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> introduction Small 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.
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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: [email protected]
> 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
THANK YOU !THANK YOU !