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

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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.

Introduction

To an oceanographer, water seeping from the land into the coastal ocean is considered ground water, regardless of its salinity or history. The same process may be considered by a hydrologist as sea water recycling, if the water has a salinity similar to ocean water. In this paper, I attempt to reconcile these viewpoints by introducing a new term: the subterranean estuary. This is defined as a coastal aquifer where ground water derived from land drainage measurably dilutes sea water that has invaded the aquifer through a free connection to the sea.

The widely accepted definition of an estuary, as proposed by Pritchard (1967), is:

A semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.

In this paper, I shall argue that some coastal aquifers can be considered subterranean estuaries because they display the most important features of surface estuaries. I shall further argue, as others have before, that fluxes of fluids from subterranean estuaries are important to the chemistry and biology of the coastal ocean (Marsh, 1977; Johannes, 1980; D'Elia et al., 1981; Simmons, 1992; Moore, 1996, Moore, 1997; Snyder, 1996).

In subterranean estuaries, chemical reactions between the mixed waters and aquifer solids modify the fluid composition; much as riverine particles and suspended sediments modify the composition of surface estuarine waters. Geochemists studying coastal aquifers have long recognized the importance of chemical reactions between aquifer solids and a mixture of sea water and fresh ground water (Runnels, 1969; Back et al., 1979). For example, mixing of sea water supersaturated with respect to calcite with fresh ground water saturated with respect to calcite can result in solutions that are either supersaturated or undersaturated with respect to calcite (Plummer, 1975). The undersaturated solutions result primarily from the non-linear dependence of activity coefficients on ionic strength and on changes in the distribution of inorganic carbon species as a result of mixing. This mechanism was proposed by Back et al. (1979)to explain the massive dissolution of limestone along the northern Yucatan Peninsula. Dissolution of submarine limestone by ground water flow creates distinctive canyons and escarpments on continental margins (Paull et al., 1990).

Calcite dissolution may also be driven by the addition of CO2 to fluids in the subterranean estuary. Burt (1993)has shown that salt water penetrating the Floridan aquifer near Savannah, GA, is enriched in inorganic carbon and calcium, as well as ammonia and phosphate, relative to sea water and fresh ground water endmembers. He attributed these enrichments to oxidation of organic carbon within the aquifer or CO2 infiltration from shallower aquifers.

Sea water–ground water mixing has been invoked to explain the formation of dolomite in coastal limestone. Surface sea water is supersaturated with respect to calcite and dolomite; yet the inorganic precipitation of these minerals from sea water is rarely observed. Hanshaw et al. (1971)and Badiozamani (1973)proposed that a mixture of sea water and ground water could be undersaturated with respect to calcite, yet remain supersaturated with respect to dolomite. They suggested that this solution could lead to the replacement of calcite by dolomite. Other studies reveal that dolomite formation is inhibited by the presence of sulfate ion (Folk and Land, 1975; Baker and Kastner, 1981). The formation of dolomite may be favored in sea water–ground water mixtures where sulfate has been reduced. Evidence of this process comes from the formation of dolomite in recent sediments in a mixing zone between relatively fresh ground water and low sulfate brines from the Dead Sea (Shatkay and Magaritz, 1987).

Biologists have also recognized the importance of sea water recycling through coastal sediments. Riedl et al. (1972)pointed out that advection of oxygenated water across the sediment surface must occur to prevent sediments from becoming reducing. These authors proposed that the circulation of sea water through sediments was driven by `surge pumping'. They estimated that the residence time of the ocean relative to circulation through continental margin sediments was 14,000 years. Other studies have revealed that ground water discharge or sea water cycling may be an important source of nutrients for coral reefs (Marsh, 1977; Johannes, 1980; D'Elia et al., 1981) or other communities on the continental shelf (Simmons, 1992; Snyder, 1996). Simmons (1992)estimated that the fluxes of nitrogen and phosphorus to the Georgia shelf from submarine ground water discharge exceeded fluxes from local rivers.

Ground water-borne nutrients may have significant effects on water quality in surface estuaries (Reay et al., 1992). Because nutrient concentrations in coastal ground water may be several orders of magnitude greater than surface waters, ground water input may be a significant factor in the eutrophication of coastal waters (Valiela et al., 1990). Input of high nitrate ground water may be linked to the initiation of intense algal blooms called brown tides in coastal waters near Long Island, NY (LaRoche et al., 1997).

Section snippets

Mechanisms of flow in subterranean estuaries

Several mechanisms have been proposed to explain the cycling of ground water–sea water mixtures through coastal sediments. The classic Ghyben–Hezberg relation predicts that the discharge of such mixtures will be restricted to a few hundred meters from shore in unconfined aquifers. However, coastal sediments often comprise a complex assemblage of confined, semi-confined and unconfined aquifers. Simple models do not consider the anisotropic nature of the coastal sediments; dynamic processes of

Use of tracers to study subterranean aquifers

Unlike their surface counterparts, subterranean estuaries cannot be seen. Evidence of their function and importance relies on signals they exchange with coastal waters. An estimate of recent salt water encroachment into these systems comes from the penetration of 14C into coastal aquifers from the adjacent ocean (Back et al., 1970; Burt, 1993). Additional evidence of exchange with coastal waters is provided by chemical tracers that are highly enriched in fluids contained in the subterranean

Case studies of tracer applications

To illustrate the use of tracers to qualitatively or quantitatively estimate the input of subsurface fluids, I will review a number of case studies. These studies provide evidence that water exchange between subterranean estuaries and ocean waters may occur throughout the coastal ocean. There are studies that reveal the input of subsurface fluids to coastal salt marshes, the inner continental shelf, the middle/outer continental shelf and even to deep troughs. The following case studies are

Effects of sea level change on the subterranean estuary

During low sea level stands, many subterranean estuaries that are now in contact with the ocean would have been above sea level. These systems, like their surface counterparts, would not be considered estuaries during low sea stand. Ground water passing through these systems should have been fresh and may have contained higher oxygen concentrations. This different chemical environment would have produced different interactions between the ground water and aquifer solids. For example, during low

Anthropogenic effects on the subterranean estuary

During the last century, subterranean estuaries, like their surface counterparts, have experienced considerable change due to anthropogenic pressure. Dredging of channels through surface estuaries and coastal regions has breached underlying confining layers and increased contact between sea water and subterranean estuaries (Duncan, 1972). Increased ground water usage has lowered potentiometric surfaces in coastal aquifers and caused infiltration of sea water into these formations (Smith, 1994).

Summary

The subterranean estuary is an important component of many coastal regions. Ground water passing through these systems is modified by mixing with sea water and reacting with aquifer sediments. The reactions release chemical tracers into fluids contained in subterranean aquifers. Injection of these fluids into the ocean may be recognized by the presence of specific tracers in coastal waters. For some regions, the injection of subsurface fluids may be an important source of nutrients to coastal

Acknowledgements

I thank Bjorn Sundby for inviting me to participate in the Symposium on Model Estuaries. Discussions with Rama, Tim Shaw, Bob Gardner, June Mirecki, Jim Krest, Andrew Crotwell, Tom Church, Bill Burnett, Jaye Cable and many others have been instrumental in shaping the ideas presented here. This research has been supported by The National Science Foundation.

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