3.5 Submarine Groundwater Discharge: A Source of Nutrients, Metals, and Pollutants to the Coastal Ocean

This chapter reviews the current knowledge on submarine groundwater discharge (SGD) and the associated fluxes of nutrients, trace metals, microbes, pharmaceuticals, and other terrestrial constituents to coastal waters. We review methods of estimating SGD, present flux estimates from different locations worldwide, and discuss how various hydrogeologic features such as topography, aquifer substrate, climate, waves, and tides affect SGD. We discuss the range of material concentrations and fluxes, their relationship to land use, and the chemical changes that nutrients and metals undergo during their seaward journey through the aquifer. Climate change is likely to affect both the quantity and the quality of SGD, and we review these effects. The chapter concludes with a discussion of active areas of SGD research, including expanding the geographic scope of SGD studies; characterizing and reducing the uncertainty associated with SGD measurements; understanding the behavior of nutrients, metals, and other pollutants in the subterranean estuary; and modeling SGD on a global scale.