Submarine groundwater discharge (SGWD): an unseen yet potentially important coastal phenomenon in Canada

Both terrestrial and fresh or marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows from confined aquifers further out on the shelf. The terrestrial and oceanic forces overlap spatially, so measured fluid advection through coastal sediments may be a result of composite forcing. Subsurface/submarine groundwater discharge (SGWD) can be defined as any and all flow of water on continental margins from the seabed to the coast, regardless of fluid composition or driving force. SGWD is typically characterized by low specific flow rates, making detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may substantially increase the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coast. Terrestrially derived fluids represent a pathway for the flux of new material to the coast. This may result in diffuse pollution in areas where the discharge of contaminated groundwater occurs. This review presents an historical context of SGWD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. This review concludes that, to fully characterize marine geochemical budgets, one must give due consideration to SGWD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGWD, pollutants to evaluate these fluxes more precisely to Canadian coast.