DEGRADATION STATE OF CARBON IN GROUNDWATERS DISCHARGING THROUGH A SANDY BEACH

It is now well known that submarine groundwater discharge (SGD) transport nutrients, carbon and trace elements which may contribute to eutrophication in coastal waters. Nevertheless transports and transformations pathways from coastal aquifers to the ocean are still poorly constrained, particularly in permeable sandy beaches which are recognized as significant biogeochemical reactive zones. In this presentation, we will focus on the behavior of groundwater-borne carbon, and specially on dissolved organic matter (DOM) which is a key component of biogeochemical and ecological processes in coastal ocean. Vertical and horizontal profiles of DOC, C/N, δ¹³C, colored dissolved organic matter (CDOM) and its optical properties were analyzed in a subterranean estuary and the associated discharging zone of a sandy beach to provide an overview of the degradation state of the exported DOM. This study takes place in Magdalen Island (St Lawrence Gulf, Quebec) where unconfined aquifers are experiencing, and are expected to experience, important anthropic pressures and hydroclimatic changes. Groundwater velocity (2 to 7 cm.d^-1), low oxygen conditions (20%) and redox oscillations induced by hydrodynamic condition at the beach surface provide an environment highly suitable for bacterial carbon oxidation and anaerobic respiration. We will show that high concentrations of DOC (160 to 4600 µM) with terrestrial signature (FI ~1.4) were observed along the subterranean estuary and in the discharge zone, leading to potentially degraded carbon discharge to the coastal ocean.