Radiological Screening Values for Effects on Aquatic Biota

at the Oak Ridge Reservation

Presented at

The Annual Meeting of DOE Biota Dose Assessment Committee

August 18, 1999

Washington, D.C.

By

Daniel S. Jones

Environmental Sciences Division

Oak Ridge National Laboratory

Overview

• Sponsor: DOE-Oak Ridge Operations’ Environmental Management Program

• Intended use: CERCLA Ecological Risk Assessments at ORO waste sites 1) show spatial extent of potential ecological effects and

2) identify the need for additional site-specific investigation.

• Derived using formulas for estimating the dose rates to representative aquatic organisms (Blaylock et al., 1993).

• Screening values based on total dose rate of 1 Rad d-1

Recommended acceptable dose rate to natural populations of aquatic biota.

Overview

• If the total dose rate from all radionuclides and pathways exceeds a recommended acceptable dose rate, further analysis is needed to determine the hazards posed by radionuclides (e.g., biological surveys and realistic exposure modeling).

• If, however, the total dose rate falls below an acceptable dose rate, radionuclides may be eliminated from further study.

Methodology

• Point Source Dose Distribution (IAEA 1976, 1979)

– Uses empirically derived dose rate formulas– Size categories of organisms are represented by

ellipsoid geometries (Table 1)

• Used to estimate the fraction of emitted energy that is absorbed by the organism

Table 1. Dimensions of organisms representing selected size categoriesOrganism Mass (kg) Length of the major axes of the ellipsoid (cm)

Small fish 0.002 3.1 × 1.6 × 0.78Large fish 1.0 45 × 8.7 × 4.9

Formulas• Dose rates (Rad d-1) from an individual isotope in

the organism (D Internal), water (D External, w), and surface sediment (D External, s) are given by:– D Int = 5.11 × 10-8 E n Co ,

– D Ext, w = 5.11 × 10-8 E n (1 - ) Cw ,

– D Ext, s = 1.92 × 10-5 E n (1 - ) Cs ,

where:

E = the average emitted energy (MeV dis-1),

n = the proportion of transitions producing an emission of energy E,

= the fraction of the emitted energy absorbed by the organism,

Co = the radionuclide concentration in the organism (pCi kg-1 wet wt),

Cw = the radionuclide concentration in water (pCi L-1), and

Cs = the radionuclide concentration in sediment (pCi g-1 dry weight).

Formulas(Exposure Assumptions)

D Int = 5.11 × 10-8 E n Co

D Ext, w = 5.11 × 10-8 E n (1 - ) Cw

• 5.11 × 10-8 = conversion factor from MeV dis-1 to Rad d-1

• Assumes water exposure from all sides, including ventrally

D Ext, s = 1.92 × 10-5 E n (1 - ) Cs ,• 1.92 × 10-5 = conversion factor from MeV dis-1 to Rad d-1

• Assumes 50% immersion in sediment – i.e.sediment exposure from bottom half

• Includes the default wet weight-dry weight conversion factor of 0.75 presented in NCRP (1991).

Formulas• For each isotope and pathway, the total dose

rate is the sum of the dose rates from each type of radiation. For example:

• D Int, total = D Int, alpha + D Int, beta + D Int, gamma

• Then, the total dose rate per isotope is the sum of the dose rates from each pathway.

• D Total = D Int, total + D Ext, w, total + D Ext, s, total

• Then, the dose rate from all isotopes can be summed.– Must account for the Relative Biological

Effectiveness (RBE) of each type of radiation, i.e. a quality factor of 20 is used for alpha particles.

Parameters• Absorbed Dose is a function of the energy emitted

(E) and the fraction absorbed ()– is a function of E and the size of the organism. – Figure 1 presents one of the empirically derived

relationships used to estimate () from E.– Average E in Table 2 can be used in place of E and n

(ICRP 1983).

– is 1 for beta in large fish and alpha in all fish. Therefore, the external dose is 1-1=0.

Table 2. Emission energies (E) and absorbed fractions () for selected radionuclides a

Absorbed Fractionsc

Emission Energies (MeV) Beta Gamma

Radionuclide (yield) Half-lifeAverageAlpha

MaximumBetab

AverageBeta

AverageGamma

SmallFish

SmallFish

LargeFish

Plutonium-239 24065y 5.23e+00 6.65e-03 7.96e-04 1 1 1Plutonium-240 6537y 5.24e+00 1.06e-02 1.73e-03 1 0.7 0.94Thorium-232 1.41e+10y 4.07e+00 1.25e-02 1.33e-03 1 0.7 0.94 Radium-228 5.75y 5.50e-02 1.69e-02 4.14e-09 1 1 1 Actinium-228 6.13h 2.08e+00 4.60e-01 9.30e-01 0.93 0.012 0.11Thorium-228 1.9131y 5.49e+00 2.05e-02 3.30e-03 1 0.7 0.94 Radium-224 3.66d 5.78e+00 2.21e-03 9.89e-03 1 0.7 0.94 Radon-220 55.6s 6.40e+00 8.91e-06 3.85e-04 1 1 1 Polonium-216 0.15s 6.91e+00 1.61e-07 1.69e-05 1 1 1 Lead-212 10.64h 5.86e-01 1.75e-01 1.48e-01 1 0.01 0.1 Bismuth-212 60.55m 2.22e+00 2.26e+00 4.69e-01 1.85e-01 0.91 0.01 0.1 Polonium-212 (64.07% of Bi-212)

0.305us 8.95e+00

Thallium-208 (35.93% of Bi-212)

3.07m 2.38e+00 5.91e-01 3.36e+00 1 0.0085 0.08

Americium-241 432.2y 5.57e+00 5.19e-02 3.24e-02 1 0.04 0.34a Selected isotopes are a subset of those presented in Blaylock et al. (1993). Indented radionuclides are thedaughter products of the preceding long-lived radionuclide, as presented in Blaylock et al. (1993). Yields, half-lives, and average energies are from ICRP (1983).

Transfer Assumptions• Uptake from water is estimated using the

biological concentration factors (BCFs) for freshwater fish.

Co = Cw x BCF

• Available BCFs primarily for fish muscle– Underestimates Co for radionuclides preferentially

sequestered in other tissues (e.g., Sr in bone)– However, most isotopes do not appear to be

preferentially sequestered in the reproductive tissues.

(Garten 1981, Garten et al. 1987, and Kaye and Dunaway 1962).

Transfer Assumptions• Uptake from Sediment: there are no standard

sediment-to-fish transfer factors– sediment-water partition coefficient (Kd) used to

derive Cw as follows: Co = Cs/Kd x BCF

– Assumes overlying Cw = interstitial Cw (conservative for lotic systems)

– Used when there is no co-located water data.

• Adsorption to sediment: also derived using KdCs = Cw x Kd

– Used to provide external dose rate from sediment if sediment data are not available

Prudently Conservative Parameters

• “Expected” BCFs and Kds were converted from a wet weight to dry weight basis

• because their derivation was not clearly defined

– BCFs x default ww:dw factor of 0.2 (NCRP 1991) – Kds / default ww:dw factor of 0.75 (NCRP 1991)

• Corrected Kd used to estimate Cs from Cw

• Uncorrected Kd used to estimate Cw from Cs

– Extreme values did not appear to be credible (e.g., the maximum BCF for thorium was 10,000), but re-evaluating the original studies was beyond the scope of this effort

Radioactive Decay Chains

• Short-lived daughter product uptake is not explicitly modeled. – Long-lived parent is modeled, short-lived progeny

are assumed to be in secular equilibrium with the parent

• Conservative for extremely long-lived parents

– Considered short-lived if < 30 day half-life (180 days used for humans;Yu 1993).

– Activity of progeny = activity of parent times the yield of the progeny (Table 2)

Benchmarks

• Concentration of an isotope that results in a total dose rate of 1 Rad d-1 (Recommended acceptable limit, NCRP 1991)

• Single-media benchmarks consider exposures from only one medium (Figure 2)– Water(w) includes internal and external exposures

from water only

– Sediment(s) includes only external exposures from sediment

– used when both water and sediment data are available at a site

External Radiationfrom Water

External Radiationfrom Sediment

CW (Measured)

CS (Measured)

Uptake (CW x BCF)

Internal Radiation

CO (Estimated)

Figure 2. Exposure pathway assumptions for the single-media benchmarks Water(W) and Sediment(S). The measured concentration in water (CW) is screened against the Water(W) benchmark, which includes estimated internal

exposures, and the measured concentration in sediment (CS) is screened against the Sediment(S) benchmark.

Benchmarks

• Multimedia benchmarks incorporate exposures from sediment and water– Water(w+s) includes exposures that are internal,

external from water, and external from sediment (Figure 3)

– Sediment(s+w) includes internal exposures and external from exposures sediment (Figure 4)

– Used when only one medium was sampled at a site

External Radiationfrom Water

External Radiationfrom Sediment

CW (Measured)

CS (Estimated)

Uptake (CW x BCF)

Adsorption (CW x K d )

Internal Radiation

CO (Estimated)

Figure 3. Exposure pathway assumptions for the multi-media benchmark Water(W+S). The

measured concentration in water (CW) is screened against the Water(W+S) benchmark , which includes estimated external exposure from sediment and the estimated internal exposure.

External Radiationfrom Water

External Radiationfrom Sediment

CW (Estimated)

CS (Measured)

Uptake (CW x BCF)

Desorption (CS / K d )

Internal Radiation

CO (Estimated)

Figure 4. Exposure pathway assumptions for the multi-media benchmark Sediment(S+W). The

measured concentration in sediment (CS) is screened against the Sediment(S+W) benchmark, which includes estimated external exposure from water and the estimated internal exposure.

Screening Process

• Calculate a hazard quotient (HQ) for each radionuclide (HQ=measured concentration divided by benchmark)

– HQ >1 indicates that the dose rate is > 1 rad d-1

• Calculate a hazard index (HI) for each medium– The HI is a measure of the total dose rate to the

organism– It is the sum of the HQs for each radionuclide– HI >1 indicates that the total dose rate is > 1 rad d-1

• Two examples are worked using the small fish benchmarks (data are from DOE 1997)

Example 1Single-Media Benchmarks

• Water and sediment data are treated as co-located samples (Table 3). – CW is divided by the Water(w) benchmark

– CS is divided by the Sediment(s) benchmark

• HI-Water is the sum of the HQs for water

• HI-Sediment is the sum of the HQs for sediment.

• HI-Total is the sum of all HQs for small fish. – The HI-Total of 0.03 suggests that the radionuclides

measured at this location pose a negligible risk to aquatic biota.

Table 3. Example 1: Use of single-media benchmarks for thecalculation of hazard quotients (HQs) and hazard indices (HIs)

Radionuclide Concentrationa Benchmarkb HQc

Water (pCi L-1)Strontium-90 1.33 6.29e+04 2.11e-05Technetium-99 327 1.94e+06 1.69e-04Thorium-228 0.144 6.01e+01 2.40e-03Thorium-230 0.117 4.13e+02 2.83e-04Thorium-232 0.081 4.78e+02 1.69e-04Uranium-233/234 37.9 4.00e+03 9.48e-03

Uranium-235 2.33 4.37e+03 5.33e-04Uranium-238 83.1 4.55e+03 1.83e-02

HI - Water 3.13e-02Sediment (pCi g-1)

Americium-241 0.06 1.67e+06 3.59e-08Cesium-137 0.18 9.32e+04 1.93e-06Technetium-99d 8.74 N/A N/AThorium-228 1.45 3.31e+04 4.38e-05Thorium-230 1.03 1.12e+08 9.20e-09Thorium-232 0.99 5.47e+04 1.81e-05Uranium-234 16.77 1.00e+08 1.68e-07Uranium-235 0.73 2.96e+05 2.47e-06Uranium-238 27.38 1.75e+06 1.56e-05

HI - Sediment 8.22e-05

HI - Total 3.14e-02aWater and sediment concentrations are from DOE (1997).bBenchmarks are the Water(W) and Sediment(S) values for small fish.cThe hazard quotient is the media concentration divided by the benchmark value. The hazard index is the sum of the hazard quotients.dTechnetium-99 does not have a Sediment(S) benchmark because external exposure is not a significant pathway.

Example 2Multi-Media Benchmarks

• Water and sediment data are evaluated separately, as if only one of them were available (Table 4).– CW is divided by the Water(w+s) benchmark

– CS is divided by the Sediment(s+w) benchmark

• HI-Total is the sum of all HQs for a medium.

• HI based on water data only is 0.0314– equal to HI-Total from Example 1

• HI based on sediment data only is 0.211– Much higher than the HI-Total in Example 1, due to

use of Kd to derive internal dose rate

Table 4. Example 2: Use of multimedia benchmarks for thecalculation of hazard quotients (HQs) and hazard indices (HIs)

Radionuclide Concentrationa Benchmarka HQc

Water (pCi L-1)Strontium-90 1.33 5.80e+04 2.29e-05Technetium-99 327 1.94e+06 1.69e-04Thorium-228 0.144 5.93e+01 2.43e-03Thorium-230 0.117 4.13e+02 2.83e-04Thorium-232 0.081 4.49e+02 1.80e-04Uranium-233/234 37.9 4.00e+03 9.48e-03

Uranium-235 2.33 4.36e+03 5.34e-04Uranium-238 83.1 4.55e+03 1.83e-02

HI - Total 3.14e-02

Sediment (pCi g-1)Americium-241 0.06 5.83e+03 1.03e-05Cesium-137 0.18 7.13e+03 2.52e-05Technetium-99 8.74 9.69e+03 9.02e-04Thorium-228 1.45 5.90e+02 2.46e-03Thorium-230 1.03 4.13e+03 2.49e-04Thorium-232 0.99 4.40e+03 2.25e-04Uranium-234 16.77 2.02e+02 8.30e-02Uranium-235 0.73 2.18e+02 3.35e-03Uranium-238 27.38 2.27e+02 1.21e-01

HI - Total 2.11e-01aWater and sediment concentrations are from DOE (1997).bBenchmarks are the Water(W+S) and Sediment(S+W) values for small fish.cThe hazard quotient is the media concentration divided by the benchmark value. The hazard index is the sum of the hazard quotients.

Recommended Usage

• Screening values only for natural populations of aquatic biota.

• Not remediation goals, which must consider other issues

• Collect co-located samples of sediment and water– Single-media benchmarks are less uncertain than multi-media

benchmarks (i.e., no Kd)

Recommended Usage

• Screening Approach– NCRP (1991) recommends a comprehensive evaluation if the dose

rate > 0.25 rad d-1

– An expert panel recommends that representative exposures be used (Barnthouse 1995)

– Possible compromise: If maximum exposure > 0.25 rad d-1, then use representative exposure

– Consider other stressors (co-contaminants, siltation, etc.) if radiological risks are possible

Copies of

these and other

ORNL Benchmarks

are available on the internet at

http://www.hsrd.ornl.gov/ecorisk/ecorisk

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References