Groundwater recharge in the North Kelantan River basin was studied using environmental isotopes and chloride data. The researchers found that groundwater in the area is mainly recharged by rainfall and nearby rivers. The isotopic signatures of the groundwater samples suggest they all come from the same source. The age of the groundwater indicates it is relatively young, with most recharge events occurring after 1953. The estimated average groundwater recharge rate in the aquifer system is around 261.5 mm per year, which is about 10.5% of the total annual rainfall.