An assessment of groundwater and surface water geochemistry in a limestone forest, Langkawi Island, Malaysia

In 2007, Langkawi Island in Malaysia was awarded UNESCO Global Geopark status, partly due to its diverse geological setting which includes tropical karst limestones. Whilst extensive studies of the geological setting at Langkawi Island have been carried out, the hydrogeochemistry and hydrogeology of the karst systems in this region have received little attention.

At the Penarak Nature Centre on Langkawi Island, the early stages of construction for an ecotourism resort within a limestone forest are taking place. At this site, water sampling and analyses were carried out, with a team of undergraduate science students, to characterise the baseline surface water and groundwater chemistry. Samples were collected from 10 locations at the limestone forest site. As no groundwater monitoring bores were available at the site, samples were instead collected from 4 identified groundwater seepage sites, in addition to 5 surface water sites and 1 seawater site. Field measurements of pH, EC, redox potential, dissolved oxygen and temperature were obtained. Samples from each site were also analysed for δ¹⁸O, δ²H and major ions.

Through a combination of geochemical plots and multivariate data analysis (Principal Components Analysis and Cluster Analysis), relationships between the water composition from different sites were identified. The water type for all of the groundwater seepage and surface water sites was Ca-HCO₃, indicating a high degree of groundwater-surface water interaction and rock dissolution. The multivariate data analysis results also provided an indication that groundwater sites located less than 300 meters apart may be flowing through separate karst conduits. One of the groundwater seepage sites, located close to construction, was identified as having higher concentrations of magnesium, calcium and sodium when compared with all of the other groundwater seepage and surface water sites. As there is little known about the karst hydrogeology and hydrochemistry on Langkawi Island, it is not clear whether the chemical differences identified at this construction site are occurring due to local heterogeneities in the karst system or due to anthropogenic impacts.

This study has been able to provide some baseline geochemical data and preliminary insights into groundwater and surface water interactions in a limestone forest at Langkawi Island. Recommendations for further research include a program of geophysical/hydrogeological mapping of the karst system and the establishment of a water monitoring network within the limestone forest.