the subterranean estuary: a reaction zone of ground water and sea...

Ž .Marine Chemistry 65 1999 111–125

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

Willard S. Moore )

Department of Geological Sciences, UniÕersity of South Carolina, Columbia, SC 29208, USA

Received 12 October 1997; received in revised form 4 November 1998; accepted 8 December 1998

Abstract

Mixing between meteoric water and sea water produces brackish to saline water in many coastal aquifers. In this mixingzone, chemical reactions of the salty water with aquifer solids modify the composition of the water; much as riverineparticles and suspended sediments modify the composition of surface estuarine waters. To emphasize the importance ofmixing and chemical reaction in these coastal aquifers, I call them subterranean estuaries. Geochemical studies withinsubterranean estuaries have preceded studies that attempt to integrate the effect of these systems on the coastal ocean. Themixing zone between fresh ground water and sea water has long been recognized as an important site of carbonatediagenesis and possibly dolomite formation. Biologists have likewise recognized that terrestrial inputs of nutrients to thecoastal ocean may occur through subterranean processes. Further evidence of the existence and importance of subterraneanestuaries comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifersthrough chemical reactions of the saline water with aquifer solids. They reach the coastal ocean as the surface andsubterranean systems exchange fluids. Exchange between the subterranean estuary and the coastal ocean may be quantifiedby the tracer distribution in the coastal ocean. Examples from the east and Gulf coasts of the U.S., as well as the Bay ofBengal, will be used to evaluate the importance of these unseen estuaries in supplying not only chemical tracers, but alsonutrients, 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 areonly beginning to be realized in this vital component of the coastal ecosystem. q 1999 Elsevier Science B.V. All rightsreserved.

Keywords: estuary; ground water; coastal aquifer; radium

1. Introduction

To an oceanographer, water seeping from the landinto the coastal ocean is considered ground water,regardless of its salinity or history. The same process

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may be considered by a hydrologist as sea waterrecycling, if the water has a salinity similar to oceanwater. In this paper, I attempt to reconcile theseviewpoints by introducing a new term: the subter-ranean estuary. This is defined as a coastal aquiferwhere ground water derived from land drainage mea-surably dilutes sea water that has invaded the aquiferthrough a free connection to the sea.

0304-4203r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0304-4203 99 00014-6

( )W.S. MoorerMarine Chemistry 65 1999 111–125112

The widely accepted definition of an estuary, asŽ .proposed by Pritchard 1967 , is:

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

In this paper, I shall argue that some coastalaquifers can be considered subterranean estuariesbecause they display the most important features ofsurface estuaries. I shall further argue, as others havebefore, that fluxes of fluids from subterranean estuar-ies are important to the chemistry and biology of the

Žcoastal ocean Marsh, 1977; Johannes, 1980; D’Eliaet al., 1981; Simmons, 1992; Moore, 1996, 1997;

.Snyder, 1996 .In subterranean estuaries, chemical reactions be-

tween the mixed waters and aquifer solids modifythe fluid composition; much as riverine particles andsuspended sediments modify the composition of sur-face estuarine waters. Geochemists studying coastalaquifers have long recognized the importance ofchemical reactions between aquifer solids and a mix-

Žture of sea water and fresh ground water Runnels,.1969; Back et al., 1979 . For example, mixing of sea

water supersaturated with respect to calcite withfresh ground water saturated with respect to calcitecan result in solutions that are either supersaturated

Žor undersaturated with respect to calcite Plummer,.1975 . The undersaturated solutions result primarily

from the non-linear dependence of activity coeffi-cients on ionic strength and on changes in the distri-bution of inorganic carbon species as a result ofmixing. This mechanism was proposed by Back et

Ž .al. 1979 to explain the massive dissolution oflimestone along the northern Yucatan Peninsula. Dis-solution of submarine limestone by ground waterflow creates distinctive canyons and escarpments on

Ž .continental margins Paull et al., 1990 .Calcite dissolution may also be driven by the

addition of CO to fluids in the subterranean estuary.2Ž .Burt 1993 has shown that salt water penetrating the

Floridan aquifer near Savannah, GA, is enriched ininorganic carbon and calcium, as well as ammoniaand phosphate, relative to sea water and fresh groundwater endmembers. He attributed these enrichmentsto oxidation of organic carbon within the aquifer orCO infiltration from shallower aquifers.2

Sea water–ground water mixing has been invokedto explain the formation of dolomite in coastal lime-stone. Surface sea water is supersaturated with re-spect to calcite and dolomite; yet the inorganic pre-cipitation of these minerals from sea water is rarely

Ž .observed. Hanshaw et al. 1971 and BadiozamaniŽ .1973 proposed that a mixture of sea water andground water could be undersaturated with respect tocalcite, yet remain supersaturated with respect todolomite. They suggested that this solution couldlead to the replacement of calcite by dolomite. Otherstudies reveal that dolomite formation is inhibited by

Žthe presence of sulfate ion Folk and Land, 1975;.Baker and Kastner, 1981 . The formation of dolomite

may be favored in sea water–ground water mixtureswhere sulfate has been reduced. Evidence of thisprocess comes from the formation of dolomite inrecent sediments in a mixing zone between relativelyfresh ground water and low sulfate brines from the

Ž .Dead Sea Shatkay and Magaritz, 1987 .Biologists have also recognized the importance of

sea water recycling through coastal sediments. RiedlŽ .et al. 1972 pointed out that advection of oxy-

genated water across the sediment surface must oc-cur to prevent sediments from becoming reducing.These authors proposed that the circulation of seawater through sediments was driven by ‘surge pump-ing’. They estimated that the residence time of theocean relative to circulation through continental mar-gin sediments was 14,000 years. Other studies haverevealed that ground water discharge or sea watercycling may be an important source of nutrients for

Žcoral reefs Marsh, 1977; Johannes, 1980; D’Elia et.al., 1981 or other communities on the continental

Ž .shelf Simmons, 1992; Snyder, 1996 . SimmonsŽ .1992 estimated that the fluxes of nitrogen andphosphorus to the Georgia shelf from submarineground water discharge exceeded fluxes from localrivers.

Ground water-borne nutrients may have signifi-cant effects on water quality in surface estuariesŽ .Reay et al., 1992 . Because nutrient concentrationsin coastal ground water may be several orders ofmagnitude greater than surface waters, ground waterinput may be a significant factor in the eutrophica-

Ž .tion of coastal waters Valiela et al., 1990 . Input ofhigh nitrate ground water may be linked to theinitiation of intense algal blooms called brown tides

( )W.S. MoorerMarine Chemistry 65 1999 111–125 113

Žin coastal waters near Long Island, NY LaRoche et.al., 1997 .

2. Mechanisms of flow in subterranean estuaries

Several mechanisms have been proposed to ex-plain the cycling of ground water–sea water mix-tures through coastal sediments. The classic Ghy-ben–Hezberg relation predicts that the discharge ofsuch mixtures will be restricted to a few hundredmeters from shore in unconfined aquifers. However,coastal sediments often comprise a complex assem-blage of confined, semi-confined and unconfinedaquifers. Simple models do not consider theanisotropic nature of the coastal sediments; dynamicprocesses of dispersion, tidal pumping and sea levelchange; or the non-steady state nature of coastal

Žaquifers due to ground water mining Moore and.Church, 1996 .

Ž .Kohout 1965 recognized that cyclic flow of seawater through coastal aquifers could be driven bygeothermal heating. He postulated that the Floridanaquifer in western Florida is in hydraulic contactwith sea water at depths of 500–1000 m in the Gulf

Ž .of Mexico. Cool sea water 5–108C entering thedeep aquifer would be heated as it penetrates theinterior of the platform. The warm water would risethrough fractures or solution channels, mix withfresh ground water and emerge as warm springs

Ž .along the Florida coast. Kohout 1967 presentedtemperature profiles in deep wells that revealed anegative horizontal temperature gradient from theinterior of the Florida platform to the margin, wherecool sea water penetration was hypothesized. Wellsnear the margin displayed negative vertical tempera-ture gradients at depths of 500–1000 m. These ob-servations were consistent with the proposed large-scale cycling of sea water–ground water mixturesthrough the aquifer.

Another mechanism that may be important forlocalized cycling is evaporation of sea water inrestricted coastal systems. This reflux mechanismŽ .Adams and Rhodes, 1960; Simms, 1984 is drivenby the increased density of isolated sea water afterevaporation. Reflux cycling can occur in response todaily, monthly, or longer sea level changes.

Cycling of sea water through coastal aquifers mayalso be driven by an inland hydraulic head. As freshwater flows through an aquifer to the sea above alayer of salt water, it encounters an irregular inter-face where mixing of the fluids is driven by diffu-

Ž .sion and dispersion Cooper et al., 1964 . This pat-tern of two-layer circulation and mixing is similar to

Žthat observed in many surface estuaries Pritchard,.1967 . Dispersion along the interface may be en-

hanced by tidal forces operating in an anisotropicmedium. Tidal forces are clearly reflected in semi-

Ž .confined coastal aquifers Burt et al., 1987 . Duringrising tide, sea water enters the aquifer throughbreaches in confining units; during falling tide, itflows back to the sea. Through each tidal cycle, thenet movement of the fresh water–salt water interfacemay be small; but during rising tide, preferentialflow of salt water along channels of high hydraulicconductivity may be caused by anisotropic perme-ability. This produces irregular penetration of saltwater into fresh water zones. Diffusion mixes the saltinto the adjacent fresh water and chemical reactionsmay ensue. During falling tide, the reverse occurs asfresher water penetrates along these same paths andgains salt by diffusion. The permeability and prefer-ential flow paths may be changed by chemical reac-tions within the aquifer. Precipitation of solids canrestrict or seal some paths while dissolution willenlarge existing paths or open new ones. The fluidexpelled to the sea may be quite different chemicallyfrom the sea water that entered.

An additional mechanism that may affect subter-ranean estuaries is the upward flow of fluids fromdeep, pressurized aquifers. The continental shelf offthe southeastern U.S. coast consists of a thick se-quence of Tertiary limestone overlain by relic sand.Drilling has revealed that substantial fresh water liestrapped in the deep sediments. The AMCOR projectŽ .Hathaway et al., 1979 drilled 19 cores on thecontinental shelf and noted that fresh or slightlybrackish waters underlie much of the Atlantic conti-nental shelf. JOIDES test hole J-1B drilled 40 kmoffshore from Jacksonville, FL, was set in the Eocene

ŽLimestone equivalent to the Floridan aquifer on-.shore 250 m below sea level. This well tapped an

aquifer that produced a flow of fresh water to aŽ .height of 10 m above sea level Kohout et al., 1988 .

Ž .Manheim 1967 has reviewed substantial evidence


for submarine discharge of water on the Atlanticcontinental slope of the southern United States.

Ž .Bisson 1994 introduced the term ‘mega-watersheds’ to describe the broadest possible catch-ment areas that may contribute water to deep reser-voirs. He explains the existence of large deep reser-voirs of fresh and brackish water in arid as well asother regions as due to tectonically induced regionalfracture permeability, largely controlled by basementtectonism. Upward leakage along fractures from thesedeep systems may provide an additional source offluids to the shallow subterranean estuary.

Like surface estuaries, subterranean estuaries areaffected strongly by sea level changes. The occur-rence of both systems today is due to the drowningof coastlines by the last transgression of the sea.During low glacial sea stands, modern surface andsubterranean estuaries did not exist in their presentlocations. Twenty thousand years ago, subterraneanestuaries were probably restricted to the edge of thecontinental shelf. At this time, river channel cutting

may have caused breaches in overlying confiningunits. If these channels were filled with coarse clas-tics during sea level rise, the channel fill may cur-rently serve as an effective conduit for shallowground water flow across the shelf and the breachesmay provide communication between the superficialand deeper confined aquifers. As the rising sea ap-proached its current level, both surface and subter-ranean estuaries developed as sea water infiltratedinto river basins and coastal aquifers.

In a provocative book titled The Hidden Sea,Ž .Chapelle 1997 pointed out that in Life on the

Mississippi, Mark Twain noted a number of remark-able aspects of the Mississippi River. One of themost puzzling was the change in river level fromdroughts to floods:

The rise is tolerably uniform down to Natchez—about 50 ft. But at Bayou La Fourche, the river risesonly 24 ft; at New Orleans only 15, and just abovethe mouth only two and one half.

Table 1Comparison of the composition of fluids extracted from the subterranean estuary with river and ocean water

226 y1Ž . Ž . Ž . Ž .Location Salinity Ba nM Ra dpm l PO mM NH qNO mM4 3 3

North Inlet, SC1–3 m wells 0.36 50 0.4 0.1 33

0.90 564 1.2 0.1 673.49 1470 4.3 1.8 74

10.28 500 3.2 1.1 13834.47 1000 10.5 8.5 93

Cape Fear, NCArtesian well 17.0 3300 9.00

Charleston, SCShallow well 7.0 1.30Deep well 24.0 5.30

Hilton Head, SCAW 64 m 20.3 1304 2.09 2.72 145AW 70 m 16.8 1345 2.14 2.66 155TS 55 m 0.4 49 0.13 4.64 36TS 64 m 11.3 775 1.09 3.27 115TS 70 m 5.3 1393 1.29 2.90 182

Pee Dee River 0.0 170 0.03 0.1 0.8Savannah River 0.0 130 0.04 0.1 0.5Atlantic Ocean 36.0 40 0.08 0.1 0.1

Ž . Ž .Data from Moore 1996 ; Moore and Shaw 1998 ; Krest, personal communication; Crotwell, personal communication; Shaw, personalcommunication.


Ž .Chapelle 1997 pointed out that because the riverchannel becomes narrower toward the mouth, justthe opposite behavior would be expected unless theriver was storing large quantities of water in theunderlying aquifer. The step from this observation tothe concept of a subterranean estuary at the rivermouth is not a long one. Changes in river level mustcause changes in the amount of fluid discharged tothe sea through the subterranean estuary.

Demand for fresh water by coastal communitieshas intensified infiltration of sea water into subter-ranean estuaries by decreasing the hydraulic pressure

Ž .in coastal aquifers Smith, 1988 . In many cases,seasonal ground water usage causes considerablefluctuation in hydraulic pressure and salt infiltration

in these systems. Changes in ground water usage arealso reflected in the position of the fresh water–saltwater interface in semi-confined coastal aquifersŽ .Smith, 1994 . Over-pumping of coastal aquifers hascaused some water to become non-potable due to saltencroachment. When such aquifers are abandoned,they may recharge with fresh water. In this case,salty fluids in the aquifer may be discharged to thesea. Examples from the South Carolina coast will bepresented later in the paper.

3. Use of tracers to study subterranean aquifers

Unlike their surface counterparts, subterranean es-tuaries cannot be seen. Evidence of their function

Fig. 1. Strategy to use tracers in the coastal ocean to assess fluid inputs from subterranean estuaries.


and importance relies on signals they exchange withcoastal waters. An estimate of recent salt waterencroachment into these systems comes from thepenetration of 14C into coastal aquifers from the

Ž .adjacent ocean Back et al., 1970; Burt, 1993 . Addi-tional evidence of exchange with coastal waters isprovided by chemical tracers that are highly enrichedin fluids contained in the subterranean estuary andrelatively non-reactive in coastal waters. Elevatedconcentrations of such tracers in the coastal oceanprovide the evidence that subterranean estuaries arefreely connected to the sea and that fluid exchange inthese systems is substantial. Several tracers havebeen used in this regard including 222 Rn, Ra iso-topes, Ba, and methane. These tracers are highlyenriched in the salty pore waters of coastal aquifers.Some examples of the contrasting composition ofsubsurface fluids with surface waters are given inTable 1.

The release of subsurface fluids to the coastalocean may create chemical anomalies that can berecognized over large areas. These chemical signalsintegrate the effects of fluid input over the studyarea. By combining the distribution of long-lived

Ž 226 228 .tracers e.g., Ra, Ra, Ba with estimates ofcoastal ocean exchange rates, the offshore flux of the

Žtracer may be estimated. For short-lived tracers e.g.,222 224 223 .Rn, Ra, Ra , decay may be more importantto the balance than exchange. The net offshore fluxor decay of the tracer must be balanced by new inputof the tracer. The input of the tracer from subter-ranean estuaries may be established after other

Ž .sources e.g., rivers, suspended sediments, diffusionare quantified. Ratios of other elements to the tracerin the advecting fluids may be used to estimate thefluxes of other elements due to this process. Thisstrategy is illustrated in Fig. 1. Currently, estimatesof fluid composition are not well constrained; thiswill change as more data become available.

4. Case studies of tracer applications

To illustrate the use of tracers to qualitatively orquantitatively estimate the input of subsurface fluids,I will review a number of case studies. These studiesprovide evidence that water exchange between sub-terranean estuaries and ocean waters may occur

throughout the coastal ocean. There are studies thatreveal the input of subsurface fluids to coastal saltmarshes, the inner continental shelf, the middleroutercontinental shelf and even to deep troughs. Thefollowing case studies are examples of the interac-tion of the subterranean estuary with the ocean.

Ž .Fanning et al. 1981 documented thermal andchemical anomalies in water from submarine springsnear Ft. Myers, FL. The warm spring water had thesame chlorinity as deep Gulf of Mexico water, butwas highly enriched in 222 Rn, 226Ra, 228Ra, andSiO , slightly depleted in Mg, and contained negligi-2

ble O . These authors concluded that the circulation2

was driven by the geothermal process envisioned byŽ .Kohout 1965 . Because the water exiting these

springs was not measurably diluted by fresh groundwater, these springs do not qualify as true examplesof subterranean estuaries. However, the geothermalcirculation process may be active in systems where

Ž .dilution of sea water is evident. Kohout 1965 clearlyintended to include the mixing of fresh ground waterand sea water for his model.

In Spencer Gulf, Australia, a system that receivesonly intermittent surface freshwater discharge Veeh

Ž . 226 Žet al. 1995 reported high Ra activities 15–25. 226dpmr100 l in surface waters. These Ra activities

could not be explained by riverrsediment input orby evaporation. They suggested that the excess 226Rain the gulf may be derived from the discharge ofground water enriched in Ra. In this arid region,intense evaporation may initiate the reflux mecha-nism discussed earlier.

The North Inlet, SC, salt marsh is a coastalsystem with negligible surface freshwater input. Here,226Ra and 228Ra activities in the tidal creeks exceedoffshore values by factors of 2 to 4. Because thesurface water in North Inlet has only a 12-h resi-dence time, a considerable internal source of 226Raand 228Ra is required to maintain the large difference

Ž .with the coastal ocean. Rama and Moore 1996computed the input of 226Ra and 228Ra to this systemby regeneration from Th parents in surface sedimentsand diffusion from deeper sediments. These sourceswere at least two orders of magnitude too low toexplain the fluxes exiting the system. They con-cluded that the subsurface advection of fluids en-riched in Ra isotopes was required to balance thefluxes from North Inlet to the coastal ocean.


Ž . 222 226Cable et al. 1996 reported high Rn and Raactivities in subsurface coastal waters near the Florida

Ž .State University Marine Laboratory FSUML . Theymodeled the measured 222 Rn and 226Ra distributionsand water residence times based on current velocitiesto derive fluxes of 222 Rn and 226Ra to the study area.The potential supply of Ra to the region by localrivers could support less than 10% of the 226Raenrichment and less than 3% could be supported bydiffusion from surface sediments. Cable et al. con-cluded that the flow of submarine ground waterhighly enriched in 222 Rn and 226Ra into the study

area was the major source of these radionuclides.They estimated that the fluid flux to this 620 km2

area was comparable to the flow of the largest riverin Florida. They also pointed out that the distributionof high 222 Rn activities in Florida shelf waters isclosely related to the distribution of known subma-rine springs.

Along the coast of South Carolina, 226Ra concen-trations exceed the amount that can be supported by

Ž .local rivers Moore, 1996 . Fig. 2 is a map of thedistribution of 226Ra in surface waters along this

Ž .coast. Moore 1996 demonstrated that less than 10%

226 Ž .Fig. 2. The distribution of Ra in surface waters of the South Atlantic Bight, July 1994. Modified from Moore 1996 .


of the excess 226Ra in these coastal waters can beexplained by river discharge and desorption fromriver-borne particles or other sediments. He used the

226 Ž .distribution of Ra in the coastal ocean Fig. 2 andan estimate of the exchange time of the inner shelfwater with offshore waters to calculate the 226Ra fluxfrom the coast. Combining this flux estimate with

226 Žmeasurements of Ra in subsurface fluids e.g.,.Table 1 led to the startling conclusion that the flux

Žof subsurface fluid that is water containing ;100226 .times more Ra than ocean surface water must

have been about 40% of the total river flow duringŽ .the period of investigation. Moore 1996 cautioned

that these estimates could not be extrapolated toother seasons because concentrations of 226Ra on theinner shelf are lower in winter and spring. Whetherthese changes are due to lower fluid inputs, a changein the composition of the fluid, or faster exchange ofcoastal waters is unknown.

Ž .Shaw et al. 1998 reached essentially the sameŽ .conclusion as Moore 1996 , based on measured

concentrations of Ba in subsurface fluids and thedistribution of Ba in these surface waters. The off-shore transport of Ba was calculated from the aver-age inner shelf concentration and the residence timeof inner shelf waters. This was compared to the Baflux calculated from measured subsurface fluid end-

Žmembers and the measured fluid flux Simmons,.1992 . The agreement between these two indepen-

dent estimates of Ba input confirmed the subsurfaceŽ .fluid flow estimate from Moore 1996 . The Ba flux

from advecting fluids was found to be on the orderŽ .of four times the river flux Shaw et al., 1998 . The

Ba concentration in subsurface fluids in the regioncan exceed saturation with respect to barite by up toa factor of 6. Coastal waters approach, but do notexceed, saturation. This work confirms that, for Ba,subsurface fluids are chemically distinct from riverwaters and very important for supplying Ba to theocean.

Exchange of subsurface fluids from subterraneanestuaries below the continental shelf may also impactdeeper regions of the shelf. Here, the input will beinto bottom waters and the signal may not reach thesurface if the water is stratified vertically. In anotherstudy of South Carolina coastal waters, Moore and

Ž .Shaw 1998 measured Ra isotopes and Ba along theŽ .Winyah Bay transect refer to Fig. 2 for location .

Concentrations of 226Ra and Ba showed two elevatedregions: on the inner shelf in -15 m depth and

Ž .40–70 km offshore at 25–45 m depth Fig. 3 . Basedon high activities of 228Ra as well as 226Ra in thedeep Ra-enriched zone, they concluded that the highconcentrations of Ra and Ba were due to submarinefluid input rather than upwelling of deeper water orthe dissolution of 226Ra from phosphorite.

Ž . 226Moore and Shaw 1998 mapped the Ra andBa distribution over a 2600 km2 area to estimate atotal deep 226Ra inventory. They used physical ob-servations and the distribution of the short lived224 Ž .Ra half lifes3.6 days in the enriched zone toestimate a residence time of 10–20 days. In this way,they converted the excess 226Ra inventory to a flux.The flux calculated for this 80 km segment of theouter shelf was similar to the flux from the entire320-km long inner shelf. Moore and Shaw alsodemonstrated that the region impacted by fluid flowexhibited high chlorophyll concentrations. They sug-gested that the leaking fluids supply nutrients tosupport primary productivity in this region. Thisstudy suggested that fluid flow onto the middle andouter shelf may be an important chemical and bio-logical process.

Studies from the north coast of the Bay of Bengalin Bangladesh reveal that subterranean estuaries arenot restricted to the southeast United States. Thecoast of Bangladesh is dominated by drainage of theGanges and Brahmaputra Rivers. The annual flux ofsediment to the ocean from these rivers is among thehighest in the world. Desorption of 226Ra and Bafrom this riverine sediment produces a major pointsource of 226Ra and Ba input to the ocean. Highest226Ra and Ba fluxes are expected to occur during

Ž .high river flow June–September when almost all ofŽ .the sediments are discharged. Moore 1997 reported

that during low discharge of the Ganges–Brahma-putra Rivers in March 1991, fluxes of 226Ra and Bato the northern Bay of Bengal were comparable toexpected river-derived fluxes during peak river dis-charge. A large non-riverine source of Ra and Bawas required to explain the high fluxes during lowriver discharge. He suggested that this source wasthe discharge of submarine fluids containing highconcentrations of 226Ra and Ba.

Ž .Tsunogai et al. 1996 reported fluid seepage at adepth of 900–1100 m in Sagami Bay, Japan. Al-


Fig. 3. Cross-section of the distribution of 226Ra along the Winyah Bay transect of the South Atlantic Bight, August 1995. Modified fromŽ .Moore and Shaw 1998 .

though they did not have Ra data, the fluid wasŽ . Ž .enriched in Ba 300 nM , NH 1500 mM and total3

Ž .CO 6 mM and depleted in chloride relative to sea2

water. The water they sampled on the sea bed at1100 m depth was similar in composition to thebrackish waters reported in Table 1. They concludedthat this fluid was a mixture of fresh ground water

and sea water that had reacted with sediment porewater and solid phases. They further concluded thatthe discharge of such fluids into the ocean is animportant factor controlling ocean chemistry.

The studies listed above which attempt to quanti-tatively estimate tracer fluxes are summarized inTable 2. This summary demonstrates that submarine

Table 2Comparison of estimated 226Ra fluxes based on different case studies

226 226Region Setting Size Ra flux Ra flux Reference2 y1 y2 y1Ž . Ž . Ž .km dpm day dpm m day

9North Inlet salt marsh 32 3=10 100 Rama and MooreŽ .1996

10FSUML near shore 620 8=10 130 Cable et al.Ž .1996

11South Atlantic Bight inner shelf 6400 2=10 30 MooreŽ .1996

11South Atlantic Bight section of 2600 3=10 100 Moore and ShawŽ .outer shelf 1998

13Bay of Bengal inner shelf, low 18,000 1=10 500 MooreŽ .river discharge 1997

12Amazon River total shelf 130,000 3=10 20 Moore et al.Ž .annual average 1995

These studies illustrate that 226Ra fluxes due to subsurface fluid input are comparable to fluxes from the Amazon River.


fluid flow occurs over different spatial scales. Whennormalized to area, most 226Ra fluxes are on theorder of 100 dpm my2 dayy1, the exception beingthe Bay of Bengal where the normalized flux isabout five times greater. By contrast, the Amazonshelf only contributes about 20 dpm my2 dayy1, inspite of the enormous water and sediment discharge.

226 Ž .Translating the Ra or other tracer flux to aflux of submarine fluid requires an estimate of the226 Ž .Ra or other tracer concentration in the leaking

Ž .fluids. Salty 2–34 ppt fluids extracted from subter-ranean estuaries along the coast of South Carolinahave 226Ra concentrations ranging from 1 to 10 dpmy1 Ž .l Table 1 . Estimates of fluid fluxes, assuming

Žconcentrations in the upper part of this range 7–10y1 .dpm l , indicate that such fluxes to the South

Carolina coast during the summer are similar to totalŽ .river fluxes Moore, 1996; Moore and Shaw, 1998 .

Additional studies of the factors that regulate thecomposition of subterranean fluids are required be-fore firm estimates of fluid fluxes are possible.

More specific information concerning the compo-sition of the water producing the 226Ra anomaly andits likely site of entry to overlying waters can beobtained using the other Ra isotopes. This approach

Ž .was used by Rama and Moore 1996 in the NorthInlet salt marsh to demonstrate that most of theexchanges occurred through sandy sediments or shellhash beneath tidal creeks or through deep wormtubes rather than through organic-rich sediments thatcap much of the marsh.

5. Effects of sea level change on the subterraneanestuary

During low sea level stands, many subterraneanestuaries that are now in contact with the oceanwould have been above sea level. These systems,like their surface counterparts, would not be consid-ered estuaries during low sea stand. Ground waterpassing through these systems should have beenfresh and may have contained higher oxygen concen-trations. This different chemical environment wouldhave produced different interactions between theground water and aquifer solids. For example, duringlow sea stand, cations adsorbed on mineral surfaces

could be exchanged with dissolved species. Thisexchange could be driven by differences in abun-dance ratios between the new low ionic strengthground water and old salty water present at high seastand. Additionally, changes in surface sites may becaused by dissolution or precipitation of mineralphases. Precipitation of Fe–Mn oxyhydroxides fromoxygenated ground water could produce high surfacearea minerals with high exchange capacity. The com-bination of low ionic strength ground water and thepresence of Fe–Mn oxyhydroxides as well as othermineral surfaces could produce an effective mecha-nism for adsorption of dissolved metals and nutrientsfrom fresh ground water passing through the aquifer.A rise of sea level would expose portions of thisaquifer to saline waters and initiate desorption ofadsorbed ions; much as a water softener is rechargedby flushing with a strong saline solution. This sce-nario could produce a flux of desorbed metals andnutrients to the ocean during sea level rise. This fluxshould continue as long as sea water was encroach-ing into the subterranean estuary and a portion of thefluid was exchanging into the ocean. During periodsof stable sea level, these effects should be dimin-ished unless sea water encroachment was beingdriven by changes in ground water flow.

6. Anthropogenic effects on the subterranean es-tuary

During the last century, subterranean estuaries,like their surface counterparts, have experienced con-siderable change due to anthropogenic pressure.Dredging of channels through surface estuaries andcoastal regions has breached underlying confininglayers and increased contact between sea water and

Ž .subterranean estuaries Duncan, 1972 . Increasedground water usage has lowered potentiometric sur-faces in coastal aquifers and caused infiltration of

Ž .sea water into these formations Smith, 1994 . Theresult has been an increased rate of salinization of

Ž .subterranean estuaries Burt, 1993 . Sea level rise,whether natural or induced, has also caused in-creased salinization of the subterranean estuary. Someof the anthropogenic changes effected over the last100 years are equivalent to one third of a glacial–in-


terglacial sea level cycle in terms of changes in thefresh–salt interface in subterranean estuaries. Thesechanges are taking place worldwide due to increased

demands of a rising coastal population for freshŽ .water Chapelle, 1997 . In this section, I shall illus-

trate some of these changes through a discussion of

Fig. 4. Changes in the potentiometric head of the Upper Floridan aquifer near the GA–SC boundary. The bottom figure shows the estimatedŽ . Ž1880 surfaces modified from Bush and Johnston, 1988 ; the top figure shows the piezometric surfaces measured in 1984 modified from

.Smith, 1988 .


effects that have been documented along the coastsof South Carolina and northern Georgia.

As people migrate to the coast, either to live orfor holiday, they require adequate fresh water fordomestic use, industry, agriculture, and to keep thelandscape attractive, especially on golf courses.Along the southeastern U.S. coast, this water haslargely been derived from ground water wells. As thedemand for ground water has increased, the potentio-

Žmetric head the level relative to sea level to which acolumn of water will rise in a tightly cased well

.drilled into the aquifer has declined sharply. EarlyŽ .in this century, artesian free-flowing wells were

Ž .common along the southeast coast Warren, 1944 ;now, they are rare. In 1880, potentiometric surfacesin the Floridan aquifer at Savannah, GA, were q10

Ž .m Bush and Johnston, 1988 ; now, they are y30 mŽ . Ž .Smith, 1988 , a drop of 40 m Fig. 4 . In 1880,fresh water discharged into Port Royal Sound east of

Ž .Savannah, GA Smith, 1988, 1994 . Now, the Flori-dan aquifer is being recharged with sea water from

Ž . 14the Sound Burt, 1993 . Analyses of C in thesesalty ground waters reveal that they contain up to74% modern carbon, in contrast to eastern and north-ern regions of the aquifer where the fraction is

Ž .-2% Burt, 1993; Landmeyer and Stone, 1995 .The salinization of the aquifer has been facilitated by

Ž .dredging. Duncan 1972 documented breaches inthe confining unit above the Floridian aquifer thatcorrelate well with channels and turning basinsdredged through Port Royal Sound. Where the aquiferis in direct contact with the ocean, water exchangewill occur. In addition to rendering the ground waternon-potable, salt water intrusion into these aquiferscauses ion exchange and other reactions to occurwhich chemically alter the intruding sea water andenrich the fluids in Ra, Ba and nutrients.

In other regions of the South Carolina coast,depression of potentiometric surfaces has causedcommunities to shift to deeper aquifers or surfacewater. An example of the effect of changing groundwater usage on coastal aquifers is provided by an

Ž .observation well HO-269 at North Myrtle Beach,Ž .SC Fig. 5 . The water level in this well decreased

Žfrom y13 m in 1977 to y35 m in 1985 data from.U.S.G.S. Water Resources Data, SC, 1977–1995 .

Superimposed on this trend are large annual fluctua-tions due to increased fresh water demand in the

Fig. 5. Changes in the water level for well HO-269 in NorthMyrtle Beach, SC, 1976–1992. Data from U.S.G.S. Water Re-sources Data, SC, 1976–1993.

summer. For example, in 1978, the water level inthis well fluctuated by 8 m. Severe drought condi-tions in 1984–1985, coupled with salinization ofother wells in this aquifer, led the city to shift tosurface water and deeper aquifers. From 1985 to1991, the water level in this well increased fromy35 m to y18 m. Unfortunately, HO-269 was lostin 1992, so additional data of the aquifer recoveryare not available.

Most models of fluid flow in coastal aquifers aredesigned to evaluate residual flow. The boundarybetween salt and fresh water is considered a planethat may move inland as potentiometric surfaces arereduced, or to the sea if they are increased. Superim-posed on the residual flow are fluctuations due todifferences in recharge and demand. An analysis ofresidual flow would smoothen the annual variationsin well HO-269. However, these variations amplifydispersion in the aquifer and may cause considerableexchange between water in the aquifer and the coastalocean.

The changes documented in these examples pro-vide an insight into how vulnerable coastal aquifersare to over pumping. The North Myrtle Beach exam-ple also reveals how quickly an aquifer may recoverif pumping is diminished. Unfortunately, recovery of


Table 3Comparison of surface and subterranean estuaries

Surface estuaries Subterranean estuaries

Mixing zone of river and sea water mixing zone of meteoric and sea water

Tidal and river forces, estuarine circulation, hydraulic and tidal forces, circulation?residual flow to sea, residual flow may be either way,short residence time long residence time

High particle concentrations lead to direct water contact with solids leadsstrong particle–water interactions to strong particle–water interactions

Sea level exerts a major control sea level exerts a major control

Human impact is often significant human impact is often significant

In contact with atmosphere, no contact with atmospherehigh oxygen, oxidized Fe and Mn low to high oxygen, high pCO ,2Ž .sediments may be different Fe and Mn may be reduced

Abundant, diverse life bacteria are primary life

Major ions dominated by sea salts major ions may reflect diagenesis

water level and recovery of water quality may pro-ceed differently.

7. Summary

The subterranean estuary is an important compo-nent of many coastal regions. Ground water passingthrough these systems is modified by mixing withsea water and reacting with aquifer sediments. Thereactions release chemical tracers into fluids con-tained in subterranean aquifers. Injection of thesefluids into the ocean may be recognized by thepresence of specific tracers in coastal waters. Forsome regions, the injection of subsurface fluids maybe an important source of nutrients to coastal waters.

There are a number of similarities as well asdifferences between surface and subterranean estuar-ies. Some of the characteristics of these systems areoutlined in Table 3.

As we become more aware that the subterraneanestuary is a vital component of the coastal ecosys-tem, ideas concerning its function and importancewill change. The purpose of this paper is not toestablish dogma concerning this system, but to open

discussion as to its role in the coastal ecosystem. Theexpertise and viewpoints of a wide variety of scien-tists are required to understand the subterranean estu-ary.

Acknowledgements

I thank Bjorn Sundby for inviting me to partici-pate in the Symposium on Model Estuaries. Discus-sions with Rama, Tim Shaw, Bob Gardner, JuneMirecki, Jim Krest, Andrew Crotwell, Tom Church,Bill Burnett, Jaye Cable and many others have beeninstrumental in shaping the ideas presented here.This research has been supported by The NationalScience Foundation.

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