Vol.:(0123456789) 1 3 Environmental Earth Sciences (2019) 78:336 https://doi.org/10.1007/s12665-019-8291-3 ORIGINAL ARTICLE The role of porewater exchange as a driver of CO 2 fux to the atmosphere in a temperate estuary (Squamish, Canada) Rowena M. Diggle 1  · Douglas R. Tait 1,2  · Damien T. Maher 1,2  · Xander Huggins 3  · Isaac R. Santos 1 Received: 15 July 2018 / Accepted: 25 April 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Porewater exchange is an important yet poorly understood component of the coastal carbon cycle. Here, a high-resolution automated radon ( 222 Rn, a natural porewater tracer) and CO 2 time series was conducted in the Squamish Central Estuary (Canada) over eight consecutive tidal cycles to assess the relative importance of porewater exchange on estuarine carbon dynamics. Radon and CO 2 observations revealed a clear tidal trend which is indicative of porewater exchange driven by tidal pumping. A radon mass balance indicated an average porewater exchange rate of 14.9 cm day −1 (4.3% of the tidal prism). The estuary was a net source of CO 2 to the atmosphere (average 212 ± 19 mmol m −2  day −1 ). Porewater exchange accounted for 9%, 5% and 30% of net dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and CO 2 exported out of the Squamish Central Estuary, respectively, while porewater inputs of free CO 2 accounted for 38% of the atmospheric evasion. These fux estimates as well as strong correlations between pCO 2 and 222 Rn suggest that porewater exchange has a strong infuence on CO 2 concentrations in the estuary even though they are a small contributor to overall DIC fuxes. Keywords Submarine groundwater discharge · Greenhouse gases · Wetlands · Permeable sediments Introduction Although occupying a relatively small area, estuaries are biogeochemical hotspots and are often sources of CO 2 emis- sions to the atmosphere; largely due to elevated levels of biological production, remineralization and allocthonous organic matter inputs (Borges 2005; Chen et al. 2013; Jiang et al. 2008; Weston et al. 2014). The global estuarine CO 2 efux is estimated to be 0.25 Pg C year −1 , roughly equal to CO 2 uptake from continental shelves (Cai 2011). However, large uncertainties still remain around estuarine fux esti- mates (± 0.25 Pg C year −1 ) (Laruelle et al. 2010; Regnier et al. 2013) and closing this knowledge gap is essential in accurately quantifying the role of estuaries in global carbon budgets. An important and often overlooked component of the carbon cycle in estuaries is the contribution of submarine groundwater discharge and/or porewater exchange which is described by Moore (2010) as any fow of water from sediments to the coastal ocean. This can include both fresh terrestrial groundwater or recirculated seawater (Sadat- Noori et al. 2016; Santos et al. 2012a). Porewater exchange can signifcantly alter estuarine biogeochemical cycling if concentrations of carbon, nutrients, contaminants, metals and pollutants in porewater are high relative to receiv- ing waters (Burnett et al. 2006; Slomp and Van Cappellen 2004; Tait et al. 2017). Despite often being volumetri- cally small, porewaters can provide a direct pathway for dissolved constituents to enter surface waters which can enhance primary production (Slomp and Van Cappellen 2004) and CO 2 evasion to the atmosphere (Macklin et al. 2014; Sadat-Noori et al. 2015a). In coastal systems, tidal pumping can be a signifcant driver of water column pore- water exchange (Burnett et al. 2006; Santos et al. 2012b). Tidal pumping is the process whereby the action of tides creates regular fushing of sediments, potentially deliv- ering solute enriched porewater to surface waters. The extent to which porewaters contribute to the partial pres- sure of CO 2 (pCO 2 ), dissolved inorganic carbon (DIC) and * Douglas R. Tait douglas.tait@scu.au 1 National Marine Science Centre, Southern Cross University, PO Box 4321, Cofs Harbour, NSW 2450, Australia 2 Southern Cross Geoscience, Southern Cross University, Lismore, NSW 2480, Australia 3 School of Engineering, University of Guelph, Guelph, ON N1G 2W1, Canada