The approach to quantify submarine groundwater discharge using Ra isotopes generally involves developing a Ra mass bal- ance in an estuary, bay or lagoon. In this work we present a 224Ra mass balance used to evaluate the importance of the sub- marine groundwater discharge (SGD) in Long Island Sound (NY, US), the third most important estuary in US, located between Long Island and Connecticut that is usually affected by summertime hypoxia in the western basin. Three surveys were conducted between April 2009 and August 2010 where 25 water stations were sampled for Ra isotopes, oxygen and Mn. Sta- tions were oriented along 4 transects: one axial extending from the western to the eastern Sound and three longitudinal tran- sects in the western, central and eastern Sound. The inventory of 224Ra in the water column in summer was circa 2 times greater than in winter, suggesting an increased 224Ra flux to the Sound in summer. A mass balance for 224Ra was constructed consid- ering tidal exchange, inputs from rivers, desorption from resuspended particles, diffusive fluxes (including bioirrigation) from bottom sediments and radioactive decay in the water column. Fluxes of 224Ra from bottom sediments were measured by incu- bating cores under oxic conditions in a continuous flow mode such that the overlying water was circulated through a Mn-oxide fiber to maintain a constant activity of 224Ra. Fluxes from muddy sediments (comprising 􏰃67% of the Sound bottom) ranged from 127 to 312 dpm m􏰁2 d􏰁1 and were 􏰃60 dpm m􏰁2 d􏰁1 in sandy sediments (33% of the Sound). Incubations under hypoxic conditions showed variable fluxes depending on reduction and mobilization of Mn. The 224Ra mass balance shows a net input of Ra to the Sound of 106 ± 50 􏰄 1012 dpm y􏰁1 in spring and 244 ± 112 􏰄 1012 dpm y􏰁1 in the summer that is attributed to SGD. Elevated 224Ra values were observed near shore and in the pore fluids of the coarse beach sands along the Long Island and Connecticut coasts, suggesting that SGD driven by tidal recirculation through the beach face is a major source of 224Ra to the Sound. Seasonal variation in this source seems unlikely, and the calculated 224Ra SGD fluxes for spring and summer over- lap within the uncertainties. Nevertheless we conclude that variations in the 224Ra water column inventories could be produced by seasonal changes in bioirrigation and/or redox cycling of Mn as well as sediment resuspension and desorption of 224Ra from resuspended particles, and that our mass balance underestimates these terms, particularly in the summer. 224Ra fluxes from sediments in estuaries, especially those with significant areas of muddy sediments and seasonal hypoxia, are important and should be well constrained in future uses of this isotope as a tracer for SGD.