The subterranean estuary: a reaction zone of ground water and sea water

Abstract

Abstract Mixing between meteoric water and sea water 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. Geochemical studies within subterranean estuaries have preceded studies that attempt to integrate the effect of these systems on the coastal ocean. The mixing zone between fresh ground water and sea water has long been recognized as an important site of carbonate diagenesis and possibly dolomite formation. Biologists have likewise recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further 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 east and Gulf coasts of the U.S., as well as the Bay of Bengal, will be used to evaluate the importance of these unseen estuaries in supplying not only chemical tracers, but also nutrients, to coastal waters. Anthropogenic effects on subterranean estuaries are causing significant change to these systems. Ground water mining, sea level rise, 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.