Compositional Change of Groundwater Chemistry in the Shallow Aquifer of Small Tropical Island Due to Seawater Intrusion Aris A. Zaharin Department of Environmental Sciences Faculty of Environmental Studies Universiti Putra Malaysia, Malaysia Abdullah M. Harun & Praveena S. Mangala School of Science & Technology, Universiti Malaysia Sabah, Malaysia Kim K.Woong Department of Environmental Science & Engineering, Gwangju Institute of Science & Technology, South Korea

> contents Introduction Statements of The Problem Objective Study Area Experimental Results & Discussion Conclusions

> 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.

> statements of the problem Pumping 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 island The 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 ~ m 2 (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) West Coast of Sabah (5°57’-5°58’ N and 115°59’-116°01’ E) Area of ~ m 2 (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 Island SABAH Malaysia

Tunku Abdul Rahman Park

Low Lying Area Forest (High Relief) Jetty Corals Resorts Manukan Island

Manukan Island features

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

Climate in this region  Warm and wet throughout the year Water resources  Dominated by rainfall recharge Average monthly rainfall distribution for study area from 1995 to 2007

> experimental Samples collection  Polyethylene 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 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- HCO 3 water type (no impact of seawater chemistry)

Experimental ParametersMethods In Situ pH & TemperaturepH 315i Electrical ConductivityCond. 315i Total Dissolved SolidsEC-TDS Scan, Eutech SalinitySal 310i Laboratory SulfateSulfaVer 4 Method (HACH) ChlorideArgentometric Method BicarbonateTitration Method Potassium, Sodium, Calcium & Magnesium Flame-AAS Method 1 1 Atomic Absorption Spectrometry Referred to APHA (1995)

> results and discussion In Situ Data ParameterUnitAbdullah et al. (1996)Present studyPresent seawater Temp.(ºC) pH ECmS/cm Salinityppt TDSmg/l

In Situ Data ParameterUnitAbdullah et al. (1996)Present studyPresent seawater Temp.(ºC) pH ECmS/cm Salinityppt TDSmg/l pH- increased from slightly acidic to alkaline EC- increased by 975% TDS- increased by 1398%

In Situ Data ParameterUnitAbdullah et al. (1996)Present studyPresent seawater Temp.(ºC) pH ECmS/cm Salinityppt TDSmg/l 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 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. MineralizationEC

Laboratory Data ParameterUnitAbdullah et al. (1996)Present studyPresent seawater Camg/l Mgmg/l Namg/l Kmg/l HCO 3 mg/l Clmg/l SO 4 mg/l  The significance increase (p < 0.05) of groundwater salinity was obviously supported by the high content of Na, Ca, Cl and SO 4 ; in fact these were the highest readings ever recorded since 1996.

Ca: 941% Na: % Cl: 2497% SO4: 1750%

Ca: 941% Na: % Cl: 2497% SO4: 1750% 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. 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

Ca: 941% Na: % Cl: 2497% SO4: 1750% 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. 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 SO 4 ) showed that overpumping of groundwater had significantly attributed to the mitigation of seawater into the fresh groundwater aquifer of the island. The increase of such major elements in seawater (i.e Na, Cl and SO 4 ) showed that overpumping of groundwater had significantly attributed to the mitigation of seawater into the fresh groundwater aquifer of the island. 2

Laboratory Data K+K+ Mg 2+ Ca 2+ Na + SO4 2- HCO3 - Cl -

Laboratory Data K+K+ Mg 2+ Ca 2+ Na + SO4 2- HCO3 - Cl - Na + and Cl - ~ 40-60% of the ions. HCO 3 - ~ 40% of the total anions in any given analysis SO 4 2- 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. Na + and Cl - ~ 40-60% of the ions. HCO 3 - ~ 40% of the total anions in any given analysis SO 4 2- 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) data Ca-HCO 3 Ca-Cl and Na-Cl

Abdullah et al. (1996) data Ca-HCO 3 Ca-Cl and Na-Cl Present data Ca-Cl and Na-Cl

Present study Major cation – Na & Ca Major anion – Cl & HCO 3 Alkalis > Alkaline earth metals Strong acids > Weak acids Na-Cl typed dominated

Simple Mixing  Simple mixing (fresh groundwater ~ seawater) – NaCl and CaCl water type  Seawater-freshwater mixing: Increased of its groundwater salinity and EC Increased in Cl and SO 4  Correlation coefficient (i.e Na, Cl, SO 4 with EC and salinity): Identified the main elements contributed to the groundwater salinity – (i.e Cl-Na, r = 0.656; Cl-SO 4, r = : p<0.05)

In freshly recharged groundwater, HCO 3 - 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/HCO 3 ionic ratio plot (Revelle, 1941), ratios of Cl/HCO 3 ~ 1.72 and and had strong positive linear relation with Cl concentrations. In freshly recharged groundwater, HCO 3 - is typically the dominant chemical constituent. In contrast, high Cl - values are associated with groundwater that has mixed with seawater.

r = p < 0.01 This linear relationship indicates the mixing of seawater and fresh groundwater

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

Cation Exchange Process  Hydrochemical 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 – X 2 → Na – X + ½Ca – X 2 From Seawater Sediment (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).

AragoniteCalciteDolomite Mean SD Min Max 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.

 Calcium & Magnesium Ca Mg TDS, EC, K & HCO 3 – correlate positively

 Calcium & Magnesium Ca Mg TDS, EC, K & HCO 3 – correlate positively r = 0.152

 Calcium & Magnesium Ca Mg TDS, EC, K & HCO 3 – correlate positively r = Aragonite Dolomite Calcite Precipitation condition 1 Took place during the cation exchange process 2 Precipitation state of dolomite, calcite and high-Mg- calcite. 3 In calcite crystals, Mg substituted Ca owing to the similarities in ionic radius charge

 Calcium & Magnesium Ca Mg TDS, EC, K & HCO 3 – correlate positively r = Aragonite Dolomite Calcite Precipitation condition ▼ Such elucidation is supported by the SI value of both calcite and aragonite which indicated supersaturation values.

 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 = – 0.887; 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).

> conclusions 1)The hydrochemical data have clearly shown that there was significant intrusion of seawater into the island’s aquifer over the ten years (1996 to ). 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 EDUCATION MALAYSIA

55 THANK YOU !