This chapter investigated groundwater quality of a coastal aquifer in the Miri region of Sarawak, Borneo Island, Malaysia. Water samples were collected from a monitoring well and adjacent lakes during three sampling periods. This study includes hourly and monthly measurements to establish baseline data for various physico-chemical parameters, major ions, trace metals, and isotopes. Results indicated that the groundwater quality generally met the Malaysian National Water Quality Standards (NWQS) for Class I water, which is suitable for drinking with minimal treatment. However, the concentrations of Cd (mean value: 0.57 mg/L) and Mn (mean value: 9.53 mg/L) exceeded permissible limits by a considerable margin. Geochemical analysis, including Gibbs and Piper diagrams, and ionic ratios, indicated a predominant influence of anthropogenic activities, likely from agricultural practices and domestic sewage discharge, as evidenced by elevated Cl− and Na+ levels, and the prevalence of Na-Cl water type. Ion exchange processes, particularly Na+ replacing Ca2+, were also identified as significant factors influencing groundwater chemistry. Isotopic signatures reveal exchange reaction between groundwater and aquifer matrix, as shown by enriched δ2H and depleted δ18O, and evaporation predominates in lake waters. Although the groundwater demonstrated excellent potential for drinking water supply based on NWQS and water quality Index (WQI) of 14.73, the presence of elevated trace metals and the influence of anthropogenic activities necessitate continued monitoring and the implementation of appropriate management strategies to ensure the long-term sustainability and safety of this vital resource.