Mining activities pose environmental impacts especially when the operation is near bodies of water, thus, affecting water quality. As these environmental impacts are growing, there is a pressing need for increased intervention studies to improve water quality. This study aimed to evaluate the effectiveness of granulated activated carbon made from coconut shells in reducing heavy metal levels and enhancing the water quality of Lake Mainit located at Agusan del Norte and Surigao del Norte, Philippines. Silica sand, pumice stones, and white marble chips were added to a glass tank with the granulated activated carbon made from coconut shells. The water sample underwent various laboratory tests. The atomic absorption spectrometry flame technique was used to analyze the heavy metals lead and cadmium. Gravimetric method was employed in total suspended solids and total dissolved solids, and nephelometric method for turbidity. Pre-treated water sample analysis regarding lead, total dissolved solids, and turbidity are within the permissible limits, however, total suspended solids and cadmium concentration surpassed the allowable limits for Class A waters. Removal efficiency in terms of heavy metal concentration and the significant difference of parameters between the water sample before and after intervention were calculated. Results showed that after the intervention, activated carbon made from coconut shells were able to reduce the cadmium level present in the water sample. It also improved the quality of water within permissible limits. Hence, the activated carbon made from agricultural waste such as coconut shells has considerable potential to provide better water quality.