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, MalaysiaKim K.WoongDepartment of Environmental Science & Engineering,Gwangju Institute of Science & Technology, South KoreaE-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 areaWest Coast of Sabah (557-558 N and 11559-11601 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 IslandSABAHMalaysia

Tunku Abdul Rahman Park

Low Lying AreaForest (High Relief)JettyCoralsResortsManukan Island

Manukan Island features

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)

Experimental1Atomic Absorption SpectrometryReferred toAPHA (1995)

ParametersMethodsIn SitupH & TemperaturepH 315iElectrical ConductivityCond. 315iTotal Dissolved SolidsEC-TDS Scan, EutechSalinitySal 310iLaboratorySulfateSulfaVer 4 Method (HACH)ChlorideArgentometric MethodBicarbonateTitration MethodPotassium, Sodium, Calcium & MagnesiumFlame-AAS Method1

> results and discussion In Situ Data

ParameterUnitAbdullah et al. (1996)Present studyPresent seawaterTemp. (C)-26.30-29.4030.40-33.20pH 6.80-6.906.59-7.978.10-8.11EC mS/cm1.4-5.60.30-12.2642.90-54.70Salinity ppt-0.29-7.4030.00-35.76TDS mg/l687-27801133-829427 000-27 2000

In Situ DatapH- increased from slightly acidic to alkaline

EC- increased by 975%

TDS- increased by 1398%

ParameterUnitAbdullah et al. (1996)Present studyPresent seawaterTemp. (C)-26.30-29.4030.40-33.20pH 6.80-6.906.59-7.978.10-8.11EC mS/cm1.4-5.60.30-12.2642.90-54.70Salinity ppt-0.29-7.4030.00-35.76TDS mg/l687-27801133-829427 000-27 2000

In Situ DatapH mainly slightly acidic to slightly alkalineTemp. between 26.3 29.4 CEC / Salinity groundwater contain high minerals; indicated that there were disturbanceTDS dominated by the salinity; falls under fresh to brackish

ParameterUnitAbdullah et al. (1996)Present studyPresent seawaterTemp. (C)-26.30-29.4030.40-33.20pH 6.80-6.906.59-7.978.10-8.11EC mS/cm1.4-5.60.30-12.2642.90-54.70Salinity ppt-0.29-7.4030.00-35.76TDS mg/l687-27801133-829427 000-27 2000

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 seawaterCover broad range of variation

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

Laboratory Data

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.

ParameterUnitAbdullah et al. (1996)Present studyPresent seawaterCa mg/l61-10360-866410-418Mg mg/l12-513-298417-450Na mg/l1-580104-27811 125-11 130K mg/l4-244-94400-429HCO3 mg/l180-353195-52491-156Cl mg/l171-909340-409919 600-19 794SO4 mg/l40-12025-6602200-3100

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. 1The 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 DataNa+ and Cl- ~ 40-60% of the ions.HCO3- ~ 40% of the total anions in any given analysisSO42- 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.

Abdullah et al. (1996) dataCa-HCO3 Ca-Cl and Na-Cl

Abdullah et al. (1996) dataCa-HCO3 Ca-Cl and Na-Cl Present dataCa-Cl and Na-Cl

Present studyMajor cation Na & CaMajor anion Cl & HCO3Alkalis > Alkaline earth metalsStrong acids > Weak acidsNa-Cl typed dominated

Simple MixingSimple mixing (fresh groundwater ~ seawater) NaCl and CaCl water typeSeawater-freshwater mixing:Increased of its groundwater salinity and ECIncreased in Cl and SO4Correlation 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

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

83% (n = 135)17% (n = 17)

Cation Exchange ProcessHydrochemical changes processes in the mixing zone of the islands 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+ + Ca X2 Na X + Ca X2

From SeawaterSediment (Aquifers Matrix)Soil Exchangerfrom (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).

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

AragoniteCalciteDolomiteMean0.510.651.11SD0.260.260.51Min-0.080.060.03Max1.091.232.35

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.

Calcium & Magnesium

Ca MgTDS, EC, K & HCO3 correlate positively

Calcium & Magnesium

Ca MgTDS, EC, K & HCO3 correlate positivelyr = 0.152

Calcium & Magnesium

Ca MgTDS, EC, K & HCO3 correlate positivelyr = 0.152 Aragonite Dolomite CalcitePrecipitation condition

Calcium & Magnesium

Ca MgTDS, EC, K & HCO3 correlate positivelyr = 0.152 Aragonite Dolomite CalcitePrecipitation 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).

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).

> conclusionsThe hydrochemical data have clearly shown that there was significant intrusion of seawater into the islands aquifer over the ten years (1996 to 2006-2007). 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

Such process reflected higher concentration of Na and Cl in groundwater compared with previous studySaturation 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 aquifers 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

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