Environ. Sci. Technol. 1909, 23, 1475-1481
Plutonium, Lead-210, and Carbon Isotopes in the Savannah Estuary: Riverborne versus Marine Sources Curtis R. Olsen,'*+Myint Thein,$ Ingvar L. Lawen,+ Philip D. Lowry,§Patrick J. Mulholland,t Norman H. Cutshall,+James 1.Byrd) and Herbert L. WindomII
Environmental Sciences Division and Environmental Compliance and Health Protection Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1, Department of Anesthesiology, Emory University School of Medicine, Atlanta, Georgia 30322, and Skidaway Institute of Oceanography, Savannah, Georgia 31415 Plutonium-238 from the Savannah River Plant labels riverborne particles, providing a unique opportunity for tracing river-ocean exchange processes. Results indicate that plutonium and lead-210 are enriched on estuarine particles and that inputs of plutonium from oceanic sources greatly exceed inputs from riverborne or drainage-basin sources as far upstream as 30 km in the Savannah River estuary. This is near the landward limit of seawater penetration. Sediment resuspension in dynamic coastal areas, sorption onto fine particles, and landward transport of particulate material along the bottom are the primary mechanisms for removing dissolved plutonium and lead210 from oceanic water and for concentrating these two radionuclides in estuarine areas. Since estuaries, bays, and intertidal areas serve as effective traps for fine particles and associated substances, this landward transport from the ocean has important implications concerning the disposal of chemically reactive substances in oceanic waters off coastlines affected by a rising sea level. W
Introduction It has been well established that deposition in estuaries traps particles, nutrients, trace substances, and contaminants from land-based sources (1-4). Less well-defined is the role of coastal estuaries in removing and trapping materials from the oceanic water column (5-7). At the estuarine interface, where fresh water mixes with seawater, the fate of substances can be affected by river flow, tidal flow, wave activity, currents, and a nontidal estuarine circulation pattern. Estuarine circulation along submergent coastlines is characterized by a lower salinity surface layer with a net seaward flow and a denser, more saline bottom layer with a net landward flow. This upstream or landward flow along the bottom causes estuaries to trap materials from both riverine and marine sources and to serve as major deposition and accumulation sites for particles and particle-associated substances (2, 6-9). Although there is a general understanding of the processes that affect the fate of chemically or particle-reactive contaminants in estuarine systems, it has been very difficult to quantify net exchange between rivers and the ocean because (1)contaminant source terms are diffuse and poorly defined, (2) contaminant transport with particles usually involves numerous episodes of deposition and resuspension in association with short-term events over long periods of time, and (3) a complex suite of physical and biogeochemical interactions take place when fresh water mixes with seawater. In this paper, we present data on plutonium (23aPuand 2399240Pu) concentrations in the Savannah River estuary and compare its distribution with Environmental Sciences Division, Oak Ridge National Laboratory. Environmental Compliance and Health Protection Division, Oak Ridge National Laboratory. 5 Emory University School of Medicine. '1 Skidaway Institute of Oceanography.
*
0013-936X/89/0923-1475$01.50/0
radiocesium (137Cs)and two naturally occurring radionuclides, 210Pband 7Be. We show that the isotopic composition of plutonium released from the Savannah River Plant (SRP) and of plutonium attached to suspended particles in the Savannah River is different from that of oceanic (fallout) plutonium, and we have used this differente to determine that inputs of oceanic plutonium to the Savannah estuary greatly exceed inputs from riverborne sources. In addition, we have used the ratio of short-lived 'Be to long-lived 210Pbas a new tool for quantifying sediment resuspension in estuarine areas and for examining the mechanisms responsible for enriching radionuclides and, by analogy, other chemically reactive contaminants on estuarine particles. Finally, we have used particulate 13'Cs concentrations and carbon isotopic [a(13C)]data to identify the contribution of inorganic and organic particles from riverborne and estuarine or marine sources. These results help provide a rational for the elevated plutonium and 210Pbconcentrations and inventories that have been found to occur in fine-grained estuarine and coastal sediments throughout the world (10-20).
Locality Information and Geochemical Tracer Methodology Plutonium. The SRP, located -256 km upstream from the mouth of the Savannah River (Figure l),is the principal plutonium production facility for the United States Department of Energy. Between 1954 and 1974, -1.4 X loll Bq (3.7 Ci) of plutonium has been released into the atmosphere from fuel reprocessing operations, and 1.1 X 1O'O Bq (0.3 Ci) has been released to surface waters, which ultimately drain into the Savannah River (21). Although concentrations of %Pu and 239v240Pu in SRP soils exhibit large variations as a result of proximity to points of release, microtopographicalheterogeneity in deposition, and sampling error, mean soil concentrations have been reported as 18 and 83 mBq/g, respectively (22). The resultant mean 238Puto 239p240Pu ratio for soils on the SRP reservation is -0.22. Concentrations of 238Puand 2391240Pu in stream and pond sediments on the SRP reservation also show large variations, ranging from 0.003 to 14 mBq/g for 238Puand 0.014 to 52 mBq/g for 239,240Pu (23). Measured 238Puto 2399240Pu ratios in these samples were often greater than 0.12 and were as high as 3.3 in the sediments of Four Mile Creek (23). These plutonium isotope ratios are significantly higher than the 238Puto 2399240Pu ratio in fallout from atmospheric testing of nuclear weapons. On the basis of soil samples collected in Raleigh, NC, Hardy and others (24) have estimated that -0.105 mCi/km2 (3.9 X lo6 Bq/km2) of =Pu were and 2.4 mCi/km2 (8.9 X lo7 Bq/km2) of 239,240Pu deposited as fallout to the southeastern United States. When extrapolated over the Savannah River drainage basin (27400 km2)this amounts to a total fallout delivery of 1.07 X 10" Bq of 23aPuand 2.4 X 10l2Bq of 2397240Pu. The recent nuclear accident at Chernobyl in the Soviet
0 1989 American Chemical Society
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Environ. Sci. Technol., Vol. 23, No. 12, 1989
1475
I _ - - - \
I
33'00'
0
L
32'00'
82'00'
81000'
Flgure 1. Large-volume water sample collection sites in the Savannah River estuary during June 1986. The freshwater sample downstream from the U.S. Department of Energy's Savannah River Plant (S-301) was collected from the river bank. All other samples were collected in the rlver channel, 1 m below the surface. A sediment box core was also collected near S-0102.
-
Union has not significantlyadded to this fallout plutonium ratio calculated from burden. The fallout 238Puto 239p240Pu these data is -0.046, which is similar to ratios that have been measured in estuarine and coastal sediments along the eastern United States (3, 7, 9, 10, 15, 18, 19). This suggests that the 238Puto 239,240Pu ratio on riverborne particles (affected by releases from the SRP) may be different from the fallout plutonium ratio on coastal particles and thus may provide a unique opportunity for separating materials transported into the Savannah River estuary from riverborne versus marine sources. Radiocesium. Global fallout from atmospheric nuclear weapons testing has also introduced 137Cs(30-yr half-life) to the Savannah River watershed, with the major influx occurring during the years 1962-1964. The 1950-1985 integrated and decay-corrected input of fallout 137Csto mid-latitude areas in the United States is -95 mCi/km2 or 3.5 X lo9 Bq/km2 (25). When extrapolated over the Savannah River drainage basin (27 400 km2)this amounts to a total fallout delivery of -9.6 X 1013Bq of 137Cs. In addition to global fallout, 137Cshas been introduced into the Savannah River watershed via operations at the SRP. It has been estimated that -500 Ci (1.9 x 1013Bq) of 137Cs has been released into surface waters on the SRP reservation between 1961and 1973, and that -18% of this total release or 90 Ci (0.3 X 1013Bq)has drained into the Savannah River (26). Previous work has shown that 137Csis rapidly sorbed to clay minerals in drainage basin soils and freshwater sediments (3) and exhibits a particle to water distribution coefficient or sorption ratio of -1 X lo5 (25). Cesium sorption on inorganic clay particles can occur at several sites, including (1)easily exchangeable sites on the particle surface, (2) fixed sites at the edges of the clay-mineral lattice, and (3) nonexchangeable, interlayer sites where 1476
Environ. Sci. Technol., Vol. 23, No. 12, 1989
137Cscan substitute for K+ in (illitic) micaceous minerals (25). Although little is known about the kinetics of 137Cs fixation into clay interlayer sites, previous work in the Susquehanna River-Chesapeake Bay system has indicated that >90% of the 137Cson bottom sediments and >70% of the 137Cson suspended particles was irreversibly sorbed into mineral lattices (25). In ocean waters, however, 137Cshas a tendency to remain in solution because of its competition with other cations (especiallyK+) for sorption sites on particles, and because of the paucity of inorganic particles in the marine water column. Consequently, 13'Cs concentrations on coastal particles and the resulting 137Csparticle to water sorption ratios in seawater are several orders of magnitude lower than respective concentrations and ratios in freshwater systems. This difference in the 137Cscontent of particles derived from the land and those derived from the marine environment suggests that 137Csmay serve as a tracer for the origin of recent inorganic particles at the land/ocean interface if its desorption from riverborne particles is relatively insignificant (27). Lead-210 and Beryllium-7. 21Pband 'Be are naturally occurring radionuclides with half-lives of 22.3 yr and 53.3 days, respectively. zloPbis one of the daughters in the naturally occurring 238U-226Ra-222Rn-214Pb decay series. As an inert gas, some z2zRnmay emanate into the atmosphere before its decay to zlOPb.The removal of 21Pbfrom the atmosphere via washout by precipitation forms a measurable flux of "excess" 210Pb(210Pb-ex)back to the earth's surface. This flux ranges from -0.2 to 2.6 mBq/cm2 on a monthly basis and annually averages 15 mBq/cm2 along the eastern coastline of the United States (25,28). Over the 32-yr mean life of 210Pb,this annual flux would support an expected steady-state 210Pb-exinventory of -480 mBq/cm2 in soils and sediments. Previous work has shown that zloPbis chemically and particle-reactive in both fresh and marine waters, and as a consequence, its distribution has been widely used to quantify physical and biogeochemical processes in estuarine and coastal systems (13, 14, 17, 20, 25, 28). Beryllium-7 is produced by cosmic-ray spallation of nitrogen and oxygen within the earth's atmosphere. Like fallout radionuclides and 210Pb-ex,7Be is primarily removed from the atmosphere via washout by precipitation. Previous work has shown that 7Be is rapidly sorbed by particulate material in aquatic systems, and exhibits a particle to water sorption ratio similar to that of plutonium and 210Pb-ex(25,28,29). Because of its short-half life, 7Be is a powerful tracer for evaluating processes that occur on short time scales (weeks to months), relative to the longer time scales (months to decades) traced by 210Pb-ex,plutonium, and radiocesium. Stable Carbon Isotopes. Stable carbon isotope ratios have been widely used as biogeochemical tracers in systems with two or more isotopically distinguishable sources of carbon (30-33). The stable carbon isotope imprint is the ratio of carbon-13 to carbon-12 and is generally expressed in the 6 notation: 6(13C) (per mil) = (Rsamp/Rstd- 1) X 1000
-
where R,, is the 13C/12Cratio in the sample and RsU is the 13C/128ratioin a PDB standard. Because the 6(13C) value for plankton from surface ocean waters off the southeastern United States (-21 per mil is heavier than the value for soil or plant carbon (-26 per mil from terrestrial sources, the W3C) value on estuarine organic materials can be used to help discern carbon derived from terrestrial drainage-basin sources from carbon produced in situ in the estuarine water column or derived from
Table I. Dissolved and Suspended-Particulate Plutonium in the Savannah River Estuarya salinity, sample S-301
L
S-0107 S-0102 S-0116 S-HH8
fresh fresh 2.1 14.5 24.6 32.5
0-2 cm
14.5
S-0101
susp part., mg/L 7.5 21.9 16.4 19.0 22.0 0.9 sediment
dissol 238Pu, rBq/L
susp part. Z38Pu, mBq/kg
dissol
susp part.
sorptn ratio
239,240pu,
239,240pu,
2aS,240pu,b
mB,/kg
L/kg
0.7 f 0.4 1.5 f 0.4 1.9 f 0.4 0.4 f 0.2 0.4 f 0.1 0.4 f 0.4
93 f 11 100 f 15 44 & 7 26 & 4 30 f 4 30 f 6
1.9 f 0.4 5.2 f 0.7 4.8 f 0.7 2.2 f 0.4 2.2 f 0.4 3.3 f 0.4
&/L
2x 5x 7x 2x 3x 1x
285 f 26 235 f 30 350 f 30 545 f 44 665 f 52 400 f 35
part. Zapu to Z39,240pu ratioc 0.33 f 0.05 0.43 f 0.08 0.13 f 0.02 0.05 f 0.01 0.05 f 0.01 0.07 f 0.02
106 104 104 105 106 106
Surface Sediment at Station S-0102 (Figure 1) 15 f 4 300 f 26
0.05 f 0.01
a Statistical counting errors are expressed in terms of la. Sorption ratio, 2aS*240Pu concentration per kg of particles/239*240Pu concentration per L of water. CParticulatePu isotope ratio, 2aPu concentration per kg of p a r t i ~ l e s / ~ ~ ~concentration * ~ ~ ~ P u per kg of particles.
marine sources via landward transport (34).
Sample Collection and Analysis We measured the particulate and dissolved distribution of 239*240Pu and 23aPuin six large-volume (850-L) water samples collected along the salinity gradient of the Savannah River, the estuary, and the adjacent coastal marine environment (Figure 1). Suspended-particulatematter was removed from these samples by continuous-flowcentrifugation. Dissolved plutonium was concentrated and removed from the water phase by coprecipitation with Fe(OH),. Radionuclide concentrations on the suspended matter and in the water phase [Fe(OH), precipitate] were analyzed separately. In addition, a sediment box core was collected in the Savannah estuary near station S-0102 (Figure 1). The plutonium analyses involved dissolution with hydrochloric acid, coprecipitation with calcium oxalate, radiochemical separation with ion-exchange columns, electrodeposition onto stainless steel disks, and a spectrometry with silicon surface-barrier detectors (35). The samples were CY counted for -21 days and yields were evaluated with a 242Putracer. 2391240Pu activities are collectively reported because the energies of the CY particles produced by the decay of 240Pu(6580-yr half-life) cannot be resolved from those produced by the decay of 239Pu(24400-yr half-life) by CY spectrometry. The concentrations of 13'Cs, 210Pb,and 7Be on the suspended-particulate and bottom-sediment samples were measured by y spectrometry using low-background,highresolution, germanium detectors equipped with a Nuclear Data Model 9900 microprocessor system programmed to record y spectra in 4096 channels. All samples were packed in 90-cm3aluminum cans or 15-cm3plastic Petri dishes, depending on the amount of material analyzed. The detectors were calibrated for the respective geometries with a certified mixed standard and the calibration procedures are described elsewhere (25). The low-energy (46.5 keV) zloPb y-ray was analyzed by use of a planar intrinsic-germanium detector and correction for self-absorption (36). This technique has a great advantage in that it does not require sample leaching or radiochemical separation and allows for the simultaneous determination of both the total zloPband the 214Pb-supportedlevel. Excess zloPb (210Pb-ex)was calculated by subtracting the 214Pbsupported level from the total 210Pbactivity. y-Emitting radionuclide data are reported in milliBequere1per gram dry weight (1mBq = 0.06 dpm = 0.027 pCi) and counting errors are f l standard deviation. The suspended-matter and sediment samples were also analyzed for stable carbon isotopes at Coastal Science Laboratories Inc. (Austin, TX), by standard mass spectrometric procedures (27).
PLUTONIUM-238
a
- - *O0
IPL:TONl~M-23$,240
k g 400 6
g qg
6 z -42;
z
3
Z a
p 4 4 -2E{
200 0
'
I fb)
g,i'2,-
w m 600
a: 3:m2l
I
I
I
I
I
I
- 0
s!?
Table 11. Savannah Estuary Box Core LPT Collected at Station 5-0102 in J u n e 1986
depth: cm
weight,b gm
0-2 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-20A 18-20B 20-24 24-28 28-32 32-36 36-40 40-44 44-48 48-52 52-56 56-61 61-66
697 65 69 65 90 64 102 86 57 46 49 58 73 94 73 95 111 76 74 63 56 67
IBe
17 f 1 7 f 4
'37CS
2.8 f 0.2 2.8 f 0.4 1.5 f 0.4 3.6 f 0.5 4.2 f 0.3 2.6 f 0.4 0.5 f 0.2 1.5 f 0.3 3.5 f 0.5 5.2 f 0.6 5.3 f 0.4 4.6 f 0.5 4.3 f 0.4 1.7 f 0.3 2.0 f 0.4 0.2 f 0.3 0.2 f 0.3 3.2 f 0.4 1.9 f O . 4 1.8 f 0.4 3.8 f 0.6 3.0 f 0.4
radioactivity, mBq/g 210Pb 214Pb 92 f 5 76 f 4 52 f 4 70 f 4 43 f 3 72 f 5 29 f 3 38 f 3 80 f 4 71 f 5 98 f 5 75 f 5 33 f 3 36 f 3 46 f 3 31 f 3 36 f 3 67 f 4 65 1 4 68 f 5 95 f 5 67 f 4
45 f 1 40 f 1 23 f 1 27 f 1 21 f 1 24 f 1 17 f 1 16 f 1 32 f 1 28 f 1 32 f 1 30 f 1 23 f 1 21 f 1 23 f 1 19 f 1 18 f 1 28 f 1 21 f 1 36 f 1 30 f 1 24 f 1
210Pb-ex 46 f 5 36 f 5 29 f 4 43 f 4 22 f 3 48 f 5 11 f 3 21 f 3 48 f 4 42 f 5 66 f 5 45 f 5 10 f 3 15 f 3 24 f 3 11 f 3 18 f 3 39 f 4 44 =k 5 33 f 5 65 f 6 43 f 4
expected inventory from atmospheric deposition or fallout
radioactivitv. mBo /cm2 b-ex 137Cs zlOP
40K
IBe
359 f 3 388 f 9 400 f 10 407 f 11 391 f 9 375 f 9 329 f 6 336 f 6 447 f 1 2 448 f 14 421 f 7 410 f 11 371 f 10 347 f 8 376 f 10 304 f 7 304 f 7 393 f 7
18.9 10.1
29
4 . 4 >82
52.0 52.6 45.2 62.4 44.7 67.7 25.7 40.7 60.8 43.7 71.9 57.3 16.4 30.2 38.3 23.8 44.3 66.4 71.9 45.8 81.3 64.3 >1107
33
320
480
388 f 10
418 f 7 395 f 11 365 f 10
3.2 4.0 2.4 5.2 2.5 3.7 1.1 3.0 4.4 5.3 5.8 5.9 7.0 3.5 3.3 0.4 0.6 5.4 3.1 2.5 4.8
Two samples were accidently labeled 18-20 cm. These were not supposed to be duplicate samples and we suspect that the measured depth increments below 20 cm are off by 2 cm. A large surface sample (620 cm2) was collected from the top of the box core for plutonium analvses. a
with 238Pu(Figure 2a) and that estuarine suspended particles are enriched in 239,240Pu (by a factor of -3) relative to respective concentrations on riverborne particles and on suspended matter in the surface waters of the ocean (Figure 2b). Plutonium isotopic ratios (238Puto 2399240Pu) for suspended particles throughout the Savannah River estuary and shelf system are also listed in Table I. In the Savannah River, the ratio of 238Puto 239@ Pu' is -0.4 (Table I). This ratio is significantly higher than the 238Puto 239,240Pu ratio in fallout (0.05),but is similar to ratios that have been measured previously in the soils and sediments around the SRP, in the Savannah River downstream from the SRP, and in some freshwater sediment cores (21-23). The 238Puto 2399240Pu ratio on suspended particles abruptly decreases to -0.13 at 2 ppt salinity (Table I). Since our data do not indicate any differential behavior between 238Puand 239p240Pu, this decrease implies that riverborne plutonium accounts for only -25% of the total plutonium on suspended particles as far as 30 km upstream from the river mouth. In the estuary, the 238Puto 239p240Pu ratio on both the suspended matter and surface sediments is similar to the ratio in fallout and on particles in the adjacent shelf environment (Table I). This is conclusive evidence that the plutonium presently enriching estuarine particles (Figure 2) comes from oceanic sources rather than riverborne sources. We suggest that dissolved plutonium is being directly removed from oceanic water by sorption onto particles within the estuary, or that coastal particulate material (containing marine plutonium) is being transported landward (upstream) along the estuarine bottom. This present-day (water-column) picture for the Savannah estuary also appears to be valid for the past as indicated by the vertical distribution of 238Puto 239,240Pu ratios in the sedimentary record. Goldberg and others (9) showed that only one out of five sediment cores collected in the Savannah estuary and adjacent marsh environment contained elevated 238Puto 2399240Pu ratios at depth in the 1478
Environ. Sci. Technol., Vol. 23, No. 12, 1989
sediment. The other sediment cores contained 238Puto 239,240Pu ratios typical of fallout (or oceanic) plutonium throughout their entire length; however, inventories were often greater than the level expected from direct fallout to the estuary surface (9). Although a 66-cm-longbox core of fine-grained estuarine sediments was also collected at S-0102 (Figure 1)during this study, only the surface (0-2 cm) was analyzed for plutonium (Table I), because the vertical profiles for 137Csand 210Pb-exwere relatively constant over the entire core length and did not decrease to nondetectable and supported levels, respectively (Table 11). As a consequence, only minimum estimates for the rate of sediment accumulation (>1cm/yr) and the 210Pb-ex inventory (>1100 mBq/cm2) could be made (Table 11). These minimum estimates, nevertheless, indicate that sediments are rapidly accumulating at this site in the estuary and that the sediment inventory of 210Pb-exis at least 2 times greater than the 210Pb-exinventory expected from its atmospheric flux to the estuary surface (Table 11). Concentrations of 210Pb-exand 7Be and 13'Cs on suspended particles as a function of salinity and location in the Savannah estuary are listed in Table I11 and illustrated in Figure 3. As in the case for 239*240Pu, Savannah estuary particles are also enriched in 210Pb-exrelative to concentrations on suspended matter in the Savannah River or in the surface waters of the coastal ocean (Figure 3a). Since the ratio of 210Pb-exto 239~240Pu is relatively uniform on suspended matter throughout the Savannah Riverestuary-shelf system (Table 111),it would appear that most of the 210Pb-exon estuarine particles also comes from oceanic sources. Input of 210Pb-exfrom the marine environment may help explain the larger than expected inventory of 210Pb-exmeasured in the estuarine sediment box core (Table 11). In contrast to zloPband 2397240Pu, the concentration of short-lived 7Be is depleted on suspended particles in the Savannah estuary (Figure 3b). Since the particle to water sorption ratio for 7Beis similar to respective sorption ratios
Table 111. Suspended-Particulate Radionuclide and Carbon Isotope Data in the Savannah River Estuary"
sample
salinity, %a
total zloPb
S-301 s-0101 S-0107 s-0102 S-0116 S-HH8
fresh fresh 2.1 14.5 24.6 32.5
130 f 7 135 f 8 125 f 6 115 f 7 125 f 9 80 f 8
0-2 cm
14.5
90
radioactivity, mBq/g supported excess ZlOpbb zloPb 'Be
*5
95 f 3 85 f 3 55 f 2 35 f 2 35 f 3 40 f 2
35 f 7 50f 8 70f7 80f7 90 f 9 40 f 8
130 f 7 115 f 4 80f6 90f7 105 f 9 150 f 12
ratio 137cs
160 f 4 120 f 2 55 f 1 15 f 1 9f1 10 f 2
Surface Sediment at Station S-0102 (Figure 1) 45 f 1 45 f 5 17 f 1 3f1
210Pb-ex/ 239,uoPu
'Be/*loPb-ex
f 0.2
125 f 27 215 f 45 200 f 45 145 f 18 135 f 17 100 f 38
3.7 f 0.9 2.2 f 0.4 1.1 f 0.1 1.1 f 0.1 1.2 f 0.1 3.8 f 0.8
-21.2 f 0.2
150 f 22
0.4 & 0.1
V3C), %a
-26.6 -26.8 -26.8 -22.8 -20.8 -21.2
f 0.2 f 0.2 f 0.2 f 0.2
f 0.2
astatistical counting errors are expressed in terms of lu. Radionuclide activities have been rounded to the nearest 5 mBq if the rounding was within the statistical counting error. bThe supported zloPbwas determined from the 214Pb activity in the sample.
BERYLLIUM-7
m
m IC
50
200
1
(Cl
i
CESIUM-137
e 8 150 E
-$
100
I
50 0
NEAR' SRP
FRESH 10 20 SALINITY '1-
30
40
Flgure 3. Plot of the concentrations of 210Pb-ex,'Be, and 137Cson suspended particles as a function of salinity and location within the Savannah River estuary. (a) As for 23e*240P~ (Figure 2b), 210Pb-exis enriched on estuarine particles relative to its concentration on suspended matter in the Savannah River or in the surface waters of the coastal ocean. (b) In contrast, the concentration of short-lived 'Be on estuarine suspended matter is depleted rattier than enriched. Since the sorption behavior of 7Be is simHar to that of 239-240Pu and 210Pkx, the enrichment of the latter two radionuclides must be a relatively long-term process (years to decades), and the depletion of 7Be may reflect the resuspension of aged bottom sedlments. (c) The sharp decrease in ls7Csconcentration indicates that as much as 95% of the 137Cson riverborne particles may be desorbed during transport into the estuary or that riverborne particles are highly diluted with particulate material produced in situ in the estuary or transported landward from marginal and marine sources.
for 210Pband 2399240Pu (3, 7, 25, 28, 29), this depletion of 7Be implies that the process responsible for enriching 239*240Pu and 210Pb-exon estuarine particles is a relatively long-term one and does not reflect short-term processes such as coagulation of dissolved radionuclides during fresh water and seawater mixing (38) or changes in particle characteristics (composition or grain size) during transport through the estuary. Concentrations of 13'Cs on suspended particles in the Savannah River estuary are listed in Table I11 and are illustrated in Figure 3c. Although the 13Tsconcentration on riverborne suspended particles is a factor of -2 higher than respective concentrations in other river-estuarine
systems along the eastern coastline of the United States (2, 3, 25), the trend of decreasing particulate 137Csconcentrations with increasing salinity (Figure 3c) is typical of most estuarine systems throughout the world. The relatively high concentration of 13Tson Savannah River particles is probably a result of 13Ts releases from SRP and the strong affinity for 13'Cs to become irreversibly sorbed within clay mineral lattices in freshwater systems. Although the kinetics for irreversible sorption are still unknown, previous work in Chesapeake Bay suggests that desorption of 137Csfrom riverborne particles during transport into estuarine areas is not significant enough to account for the order of magnitude decrease illustrated in Figure 3c (25). We suggest that this decreasing trend in particulate 137Csconcentrations with increasing salinity results from the dilution of riverborne particles with particles produced in situ in the estuary, eroded from the estuarine margins, or transported landward from marine sources. Assuming that 137Csdesorption from riverborne particles is relatively insignificant, it would appear that estuarine or marine sources contribute -40% of the particulate material in the upper estuary to 100% in the lower estuary (27). Dilution of riverborne particles by estuarine or marine materials is also supported by the stable carbon isotope data in Table 111. The 6(13C) signature of freshwater particles (-27 %o) clearly indicates a terrestrial watershed source, whereas particles in the estuary have 6(13C)values typical of marine phytoplankton (-21 $Ti). Estimates for the extent of dilution of riverborne organic materials by in situ productivity within the estuarine or coastal environment are complicated, because V3C) values for C-4 plants, such as Spartina alternaflora, which dominate the flora of salt marshes, are considerably heavier (-15 %o) than marine or estuarine phytoplankton. Consequently, we cannot discount the possibility that a combination of terrestrial and salt marsh detritus could be responsible for the particulate 8(13C)values in the Savannah estuary. A much more detailed discussion on the sources for organic and inorganic particles in the Savannah River estuary is presented elsewhere, based on many more lNCs, 6(13C),and 6(15N)analyses for sediment samples recently collected in the estuary (27).
Sediment Resuspension and Radionuclide Enrichment Mechanisms One possible mechanism for depleting the concentration of 7Be and enriching the concentration of oceanic m*240Pu and 210Pb-exon estuarine particles involves radionuclide sorption onto resuspended bottom materials during landward transport into or within estuarine systems. Although plutonium, 210Pb,and 7Be are chemically and particle-reEnviron. Sci. Technol., Vol. 23, No. 12, 1989
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active in oceanic water, the paucity of suspended matter causes these nuclides to remain in solution. For example, when suspended-matter concentrations are less than 1 mg/L (typical of the marine environment), there is lo6 times more water than suspended matter in a liter. As a consequence, only 10% of a particle-reactive radionuclide (having particle to water distribution coefficient or sorption ratio of -lo5) will be associated with the suspended matter and the remaining 90% will be in the water phase. Previous work has shown that more than 90% of the fallout plutonium delivered (primarily in the mid1960s) to the surface ocean is still in the water column, but that dissolved concentrations of plutonium decrease sharply as you approach the coastal zone (10,18). Nutrient loading, and wave and current action in estuarine and coastal areas, increase suspended-matter concentrations by 1-2 orders of magnitude by enhancing biological production and sediment resuspension. We suggest that plutonium, 210Pb,and 7Be are being continually removed from the oceanic water phase by sorption onto particles in turbid estuarine and coastal areas. This removal probably occurs over long time periods as bottom sedimentary materials undergo repeated episodes of resuspension during landward transport on the inner shelf and upstream transport in the estuary. Sorption onto particles during repeated episodes of sediment resuspension would enrich the concentration of long-lived 239i240puand 210Pb-ex on estuarine particles (Figures 2b and 3a) but would dilute the concentration of short-lived 7Be (Figure 3b) by resuspending older materials from the bottom that have been depleted in 7Be by radioactive decay. These results also imply that the ratio of short-lived 7Be to long-lived 210Pb-ex(Table 111)can be used as a new tool for quantifying the amount of resuspended material in the estuarine water column. Because of its short half-life and because of its dilution with older sediment particles, the concentration of 7Be on surface bottom sediments is much less than its concentration on suspended particles, whereas the concentration of long-lived 210Pb-exis nearly the same on fine-grained sediments and suspended matter. The 7Be to 210Pb-exratio on surface bottom sediments in the Savannah estuary is 0.4, which is an order of magnitude lower than the 7Beto 21”Pb-exratio on planktonic-rich suspended matter (-3.8) in the Savannah River and in surface water on the inner shelf (Table 111). Suspended matter in the Savannah estuary has a 7Be to 210Pbratio of 1.0 (Table 111). If we assume that the suspended particles in the estuary is a mixture of plankton (with a 7Be to 210Pbratio of -4.0) and resuspended bottom material (with a ratio, of 0.4),then the measured ratio of 7Be to 210Pb-exon estuarine suspended particles (Table 111) suggests that resuspended bottom material may account for more than 80% of the suspended matter in the estuarine water column. Although this is certainly a rough estimate, it nevertheless illustrates the potential use of this natural radionuclide technique for quantifying the amount of particles produced in the surface water from the amount of particles resuspended from the bottom. In addition, these results illustrate the important contribution that sediment resuspension makes to turbidity within estuarine systems.
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Conclusions
Our results indicate that the input of plutonium and 210Pb-exinto the Savannah estuary via landward transport from marine sources greatly exceeds the input of these two radionuclides via river discharge from drainage-basin sources. We suggest that these radionuclides are being scavenged from oceanic water by sorption onto particles in turbid estuarine and coastal areas. Scavenging, in as1480
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sociation with repeated episodes of fine-particle deposition and resuspension, will deplete the concentration of 7Beand enrich the concentration of 239mPuand 21”Pb-ex(over long time periods) on suspended particles and sediments. Net landward transport along the bottom traps inorganic and organic materials from marine sources and causes coastal bays and estuaries to serve as major deposition and accumulation sites for marine, as well as riverborne substances. These results are consistent with previous studies that have shown elevated 2399240P~ and 210Pb-exconcentrations and inventories in fine-grained sediments along the coastlines of England (5), France (11), and various other locations around the world (!9-20). Since other chemically or particle-reactive trace substances may also be scavenged from oceanic waters by sorption in turbid coastal areas and, subsequently, transported landward to accumulate within estuarine systems, it is important to quantify the magnitude of this process in models that simulate the transport and biogeochemical fate of river inputs to the ocean. To date, inputs of carbon, synthetic organic compounds, trace metals, and radionuclides from oceanic sources have largely been ignored in models simulating the fate of contaminants in estuaries. In addition, these results have extremely important implications for the landward transport and accumulation of chemically reactive substances disposed of in oceanic waters off coastlines affected by a rising sea level. Acknowledgments
We thank E. D. Goldberg of Scripps Institute of Oceanography, G. R. Helz of the University of Maryland, P. Santschi of Texas A&M University, K. L. Von Damm and T.-H. Peng of Oak Ridge National Laboratory, and G. Saunders of the U. S. Department of Energy for their reviews and comments on earlier drafts of this paper. C.R.O. would also like to thank H.J. Simpson of Columbia University and R. F. Bopp of Barnard College for their advice regarding the use of geochemical tracers for estuarine processes. Registry No. 238Pu,13981-16-3; 239Pu,15735-80-5; 240Pu, 14119-33-6;137Cs,10045-97-3;210Pb,14255-04-0;7Be,13966-02-4; 40K, 13966-00-2;13C,14762-74-4. Literature Cited River Inputs to Ocean S y s t e m ; Martin, J.-M., Burton, J. D., Eisma, D., Eds.; United Nations Environment Pro-
gramme: Switzerland, 1981. Bopp, R. F.; Simpson, H. J.; Olsen, C. R.; Trier, R. M.; Kostyk, N. Enuiron. Sci. Technol. 1982, 16, 666-676. Simpson, H. J.; Linsalata, P.; Olsen, C. R.; Cohen, N.; Trier, R. M. In Environmental Research on Actinide Elements; Pinder,J. E., 111,Alberta,J. J., McLeod, K. W., Schreckhise, R. G., Eds.; U S . Department of Energy: Springfield,VA, 1987; CONF-841142, pp 273-297. Smith, J. N.; Ellis, K. M.; Nelson, D. M. Chem. Geol. 1987, 63, 157-180. Peirson, D. H.; Cambray, R. S.; Cawse, P. A.; Eakins,J. D.; Pattenden, N. J. Nature 1982, 300,27-31. Meade, R. H. In Framework of Coastal Plain Estuaries; Nelson, B. W., Ed.; Geology -. Society of America: Boulder, CO,1972; pp 91-120. Olsen, C. R.; Simpson, H. J.; Trier, R. M. Earth Planet. Sci. Lett. 1981. 55. 377-392. Sinex, S. A.; Helz, G. R. Enuiron. Sci. Technol. 1982,16, 820-825. Goldberg, E. D.; Griffin, J. J.; Hodge, V.; Koide, M.; Windom, H. L. Enuiron. Sci. Technol. 1979, 13, 588-594. Buesseler, K. 0.;Livingston, H. D.; Sholkovitz, E. R. Earth Planet. Sci. Lett. 1986, 76, 10-22. Jeandel, de C.; Martin, J.-M.; Thomas, A. J. C. R. Acad.
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Aston, S. R.; Stanners, D. A. Nature 1981,289, 581-582. Beasley, T. M.; Carpenter, R.; Jennings, D. C. Geochim. Cosmochim. Acta 1982,46, 1931-1946.
Scott, M. R.; Salter, P. F.; Halverson, J. E. Earth Planet. Sci. Lett. 1983, 63, 202-222.
Santachi, P. H.; Li, Y.-H.;Bell, J. J.; Trier, R. M.; Kawtaluk, K. Earth Planet. Sci. Lett. 1980, 51, 248-265. Aston, S. R.; Assinder, D. J.; Kelly, M. Estuarine, Coastal Shelf Sci. 1985, 20, 761-771. Smith, J. N.; Boudreau, B. P.; Noshkin, V. Earth Planet. Sci. Lett. 1987, 81, 15-28. Sholkovitz, E. R.; Mann, E. R. Estuarine, Coastal Shelf Sci. 1987, 25, 413-434. Linsalata, P.; Wrenn, M. E.; Cohen, N.; Singh, N. P. Enuiron. Sci. Technol. 1980, 14, 1519-1523. Kuehl, S. A.; DeMaster, D. J.; Nittrouer, C. A. Cont. Shelf Res. 1986, 6 , 209-225. Hayes, D. W.; Sackett, W. M. Estuarine, Coastal Shelf Sci. 1987, 25, 169-174.
Pinder, J. E., 111; Paine, D. In Transuranic Elements in the Enuironment; Hanson, W. C., Ed.; U.S. Department of Energy: Springfield,VA, 1980; pp 165-172. Alberts, J. J.; Halverson, J. E.; Orlandini, K. A. J.Environ. Radioact. 1986, 3, 249-271. Hardy, E. P.; Krey, P. W.; Volchok, H. L. Nature 1973,241, 444-445.
Olsen, C. R.; Larsen, I. L.; Lowry, P. D.; McLean, R. I.; Domotor, S. L. Radionuclide Distributions and Sorption Behavior in the Susquehanna-Chesapeake Bay System. Maryland Power Plant and Environmental Review PPER-R-12; Annapolis, MD, 1989. Marter, W. L. Radioactivity from SRP Operations in a Downstream Savannah River Swamp. Atomic Energy Commission Report DP-1370. E. I. du Pont de Nemours Co.: Aiken, SC, 1974.
Mulholland, P. J.; Olsen, C. R. submitted for publication in Estuarine, Coastal Shelf Sci. Krishnaswami, S.; Benninger, L. K.; Aller, R. C.; Von Damm, K. L. Earth Planet. Sci. Lett. 1980,47,307-318. Olsen, C. R.; Larsen, I. L.; Lowry, P. D.; Cutshall, N. H.; Nichols, M. M. J. Geophys. Res. 1986, 91, 896-908. (30) Craig, H. Geochim. Cosmochim. Acta 1953, 3, 53-92. (31) Gearing, J. N.; Gearing, P. J.; Rudnick, D. T.; Requejo, A. G.; Hutchins, M. J. Geochim. Cosmochim. Acta 1984,48, 1089-1098. (32) Hedges, J. I.; Parker, P. L. Geochim. Cosmochim. Acta 1976, 40, 1019-1030. (33) Krom, M. D.; Bennett, J. T. Estuarine, Coastal Shelf Sci. 1985,21, 325-326. (34) Turekian, K. K.; Benoit, G. J. In Flux of Organic Carbon
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Received for review November 9,1988. Accepted June 29,1989. This research was sponsored by the Ecological Research Division, Officeof Health and Environmental Research, U S . Department of Energy, under contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Znc., and under contract DE-FGOS86-ER60435 with Skidaway Institute of Oceanography. Publication No. 3341, Environmental Sciences Division, Oak Ridge National Laboratory.
Modeling and Field Evidence of Pressure-Driven Entry of Soil Gas into a House through Permeable Below-Grade Walls Karina Garbed' and Richard G. Sextro Indoor Environment Program, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Modeling and field evidence are presented that indicate that soil gas can enter houses with basements at significant rates through permeable below-grade walls. Entry via this pathway could result in elevated indoor concentrations of radon and other pollutants. By use of artificial depressurization of the basement (-25 to -30 Pa), field measurements were made of pressure coupling between a basement and the surrounding soil and of soil-gas entry into the house. A two-dimensional, steady-state finite element model of fluid flow through porous media was used to simulate the experimental conditions, assuming air flow occurs through permeable substructure walls. Given a basement wall permeability consistent with prior experimental research, the model predicts 32 5% pressure coupling 0.5 m from the basement wall at a depth of 0.5 m, in agreement with pressure coupling measured a t the site. Under the same conditions the model predicts a soil-gas entry rate of 2.5 m3 h-l, within the range estimated by tracer-gas measurements. The presence of a horizontal, low-permeability soil layer just above basement floor level explains the high pressure coupling observed at 3-m depth even out to 14 m west of the house. Introduction
Soil gas is an important source of indoor air pollution. Research on sources of human exposure to radon indicate 0013-936X/89/0923-1481$01.50/0
that soil is the primary source of indoor radon in singlefamily houses in the United States ( I ) . Pressure-driven flow is a principal means by which soil gas enters houses; it is expected to be the predominant source of radon in houses with elevated concentrations (2-4). Recent studies indicate that entry of volatile organic contaminants via the soil-gas pathway could pose a public health risk in residences located near landfiis, even those designed to accept only nonhazardous waste (5, 6 ) . Pressure-driven flow of soil gas into houses results from the depressurization of the substructure of the hquse with respect to the surrounding soil. There are three principal causes of basement depressurization: thermal differences between indoors and outdoors, wind loading on the building superstructure, and imbalanced building ventilation ( 2 , 4 ) . Field measurements have shown that under normal operating conditions of houses during the winter the temperature effect alone can result in consistent substructure underpressures of between 2 and 6 Pa (7, 8). Other factors being equal, pressure-driven entry is likely to be most important in houses with basements because they provide a large interface with the soil. Soil-gas entry due to basement depressurization has been experimentally demonstrated by Turk et al. (9) and Nazaroff et al. (10). Significant pressure-driven entry of radon from soil has also been reported for houses with crawl spaces (11). Entry pathways have been assumed to be penetrations, gaps, or
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