Compositional Change of Groundwater Chemistry in the Shallow Aquifer of Small Tropical Island Due to...

Compositional Change of Groundwater Chemistry in the Shallow Aquifer of Small Tropical Island Due to Seawater Intrusion

Aris A. ZaharinDepartment of Environmental SciencesFaculty of Environmental StudiesUniversiti Putra Malaysia, Malaysia

Abdullah M. Harun & Praveena S. MangalaSchool of Science & Technology,Universiti Malaysia Sabah, Malaysia

Kim K.WoongDepartment of Environmental Science & Engineering,Gwangju Institute of Science & Technology, South Korea

E-Mail: zaharin@env.upm.edu.my

> contentsIntroductionStatements of The ProblemObjectiveStudy AreaExperimental Results & DiscussionConclusions

> introductionSmall islands ~ special physical, demographic and economic features.The most prevalent issue - freshwater supply.Generally, small islands such as Manukan rely on groundwater and rain collection as the only way to get the natural water source.

> statements of the problemPumping from the upper phreatic zone is widely practiced on the island.Groundwater usage has drastically increased over the last decade due to the rapid increase in visitors to the islandThe increasing exploitation causes deterioration of groundwater.The driving force of seawater intrusion into the aquifer is due to excess pumpage, which lowers the freshwater table, changing the direction and magnitude of groundwater gradients.

> objectives

> background of study area

• West Coast of Sabah (5°57’-5°58’ N and 115°59’-116°01’ E)• Area of ~ 206 000 m2 (80% covered by forest particularly

on the high relief side)• Consists of interbedded sandstone and shale classified as

the Crocker Formation deposited during Late Eocene to Middle Miocene (Basir et al., 1991; Abdullah et al., 1997)

Manukan IslandManukan Island

SABAHMalaysia

Tunku Abdul Rahman Park

Low Lying Area

Forest (High Relief)

Jetty

Corals

Resorts

Manukan Island

Manukan Island features

m.s.l

sandstone

sands

0 90 180 m

30

15

0 m

water table

Y X

20 m

40 m

60 m

20 m

m 1500

M anukan I sland

NORT H

The sediment of the island is loose, not cemented and act as good water storageSmall area and low elevations – limited water storage

Climate in this regionWarm and wet

throughout the yearWater resourcesDominated by rainfall

recharge

Average monthly rainfall distribution for study area from 1995 to 2007

> experimentalSamples collectionPolyethylene bottles were used (APHA,

1995)All samples were filtered and split in a

different polyethylene bottles for subsequent analyses of cations and anions

162 groundwater samples from 9 wells on Manukan island

Sampling wells located on the low lying area

of the island

The extreme fresh groundwater chemistry used in this study was based on data presented by Abdullah et al., (1996) - was set as a threshold data value for each constituent species.

• the water was largely characterized as Ca-HCO3 water type (no impact of seawater chemistry)

ExperimentalParameters Methods

In SitupH & Temperature pH 315i

Electrical Conductivity Cond. 315i

Total Dissolved Solids EC-TDS Scan, Eutech

Salinity Sal 310i

LaboratorySulfate SulfaVer 4 Method (HACH)

Chloride Argentometric Method

Bicarbonate Titration Method

Potassium, Sodium, Calcium & Magnesium

Flame-AAS Method1

1Atomic Absorption Spectrometry

Referred toAPHA (1995)

> results and discussion In Situ Data

Parameter Unit Abdullah et al. (1996) Present study Present seawaterTemp. (ºC) - 26.30 - 29.40 30.40 - 33.20

pH 6.80 - 6.90 6.59 - 7.97 8.10 - 8.11

EC mS/cm 1.4 - 5.6 0.30 - 12.26 42.90 - 54.70

Salinity ppt - 0.29 - 7.40 30.00 - 35.76

TDS mg/l 687 - 2780 1133 - 8294 27 000 - 27 2000

In Situ DataParameter Unit Abdullah et al. (1996) Present study Present seawaterTemp. (ºC) - 26.30 - 29.40 30.40 - 33.20

pH 6.80 - 6.90 6.59 - 7.97 8.10 - 8.11

EC mS/cm 1.4 - 5.6 0.30 - 12.26 42.90 - 54.70

Salinity ppt - 0.29 - 7.40 30.00 - 35.76

TDS mg/l 687 - 2780 1133 - 8294 27 000 - 27 2000

pH - increased from slightly acidic to alkaline

EC - increased by 975%

TDS - increased by 1398%

In Situ DataParameter Unit Abdullah et al. (1996) Present study Present seawaterTemp. (ºC) - 26.30 - 29.40 30.40 - 33.20

pH 6.80 - 6.90 6.59 - 7.97 8.10 - 8.11

EC mS/cm 1.4 - 5.6 0.30 - 12.26 42.90 - 54.70

Salinity ppt - 0.29 - 7.40 30.00 - 35.76

TDS mg/l 687 - 2780 1133 - 8294 27 000 - 27 2000

• pH – mainly slightly acidic to slightly alkaline

• Temp. – between 26.3 – 29.4 °C

• EC / Salinity – groundwater contain high minerals; indicated that there were disturbance

• TDS – dominated by the salinity; falls under fresh to brackish

• Consisted of medium to very concentrated solutions of TDS.

•The water; more saline compared with Abdullah et al. (1996)

•Which more vulnerable to contamination by seawater

•Cover broad range of variation

• Contents of dissolved salts had increased in the groundwater at all pumping locations.

Laboratory DataParameter Unit Abdullah et al. (1996) Present study Present seawaterCa mg/l 61 - 103 60 - 866 410 - 418

Mg mg/l 12 - 51 3 - 298 417 - 450

Na mg/l 1 - 580 104 - 278 11 125 - 11 130

K mg/l 4 - 24 4 - 94 400 - 429

HCO3 mg/l 180 - 353 195 - 524 91 - 156

Cl mg/l 171 - 909 340 - 4099 19 600 - 19 794

SO4 mg/l 40 - 120 25 - 660 2200 - 3100

The significance increase (p < 0.05) of groundwater salinity was obviously supported by the high content of Na, Ca, Cl and SO4; in fact these were the highest readings ever recorded since 1996.

As well as with the other major elements, Mg and K also showed their presence in the groundwater in relatively higher concentration in this present study as compared to 1996 data.

1

As well as with the other major elements, Mg and K also showed their presence in the groundwater in relatively higher concentration in this present study as compared to 1996 data.

1

The increase of such major elements in seawater (i.e Na, Cl and SO4) showed that overpumping of groundwater had significantly attributed to the mitigation of seawater into the fresh groundwater aquifer of the island.

2

Laboratory Data

Laboratory Data

• Na+ and Cl- ~ 40-60% of the ions.

• HCO3- ~ 40% of the total anions in any given analysis

• SO42- ion was never preponderant in these waters with very

high mineral contents and the high conc. were always combined with high chloride levels in Na-Cl water type.

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Abdullah et al. (1996) data

• Ca-HCO3

• Ca-Cl and

• Na-Cl

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Abdullah et al. (1996) data

• Ca-HCO3

• Ca-Cl and

• Na-Cl

Present data

• Ca-Cl and

• Na-Cl

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Present study

• Major cation – Na & Ca

• Major anion – Cl & HCO3

• Alkalis > Alkaline earth metals

• Strong acids > Weak acids

• Na-Cl typed dominated

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Mg2+

Na+ + K

+CO3

2- + HCO3

-

SO42-

Cl-

SO4

2- + C

l- Ca 2+ + M

g 2+

EXPLANATIONGroundwater (1996)Groundwater (2006-2007)Seawater

Ca2+

Cl-

r = 0.6(Gimenez & Morell, 1997)Indicates active seawater intrusion.

Na+

Cl-

Reflects from the cationexchange process

Simple MixingSimple mixing (fresh groundwater ~

seawater) – NaCl and CaCl water typeSeawater-freshwater mixing:

• Increased of its groundwater salinity and EC• Increased in Cl and SO4

Correlation coefficient (i.e Na, Cl, SO4 with EC and salinity):

• Identified the main elements contributed to the groundwater salinity

– (i.e Cl-Na, r = 0.656; Cl-SO4, r = 0.757 : p<0.05)

In freshly recharged groundwater, HCO3- is typically the

dominant chemical constituent. In contrast, high Cl- values are associated with groundwater that has mixed

with seawater.

Based on the Cl vs Cl/HCO3 ionic ratio plot (Revelle, 1941), ratios of Cl/HCO3 ~ 1.72 and 23.12 and had strong positive linear relation with Cl concentrations.

In freshly recharged groundwater, HCO3- is typically the

dominant chemical constituent. In contrast, high Cl- values are associated with groundwater that has mixed

with seawater.

r = 0.972p < 0.01

This linear relationship indicates the mixing of seawater and fresh groundwater

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

0 20 40 60 80 100 120 140

Ioni

c ra

tio (

Cl/H

CO3)

Cl (meq/ l)March'06 May'06 July'06 Sptember'06 November'06 January'07

Not affected

Slightly / moderatelyaffected

Strongly affected

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

0 20 40 60 80 100 120 140

Ioni

c ra

tio (

Cl/H

CO3)

Cl (meq/ l)March'06 May'06 July'06 Sptember'06 November'06 January'07

Not affected

Slightly / moderatelyaffected

Strongly affected

83% (n = 135)

17% (n = 17)

Cation Exchange ProcessHydrochemical changes processes in

the mixing zone of the island’s aquifer were complex and displayed a heterogeneous pattern of the studied ions, spatially and temporally.

The most marked pattern could be observed in Na and Ca ions,

The excess value of Na in the groundwater was probably attributed to the direct cation exchange process at the seawater-freshwater interface

The lower concentration of Ca compared to Na, is a result from the cation exchange process that occurs naturally when seawater intrudes into the aquifer system.

Na+ + ½Ca – X2 → Na – X + ½Ca – X2

From SeawaterSediment

(Aquifer’s Matrix)

Soil Exchanger

from (Appelo & Postma, 2005)

Presuming that Ca is the dominant ion for the aquifer matrix of the study area;

When Ca exchanged with Na, the water becomes saturated for calcite and precipitation results (Back, 1966; Chappelle, 1983).

Aragonite Calcite Dolomite

Mean 0.51 0.65 1.11SD 0.26 0.26 0.51Min -0.08 0.06 0.03Max 1.09 1.23 2.35

When Ca exchanged with Na, the water becomes saturated for calcite and precipitation results (Back, 1966; Chappelle, 1983).

the waters are supersaturated with respect to dolomite and calcite and the dolomite SI values are higher than the calcite SI values.

the waters are supersaturated with respect to dolomite and calcite and the dolomite SI values are higher than the calcite SI values.

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

SaturatedUnsaturated

Satu

rate

dUn

satu

rate

dDolomiteDolomite

Calci

teCa

lcite

Calcium & MagnesiumCa Mg

TDS, EC, K & HCO3 – correlate positively

Calcium & MagnesiumCa Mg

TDS, EC, K & HCO3 – correlate positively

r = 0.152

Calcium & MagnesiumCa Mg

TDS, EC, K & HCO3 – correlate positively

r = 0.152 • Aragonite• Dolomite• Calcite

Precipitation condition

Calcium & MagnesiumCa Mg

TDS, EC, K & HCO3 – correlate positively

r = 0.152 • Aragonite• Dolomite• Calcite

Precipitation condition

At high pH, Ca and Mg are usually transferred to a solid phase, therefore, their concentrations are controlled by mineral precipitation (Lee et al., 2001).

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

SI V

alue

pH

Aragonite(a)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

SI V

alue

pH

Calcite(b)

0

0.5

1

1.5

2

2.5

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

SI V

alue

pH

Dolomite(c)

Strong correlations (r = 0.795 – 0.887; p < 0.01) between pH and SI values of aragonite, calcite and dolomite, suggesting that the precipitation of those minerals species were due to the increasing alkalinity of groundwater (increasing pH).

> conclusions1) The hydrochemical data have clearly

shown that there was significant intrusion of seawater into the island’s aquifer over the ten years (1996 to 2006-2007).

2) The shallow groundwater undergoes a compositional change from Ca-rich to Na-rich which mostly by simple mixing process between seawater and fresh groundwater and by simultaneously cation exchange process

3) Such process reflected higher concentration of Na and Cl in groundwater compared with previous study

4) Saturation indices of major carbonate minerals are of positive values and near to equilibrium states, indicating that the supersaturation of water by these minerals as an extended effect to the direct cation exchange process occurred between the seawater and aquifer’s exchange media

A diversity of geochemical processes that took place in the fresh groundwater-seawater mixing zone in the aquifer altered the fresh groundwater and seawater mixture away from the theoretical composition.

special thanks to

MINISTRY OF SCIENCE, TECHNOLOGY & INNOVATION and MINISTRY OF HIGHER EDUCATIONMALAYSIA

55

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