Transport and transformations of groundwater-borne carbon discharging through a sandy beach to a coastal ocean

Submarine groundwater discharge (SGD) is now recognized as an important coastal process affecting local and regional sources of solutes to coastal oceans. The objective of this study was to compare SGD estimates through Martinique Beach (Îles-de-la-Madeleine, QC, Canada) using different sampling techniques in order to quantify SGD fluxes of dissolved organic and inorganic carbon (DOC, DIC) exported to a coastal embayment. These fluxes were estimated using hydrogeological and geochemical methods, including direct measurement via seepage meters and hydrogeological estimation using Darcy’s law. Darcy estimates led to a fresh SGD flow ranging from 0.020 m3/s at the shoreline to 0.030 m3/s at the beach face. Direct measurements by seepage meters revealed higher SGD flows, with a mean value of 0.090 m3/s. These SGD flows were mainly composed of recirculating seawater flushing by the falling tide. The unconfined aquifer of Martinique Beach is a biogeochemically reactive zone, where DOC and DIC are nonconservative. The groundwater residence time (~32 days), low oxygen conditions (~20%) and high DOC concentrations (~ 2 mmol/L) represent a suitable environment for biogeochemical reactions to occur, and subsequently alter groundwater-borne carbon concentrations. We calculated a fresh-groundwater carbon load of 147 kg/d of DIC and 27 kg/d of DOC at the beach face. Even though recirculating seawater dominated the volume of total SGD, fresh SGD was an important carbon pathway, accounting for 12 and 20% of total DIC and DOC, respectively. This site-specific study is the first attempt to estimate volumetric and chemical groundwater fluxes to a coastal Canadian ocean, and demonstrates the strong interaction between fresh groundwater and coastal systems.