Coastal areas in Hong Kong are experiencing ocean acidification, with submarine groundwater discharge (SGD) playing a significant role in this process. SGD can contribute up to 45% of seawater pH decline by directly adding carbonate species. Air-sea CO2 exchange has minimal impact on seawater pH, but air CO2 uptake can indirectly affect coastal pH through bay-open ocean water exchange. Aerobic respiration is the main cause of seawater pH decline, with SGD acting as a major nutrient source. Managing groundwater is crucial to combat rapid coastal ocean acidification.