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Groundwater Chemistry Change in Small Island due to Seawater Intrusion

This study investigates the compositional changes in groundwater chemistry in Manukan Island, Sabah, Malaysia, resulting from seawater intrusion. The study explores the driving forces and impact of excess pumpage on groundwater quality and presents empirical data on the increase in salinity and major ion concentrations in the aquifer. The findings highlight the need for sustainable water management practices on small tropical islands.

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Groundwater Chemistry Change in Small Island due to Seawater Intrusion

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  1. 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, UniversitiMalaysia Sabah, Malaysia Kim K.Woong Department of Environmental Science & Engineering, GwangjuInstitute of Science & Technology, South Korea E-Mail: zaharin@env.upm.edu.my

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

  3. > 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.

  4. > 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.

  5. > objectives To report on the geochemical processes that changed the groundwater chemistry observed in Manukan, Sabah

  6. > background of study area SABAH Malaysia Manukan Island • 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)

  7. Tunku Abdul Rahman Park

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

  9. Manukan Island features

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

  11. 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

  12. > 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

  13. Sampling wells located on the low lying area of the island

  14. 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)

  15. Experimental Referred to APHA (1995) 1Atomic Absorption Spectrometry

  16. > results and discussion • In Situ Data

  17. In Situ Data pH - increased from slightly acidic to alkaline EC - increased by 975% TDS - increased by 1398%

  18. In Situ Data • 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

  19. 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.

  20. 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.

  21. 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.

  22. 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. 2 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.

  23. Laboratory Data

  24. 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.

  25. Abdullah et al. (1996) data • Ca-HCO3 • Ca-Cl and • Na-Cl

  26. Abdullah et al. (1996) data • Ca-HCO3 • Ca-Cl and • Na-Cl Present data • Ca-Cl and • Na-Cl

  27. Present study • Major cation – Na & Ca • Major anion – Cl & HCO3 • Alkalis > Alkaline earth metals • Strong acids > Weak acids • Na-Cl typed dominated

  28. 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 SO4 • Correlation coefficient (i.e Na, Cl, SO4 with EC and salinity): • Identified the main elements contributed to the groundwater salinity • (i.eCl-Na, r = 0.656; Cl-SO4, r = 0.757 : p<0.05)

  29. 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.

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

  31. r = 0.972 p < 0.01 This linear relationship indicates the mixing of seawater and fresh groundwater

  32. 83% (n = 135) 17% (n = 17)

  33. 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

  34. 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.

  35. Presuming that Ca is the dominant ion for the aquifer matrix of the study area; Na+ + ½Ca – X2 → Na – X + ½Ca – X2 From Seawater Sediment (Aquifer’s Matrix) Soil Exchanger from (Appelo & Postma, 2005)

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

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

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

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

  40. Ca Mg • Calcium & Magnesium TDS, EC, K & HCO3 – correlate positively

  41. Ca Mg • Calcium & Magnesium TDS, EC, K & HCO3 – correlate positively r = 0.152

  42. Ca Mg • Calcium & Magnesium TDS, EC, K & HCO3 – correlate positively Precipitation condition • Aragonite • Dolomite • Calcite r = 0.152

  43. Ca Mg • Calcium & Magnesium TDS, EC, K & HCO3 – correlate positively Precipitation condition • Aragonite • Dolomite • Calcite r = 0.152

  44. 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).

  45. 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).

  46. > conclusions • 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). • 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

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