Total Alkalinity and Dissolved Inorganic Carbon Production in Groundwaters Discharging through a Sandy Beach

The paper presents evidence of the complexity of describing carbon transport from submarine groundwater discharge and chemical reactions in the subterranean estuary. Vertical and horizontal profiles of total alkalinity (TA) and dissolved inorganic carbon (DIC) concentrations were analyzed in a cross-shore transect of a sandy beach and an evaluation of DIC and TA fluxes to coastal waters is proposed based on groundwater discharge velocities and beach hydrogeology. The study was conducted in the Magdalen Islands in the Gulf of St. Lawrence (QC, Canada) where an unconfined sandstone aquifer rapidly discharges to the coastal ocean with a rate of ∼3000 m3 d-1. Increases in DIC and TA observed along the discharge pathway exceed the expected conservative mixing between inland groundwaters and seawater. This local production is attributed to anaerobic respiration. Slow groundwater velocity (1 to 9 cm d-1), low oxygen conditions (∼20%) and redox oscillations probably induced by tidal pumping present a suitable environment for bacterially-mediated carbon oxidation and anaerobic respiration. Depleted δ13C-DIC (from −14‰ to −28‰) and the characteristic odour of H2S during sample collection support the idea that sulphate reduction may generate high alkalinity and DIC concentrations at the seepage face, leading to potentially high discharge to the coastal ocean at this beach (2.0 to 8.2 mol DIC/day and 1.9 to 7.9 mmol TA/day). To our knowledge, this study is the first attempt to estimate the transport and transformations of dissolved inorganic carbon by biogeochemical processes in the subterranean estuary of a northern sandy beach.