Submarine Groundwater Discharge from an Oceanographic Perspective

Submarine groundwater discharge (SGD) is an important pathway for freshwater and land-based tracers to the ocean. While estimates of total discharge across the world vary considerably, it is known that SGD can be the dominant control of freshwater and nutrient content on local scales. This has important implications for local ecosystems and water quality. Although bulk discharge estimates using budgets of terrestrial water or radioactive tracers provide insight on regional scales, small-scale local discharge conditions determine the mechanics of the interaction between ocean and groundwater and therefore have important influence on larger scales. For example, in locations with karstic geology, SGD can be concentrated into localised submarine springs which interact with the ocean in a dramatically different way than slow seepage for the same bulk freshwater output. From an oceanographic perspective, these springs affect the local ocean conditions by providing a flux of freshwater and nutrients while also producing a buoyant plume that rises and mixes with its surroundings. In addition, the discharge itself is influenced by the ocean through changing hydraulic head and subterranean mixing with seawater. This work provides a perspective on what can be learned about SGD from examining the ocean physics of a submarine spring plume. Specifically, measurements of water velocity along the axis of a ~ 1 m deep rising submarine spring plume are used to infer the size, rate, freshwater content, and tracer flux of the discharge using theoretical predictions of the plume dynamics. Furthermore, the observations are compared to and supplemented by a numerical simulation of the plume to investigate its influence on the surrounding ocean.