A detailed groundwater, seawater and sediment study were undertaken to examine
the evolution of groundwater in the shallow aquifer of Manukan island, Sabah,
Malaysia. Its aquifer is often exposed to heavy pumping and that could lead to
seawater intrusion. The continuous pumping of groundwater on this island has
contributed to enrichment of some constituents found in seawater. As a consequence
of indiscriminate exploitation, the groundwater quality of this island has deteriorated.
Major ion chemistry analysis shows that the groundwater quality of the island
experiences some changes attributed to seawater intrusion. Simple mixing between
seawater-freshwater is complicated by the cation exchange process and highly
associated with groundwater w itdrawal. This study indicates that the groundwater is
classified as Na-Cl and Ca-Cl types. The groundwater has undergone a compositional
change from Ca-rich to Na-rich which can be explained mostly by the cation
exchange process. This study shows that the rise of Na and Cl composition in the
groundwater is not only controlled by seawater intrusion, but also controlled by rapid
cation exchange processes. Strong correlations exist among the major elements (Na,
Mg, K, Cl and SO₄) and salinity with/or EC suggest that the impact of seawater
intrusion to these major elements are more significant due to highly competitive
relationship between ions. These relationships clearly identify the main elements
contributing to the groundwater salinity and their tendency to depict a similar trend
of salinization pattern. From the PHREEQC calculation, calcite, dolomite and
aragonite solubility showed positive values of the saturation indices (SI), indicating
supersaturation that lead to mineral precipitation condition of water by these
minerals. Intensive exploitation of groundwater from Manukan Island's aquifer has
disturbed the natural equilibrium between fresh and saline water, and has resulted in
the increase of groundwater salinity and leap to the hydrochemical complexities of
freshwater-seawater contact. It was observed that the mixing between fresh water seawater created diversity in the geochemical processes of Manukan Island's aquifer
and altered the freshwater and seawater mixture away from the theoretical
composition line. This explained the most visible processes taking place during the
displacement. The results from reactive transport modelling confirmed that the
migration of seawater into the fresher parts of the aquifer apparently leads to a
calcification of the aquifer despite the seawater being supersaturated for carbonate
minerals and shows that the composition of the near coast zone and further
landward area may vary and have a significant effect on the processes during the
intrusion
