Abstract:
Submarine groundwater studies that have employed new methods developed over the last two decades have shown high spatial and temporal variability in flow and discharge. Some of this variability is driven by onshore hydrostratigraphy, climate, topography, vegetation, and land use. Offshore parameters that control spatial and temporal variability, however, have not generally been conceptualized as clearly or quantified as extensively. Variability of submarine groundwater flow and discharge on passive continental margins can be examined effectively by considering the following three distinct spatial scales: (1) the nearshore or beach scale, spanning approximately 0-10 m offshore, and including the unconfined surficial aquifer and the intertidal recirculation cell, where present; (2) the embayment or inner shelf scale, spanning approximately 10 m to 10 km offshore, and including the uppermost confined submarine aquifer and its terminus; and (3) the shelf scale, spanning the width (generally approximately 80 km) and thickness of the aquifers of the entire continental shelf, from the base of the uppermost confined aquifer downward, and including influences of geothermal convection and glacio-eustatic sea-level change. With the exception of fluid overpressure studies in the Gulf of Mexico in 2005 (Expedition 308), recent or scheduled expeditions of the Integrated Ocean Drilling Program (IODP) that have included primarily hydrogeological objectives have concentrated on mid-ocean ridges (e.g., Juan de Fuca, Mid-Atlantic) and subduction zones (e.g., Cascadia margin, Costa Rica margin). Barge-based drilling is necessary to access all but the smallest and largest scales of marine hydrogeologic phenomena on continental margins. Intermediate-scale phenomena associated with submarine aquifers that exist beneath estuaries, coastal lagoons, and inner continental shelf settings, including permafrost-bearing shelves, may be suitable targets for future IODP expeditions that use alternative platforms such as drilling rigs mounted on shallow-draft and jack-up barges.