Submarine groundwater discharge and associated fluxes of alkalinity and dissolved carbon into Moreton Bay (Australia) estimated via radium isotopes Benjamin T. Stewart, Isaac R. Santos ⁎, Douglas R. Tait, Paul A. Macklin, Damien T. Maher School of Environment, Science and Engineering, Southern Cross University, PO Box 4321, Coffs Harbour, 2450 NSW, Australia abstract article info Article history: Received 18 December 2014 Received in revised form 30 March 2015 Accepted 31 March 2015 Available online 2 April 2015 Keywords: Permeable sediments Mangrove Subterranean estuary Carbon cycle Submarine groundwater discharge (SGD) can release solutes into the coastal ocean. This study used radium isotopes ( 224 Ra, 223 Ra, and 226 Ra) 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 res- idence 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 226 Ra mass balance, a very minimum SGD flux of 1.1 × 10 7 m 3 d -1 (or 0.7 cm/day) was calculated relying on extreme assumptions including the use of maximum 226 Ra concentra- tion for the groundwater endmember attained from 45 spatially distributed samples. Using more reasonable as- sumptions (i.e., the average 226 Ra concentration in the groundwaters as the end member), a total SGD rate of 6.7 × 10 7 ± 2.2 × 10 7 m 3 /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/m 2 /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 hydro- logical 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The flow of carbon and nutrients from major rivers to the ocean has been relatively well investigated (Cai et al., 2008; Seitzinger et al., 2005; Smith et al., 2003). While not as obvious as rivers, groundwater also dis- charges directly into the coastal ocean. Groundwater fluxes and their impacts on the surrounding marine environment have historically been difficult to constrain due to their patchy spatial distribution and temporal variability (Burnett et al., 2006). Submarine groundwater discharge (SGD) is defined as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid compo- sition or driving force (Moore, 2010a). These advective flows can include both recirculated saline and fresh terrestrial submarine ground- water (Santos et al., 2009). The drivers of seawater recirculation (or porewater advection) in permeable sediments can include wave action, tides, currents and bio-irrigation, while deeper flows are often driven by hydraulic gradients, convection and tidal pumping (Moore and Wilson, 2005; Santos et al., 2012b). While groundwater flows directly into the ocean wherever an aquifer is connected to the sea, coastal aquifers are usually arranged in a complex matrix of confined, semi-confined and unconfined systems (Li et al., 2009). Reviews by Burnett et al. (2006), Moore (2010a) and Santos et al. (2012a,b) have demonstrated the potential importance of SGD to the global ocean. For example, the annual average total SGD flux along a 600 km stretch of the South Atlantic Bight (USA) was found to be three times greater than the river flux (Moore, 2010b). However, our knowledge about the importance of SGD as a major part of hydrological and biogeochemical cycles is still evolving. Groundwater and porewater that flows through coastal sediments may release large amounts of trace metals, nutrients, carbon and other dissolved species to the coastal ocean at rates that may be comparable to surface water flows (Slomp and Van Cappellen, 2004). The potential importance of SGD to the bio- geochemistry of coastal systems may be significant, especially in bodies of water with limited circulation such as coastal embayments (Charette et al., 2013; Liu et al., 2012; Santos et al., 2014). Marine Chemistry 174 (2015) 1–12 ⁎ Corresponding author at: National Marine Science Centre, School of Environment, Science and Engineering, Southern Cross University, PO Box 4321, Coffs Harbour, 2450 NSW, Australia. E-mail address: isaac.santos@scu.edu.au (I.R. Santos). http://dx.doi.org/10.1016/j.marchem.2015.03.019 0304-4203/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem