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Ecological Risk Assessment ofRadiological Exposure to DepletedUranium in Soils at a Weapons TestingFacilityJING-JY CHENG a , IHOR HLOHOWSKYJ a & CHIH LUN TSAO ba Environmental Assessment Division Argonne National Laboratory ,Argonne, IL, USAb CH2M Hill , Sacramento, CA, USAPublished online: 10 Aug 2010.
To cite this article: JING-JY CHENG , IHOR HLOHOWSKYJ & CHIH LUN TSAO (2004) Ecological RiskAssessment of Radiological Exposure to Depleted Uranium in Soils at a Weapons Testing Facility, Soiland Sediment Contamination: An International Journal, 13:6, 579-595
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Soil & Sediment Contamination, 13:579–595, 2004Copyright © ASPISSN: 1058-8337 print / 1549-7887 onlineDOI: 10.1080/10588330490519455
Ecological Risk Assessment of RadiologicalExposure to Depleted Uranium in Soils at a Weapons
Testing Facility
JING-JY CHENGIHOR HLOHOWSKYJ
Environmental Assessment DivisionArgonne National LaboratoryArgonne, IL, USA
CHIH LUN TSAO
CH2M HillSacramento, CA, USA
The potential for unacceptable risks to biota from radiological exposure to depleteduranium (DU) in soils was evaluated at two sites where DU weapons testing had beenconducted in the past. A screening risk assessment was conducted to determine if mea-sured concentrations of DU-associated radionuclides in site soils exceed radionuclidelevels considered protective of biota. While concentrations of individual radionuclidesdid not exceed acceptable levels, total radionuclide concentrations could result in po-tentially unacceptable doses to exposed biota. Thus, a receptor-specific assessment wasconducted to estimate external and internal radiological doses to vegetation and wildlifeknown or expected to occur at the sites. Wildlife evaluated included herbivores, omni-vores, and top-level predators. Internal dose estimates to wildlife considered exposurevia fugitive dust inhalation and soil and food ingestion; root uptake was the primaryexposure route evaluated for vegetation. Total doses were compared with acceptabledose levels of 1.0 and 0.1 rad/day for vegetation and wildlife, respectively, with poten-tially unacceptable risks indicated for doses exceeding these levels. All estimated doseswere below or approximated acceptable levels, typically by an order of magnitude ormore. These results indicate that current levels of DU in soils do not pose unacceptableradiological risks to biota at the sites evaluated.
Keywords Depleted uranium, ecological risk assessment, ecological dose modeling,radiation exposure, biota dose limits.
The submitted manuscript has been created by the University of Chicago as Operator of ArgonneNational Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department ofEnergy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive,irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copiesto the public, and perform publicly and display publicly, by or on behalf of the Government.
Work supported under a military interdepartmental purchase request from the U.S. Departmentof Defense through U.S. Department of Energy Contract W-31-109-Eng-38.
Address correspondence to Ihor Hlohowskyj, Environmental Assessment Division, ArgonneNational Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA. E-mail: [email protected]
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580 J.-J. Cheng et al.
Introduction
Past weapons testing activities at a military facility in California have included the useof munitions containing depleted uranium (DU). Depleted uranium is a byproduct of theuranium enrichment process, which is used to increase the proportion of U-235 in theproduct, enriched uranium. Because of the stripping of U-235, the U-235 proportion inthe depleted uranium is less than the 0.7% it comprises in natural uranium. A number ofenvironmental investigations were conducted to characterize the environmental distributionof DU that resulted from past weapons testing using DU munitions (unpublished; alsosee Johnson et al., 2004). In addition to these characterization investigations, the potentialfor unacceptable risks to human health (Callahan et al., 2004) and ecological resourcesfrom radiation exposure associated with the DU munitions has also been evaluated. Thispaper presents an ecological risk assessment (ERA) that was conducted for two areas, theKS Impact Area and the G-6 Test Area (G-6). The ERA was conducted specifically todetermine whether biota inhabiting these locations may receive unacceptable radiologicaldoses from DU-associated radionuclides that could affect reproduction, survival, or growth.Non-radiological toxicity associated with the DU exposure was not evaluated in the ERA.
Environmental Setting
The KS site is approximately 6.2 km2 and can be characterized as an alkaline basin withhopsage scrub plant communities. A total of 3,411 rounds of DU penetrators were firedat two rows of various types of armored vehicle targets at this site. Penetrators that hitthe vehicles aerosolized, contaminating the vehicles and the soils around them. Penetratorsthat missed the vehicles struck the ground surface and either remained intact or fracturedinto fragments. Some of these penetrators and fragments remain on the ground surface,while others are buried in surface soils (at 30 to 45 cm below ground surface) (unpublisheddata; also see Johnson et al., 2004). Because of the nature of the testing, the majority ofpenetrators are expected to occur around or behind (due to overshoot) the target vehicles,and much of the entire KS area is expected to have no DU penetrators present.
The G-6 site covers about 123 km2 and is characterized by creosote bush scrub vege-tation. About 100,000 DU rounds were fired from a land-based gun system in a 45-degreearc over a radius of about 15,000 m out over the land surface rather than at armored vehicletargets (unpublished data). Thus, most rounds are widely scattered and are expected to beintact and to occur on the ground surface or at very shallow depths. No DU testing hasoccurred at either location since the mid-1980s.
The test facility is located in a portion of the Mojave Desert between the Sierra Nevadaand the Argus Ranges in California. The geography of the region is characterized by flat,dry lakebeds, dry washes, alluvial fans and mountains. Climate conditions are typical ofthe desert regions of the Southwest. The annual rainfall ranges from 3 to 6 in. The averagetemperature in the valley is 64◦F, with an annual temperature range from 0 to 118◦F. The KSsite is located along the shoreline of a Pleistocene lake, with soils composed primarily ofcoarse-grained sandy to pebbly beach deposits with fine-grained playa and lacustrine clayswith associated salts (Buck et al., 2004; Johnson et al., 2004). The G-6 site is located on thedistal portion of an alluvial deposit, with soils having a relatively uniform texture of 80%sand and 20% silt. Detailed information on the soils of both sites can be found in Johnsonet al. (2004). The predominant vegetative community of the KS site is hopsage scrub, whilethe G-6 site is dominated by creosote bush scrub but also includes Great Basin mixed scruband Mojave sand field, and alkaline basin scrub plant communities (unpublished data). No
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Ecological Risks of Depleted Uranium Exposure 581
surface waters occur at either site, and wildlife of the area are expected to be typical of thatfor the Mojave Desert.
Risk Assessment Methods
Risk Assessment Approach
The assessment of ecological risks associated with radiological exposure at the KS andG-6 sites followed the U.S. Environmental Protection Agency (USEPA) framework forconducting ecological risk assessments (USEPA, 1997). The focus of this risk assessmentwas to evaluate the potential for ecological resources that are known or expected to occurat the KS and G-6 sites to incur radiological doses sufficient to potentially result in adverseecological effects. While uranium is known to elicit chemotoxic effects in vertebrates, thepotential for such effects was not evaluated in this assessment.
Radiological risks to ecological resources were evaluated in two phases. First, ascreening-level ERA (consistent with Steps 1 and 2 of the USEPA framework) was con-ducted that compared the maximal DU-related radionuclide concentrations in site soilsto radionuclide-specific soil concentration limits that would not result in unacceptableradiological risks to biota. This approach implicitly assumes that the entire site is uni-formly contaminated at the maximal level, and all receptors receive maximal exposures. Ifthe radionuclide-specific soil concentration exceeded the corresponding soil concentrationlimit, or if the sum of the radionuclide-specific soil concentration/soil limit ratios exceededa value of 1.0, then a receptor-specific ERA was conducted.
The receptor-specific ERA represents the initial activities (Step 3 of the USEPA frame-work) conducted for a baseline ecological risk assessment, namely a reevaluation of thescreening risk assessment using more realistic and site-specific exposure assumptions, suchas a mean rather than maximal radionuclide concentration, and considerations of receptor-specific ecological and physiological parameters (USEPA, 1997). The receptor-specificERA employed dose modeling to evaluate potential radiation exposures to individual eco-logical receptors.
Internal, external, and total dose estimates were calculated for terrestrial vegetation andfor a variety of wildlife species. Internal doses were estimated by modeling uranium isotopeuptake to predict radioisotope tissue concentrations, which were then used to calculateinternal radiation doses. External radiation doses were calculated considering receptor-specific life history parameters, such as home range and fractions of time spent on, above,or below the ground surface. The internal and external doses were then added and the sumwas compared to the applicable dose limit (1.0 rad/d and 0.1 rad/d for terrestrial vegetationand wildlife, respectively (USDOE, 2002)). Potentially unacceptable radiological risks wereindicated for an ecological receptor if the total dose level exceeded the appropriate doselimit.
Ecological Receptors
The ecological receptors evaluated in this risk assessment included vegetation, as a wholerather than as individual species, and ten wildlife species known or expected to occur atthe KS and G-6 areas (Table 1). The individual wildlife species were selected to serveas surrogates for evaluating potential risks to the variety of wildlife receptor categoriesknown or expected to occur at the sites. These categories reflect the various trophic levelcomponents of the ecosystems of these sites, including herbivores, omnivores, and top-level
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Table 1Wildlife species evaluated at the KS and G-6 areas
Receptor species Food habits Foraging mode
ReptilesDesert iguana
(Dipsosaurus dorsalis)Chiefly vegetation; also
macroinvertebratesGround surface, shrub
layerDesert tortoise (Gopherus
agassizii)Vegetation Ground surface
Gopher snake (Pituophiscatenifer)
Mammals and birds; also lizardsand macroinvertebrates
Ground surface andsubsurface
BirdsAmerican kestrel (Falco
sparverius)Small mammals, birds, lizards,
and macroinvertebratesGround surface
Mourning dove (Zenaidamacroura)
Mostly plant materials; alsosome macroinvertebrates
Ground surface
Peregrine falcon (Falcoperegrinus)
Small birds Air
MammalsPallid bat (Antrozous
pallidus)Macroinvertebrates Ground surface, shrub
layer, tree canopy, airKit fox (Vulpes macrotis) Small mammals, birds, and
vegetationGround surface and
subsurfaceCoyote (Canis latrans) Small mammals, birds, reptiles,
macroinvertebrates, andvegetation
Ground surface andsubsurface, shrublayer
Merriam’s kangaroo rat(Dipodomys merriami)
Macroinvertebrates and seeds Ground surface
Sources: Alsop (2001); Burt and Grossenheider (1976); Chapman and Feldhamer (1982); Short(1983); Stebbins (2003).
predators (carnivores). Because each of these trophic level categories is important to theoverall condition of the ecosystem, representative species from each category were selectedfor evaluation. The species evaluated in this assessment were selected, to the extent possible,on the basis of the following conditions:
� the surrogate species is known to occur or is likely to occur at the site;� the surrogate species is representative of a specific taxonomic group, trophic level,
and/or species of concern (e.g., a listed species);� ecological information is readily available for the surrogate species; and� the surrogate species is known or considered to be radiosensitive.
Species-specific exposure factors (e.g., body weight; food, water, and soil ingestion rates;home range and diet composition) were used to model radiological doses to each receptor.These exposure factors were obtained from a variety of literature sources (e.g., Mautzand Nagy, 1987; CEPA, 2002; USEPA, 1993); if unavailable they were either empiricallyderived using allometric equations (USEPA, 1993) or taken from a closely related species,as appropriate.
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Ecological Risks of Depleted Uranium Exposure 583
Exposure Scenarios
The DU at the KS and G-6 areas occurs primarily in the surface and shallow subsurfacesoils. Biota occurring at these areas may be exposed by either coming into direct contactwith the DU-containing soils or by coming close enough to receive external radiation (nodirect contact necessary). Four primary exposure routes were identified by which ecologicalreceptors could be exposed to DU-related radionuclides:
� Root uptake from soil by vegetation;� Ingestion of soil and vegetation containing DU-related radionuclides and the subse-
quent food chain transfer to upper trophic level receptors;� Inhalation of dust particles contaminated with DU-related radionuclides; and� Direct radiation.
A generalized conceptual site model depicting these exposure pathways and the receptorscategories evaluated in this risk assessment is shown in Figure 1. No known groundwatercontamination by DU or uranium progeny radionuclides exists (unpublished data; Johnson etal., 2004), thus exposures to uranium from ingesting surface water recharged by groundwateris not considered.
Exposure Point Concentrations
The evaluation of ecological risks was based on the use of an exposure point concentration(EPC) that was considered representative of the soil concentration to which biota could beexposed. Maximum reported contaminant concentrations are typically employed as EPCsfor screening level assessments, while mean (or some other central tendency estimate [CTE])concentrations are used as EPCs in more site-specific assessments (USEPA, 1993, 1997).For the KS and G-6 areas, the use of maximum and mean soil concentrations as the EPCswas problematic because of the extremely heterogeneous distribution of DU penetratorsand DU-related radionuclides.
Previous studies that examined the distribution and mobility of DU at the KS and G-6areas (Buck et al., 2004; Johnson et al., 2004) found relatively high DU soil concentrations
Figure 1. Conceptual site model depicting radiological exposures of ecological resources to DU-related radionuclides at the KS Impact Area and G-6 Test Area. Soil invertebrates are considered onlyas prey for insectivorous wildlife and assumed to have tissue radionuclide concentrations equal toradionuclide concentrations in site soils.
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584 J.-J. Cheng et al.
(i.e., 1,647 µg DU/g soil) to be very discretely located immediately around individualpenetrators, and that DU concentrations exponentially decreased to very low levels (i.e.,6.2 µg/g) over relatively short distances (i.e., typically less than 60 cm and in some casesless than 8 cm) from the penetrator (Johnson et al., 2004). In addition, maximum DUconcentrations were often associated with large DU fragments or flakes that would not bereadily bioavailable to ecological receptors (Buck et al., 2004; Johnson et al., 2004).
Because of this highly discrete distribution and the large variation in concentrationsover very short distances, the average DU soil concentration for any sampling area wouldbe dominantly influenced by the very high DU concentrations at penetrator locations, eventhough these areas constitute only a very small fraction of the total sampling area. However,if the very high DU concentrations are considered as outliers and excluded from EPCconsideration, the resultant EPC would only slightly exceed the background levels of naturaluranium. Therefore, it was determined that the EPCs would be best represented by thepercentile values rather than the maximum and mean values of the sampling data. The90th percentile DU-related radionuclide concentration (i.e., the concentration below which90% of the soil DU-related radionuclide concentration data would be expected to occur)was selected as the representative EPC for a reasonable maximum exposure (RME). The50th percentile concentration (i.e., the concentration below which 50% of the radionuclideconcentrations would be expected to occur) was selected as the EPC for the site-specificassessments. This 50% EPC was considered to be representative of a CTE exposure of biotato DU.
For the KS area, the RME and CTE uranium concentrations were developed using soilsamples (n = 250) collected from a 12 m × 15 m sampling grid centered on an armoredvehicle target where previous DU characterization studies indicated an area of maximumDU penetrator occurrence (unpublished data). For the G-6 area, the RME and CTE concen-trations were developed using sampling data for individual penetrators pooled together toprovide a larger data set. The 50th percentile CTE concentrations are much greater than theactual mean radionuclide concentrations for these areas because uranium concentrationsacross very large portions of these areas are at background levels. Uranium concentrationsin sedimentary soils (similar to those at the KS and G-6 sites) in the Mojave desert aretypically in the range of 3.0 to 4.4 ppm (unpublished data; Rudin et al., 1997; Johnson andRudin, 2001; Johnson et al., 2004).
To calculate the RME and CTE concentrations of the DU-related radionuclides, thepercentile concentration values for total uranium were first determined from laboratoryanalyses of soil samples (Johnson et al., 2004). The corresponding isotope (U-234, U-235,and U-238) concentrations were then derived using the average activity ratios among theisotopes. The RME and CTE concentrations developed for total uranium and the uraniumisotopes are presented in Table 2.
Radiation Dose Estimation
Radioisotopes Uptake and Radiation Exposures. Internal and external radiation doseswere estimated during the receptor-specific ERA. Internal dose estimates considered in-halation and ingestion pathways for wildlife and root uptake by vegetation, while dermalabsorption of radionuclides across body surfaces (skin and plant cuticles) was consid-ered insignificant and not evaluated. Internal radiation doses were calculated by multiply-ing predicted radionuclide tissue concentrations by appropriate radionuclide-specific inter-nal dose conversion factors (DCFs) developed by the U.S. Department of Energy (DOE)
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Ecological Risks of Depleted Uranium Exposure 585
Table 2Uranium concentrations for KS Impact Area and G-6 Test Areaa
Area and representative Total uranium U-238 U-235 U-234percentile values (µg/g) (Bq/g) (Bq/g) (Bq/g)
KS impact area—Maximum concentrationb 101,000 1,241 19 1,42990th percentile RME concentration 3,500 43 0.70 5050th percentile CTE concentration 57 0.71 0.01 0.81Minimum concentrationc 8.7 0.11 0.002 0.12
G-6 test area—Maximum concentrationb 805 9.9 0.35 10.890th percentile RME concentration 440 5.5 0.20 6.050th percentile CTE concentration 15.3 0.2 0.01 0.20Minimum concentrationc 4.2 0.05 0.002 0.06
aActivity concentrations (Bq/g) of uranium isotopes, U-238, U-235, and U-234 were calculated onthe basis of the total uranium concentration and the average activity ratios among the isotopes. Activityratios for each sample were first calculated, then the ratios were summed across all the samples toobtain the average values. The average activity ratios for the KS Impact Area were U-238 : U-235 : U-234 = 1 : 0.016 : 1.15. The average activity ratios for the G-6 Test Area were U-238 : U-235 : U-234 =1 : 0.0357 : 1.09. Therefore, the activity concentration of U-238 (Bq/g) can be obtained by dividingthe total uranium concentration (µg/g) with 81.37 for the KS impact area, and with 81.61 for the G-6test area.
bMaximum of the sampling data used to derive the percentile values.cMinimum of the sampling data used to derive the percentile values.
(USDOE, 2002). The DOE DCFs are conservative estimates of the energy absorbed perunit body weight per unit time from per unit tissue concentration and are derived assumingthat the exposed organism is infinitely large in size so that all of the energies emitted fromradioactive decay are absorbed by the tissues of the organism. The receptor-specific ERAalso considered potential radiation doses from short-lived uranium decay progenies (Th-234, Pa-234m, Pa-234, and Th-231). The decay progenies were assumed to be in secularequilibrium with their parent radionuclides and have the same tissue concentrations. Theinternal DCFs for uranium isotopes that included contributions from decay progenies wereused in the dose estimates.
External dose estimates were calculated by multiplying the soil radionuclide concen-tration by an appropriate external DCF (USDOE, 2002). The external DCFs were developedassuming the receptor burrowed in the contaminated soils for the entire year, the soils wereuniformly contaminated and infinite in extent, and that the receptor was infinitely small sothat attenuation of radiation by tissues would be negligible. The external dose estimates alsoconsidered the amount of time (time fraction) a receptor was expected to occur on, below,and/or above the ground surface as well as the amount of time the receptor might actuallyspend on the KS or G-6 areas. As with the internal DCFs, the external DCFs includedcontributions from decay progenies, which were assumed to be in secular equilibrium andto have the same soil concentrations as their parent uranium isotopes.
Internal dose estimation for vegetation considered root uptake as the primary exposureroute. Translocation of radionuclides from foliage surfaces (resulting from deposition ofairborne dust particles) to plant tissues was considered insignificant compared with theroot uptake mechanism and not evaluated. The plant tissue concentration was multiplied
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by the appropriate internal DCF to estimate the internal radiation dose. External radiationexposures to vegetation were estimated by multiplying the soil concentrations by the externalDCFs (USDOE, 2002) without any adjustment for above- and below-ground time fraction,assuming the root system to be the most critical organ of the plant species.
Estimation of Radionuclide Tissue Concentrations. Tissue concentrations in wildlife wereconservatively estimated assuming radionuclide concentrations in invertebrate food itemswere equal to the soil EPC, and that all food ingested by wildlife was contaminated. Rootuptake was estimated using a radionuclide-specific root transfer factor that was developedusing the negative correlation with the radionuclide soil/water distribution coefficient (Kd)(Baes et al., 1984). Representative site-specific Kd values (unpublished data; Johnson et al.,2004) were used to develop radionuclide-specific root transfer factors for each area. Thesoil radionuclide EPC was then multiplied by the appropriate root transfer factor to estimatethe plant radionuclide tissue concentration.
The estimation of radionuclide tissue concentrations in wildlife receptors requiredconsideration of food uptake along food chains and food webs across multiple trophiclevels (Figure 1), and the estimation of radionuclide tissue concentrations of prey biota. Theestimated radionuclide concentrations of kangaroo rats, mourning doves, and desert iguanaswere used to represent concentrations in small mammal, bird, and reptile prey, respectively,for predator species. The radionuclide tissue concentration in a wildlife receptor (eitherprey or predator) was calculated with the following equation, which was based on theradionuclide tissue concentration model developed by DOE (USDOE, 2002) and modifiedto include radionuclide uptake from multiple food sources and multiple trophic levels:
CT = [{(CS × IP × B) + (CS × IS) + �(CK × IK) + (IAF × IA × M × CS)}A]
(KR + KB) × W
× (1 − e−t(Kr+Kb)
)
where:
CT = radionuclide tissue concentration in the receptor (Bq/kg),IP = vegetation ingestion rate (kg/d),B = radionuclide root uptake transfer factor,IS = soil ingestion rate (kg/d),
CS = radionuclide soil concentration (Bq/kg),IK = ingestion rate of prey item k (kg/d),
CK = radionuclide concentration in prey item k (Bq/kg),IAF = inhalation adjustment factor,
IA = inhalation rate (m3/d),M = mass loading factor for dust inhalation (kg/m3),A = radionuclide assimilation fraction to receptor tissues,
KR = radionuclide radioactive decay constant (1/d),KB = radionuclide biological decay constant (1/d),
t = receptor life span (d), andW = receptor body weight (kg).
The receptor-specific biological decay constant was derived using the allometric equa-tion KB = 0.693/(0.8 W0.28), where KB = receptor-specific biological decay constant andW = the body weight (g) of the receptor (USDOE, 2002). The receptor-specific wildlifeexposure factors are presented in Table 3. Whenever minimum and maximum exposure
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Tabl
e3
Spec
ies-
spec
ific
expo
sure
fact
ors
used
for
dose
mod
elin
gto
wild
life
rece
ptor
s
Soil
diet
%of
time
%of
time
spen
tB
ody
Food
inge
stio
nfr
actio
nH
ome
spen
tbel
owon
grou
ndIn
hala
tion
Rec
epto
rw
eigh
t(g)
ara
te(k
g/d)
b(%
ofdi
et)
rang
e(h
a)gr
ound
surf
ace
Lif
espa
n(d
)ra
te(m
3/d
)bD
ietc
ompo
sitio
n(%
)
Mer
riam
’s36
.2–4
1.1
0.00
47–0
.005
1<
2.0c
0.09
d75
e25
e63
0–69
0a0.
038–
0.04
25M
acro
inve
rteb
rate
s:4
f
Kan
garo
oSe
eds:
96ra
tK
itfo
x1,
930–
2,03
00.
118–
0.12
32.
8c3,
248–
3,47
3d50
e50
a1,
440–
2,52
0a0.
926–
0.96
3M
amm
als:
66a
Bir
ds:1
0M
acro
inve
rteb
rate
s:24
Palli
dba
t25
.2–3
4.7
0.00
3–0.
004
0g8.
9–13
.7d
0g0.
5e17
,100
a0.
029–
0.03
7M
acro
inve
rteb
rate
s:10
0C
oyot
e10
,200
–12,
700
0.46
4–0.
555
2.8c
31,1
23–4
1,93
4d30
e70
e5,
400a
3.50
–4.1
7M
amm
als:
60h
Bir
ds:1
6M
acro
inve
rteb
rate
s:6
Veg
etat
ion:
16M
isc:
2A
mer
ican
86–1
640.
012–
0.01
80g
13.4
–32.
2i0g
0.5e
3,54
0a0.
062–
0.10
2M
acro
inve
rteb
rate
s:32
a
kest
rel
Mam
mal
:25.
7B
irds
:16.
6R
eptil
es:2
6Pe
regr
ine
611–
952
0.04
2–0.
056
0g18
8–34
2i0g
0g4,
410
0.28
–0.3
9B
irds
:100
falc
onM
ourn
ing
99.3
–125
0.01
3–0.
015
10.4
c7.
3–9.
5i0e
50e
540a
0.06
9–0.
083
Veg
etat
ion:
97a
dove
Mac
roin
vert
ebra
tes:
3D
eser
t54
–60
0.00
05–0
.000
64.
5j
0.00
1k75
e25
e2,
700–
3,06
0a0.
009–
0.01
0V
eget
atio
n:99
j
igua
naM
acro
inve
rteb
rate
s:1
Des
ert
1,57
8–2,
290
0.00
9–0.
013
4.5a
0.02
6–0.
04k
75e
25e
18,7
00a
0.03
–0.0
4V
eget
atio
n:10
0j
tort
oise
Gop
her
548–
655
0.00
38–0
.004
44.
5c3.
5m75
e25
e6,
120
0.01
–0.0
2R
eptil
es:7
4n
snak
eM
amm
als:
26
aC
EPA
,200
2;bes
timat
edus
ing
allo
met
ric
equa
tions
from
USE
PA,1
993;
food
inge
stio
nra
teba
sed
ondr
yw
eigh
t;c B
eyer
etal
.,19
94;d
estim
ated
usin
gal
lom
etri
ceq
uatio
nin
Har
esta
dan
dB
unne
l,19
79;
e assu
med
;fba
sed
onO
rd’s
kang
aroo
rat
(Gar
riso
nan
dB
est,
1990
);gas
sum
edne
glig
ible
onba
sis
ofre
cept
orfo
ragi
ngm
ode;
hD
rew
ek,
1980
;i Sc
hoen
er,
1968
;j M
autz
and
Nag
y,19
87;
kT
urne
ret
al.,
1969
;l B
eck
and
Pete
rson
,19
95;
mba
sed
ontig
erra
ttles
nake
(Cro
talu
sti
gris
);nR
odri
guez
-Rob
les
and
Gre
ene,
1999
.
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588 J.-J. Cheng et al.
factor values were available, an average value was calculated and used to estimate the tissueconcentration.
The inhalation rate, IA, for each receptor was derived using the allometric equationIA = aWb, where W is the weight of the receptor and a and b are constants (USEPA, 1993).The receptor-specific ingestion rate of individual food items (vegetation, invertebrates, orvertebrates) was calculated by multiplying the total ingestion rate (dry weight-based; seeTable 3) of the receptor by the percentage contribution of each food category of the total dietof the receptor, and then adjusting the resultant dry weight ingestion rate for total weight.
Estimation of Radiological Doses. The total radiation dose incurred by an ecological recep-tor was estimated by summing the internal and external dose estimates (USDOE, 2002) andapplying a site use factor so that the estimated total dose reflected the temporal occurrenceof the receptor at the site. Total radionuclide-specific radiation dose was estimated usingthe equation:
DT = (DI + DE) × SUF
where:
DT = radionuclide-specific total radiation dose (Gy/d),DI = radionuclide-specific internal radiation dose (Gy/d),DE = radionuclide-specific external dose (Gy/d), and
SUF = receptor-specific site use factor.
The SUF reflects the percentage of time that a receptor may be expected to occur onthe contaminated site over the course of its normal activities. Biota with home range areassmaller than the contaminated site may be exposed to site contaminants 100% of the time. Incontrast, biota with home ranges much larger than the contaminated site would be expectedto occur on site for only a fraction of time and thus have a much lower exposure to sitecontaminants. For biota with home ranges less than the area of the contaminated site, theSUF is set at 1.0 (100% exposure). For biota with home ranges larger than the site, the SUFis calculated as the ratio of the contaminated site (m2) to the home range (m2) and will beless than 1.0 (<100% exposure).
The radionuclide-specific internal dose was calculated by multiplying the estimatedtissue concentration by the appropriate internal DCF. The radionuclide-specific externaldose to a wildlife receptor was calculated as:
DE = CS × DCFE(1/2ta + tu)
where:
DE = external dose (Gy/d),CS = radionuclide soil concentration (Bq/kg),
DCFE = radionuclide-specific external dose conversion factor ([Gy/d]/[Bq/kg]),ta = time fraction spent above ground by the receptor, andtu = time fraction spent underground by the receptor.
The use of the term (1/2ta + tu) considers the fraction of time that a specific receptor canbe expected to incur an external radiation dose while at a contaminated site, taking intoaccount the foraging and life history behavior of the receptor (e.g., aerial feeder, groundsurface forager, burrower). The DCFs used in the receptor-specific ERA are presented inTable 4.
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Ecological Risks of Depleted Uranium Exposure 589
Table 4Parameter values used in site-specific ERA for the KS Impact Area
Input parameters Values
External DCFs (Gy/y)/(Bq/kg)a
U-238 4.6 × 10−6
U-235 1.8 × 10−6
U-234 6.5 × 10−8
Internal DCFs (Gy/y)/(Bq/kg)a
U-238 4.4 × 10−4
U-235 4.5 × 10−4
U-234 4.9 × 10−4
Body assimilation rate for uraniuma 0.05Correction factor for inhalation to ingestiona
U-238 4.0 × 103
U-235 3.5 × 103
U-234 7.0 × 103
Root uptake transfer factor for uranium (wet plant to dry soil)b 7.55 × 10−3
Dust loading factor (ratio of the air conc. to soil conc.) (kg/m3) 1.0 × 10−7
aSource: USDOE, 2002.bRoot uptake transfer factor was calculated with the correlation with Kd: ln(Kd) = 2.1 + 0.56 ×
ln(Biv), where Biv is the root uptake transfer factor, and 2.1 and 0.56 are coefficient values for sandysoil (Baes et al., 1994).
Risk Characterization
Hazard Quotient Approach. The potential for unacceptable radiological risks to vegetationand wildlife was estimated as the hazard quotient (USEPA, 1997), which was calculated as:
HQ = EPC/BCG or D/dose limit
where:
HQ = hazard quotient,EPC = radionuclide-specific exposure point concentration,BCG = radionuclide-specific soil concentration limits,
D = radionuclide-specific estimated dose, anddose limit = acceptable dose limit for vegetation or wildlife.
An HQ ≤ 1.0 indicates no unacceptable ecological risk from radiation exposure, andfurther evaluation or remediation is not warranted. An HQ > 1.0 indicates that the radionu-clide is present at a concentration in soil that may pose unacceptable radiological risks toexposed biota and that further evaluation or remediation may be warranted.
For the screening-level ERA, HQs were calculated by comparing DU-related ura-nium isotope soil concentrations with radionuclide-specific soil concentration limits. Forthe receptor-specific ERA, HQs were calculated for each uranium isotope by compar-ing the total estimated receptor-specific dose with a biota dose limit. For both levels ofERA, the HQs of the three uranium isotopes were summed to provide a hazard index(HI), which is an estimate of the potential for unacceptable risks from exposure to all ofthe DU-related uranium isotopes. An HI ≤ 1.0 indicates that the soil concentrations of the
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590 J.-J. Cheng et al.
DU-related uranium isotopes are not expected to result in a total radiation dose that poses anunacceptable ecological risk. Alternately, an HI > 1.0 indicates a potential for unacceptableecological risks.
Soil Concentration and Biota Dose Limits. In the screening-level ERA, the uranium isotopesoil concentrations were compared with radionuclide-specific soil concentration limits. Thelimits used were the screening biota concentration guides (BCGs) developed by DOE forevaluating radiological risks to ecological resources (USDOE, 2002). For each DU-relateduranium isotopes the BCG represents the limiting medium (i.e., lowest soil) concentration,derived considering a range of terrestrial species, which would not result in recommendeddose limits for biota to be exceeded.
For the receptor-specific ERA, the dose limits used were 0.01 Gy/d (1 rad/d) and0.001 Gy/d (0.1 rad/d) for terrestrial vegetation and wildlife, respectively, and are consideredto be protective of growth, reproduction, and survival from chronic radiation exposure.The dose limit for terrestrial wildlife is less than that for terrestrial vegetation primarilybecause wildlife are considered to be more radiosensitive (USDOE, 2002). DOE establishedthese radiation dose limits for use in conducting radiological ERAs (USDOE, 2002), and thelimits are identical to the recommendations of the National Council on Radiation Protection(NCRP, 1991), the International Atomic Energy Agency (IAEA, 1992), the United NationsScientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1996), and theCanadian Nuclear Safety Commission (CNSC, 2002).
Results and Conclusions
Screening Level Risk Assessment
Examination of the HQs derived using RME radionuclide soil concentrations at the G-6 andKS sites indicated none of the uranium isotopes to individually pose potentially unacceptableradiological risks (all HQs < 1.0) to ecological resources at either area. The RME-basedHI for the G-6 area (0.127) was well below the HI threshold value of 1.0, indicating nopotential for unacceptable radiological risks to ecological resources at this site. Because ofthe low RME-based HI for the G-6 area, calculation of a CTE-based HI was not warrantedfor this area.
The RME-based HI calculated for the KS (HI = 1.01) only slightly exceeded thethreshold HI value of 1.0, while the CTE-based HI for this area was less than 0.02 (Table 5).The RME-based HI value suggests that there is a very slight potential for unacceptableradiological risks to biota from exposure to DU-related radionuclides in site soils at KS,and that further evaluation of radiological risks to biota may be warranted. Because of
Table 5Screening level ERA results
Radionuclide BCG (Bq/g) KS RME HQ KS CTE HQ G-6 RME HQ
U-238 58.4 0.74 0.012 0.094U-235 105 <0.01 <0.01 <0.01U-234 190 0.26 <0.01 0.03Total U HI NA 1.01 0.02 0.13
aNA = not applicable.
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Tabl
e6
Rec
epto
r-sp
ecifi
cE
RA
resu
ltsfo
rK
SIm
pact
Are
aba
sed
onre
ason
able
max
imal
expo
sure
(RM
E)
uran
ium
conc
entr
atio
ns
Rec
epto
rs
Kan
garo
oM
ourn
ing
Des
ert
Des
ert
Palli
dK
itPe
regr
ine
Am
eric
anG
ophe
rR
esul
tsV
eget
atio
nra
tdo
veig
uana
tort
oise
bat
Coy
ote
fox
falc
onke
stre
lsn
ake
Tis
sue
conc
entr
atio
na
U-2
38(B
q/kg
)3.
26×
102
5.82
×10
12.
05×
102
7.91
1.12
×10
18.
22×
102
1.62
×10
23.
33×
102
1.78
×10
13.
71×
102
4.88
U-2
35(B
q/kg
)5.
219.
23×
10−1
3.26
1.25
×10
−11.
78×
10−1
1.31
×10
12.
575.
302.
78×
10−1
5.92
7.76
×10
−2
U-2
34(B
q/kg
)3.
75×
102
6.94
×10
12.
37×
102
9.52
1.30
×10
29.
48×
102
1.90
×10
23.
86×
102
2.30
×10
14.
29×
102
5.74
Tota
lU(m
g/kg
)2.
63×
101
4.69
1.65
×10
16.
38×
10−1
9.00
×10
−16.
63×
101
1.30
×10
12.
681.
442.
99×
101
3.93
×10
−1
Dos
e(G
y/d)
U-2
389.
38×
10−4
5.48
×10
−43.
14×
10−5
4.87
×10
−44.
91×
10−4
6.08
×10
−51.
04×
10−8
1.66
×10
−75.
58×
10−8
1.36
×10
−59.
57×
10−5
U-2
359.
83×
10−6
4.12
×10
−63.
99×
10−7
3.14
×10
−63.
21×
10−6
9.91
×10
−71.
02×
10−1
01.
86×
10−9
8.92
×10
−10
2.21
×10
−76.
10×
10−7
U-2
345.
13×
10−4
1.01
×10
−42.
63×
10−5
2.05
×10
−52.
52×
10−5
7.80
×10
−54.
93×
10−9
1.08
×10
−78.
04×
10−8
1.74
×10
−53.
06×
10−6
Tota
lU1.
46×
10−3
6.53
×10
−45.
81×
10−5
5.11
×10
−45.
19×
10−4
1.40
×10
−41.
54×
10−8
2.75
×10
−71.
37×
10−7
3.12
×10
−59.
94×
10−5
Haz
ard
inde
x0.
150.
650.
06<
0.00
10.
520.
14<
0.00
1<
0.00
1<
0.00
10.
030.
10
aT
hedr
y-w
eigh
tins
ectt
issu
eco
ncen
trat
ions
wer
eas
sum
edto
beth
esa
me
asth
eso
ilco
ncen
trat
ions
.The
wat
erco
nten
tin
tissu
esw
asas
sum
edto
be0.
65.T
hetis
sue
conc
entr
atio
nsfo
rth
ew
ildlif
ere
cept
ors
wer
eca
lcul
ated
assu
min
gth
atal
lth
efo
odin
gest
edan
dai
rin
hale
dw
ere
cont
amin
ated
.Exp
osur
epa
ram
eter
sar
elis
ted
inTa
ble
2.
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Tabl
e7
Rec
epto
r-sp
ecifi
cE
RA
resu
ltsfo
rK
SIm
pact
Are
aba
sed
once
ntra
lten
denc
yex
posu
re(C
TE
)ur
aniu
mco
ncen
trat
ions
Rec
epto
rs
Kan
garo
oM
ourn
ing
Des
ert
Des
ert
Palli
dK
itPe
regr
ine
Am
eric
anG
ophe
rR
esul
tsve
geta
tion
rat
dove
igua
nato
rtoi
seba
tC
oyot
efo
xfa
lcon
kest
rel
snak
e
Tis
sue
conc
entr
atio
na
U-2
38(B
q/kg
)5.
369.
55×
10−1
3.36
1.30
×10
−11.
83×
10−1
1.35
×10
12.
665.
462.
92×
10−1
6.09
8.01
×10
−2
U-2
35(B
q/kg
)7.
55×
10−2
1.34
×10
−24.
72×
10−2
1.82
×10
−32.
58×
10−3
1.90
×10
−13.
73×
10−2
7.68
×10
−24.
03×
10−3
8.58
×10
−21.
12×
10−3
U-2
34(B
q/kg
)6.
121.
133.
871.
55×
10−1
2.11
×10
−11.
54×
101
3.10
6.29
3.75
×10
−16.
999.
35×
10−2
Tota
lU(m
g/kg
)4.
30×
10−1
7.67
×10
−22.
70×
10−1
1.04
×10
−21.
47×
10−2
1.08
2.13
×10
−14.
38×
10−1
2.35
×10
−24.
89×
10−1
6.43
×10
−3
Dos
es(G
y/d)
U-2
381.
54×
10−5
8.99
×10
−65.
15×
10−7
8.00
×10
−68.
06×
10−6
9.99
×10
−71.
7×
10−1
02.
72×
10−9
9.17
×10
−10
2.23
×10
−11.
57×
10−6
U-2
351.
42×
10−7
5.98
×10
−85.
79×
10−9
4.55
×10
−84.
65×
10−8
1.44
×10
−81.
47×
10−1
22.
70×
10−1
11.
29×
10−1
13.
20×
10−9
8.85
×10
−9
U-2
348.
35×
10−6
1.64
×10
−64.
29×
10−7
3.35
×10
−74.
10×
10−7
1.27
×10
−68.
04×
10−1
11.
75×
10−9
1.31
×10
−92.
83×
10−7
4.99
×10
−8
Tota
lU2.
39×
10−5
1.07
×10
−59.
50×
10−7
8.38
×10
−68.
52×
10−6
2.28
×10
−62.
52×
10−1
04.
50×
10−9
2.24
×10
−95.
09×
10−7
1.63
×10
−6
Haz
ard
inde
x<
0.01
0.01
<0.
01<
0.01
<0.
01<
0.01
<0.
01<
0.01
<0.
01<
0.01
<0.
01
The
dry-
wei
ghti
nsec
ttis
sue
conc
entr
atio
nsw
ere
assu
med
tobe
the
sam
eas
the
soil
conc
entr
atio
ns.T
hew
ater
cont
enti
ntis
sues
was
assu
med
tobe
0.65
.The
tissu
eco
ncen
trat
ions
for
the
wild
life
rece
ptor
sw
ere
calc
ulat
edas
sum
ing
that
all
the
food
inge
sted
and
air
inha
led
wer
eco
ntam
inat
ed.E
xpos
ure
para
met
ers
are
liste
din
Tabl
e2.
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Ecological Risks of Depleted Uranium Exposure 593
the conservatism associated with the RME-based dose, HQ, and HI estimates, this slightpotential for unacceptable radiological risks is very likely overestimated. Actual soil con-centrations of DU-related uranium isotopes are much less than the RME concentrations usedto estimate HQs and HIs, with most of the site at natural background radiation levels (un-published data). Measured background concentrations of uranium at the test facility variesby location, and averages about 3 ug/g, depending on local geological and mineralogicalconditions (unpublished data; Johnson et al., 2004).
In addition, the dose, HQ, and HI estimates were derived assuming that the entire KSsite has a DU-related uranium isotope soil concentration equal to the RME concentrationthat was determined using data from a tank target area that exhibited maximum penetratoroccurrence (unpublished data). In addition, the BCG values used to calculate the HQs andHIs were derived using very conservative exposure assumptions (USDOE, 2002). Few, ifany, biota at KS would be expected to actually incur such exposure. Thus, the actual potentialfor unacceptable radiological risks to biota is very likely well below the level suggested bythe HI. However, because the screening-level ERA did result in a HI value exceeding thethreshold limit of 1.0, the potential for unacceptable radiological risks to biota at KS wasfurther evaluated by conducting a receptor-specific ERA.
Receptor-Specific Risk Assessment
The receptor-specific ERA for KS evaluated potential radiological risks to terrestrial vege-tation and ten wildlife species (representing major trophic levels and biota categories at thesite) using the RME and CTE uranium isotope concentrations in site soil (Table 2) and dosemodeling to estimate internal, external and total exposure. The species-specific exposurefactors for the 10 wildlife species evaluated are presented in Table 3.
Estimated RME-based tissue concentrations and total dose estimates for vegetationand the wildlife receptors are presented in Table 6; none of the HI values exceeded thethreshold limit of 1.0. The highest HI values were 0.65, 0.52, and 0.51, calculated for thekangaroo rat, desert tortoise, and desert iguana, respectively. The HIs for these three wildlifereceptors are greater than for the other wildlife receptors because they were assumed to spend100% of their lifetime in the contaminated area at the RME soil concentration (because thehome ranges of these species can be fully encompassed within KS area), and because allconsumed food and inhaled air were assumed to be contaminated. For wildlife species suchas the coyote, kit fox, and peregrine falcon that have home ranges much larger than the KSarea, the HIs were very small (<0.001).
Tissue concentrations, dose estimates, and HI estimates were also determined usingthe CTE-based uranium isotope soil concentrations, and are presented in Table 7. On thebasis of the low values of the RME-based HIs, none of the CTE-based HIs were expectedto exceed the threshold limit of 1.0. The highest CTE-based HI was 0.01, calculated for thekangaroo rat.
Conclusions
The KS and G-6 sites were conservatively evaluated for potential unacceptable radiologicalrisks to ecological resources from exposure to DU contamination in soils resulting from pastweapons testing activities. The screening level risk assessment identified a slight potentialfor unacceptable risks only at KS and only under near-maximum DU-related radionuclidesoil concentrations. In contrast, the receptor-specific ERA for KS, using more realisticreceptor-specific exposure scenarios and uptake and dose models, indicated no unacceptable
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594 J.-J. Cheng et al.
ecological risks from the near-maximum DU-related radionuclide concentrations. Thus,potential radiation exposures of biota under current conditions at the KS and the G-6 areasare well below levels that would result in potentially unacceptable ecological risks, and thusdo not warrant either further evaluation or remediation. Any removal of DU penetrators fromthese locations can be expected to decrease ecological exposures and thus reduce potentialradiological risks to ecological resources to even lower levels than those identified in thisrisk assessment.
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