Environ. Sci. Technol. 2005, 39, 4894-4899
Enrichment of Excess 210Po in Anoxic Ponds G U E B U E M K I M , * ,† S U - J I N K I M , † KOH HARADA,‡ MICHAEL K. SCHULTZ,§ AND WILLIAM C. BURNETT| School of Earth and Environmental Sciences/RIO, College of Natural Sciences, Seoul National University, Seoul 151-742, Korea, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, Radioactivity Group, National Institute of Standards and Technology, United States Department of Commerce, Building 245, Room C114, Gaithersburg, Maryland 20899, and Environmental Radioactivity Measurement Facility, Department of Oceanography, Florida State University, Tallahassee, Florida 32306-3048
We have investigated the cycling of naturally occurring 210Po in waters from seasonally anoxic Pond B (South Carolina) and permanently anoxic Jellyfish Lake (Palau Islands, western Pacific Ocean). The maximum 210Po activity in Pond B was about 14 mBq L-1 during summer. This activity was much higher than its parent 210Pb activity, indicating excess 210Po inputs from bottom sediments. The summertime excess 210Po activity was accompanied by increases in Fe and Mn, suggesting 210Po diffusion from bottom sediments under reducing conditions. Activity of 210Po was much lower under winter oxic conditions, most likely a consequence of efficient coprecipitation with Fe and Mn oxides that occurs with destruction of Pond B stratification. In permanently anoxic Jellyfish Lake, the maximum activity of 210Po was 133 mBq L-1, among the highest reported from any surface aqueous environment. A box model suggests that the release of only 2% of 210Po, produced from the 210Pb in the bottom sediments, would account for the observed excess. Our results suggest that 210Po can be naturally enriched in anoxic environments to a high level, perhaps in concert with the Fe and Mn redox cycles.
Introduction An R-emitting radionuclide in the 238U decay series, 210Po (t1/2 ) 138 days) is the decay product of 210Pb (t1/2 ) 22.3 years) via 210Bi (t1/2 ) 5 days). Since the residence time of aerosols is shorter than 10 days in the atmosphere (1-4), 210Po produced from 210Pb is negligible in precipitation or fallout particles (210Po/210Pb activity ratio 97%) rather than directly from intake of seawater. A 210Po-sulfur covalent bond may be formed in the hepatopancreas of marine invertebrates (23). High enrichment of radioactive 210Po can have a deleterious effect on the health of living organisms (19, 20, 24, 25). Thus, it is important to understand the processes that influence the geochemical cycle of poloniumsespecially processes that may result in a large increase in the dissolved Po species. We attempt here to elucidate the mechanism of a high enrichment of 210Po in anoxic aqueous environments.
Experimental Procedures Sampling Sites. This study was conducted in two anoxic water environments, Pond B and Jellyfish Lake. Pond B is a seasonally anoxic artificial pond, located in South Carolina. This pond was constructed in 1961 and had been used as a cooling reservoir for a nuclear production reactor until 1964. Since there has been no public access to Pond B, located within the U.S. Department of Energy’s Savanna River Site (an access-restricted area), the pond is quite undisturbed. The surface area of the pond is approximately 0.81 km2, with a maximum depth of about 12 m and a mean depth of 4.4 m. There are no permanent streams entering the pond from the surrounding 3.5 km2 watershed, and runoff events are rare due to well-drained sandy soils. Jellyfish Lake is a meromictic-marine lake, comprising permanently stratified oxic (surface) and anoxic (bottom) layers. The lake is located on the uninhibited island of Eil Malk, Palau Islands in the western Pacific Ocean. The water depth of the lake is approximately 30 m, with an area of 0.04 km2. The lake is in continuous contact with seawater by way of infiltration through underground channels in the porous limestone upon which the island is built (26). The surface layer has extremely high biological productivity and a large population of a restricted member of the metazoans (27). A layer 1-2 m thick of purple pigmented, probably phototrophic, and sulfur-oxidizing bacteria exists at the chemocline (28). The benthic environment is characterized by permanently anoxic bottom water, with high H2S concentrations (up to 2.5 mM) and sediments consisting of high organic matter contents (29). Because Jellyfish Lake is a relatively controlled natural environment having a stable climate, extreme anoxic conditions, and restricted inputs and outputs, it seems to be an ideal environment to 10.1021/es0482334 CCC: $30.25
2005 American Chemical Society Published on Web 05/28/2005
TABLE 1. Activity of
210Po
in Various Aquatic Environments
location
water type
depth (m)
Atlantic Ocean Pacific Ocean Indian Ocean North Sea Framvaren Fjord Jellyfish Lake Lake Sammamish Crystal Lake Bickford Pond Pond B
oxic seawater oxic seawater oxic seawater oxic seawater permanently anoxic seawater permanently anoxic seawater seasonally anoxic freshwater seasonally anoxic freshwater seasonally anoxic freshwater seasonally anoxic freshwater
0-4200 0-5420 0-5587 5-45 0-30 0-30 0-32 0-20 0-12 0-12
210Po
(mBq L-1)
1-2 1-5 1-2 1-5 0-17 1-133 0-3 1-5 1-17 1-14
ref 44 17 47 48 35 this paper 40 49 34 this paper
FIGURE 1. Depth-time isopleths of temperature (a) and dissolved oxygen concentrations (b) in Pond B for the year 2000. Depth profiles of Fe (c) and Mn (d) in Pond B in August 1999 and February 2000. investigate the geochemical cycling of redox-sensitive elements (30). Methods. In Pond B, water temperature, dissolved oxygen (DO), and bulk Fe and Mn concentrations were measured bimonthly. The concentrations of Fe and Mn were measured with an Elan 5000 inductively coupled plasma mass spectrometer (ICP-MS, Perkin-Elmer Instruments, Thornhill, Ontario, Canada). About 20 L water samples were taken for 210Po and 210Pb analyses from Pond B and Jellyfish Lake. The samples were acidified (pH < 1) with nitric acid, and then the 209Po yield tracer (1 Bq), Pb carrier (30 mg), and Fe3+ carrier (50 mg) were added. After vigorous stirring for 1 h for equilibration of the spike and carriers, samples were allowed to stand for an additional duration of 4 h to ensure complete chemical equilibrium of tracers, carriers, and analytes. Po and Pb were coprecipitated with Fe(OH)3 at pH ∼8 by adding NH4OH. After allowing the precipitate to settle for 4-5 h, the supernatant was siphoned off, and the residual mixture was filtered through Whatman 54 quantitative grade paper. The precipitate was dissolved in 0.5 N HCl, and the solution was transferred to a Teflon beaker. After adding 0.5 g of ascorbic acid, the solution was heated to 90 °C, and Po was spontaneously plated onto a silver disk. The remaining solution was purified for Pb using an anion exchange column and then was stored for more than 6 months for 210Po ingrowth from the 210Pb (31). This 210Po was then measured to determine the 210Pb activity. The chemical yield of Pb was determined by measuring the stable Pb recovery on a flame atomic absorption spectrometer. Further details of the analytical methods and calculations can be found in Kim et al. (31). Sediments from Jellyfish Lake were collected by a gravity corer (core #7) and diver-operated coring device (core #8). The upper 10-13 cm of core #7 was lost during collection by the gravity corer. After sectioning the cores, sediment samples were dried, ground, and subjected to 210Pb analysis.
The 210Pb in sediments was measured by the methods of McCabe et al. (32) and Narita et al. (33).
Results and Discussion The activity of 210Po in normally oxic waters ranges from 1 to 5 mBq L-1 and is higher, up to 17 mBq L-1, in seasonally anoxic ponds such as Pond B (this paper), Bickford Pond (34), and permanently anoxic seawater such as the Framvaren Fjord (35) (Table 1). In permanently anoxic Jellyfish Lake, we found that the maximum activity of 210Po was 133 mBq L-1, among the highest found in natural waters, except for some groundwater (15). Thus, high enrichment of 210Po in living biota (i.e., jellyfish) may occur in this environment since concentration factors of 210Po generally increase through the food web (18-22). Po Cycling in Seasonally Anoxic Pond B. Pond B waters are slightly acidic (pH 5-7), and the conductivities increase from the surface waters (20-30 µS cm-2) to the bottom anoxic waters (140 µS cm-2) (36, 37). Because of summer stratification, the bottom layer becomes anoxic from May to October (Figure 1a,b). In the summer anoxic season, the concentrations of Fe and Mn increase sharply from the surface to the bottom layer (Figure 1c,d). This is likely due to dissolution of metal oxides and oxyhydroxides (if present) of Fe and Mn in the reducing bottom sediments, followed by diffusion to the overlying water column (38, 39). The contrast between the seasons has a dramatic effect on the distribution of 210Po in the water column. In summer, the activity of 210Po increases sharply from the surface to the bottom layer, with a maximum activity of 14 mBq L-1, while it is vertically uniform and low in winter (Figure 2). The 210Po activity was much higher than 210Pb activity, indicating excess 210Po inputs from bottom sediments. It has been reported that Po (IV) is insoluble and is reduced to Po (II) near the potential that Mn (IV) is reduced to Mn (II) (34, 40). Therefore, redox sensitive elements, such as Po, S, Fe, and Mn, appear VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Vertical profiles of 210Po and 210Pb activity in the Pond B water column. to diffuse into Pond B bottom waters from sediments under summer anoxic conditions by forming reduced soluble species. In contrast, Fe and Mn oxides/oxihydroxides that settle under winter oxic conditions may serve as an effective carrier of Po. Differences in the shapes of the metal vertical profiles may be due to chemical fractionations. For example, Mn is reduced to the divalent state (from Mn4+) at higher oxygen saturation (similar ambient conditions, pE ∼11) values than Fe in freshwater systems (similar ambient conditions, pE ∼1) (41). Alternatively, the cycling of Po may be associated with microbial activity in the bottom sediment. It has been suggested based on laboratory experiments that Po is highly enriched in sulfur-enriched bacteria and often forms methylated species (7, 12, 16). In Jellyfish Lake, H2S concentrations increase while SO42- concentrations decrease toward the lake bottom (29). Recent work on Mn also suggests that this species may be closely coupled to the diagenetic cycling of sulfur (42). Therefore, high 210Po activity in the water column could be due to efficient regeneration of 210Po in the bottom sediment through microbial activities and then diffusion and advection to the upper water column. In contrast, 210Pb is vertically uniform in Pond B and does not increase in the anoxic layer. Schultz (43) also showed similar trends for Cs, Al, Na, and Cu. Therefore, it is likely that these elements are not significantly affected by the seasonal changes in redox conditions. To calculate summer-winter turnover fluxes of 210Po between the water column and sediments, we constructed a simple box model. The change of 210Po in the water is given by
dPo ) IPo + PbλPo - PoλPo - Pok dt
(1)
where IPo is the atmospheric depositional flux of 210Po, Po (Bq m-2) and Pb (Bq m-2) are the inventories of total 210Po and 210Pb in the water column, respectively, λPo (1.83 year-1) is the decay constant of 210Po, and k is the turnover rate constant (downward scavenging constant or upward diffusion constant) of 210Po (44). If the flux of atmospheric 210Po is negligible in eq 1, the turnover fluxes (Bq cm-2 year-1) of 210Po (PPo) are expressed as follows:
PPo ) λPo
[Pb(1 - e-λPot) + Pot1e-λPot - Pot2] 1-e
-λPot
(2)
where Pot1 and Pot2 are the inventory of 210Po for the summer (t1) and winter (t2) observations, respectively. In this equation, the 210Pb inventory is from the average of 210Pb at the first (t1) and second (t2) observations, t is the interval (t2-t1) of 6 months. The estimated PPo is about 18 mBq cm-2 year-1, 4896
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FIGURE 3. Vertical profile of water column.
210
Po activity in the Jellyfish Lake
FIGURE 4. Vertical profiles of 210Pb activity in Jellyfish Lake sediment column. which is a small fraction of the sedimentary 210Po inventory (380 mBq cm-2) produced from the 210Pb (annual atmospheric deposition of 210Pb is about 12 mBq cm-2 in the eastern U.S.; ref 45). This indicates that the actual amount of labile 210Po fluxing into the water column is a small fraction of the 210Po pool in the bottom sediment. Po Enrichment in Permanently Anoxic Saline Lake. The enrichment mechanism of 210Po in Jellyfish Lake may be similar to that in Pond B as discussed previously. However, Jellyfish Lake may have a greater 210Pb sediment inventory because of its larger drainage area to lake area ratio as compared to Pond B and thus would have more labile 210Po (Figure 3). To determine the reason for the significantly higher 210Po enrichment in Jellyfish Lake, we estimated the inventory of 210Pb, the parent of 210Po, in the sediment column. The activity of 210Pb decreases systematically from the top to the bottom of the core samples (Figure 4). Using the decreasing exponential shape of excess 210Pb in the sediment, the sediment accumulation and average mass accumulation rates were determined to be 0.70 cm year-1 and 0.05 g cm-2 year-1, respectively. The inventory of 210Pb in surface sediment may be expressed as follows:
I)
∫A D
0
0
( DS)dD
exp -λ
(3)
where I is the inventory of 210Pbex in the sediment (Bq cm-2), D is the cumulative mass of the sediment (g cm-2), A0 is the initial concentration of 210Pbex (Bq g-1), λ is the decay constant of 210Pb (year-1), and S is the sedimentation rate (g cm-2 year-1). The 210Pb inventory may also be expressed as follows
FIGURE 5. Schematic diagram illustrating the fluxes of 210Pb and 210Po in Jellyfish Lake. Fluxes include atmospheric deposition, inflow from surrounding area, ingrowth of 210Po in sediments, and diffusion/advection of 210Po from underlying anoxic sediments. Kz denotes the eddy diffusivity of the anoxic layer, and 210Pb inv. denotes the inventory of excess 210Pb in the sediment column. using the average concentration of sediment.
210Pb
(Bq g-1) in the
I ) average concentration × D
(4)
Using eqs 3 and 4, A0 was estimated to be about 3.6 Bq g-1, and the resulting flux of 210Pb from the lake water to the bottom sediment is determined to be approximately 0.33 Bq cm-2 year-1 (Figure 5), which is much greater than the atmospheric flux of approximately 17 mBq cm-2 year-1 (4, 46). Thus, the 1 order of magnitude higher inventory of 210Po (11 Bq cm-2) in Jellyfish Lake water as compared to Pond B is consistent with an order of magnitude higher 210Pb inventory in the bottom sediment. Sources of 210Pbex to the bottom sediment can be divided into (1) direct deposition from the atmosphere to the surface of the lake, (2) removal of 210Pb generated from 222Rn in the water column and sediment pore solutions, (3) removal of 210Pb transported from the open ocean into the lake by water exchange, and (4) input of 210Pb deposited on the surrounding area from the atmosphere to this area with subsequent transport of 210Pb-bearing particles into the lake. The deposition rate of 210Pb from the atmosphere in this area is approximately 17 mBq cm-2 year-1 (4, 46), much lower than the calculated flux from the sedimentation rates and inventory. The supply of 210Pb from 222Rn cannot be estimated since we did not measure 226Ra and 222Rn in the water column. However, if production of 210Pb is fully supported by the 222Rn in the water column, the average activity of 222Rn would have to have been about 14 Bq L-1, which is very unlikely. Therefore, the 210Pbex is most likely from sources (3) and (4). We suggest that the high rainfall of this region, together with the presence of a large drainage basin, causes a focusing of 210Pb from the surrounding countryside into Jellyfish Lake. We can calculate the upward flux of 210Po from sediment by a simple budget calculation. The shape of the profile (Figure 3) clearly indicates that the source of 210Poex is from bottom sediment by diffusion and/or advection. While Po in most natural waters is a particle-reactive element, it apparently exists in a more soluble form in the anoxic portion of Jellyfish Lake. For the purpose of the calculation, we assume conservative behavior for Po (i.e., we consider that the only
sinks for the soluble form of Po are radioactive decay and mixing with the overlaying water). The highly stratified nature of Jellyfish Lake suggests that mixing is insignificant. Therefore, in steady-state conditions, radioactive decay of 210Po in the water column should just balance the flux from the bottom sediment. On the basis of the inventory of 210Po in the anoxic layer (0.1 Bq cm-2), we estimate that the upward diffusive/advective flux of 210Po from the bottom sediment is approximately 0.18 Bq cm-2 year-1. The production rate of 210Po in the sediment is estimated to be about 9 Bq cm-2 year-1 based on the sedimentary inventory of 210Pb (Figure 5). Thus, only 2% of 210Po, produced from the 210Pb in sediments, could fully account for the excess 210Po observed in the overlying water column. The benthic fluxes of 210Po to the overlying column may be controlled either by diffusion or by porewater advection due to tidal pumping of groundwater. Further studies would be required to determine the main mechanisms of 210Po transport from bottom sediments using direct measurement tools such as benthic chambers. Overall, our results suggest that 210Po can be naturally enriched in anoxic environments to a very high level. If we assume that the 210Po in the water is entirely from the bottom sediment, the eddy diffusivity of bottom waters (between 15 and 30 m) can be calculated on the basis of the vertical gradient of 210Po in the water column using the following equation:
C2 ) C1 exp[-z(λ/Kz)1/2]
(5)
where C1 and C2 are the concentrations at the two depths, z is the depth interval, λ is the decay constant of 210Po, and Kz is the vertical eddy diffusivity. With these assumptions, the vertical eddy diffusivity was measured to be ∼0.2 cm2 s-1 between 15 and 30 m, confirming high stratification of the Jellyfish Lake bottom layer (Figure 5).
Acknowledgments The preparation of this manuscript was supported by a research grant from the KOSEF/KRF (R08-2003-000-103280), Korea. Financial support for the field programs in Palau VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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was provided by grants to W.C.B. from ACS-PRF and Earthwatch. Financial support for field sampling at Pond B was supported by Financial Assistance Award Number DE-FC09-96SR18546 from the U.S. Department of Energy to the University of Georgia Research Foundation. Many thanks to Tom Hinton and the staff of the Savannah River Ecology Laboratory for assistance with field sampling at Pond B.
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Received for review November 12, 2004. Revised manuscript received April 25, 2005. Accepted April 27, 2005. ES0482334
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