The influence of sediments on the determination of SGD by using Ra isotopes was investigated in the Port of Maó (Balearic Islands, NW Mediterranean). This natural harbor was selected because SGD occurs all along its southern boundary and it is covered by fine-grained sediments that are frequently resuspended due to vessel maneuvering. Comprehensive seasonal Ra mass balances were constructed for the waters of the Port of Maó using both short- (224Ra) and long- (228Ra) lived Ra isotopes. SGD flows to the Port of Maó obtained by using 228Ra revealed a seasonal pattern, likely dominated by the recharge cycle, with maximum SGD rates during the wet seasons ((180 ± 100) · 103 m3 · d− 1 in fall) and minimum flows during summer ((56 ± 35) · 103 m3 · d− 1). The results also showed that the Ra flux from bottom sediments, through diffusion and due to releases associated to resuspension events, represented a significant source of Ra to the harbor waters. This sedimentary source accounted for a major fraction of the 224Ra supplied to the system (30 - 90%, depending on the season), whereas the sediment influence on the 228Ra mass balance was significantly lower (10 - 40%) due to its slower production rate. These findings suggested that attributing Ra inputs to the water column solely to SGD in systems covered by fine-grained sediments and/or affected by processes that favor Ra exchange across the sediment-water interface might not be accurate, requiring a detailed evaluation of the sediment sources. The inputs from sediments are often difficult to quantify, but using long-lived Ra isotopes to estimate the SGD flow may minimize the effect of a poor characterization of the sediment source.