4 (2007) 27–42www.elsevier.com/locate/marchem

Marine Chemistry 10

Seasonal variations on the residence times and partitioning ofshort-lived radionuclides (234Th, 7Be and 210Pb) and

depositional fluxes of 7Be and 210Pb inTampa Bay, Florida

M. Baskaran a,⁎, P.W. Swarzenski b

a Department of Geology, Wayne State University, Detroit, MI 48202, United Statesb U.S. Geological Survey, St. Petersburg, FL 33701, United States

Received 18 January 2006; received in revised form 23 June 2006; accepted 23 June 2006Available online 9 August 2006

Abstract

Historically, Tampa Bay has been impacted heavily by a wide range of anthropogenic perturbations that may include,agricultural-, shipping-, phosphate mining/distribution-related activities, as well as a burgeoning coastal population. Due to thepresence of U-rich underlying sediments, elevated activities of U- and Th-series daughter products may be naturally released intothis system. This region is also known for summer thunderstorms and corresponding increases in precipitation and surface waterrunoff. Only limited work has been conducted on the partitioning of particle-reactive radionuclides (such as 7Be, 210Pb, and 234Th)in such a dynamic coastal system. We investigated both the removal residence time and partitioning of these radionuclides betweenfilter-retained particulate matter (≥0.5 μm) and the filtrate (b0.5 μm) phase during late spring (June 2003) and mid summer(August 2003) in the water column of Tampa Bay.

Our results indicate that the partitioning of 7Be, 210Pb, and 234Th between filtrate and filter-retained phase is controlled foremostby enhanced bottom resuspension events during summer thunderstorms. As a consequence, no significant relationship existsbetween the distribution coefficients (Kd values) of these isotopes and the concentration of suspended particulate matter (SPM).Relatively faster recycling rates of atmospheric water vapor derived from the ocean results in lower atmospheric depositional fluxesof 210Pb to the study site than predicted. The relationship between 7Be and 210Pb in bulk (wet+dry) deposition is compared to theirrespective water column activities. The residence times of particulate and dissolved 234Th, 7Be and 210Pb, as well the distributioncoefficients of these radionuclides, are then compared to values reported in other coastal systems.© 2006 Elsevier B.V. All rights reserved.

Keywords: Tampa Bay; Scavenging of particle-reactive radionuclides; Be-7; Pd-210; Th-234; Distribution coefficients; Residence times of nuclides

1. Introduction

Particle-reactive radionuclides in the U- and Th-seriesand 7Be, have been used extensively as tracers in estuarine

⁎ Corresponding author.E-mail address: Baskaran@wayne.edu (M. Baskaran).

0304-4203/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.marchem.2006.06.012

and coastal environments to evaluate the biogeochemicalreactivity of chemical constituents, as well as the pro-venance of suspended particulate matter, SPM (Broeckeret al., 1973; Santschi et al., 1979, 1980; Aller et al., 1980;Olsen et al., 1982, 1986; Baskaran and Santschi, 1993;Baskaran et al., 1997; Feng et al., 1999; Swarzenski et al.,1999; Turner and Millward, 2002; Radakovitch et al.,

28 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

2003). The particulate matter composition within anestuary may consist of lithogenic (weathered crustalmaterial, mainly quartz and silicate minerals), hydrogenic(coatings of oxyhydroxides of Fe and Mn, carbonates,sulfides and humic aggregates), biogenic (produced bybiological processes from organisms such as bacteria,fungi, protozoans, plankton, fecal matter and other re-mains of decaying organisms, lipids, proteins, etc.), andanthropogenic (particulate pollutants such as sewagesolids, surfactants, trace elements, coal dust, fly ash, etc.)components (Turner and Millward, 2002). The partition-ing of reactive chemical species depends directly on theconcentration, composition, and fate of SPM (Turekian,1977). SPM concentrations are typically 2–4 orders ofmagnitude higher in coastal waters (1–100 mg L−1) thanin the open ocean (10–100 μg L−1), and as SPM has amuch larger surface area per unit mass compared to largerparticles that may undergo faster settling, SPM tends tostay in the water column long enough to effectively sca-venge particle-reactive chemical species. Thus, most ofthe particle-reactive radionuclides in estuarine and coastalwaters are associated with SPM, and their dynamicinteraction can be investigated using a suite of particle-reactive radionuclides.

One of the more useful techniques for quantifying thefate of particle-reactive radionuclides is to apply informa-tion from natural U- and Th-series and cosmogenic radio-nuclides, either bymaking use of parent–daughter nuclidedisequilibria, or by measuring precise fluxes and standingcrops (inventories) of atmospherically-delivered radio-nuclides (i.e., 210Pb, 7Be). For example, the disequilibriumbetween 234Th (t1/2=24.1 days) and

238U (or 228Th/228Ra),and the determination of fluxes and standing crops ofatmospherically-delivered 210Pb (t1/2=22.1 years) andcosmogenic 7Be (t1/2=53 days) are powerful tools forestimating the rates of processes that affect the estuarinedistribution of pollutants (Broecker et al., 1973; Aller andCochran, 1976; Bacon et al., 1976; Simpson et al., 1976;Turekian, 1977; Olsen et al., 1980, 1982; Li et al., 1979;Santschi et al., 1980; Minagawa and Tsunogai, 1980; Alleret al., 1980; Benninger andKrishnaswami, 1981; Kaufmanet al., 1981; Baskaran and Santschi, 1993; Baskaran et al.,1997; Feng et al., 1999).

In estuarine environments, the primary sources ofthese radionuclides include, i) radiogenic productionfrom the parent (i.e., 234Th from 238U, and 210Pb from222Rn after several daughter products), ii) directatmospheric deposition (fall out of 210Pb and 7Be), iii)erosional input from the watershed through the dischargeof streams and rivers, and iv) radionuclides present inoff-shore waters that may enter the estuary or baythrough physical exchange processes. Seasonal events,

such as major summer thunderstorms, appear todominate the distribution of these radionuclides both inthe water column and bottom sediments. Their distribu-tion between the particulate and dissolved phases in thewater column provides information on the role ofseasonal events on the distribution of other particle-re-active species. In addition, the present study site, TampaBay, is among the largest estuaries in Florida (drainagebasin=∼7500 km2), and may be utilized as a model forthe investigation of the fate of U- and Th-seriesradionuclides in an estuary where U activities may beboth naturally and anthropogenically enriched. This bayreceives very low sediment input (Schoellhamer, 1995)with a very low drainage basin area/sediment input ratio.The presence of two phosphate distribution facilitiesdirectly adjacent to Tampa Bay may provide anadditional source of U- and Th-series radionuclidesabove levels present in ubiquitous U-rich sediments(Fanning et al., 1982; Swarzenski and Baskaran, 2007).We have analyzed water samples from 17 stationsthroughout Tampa Bay to test the hypothesis that in sucha particle-depleted estuarine system, the particle–concentration effect (Honeyman and Santschi, 1989)will not be as pronounced as compared to other coastalsystems where SPM concentrations are relatively high.Specific objectives were to investigate the range incalculated particle residence times using 7Be, 210Pb, and234Th, and to evaluate the partitioning of 7Be, 210Pb and234Th between the dissolved water and SPM during twoseasons, late spring and summer of 2003. We also reportvariations in the atmospheric depositional input of 210Pband 7Be to the water column and assess the relationbetween 7Be and 210Pb in the bulk atmosphericdepositional flux as well as in the water column.

2. Materials and methods

2.1. Measurements of 7Be and 210Pb from Tampa Bay

Tampa Bay is a shallow (less than 3 m) estuarinesystem and the water column is generally well mixedvertically. Water samples from 17 stations within the baywere collected and analyzed for U- and Th-seriesradionuclides and 7Be (Fig. 1) during June and August2003. For 7Be and 210Pb analyses, ∼150 L of watersamples from Tampa Bay were filtered through a pre-cleaned 0.5 μm (median pore size) prefilter and collectedin precleaned 20-L containers and transferred into a 200-L drum for Fe(OH)3 precipitation at the shore-basedlaboratory. The cubitainers were rinsed with 6 M HCl toremove any adsorbed 7Be or 210Pb. To the solution, 1 mLof stable Pb (≡1 mg, Aldrich 05618DR) and Be (≡1 mg,

29M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

Baker 6921-01) were added. The spikes were allowed toequilibrate for approximately 4–5 h and the Fe(OH)3precipitation was conducted using ammonia solution at apH of ∼7. After approximately 12 h, the precipitate andsolution were separated by first decanting the superna-tant and then by centrifugation. The precipitate wasdissolved in a small volume of 6 M HCl and completelydried. The residue was quantitatively transferred to agamma counting vial and assayed for 7Be and 210Pb.Although 234Th also would co-precipitate in the Fe(OH)3precipitation method, we did not utilize the 234Th dataobtained from this method, as co-precipitation ofvariable amounts of 238U could contribute to varyingamounts of 234Th from the decay of 238U.

Fig. 1. Station location map for Tampa Bay showing water-

For the measurement of dissolved 234Th, large-volume (300–500 L) water samples were filteredthrough a filtration and extraction system consisting ofa pre-cleaned polypropylene prefilter cartridge (0.5 μmmedian diameter) and two MnO2-impregnated polypro-pylene filter cartridges, connected in series. Details onthe cleaning and impregnation of MnO2 on to cartridgesare given in Baskaran et al. (2003) and Trimble et al.(2004). Radionuclide blanks in the MnO2-impregnatedcartridges as well as prefilter cartridges were found to bebelow detection limits, and therefore were not consid-ered. 234Th activities in the filtrate fraction werecalculated assuming constant extraction efficiency (i.e.,85% for 234Th, 85±8%, range: 74–99%, n=13). This

column sites sampled during June and August, 2003.

30 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

value is based on results from a number of experimentsthat have directly evaluated the extraction efficiency ofMnO2 for Th (Baskaran et al., submitted for publication).These assumptions were necessary since the activitiesmeasured in the second extractor cartridge wereconsistently below detection limits, and thus the two-extractor cartridge method described in Baskaran et al.(1993a) and elsewhere could not be used. The variousmethodologies, including the use of dual MnO2-coatedcartridges and their potential limitations for measuringdissolved and particulate 234Th are given in a recentreview paper by Rutgers van der Loeff et al. (2006). Theused filters were first rinsed with distilled water and thenashed at 550 °C. All the radiochemically-processedsamples were counted for ∼24 h in 10 mL gammacounting vials in a high-purity Ge-well detector(Canberra) coupled to a Canberra InSpector multi-channel analyzer. Details on the peak background,resolution and Peak/Compton ratio, and the resolutionof the detector are given in McNeary and Baskaran(2003). The matrix densities of the standards andsamples were comparable, and hence self-absorptioncorrections were not applied. The gamma ray detectorwas calibrated with standard solutions (RGU-1 (for234Th and 210Pb) and RGTh-1 from IAEA). For 7Be(477.6 keV), the counting efficiency was obtained fromthe linear extrapolation of the counting efficienciesobtained for 214Pb at 351.6 keV and 214Bi at 609.4 keV.This counting efficiency was verified with valuesobtained ∼3 years ago using the IAEA RGU-1 Ustandard and the agreement was found to be very good.

Excess 210Pb and 234Th activities (i.e., total measuredminus parent-supported 210Pb or 234Th) in the prefilterswere obtained by subtracting the estimated parentsup-ported activities in suspended particles (i.e., 222Rn es-timated from 226Ra and 238U, respectively) from themeasured total activities in filtered particles. Although wedo not know the 222Rn/226Ra ratio in SPM, a value of 0.5has been reported in surface sediments of lakes and thedeep sea (Key et al., 1979; Imboden and Stiller, 1982) andhas been used here to calculate 210Pbxs. If the actual ratiois higher than 0.5, then that would imply correspondinglylower 210Pbxs values and our calculated value of 210Pbxswould thus represent a higher estimate (e.g., Baskaran andSantschi, 1993).

2.2. Determination of the atmospheric depositional fluxesof 210Pb and 7Be

A bulk rain collector (200-L polyethylene drum withsurface area of 2800 cm2) was deployed on the roof ofU.S. Geological Survey office in St. Petersburg, FL

(27°45.828′N, 82°38.284′W; ∼17 m above mean sealevel) from July 2003 through July 2004. The lid of thedrum was simultaneously deployed as the dry fall outcollector in October 2003, about 10 m away from thebulk collector site. To prevent any potential adsorptionof 7Be and 210Pb on to the walls of the collector, bothdry and wet collectors were acidified prior todeployment with concentrated HCl and chemical yieldspikes [1-mg of stable Pb and 1-mg of stable Be] wereadded. Bulk rain samples were collected after eachmajor precipitation event or roughly every month. Thedry collector was sampled after every ∼10-day periodof dry weather. Immediately after collection, the drumand/or the lid were cleaned repeatedly with 6 M HCl toremove adsorbed Be and Pb isotopes from the surfaceof the drum and/or lid. These rinses were combinedwith the rain sample, and were subsequently processedin the laboratory as per methods described in McNearyand Baskaran (2003). Briefly, ∼150 mg Fe (in the formof FeCl3) and ∼5 g of NH3Cl were added to the sample.The solution was allowed to equilibrate for a minimumof ∼3–4 h, and then the Fe(OH)3 precipitation wasconducted by adding NH3OH to attain a pH of ∼7.After ∼12 h, the precipitate and solution were separatedby decanting the supernatant and by filtration through aWhatman filter paper (#42) using a glass vacuumfiltration flask assembly. After the filtration, the filterpaper containing Fe(OH)3 precipitate was transferred toa Teflon beaker and the precipitate was dissolved in aminimum amount of 6 M HCl. The filter paper wasremoved from the beaker and rinsed with 6 M HCl anddistilled water. The solution in the beaker was driedcompletely and the residue was quantitatively trans-ferred to a gamma counting vial. After gamma countingthe sample, the residue was dissolved in 3 M HCl to avolume of 100 mL. About 5 mL of this solution wasanalyzed for stable Be and Pb using an AAS method todetermine the chemical yield. The chemical yield wasused in the final calculation of the activities of 7Be and210Pb.

We also collected a suite of 1-L water samples fromTampa Bay during June and August 2003 for thegravimetric determination of SPM concentrations usinga preweighed 0.4 μm, 47-mm Nuclepore filter. Thesalinity of each sample was determined using an YSImulti-probe, with a precision of about 0.01.

3. Results and discussion

The salinities, concentrations of SPM, and activitiesof particulate excess 234Th (234Thxs) and excess 210Pb(210Pbxs; in the case of particulate, only excess for

234Th

1 Total (210Pb or 234Th)=dissolved (210Pb or 234Th)+excess (210Pbor 234Th) in the particulate phase.

31M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

and 210Pb are reported and discussed here and elsewherein the paper) and dissolved 234Th, 7Be and 210Pbcollected during the two seasons, June 2003 and August2003, are given in Tables 1 and 2, respectively. Weanalyzed 238U activities during June and August 2003using ICP–MS (Swarzenski and Baskaran, 2007).Although there is generally a strong relationship betweenU and salinity in the world oceans (Ku et al., 1977; Chenet al., 1986), non-conservative behavior of U has beenobserved in select estuarine and coastal waters (Maedaand Windom, 1982; McKee et al., 1987; Toole et al.,1987; Carroll and Moore, 1994; Swarzenski et al., 1995,2004; summarized in Swarzenski and Baskaran, 2007).Non-conservative behavior of U has been observed inTampa Bay waters and has been attributed, in part, toenhanced fluid exchange across the sediment/waterinterface (Swarzenski and Baskaran, 2007). In TampaBay, SPM concentrations varied between 2.5 and13.6 mg L−1 with a mean value of 6.6 mg L−1 observedduring June (2003), while in August 2003 SPMconcentrations varied between 2.1 and 7.1 mg L−1,with a mean value of 4.6 mg L−1. Chen et al. (2007-thisvolume) also reported low SPM concentrations inTampa Bay, and evaluated colored dissolved organicmatter (CDOM) and dissolved organic carbon (DOC)sources relative to SPM. The SPM concentration inriver waters is considerably lower during highdischarge seasons, indicating that the SPM concentra-tion in the water column is at least partially governedby the riverine input. However, it has been shown thatthe concentration of SPM is generally higher than theambient concentration during periods of large windwaves generated by tropical storms (Schoellhamer,1995) and our result is in contrast with the observationreported by Schoellhamer (1995). The daily mean streamflow from June to September 2003 is shown in Fig. 2.Salinity values were significantly higher (average salin-ity=24.99) in June 2003 at most stations compared toAugust 2003 (average salinity=16.60) and are attributedto higher riverine discharge during August (Fig. 2).

3.1. Activities of 234Th, 210Pb and 7Be in the dissolvedand particulate phases

During the June sampling, the mean activity of dis-solved 234Th (4.6 dpm 100 L−1; n=5; 60 dpm=1 Bq)and particulate 234Th (7.7 dpm 100 L−1; n=14), werelower than the corresponding values observed duringAugust (dissolved: 234Th=20.9 dpm 100 L−1, n=2; andparticulate: 234Th=16.8 dpm 100 L−1, n=16) (Tables 1and 2). The mean activity of dissolved and particulate 7Beduring June 2003, 19.4 dpm 100 L−1 (n=9) and

1.5 dpm 100 L−1 (n=15), respectively, were alsosignificantly lower than the corresponding values inAugust, 61.5 dpm 100 L−1 (n=15) and 5.7 dpm 100 L−1

(n=16), respectively (Tables 1 and 2). The mean activityof dissolved and particulate 210Pb during June 2003,4.7 dpm 100 L−1 (n=17) and 4.9 dpm 100 L−1 (n=17),respectively, were significantly higher in August, 7.2dpm 100 L−1 (n=8) and 7.5 dpm 100 L−1 (n=16),respectively. Both dissolved and particulate activities of7Be and 210Pb were distinctly higher during August ascompared to June 2003. Higher activities of dissolved andparticulate 7Be and 210Pb can be attributed to higherdepositional fluxes from enhanced precipitation and alsoto elevated freshwater discharges (Fig. 2). Significantlyhigher particulate 210Pb activities compared to 7Be couldbe attributed to the higher affinity of Pb to particulatematter or to episodic resuspension events that wouldpreferentially enrich 210Pb (Baskaran et al., 1997). Thebenthic sediment column contains much higher activitiesof 210Pbxs (32 times the annual atmospheric depositionalflux for 210Pb), if all direct atmospheric depositional fluxis removed in situ (discussion on 210Pbxs in thesedimentary column is presented in Campbell et al.,2006-this issue) compared to 7Be (0.21 times the annualatmospheric depositional flux of 7Be). The fraction ofparticulate 7Be (=7Bep/(

7Bep+7Bed) for the whole data

set was obtained by plotting particulate 7Be against total7Be (total=particulate+dissolved), yielding a value of4.3% (Fig. 3a). The particulate 210Pbxs plotted againsttotal1 210Pb (=210Pbp, excess

210Pb in the particulatephase, +210Pbd) indicate that ∼57% of the total 210Pbresides in the particulate phase (Fig. 3b).

3.2. Specific activities of 234Th, 210Pb and 7Be

The specific activity (activity per gram of SPM) of234Thp observed during both seasons varied between4.2 dpm g−1 and 17.9 dpm g−1, with a mean value of11.7 dpm g−1. The specific activities of 234Thxs duringAugust were significantly higher than those in June 2003.In our previous sediment core sampling, we did not findany measurable 234Thxs, and thus, the SPM collected isnot representative of bottom sediments (Campbell et al.,2006-this issue). However, due to sediment winnowing,some of the fine sedimentary material (such as clayminerals) could get resuspended in the water column,leading to increased scavenging of Th and other particle-reactive radionuclides from the water column. The

Table 1Salinity, concentrations of SPM and dissolved, particulate and total 234Th, 210Pb, and 7Be in Tampa Bay, FL (June 2003)

Samplecode

Salinity SPM(mg L−1)

234Thd(dpm 100 L−1)

234Thp(dpm 100 L−1)

234Tht(dpm 100 L−1)

210Pbd(dpm 100 L−1)

210Pbp(dpm 100 L−1)

210Pbt(dpm 100 L−1)

7Bed(dpm 100 L−1)

7Bep(dpm 100 L−1)

7Bet(dpm 100 L−1)

June '03TB01 23.1 9.5 1.60±1.34 15.1±1.8 16.6±2.3 1.06±0.76 8.13±0.73 9.2±1.1 NM 2.32±0.49 –TB02 21.6 10.0 6.50±2.13 12.5±2.0 19.0±2.9 3.06±1.03 12.6±0.9 15.7±1.4 NM 1.28±0.36 –TB03 28.4 8.8 BD 3.65±0.74 3.65±0.74a BD 1.41±0.30 1.41±0.30a 21.9±6.9 0.74±0.25 22.6±6.9TB04 26.0 5.2 2.66±0.90 5.39±0.75 8.05±1.18 2.82±0.63 2.92±0.33 5.74±0.71 10.7±3.0 0.70±0.21 11.4±3.0TB05 28.0 2.9 BD 5.04±0.52 5.04±0.52a BD 1.47±0.18 1.47±0.18a 12.9±2.8 0.80±0.15 13.7±2.8TB06 27.7 3.3 3.99±1.81 5.88±0.54 9.87±1.89 BD 2.34±0.20 2.34±0.20a NM 0.93±0.14 –TB07 22.1 8.6 BD 4.94±1.22 4.94±1.22a 6.29±0.69 7.71±0.51 14.0±0.9 17.6±2.9 0.30±0.22 17.9±2.9TB08 21.5 13.6 8.45±1.86 10.3±1.4 18.7±2.4 10.9±0.6 7.95±0.63 18.9±0.9 84.8±7.0 1.49±0.34 86.3±7.0TB09 22.5 6.2 BD 5.75±4.58 5.8±4.6 a 6.73±1.2 7.67±1.30 14.4±1.8 5.5±1.6 5.25±0.79 10.7±1.8TB10 24.1 5.9 0.00 4.18±0.95 4.18±0.95a 6.2±2.1 1.84±0.24 8.1±2.1 BD BD BDTB11 25.2 6.3 BD 11.0±1.2 11.0±1.2a BD 3.30±0.29 3.30±0.29a BD 0.84±0.16 0.84±0.16a

TB12 24.5 10.9 BD 13.4±1.1 13.4±1.1a 2.36±0.77 6.45±0.33 8.81±0.84 6.3±2.4 2.19±0.22 8.5±2.4TB13 25.2 6.3 BD 8.6±1.3 8.6±1.3a BD 4.00±0.27 4.00±0.27a 5.2±1.5 1.13±0.23 6.3±1.5TB14 25.2 8.1 BD BD BD 2.9±1.4 3.35±0.50 6.2±1.5 9.5±1.7 1.86±0.34 11.4±1.8TB15 27.4 6.3 BD BD BD BD 3.15±0.30 3.15±0.30a NM 1.78±0.24 –TB16 29.4 2.5 BD 3.08±0.88 3.08±0.88a BD 1.72±0.17 1.72±0.17a NM 1.53±0.15 –TB17 23.0 10.4 BD 28.4±4.9 28.4±4.9a BD 7.94±0.75 7.9±0.8a NM BD –

Site locations are provided in Fig. 1.NM: not measured. BD: below detection limit.1 Bq=60 dpm. We have used the dpm unit to be consistent with other articles in this issue.238U activities are taken from Swarzenski and Baskaran (2007).a Only dissolved activity was below detection limit.

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Table 2Salinity, concentrations of SPM and activities of dissolved, particulate and total 234Th, 210Pb and 7Be in Tampa Bay, FL (August 2003)

Samplecode

Salinity SPM(mg L−1)

234Thd(dpm 100 L−1)

234Thp(dpm 100 L−1)

234Tht(dpm 100 L−1)

210Pbd(dpm 100 L−1)

210Pbp(dpm 100 L−1)

210Pbt(dpm 100 L−1)

7Bed(dpm 100 L−1)

7Bep(dpm 100 L−1)

7Bet(dpm 100 L−1)

August '03TB01 10.7 2.23 BD 45.7±3.1 45.7±3.1 a 8.76±3.29 14.5±1.0 23.3±3.4 123.6±16.7 21.93±0.86 145.5±16.7TB02 11.4 4.23 BD 52.1±1.5 52.1±1.5 a 6.05±2.10 5.37±0.36 11.4±2.1 166.6±18.2 6.19±0.30 172.8±18.2TB03 25.3 2.16 20.6±7.9 13.7±2.2 34.3±8.2 BD 6.12±0.51 6.12±0.51 a 21.0±7.2 4.38±0.38 25.4±7.3TB04 20.3 3.45 BD 7.82±0.78 7.82±0.78 a BD 4.25±0.22 4.25±0.22 a 25.3±11.6 2.49±0.18 27.8±11.6TB05 21.4 3.11 BD 20.2±3.5 20.2±3.5 a 4.34±2.35 8.00±0.73 12.3±2.5 12.1±5.5 4.14±0.49 16.2±5.6TB06 15.6 3.04 BD 15.4±1.4 15.4±1.4 a 7.47±3.49 7.92±0.54 15.4±3.5 34.3±10.1 5.80±0.39 40.1±10.1TB07 11.2 7.03 BD 21.7±1.2 21.7±1.2 a 10.2±3.1 6.51±0.39 16.7±3.1 49.0±12.9 3.94±0.27 53.0±12.9TB08 12.1 5.32 BD 9.91±1.37 9.91±1.37 a 5.93±3.30 10.4±0.5 16.3±3.3 23.0±7.9 2.96±0.29 25.9±7.9TB09 12.9 6.42 BD 6.09±0.73 6.09±0.73 a 9.7±2.0 8.42±0.30 18.1±2.0 27.3±6.5 2.73±0.19 30.0±6.5TB10 15.3 6.78 BD 6.94±2.21 6.94±2.21 a 5.1±2.3 12.8±0.9 17.9±2.5 109.6±11.4 7.63±0.57 117.2±11.4TB11 16.7 5.98 BD 12.9±1.9 12.9±1.9 a BD 6.10±0.61 6.10±0.61 a 30.9±6.6 7.70±0.52 38.6±6.6TB12 17.3 7.11 BD 8.16±0.79 8.16±0.79 a BD 3.59±0.27 3.59±0.27 a 135.8±18.5 3.15±0.23 138.9±18.5TB13 18.7 6.24 21.2±12.1 11.6±1.7 32.7±12.2 BD 4.85±0.52 4.85±0.52 a 51.3±8.7 5.13±0.40 56.4±8.7TB14 19.4 4.35 BD 7.77±1.04 7.77±1.04 a NM 3.60±0.26 NM NM 1.84±0.19 NMTB15 20.7 3.67 BD 11.0±1.9 11.0±1.9 a BD 5.34±0.41 5.34±0.41 a 27.3±19.3 2.29±0.27 29.6±19.3TB16 NM 2.13 NM NM NM NM NM NM NM NM NMTB17 11.6 4.23 BD 17.0±1.6 17.9±1.6 a BD 11.6±0.6 11.6±0.6 a 84.6±13.3 8.48±0.41 93.1±13.3

NM: not measured. BD: below detection limit.238U activities are taken from Swarzenski and Baskaran (2007).a Only dissolved activity was below detection limit.

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Fig. 2. Daily precipitation (July 2003–July 2004) and (inset) daily mean stream flow (June–August 2003).

34 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

specific activity of particulate 210Pbxs varied between 1.6and 9.0 dpm g−1 (mean: 6.6 dpm g−1, n=16) in June and5.0 and 65.0 dpm g−1 (mean:=19.0 dpm g−1, n=16) inAugust 2003. The distinctly higher activities in June 2003could be attributed to the more frequent recycling of veryfine particulate matter resulting in enrichment ofparticulate 210Pbxs. The activities of particulate 210Pbxsin SPM from shallow estuarine systems could beeffectively utilized to obtain information on the extentof resuspension of the particulate matter. The specificactivity of 7Be in SPM varied between 0.8 and8.5 dpm g−1 (mean: 2.5 dpm g−1) in June and between4.3 and 98.3 dpm g−1 (mean: 16.0 dpm g−1) in August2003. Higher precipitation in August (Fig. 2) resulted inincreased atmospheric depositional flux of 210Pb and 7Be,resulting in more particulate 210Pbxs and

7Be.

3.3. Atmospheric depositional flux of 7Be and 210Pb

The amount of precipitation, collection interval, specificactivities, and depositional fluxes of 210Pb and 7Be for the20 bulk and 9 dry deposition samples collected betweenJuly 2003 and July 2004 are given in Table 3. As discussedearlier, the total activities of 7Be and 210Pb in Tampa Bayvaried over an order of magnitude and this variation mustbe related to input variations. The depositional flux of 210Pbvaried by a factor of 40, from0.10 to 3.98dpmcm−2 year−1

(Table 3). The corresponding depositional flux of 7Bevaried by a factor of ∼14, from 3.9 to 53.7 dpm cm−2-

year−1. The relatively large variation in the depositionalflux of 210Pb at our study site is likely due to variations in

the source of air masses. Due to low exhalation rates of222Rn over the ocean as compared to continental areas,oceanic air masses are typically depleted in 222Rn and itsdaughter products, including 210Pb. As a consequence, thestanding crop of 210Pb in the atmosphere has a stronglongitudinal dependency (Turekian et al., 1977). However,7Be is of cosmogenic origin, and its flux to the Earth'ssurface has a latitudinal dependence and hence itsatmospheric flux to the Earth's surface should beindependent of geography for any given latitude. Theannual bulk depositional flux for 210Pb is estimated to be0.74 dpm cm−2 year−1 (based on 0.56 dpm cm−2 during277 days of sampling in 2003–2004 with 113 cm of rain,Table 3). This value is distinctly lower than otherGulf coastsites, such as Galveston, Texas, where a value of1.03 dpm cm−2 year−1 was reported (Baskaran et al.,1993b) based on about 3 years of atmospheric depositionalflux data. It is likely that a major component of the airmasses above St. Petersburg are derived from the adjacentoceans (the global flux of 222Rn to the atmosphere isestimated to be 8–11 dpm cm−2 day−1 from continentalareas while it is∼0.1 dpm cm−2 day−1 from oceanic areas;Samuelsson et al., 1986; Nazaroff, 1992). In contrast,Baskaran et al. (1993b) reported that a major portion of airmasses that brings precipitation to Galveston is ofcontinental origin. Our annual 7Be depositional flux of8.81 dpm cm−2 year−1 is also significantly lower than thevalue reported for Galveston (14.7 dpm cm−2 year−1) andthis difference is likely due to differences in cloudcondensation heights and the residence time of watervapor in the atmosphere. During summer months, the

Fig. 3. (a) Particulate 210Pb versus total 210Pb; (b) particulate 7Beversus total 7Be in water samples from Tampa Bay, FL.

35M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

source of water vapor for afternoon thunderstorms isgenerally from the ocean and subsequent precipitation mayoccur within a few hours. The residence time of watervapor in this region during summer months is likely veryshort. Due to the occurrence of almost daily summerthunderstorms, the atmosphere is likely effectively flushedand thus 7Be activities can not build up. Lateral movementofmarine airmasses coupledwith the verticalmovement ofwater vapor towards the cloud condensation height couldresult in relatively lower activities of 7Be in the air (dilutioneffect). This could explain lower atmospheric depositionalflux values for 7Be. The dry depositional flux for 7Be and210Pb varied between 0.13 to 2.74 dpm cm−2 year−1, andbelow detection limit to 0.298 dpm cm−2 year−1, respec-tively. These values (Table 3) correspond to a relativelysmall percentage (b10%) of the bulk depositional flux.

3.4. Relationship between 7Be and 210Pb in the bulkprecipitation and water column

It has been shown that the removal mechanisms of 7Beand 210Pb in the atmosphere are similar and there is a

strong correlation between the bulk depositional fluxes of7Be and 210Pb (e.g., Baskaran et al., 1993b;McNeary andBaskaran, 2003; Kim et al., 2000). Baskaran et al. (1993b)argued that a weaker correlation between 7Be and 210Pbcould indicate mixing of continental and oceanic airmasses. Our data indicate a strong correlation betweenthese radionuclides (Fig. 4, r=0.78, Pb0.001). However,the relationship between these radionuclides in the watercolumn is weak (Fig. 5, r=0.39), compared to the bulkprecipitation. We attribute the difference in the relation-ship between the activity of 7Be and 210Pb in the watercolumn and the atmospheric depositional flux to episodicresuspension events as well as differences in thepartitioning of Pb and Be between particulate matter(see Section 3.6) and the dissolved phase.

3.5. Residence times of 234Th, 210Pb and 7Be

The residence times of 234Th, 7Be and 210Pb in theTampa Bay water column can be evaluated from themeasured activities of these radionuclides in theparticulate and dissolved phases, along with theircalculated production rates (for 234Th) and depositionalflux (for 7Be and 210Pb). Assuming that scavenging isirreversible (reverse rate constant is much longercompared to forward rate constant) and that advectionand diffusion are negligible, then, the residence time oftotal (τt) and particulate (τp)

234Th in the coastal waterscould be calculated using the following equations (e.g.,Baskaran and Santschi, 1993; Baskaran et al., 1996):

st ¼ 234Tht=½ð238U−234Th

tÞkTh� ðiÞ

and

sp ¼ 234Thp= ½ð238U−234Th

dÞ−234Th

p�kTh

n oðiiÞ

where λTh is the 234Th decay constant (0.029 day−1),and 238U, 234Thd,

234Thp, and 234Tht representdissolved 238U, and dissolved, particulate, and total234Th activities (=excess particulate+dissolved), re-spectively. The calculated residence time of total 234Thvaried between 0.6 and 6.6 days (mean: 2.3 days, n=15)and 1.3 and 14.8 days (mean: 5.0 days, n=16), in June andAugust 2003, respectively (Table 4). The residence time ofparticulate 234Th varied between 0.6 and 3.3 days (mean:1.6 days, n=14) and between 1.3 and 14.8 days (mean:4.3 days, n=16) during June and August 2003, respec-tively. The distinctly higher values of particulate anddissolved 234Th during August 2003 are attributed toelevated precipitation and associated runoff that can resultin increased sediment resuspension rates, although SPMconcentrations are generally lower than that of June. These

Table 3Sampling time intervals, precipitation rates, annual depositional fluxes, and activities of 7Be and 210Pb measured in St. Petersburg, FL, from July 2003through June 2004

Collection interval Days incollection

Number ofrainy days

Rainfall(cm)

7Be(dpm L−1)

210Pb(dpm L−1)

7Be/210Pbactivity ratio

7Be flux(dpm cm−2 year−1)

210Pb flux(dpm cm−2 year−1)

Wet fall outJuly 16 – July 18,2003

3 3 2.62 57.1 5.11 11.2 42.3 3.79

July 18 – July 31,2003

14 5 2.97 104 5.88 17.7 9.87 0.56

July 31 – Aug. 6,2003

6 1 2.79 84.2 5.18 16.3 28.2 1.73

Aug. 9 – Aug. 12,2003

4 3 9.70 82.7 6.13 13.5 53.7 3.98

Aug. 25 – Aug. 29,2003

5 4 7.34 36.6 1.38 26.5 23.1 0.87

Aug. 29 – Sept. 3,2003

5 2 4.22 34.0 1.54 22.1 19.5 0.89

Sept. 9 – Sept. 11,2003

3 2 11.7 7.05 0.41 17.2 15.0 0.87

Sept. 11 – Sept. 27,2003

17 7 7.39 56.4 2.75 20.5 11.7 0.57

Sept. 27 – Oct. 5,2003

8 4 2.90 78.8 11.1 7.1 14.7 2.07

Oct. 5 – Nov. 2,2003

28 4 3.40 49.3 4.06 12.1 2.61 0.22

Nov. 2 – Nov. 30,2003

29 7 2.21 30.3 2.29 13.2 1.30 0.098

Dec. 11 – Jan. 25,2004

46 8 8.46 53.5 3.58 14.9 3.59 0.24

Jan. 25 – Feb. 18,2004

25 9 6.86 67.4 4.99 13.5 6.75 0.50

Feb. 24 – Feb. 29,2004

6 3 7.52 48.5 1.78 27.2 32.9 1.21

March 15 – March18, 2004

4 2 2.95 26.8 2.17 12.4 10.4 0.84

April 12 – May 8,2004

26 5 7.34 74.3 3.13 23.7 6.08 0.26

May 8 – May 25,2004

17 3 5.72 122 8.01 15.2 11.8 0.77

May 25 – June 18,2004

24 5 3.07 61.7 6.33 9.7 3.90 0.40

June 18 – July 6,2004

18 7 11.84 73.8 4.62 16.0 12.3 0.77

July 6 – July 12,2004

7 3 4.06 4.33 2.86 1.51 3.98 2.63

Dry fall outOct. 5 – Nov. 13,2003

39 – – – – 1.27 0.131 0.103

Nov. 17 – Dec. 3,2003

16 – – – – – 0.274 BD

Dec. 12 – Dec. 30,2003

18 – – – – 3.95 0.288 0.073

Jan. 11 – Feb. 23,2004

43 – – – – 2.66 0.133 0.050

March 3 – March14, 2004

11 – – – – 3.78 0.314 0.083

March 17 – March30, 2004

– – – – – 8.16 0.734 0.090

April 6 – April 22,2004

16 – – – – 11.5 0.229 0.020

36 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

Fig. 4. Bulk depositional fluxes of 7Be versus 210Pb in St. Petersburg,FL, from July 2003 through July 2004.

Table 3 (continued)

Collection interval Days incollection

Number ofrainy days

Rainfall(cm)

7Be(dpm L−1)

210Pb(dpm L−1)

7Be/210Pbactivity ratio

7Be flux(dpm cm−2 year−1)

210Pb flux(dpm cm−2 year−1)

April 24 –May 10,2004

16 – – – – 5.56 0.328 0.059

May 14 – May 25,2004

11 – – – – 9.19 2.74 0.298

The precipitation and river discharge are shown in Fig. 2.BD: below detection limit.

37M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

residence times are comparable to values reported for otherestuarine systems, such as Long Island Sound (∼1 day;Aller and Cochran, 1976), Narragansett Bay (b1 to20 days; Santschi et al., 1979), the Yangtze River estuary(b1 day; McKee et al., 1984), estuaries and bays in Texas(0.9 to 7.8 days; Baskaran and Santschi, 1993), and theHudson River estuary (1.7 to 12 days; Feng et al., 1999).

In coastal waters, most of the 210Pb and 7Be present inthe water column is derived from atmospheric depositionand hence data on the activities of these radionuclides inthe water column along with the atmospheric input can beutilized to obtain estimates of their residence times.Riverine inputs of 210Pb and 7Be as well as in situ pro-duction from 222Rn (for 210Pb) can be assumed negligible(e.g., Baskaran and Santschi, 1993). Under steady stateconditions, if the in situ production rate of 210Pb from222Rn is negligible compared to the atmospheric falloutrate, then the residence times, τ, of 210Pb (or 7Be) can becalculated, as follows:

sBe;Pb ¼ ln2� APb=Be � h=IPb=Be ðiiiÞ

where APb/Be is the total activity of 210Pb or 7Be(dpm m−3), IPb/Be is the atmospheric input rate of 210Pbor 7Be (dpm m−2 day−1), and h is the mean depth (m) ofthe well-mixed station. The total residence time of 210Pbvaried between 3.0 and 54.8 days (mean: 19.6 days,n=17) and 4.1 and 27.7 days (mean: 12.1 days, n=16)during June and August 2003, respectively. The cor-responding residence times of 7Be varied between 1.6 and20.4 days (mean: 4.4 days, n=9) and 2.8 to 58.7 days(mean: 19.6 days, n=15), respectively, during June andAugust 2003. The residence times of 210Pb and 7Be aresignificantly higher than those of 234Th and this is attrib-uted to resuspension of bottom sediments. The resuspen-sion events affect those radionuclides with the longesthalf-lives the most, as the sediments retain much largeramounts (compared to depositional input) of long-livedradionuclides. Since 210Pb and 7Be are steady-state trac-ers, both radionuclides were present in the bay prior to

water sampling efforts. However, 1 year after deposition,N97% of the 7Be would have decayed away, while only∼3% of the 210Pb would have decayed. The mean-life of210Pb is 33 years while 7Be is 77 days and hence theamount of 210Pb retained in the upper sedimentary layerwill be much higher compared to the annual atmosphericdepositional input. The range of total 210Pb residencetimes are comparable to values reported for other estu-arine systems, such as Galveston Bay (4 to 28 days,Baskaran and Santschi, 1993), Narragansett Bay (10 to60 days, Santschi et al., 1979), and the Sabine–NechesEstuary (5.1 to 39 days, Baskaran et al., 1997). Similarly,the residence times of total 7Be are comparable to thevalues reported for other estuaries such as New YorkHarbor (8 to 17 days; Olsen et al., 1986), the James RiverEstuary (2 to 4 days; Olsen et al., 1986), Raritan Bay (7 to17 days; Olsen et al., 1986), Chesapeake Bay (5 to52 days; Dibb and Rice, 1989), Galveston Bay (0.9 to1.8 days; Baskaran and Santschi, 1993), Sabine–NechesEstuary (0.8 to 10.5 days; Baskaran et al., 1997), and theHudson River Estuary (0.7 to 9.5 days; Feng et al., 1999).

Fig. 5. Activities of total 7Be versus total 210Pb in the water column,from Tampa Bay.

38 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

The residence times of total 234Th, 7Be and 210Pb areplotted against SPM concentrations for June and August2003 in Fig. 6. Generally, there is a significant correlationbetween the total residence times of 234Th, 210Pb and 7Beand concentrations of SPM for the June sampling, whileno such relation is apparent for the August samples. Alinear relation between the radionuclide residence timesand SPM suggests that the amount of SPM controls theresidence times of these radionuclides. However, resus-pension events may significantly impact SPM concentra-tions, as well as the total activities of these radionuclides,and thus, no relation would be expected when resuspen-sion events are frequent and significant. It appears that theeffect of wind- and tide-generated sediment resuspension

Table 4Residence times of 234Th, 7Be and 210Pb

Samplecode

Depth(m)

234Th (day) 234Th (day) 7Be

June 2003 August 2003 June

TB01 2.8 0.8±0.1 2.6±0.2 –TB02 3.1 1.0±0.1 3.0±0.1 –TB03 4.6 0.16±0.04 1.6±0.4 4.8±TB04 3.8 0.4±0.1 0.38±0.04 1.9±TB05 3.0 0.22±0.02 0.9±0.2 1.8±TB06 3.5 0.4±0.1 0.8±0.1 –TB07 2.9 0.2±0.1 1.2±0.1 2.3±TB08 4.4 0.9±0.1 0.5±0.1 20.4TB09 5.3 0.3±0.2 0.28±0.04 2.5±TB10 2.6 0.19±0.04 0.3±0.1 –TB11 5.1 0.49±0.06 0.7±0.1 0.19TB12 4.4 0.6±0.1 0.4±0.1 1.6±TB13 7.7 0.4±0.1 1.8±0.7 2.1±TB14 4.0 – 0.39±0.05 2.0±TB15 3.8 – 0.5±0.1 –TB16 3.3 0.13±0.04 – –TB17 4.8 1.4±0.2 0.9±0.1 –

events on the residence time of particle-reactive radio-nuclides could be inferred using the relationship betweenthe residence time of a particle-reactive nuclide and SPMconcentration.

3.6. Distribution coefficients of 234Th, 210Pb and 7Be

The partitioning of 234Th, 7Be and 210Pb betweenfilter-passing and filter-retained particulate phases canbe evaluated from the calculation of the distributioncoefficient, Kd, given by the relation,

Kd ¼ ðAp=AdÞð1=SPMÞ ðivÞ

where Ap and Ad are the activities of a nuclide in theparticulate and dissolved phases (dpmL−1), and SPM is thefilter-retained particle concentration (g cm−3). The distri-bution coefficient for 234Th varied between 0.9×105 and10×105 cm3 g−1 (mean: 4.2×105 cm3 g−1) during June,and between 0.9×105 and 3.1×105 cm3 g−1 (mean:2×105 cm3 g−1) observed during August 2003. Thesedistribution coefficients are comparable to the valuesreported for Texas bays and estuaries observed during thesummer season, but are higher than those reported for thespring season. The distribution coefficients of 210Pb variedbetween 0.5×105 and 8.1×105 cm3 g− 1 (mean:2.5 × 105 cm3 g− 1) and between 0.9 × 105 and5.9×105 cm3 g−1 (mean: 3.5×105 cm3 g−1) duringJune and August 2003, respectively. TheKd values of

7Bevaried between 0.1×104 and 34×104 cm3 g−1 (mean:11 × 104 cm3 g− 1) and between 0.3 × 104 and

(day) 7Be (day) 210Pb (day) 210Pb (day)

2003 August 2003 June 2003 August 2003

31.4±5.3 17.2±2.1 18.9±2.845.9±8.2 32.0±2.8 10.0±1.9

1.5 6.9±2.3 4.3±0.9 8.1±0.70.5 6.2±2.9 14.5±1.8 4.7±0.20.4 2.8±1.0 3.0±0.3 10.7±2.2

8.6±2.4 5.5±0.5 15.7±3.60.3 9.2±2.5 26.5±1.7 13.7±2.5±2.2 6.6±2.3 54.8±2.6 20.4±4.20.5 9.7±2.5 51.0±6.4 27.7±3.1

20.6±2.6 13.7±3.6 13.1±1.9±0.03 12.5±2.5 11.2±1.0 9.0±0.90.5 58.7±15.4 25.8±2.5 4.6±0.30.5 33.7±8.1 20.3±1.4 10.7±1.10.3 – 16.3±4.0 4.1±0.3

6.7±5.0 8.0±0.8 5.9±0.5– 3.8±0.4 –35.2±7.8 25.1±2.5 15.9±0.9

39M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

3.7×104 cm3 g−1 (mean: 3.7×104 cm3 g−1) during Juneand August 2003, respectively. TheKd values for

7Be and234Th are higher in June while it is higher for 210Pb inAugust, although differences in the mean values betweenthe two sampling periods are marginal. This differencecould be attributed to the limited data for 234Th anddifferences in the particle reactivity between 7Be and210Pb during dynamic sediment resuspension in August.The partitioning of Th, Pb and Be in Tampa Bay followsthe sequence: Kd(Th)∼Kd(Pb)NKd(Be). The range ofvalues are comparable to the values reported in othercoastal systems (e.g., McKee et al., 1986; Dibb and Rice,1989; Baskaran and Santschi, 1993). The wide range ofobserved Kd values for Th, Be and Pb indicates thatseveral variables likely affect the solubility partitioning ofthese radionuclides between the particulate and dissolvedphases. In particular, changes in pH, salinity, theconcentration and composition of SPM, the nature andcomposition of dissolved organic carbon, the hydraulicresidence time and the extent and frequency of resuspen-sion of bottom sediments all likely affect the partitioningof these radionuclides. It has been shown that the par-

Fig. 6. Total residence times of 234Th, 210Pb and 7Be plotted against SPM inmeasured in June (correlation coefficient shown) while no significant correl

titioning of Be depends directly on pH (You et al., 1989;Vesely et al., 2002), with more favorable partitioning intothe aqueous phase at higher pH values. The high pH ofseawater relative to river water should result in decreasingparticulate 7Be with salinity (or increase of dissolved 7Bewith increase in salinity). For example, Bloom andCrecelius (1983) observed that the amount of particulate7Be was proportional to the SPM concentration, and asmuch as∼50% of 7Be was found on the particulate phasewith SPMconcentrations of∼20mgL−1. The percentageof particulate 7Be varied between 1% and 86% (Dibb andRice, 1989), with varying suspended load concentrations(summarized in Kaste et al., 2002). A summary of all Kd

values for 7Be observed in marine environments ispresented in Kaste et al. (2002), with values ranging from3.2×103 to 106 cm3 g−1.

The distribution coefficients for 234Th, 210Pb and 7Beare plotted against log(SPM) concentration in Fig. 7.There is an apparent inverse correlation between log Kd

of 210Pb (August) and 7Be (August) and log(SPM)concentration, while no such trend exists for log Kd of210Pb (June) and 7Be (June) and log(SPM). Such inverse

Tampa Bay, FL. There is significant correlation between radionuclidesation is found for the samples collected in August.

Fig. 7. The log distribution coefficients (Kd=(Ap/Ad)(1/SPM); see text for explanation) for 234Th, 210Pb and 7Be as a function of log(SPM)concentration.

40 M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

correlations between Kd of particle-reactive species and[SPM] have been previously reported (McKee et al.,1986; Honeyman and Santschi, 1989; Baskaran andSantschi, 1993) and attributed to a ‘particle–concentra-tion–effect’ (e.g., O'Connor and Connolly, 1980;Honeyman and Santschi, 1988, 1989) due to the occur-rence of colloids in the filtrate fraction.

4. Conclusions

We have measured the activities of 7Be, 234Th and210Pb in the particulate and dissolved phase during JuneandAugust 2003 inTampaBay, FL. The activities of 238U,and thereby the production rates of 234Th, are among thehighest measured in any coastal and/or estuarine system.Based on our data, we draw the following conclusions:

1) The activities of dissolved and particulate 210Pb and7Be in the water column has been found to vary withprecipitation and river discharge, which suggests thatthe depositional flux and episodic resuspension eventscontrol the fate of these particle-reactive species in thewater column during the summer months.

2) The precipitation-normalized atmospheric depositionalflux of 7Be and 210Pb are significantly lower comparedto other Gulf coastal systems, and this is attributed to arelatively higher fraction of oceanic versus continentalair mass at the sampling site, and a shorter residencetime of water vapor that brings rain to coastal Florida.

3) The average residence time of total as well as parti-culate 234Th in Junewas lower than that inAugust. Thelonger residence time in August is attributed toresuspension of 234Th-enriched particles due to higheramounts of riverine discharge and precipitation. Incontrast, the mean residence time of total 210Pb duringJune is longer than that in August, and this is attributedto higher inputs of 210Pb from atmospheric depositionin August as well more resuspension during August ascompared to June.

4) There is considerable scatter on the relationship bet-weenKd of

7Be and 210Pb and [SPM], indicating a lackof a particle concentration effect. This is likely due thegeneral paucity of particles and colloids present withinthe bay, as well as due to extensive recycling of SPMduring frequent thunderstorms observed during sum-mer months.

41M. Baskaran, P.W. Swarzenski / Marine Chemistry 104 (2007) 27–42

Acknowledgements

We specially thank Carl Henderson (USF) forcollecting and processing the wet/dry fall out samples.Marci Marot (USGS), Chris Reich (USGS) and BrianBlake-Collins (ETI) ably assisted in field and laboratorywork. MB acknowledges the support from USGS on aco-operative agreement between Wayne State Universi-ty and USGS. PWS acknowledges continued supportfrom the USGS Coastal and Marine Geology Programand Kim Yates (USGS) for her support in the TampaBay Pilot Project. Constructive comments and sugges-tions from two anonymous reviewers and Peter Santschiare appreciated. The use of trade names is for descriptivepurposes only and does not imply endorsement by theU.S. Government.

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