Impact of Environmental Curium on Plutonium Migration and Isotopic Signatures

Impact of Environmental Curium on Plutonium Migration andIsotopic SignaturesHiromu Kurosaki,†,‡ Daniel I. Kaplan,§ and Sue B. Clark*,†

†Department of Chemistry, Washington State University, Post Office Box 644630, Pullman, Washington 99164, United States§Savannah River National Laboratory, Aiken, South Carolina 29808, United States

*S Supporting Information

ABSTRACT: Plutonium (Pu), americium (Am), and curium (Cm)activities were measured in sediments from a former radioactive wastedisposal basin located on the Savannah River Site, South Carolina, and insubsurface aquifer sediments collected downgradient from the basin. Insitu Kd values (Pu concentration ratio of sediment/groundwater) derivedfrom this field data and previously reported groundwater concentrationdata compared well to laboratory Kd values reported in the literature. Puisotopic signatures confirmed multiple sources of Pu contamination. Theratio of 240Pu/239Pu was appreciably lower for sediment samplescompared to the associated groundwater. This isotopic ratio differencemay be explained by the following: (1) 240Pu produced by decay of244Cm may exist predominantly in high oxidation states (PuVO2

+ andPuVIO2

2+) compared to Pu derived from the disposed waste effluents,and (2) oxidized forms of Pu sorb less to sediments than reduced forms of Pu. Isotope-specific Kd values calculated frommeasured Pu activities in the sediments and groundwater indicated that 240Pu, which is derived primarily from the decay of244Cm, had a value of 10 ± 2 mL g−1, whereas 239Pu originating from the waste effluents discharged at the site had a value of 101± 8 mL g−1. One possible explanation for the isotope-specific sorption behavior is that 240Pu likely existed in the weaker sorbingoxidation states, +5 or +6, than 239Pu, which likely existed in the +3 or +4 oxidation states. Consequently, remediation strategiesfor radioactively contaminated systems must consider not only the discharged contaminants but also their decay products. In thiscase, mitigation of Cm as well as Pu will be required to completely address Pu migration from the source term.

■ INTRODUCTION

Among anthropogenic radioactive contaminants, Pu typicallygenerates much interest. It can be derived from global fallout aswell as localized inputs resulting from various industrialactivities, such as production of heat sources for deep spaceexploration, waste management practices associated withnuclear energy production, and the development and testingof military devices. The isotopic ratio 240Pu/239Pu can be oneindicator of its origin and purpose of production.1,2 Also, 239Puand 240Pu are radiogenic progeny of isotopes with much shorterhalf-lives, e.g., 243Cm (half-life = 29.1 years) and 244Cm (half-life = 18.1 years), respectively. Therefore, when these isotopesof Pu and Cm coexist in an environmental system, the240Pu/239Pu isotopic ratio changes with time. The Pucontamination in environmental systems is frequently derivedfrom multiple sources,3 although estimates of its transport andmigration for environmental assessment purposes may notconsider this fact. In a different area of environmentalradiochemistry, sometimes called isotope geochemistry ornuclear forensics, the question of multiple source terms isexplicitly considered and isotopic signatures are used toattribute Pu in the groundwater, soil, or sediment to specificsources.4−6 Knowledge of both the total Pu concentration andits isotopic composition are necessary for estimating Pu

ecological risk, because the activities of each isotope contributedifferently to the total risk and, overall, long-term risk can bedetermined by radiogenic relationships.7

Actinide activities and isotopic ratios in F-Area groundwaterlocated in the Savannah River Site (SRS) have been reported.1,8

Two sources of 240Pu were reported: one source is from thedecay of 244Cm, and the second source is from the directdischarge of 240Pu in process effluents from the F-Area chemicalseparation facility (referred to as weapons-grade Pu). Unlike244Cm, very little 243Cm is found in SRS waste effluents and thelocal environment because the quantities produced in the SRSreactors are very small relative to 244Cm. On the basis of theobservation that there was proportionally greater 240Pu than239Pu further downstream of the source than closer to thesource, it was hypothesized by Buesseler et al.1 that most of thedownstream 240Pu originated from the decay of the relativelymore mobile 244Cm. They also hypothesized that thedownstream 240Pu had a higher oxidation state than the 240Puoriginating directly from the discharged waste, which was

Received: February 25, 2014Revised: September 7, 2014Accepted: October 28, 2014Published: October 28, 2014

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© 2014 American Chemical Society 13985 dx.doi.org/10.1021/es500968n | Environ. Sci. Technol. 2014, 48, 13985−13991

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predominantly weapons-grade Pu. The mechanism by whichthe +5 or +6 oxidation states predominate involves electronstripping after decay and is referred to as the Szilard−Chalmersprocess.8 Another interpretation of their observations is that themigration of trivalent Cm occurred prior to radioactive decay,and the observed 240Pu downgradient of the basin is the resultof greater Cm than Pu mobility and subsequent radioactivedecay to the Pu daughter isotope rather than the mobility of thePu itself. It is not clear which mechanism predominates. In theenvironmental assessment field, the notion that the source ofPu makes a difference in Pu mobility has been referred to as“source-dependent” geochemical behavior.9

The objective of this study was to evaluate whether the Puisotopes in the F-Area partitioned between the aqueous andsolid phases in a manner consistent with source-dependentgeochemical behavior, i.e., that 240Pu would tend to exist in theaqueous phase [presumably as Pu(V/VI)] and 239Pu wouldtend to exist in the solid phase [presumably as Pu(III/IV)]. Inthis field study, Pu, Am, and Cm activities as well as Pu isotopicratios were measured in sediments collected from in or near theF-Area seepage basins. Using our sediment concentrationresults along with groundwater concentration data reported byBuesseler et al.,1 in situ distribution coefficients (Kd values, theratio of the Pu concentration in sediment versus groundwater)were calculated. In addition, the distribution of the Pu and Cmisotopes after decay correction provided additional detail aboutmultiple sources of Pu and the migration of 240Pu in thissystem. These results are presented and discussed in thecontext of contaminant source attribution and radiological risk.

■ MATERIALS AND METHODSMaterials. Sediments. Sediments from two Department of

Energy facilities were used in this study. One set of sedimentswas collected from the Idaho National Laboratory (INL) site,adjacent to a radioactive waste disposal area called theSubsurface Disposal Area (SDA). These sediments wereanalyzed previously for 241Am, 244Cm, 239Pu, and 240Pu10 andwere used to validate the highly sensitive (sub-femtomoles pergram of soil) analytical technique applied in this study. Twodifferent INL sediments were studied. One was collected from adepth of 0−4 cm adjacent to the SDA, near where wastes fromRocky Flats were disposed. This sample was contaminated withknown quantities of 241Am and 239 + 240Pu11 and was used tovalidate the radioanalytical procedures employed. Unfortu-nately, no reference sediments containing Cm or a samplecontaining known quantities of Cm were available. Con-sequently, a method validation sediment was prepared byspiking an INL sediment with a known amount of 244Cm, asdescribed in the Methods section. This sediment was ahomogenized mixture of sediments collected from five differentlocations 1.3 km away from the SDA. This sediment had nodetectable Am or Pu.12

The focus of this study involved the second set of samplesthat was obtained from SRS, a Department of Energy facility inSouth Carolina. These were the samples used to study Pumigration, and they were collected in and around the F-Areaseepage basins (Figure 1). The seepage basins were unlined pitsused between 1955 and 1988 to manage low-level wasteeffluents from a radiochemical separation plant at the SRS.13 Asa result of this waste management practice, small quantities ofactinides, fission products, and tritium, along with nitrate, andvarious mineral acids were released to the unlined basins andallowed to seep into the vadose zone. Approximately 12 Ci

(444 GBq) of 239 + 240Pu, 0.23 Ci (8.51 GBq) of 241Am, and0.35 Ci (12.95 GBq) of 244Cm were released to the basinsduring the 30+ years of operation.13 As will be described inmore detail below, the 239 + 240Pu and 241Am concentrationswere not decay-corrected because of their long half-lives (>400years), but 244Cm concentrations were corrected to the year ofgreatest release to the seepage basins, 1973. Five sedimentsamples were collected from the F-Area, one directly from theseepage basin bottom (prior to closure in 1988) and the otherfour from well borehole samples that were 15−40 m below thesurface (Figure 1). The acidic and oxic plume emanating fromthe seepage basins form a pH (pH 3.20−6.77) and Eh (654−381 mV) gradient as it travels 0.7 km before resurfacing at FourMile Branch (Figure 1).14

Chemicals. 243Am (Eckert & Ziegler Isotope Products,Valencia, CA) was used as a chemical yield tracer. Similarly,242Pu (NIST SRM4334H, National Institute of Standard andTechnology, Gaithersburg, MD) was used as a chemical yieldtracer for Pu isotopes. 244Cm was obtained from Eckert &Ziegler Analytics (Atlanta, GA). All reagents were analytical-grade. Water was purified with a LABCONCO Water Pro PSsystem (Kansas City, MO). Radioanalytical separations usedTEVA and TRU resins (Eichrom Technologies, Inc., Lisle, IL)and 2 mL plastic columns (BioRad, Hercules, CA).

Methods. Preparation of 244Cm-Spiked Sediment Sam-ples. A known amount of 244Cm solution was added to 10 g ofthe INL sediment (0−0.04 m depth) and then permitted to air-dry completely. Care was taken to minimize the loss of 244Cm

Figure 1. Study area (SRS F-Area) showing the three seepage basins,the five sediment sample locations, sediment and groundwater240Pu/239Pu isotopic ratios, and general groundwater flow direction.

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because of sorption onto the beaker by limiting the quantity of244Cm solution used and taking precaution in minimize contactbetween the added Cm solution and the beaker. Oncecompletely dry, the 10 g aliquot of the spiked 244Cm samplewas mixed with approximately 500 g of bulk sediment from thesame source in a blender for several hours. After mixing, thesediment was analyzed using the procedure described below;measured values were compared to calculated values based onthe amount of radionuclide spiked into sediment. In addition,different portions of sediment (top, middle, and bottom of thebottle) were analyzed to demonstrate homogeneity of thespiked sediment sample.Sediment Fusion. Sediment samples were weighed in

platinum crucibles and then spiked with 243Am and 242Putracer solutions. Sediments were wet-ashed 3 times withconcentrated nitric acid and 3 times with concentratedhydrofluoric acid. The ashed materials were then coveredwith 10 g of potassium fluoride and fused over a Bunsenburner. Once clear melts were obtained, the cake was cooled toroom temperature. Approximately 15 mL of sulfuric acid wasadded to the cake to dissolve it (with heat if necessary),followed by 10 g of sodium sulfate. This mixture was heatedwith the burner until a transparent orange melt was obtained.This fused sample was then cooled to room temperature.Dissolution of Cakes and Fluoride Precipitation. Once

cooled to room temperature, the fused cake was dissolved inboiling 1 M hydrochloric acid. A total of 4 mL of concentratedhydrofluoric acid was added to the solution to separate thelanthanides and actinides as fluoride precipitates. Fluorideprecipitates were then separated from suspension bycentrifugation for 30 min at 3400 rpm. Precipitates wererepeatedly wet-ashed with concentrated nitric acid until a clearsolution was obtained. The Pu oxidation state was adjusted inthe 3 M nitric acid background solution to +IV with sodiumnitrite prior to introducing the solution on the separationcolumn (described below).Chromatographic Separation of Actinides. Actinides were

separated chromatographically using Eichrom TEVA/TRUextraction resins. Plastic columns (2 mL) were filled witheither TEVA or TRU and then pre-conditioned with 10 mL of3 M nitric acid solution. The TRU columns were placeddowngradient of the TEVA columns, so that elution from theTEVA columns flowed into the TRU columns. Eluent from theTRU columns was discarded.After pipetting the dissolved samples onto the TEVA

columns, the beakers were rinsed 3 times with 1 mL of 3 Mnitric acid, and each rinse was transferred to the TEVA columnsafter the prior solution completely drained. Once all TEVA andTRU columns were drained completely, the two columns were

separated. TEVA columns were washed with another 10 mL ofnitric acid, followed by 20 mL of 9 M hydrochloric acid.Plutonium was eluted from TEVA columns with 20 mL of a 0.3M sodium formaldehyde sulfoxylate, 0.1 M hydrochloric acid,and 0.1 M hydrofluoric acid solution. Americium and curiumwere eluted together from TRU columns with 15 mL of 4 Mhydrochloric acid.Each actinide fraction was evaporated to dryness, wet-ashed

with concentrated nitric acid, and then re-constituted with 1 Mhydrochloric acid. About 50 μg of neodymium carrier wasadded to each fraction, and 1 mL (for Pu) or 3 mL (for Am/Cm) of concentrated HF was added to form a fluorideprecipitate. Precipitates were allowed to form for 30 min andthen filtered from suspension with cellulose nitrate membranefilters (Cole-Parmer Instrument Company, Vernon Hills, IL).Filters were washed with 80% methanol and then air-dried for αspectrometry measurements.

Spectrometry. α activities were measured using an ORTECOCTETE Plus α spectrometry system (ORTEC, Oak Ridge,TN). Most samples were counted for 3−5 days, dependingupon the activity level. Because 239Pu and 240Pu cannot beresolved by α spectrometry, their activities are reported ascombined 239 + 240Pu activity. Background activities weredetermined by counting for a few months; this activity wassubtracted from each spectrum to determine the final activityvalues. After α spectrometry measurements of the Pu samples,the filters were dissolved with concentrated nitric andperchloric acids on a hot plate. This solution was then re-constituted with 1 mL of 2% nitric acid for inductively coupledplasma−mass spectrometry (ICP−MS) analysis. Plutoniumisotopic ratios were measured using Thermo Finnigan Element2 sector field ICP−MS (Thermo Electron Corp., Bremen,Germany). Additional details about the α spectroscopy andICP−MS are provided in the Supporting Information.

■ RESULTS AND DISCUSSION

Study Area: SRS Sediments. Although some actinideconcentration data exist for groundwater downgradient of theF-Area seepage basins,1,8,15 no data are available for the activityof these contaminants in the sediments. In this study, sedimentscollected directly from the seepage basin as well as sedimentscollected during the installation of several wells downgradientof the basins were analyzed. Figure 1 shows the study site,groundwater flow direction, seepage basins (source term), andsample locations.

Method Validation: INL Sediment. Before attempting toquantify the very low concentrations of Pu, Am, and Cmcontamination in the SRS sediment samples, we confirmed ourradioanalytical procedures with previously analyzed sediments

Table 1. Actinide Activities Measured in Sediments Collected from INL SDA and SRS F-Area

sediment distance from F-Area basin (m) sample depth (m) 241Am (Bq g−1) 244Cm (Bq g−1) 239 + 240Pu (Bq g−1)

INL SDA reporteda 0−0.04 2.3 ± 0.3 0.32 ± 0.01b 0.48 ± 0.09measuredc 0−0.04 2.0 ± 0.3 0.33 ± 0.06 0.42 ± 0.02

SRS F-Area basin 0 0−0.1 0.98 ± 0.06 0.7 ± 0.1 3.2 ± 0.7FEX-9 95 21.6−22.0 0.02 ± 0.02 0.026 ± 0.007 0.0011 ± 0.0004FEX-10 (18 m) 77 18.2−18.6 0.003 ± 0.002 0.0022 ± 0.0003 0.0011 ± 0.0003FEX-10 (30 m) 77 30.2−30.5 0.0005 ± 0.0004 0.0002 ± 0.0002 NDd

FSB-79A 375 16.5−16.8 0.001 ± 0.0005 0.0011 ± 0.0008 NDFSB-95C 15 22.9−23.2 0.006 ± 0.001 0.005 ± 0.001 ND

aFrom ref 12. bKnown quantity of 244Cm added to the sediment. 241Am and 239 + 240Pu are existing sediment contaminants. cThe measured value wasdetermined in this study to compare to the previously reported value. dND = not detected.



collected near the SDA at INL.11 The measured values for theINL SDA sediment sample agreed within the uncertainties ofthe reported values (Table 1). The concentration of 241Am wasabout an order of magnitude greater than that of either 244Cmor 239 + 240Pu. These results provided confidence in theanalytical methods to quantify sediment Pu and Amconcentrations. For Cm, our measured value agreed well withthe concentration estimated on the basis of the spike additionto a known sediment mass (see footnote b in Table 1).Actinide Measurements by α Spectrometry. Table 1

shows α activities with uncertainties for 241Am, 244Cm, and239 + 240Pu in sediment samples collected from differentlocations. As expected, the sample from the basin (where thewaste solution was initially discharged) had the highestactivities for all three isotopes. On the basis of the observationsthat Pu activities were greater than Am and Cm activities in thebasin and that very little or no Pu was detected in three of thedowngradient sediments, while Am and Cm were detected inall five sediments, we deduced that Pu migration from the basinwas slower than Am and Cm migration.Cm concentrations in the sediments were not inversely

correlated with the distance from the basins (Table 1 andFigure 1). The highest Cm activity downstream from the basinwas found in FEX-9 well, which is 95 m from the seepage basinedge. Cm concentrations, from the highest to the lowest, wereas follows (distance from the basin edge): FEX-9 (95 m) >FSB-95C (15 m) > FEX-10 (77 m) > FSB-79A (375 m) (Table1 and Figure 1). The lack of correlation with distance can likelybe attributed to the limited number of samples analyzed, whichdo not accurately delineate the spatial extent of the Cm plume.The lack of correlation may also be attributed to secondarysample effects, such as (1) samples originating from differentdepths (Table 1), (2) sediment sample heterogeneity (and,therefore, sorption heterogeneity, despite efforts to ensuresample textural uniformity), and (3) the plume not makinguniform contact with each of the samples.The trivalent cations, e.g., Am and Cm, exhibit similar

migration behaviors, as expected. Figure 2 demonstrates the 1:1correlation between Am and Cm, suggesting that they aremoving in the aquifer at about the same rate.Pu Isotopic Measurement by ICP−MS. Reactors in SRS

were used to produce weapons-grade Pu, which has a low

isotopic ratio of 240Pu/239Pu of approximately 0.0628 (Table 2).The isotopic ratio for the basin sediment was 0.0729 ± 0.0003,

which is distinctly higher than the historic 240Pu/239Pu ratio.8

This higher sediment isotopic ratio can be attributed to twofactors: (1) contribution of atmospheric Pu fallout (the regional240Pu/239Pu ratio is 0.18−0.19), and (2) contribution of 240Puderived from the decay of co-disposed 244Cm (Table 1). 244Cm-decay-corrected Pu isotopic ratios were calculated to provide anidea of the contribution of 244Cm decay to the observed240Pu/239Pu ratios. This correction was made by assuming thatall 244Cm was released into the seepage basins during the yearof greatest release, 1973. The 244Cm half-life (18.1 years) wasthen used to estimate the amount of 240Pu produced as a resultof 244Cm decay during the time elapsed from the sampling dateto 1973. When the contribution of 240Pu ingrowth from 244Cmdecay is corrected, the isotopic ratio was re-calculated to be0.0719 ± 0.0003, which is still significantly greater thanpreviously reported for groundwater samples8 (Table 2). Forthe basin sediment sample, the effect of 244Cm ingrowth on theisotopic ratio is only about 1%. This effect is small because therelative activity of basin 239 + 240Pu is high compared to that of244Cm. However, when precise activity ratios are required, theseingrowth corrections are necessary.Pu isotopic ratios for sediments collected from downgradient

wells were also calculated in two well locations, FEX-9 (95 mdowngradient) and FEX-10 (77 m downgradient; Table 2).The isotopic ratios in these downgradient sediments werehigher than that of the basin sediment. FEX-9 sediment had anisotopic ratio of 0.39 ± 0.05, and FEX-10 had an isotopic ratioof 0.23 ± 0.05. To evaluate whether Pu in the FEX-10 sampleoriginated from decay of 244Cm or from 239 + 240Pu dischargedto the seepage basin, the isotopic ratios were corrected for240Pu ingrowth (as described above), resulting in a decay-corrected isotopic ratio of 0.20 ± 0.05. The 244Cm-decay-corrected Pu isotopic ratio for the FEX-9 sample was identicalto that for FEX-10, 0.20 ± 0.05. These corrected values suggestthat Pu in the two downstream sediment samples did notoriginate primarily from Pu introduced into the seepage basins,e.g., 239Pu, but rather from the decay of 244Cm decay withcontributions from global fallout (which has a 240Pu/239Pu ratioof 0.18−0.19).For the FEX-10 sediment, 240Pu originating from 244Cm

decay accounted for 15% of all 240Pu in the sample, whereasFigure 2. 241Am and 244Cm activities in SRS F-Area sediments.

Table 2. Isotopic Ratio before and after Correction for244Cm Decay

240Pu/239Pu

sedimentsamplea

depth(m)

directmeasurement

corrected for 244Cmdecay

basin 0.1 0.0729 ± 0.0003 0.0719 ± 0.0003FEX-9 22 0.39 ± 0.05 0.20 ± 0.05FEX-10 18 0.23 ± 0.05 0.20 ± 0.05FEX-10 39 NDb

FSB-79A 17 NDFEB-95C 23 NDF-reactorc 0.062global falloutd 0.18−0.19

aBasin, F-reactor, and global fallout were surface sediment samples.FEX-9, FEX-10, FSB-79A, and FEB-95C were sediment samplesrecovered from a borehole drilled during well development. bND =not detected. cFrom ref 8. dFrom ref 2.



49% of 240Pu in FEX-9 can be attributed to 244Cm decay.Clearly, 244Cm progeny is affecting the observed Pu isotopicratio for these samples. Consequently, Pu sorbed to sediments95 m downgradient from the seepage basin likely did notoriginate from the basins but rather is derived primarily from244Cm decay.In this study area, two 244Cm-corrected Pu isotopic ratios

were found to be very different, i.e., 0.0719 ± 0.0003 for thebasin sediment and 0.20 ± 0.05 for the downgradientsediments. The lower ratio of the basin sediment reflects thecomposition of the waste stream discharged to the basin, whichwas derived from the processing of targets irradiated in the SRSreactors. The Pu isotopic ratio found in the downgradientsediments was, however, similar to that of the global falloutvalue, 0.18−0.19, indicating little to no release of Pu from theseepage basins. This is consistent with the total Puconcentrations at each location; e.g., the largest concentrationof Pu in the samples that we analyzed was found in thesediments collected within the basin.In Situ Kd Value Estimations. In situ distribution

coefficients (Kd) [units = (Bq/g)/(Bq/mL) = mL/g], asteady-state parameter, were calculated by combining thesediment activity values measured in this study (Table 1) withgroundwater activity levels reported by Buesseler et al.1

=in situ K[An]

[An]dsediment

water (1)

where [An]sediment and [An]water are the actinide concentrationsassociated with the sediment and water, respectively. Thegroundwater actinide concentration data were from well 2 byBuesseler et al.,1 which is about 34 m from well FEX-9. A totalof 2 years elapsed between the collection of samples byBuesseler et al.1 and our sampling. It is important to note thatthe samples used to calculate the in situ Kd values were notprecisely paired, as is the case with laboratory (ad)sorption testsof batch equilibrium Kd values. Given these caveats, it would bereasonable to assume that the uncertainty associated with theestimated in situ Kd values is greater than that associated withthe laboratory Kd value. However, in our conceptual model forthis construct, we assumed that the Pu plume was movingslowly (i.e., the 2 year difference between groundwater andsediment determinations does not greatly influence the Kdestimate). A shortcoming of this assumption is that seasonaland water-level changes in water chemistry are well-documented.16 A comparison of contaminant characteristicsbetween well 2 and well FEX-9, e.g., pH range of 3.2−3.4 andnitrate−nitrite concentrations of ∼50 mg L−1, supports ourassumption of uniformity of the groundwater in the region ofthose two wells. Another note about the in situ Kd values (eq 1)is that they differ from the “calibrated field Kd” in that they arenot based on an advection−dispersion equation and ground-water flow information to calculate a retardation factor.17

In situ Kd values were calculated using eq 1 and the activityconcentrations of 244Cm, 239Pu, and 240Pu, as shown in Table 3.239Pu and 244Cm Kd values were compared to reported batchstudies conducted with SRS sediments with similar pH values(pH 4.58).18 The in situ Kd values, using the field data,compared unexpectedly well to the literature Kd values;

18 thetwo different methods of estimating Kd values were notexpected to yield such similar values. Batch laboratoryexperiments,18 with complete mixing resulted in Kd valuesthat were only twice as great as in situ Kd values (Table 3).

Sediment pH for the FEX-9 sample was 4.58, and the sedimentcollected from an uncontaminated portion of the sameformation had a similar pH of 4.53. The batch Kd and the insitu Kd measurements did not require 244Cm correction,because the former was conducted in a 244Cm-free systemthat included primarily 239Pu and the latter was a 239Pu-specificKd. The smaller distribution coefficient for 240Pu compared to239Pu is consistent with the hypothesis by Buesseler et al.1 that∼75% of 240Pu in the F-Area groundwater exists in the oxidizedforms, i.e., Pu(V) or Pu(VI), whereas 239Pu in the samegroundwater existed almost exclusively in the reduced forms,Pu(III) and Pu(IV). As noted by Buesseler et al.,1 it is not clearwhy the isotopes have unique oxidation states after they havehad presumably years to come to steady state. As mentionedabove, the isotopic-specific geochemical behavior is attributedto the presence of 244Cm, which decays to 240Pu. 240Pu derivedfrom parent 244Cm tends to be in the oxidized form, ascompared to the steady-state reduced forms of 239Pu originatingfrom the waste stream effluents.1

Interestingly, 240Pu/239Pu isotopic ratios determined indowngradient sediments were markedly different from thosereported in nearby groundwater samples (Table 2). Ourmeasurements indicate that two sediments in which Pu wasdetected had Pu isotopic ratios, 0.39 ± 0.05 and 0.23 ± 0.05,that were significantly greater than regional global falloutvalues, 0.18−0.19, primarily because of the contribution from244Cm decay. Dai et al.8 reported Pu isotopic ratios ingroundwater in this area to be much higher (ratios = 3−8).This isotopic disequilibrium between Pu in groundwater andsediment may be explained by considering the following: (1)240Pu produced by decay of 244Cm may be in a higher oxidationstate (PuVO2

+ and/or PuVIO22+) as a result of the electron-

stripping Szilard−Chalmers process,8 and (2) oxidized forms ofPu have much lower tendencies to sorb to sediments thanreduced forms of Pu.18 We do not provide direct evidence ofthe higher oxidation state of 240Pu, but rely on previousmeasurements made in this aquifer by Buesseler et al.1

From this information, it can be hypothesized that oxidized240Pu is produced by the decay of sediment 244Cm, whichremains in groundwater (as opposed to sediment bound)because of its higher solubility. Conversely, 239/240Pu derivedfrom the original disposed effluent is reduced. Plutonium in thelower oxidation states, Pu(III) and Pu(IV), is more inclined to

Table 3. In Situ and Published 239Pu, 240Pu, and CmDistribution Coefficients (Kd Values)

239Pu 240Pu 244Cm

sedimentconcentration(mBq g−1)

0.50 ± 0.03 0.6 ± 0.1 26 ± 7

water concentration(mBq mL−1)a

0.005 ± 0.0003 0.063 ± 0.003 0.62 ± 0.03

in situ Kd (this study;mL g−1)b

101 ± 8 10 ± 2 40 ± 10

batch (ad)sorptionKd

18220c 40d

aFrom ref 8; well 2. bEquation 1; well FEX-9 sediment; and pH 4.58.cFrom ref 18; subsurface sandy sediment collected from the sameaquifer as the F-Area; pH 4.53; 97% sand, 2% silt, and 1% clay; andspike = 10−9 M 239Pu(IV). dFrom ref 1; subsurface clayey sedimentcollected from the same aquifer as the F-Area; pH 3.9; 58% sand, 30%silt, and 12% clay; spike = 100 ppb 244Cm; and 1:15 solid (g)/liquid(mL).



sorb to the sediment. This isotopic difference in oxidationstates results in a natural isotopic fractionation between thegroundwater and sediment, which has implications on riskcalculations and remediation decisions. In the case of Pu, thehuman risk presented by exposure to 239Pu (half-life = 24 100years) is not the same as that posed by 240Pu (half-life = 6560years). Additionally, the metal toxicity imposed by oxidized Puis greater than that imposed by reduced Pu. It may be incorrectto assume that (easier acquired and analyzed) groundwatersamples provide insight into the actual long-term risk imposedby radioactive contaminants in the subsurface sediments.Figure 3 presents a schematic of a proposed radio-

geochemical behavior of 244Cm, 239Pu, and 240Pu in down-

gradient (well FEX-9) groundwater based on this study andpublished data.1,8 First, Pu and Cm were discharged from SRSF-Area facility to the seepage basins.13,19 The plume pH at thestudy site, as mentioned above, is quite low, pH 3−4.4. Cm(III)sorbs less at lower pH levels and has a sharper pH sorptionedge than Pu(IV);1 consequently, Cm would be expected totravel further in the subsurface environment. The in situ 244CmKd value was 40 ± 10 mL g−1 (FEX-9 well in Table 3), which issimilar to the laboratory batch measurement reported byBuesseler et al.,1 40 mL g−1. 239 + 240Pu in the far field originatedprimarily from global fallout with a 240Pu/239Pu isotopic ratio of0.18 and a Kd value of 101 ± 8 mL g−1 (FEX-9 well in Table 3).Comparing these in situ Kd values suggests that

244Cm shouldbe more mobile in the study site than 239 + 240Pu. Additionally,Pu in the far field existed with elevated 240Pu concentrations, aresult of 244Cm decay. 240Pu derived from 244Cm has previouslybeen shown to exist primarily in the oxidized form, a resultpresumably because of the electron-stripping Szilard−Chalmersprocess.1 What is not clear is why 240Pu remains oxidized in theSRS subsurface environment. Buesseler et al.1 measuredreduced Pu(III/IV) but primarily oxidized Pu(V/VI) in theF-Area plume. The Eh (0.654−0.381 V) and pH (3.20−6.77)conditions of the F-Area plume14 would suggest that Pu(IV)would likely dominate the solid phase. Furthermore, laboratory

experiments with SRS sediments promoted rapid reduction ofPu(V) to Pu(III/IV).20,21 Additional research is required toidentify why 240Pu remains oxidized under these conditions thattypically promote reduction.The in situ Pu Kd values were much lower in the groundwater

that had a higher concentration of 240Pu, 10 ± 2 mL g−1.Because of this additional supply of 240Pu to the downgradientsystem, the Pu isotopic ratio in the sediment was a higher valuethan that of global fallout. In the groundwater, the isotopic ratiohad a much higher value of 3.4.1 This was due to the additionalsupply of 240Pu from the decay of 244Cm and the fact that 240Puproduced by 244Cm was likely in the oxidized form, which wasless prone to sorb to sediments than the reduced form. Notshown in Figure 3 is that mobile colloids influence Pu transportthrough this subsurface system.1,8,15

These results have implication to radiological risk calcu-lations and the need for environmental remediation insofar thatthe radiological isotopes in groundwater, which is mostcommonly sampled during site characterization, may not be agood indication of the typically more abundant isotopes insediments. At issue is that not all isotopes of an element posethe same risk. In the case of Pu, the risk (or radioactivity) posedby equal masses of 239Pu (half-life = 24 100 years) and 240Pu(half-life = 6560 years) are not the same because the latter hasan appreciably greater specific activity. These results underscorethe importance that, under conditions where multiple isotopesof varying toxicity are present, it is important to not onlysample and characterize the groundwater but also the sediment,which typically contains the vast majority of contaminants.

■ ASSOCIATED CONTENT

*S Supporting InformationInformation about the sampling locations at the INL SDA andthe SRS F-Area (Table S1) and instrument parameters forICP−MS (Table S2). This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Telephone: 509-335-1411. Fax: 509-335-8867. E-mail: [email protected].

Present Address‡Hiromu Kurosaki: Oak Ridge National Laboratory, Post OfficeBox 2008, MS 6105, Oak Ridge, Tennessee 37831, UnitedStates.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge Dr. Evgeny Taskev of Eckert &Ziegler Analytics for providing 244Cm standard solution. Thisproject was funded by the United States Department of Energy,Basic Energy Science (DE-FG02-06ERI15782). Sue B. Clarkalso acknowledges support from the United States Departmentof Homeland Security, Academic Research Initiative (Contract2009DN077-ARI03302). Daniel I. Kaplan received fundingfrom the Department of Energy’s Subsurface BiogeochemistryResearch Program within the Office of Science (ContractSCW-0083).

Figure 3. Proposed Cm and Pu radiogeochemical behavior down-gradient of the SRS F-Area seepage basins. Using well FEX-9, located95 m downgradient of the seepage basins, it has Pu isotopic signatures,suggesting that Pu comes from either global fallout or 244Cm and notfrom the seepage basins. 244Cm decay produces primarily 240Pu(V/VI),whereas global fallout contains primarily 240+239Pu(III/IV), which mayexist in dissolved, sorbed, or mobile colloid forms. The role of mobilecolloids in transporting Pu is not depicted.



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