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
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