ORIGINAL ARTICLE

Hydrochemical changes in a small tropical island’s aquifer:Manukan Island, Sabah, Malaysia

A. Z. Aris Æ M. H. Abdullah Æ K. W. Kim ÆS. M. Praveena

Received: 5 December 2007 / Accepted: 26 February 2008 / Published online: 13 March 2008

� Springer-Verlag 2008

Abstract Small islands groundwater are often exposed to

heavy pumpings as a result of high demand for freshwater

consumption. Intensive exploitation of groundwater from

Manukan Island’s aquifer has disturbed the natural equi-

librium between fresh and saline water, and has resulted

increase the groundwater salinity and leap to the hydro-

chemical complexities of freshwater–seawater contact. An

attempt was made to identify the hydrochemical processes

that accompany current intrusion of seawater using ionic

changes and saturation indices. It was observed that the

mixing between freshwater–seawater created diversity in

geochemical processes of the Manukan Island’s aquifer and

altered the freshwater and seawater mixture away from the

theoretical composition line. This explained the most vis-

ible processes taking place during the displacement.

Keywords Groundwater � Hydrochemistry �Seawater intrusion � Small island

Introduction

The presence of freshwater in small island usually depends

on the quantity, its surface and subsurface storage. For

small islands, the interaction between infiltration and

groundwater resources is needed in order to sustain the

groundwater freshness. This is critical in place where sur-

face water does not exist in an exploitable form, whereas

the groundwater resource itself is very limited. Fresh

groundwater is only option which can satisfy small amount

of water.

Groundwater usage for daily supply in the small island

of Manukan, Sabah, well known as diver’s paradise, has

drastically increased over the last decade due to the rapid

increase in visitors to the island. It receives steep rise in

tourists number, especially over a span of 8 years from

1997 (125,000 number of tourists) to 2004 (175,000

number of tourists). Thus demand for groundwater supply

has increased tremendously to meet the domestic needs.

This has resulted in an enormous leap on groundwater

extraction, leading to a contamination of the wells in the

island. With this current practice, incursion of seawater

into the island’s aquifers especially in the low-lying area

of the island, is an expected significance consequence. In

general, small islands are highly susceptible to seawater

intrusion due to its highly permeable aquifer. While it is

unseen phenomenon, the influence of seawater intrusion

on the ecology of coastal systems especially small islands

may be more important than once thought, due to the

potential impacts resulting from chemical alterations

during the freshwater-seawater mixing. The groundwater

of small islands is also vulnerable to tidal waves and

tsunami, which can cause salinization and seawater

encroachment (Falkland and Brunel 1993; Abdullah et al.

1997a).

A. Z. Aris (&)

Department of Environmental Sciences,

Faculty of Environmental Studies,

Universiti Putra Malaysia, UPM Serdang,

43400 Selangor, Malaysia

e-mail: zaharin@env.upm.edu.my

M. H. Abdullah � S. M. Praveena

Environmental Science Programme,

School of Science and Technology,

Universiti Malaysia Sabah, 88999,

Kota Kinabalu, Sabah, Malaysia

K. W. Kim

Department of Environmental Science & Engineering,

Gwangju Institute of Science and Technology,

Gwangju 500-712, South Korea

123

Environ Geol (2009) 56:1721–1732

DOI 10.1007/s00254-008-1275-3

Any environmental hazard such as water quality dec-

lination cannot be avoided or controlled altogether, but

can be mitigated by taking appropriate measures like

proper water use planning, management and development.

Many of the previous studies on small islands have

focused on groundwater flow in the subsurface system,

but did not indicate the fate of its geochemical processes.

These include studies done by Abdullah et al. (1997b),

Singh and Gupta (1999) and Babu et al. (2002). Some of

the recent related studies pertaining to groundwater

chemistry in small islands have been encountered in lit-

eratures by Cruz and Amaral (2004) and Xie et al. (2005).

Despite these examples, relatively little is known about

seawater intrusion and hydrochemistry status of fresh

groundwater affected with saline water in small tropical

islands. Hence, an attempt is made to characterize the

sources of salinity and mechanisms of their mobility into

the groundwaters in the investigated area. The study

focused on the determination of the principal determinant

of groundwater salinization due to seawater intrusion

by taking into consideration the hydrochemical processes

that modify the theoretical freshwater–seawater mixture.

Increased knowledge of geochemical processes that con-

trol groundwater chemical composition in tropic region

especially in small islands environment could lead to

improved understanding of hydrochemical systems in such

an area. Such an effort contributes to effective manage-

ment and utilization of the groundwater resource by

clarifying relations among the associated parameters.

Therefore, to protect island ecological balance, the

mixing mechanisms of saline water and fresh groundwater

attributed by seawater intrusion needs to be identified and

investigated so that the states of the reserves are not

compromised.

Geography and climate

The island is located offshore of Kota Kinabalu in Sabah,

east Malaysia on the island of Borneo (Fig. 1), Manukan is

one of the islands under Tunku Abdul Rahman Parks.

Covering an area of 206,000 m2, Manukan Island (5�570–5�580N and 115�590–116�010E) is the park’s second largest

island after Gaya Island. It is surrounded by other mag-

nificent small islands namely Sapi, Mamutik and Sulug.

Almost 80% of the area is covered by dense vegetation in

the high relief area (west side of the island), while 20% of

the area located on the low lying area of the island is

developed for tourism activities. The topography of the

island is relatively hilly land with maximum elevations of

approximately 60 m in the western and decreasing eleva-

tion towards the eastern coast. The island is a crescent

shaped, one and half kilometer long and three kilometer

wide in the middle. The island is enacted under govern-

ment’s Parks Enactment 1978 and under the management

of The Sabah Parks Trustees.

The area experiences a warm and humid climate with

annual rainfall range between 2,000 and 2,500 mm, and

large amount of the precipitation reaches the groundwater.

The monthly average rainfall distribution from 1995 to

2007 is shown in Fig. 2. The yearly temperature ranges

from 21�C to 32�C with humidity between 80 and 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 March, and the southwest monsoon from May to Sep-

tember and usually the periods between the monsoons are

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.

Towards the end of Middle Miocene happened about one

million years ago, changes of the sea level occurred,

resulting in portions of the mainland being cut off by the

sea, thus deposited the formations today (Basir et al. 1991).

The soil map profile of study location is shown in Fig. 3.

The main association of Manukan Island is characterized

by Lokan association with approximately 85% of the area.

Low lying area of Manukan Island is characterized as

Tanjung Aru association. The parent materials that can be

found in the high relief side of the island are sandstone and

mudstone while in the low relief side is alluvium. The

lower sequence of the island consists of thick yellowish

brown shale and dark gray in color and, interbedded with

black gray thin sandstone while the weathered sandstones

are yellowish in color, whereas the shales are brown and

dark (Abdullah et al. 1997b). Exposed sandstone outcrops

still feature the coasts of this island forming cliffs and

deep crevasses along the shore. The sedimentary rock of

Manukan Island dips towards the low relief area (east–

northeast), with dipping angles of 15�–45�. The fold forms

a slight symmetrical syncline in the low relief are and small

scale normal faults and joint sets can be observed in several

locations around the island (Abdullah et al. 1997b).

An early study on the morphological of the island’s

aquifer conducted by Abdullah et al. (2002) found that the

thickness of the overlying rocks are 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 profiles at low lying area are thinner compared

to that of hilly area (Fig. 4) as reported by Abdullah et al.

(1997b). The changes in sea level that occur during

1722 Environ Geol (2009) 56:1721–1732

123

20 m

40 m

60 m

20 m

m 1500

Manukan Island

3.47

Gaya Island

Sapi Island

Manukan Island

Mamutik Island

Sulug Island

Kota KinabaluSABAH

SOUTH CHINA SEA

0 km

MALAYSIA

SABAH

Kota Kinabalu

0 65km

5.9754

5.9756

5.9758

5.976

5.9762

116.0052116.0054116.0056116.0058116.006116.0062116.006411 6.0066

PK 1

PK 2

PK 9PK 8

PK 5

PK 7

PK PK 4 3

PK 6Sampling Area

60 m

Countour

Sampling Point

LEGEND

Fig. 1 Schematic map showing the geographical locality of Manukan Island and sampling locations

Fig. 2 Average monthly rainfall distribution for study area from 1995 to 2007

Environ Geol (2009) 56:1721–1732 1723

123

Quaternary, caused the formation of limestone terraces in

coastal areas where Manukan Island is formed of carbonate

rocks which originated from coral deposits and overlain by

Quarternary alluvium (Basir et al. 1991). The alluviums are

loose, not cemented and act as sufficient water storage

which entirely depends on its thickness. A study by Ab-

dullah et al. (2002) indicates that the medium of the aquifer

consist of fine to coarse sand mixed with some fine gravel.

On the lowland, the sandstone has about the same thickness,

with shale and carbonate deposits. Small aquifers may occur

in the sandstone alluvium regions that often occur at sites

near the coast. Small areas and low elevations of Manukan

Island lead to very limited water storage (Aris et al. 2007a).

Presently, Manukan Island depends on shallow aquifer

for its groundwater supply. Dug wells are used for

extracting groundwater from its sandy aquifer. The wells

have a diameter of 150 cm and heights between 55 and

78 cm from ground surface level. The groundwater level of

the study area fluctuates between 0.03 and 0.79 m above

mean sea level (a.m.s.l.) (Abdullah 2001).

Sampling and analyses

A total of 162 groundwater samples were obtained from

nine existing wells located on the low lying area of the

island. Samplings were done from March 2006 to January

2007. Polyethylene bottles that soaked with 1:10 HNO3

acid wash and pre rinsed with distilled water were used to

store groundwater samples based on the methods described

in APHA (1995).

The analysis of water samples were carried out to assess

pH, temperature, electrical conductivity (EC), salinity and

total dissolved solids (TDS) and ions, namely sodium (Na),

potassium (K), calcium (Ca), magnesium (Mg), bicarbon-

ates (HCO3), chloride (Cl) and sulphate (SO4). An average

fresh groundwater composition, published by Abdullah

et al. (1996) was used as the threshold value for fresh

groundwater of the study area. That data indicated that the

water was characterized as Ca–HCO3 water type and has

no significant impact of seawater chemistry.

Saturation indices for selected minerals and ionic con-

centration changes were calculated in order to better

understand the hydrochemical processes that take place in

the aquifer during the freshwater–seawater mixing. The

hydrochemical processes during freshwater–seawater dis-

placement can be evaluated based on calculation of the

expected composition based on conservative mixing of

seawater and freshwater, and then comparing the result

with actual compositions found in the studied groundwater

samples (Appelo and Postma 2005). The seawater

contribution was used for calculating the theoretical

Fig. 3 Soil map of Kota Kinabalu and surrounding area

1724 Environ Geol (2009) 56:1721–1732

123

concentration of each ion (i) in conservative mixing of

seawater and freshwater:

ei;mix ¼ fseaei;sea þ 1� fseað Þei;fresh ð1Þ

where ei (in meq/L) is the concentration of specific ion (i),

fsea is the fraction of seawater in mixed freshwater–

seawater, and subscripts mix, sea and fresh indicate the

conservative mixture of seawater and freshwater. Any

change in concentration (ionic change), ei, change as a

result of chemical reaction then becomes (Fidelibus et al.

1993):

echange ¼ ei;sample � ei;mix

¼ ei;sample � fseaei;sea þ 1� fseað Þei;fresh

� �ð2Þ

where ei,sample is the actual observed concentration of

specific ion in the water sample. The fraction of seawater is

normally based on Cl concentration of the sample. These

fraction calculations can be made because Cl can be

assumed to be a conservative element and it is not usually

removed from the system due to its high solubility (Appelo

and Postma 2005). The theoretical seawater fraction was

calculated by reckoning the seawater contribution from the

sample Cl concentration (eCl,sample), the freshwater Cl

concentration (eCl,fresh) and seawater Cl concentration

(eCl,sea) where Cl concentration is expressed in

milliequivalents per liter (meq/L) (Appelo and Postma

2005):

fsea ¼eCl;sample � eCl;fresh

eCl;sea � eCl;fresh

: ð3Þ

Results and discussion

The overall data obtained in this study are shown in

Table 1. The pH values are found to be between 6.59 and

7.97. It shows that there is no distinct grouping of these

values. The temperatures of the groundwater are generally

between 26�C and 29�C. Significance differences are found

in the salinity values, where the lowest reading was

recorded in the month of July 2006 (0.29 ppt) and the

average value is 2.36 ppt. The electrical conductivity (EC)

ranged between 0.99 and 12.26 mS/cm. This is a good

illustration of the intrusion of salt water as a result of a

single pumping operation. It can be seen in general manner

that the high values of conductivity observed (EC [ 5 mS/

cm) are related to seawater intrusion, as supported by high

NaCl levels too. The EC values between 2 and 5 mS/cm

corresponds to mixtures of varying proportions of fresh

water and seawater after pumping (Freeze and Cherry

1979; Custodio 1991; Dazy et al. 1997). Wide ranges and

great standard deviations occurred for all parameters,

where the highest value was recorded in January 2007, and

the lowest value was in July 2006. The basic properties of

groundwater (pH, temperature, EC, salinity and TDS) and

major constituents of the groundwater (Ca, Mg, Na, and K)

as well as with seawater data are given in Tables 1 and 2,

respectively. The wide distributions among major compo-

sition of studied groundwaters indicated that the chemical

composition was affected by multiple processes, including

seawater–freshwater mixing (Aris et al. 2007b). The pre-

dominance of Na and Cl in the groundwater indicates

strong saline water impact. Of the Cl and SO4 concentra-

tions, 100 and 57%, respectively exceeded the Malaysian

National Standard for raw water quality standard viz Cl;

250 mg/L and SO4; 250 mg/L (MOH 2004). Despite the

wide ranges of values, most samples showed lognormal

density distribution (Fig. 5).

Piper diagram and water type characteristics

Based on Table 1 and Fig. 6, apparently the groundwater

are saline, as indicated by its Na–Cl water type. Large

proportions of the groundwaters showed Na–Cl, which

generally indicates a strong seawater influence to the

aquifer. During the sampling period, groundwater with

Ca–Cl and Mg–Cl facies were also encountered. The

dominant water type at all sampling locations was Na–Cl

Fig. 4 The soils profiles of the lowland and the hilly area of

Manukan Island

Environ Geol (2009) 56:1721–1732 1725

123

Table 1 The physio-chemical

properties and major ions of

groundwater in the study area

(n = 162)

Sampling period

March 2006 May 2006 July 2006 September 2006 November 2006 January 2007

Temperature (�C)

Mean 27.8 28.3 28.0 27.9 27.6 27.7

SD 0.81 0.69 0.66 0.81 0.83 0.62

Range 26.3–28.7 27.1–29.4 27.0–29.0 26.8–29.0 26.5–29.0 26.7–28.5

pH

Mean 7.55 7.35 7.18 7.03 7.36 7.16

SD 0.22 0.19 0.15 0.22 0.20 0.15

Range 7.18–7.97 7.08–7.60 6.98–7.48 6.59–7.37 7.11–7.65 6.94–7.43

EC (mS/cm)

Mean 5.59 8.36 4.12 3.23 1.03 6.43

SD 2.14 3.01 1.43 1.00 0.44 3.14

Range 1.66–8.71 4.27–12.26 0.99–6.31 1.06–4.33 0.30–1.59 1.64–9.81

Salinity (ppt)

Mean 3.55 4.59 2.36 3.39 3.72 5.10

SD 1.55 1.99 1.01 0.71 1.10 1.94

Range 0.90–5.90 1.70–7.10 0.29–3.90 1.81–4.33 2.08–5.24 2.35–7.40

TDS (mg/L)

Mean 4,201 4,641 3,390 4,172 4,964 5,843

SD 1,366 2,025 926 920 1,655 2,145

Range 2,449–6,811 2,195–8,263 1,133–4,632 2,420–5,418 2,518–6,999 2,979–8,294

Ca (mg/L)

Mean 309 248 213 552 496 523

SD 83 56 76 121 144 78

Range 189–467 128–298 60–331 411–866 256–776 354–640

Mg (mg/L)

Mean 120 179 101 80 92 88

SD 58 92 42 21 42 54

Range 27–221 32–298 35–163 34–108 30–157 3–174

Na (mg/L)

Mean 1,618 1,205 352 1,006 1,150 1,756

SD 504 609 159 309 479 845

Range 1001–2,780 482–2,606 104–642 316–1,420 434–1,814 552–2,761

K (mg/L)

Mean 38 55 29 24 32 40

SD 21 28 12 7 15 20

Range 11–83 14–94 4–44 7–32 8–56 8–63

HCO3 (mg/L)

Mean 350 330 327 304 325 331

SD 65 80 48 40 43 37

Range 278–520 14–94 268–405 254–386 266–410 281–400

Cl (mg/L)

Mean 1,467 2,290 2,194 1,875 2,059 2,825

SD 814 1,127 719 392 608 1,071

Range 340–2,774 550–4,074 425–3,199 1,000–2,399 1150–2899 1299–4099

SO4 (mg/L)

Mean 301 356 170 187 231 292

SD 136 211 58 56 116 154

Range 80–500 30–660 60–240 50–250 50–400 25–475

1726 Environ Geol (2009) 56:1721–1732

123

in March 2006 and May 2006. However, in July 2006 Ca–

Cl water type was found at PK 1 and PK 2, and Mg–Cl at

PK 8. In September 2006, PK 3 and PK 8 observation wells

presented Ca–Cl water type, an indication of ion exchange

occurrence between freshwater–seawater contacts. In

November 2006 and January 2007, the groundwater from

all observation wells was of Na–Cl type. PK 8 showed

Ca–Cl water type in January 2007. Unlike the other

observation wells, PK 8 well experienced the rapid ionic

exchange process. The change of Na–Cl to Ca–Cl water

type shows a deficit in sulfate content and can be observed

from March 2006 to January 2007. Such an evolution

patterns indicate that the groundwater chemistry were

controlled by cation exchange reaction as well as by simple

mixing between two end member (Vengosh et al. 1991;

Appelo and Postma 2005). It is obvious that from these

Table 2 Seawater chemistry data (all values in mg/L except for temperature: �C, EC: mS/cm, salinity: ppt and pH)

Temperature pH EC Salinity TDS Ca Mg Na K HCO3 Cl SO4

Mean 30.4 8.10 42.90 27.80 45,764 422 393 25,882 429 90.58 16,347 2,200

SD 0.00 0.01 0.00 0.00 1.00 3.77 0.67 162.30 4.41 0.47 2.27 0.00

Min 30.4 8.10 42.9 27.8 45,763 418 391 25,700 425 90.28 16,345 2,200

Max 30.4 8.11 42.9 27.8 45,764 425 394 26,012 435 91.12 16,349 2,200

3.002.752.502.252.001.75

Log Ca (mg/l)

40

30

20

10

0

Fre

qu

ency

Mean = 2.5444Std. Dev. = 0.21567N = 162

2.502.001.501.000.500.00

Log Mg (mg/l)

40

30

20

10

0

Fre

qu

ency

Mean = 1.952Std. Dev. = 0.32407N = 162

3.503.253.002.752.502.252.00

Log Na (mg/l)

25

20

15

10

5

0

Fre

qu

ency

Mean = 2.9801Std. Dev. = 0.31387N = 162

2.001.801.601.401.201.000.800.60

Log K (mg/l)

30

25

20

15

10

5

0

Fre

qu

ency

Mean = 1.4818Std. Dev. = 0.28519N = 162

2.802.702.602.502.402.302.20

Log HCO (mg/l)

30

25

20

15

10

5

0

Fre

qu

ency

Mean = 2.5101Std. Dev. = 0.06824N = 162

3

3.753.503.253.002.752.50

Log Cl (mg/l)

30

25

20

15

10

5

0

Fre

qu

ency

Mean = 3.2741Std. Dev. = 0.23394N = 162

3.002.502.001.50

Log SO (mg/l)

30

25

20

15

10

5

0

Fre

qu

ency

Mean = 2.3215Std. Dev. = 0.30328N = 162

4

Fig. 5 Distribution of the major ions

Environ Geol (2009) 56:1721–1732 1727

123

data and comparison with previous record (Abdullah et al.

1996), the island’s aquifer has suffered significant seawater

intrusion.

Ionic changes

From the Piper diagram, it is clear that the groundwater

composition of Manukan Island is highly marked by sea-

water characteristic. In order to determine the main process

that mainly control the evolution of the groundwater

chemistry in the island, the concentrations of major ele-

ments are presented as a function of Cl. Cl is assumed to be

a conservative parameter. While the salinity of seawater

may vary, the ratios among the ions remain constant for all

major ions and can be used to represents the mixture pro-

portion of the mixture between seawater and freshwater.

The correlation coefficient matrix between the examined

major ions, which were calculated using linear regression

analysis, is shown in Table 3. It was found that the corre-

lations between Cl and/with the major components of

seawater (Na and SO4) were strong (Cl–Na, r = 0.656;

Cl–SO4, r = 0.757); an indication of seawater influence on

the groundwater salinity. The variation of these relationships

may indicate the complexity of the hydrochemical compo-

nents of groundwater. The weak correlationship between

HCO3 with all the studied parameters suggested that the

seawater had insignificant impact on the chemistry and

behavior of HCO3 in the system.

The chemical reactions during freshwater-seawater dis-

placement can be deduced more precisely by calculating

the expected composition based on conservative mixing of

seawater and freshwater, and then compared with the

measured concentrations in the groundwater sample. The

resulting ionic change and seawater fraction in all obser-

vation wells based on Eqs. (1)–(3) is shown in Table 4.

The mixing rates of seawater intrusion in all observation

wells (PK 1–PK 9) during the field sampling period were

8% in March 2006, 13% in May 2006, 13% in July 2006

and 11% in September 2006, 12% in November 2006 and

16% in January 2007, respectively. Figure 7 shows the

ionic changes (echange) calculated for Na, Ca, Mg, K, HCO3

and SO4 for all observation wells in March 2006 to January

2007.

The hydrochemical changes process in the mixing zone

of the island’s aquifer were complex and displayed a het-

erogeneous pattern of the studied ions, spatially and

temporally. The most marked pattern could be observed in

Na and Ca ions. The heterogeneous patterns shown by the

two ions were most probably due to direct contact between

Na-highly enriched water (seawater) with Ca-highly enri-

ched water (fresh groundwater). The echange of Na was

usually positive since fresh groundwater contains only

small amount of Na. However, most of the wells presented

negative values except for PK 5 and PK 8 in March 2006

and in PK 1 in May 2006 (Fig. 7). The negative value of

echange indicated that the groundwater was experiencing

simple mixing with seawater, as indicated by the high

salinity values recorded.

The excess value of Na in the groundwater was probably

attributed to the direct cation exchange process at the

seawater–freshwater interface. Usually, the composition of

fresh groundwater in coastal area is often dominated by Ca

and HCO3 ions which resulted from carbonate aquifer

Ca2+

Mg2+

Na+

+ K+

CO32-

+ HCO3-

SO42-

Cl-

SO 4

2-+

Cl

-

Ca 2+

+M

g 2+

EXPLANATIONMarch'06May'06July'06September'06November'06January'07Seawater

Fig. 6 Piper plot for studied groundwater of Manukan Island

Table 3 Correlations among

the major ions of the

groundwater (n = 162)

a Value in the upper triangle of

the matrix is significant value

(p \ 0.01** and p \ 0.05*)b Correlation values (in lower

triangle)

Ca Mg Na K HCO3 Cl SO4

Ca 1 0.054a 0.000** 0.931** 0.191** 0.001** 0.265**

Mg -0.152 1 0.000** 0.000** 0.020* 0.000** 0.000**

Na 0.366 0.459 1 0.000** 0.017* 0.000** 0.000**

K 0.007 0.905 0.618 1 0.000** 0.000** 0.000**

HCO3 -0.103 -0.183 -0.187 -0.299 1 0.000** 0.001**

Cl 0.251 0.641 0.656 0.777 -0.340 1 0.000**

SO4 0.088 0.886 0.696 0.952 -0.219 0.757b 1

1728 Environ Geol (2009) 56:1721–1732

123

dissolution. In seawater, the most dominant ions are Na and

Cl, and sediment in direct contact with seawater due to

seawater intrusion will have mostly Na on the aquifer’s

matrix. When seawater intrudes into coastal aquifer, an

exchange of cations occur (Appelo and Postma 2005) as

follows:

Naþ þ 1

2Ca� X2 ! Na� Xþ 1

2Ca2þ ð4Þ

where X indicates the soil exchanger. This process is

indicated by the relationship between Na and Ca where

particularly samples with negative Nachange generally have

positive Cachange. Such a process is highly influenced by

seawater signature where the more seawater intrudes the

aquifer, the negative value of Nachange increases (Fig. 8a).

Adams et al. (2001) indicated that the presence of suitable

exchange media in the aquifer, such as through seawater

encroachment, would increase the amount of Na signifi-

cantly through the cation exchange process. In the present

study, there are about 50–99% increase of Na concentra-

tions when compared to Abdullah et al. (1996) earlier

record. The element of K, shows negative Kchange in sam-

ples that highly marked by seawater signature (Fig. 8b)

except in the less mineralized samples, where they are

relatively small positive values close to 0. The downward

trend for both Na and K with the increase fraction of

seawater shows that the groundwater is dominated by

seawater as can be observed from the negative value of

echange for Na and K.

For the Mgchange, the values generally positive, except

for PK 5 in Mei’06, PK 1 and PK 7 in July 2006, PK 8 in

November 2006 and PK 5, PK 8 and PK 9 in January 2007

where recorded negative values. The negative values were

due to low concentration of Ca in groundwater as a result

of direct cation exchange process with Na. The origin of Ca

and Mg was from the dissolution of carbonate minerals,

calcite (CaCO3). Calcite was likely to be the dominant

mineral present as indicated by the high concentration of

Ca whereas Mg concentrations were observed to be low.

Probably, some Mg from mineral may be lost due to cation

exchange with Na as indicated by the seawater fraction plot

(Fig. 8d).

Table 4 Fraction of seawater in Manukan Island aquifer (meq/L)

Seawater Sample Freshwater Mix Change

March 2006

Ca 21.05 15.40 3.04 4.49 10.91

Mg 32.30 9.87 0.99 3.50 6.38

Na 1126.00 70.39 0.04 90.22 -19.83

K 10.96 0.98 0.12 0.99 -0.01

HCO3 1.49 5.73 5.83 5.49 0.24

Cl 461.11 41.37 4.82 41.37 0.00

SO4 45.80 6.26 0.83 4.43 1.83

f sea 100.00 8.01

May 2006

Ca 21.05 12.37 3.04 5.40 6.97

Mg 32.30 14.75 0.99 5.09 9.66

Na 1126.00 52.43 0.04 147.53 -95.10

K 10.96 1.41 0.12 1.54 -0.13

HCO3 1.49 5.41 5.83 5.26 0.15

Cl 461.11 64.59 4.82 64.59 0.00

SO4 45.80 7.41 0.83 6.72 0.69

f sea 100.00 13.10

July 2006

Ca 21.05 10.62 3.04 5.30 5.33

Mg 32.30 8.28 0.99 4.91 3.38

Na 1126.00 15.33 0.04 140.91 -125.59

K 10.96 0.75 0.12 1.48 -0.73

HCO3 1.49 5.36 5.83 5.29 0.07

Cl 461.11 61.91 4.82 61.91 0.00

SO4 45.80 3.55 0.83 6.46 -2.91

f sea 100.00 12.51

September 2006

Ca 21.05 27.54 3.04 4.94 22.60

Mg 32.30 6.62 0.99 4.29 2.33

Na 1126.00 43.78 0.04 118.68 -74.90

K 10.96 0.62 0.12 1.26 -0.64

HCO3 1.49 4.99 5.83 5.38 -0.39

Cl 461.11 52.90 4.82 52.90 0.00

SO4 45.80 3.89 0.83 5.57 -1.68

f sea 100.00 10.54

November 2006

Ca 21.05 24.74 3.04 5.15 19.60

Mg 32.30 7.55 0.99 4.64 2.90

Na 1126.00 50.02 0.04 131.43 -81.41

K 10.96 0.81 0.12 1.39 -0.58

HCO3 1.49 5.33 5.83 5.33 0.00

Cl 461.11 58.07 4.82 58.07 0.00

SO4 45.80 4.80 0.83 6.08 -1.28

f sea 100.00 11.67

January 2007

Ca 21.05 26.10 3.04 6.00 20.10

Mg 32.30 7.27 0.99 6.12 1.15

Table 4 continued

Seawater Sample Freshwater Mix Change

Na 1126.00 76.38 0.04 184.60 -108.23

K 10.96 1.03 0.12 1.90 -0.86

HCO3 1.49 5.42 5.83 5.12 0.30

Cl 461.11 79.62 4.82 79.62 0.00

SO4 45.80 6.07 0.83 8.20 -2.13

f sea 100.00 16.37

Environ Geol (2009) 56:1721–1732 1729

123

Saturation indices

Saturation indices (SI) for calcite and aragonite were

calculated in order to show the extend of the effect of

seawater intrusion to the saturation state of the freshwa-

ter–seawater mixing process in a closed-system. The

PHREEQC code (Parkust and Appelo 1999) was used to

model the calcite and aragonite saturation state the in

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 aragonite. Figure 9 shows the

calcite and aragonite saturation indices as a function of

the proportion of seawater on the mixture. The theoretical

line of conservative mixing is also shown in the plot. The

obvious difference between the theoretical line for con-

servative saturation states with the calculated values for

the samples is due to the non conservative nature of

dissolution and precipitation of the calcite and the CO2

flux (Leboeuf 2004).

For calcite plot, the lowest conservative mixing is

between 10 and 75% mixtures containing seawater which

occurs in undersaturated states. Calculated SIs for calcite

are found to be more highly saturated then the predicted.

The over saturated values were probably due to more

carbonate minerals dissolved under closed conditions as

the initial water enters from intermediate to deep aquifer.

The carbonate minerals dissolution simultaneously

exchanged Ca and Mg with Na that led the water remains at

or slightly above saturations states with respect to the

carbonate minerals. pH of groundwater influenced the

saturation state of calcite and aragonite, where Ca and Mg

are usually transferred to a solid phase at high pH value,

and therefore their concentrations are controlled by mineral

precipitation (Hem 1989; Lee et al. 2001). The trend of

aragonite species distribution is similar: the lowest con-

servative mixing of aragonite is between 10 and 75%

mixtures of seawater which occurs at undersaturated state.

This trend suggests that the dissolution of carbonate min-

erals from infiltration was not the main contributing factor

for the high presence of Ca and Mg in the groundwater.

When infiltration water enters the aquifer and dissolves

more carbonate bedrocks under closed condition, the water

remains at or slightly above saturation conditions with

respect to the carbonate minerals even with the increase of

seawater proportion penetrates into the aquifer. The water

remains at or slightly above the saturation condition due to

simultaneously exchange of Ca and Mg with Na. Hence,

the significant difference between the theoretical line for

conservative saturation state and the calculated values

indicates that the seawater intrusion itself was not the only

factor contributing to the dissolution of minerals. It was

possibly one of the factors that significantly control the

saturation state of the carbonate minerals.

The low significant correlation (r = -0.152) between

Ca and Mg (Table 2) is possibly attributed to the precipi-

tation state of calcite and aragonite. Precipitation took

place during the cation exchange process as an extending

effect from seawater intrusion as discussed previously. In

calcite crystals, Mg substituted Ca owing to the similarities

in ionic radius charge. Such elucidation is supported by the

-200

-150

-100

-50

0

50

100

-10

-5

0

5

10

15

20

25

30

35

40

PK

1P

K 2

PK

3P

K 4

PK

5P

K 6

PK

7P

K 8

PK

9P

K 1

PK

2P

K 3

PK

4P

K 5

PK

6P

K 7

PK

8P

K 9

PK

1P

K 2

PK

3P

K 4

PK

5P

K 6

PK

7P

K 8

PK

9P

K 1

PK

2P

K 3

PK

4P

K 5

PK

6P

K 7

PK

8P

K 9

PK

1P

K 2

PK

3P

K 4

PK

5P

K 6

PK

7P

K 8

PK

9P

K 1

PK

2P

K 3

PK

4P

K 5

PK

6P

K 7

PK

8P

K 9

Ion

ic C

han

ge

for

Ca,

Mg

, K, H

CO

3 an

d S

O4

(meq

/L)

Observation Wells

Ca Mg K Na

MARCH'06 MAY'06 JULY'06 SEPTEMBER'06 NOVEMBER'06 JANUARY'07

SO4 HCO3

Ion

ic Ch

ang

e for N

a (meq

/L)

Fig. 7 Ionic changes (echange)

of groundwater samples from

Manukan Island

1730 Environ Geol (2009) 56:1721–1732

123

SI value of both calcite and aragonite which indicated

supersaturation values. The increase of Ca concentration in

the groundwater samples recorded in March 2006–January

2007 were probably not due to calcite dissolution since the

HCO3 did not increase much when compared to the

November 2006 and January 2007 data. Such a weak

correlationship (r = -0.103) among major anions and

cations reveals that dissolved salts from seawater had

insignificant impact on the HCO3 concentration in the

groundwater.

-250

-200

-150

-100

-50

0

50

100

150

200

250

0302010

Na c

han

ge

(meq

/l)

% seawater

March'06 May'06 July'06 September'06 November'06 January'07

(a)

-2

-1

0

1

2

0302010

Kch

ang

e(m

eq/l)

% seawater

March'06 May'06 July'06 September'06 November'06 January'07

(b)

-10

0

10

20

30

40

50

0302010

Ca c

han

ge

(meq

/l)

% seawater

March'06 May'06 July'06 September'06 November'06 January'07

(c)

-5

0

5

10

15

20

0302010

Mg

chan

ge

(meq

/l)

% seawater

March'06 May'06 July'06 September'06 November'06 January'07

(d)

Fig. 8 echange for Na (a), K (b), Ca (c) and Mg (d) for the samples taken in March 2006 to January 2007

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 20 40 60 80 100

SI f

or

Cal

cite

% seawaterMarch'06 May'06 July'06 September'06 November'06 January'07

(a)

theroretical line

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100

SI f

or

Ara

go

nit

e

% seawaterMarch'06 May'06 July'06 September'06 November'06 January'07

(b)

theroretical line

Fig. 9 a Calcite and b Aragonite saturation indices of studied groundwaters with the theoretical saturation indices for conservative mixing

between seawater and freshwater

Environ Geol (2009) 56:1721–1732 1731

123

Conclusions

The groundwater of Manukan Island experienced miner-

alization, an indication of marine water intrusion into the

island’s aquifer and marked by the presence of Na–Cl

water type. The mixing of freshwater–seawater creates a

diversity of geochemical processes that has alter the theo-

retical composition of the freshwater and seawater mixture

in the Manukan island’s aquifer.

The processes involved were complex and varies, spa-

tially and temporally. However, the most dominant process

that took place within the freshwater-seawater mixing was

the cation exchange which occurred mainly as direct

exchange between Na with Ca and Mg.

The obvious difference between the theoretical compo-

sitions for conservative saturation states with the calculated

values indicates that seawater intrusion itself was not the

sole factor that contributed to the dissolution of minerals.

Thus, seawater intrusion process was found to be one of the

factors that control the saturation states of the carbonate

minerals. The effect of flushing process could be one that

significantly attributed to the minerals dissolution process

in the aquifer’s system.

Saturation indices of calcite and aragonite were in

positive values and near to the equilibrium states with

increasing proportion of seawater, indicating the supersat-

uration of studied water by these minerals. This is an

extend effect of direct cation exchange process between the

seawater and carbonate minerals found in the aquifer.

Acknowledgments This research was financially supported by the

Ministry of Higher Education (MOHE), Malaysia through funda-

mental research grant of FRG0050-ST-1/2006. Permission from the

Sabah Parks Trustees for the study site exploration is highly

acknowledged. A special thanks to Mr Ahmad Fairudz Jamaludin and

Mrs Maimunah Mohamad from Climatological and Hydrological

Division, Malaysian Meteorological Department for the rainfall data.

The authors also thank anonymous reviewers for the helpful

comments.

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