**Abstract**

The chapter aims to evaluate the groundwater quality levels in Kapas Island, Terengganu, Malaysia during the monsoon changes of the Southwest Monsoon (SWM), Monsoon Transition (MT) and Northeast Monsoon (NEM) in 2018. Four locations were used for groundwater sampling namely, the Kapas Coral Beach Resort, Kapas Beach Chalet, Pak Ya Seaview Chalet, and Kapas Island Resort. Three water samplings at each station for every month in the monsoon. Six parameters of the Malaysian Water Quality Index (WQI), i.e., dissolved oxygen (DO), pH, biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS) and ammoniacal nitrogen (NH3-N), were used to evaluate the water quality. The findings showed the groundwater quality parameters are in Class I and II. However, according to WQI Malaysia, the water quality status during the three monsoons is slightly polluted. During the SWM, the WQI value was 76 (Class III), the MT was 77 (Class II), and the NEM was WQI 71 (Class III). Given this status, it requires more intensive water treatment as it is not suitable for direct drinking water supply. The implications of the study show that the quality of groundwater in Kapas Island has to improve by the tour operators.

**Keywords:** water quality index, groundwater, slightly polluted, water treatment, island tourism
