Submarine groundwater discharge and associated fluxes of alkalinity and dissolved carbon into Moreton Bay (Australia) estimated via radium isotopes

Submarine groundwater discharge (SGD) can release solutes into the coastal ocean. This study used radium isotopes (²²⁴Ra, ²²³Ra, and ²²⁶Ra) to investigate SGD and its influence on alkalinity (TAlk), dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) inputs to Moreton Bay, Queensland, Australia. The average residence time of the bay was estimated to be about 40 days using radium isotope ratios, which is comparable to a previous physical model that revealed average residence times of 50 days. Radium isotopes identified several SGD hotspots around the bay. Using a ²²⁶Ra mass balance, a very minimum SGD flux of 1.1 × 10⁷ m³ d^{− 1} (or 0.7 cm/day) was calculated relying on extreme assumptions including the use of maximum ²²⁶Ra concentration for the groundwater endmember attained from 45 spatially distributed samples. Using more reasonable assumptions (i.e., the average ²²⁶Ra concentration in the groundwaters as the end member), a total SGD rate of 6.7 × 10⁷ ± 2.2 × 10⁷ m³/day (or 4.4 ± 1.5 cm/day) was estimated, which was ~ 18 times greater than the average annual discharge of all the major river inputs into the bay (i.e., the sum of Brisbane, Caboolture, Pine, and Logan Rivers). The average groundwater concentrations of TAlk, DIC and DOC were 1.5, 1.7 and 6.7 times greater than Moreton Bay surface waters, respectively. Fluxes of SGD-derived TAlk, DIC and DOC were estimated to be 161, 156, and 36 mmol/m²/day, respectively. When upscaled to the Bay area, these SGD fluxes became 20 to 38 times higher than the estimated annual input of all the major rivers. SGD is regionally important from a hydrological and carbon cycle perspective even if extreme assumptions are made to minimize SGD estimates. However, it remains unclear whether the SGD traced by radium isotopes in Moreton Bay is composed of fresh or saline groundwater.