Extent and severity of groundwater contamination based on hydrochemistry mechanism of sandy tropical coastal aquifer

Small islands are susceptible to anthropogenic and natural activities, especially in respect of their freshwater supply. The freshwater supply in small islands may be threatened by the encroachment of seawater into freshwater aquifers, usually caused by over pumping. This study focused on the hydrochemistry of the Kapas Island aquifer, which controls the groundwater composition. Groundwater samples were taken from six constructed boreholes for the analysis and measurement of its in-situ and major ions. The experimental results show a positive and significant correlation between Na-Cl (r = 0.907; p < 0.01), which can be defined as the effect of salinization. The mechanisms involved in groundwater chemistry changes were ion exchange and mineralization. These processes can be demonstrated using Piper's diagram in which the water type has shifted into a Na-HCO3 water type from a Ca-HCO3 water type. Saturation indices have been calculated in order to determine the saturation condition related to dissolution or the precipitation state of the aquifer bedrock. About 76% of collected data (n = 108) were found to be in the dissolution process of carbonate minerals. Moreover, the correlation between total CEC and Ca shows a positive and strong relationship (r = 0.995; p < 0.01). This indicates that the major mineral component in Kapas Island is Ca ion, which contributes to the groundwater chemical composition. The output of this research explains the chemical mechanism attributed to the groundwater condition of the Kapas Island aquifer.

► The hydrogeological interactions between static and dynamic components determine the groundwater quantity and quality. ► Major cations in groundwater were derived from the chemical reaction of the deposited carbonate. ► Dissolution of carbonate, weathering and cation exchange processes explain the changes in groundwater compositions. ► Cation exchange mechanism is the key factor that modifies and controls the concentration of major cations in groundwater.