Abstract: A mass balance model of naturally-occurring short-lived and long-lived radium isotopes (224 Ra and 226 Ra) for the Jiulong River Estuary is developed to estimate the submarine groundwater discharge (SGD)to the estuary.All likely source and sink terms are considered in the model establishment.The source terms for Ra include river dis-charge,desorption from riverine suspended particles,desorption from resuspended particles,diffusion from subtidal sediments,and input from open sea and SGD.Ra can also be removed through radioactive decay and with ebb tide. The modeling result shows that up to 41.9% to 56.9% of the total Ra input can be attributed to the contribution of submarine groundwater in the estuary,and this has to be sustained by a discharge of 1.65×10⁸ to 1.83×10⁸ m³/d in groundwater input.Such groundwater amounts can be four times that of river discharge in the estuary.Assuming that fresh groundwater can account for 10% of the total amount of groundwater,then the model calculated inorganic nutri-ent fluxes are 4.61×10⁶ mol/d for dissolved inorganic nitrogen (DIN),0.22×10⁶ mol/d for dissolved inorganic phosphate (DIP),and 6.94×10⁶ mol/d for dissolved inorganic silicon (DSi),respectively.And such inorganic nutrient fluxes are 23%,28% and 77% of that delivered by river discharge in the estuary,respectively.The result suggests that a considerable amount of nutrients in the Jiulong River Estuary is coming from the contribution of SGD.The latter itself is also significant in relation to the total river discharge in the estuary.Nutrients contributed by SGD could potentially cause environmental concerns of estuary and coastal marine eutrophication.Such an environmental issue must be considered in the future management plan.