Land or ocean?: Assessing the driving forces of submarine

groundwater discharge at a coastal site in the Gulf of Mexico

Isaac R. Santos,1 William C. Burnett,1 Jeffrey Chanton,1 Natasha Dimova,1

and Richard N. Peterson1

Received 22 July 2008; revised 4 February 2009; accepted 17 February 2009; published 21 April 2009.

[1] Submarine groundwater discharge (SGD) is an important pathway of trace elementand nutrient cycling in the coastal ocean. Since terrestrial and marine SGD drivingforces are usually superimposed, it is difficult to separate their relative contributions. Thiswork uses continuous, long-term (�2 years) records of seawater 222Rn to estimate totalSGD rates and hydrological observations to examine the driving forces of SGD at acoastal site on the Gulf of Mexico. While seasonal cycles cannot be clearly identified,short-term fluctuations were ubiquitous. Radon-derived SGD was strongly associatedwith the neap spring tidal cycle, at least during a period of extreme drought whenminimum external disturbances were present. We applied multiple independentapproaches to separate the relative contribution of fresh and saline SGD. A salinity mixingmodel indicated that the contribution of fresh SGD ranged between 3 and 12%. Therate of freshening of water within a seepage meter required vertical freshwater advectionof �0.9 cm d�1 at low tide, which is <10% of the total SGD estimated from 222Rn(�11 cm d�1). Darcy’s law calculations and water table decline rate also suggested a freshSGD contribution <5%. The temporal trends in total SGD and the reasonable agreement ofour multiple approaches indicate that transient marine forces, likely tidal pumping,dominate benthic advective exchange in these permeable sediments. Compared to regionalriver inputs, our conservatively extrapolated saline SGD is on the same order ofmagnitude, but fresh groundwater only represents a small fraction. Future investigationsshould refine techniques for scaling up local observations.

Citation: Santos, I. R., W. C. Burnett, J. Chanton, N. Dimova, and R. N. Peterson (2009), Land or ocean?: Assessing the driving

forces of submarine groundwater discharge at a coastal site in the Gulf of Mexico, J. Geophys. Res., 114, C04012,

doi:10.1029/2008JC005038.

1. Introduction

[2] Submarine groundwater discharge (SGD) is an im-portant pathway of trace element and nutrient cycling in thecoastal ocean [Charette and Sholkovitz, 2006; Kim et al.,2008; Santos et al., 2008a; Slomp and Van Cappellen,2004]. The term ‘‘submarine groundwater discharge’’caused confusion until it was defined as ‘‘any and all flowof water on continental margins from the seabed to thecoastal ocean, regardless of fluid composition or drivingforce’’ [Burnett et al., 2003]. The misunderstandings arosefrom the divergent perspectives in which different disci-plines see this phenomenon. Oceanographers tend to viewany subsurface fluid as groundwater (pore water), whereashydrologists have often thought of SGD as only thatmeteoritic fresh water that soaks into the ground, becomingpart of an aquifer [Church, 1996; Moore, 1996]. The currentdefinition of SGD thus includes submarine fresh ground-

water discharge (the terrestrial component) and recirculatedsaline groundwater discharge (the marine component).[3] Since terrestrial and marine driving forces are usually

superimposed, it is difficult to separate their relative con-tributions. In the last decade or so, SGD assessmentmethods have been developed and refined [Burnett et al.,2006; Cable et al., 1996b; Moore, 1996]. A number ofstudies have highlighted its environmental significance (seereviews by Burnett et al. [2006] and Swarzenski et al.[2007]), but little information about the SGD driving forcesis available. The single most studied factor driving SGD isthe change in pressure (hydraulic) head, which drives freshSGD. Saline SGD may result from tidal pumping, wavesetup, current-induced pressure gradients [Huettel et al.,2003; Li et al., 1999; Taniguchi, 2002]; and water leveldifferences across permeable barriers [Chanton et al.,2003]. Convection induced by denser water overlying lessdense groundwater [Smith, 2004], anthropogenic alterationsin the water cycle [Santos et al., 2008b], and bioirrigation[Martin et al., 2006] can also drive groundwater discharge.Seasonal saline SGD cycles may be related to a shift in thefreshwater-saltwater interface driven by changes in thewater table on land. At Waquoit Bay, Massachusetts, for

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1Department of Oceanography, Florida State University, Tallahassee,Florida, USA.

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example, seawater recharges the aquifer in the winter and isdischarged back into coastal waters in the summer [Michaelet al., 2005].[4] Many dissolved species, including nutrients, can be

enriched in fresh groundwater, so determining its relativecontribution in SGD is important for coastal managers. Inspite of the increasing number of SGD studies, only a fewinvestigators have attempted to produce separate estimatesof fresh SGD in relation to total SGD [Martin et al., 2007;Michael et al., 2003; Smith, 2004; Taniguchi et al., 2006].In a recent analysis, Moore et al. [2008] estimated from228Ra profiles that groundwater inputs into the entireAtlantic Ocean are 80–160% of the total riverine input,but offered little insight whether this SGD is fresh or saline.Rarely have two or more approaches been employed in anyone study to determine total SGD [Burnett et al., 2003].Recently, some studies have applied multiple approaches tocalculate total SGD [Burnett et al., 2006; Dulaiova et al.,2006; Mulligan and Charette, 2006; Peterson et al., 2008;Swarzenski et al., 2007], but we are not aware of anystudies that have attempted to apply more than oneapproach to estimate the relative contribution of fresh versussaline SGD.[5] As the magnitude of SGD at any one site may vary

over time scales ranging from hours to years, we hypoth-esize that intensive, long-term observations will provideinsights into SGD driving forces beyond that which occa-sional sampling can provide. In this paper, we combinelong-term (�2 years) hydrological and geochemical obser-vations at a coastal site in the Gulf of Mexico to separate thedifferent groundwater fractions in hope that this will provideinsights into the driving mechanisms of SGD. In a com-panion paper, we qualitatively discuss the temporal vari-ability of 222Rn and CH4, including the effect of storms onthese gaseous SGD tracers in coastal seawater [Santos et al.,2009a]. Here, we use the 222Rn time series to quantitativelyestimate total SGD rates and discuss its driving forces. Wealso compare the 222Rn-derived total SGD with multipleindependent approaches to separate the relative contributionof fresh and saline SGD.

2. Material and Methods

2.1. Site description

[6] The experimental site (N29� 540; W84� 300) is locatedin the northeastern coastal Gulf of Mexico (Florida StateCoastal and Marine Laboratory (FSUCML)). This area iswell known for the presence of both diffuse seepagenearshore (from the unconfined aquifer) and submarinesprings (from the confined Floridan Aquifer) [Burnett andDulaiova, 2003; Cable et al., 1996b; Lambert and Burnett,2003]. Annual mean precipitation for the region is�150 cm,with most rainfall usually occurring from June to Octoberand the lowest precipitation in November and December, aswell as March to May. The tides are mixed with an averagerange of 0.85 m. The seafloor is characterized by gentlysloping topography away from the coast resulting in a waterdepth of only �2 m as far as 1 km offshore [Lambert andBurnett, 2003]. The upper �2.5 m of the shallow aquiferconsists of permeable sands, while an impermeable mudlayer occurs between �3 and 5 m [Santos et al., 2008a].This surface sand aquifer sits atop a layered limestone and

dolomite platform, which hosts one of the most prolificaquifers in the world (the Floridan Aquifer).

2.2. Approach

[7] Coastal seawater was continuously pumped from anintake (1.6 m deep; 300 m offshore) to a shore-basedlaboratory for almost 2 years (from 18 November 2005 to15 October 2007). Seawater 222Rn (1-hour intervals), tem-perature, and salinity (15-min intervals) were measured.Radon is an excellent natural SGD tracer because it is2–4 orders of magnitude more concentrated in groundwaterthan seawater, its half-life (3.8 days) is on the same order asmany coastal processes, and it is conservative except forexchange with the atmosphere [Cable et al., 1996b]. Weused a portable continuous radon in air monitor modified forradon in water (RAD-7, Durridge Company) [Burnett et al.,2001]. This system is based on the equilibration of a streamof flowing water with a stream of air that is recirculatedthrough an air-water exchanger and the RAD-7. The Rnequilibration partitioning between the gas and the liquidphase is temperature dependent, so we determine the solu-bility coefficients on the basis of the continuous temperaturemeasurements. We also report results from a separateexperiment designed to test whether the radon observationsat the seawater intake are representative of a larger area. Inaddition to the intake at 300 m from shore, we set up othercontinuous monitoring systems for about two days at 10, 55,and 110 m from the high tide mark. In order to do that, threeboats were positioned off a beach about 500 m east from thelong-term monitoring intake.[8] A complete description of the calculations required

for estimating total SGD on the basis of 222Rn can be foundelsewhere [Burnett and Dulaiova, 2003]. Briefly, the tem-poral variability of the total inventory of 222Rn in the watercolumn is assumed to be balanced by the difference betweenits inputs (SGD, 226Ra decay in the water column, anddiffusion from sediments) and losses (atmospheric evasion,decay, and mixing with offshore waters). Inventories aredefined as the total amount of radon per unit area of seabottom and calculated simply by multiplying radon concen-tration by water depth (units of dpm/m2). Previous studies atthis site demonstrated that SGD accounts for at least 1 orderof magnitude more 222Rn than the other source terms[Lambert and Burnett, 2003]. Atmospheric losses andmixing are the two major losses influencing 222Rn fluxes[Burnett et al., 2007]. The loss to the atmosphere wascalculated as a function of wind speed, diffusion coeffi-cients and air-sea concentrations gradients [Dulaiova andBurnett, 2006; MacIntyre et al., 1995]. The mixing losseswere estimated from inspection of the net radon fluxes overtime, i.e., residual negative fluxes after accounting for otherlosses were assigned to mixing with offshore waters.Assuming that benthic fluxes of radon are driven mainlyby groundwater inputs, we converted calculated 222Rnfluxes to water advection rates by dividing the estimatedtotal 222Rn fluxes by the estimated groundwater radon end-member concentration (129,000 dpm/m3 [Lambert andBurnett, 2003]). This general approach has been success-fully applied in many coastal marine settings by our group[Burnett and Dulaiova, 2003; Burnett et al., 2006] andothers [Mulligan and Charette, 2006; Swarzenski et al.,2007; Tse and Jiao, 2008].

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[9] Water depth variations (10-min intervals) were mea-sured with an ultrasonic water level meter (Infinities USA,Inc). Precipitation, atmospheric pressure, and wind speeddata were obtained online (www.wunderground.com) froma weather station (5-min intervals) located at AlligatorPoint, about 10 km from our study site. Long-term ground-water level, temperature, and salinity monitoring (10 minintervals) was also conducted using ceramic CTD divers(Van Essen Instruments

1

). Spatially intensive hydraulicconductivities of the shallow aquifer were obtained usingGeoprobe pneumatic slug tests. Salinity and temperaturemeasurements inside the seepage meter chamber and in the

surrounding seawater were made for estimating the fresh-water fraction of SGD [Garrison et al., 2003].

3. Results

3.1. Total SGD Rates

[10] We performed over 15,000 222Rn measurements overalmost 2 years of continuous observations. The 222Rn-derived SGD rates ranged between 0 and 155 cm d�1 (meanof 11 cm d�1). Temperature was the only variable with aclear seasonal trend (Figure 1). While seasonal cyclescannot be clearly identified in our SGD data set, short-termfluctuations were very common. In most days both 222Rnactivities and total SGD rates were related to the water level,i.e., high tracer concentration during low tide and low tracerconcentration during high tide, as illustrated in Figure 2.This provides preliminary evidence that tidal pumping is animportant SGD driver. We emphasize, however, that thispattern is not constant all the time, probably as a result ofmultiple, overlapping processes controlling SGD over dif-ferent time scales.[11] In order to gain insights into the area integrated by

our long-term 222Rn measurements, we concurrently mon-itored 222Rn and associated variables for 48 hours at fourlocations along a transect extending up to 300 m offshore(Figure 2). The results indicated very similar patterns atthe four locations. The average depths ranged from 0.89 to1.99 m, while the 222Rn activities ranged between 3700 and4785 dpm/m3. The slightly higher activities nearshorecompensate the shallower depths, leading to vey similar

Figure 1. Time series of coastal (a) groundwater level,(b) seawater temperature, and (c) seawater 222Rn betweenNovember 2005 and October 2007. We also show the(d) atmospheric and (e) mixing losses, the two mostimportant terms of the nonsteady state box model, and the(f) radon-derived total SGD advection rates. Small circlesand the gray error bars represent 24-hour averaged valuesand their respective standard deviations. The thicker lineshows a 30-day smoothing.

Figure 2. Time series of (a) water level, (b) radonactivities, and (c) radon-derived total SGD rates at foursites located 10, 55, 120, and 300 m from the shore.

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radon inventories up to 120 from the shore. The average222Rn inventories were 4020, 3900, 3920, and 7100 dpm/m2

at 10, 55, 120, and 300 m from shore, respectively. Theradon-derived SGD trends compared well, indicating thatour long-term monitoring system is a good representation ofthe processes taking place within at least 300 m from shore.[12] A closer inspection of Figure 1 indicates that 222Rn

patterns and SGD rates occasionally followed biweeklycycles. Even though this was not always constant, these�15-day cycles were extremely clear between May andAugust 2007, when SGD rates were visibly associated withthe neap spring tidal cycle (Figure 3). The autocorrelationfunction (ACF) of both tidal ranges and total SGD ratesoffered strong quantitative evidence supporting tidal pump-ing as the main SGD driver at this site, as both variablessignificantly correlated with time lags ranging between13 and 18 days (Figure 4). Our attempts to apply ACFover longer time scales were unfruitful as our whole data setappears to be too complex to be explained as a function ofany single variable. We suspect that the significant ACFshown in Figure 4 can be more easily observed for a systemnot disturbed by additional external processes, such asstorms, precipitation, river, and spring inputs. Indeed, theperiod between May and August 2007 was extremely drywith only one significant rainfall (45 mm in April 2007). Inthe absence of storms, the system behaves more predictablyand the influence of wind on water levels is likely dimin-ished. In addition, under these circumstances the uncertain-ties of our 222Rn model are reduced and the individual SGDestimates seem more reliable [Burnett et al., 2007].

3.2. Salinity Mixing Model in Seepage Chambers

[13] We deployed continuous CTD sensors within seep-age meter (benthic) chambers on a number of occasions.During a January–February 2006 deployment, the salinityinside the chamber was slightly lower than the ambientseawater, indicating a small freshening of SGD (Figure 5).We applied a simple salinity mixing model to calculate therelative contribution of fresh SGD. The results indicate thatthe contribution of fresh SGD ranges between �3 and 12%.The absolute values change depending upon our assumptionconcerning the seawater end-member salinity. If we assume

Figure 3. Time series of tidal ranges and SGD rates for aperiod when spring tides were associated with higher SGDrates (three-point smoothing).

Figure 4. Autocorrelation function for the data fromFigure 3 showing that the periodicity of the SGD rates aresimilar to the tidal range periodicities at least for thisselected part of the data set. Values outside the dashed lineare significant at 99%.

Figure 5. The freshwater fraction of SGD using a salinitymixing model ((a) water levels, (b) salinity of the waterinside and outside the seepage meter, and (c) ranges in thefresh SGD according to a simple mixing model). Thesalinity drops used for estimating fresh SGD according tothe ‘‘freshening of seepage meter water’’ (section 3.3) areidentified in Figure 5b; D refers to the change in salinityover time.

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that seawater salinity changes over time, we obtain lowervalues (average 4.4%). If we assume that the seawatersalinity is constant and equal to the highest seawater salinityobserved during the deployment (31.9), as done by otherinvestigators [Taniguchi et al., 2008], we obtain a highercontribution of fresh SGD (average 8.2%). Regardless ofthe seawater end-member value, the temporal trends of ourminimum and maximum estimates were similar, indicatingslightly higher inputs of freshwater during low tides. Therainfall that occurred on 2 February 2006 led to incoherentestimations of the minimum fresh SGD (negative values)because of seawater freshening (Figure 5b). However, theseawater dilution caused by this rainfall was not accompa-nied by freshening inside the chamber. This indicates thatflows were not reversed during this deployment, i.e., waterfluxes into chambers occurred only from the bottom up.This also suggests that short circuits of seawater recircula-tion under the chamber did not occur, as this would alsolead to a freshening of seepage meter water after the rainevent.

3.3. Rate of Freshening of Seepage Meter Water

[14] An independent way to calculate the freshwatercontribution of SGD is by estimating the volume of fresh-water that accounts for the dilution of seepage meter waterduring falling tides. Salinity drops occurred during fourconsecutive low tides in the seepage meter water, but not inthe overlying seawater (Figure 5b). The steepest drop insalinity occurred on 31 January 2006, coinciding with thelowest low tide. As the volume of water in our seepagemeters is about 0.025 m3, this salinity drop (from 29.5 to28.2) can be explained by a total freshwater input of 1.1 L in5.6 hours. By dividing the rate of freshening (�0.2 L h�1)by the seepage meter area (0.025 m2), we obtain a freshgroundwater upwelling velocity of 1.9 cm d�1. The subse-quent drops (drops 2 to 4 in Figure 5b) were less marked,and the low tides stood at higher levels. These drops can beexplained by vertical advection rates of fresh groundwaterranging from 0.4 to 0.7 cm d�1 of freshwater. During thisperiod, total SGD rates in this particular seepage meterwas 12 ± 8 cm d�1, while the 222Rn derived total SGD was8 ± 5 cm d�1. Hence, by comparing the advection ratesderived from ‘‘freshening of seepage meter water’’ (overallaverage 0.9 cm d�1 for four ebb tides) approach with the

222Rn-derived total SGD rates, we find further evidence thatthe freshwater contribution to SGD is higher than �10%only in extreme situations. We emphasize that such drops insalinity did not necessarily occur in other weeklong deploy-ments (data not shown).

3.4. Darcy’s Law Estimation of Fresh SGD

[15] Darcy’s law is a widely applied [Oberdorfer et al.,2008; Taniguchi and Iwakawa, 2004] approach to estimatesteady state fresh SGD. It is defined as the product of thehydraulic conductivity, the hydraulic gradient at a given time,and the shore normal area across which water flows towardthe ocean. Slug tests at this site indicate that hydraulicconductivity ranges from 0.16 to 25 m d�1, with a geometricmean of 1.2 [Caulkins, 2004; Li et al., 2009]. Since ourmonitoring wells are very close to the ocean, tidal oscillationsnot only affect the vertical gradient, but also the horizontaldistance between the monitoring well and the sea, thusinfluencing the hydraulic gradients. As an impermeablemud layer is found at about 3 m [Santos et al., 2008a], weassume that the diffuse fresh groundwater seepage originatesonly from the upper permeable aquifer. This neglects leakagefrom the underlying confined aquifer, which is known tooccur further offshore in this area [Moore, 2003]. By apply-ing Darcy’s law using the range of hydraulic conductivitiesand terrestrial gradients (0.02 to 0.05; average of 0.03), wefind that the volumetric fluxes of fresh SGD range between0.1 and 3.7 m3 d�1 per m of shoreline. We used differenthydraulic conductivities for the different sediment layers(25m d�1 for the upper meter of the aquifer; 5 m d�1 between1 and 2 m; and 1 m d�1 between 2 and 3 m [Li et al., 2009]),leading to an average fresh SGD value of 0.9 m3 d�1 per mof shoreline. About 80% of this estimated fresh SGD isderived from the upper meter of the aquifer.

3.5. Water Table Decline Rate

[16] We may also derive the freshwater component ofSGD by using continuous observations of groundwaterlevel from a freshwater well (2.5 m deep) located 15 monshore from the high tide mark (Figure 1a). The localwater table increases sharply (on a scale of hours) after rainevents. While the water table declines rapidly immediatelyafter rainfall, it declines at a slower rate after several hours.By taking the first derivative of the water table level time

Figure 6. (a) Schematic representation of the approach used to estimate fresh SGD following the watertable decline rate and (b) histogram showing the water table decline rate on the basis of the first derivativeof the aquifer level time series (Figure 1a).

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series, we find that during 80% of the time the water tabledeclines at a rate of <2 cm d�1. During only 4% of the time,it declines at rates higher than 4 cm d�1. We use theapproach illustrated in Figure 6 to roughly estimate thefresh SGD rate. The average rate of decline of the shallowaquifer was 1.5 cm d�1. In computing this average, we havenot included the values obtained when the aquifer level wasrising during precipitation events, as including rechargeevents would lead to incorrect decline rate estimations. Thisassumption has little influence on our overall calculations asthe aquifer level was rising for only �6% of the time. Forexample, the aquifer rose about 70 cm in less than 24 hoursduring the strongest precipitation event observed; after thisrecharge, it took almost 2 months for the aquifer to drop toits original level. After accounting for the aquifer specificyield (0.2 [Li et al., 2009]), we obtain a drop rate of only0.3 cm d�1. As the evapotranspiration rate for this area is0.2 cm d�1 [Kroeger et al., 2007], only about 0.1 cm d�1

can be discharged as fresh SGD if the seepage face andaquifer source have the same area. By aquifer source, werefer to the area where groundwater discharges into theocean before completely returning to the atmosphere, notthe recharge area. By comparing these estimates with ourradon-derived advection rates (�11 cm d�1), we estimatethat the freshwater contribution to total SGD would be nil orvery small (�1%).

4. Discussion

4.1. Fresh and Saline SGD Volumetric Fluxes

[17] Converting upward groundwater advection rates(units of cm d�1) derived from geochemical tracers intovolumetric groundwater fluxes (in units of m3 d�1 per m ofshoreline or m3 m�1 d�1) is a major challenge, as it requiresextrapolation over the area of the seepage face. In spite ofthe large amount of data available for this site, the width ofthe active seepage face is not precisely known. It has beenmeasured with seepage meters [Taniguchi et al., 2003] and

estimated at that time to be about 200 m wide. Our radondata indicates that SGD advection rates are fairly uniformwhen measured out to at least 300 m (Figure 2). Whenseepage meters where deployed further than 200 m offshore,seepage rates of about 3 cm d�1 were still observed between500 and 1000 m [Cable et al., 1997]. However, these ratesare near the limit of detection and could be due to pressurehead induced artifacts. Modeling radon inventories on theshelf at a site located 24 km offshore indicated advectionrates ranging from 2 to 12 cm d�1 [Bugna et al., 1996;Cable et al., 1996a]. SGD is therefore a widespreadphenomenon in this coastal area. We elected to use themost conservative estimate of the seepage face (e.g., 200 mas derived from seepage meters [Taniguchi et al., 2003]) toestimate volumetric groundwater fluxes from the nearshorezone. In doing so, the average upward advection ratesestimated from the 222Rn model (11 cm d�1) translateconservatively into average volumetric fluxes of 22 m3

d�1 per m of shoreline.[18] Our combination of geochemical and hydrological

observations allowed us to determine the relative contribu-tion of fresh, terrestrially derived SGD in a number of ways.All four approaches (e.g., salinity mixing model, fresheningof seepage meter water, water table decline rate, andDarcy’s law) compare reasonably well (Table 1) and indi-cate that the freshwater contribution to the total SGD wasonly about 5% at this site. If we extrapolate our nearshore,diffuse SGD volumetric fluxes to an estimated coastline ofFlorida’s Gulf of Mexico (�1200 km long), the dischargeswould represent a total groundwater flux of at least 2.7 �107 m3 d�1, with only �0.2 � 107 m3 d�1 being freshwater.A severe drought that occurred during the 2-year period ofour observations may partially explain the low contributionof fresh SGD at this site, as the total precipitation during our696 days of continuous observations was only 65% of thehistorical average.[19] Compared to the average total riverine input to the

same area (Florida West Coast) during our measurement

Table 1. Percentage of Fresh Groundwater Discharge in Relation to Total SGD in FSUCML Estimated From Multiple Approaches

Compared to Other Coastal Environments

Location Site Description Percent of Fresh SGD Approach Source

FSUCML, Florida Coastal plain sands 2–12 Salinity mixing model This studyFSUCML, Florida Coastal plain sands <16 Freshening of seepage meter

water divided by 222Rn modelingThis study

FSUCML, Florida Coastal plain sands �1 Water table rate of decaydivided by 222Rn modeling

This study

FSUCML, Florida Coastal plain sands �4 (<17) Darcy’s Law divided by 222Rnmodeling (200 m seepage face)

This study

Osaka Bay, Japan Muddy sand sediments 4–29 Darcy’s law SGD divided byseepage meter observations

Taniguchi andIwakawa [2004]

Yatsushiro Sea, Japan Volcanic rocks <40 Salinity mixing model Taniguchi et al. [2006]Atlantic Bight,United States

Permeable relict sands <4 Theoretical calculations Li et al. [1999]

Hypothetical aquifer Many input variables 25–100 Modeling Smith [2004]Israel Karst and sand aquifer 38–49 Modeling Prieto [2001] as cited by Smith [2004]Kahana Bay, Hawaii Calcareous and volcanic

alluvial deposits<20 Salinity mixing model Garrison et al. [2003]

Huntington Beach,California

Sandy, wavy coast <26 Analytical models Boehm et al. [2006]

Waquoit Bay,Massachusetts

Gravel and sandglacial outwash

<80 Darcy’s law dividedby 222Rn modeling

Mulligan and Charette [2006]

Waquoit Bay,Massachusetts

Same as above <23 Seepage meters spatiallydistributed

Michael et al. [2003]

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period (about 9 � 107 m3 d�1), recirculated SGD is of thesame order of magnitude but fresh SGD represents just asmall fraction. We emphasize that our fluxes are extremelyconservative and represent only nearshore (e.g., 200 m fromshore), diffuse SGD processes. In actual fact, the deep andshallow aquifers may communicate via breaches in theaquitard, generating springs that deliver fresh groundwaterdirectly to the shelf [Moore, 2003; Swarzenski et al., 2001].For example, the largest spring system in Florida (SpringCreek, about 50 km East of the FSUCML site) has anaverage discharge of �0.4 � 107 m3 d�1, a value higherthan our extrapolated diffuse fresh SGD. Even thoughsprings are widespread off western Florida, they are thoughtto have a small or negligible influence on our nearshoreobservations. Hence, scaling up our SGD fluxes is clearlyan underestimation of the regional fresh SGD as we do notaccount for these point sources.

4.2. Model Uncertainties

[20] Burnett et al. [2007] discussed the limitations in theuse of 222Rn as a groundwater tracer and suggested thatthe groundwater end-member concentration may representthe largest uncertainty for estimating radon-derived SGD.Even though radon in groundwater depends essentially onthe physical properties of sediments, there may be a largevariability in the radon concentration as a result of mixing ofdifferent water sources and changes in the geochemicalconditions controlling 226Ra in the subterranean estuary[Dulaiova et al., 2008]. Indeed, by sampling subterraneanestuary groundwater over a complete tidal cycle, we foundthat radon may vary over 1 order of magnitude spatially andtwofold temporally [Santos et al., 2009a], illustrating thecomplexity of determining SGD end-member concentra-tions. To determine such an end-member, one normallyrelies on observations from monitoring wells, water sampledfrom seepage meters, or sediment equilibration experiments.[21] Where diffuse seepage is the main mode of dis-

charge, equilibrating seawater with sediments should bean excellent manner to estimate the end-member radonactivity. With flow rates on the order of a few centimetersper day, there should be sufficient time for radon toequilibrate with 226Ra in the sediment [Burnett et al.,2007]. However, where tidal pumping dominates SGD,residence times of groundwater are likely to be shorter than�20 days (the time for radon equilibration). Indeed, thesediment equilibration experiments results for the FSUCMLsite (129 dpm/L) are somewhat higher than measuredseepage meter radon concentrations (ranging from 80 to115 dpm/L [Santos et al., 2009a]). Hence, as our assumedradon end-member probably represents an upper limit, the222Rn-derived total SGD rates should represent minimumvalues.[22] Most prior estimates of fresh SGD have been made

by salinity measurements inside seepage meters. The mainproblem associated with the salinity mixing model is thatSGD is invariably patchy. As seepage meters cover only asmall area (0.25 m2 in our case), the results may not berepresentative of the larger area integrated by 222Rn andother geochemical tracers [Burnett et al., 2006]. In addition,the presence of seepage meters may induce upward advec-tion in settings with strong currents and waves [Cable et al.,2006; Shinn et al., 2002]. We deployed seepage meters on a

number of occasions. Even though salinities inside thechamber were always slightly lower than seawater salinities,obvious freshening in the seepage meter water, as shown inFigure 5b, were infrequent. This implies that the pulses offresh SGD at low tide are either patchy or uncommon.Hence, our seepage meter freshening approach detects onlythe highest fresh groundwater advection rates and may notwork in some situations.[23] A common drawback associated with the Darcy’s

law approach is that hydraulic conductivities are oftenhighly variable and poorly constrained. We feel we ade-quately overcame this problem by acquiring a large numberof independent estimations over a small area [Caulkins,2004; Li et al., 2009] and by using different values for thedifferent sediment layers.[24] While the groundwater decline rate approach is

inherently limited, it illustrates that there is very little waterstored in the surface aquifer available for discharge into theocean before it is lost to the atmosphere via evapotranspi-ration. A major uncertainty of this approach is that we donot have accurate information about how the aquifer sourceand the seepage face areas compare. For purposes ofillustration, we assumed that these areas are about the same.If the aquifer source area is significantly larger than theseepage face, our fresh SGD estimation from section 3.4(about 0.1 cm d�1) would be largely underestimated.However, a small source area is supported by the aquiferdecline rate in well P1, located about 100 m onshore of thehigh tide mark (data not shown). Decline rates in well P1were slightly lower than well A2 (located only 15 m fromthe high tide mark; Figure 1a) and could be explainedessentially by evapotranspiration, indicating that the freshSGD source area (only for the surface aquifer) extends only�100 onshore. A small source area is also supported byDarcy’s velocities. By using the average local hydraulicgradients (0.03) and hydraulic conductivities (�5 m d�1),we estimate that shallow groundwater moves at averagevelocities of about only 0.15 m d�1 toward the ocean. Thisimplies that groundwater from a location 100 m inlandwould need over 1 year to reach the ocean; more thanenough time for this water to be lost to the atmosphere viaevapotranspiration. Another problem of the ‘‘decline rate’’approach is that groundwater may still flow toward theocean even if the water table has a constant level, as long asthat level is above sea level. This would require a ground-water source further away from the ocean and lying at ahigher level. In the coastal plain surficial aquifer studiedhere, however, this does not seem to be the case as theterrain rises about 3 m above sea level in less than 100 mfrom the shore, but remains relatively flat as one movesseveral km inland, limiting the amount of fresh wateravailable for discharge.

4.3. Comparison to the Literature

[25] Relatively large proportions of saline SGD were alsoinferred for a few other settings (Table 1). For example, asummer investigation using radium isotopes in HuntingtonBeach, California, suggested that recirculated seawateraccounted for at least 74% of the total SGD [Boehm etal., 2006]. In Osaka Bay, Japan, a comparison betweenDarcy’s law and seepage meter observations indicated thatthe freshwater component ranged between 4 and 29% of the

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total SGD [Taniguchi and Iwakawa, 2004]. In the IndianRiver Lagoon, Florida, saline inputs were 2 orders ofmagnitude greater than terrestrial SGD [Martin et al.,2007]. A higher contribution of fresh SGD may be expectedwhere conduits (such as lava tubes and karst caves) arepresent and hydraulic gradients are high [Garrison et al.,2003; Lee and Kim, 2007; Peterson et al., 2009]. Models ofidealized aquifers (e.g., homogeneous and nontidal) esti-mate that freshwater may account for 25–100% of totalSGD [Smith, 2004]. In fact, our observations are that thehydrological models currently available tend to providerelatively lower total flows and higher contributions of freshSGD. This is likely the case because models often do notaccount for some of the transient oceanic SGD drivers,especially tidal pumping. Our results and others’ thusdemonstrate that saline groundwater may dominate benthicexchange especially when total SGD is estimated fromgeochemical tracers, such as 222Rn and radium isotopes,that integrate the whole spectrum of SGD driving forces.[26] The dominance of recirculated seawater at some sites

does not imply that SGD is not biogeochemically important.While fresh SGD provides new nutrient additions and maybe a source of land-derived contaminants, the chemicalcomposition of recirculated seawater in aquifers is dramat-ically altered relative to the original seawater. Permeablesediments operate as biocatalytical filters, efficientlydecomposing particulate organic matter and releasing largeamounts of nutrients to surface waters [Huettel et al., 2003;Rusch et al., 2006]. In addition, saline waters flushingcoastal aquifers may be the primary source of radium,nutrients, and a number of other elements to coastal waters[Moore, 1996; Moore et al., 2008]. In a recent analysis, weestimated that the magnitude of nitrogen and dissolvedorganic carbon from this nearshore, mostly saline SGDalone is comparable to the input of nutrients from localrivers [Santos et al., 2008a]. Tidal pumping supplies oxygenand organic matter to the shallow aquifer, enhancing sub-terranean biogeochemical reactions that release largeamounts of nutrients to seawater [Santos et al., 2009b].We emphasize, however, that while rivers are a net source ofnutrients, saline SGD is likely to represent a source ofnutrients recycled in the subterranean estuary rather thannew nutrient additions. Combined, the diffuse saline SGDdiscussed here and point sources of fresh SGD offshorerepresent a still largely neglected, but major mechanismdelivering dissolved species to coastal waters in Florida andelsewhere.

4.4. What is Tidal Pumping?

[27] The temporal variability of 222Rn and SGD followingdaily (Figure 2) and biweekly tidal cycles (Figure 3) pro-vided strong evidence that tidal pumping is the primarydriver for seawater recirculation in the shallow aquifer at thissite. The observed spring neap SGD periodicity (Figure 4) isconsistent with models obtained for a subterranean estuary inAustralia, where changes in the tidal excursion over thespring neap cycle induce significant variations in the inter-tidal salinity structure [Robinson et al., 2007a]. Marinedrivers other than tidal pumping cannot explain the observedtemporal variability of SGD. Wave setup, for example, isprevented at this site by offshore bars except during largestorms. Even though convection probably accounts for an

important fraction of our total fluxes, it does not seem toexplain the temporal patterns observed. While seawaterdensities ranged from 1021 to 1027 g L�1 in the summerand winter, respectively, groundwater densities rangedbetween 998 and 999 g L�1. Therefore, seawater was onaverage 2.8%more dense than groundwater in the winter and2.3% in the summer. Density gradients should thereforedisplay limited temporal variability, which should lead torelatively constant convective exchange over time scalesranging from days to months. This is supported by modelsindicating significant but relatively steady convectiveexchange in coastal sediments [Rasmussen, 1998].[28] Further evidence supporting tidal pumping as the

primary mechanism driving SGD at the FSUCML site wasobtained by a density variable numerical code applied tomodel SGD rates nearshore [Li et al., 2009]. By accountingfor tidal pumping as a SGD driver, these authors werecapable of accurately predicting the observed salinity dis-tributions in the subterranean estuary. Tidal pumping hasbeen evoked as an important SGD driver in other environ-ments, but little quantitative evidence is available. Forexample, the temporal variation of 222Rn in Korea wasrelated to a spring neap tidal oscillating pumping force inthe fall, but not in the summer [Kim and Hwang, 2002].What exactly is meant by the term ‘‘tidal pumping’’? Wesuggest that groundwater-seawater exchange driven by tidalpumping can be separated into at least the following threemajor components: (1) hydraulic gradient-driven fresh SGD,e.g., steeper hydraulic gradients at low tide leading to higherfresh groundwater seepage at low tide; (2) seawater recircu-lation; e.g., seawater infiltrating the beach face at high tideand discharging at low tide; (3) current-driven recirculation,e.g., benthic exchange is enhanced when seawater currentvelocities are stronger; this likely occurs mostly during ebband flood tide and during spring tides.[29] Another type of exchange may be related to the

‘‘subtidal pump’’ mechanism [Riedl et al., 1972], a one-dimensional advective exchange phenomenon driven bywaves on the continental shelf at depths shallower than200 m. However, this does not normally apply to FSUCMLbecause wave action is minimal as this site is sheltered byoffshore bars. Some authors imply that tidal pumping refersto a change in the terrestrial hydraulic gradient in responseto tidal fluctuations [Kim and Hwang, 2002; Taniguchi andIwakawa, 2004]. If this is the case, tidal pumping would beprimarily a source of fresh SGD at low tide. Other authorstend to see tidal pumping as recirculation of seawaterthrough the shallow aquifer, e.g., the aquifer recharges withseawater at high tide and discharges at low tide [Boehm etal., 2006; Li et al., 1999; Robinson et al., 2007b]. Our datasuggests that even though these processes must occursimultaneously, the first component represents the lowervolumetric contribution and therefore tidal pumping isprimarily a source of saline rather than fresh SGD at thissite.[30] The influence of tides on terrestrial hydraulic gra-

dients is illustrated in Figure 7. After the highest rainfall(75 mm) recorded during our experiment, the groundwatertable was elevated 80 cm and the hydraulic gradientincreased by about 20%. By applying Darcy’s law, thesehydraulic gradients are equivalent to a fresh SGD flux of 0.8and 1.1 m3 m�1 d�1 before and after the raise in the aquifer

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level, respectively. These differences are minor when super-imposed on the total SGD of about 22 m3 m�1 d�1 (or11 cm d�1). In addition, this steep change in the hydraulicgradients produced no detectable effect on the 222Rn activ-ities measured in seawater, providing further evidence thatthe terrestrial forces are small at this coastal plain site. It isthus clear that the smaller amplitude oscillations of thehydraulic gradient driven by sea level fluctuations have aneven less pronounced effect on fresh and total SGD rates atthis site.

[31] The second component of tidal pumping (e.g., sea-water recharge/discharge cycles) can be estimated followingthe approach shown in Figure 8. We deployed continuousCTD divers in shallow wells installed across the beach face[Santos et al., 2009b]. Because of high sediment perme-ability, the beach groundwater level responded almostimmediately to the flood tide. During the ebb tide, thegroundwater table decreased quickly during the first couplehours and slowed later (Figure 8b). The integration of therecharge area at the beach face provides an estimate of thevolume of seawater exchanged on tidal time scales. About3 m3 m�1 d�1 is recirculated at the beach face, a value 3times higher than the Darcy’s law-derived fresh SGD butstill just a minor fraction (�15%) of the total SGD derivedfrom 222Rn. We emphasize that this approach (Figure 8)represents a minimum estimate of recirculated seawater, asit assumes no leaking into or out of the aquifer, which likelyoccurs all the time. In addition, this calculation only con-siders fluxes right at the shoreline (e.g., within 10 m fromthe high tide mark). As exchange is known to occur at leastwithin 200 m offshore, our data indicates that the remainingSGD fluxes are due to a combination of other marineprocesses, e.g., current- and topography-induced recircula-tion [Huettel et al., 2003], bioirrigation [Martin et al.,2006], and a nearly steady density-driven recirculation[Rasmussen, 1998; Smith, 2004]. Of these, only topography-induced recirculation would contribute to the tidal-scalepatterns revealed in our data.

5. Conclusions

[32] Our combination of long-term (almost 2 years)geochemical and hydrological observations allowed us toestimate the relative contribution of fresh SGD. All fourapproaches (e.g., salinity mixing model, freshening ofseepage meter water, water table decline rate, and Darcy’slaw) compare reasonably well in spite of their inherentindividual limitations. Together, these approaches providestrong evidence that transient marine forces account for themajority (nearly 95%) of the 222Rn-derived total SGD at thiscoastal plain site. Combining independent approaches toestimate fresh SGD is important as the spatial and temporalscales of SGD are highly variable. For example, salinity

Figure 7. Temporal variability of the hydraulic gradientbefore and after a large rainfall event demonstrating thathydraulic gradients (dh/dx) have little effect on 222Rnactivities at this coastal plain site. The drop in radon on13 June 2006 was associated with high wind speeds. Afterthe storm, radon returned to its original baseline activities.

Figure 8. (a) Schematic representation of the strategy used to quantify the volume of water thatinfiltrates the beach face at high tide and discharges at low tide. (b) Example of groundwater level timeseries used to estimate the recharge area following the equation in Figure 8a.

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observations in seepage meters are representative of a verysmall area, while the aquifer decline rate, approach encom-passes a much larger but unknown area.[33] The extremely variable total SGD rates were largely

associated with fluctuations over short (e.g., hourly to daily)time scales rather than seasonal cycles, suggesting tidalpumping as an important modulator of SGD at this site.We demonstrated that tidal pumping is driven primarily byseawater recirculation rather than oscillating hydraulic gra-dients leading higher fresh SGD at low tide. Extrapolated tothe entire Florida Gulf of Mexico coastline, total groundwa-ter volumetric additions where within the same order ofmagnitude of riverine inputs, but the fresh SGD addition wasjust a small fraction. We emphasize, however, that our SGDextrapolation was extremely conservative, as it onlyaccounted for diffuse seepage within 200 m from theshoreline while it is known that point sources of freshgroundwater and seawater recirculation are widespread fur-ther offshore in Florida. Future investigations should refinetechniques for scaling up local observations and account fortemporal changes in the groundwater radon end-member.

[34] Acknowledgments. This project was sponsored by NSF(OCE05–20723). I.R.S. held a Fulbright/CAPES fellowship. We thankthe Florida State University Coastal and Marine Laboratory (FSUCML)personnel for their helpful support and Bill Hu and Xinya Lee for fertilediscussions.

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�����������������������W. C. Burnett, J. Chanton, N. Dimova, R. N. Peterson, and I. R. Santos,

Department of Oceanography, Florida State University, Tallahassee, FL32306-4320, USA. (issac.santos@scu.edu.au)

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