The Subterranean Estuary: A reaction zone of groundwater and ocean water

Willard S. Moore, Ph.D.
University of South Carolina

Wednesday, January 12, 2000
3:00 p.m.—Pacific Forum

Mixing between meteoric water and seawater produces brackish to saline water in many coastal aquifers. In this mixing zone, chemical reactions of the salty water with aquifer solids modify the composition of the water, much as riverine particles and suspended sediments modify the composition of surface estuarine waters. To emphasize the importance of mixing and chemical reaction in these coastal aquifers, I call them subterranean estuaries.

Evidence of the existence and importance of subterranean estuaries comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers through chemical reactions of the saline water with aquifer solids. They reach the coastal ocean as the surface and subterranean systems exchange fluids. Exchange between the subterranean estuary and the coastal ocean may be quantified by the tracer distribution in the coastal ocean. Examples from the southeast coast of the United States and Monterey Bay, California, will be used to evaluate the importance of these unseen estuaries in supplying not only chemical tracers, but also nutrients, to coastal waters. Recent evidence of semidiurnal temperature variations in a submarine aquifer strengthens the conclusion that these systems rapidly exchange fluids.

Anthropogenic effects on subterranean estuaries are causing significant change to these systems. Groundwater mining, sea level rise, waste disposal, and channel dredging impact these systems directly. The effects of these changes are only beginning to be realized in this vital component of the coastal ecosystem.

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 Last updated: December 19, 2000