7.97. It is found that there is no distinct grouping of these values. The temperatures of the groundwater are generally between 26uC and 29uC. Significant differences are found in the salinity values, where the lowest


68

Table 1.


The physiochemical properties and major ions of groundwater and seawater in the study area.

Variable

Seawater Samples

Groundwater Sampling Periods March ’06

May ’06

July ’06

Sept. ’06

Nov. ’06

Jan. ’07

27.8 0.81 26.3–28.7

28.3 0.69 27.1–29.4

28.0 0.66 27.0–29.0

27.9 0.81 26.8–29.0

27.6 0.83 26.5–29.0

27.7 0.62 26.7–28.5

Temp. (uC) Mean SD Range

30.4 0 30.4–30.4

pH Mean SD Range

8.10 0.01 8.11–8.10

7.55 0.22 7.18–7.97

7.35 0.19 7.08–7.60

7.18 0.15 6.98–7.48

7.03 0.22 6.59–7.37

7.36 0.20 7.11–7.65

7.16 0.15 6.94–7.43

42.9 0 42.9–42.9

5.59 2.14 1.66–8.71

8.36 3.01 4.27–12.26

4.12 1.43 0.99–6.31

3.23 1.00 1.06–4.33

1.03 0.44 0.30–1.59

6.43 3.14 1.64–9.81

27.8 0.00 27.8–27.8

3.55 1.55 0.90–5.90

4.59 1.99 1.70–7.10

2.36 1.01 0.29–3.90

3.39 0.71 1.81–4.33

3.72 1.10 2.08–5.24

5.10 1.94 2.35–7.40

EC (mS/cm) Mean SD Range Salinity (ppt) Mean SD Range TDS (mg/L) Mean SD Range

45,764 1.00 45,763–45,764

4201 1366 2449–6811

4641 2025 2195–8263

3390 926 1133–4632

4172 920 2420–5418

4964 1655 2518–6999

5843 2145 2979–8294

414 0.86 414–415

309 83 189–467

248 56 128–298

213 76 60–331

552 121 411–866

496 144 256–776

523 78 354–640

825 0.35 824–826

120 58 27–221

179 92 32–298

101 42 35–163

80 21 34–108

92 42 30–157

88 54 3–174

11,121 0.96 11,120–11,122

1618 504 1001–2780

1205 609 482–2606

352 159 104–642

1006 309 316–1420

1150 479 434–1814

1756 845 552–2761

416 0.39 416–418

38 21 11–83

55 28 14–94

29 12 4–44

24 7 7–32

32 15 8–56

40 20 8–63

125 0.25 125–126

350 65 278–520

55 28 14–94

327 48 268–405

304 40 254–386

325 43 266–410

331 37 281–400

19751 2.73 19,750–19,752

1467 814 340–2774

2290 1127 550–4074

2194 719 425–3199

1875 392 1000–2399

2059 608 1150–2899

2825 1071 1299–4099

2600 0.00 2600–2600

301 136 80–500

356 211 30–660

170 58 60–240

187 56 50–250

231 116 50–400

292 154 25–475

Ca (mg/L) Mean SD Range Mg (mg/L) Mean SD Range Na (mg/L) Mean SD Range K (mg/L) Mean SD Range HCO3 (mg/L) Mean SD Range Cl (mg/L) Mean SD Range SO4 (mg/L) Mean SD Range

reading was recorded in the month of July 2006 (0.29 ppt) and the average value is 2.36 ppt. The EC ranged between 0.99 and 12.26 mS/cm. This is a good illustration of the intrusion of saltwater as a result of a single pumping operation. It can be seen in general that the high values of conductivity observed (EC . 5 mS/cm) are related to seawater intrusion, as can be seen from the high NaCl levels in the analyses. The EC values

between 2 and 5 mS/cm corresponds to mixtures of varying proportions of freshwater and seawater after pumping (Custodio, 1991; Dazy et al., 1997; Freeze and Cherry, 1979). Wide ranges and great standard deviations occurred for most parameters, where the highest value was recorded in January 2007 and the lowest value in July 2006. The basic properties of groundwater (pH, temperature, EC, salinity, and TDS) and



Table 2.

69

The ionic composition of potential salinization sources. Hydrochemical Ratio

Sampling Period

Na/Cl

SO4/Cl

Mg/Ca

Cl/HCO3

Ca/(HCO3 + SO4)

March ’06 Mean SD Range

2.48 0.37 0.86–2.70

0.16 0.03 0.12–0.24

0.64 0.29 0.19–1.18

7.59 4.46 1.72–14.87

1.31 0.34 0.88–1.92

0.91 0.41 0.49–1.99

0.12 0.04 0.01–0.21

1.16 0.52 0.39–1.78

12.62 6.25 2.77–22.98

1.05 0.32 0.71–1.73

0.26 0.08 0.10–0.38

0.06 0.02 0.04–0.10

1.02 0.77 0.22–2.63

12.10 4.67 1.85–18.35

1.18 0.41 0.39–1.87

0.80 0.12 0.49–0.91

0.07 0.01 0.04–0.09

0.25 0.07 0.13–0.32

10.85 2.76 4.73–14.84

3.15 0.88 2.41–5.86

0.83 0.13 0.58–0.97

0.08 0.02 0.03–0.11

0.31 0.12 0.11–0.50

10.18 3.82 5.46–15.85

2.47 0.59 1.50–3.60

0.91 0.14 0.66–1.04

0.07 0.02 0.01–0.09

0.27 0.15 0.01–0.52

15.17 6.50 5.32–23.12

2.38 0.60 1.83–3.74

May ’06 Mean SD Range July ’06 Mean SD Range Sept. ’06 Mean SD Range Nov. ’06 Mean SD Range Jan. ’07 Mean SD Range Seawater (this study) Seawater (other studies) Deep brines upconing

0.87 0.86–1.00*,{ ,0.81 ,0.86*,** 0.5–0.8{

0.10 0.05*.{ ,0.05{,1

3.50 .5.001 .1.00*

271.62

0.37 0.35–1.00* .1.00*,{,1,**

* Mandel and Shiftan (1981). { Vengosh and Rosenthal (1994). { Vengosh et al. (1999). 1 Vengosh and Ben-Zvi (1994). ** Mercado (1985).

major constituents of the groundwater (Ca, Mg, Na, K) are given in Table 1. The wide distributions among major composition of studied groundwaters indicated that the chemical composition was affected by multiple processes, including seawater–freshwater mixing (Aris et al., 2007).

DISCUSSION Ionic Ratios Analysis To delineate the processes contributing to the increasing salinity in the study area due to the influence of seawater, researchers use the ionic ratios of Na/Cl, SO4/Cl, K/Cl, Ca/ (HCO3 + SO4), Mg/Ca, and Cl/HCO3 to assess the impacts of seawater on aquifer chemistry (Hem, 1985 Mandel and Shiftan; 1981; Revelle, 1941; Vengosh and Rosenthal, 1994). Ionic ratios were computed from analyzed groundwater chemical constituents, using the ionic concentrations in milliequivalents per liter. Because the investigated area is surrounded by a marine environment, the ratios of Na/Cl, SO4/ Cl, K/Cl, Ca/(HCO3 + SO4), Mg/Ca, and Cl/HCO3 are used to

distinguish the factors responsible for the variation of groundwater chemistry with respect to the different potential of salinization sources. Table 2 shows the ranges of hydrochemical ratios for the studied groundwater. Continuous fresh groundwater abstractions have resulted in significant seawater intrusion because of the nature of the medium of shallow and permeable aquifers. In general, conservative seawater–freshwater mixing is expected to show a linear increase in Na and Cl (Sanchez, Pulido, and CalaforraChordi, 1999). This notable linear increase can be observed from this study because it shows positive and significant correlation (r 5 0.999, p , 0.01; Figure 5). Seawater mixed with fresh groundwater has distinguishable hydrochemical characteristics. Seawater in this study has distinct ionic ratios such as Na/Cl 5 0.87, SO4/Cl 5 0.10. Groundwater ratio values that are less than the seawater ratio (0.867 6 10%) indicate that fresh groundwater was contaminated with saline water (Mandel and Shiftan, 1981; Mercado, 1985; Vengosh and Rosenthal, 1994). The lowest Na/Cl value was recorded during July 2006 (mean: 0.26) and was the result of the dilution of contaminated groundwater by heavy precipitation (691 mm)


70

Figure 5.


Ionic ratio of (a) Na/Cl vs. Cl concentration in meq/L.

recorded before the sampling was conducted. Na/Cl ratios were mildly influenced by the rainfall because a moderate correlation coefficient between Na/Cl and rainfall was found (r 5 0.36). Table 1 clearly shows that the effect of Ca-rich water (recharge water) on the composition of groundwater that was severely contaminated with seawater was obviously the result of the low Na concentrations observed in July 2006 compared with other sampling months. Because Cl ion is considered to be a conservative ion, chloride concentration is not affected by ion exchange whereas the effects are clearer in the Ca-rich and Narich domination waters relationship. Thus, low Na/Cl ratios could be an indicator of preexisting salts during the early stage of salinization deposited from carbonate precipitation. The Na/ Cl ratios for groundwater in this study ranged from 0.10–2.70, indicating that fresh groundwater composition in Manukan Island was controlled by hydrochemical processes such as simultaneous seawater intrusion and cation exchange. Values close to the seawater ratio indicate a recent intrusion of seawater into the aquifer. The effect of seawater on fresh groundwater can be clearly demonstrated using a Piper diagram (Figure 6).The fluctuation in the Na/Cl ratios could be related to some regular and heavy pumping activities. From the Piper plot of the chemistry data, a total of 93% are classified as Na-Cl and remaining 7% are classified as the Ca-Cl water type. All these types of groundwater are indicative of seawater intrusion of varying degrees. The bivariate plots of Na/Cl ratio of groundwaters showed that these cation concentrations generally increased with Cl concentration (Figure 5), which indicates that mixing with seawater is one of the factors controlling groundwater composition, other than cation exchange process, as an extended effect. Large numbers of samples with values of Na/Cl ratios higher than the mean sea value ratio are indicative that a cation exchange process is simultaneously taking place. In combination, the depletion and enrichment of cations (Ca and Na) in the groundwater and the composition of the exchanger as postulated by Aris et al. (2010) suggest the development of a chromatographic ion exchange pattern in the Manukan Island aquifer. In seawater, the most dominant ions are Na and Cl, and sediment in direct contact

Figure 6.

Piper diagram for groundwaters studied at Manukan island.

with seawater due to seawater intrusion will have mostly Na on the aquifer’s matrix (Appelo and Postma, 2005) and the inferred sequential reactions are as follows: 1 1 Naz z Ca{X2 ?Na{Xz Ca2z , 2 2

ð1Þ

Ca2z zMg-X2 ?Mg2z zCa{X2 ,

ð2Þ

where X indicates the soil exchanger. Equation (1) was normally observed at the aquifer affected with seawater transition, while Equation (2) normally indicates that the aquifer is experiencing freshening as shown by its Ca–Cl water type. During seawater intrusion, when seawater mixes with freshwater, the cation affinity order is normally Na . K . Ca . Mg with Ca being displaced from the exchanger in the first order and Na eventually dominating the water and the exchanger. An adverse sequence will be observed in a freshening aquifer because there is a dilution effect as aquifers recharge, and it was found that some of the dissolved Na and K in the displacing seawater may exchange directly with adsorbed Ca on the exchanger as a result of dispersion (Aris et al. 2010). This may explain the variation of Na/Cl ratios in the study area. The variation in the ionic ratios of SO4/Cl is evaluated with respect to the distribution of Cl concentrations. It was found that more than 70% (n 5 114 over 162) of samples fall below the seawater ratio value (0.10), an indication of seawater contamination. The ratio of SO4/Cl ranges between 0.01 and 0.24, and it increases with Cl concentrations (r 5 0.551, p , 0.01; Figure 7). According to Mandel and Shiftan (1981), the solubility product of CaSO4 is not attained in the range of normal salinities and SO4 concentrations continue to increase but Cl concentrations increase more rapidly until Cl becomes the dominant ion. This can be seen from the rapid increases of Cl concentration found in the studied groundwater samples.



Figure 7.

Ionic ratio of (b) SO4/Cl vs. Cl concentration in meq/L.

The salinized Na-Cl type groundwater showed SO4/Cl ratios similar to or less than the seawater value. The ratios of Cl/HCO3 ranged between 1.72 and 23.12 and had a strong positive linear relation with Cl concentrations (r 5 0.972, p , 0.01; Figure 8). This linear relationship indicates the mixing of seawater and fresh groundwater. The effect of salinization of the groundwater was classified using Cl/HCO3 ratios, which includes groundwater having Cl/HCO3 ratios less than or equal to 0.5 for unaffected groundwater, 0.5–6.6 for slightly and moderately affected, and .6.6 for strongly affected (Revelle, 1941). Considering the classification adopted by Revelle (1941), 17% of the groundwater samples (n 5 27) were slightly or moderately affected by seawater and 83% (n 5 135 out of 162 groundwater samples) were strongly affected by seawater. This result implies that the fresh groundwater was contaminated by seawater. Decreasing the Ca content by ion exchange reactions and precipitation of carbonate minerals results in a Ca/(HCO3 + SO4) ratio .1, which is an indicator of the Ca-Cl water type that occurs at some sampling locations. The precipitation of carbonate minerals is likely close to the equilibrium state, which is an indicator of ion exchange reactions with the parent rock minerals (carbonate). This causes an increase in Na and a decrease in Ca and Mg, where Ca and Mg are exchanged for Na. This will result in an increase in the Na/Cl ratio and a decrease in the Ca/(HCO3 + SO4) ratio. The extended effect of seawater influence in fresh groundwater can be seen from the precipitation of carbonate minerals that occurs when seawater highly enriched with Na mixes with fresh groundwater that contains high concentrations of Ca, which would be a matrix for ion exchange reactions. Thus, geochemical analysis was used to show the seawater influence in fresh groundwater via saturation states of the freshwater–seawater mixing process in Manukan Island. The decrease in Cl concentration is due to a reduction in pumping rate (25%) with an increase in recharge rate as reported by Praveena et al. (2009) can be attributed to the dilution of precipitated minerals, modifying its permeability by widening the pores or fractures, allowing more water of lower chloride concentrations and diluting it (Ong’or, Shu, and

Figure 8.

71

Ionic ratio of Cl/HCO3 vs. Cl concentration in meq/L.

Liu, 2007). According to Samsudin et al. (2008), heavy rainfall that directly recharges the aquifer flushes out most of the salts from the aquifer.

Geochemical Analyses Using PHREEQC Most coastal aquifers are composed of some carbonate materials, either of calcareous clastics in which the cement is made of carbonate phases or of limestone in which calcium carbonate is the predominant mineral (Jones et al., 1999). Dissolution of carbonate minerals in the freshwater–seawater mixing zone below carbonate islands and coastal aquifers creates unique formation features in comparatively short times (Ramanov and Dreybrodt, 2006). Mylroie and Carew (1990) reported that tropical carbonate islands, with exposed young porous limestone such as found at Isla de Mona, Puerto Rico, exhibit numerous flank-margin caves at their coastline. In their further investigation, Mylroie and Carew (1990) reported that such caves can evolve within a few 10,000 years to a size reaching several ten of meters landward. This cave evolved at the outflow of brackish groundwater from the island, where mixing between seawater and freshwater creates a sharp transition zone. Mixing of groundwater, in equilibrium with calcite, and of seawater supersaturated with respect to calcite causes undersaturation of the mixed solutions that could result in dissolutional features of the carbonate minerals in the entire transition zone along the freshwater lens (Romanov and Dreybrodt, 2006). Mixing of seawater and fresh groundwater has been interpreted as being responsible for carbonate dissolution and precipitation in the coastal aquifers, and this has been clearly discussed by several authors such as Back et al. (1979), Chapelle (1983), Smart, Dawans, and Whiater (1988), and Wicks and Herman (1996). Because the groundwater was supersaturated with the respect to the carbonate minerals, as in the case of the present study, dissolution of calcite, aragonite, and dolomite are not predicted for the mixing zone in this situation. Saturation indices (SI) for calcite, aragonite, and dolomite were calculated to show the extent of the effect of seawater


72

Table 3. Sampling Period


SI of calcite, aragonite, dolomite, and gypsum of the study area.

Calcite

Aragonite

Dolomite

Gypsum


0.87 0.16 0.58–1.14

0.73 0.16 0.44–0.99

1.66 0.41 0.81–2.35

21.11 0.25 21.68–(20.82)

0.56 0.15 0.30–0.80

0.42 0.15 0.16–0.66

1.31 0.42 0.47–2.00

24.41 0.46 25.29–(23.73)

0.37 0.26 0.06–0.96

0.23 0.26 20.08–0.81

0.82 0.33 0.47–1.61

21.41 0.27 22.11–(21.10)

0.57 0.19 0.24–0.84

0.44 0.19 0.10–0.69

0.69 0.38 0.03–1.31

1.07 0.18 21.55–(20.92)

May ’06 Mean SD Range July ’06 Mean SD Range Sept. ’06 Mean SD Range

Figure 9. periods.

Average SI trends of selected minerals over the sampling

mwe ~ mgw { msw ,

Nov. ’06 Mean SD Range

0.86 0.18 0.63–1.23

0.72 0.18 0.49–1.09

1.34 0.32 0.87–2.03

21.07 0.27 21.56–(20.80)

0.68 0.17 0.39–0.98

0.53 0.17 0.25–0.83

0.81 0.31 0.07–1.19

21.03 0.31 21.81–(20.79)

Jan. ’07 Mean SD Range

toward saturation states of carbonate dissolution and precipitation. The PHREEQC code (Pharkurst and Appelo 1999) was used to model the calcite and aragonite saturation state in the seawater-affected system. SI is defined as log (IAP/Ksp), where IAP is the ion activity product and Ksp is the equilibrium solubility product. From the calculation, most of the groundwater samples are at or close to saturation with respect to calcite and dolomite (Table 3), while SI trends for selected minerals over the sampling periods shows that the groundwater is experiencing chemical alteration to its composition over time (Figure 9). The ANOVA test shows that sampling episodes affect the studied SI values of selected minerals significantly (p , 0.05). Because the investigated area is possibly recharged by subsurface inflow from the high topography area into the shallow aquifers at the low relief area, the ratio of Mg/Ca is used to distinguish the factor responsible for the variation of groundwater quality with respect to the influence of seawater as suggested by Hem (1985). The ratios of Mg/Ca for the study area ranged between 0.25 and 0.31. For waters flowing through limestone aquifers, the ratio of Mg/Ca is normally in the range of 0.5–0.7. A ratio that exceeds 0.7 indicates the admixture of seawater. In this brackish groundwater, an increase of the Mg/ Ca ratio in the aquifer may be caused by the precipitation of calcium carbonate minerals, if concentrations are sufficiently high (Mandel and Shiftan, 1981). To delineate the effect of weathering of carbonate minerals, the contributions of the major elements of carbonate minerals (Ca and Mg) to the groundwater can be obtained by subtracting the seawater influence of Ca and Mg from that present in the groundwater by the relation (Chua et al., 2007)

ð3Þ

where m refers to molar concentration of a major ion and the subscripts gw, sw, and we refer to the contribution from groundwater, seawater, and effects of weathering, respectively. The contribution of seawater (msw) is calculated from: msw ~ seawater content|½msw ,

ð4Þ

where [msw] is the major ion concentration in seawater, where the brackets indicate molar concentration. Seawater content was calculated based on the ratio between chloride concentration in the groundwater and seawater. The ratios of Cawe/Cagw and Mgwe/Mggw for the studied groundwaters are given in Table 4. Based the calculated ratios for Cawe/Cagw and Mgwe/Mggw, we found that most of the samples fall in the range between 0.72 and 0.89 and between 20.85 and 0.71, respectively. This indicates that the groundwater is experiencing various degree of seawater influence. The ratio of Cawe/Cagw is found approximately less far from unity for majority of samples. This indicates Ca and Mg contents in groundwater are largely a result of calcite, aragonite, and dolomite precipitation as a result of cation exchange process. From the computed SI values, the groundwater of Manukan Island is in oversaturation condition where the SI values for calcite, aragonite, and dolomite are in the range of 0.06–1.23, 20.08–1.09 and 0.03– 2.35, respectively. Therefore, the Ca and Mg contents in groundwater are solely controlled by the cation exchange process as a result of seawater influence. The average saturation states of selected minerals (calcite, aragonite, and dolomite) with respect to the Cawe/Cagw and Mgwe/Mggw ratios for the studied groundwaters are shows in Figure 10. These time variations in the ionic ratios could be partly explained by seasonal factors where the two-way ANOVA test shows that the sampling episodes affect the physicochemical parameters values and major ions concentration significantly (p , 0.05), or modifications in fresh groundwater extraction. On the contrary, both ratios (Cawe/Cagw and Mgwe/Mggw) grew in the same directions as indicated in Figure 10. This suggests that the ratios were more influenced by the pumping rate, which is



Table 4.

73

Molar contributions from groundwater and weathering effects to selected carbonate minerals of sampled groundwaters. Saturation Indices

Sampling Period

Ionic Strength

Aragonite

Molar Ratios

Calcite

Dolomite

Cawe/Cagw

Mgwe/Mggw


5.96 2.32 3.95–11.96

0.73 0.16 0.44–0.99

0.87 0.16 0.58–1.14

1.66 0.41 0.81–2.35

0.88 0.06 0.75–0.96

0.71 0.05 0.60–0.80

3.91 0.92 2.70–5.97

0.42 0.15 0.16–0.66

0.56 0.15 0.30–0.80

1.31 0.42 0.47–2.00

0.76 0.10 0.63–0.95

0.63 0.28 20.18–0.87

1.63 0.53 0.75–2.52

0.23 0.26 20.08–0.81

0.37 0.26 0.06–0.96

0.82 0.33 0.47–1.61

0.72 0.13 0.34–0.86

0.36 0.39 20.55–0.88

2.59 0.70 1.18–3.35

0.44 0.19 0.10–0.69

0.57 0.19 0.24–0.84

0.69 0.38 0.03–1.31

0.91 0.02 0.89–0.95

0.43 0.06 0.27–0.50

3.14 1.06 1.79–5.38

0.72 0.18 0.49–1.09

0.86 0.18 0.63–1.23

1.34 0.32 0.87–2.03

0.89 0.04 0.78–0.93

0.39 0.19 20.08–0.57

4.42 1.58 1.84–6.51

0.53 0.17 0.25–0.83

0.68 0.18 0.39–0.98

0.81 0.31 0.07–1.19

0.86 0.04 0.81–0.94

20.85 2.98 29.21–0.46

0.91 0.003 0.91–0.92

0.60 0.004 0.60–0.61

0.46 0.005 0.46–0.47

1.63 0.05 1.63–1.64

—

May ’06 Mean SD Range July ’06 Mean SD Range Sept. ’06 Mean SD Range Nov. ’06 Mean SD Range Jan. ’07 Mean SD Range Seawater Mean SD Range

attributed to the seawater influence and extended to the cation exchange process.

CONCLUSION The mechanism of seawater intrusion into the Manukan Island aquifer is relatively well known and is rather complicated by the processes that are taking place within the seawater and freshwater mixing zone. Monitoring and early detection of salinity sources in the aquifer are crucial for water management and successful protection. Influences of seawater on the chemical composition in fresh groundwater are elaborated via ionic states and saturation states. The ratios of Na/Cl, SO4/Cl, K/Cl, Ca/(HCO3 + SO4), Mg/Ca, and Cl/HCO3 are used to distinguish the variations of groundwater composition. The Na/Cl ratios for groundwater studied ranged from 0.10 to 2.70, implying that the fresh groundwater in Manukan Island was affected by the seawater signature. Values close to the seawater ratio indicate an influence of seawater into the aquifer. Additionally, mixing of seawater and fresh groundwater has been interpreted as being responsible for carbonate precipitation in the coastal aquifers as indicated by the SIs for calcite, aragonite, and dolomite. The groundwater of Manukan Island is in an oversaturated condition, where the SI values for calcite, aragonite, and dolomite are in the range of 0.06–1.23, 20.08–1.09, and 0.03–2.35, respectively. This indicates that

—

the groundwater is experiencing various degrees of seawater influence. The results presented show that the migration of seawater into the aquifer apparently leads to precipitation of the aquifer despite the seawater being supersaturated for carbonate minerals, an indication of seawater chemistry influence. This study revealed that the Ca and Mg content in groundwater is largely a result of calcite, aragonite, and dolomite precipitation resulting from the cation exchange process. Based on the calculated ionic ratios, the results obtained in this study confirmed the influence of seawater on the groundwater chemical composition of Manukan Island. This study also showed that the water composition of the study area may vary and have a significant effect on the several processes (i.e., cation exchange process and precipitation of carbonate) during the intrusion. The available data clearly pointed out that the influence of seawater is the main source contributing to groundwater chemical composition.

ACKNOWLEDGMENTS This research was supported by the Ministry of Science, Technology, and Innovation, Malaysia (MOSTI) and the Universiti Malaysia Sabah through fundamental research project FRG0050-ST-1/2006. Approval from the Sabah Parks Trustees for the study site exploration is acknowledged.


74


Figure 10. Average saturation states of calcite, aragonite, and dolomite with respect to Cawe/Cagw and Mgwe/Mggw.

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