Transcript ORNL Radiological Screening Values for Effects on Aquatic Biota

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 categories
Organism
Small fish
Large fish
Mass (kg)
0.002
1.0
Length of the major axes of the ellipsoid (cm)
3.1 × 1.6 × 0.78
45 × 8.7 × 4.9
Formulas
• Dose rates (Rad d ) from an individual isotope in
the organism (D ), water (D
), and surface
sediment (D
) are given by:
-1
Internal
External, w
External, s
– D Int = 5.11 × 10-8 E n M Co ,
– D Ext, w = 5.11 × 10-8 E n (1 - M) Cw ,
– D Ext, s = 1.92 × 10-5 E n (1 - M) Cs ,
where:
E = the average emitted energy (MeV dis-1),
n = the proportion of transitions producing an emission of energy E,
M
= 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 M Co
D Ext, w = 5.11 × 10-8 E n (1 - M) 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 - M) 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 (M)
– M is a function of E and the size of the organism.
– Figure 1 presents one of the empirically derived
relationships used to estimate M () from E.
– Average E in Table 2 can be used in place of E and n
(ICRP 1983).
– M 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 radionuclidesa
Absorbed Fractionsc
Beta
Gamma
Small
Small
Large
Fish
Fish
Fish
1
1
1
1
0.7
0.94
1
0.7
0.94
1
1
1
0.93
0.012
0.11
1
0.7
0.94
1
0.7
0.94
1
1
1
1
1
1
1
0.01
0.1
0.91
0.01
0.1
Emission Energies (MeV)
Average Maximum Average Average
Radionuclide (yield) Half-life Alpha
Beta b
Beta Gamma
Plutonium-239
24065y 5.23e+00
6.65e-03 7.96e-04
Plutonium-240
6537y 5.24e+00
1.06e-02 1.73e-03
Thorium-232
1.41e+10y 4.07e+00
1.25e-02 1.33e-03
Radium-228
5.75y
5.50e-02 1.69e-02 4.14e-09
Actinium-228
6.13h
2.08e+00 4.60e-01 9.30e-01
Thorium-228
1.9131y 5.49e+00
2.05e-02 3.30e-03
Radium-224
3.66d
5.78e+00
2.21e-03 9.89e-03
Radon-220
55.6s
6.40e+00
8.91e-06 3.85e-04
Polonium-216
0.15s
6.91e+00
1.61e-07 1.69e-05
Lead-212
10.64h
5.86e-01 1.75e-01 1.48e-01
Bismuth-212
60.55m 2.22e+00 2.26e+00 4.69e-01 1.85e-01
Polonium-212
0.305us 8.95e+00
(64.07% of Bi-212)
Thallium-208
3.07m
2.38e+00 5.91e-01 3.36e+00
1
0.0085
0.08
(35.93% of Bi-212)
Americium-241
432.2y 5.57e+00
5.19e-02 3.24e-02
1
0.04
0.34
a Selected isotopes are a subset of those presented in Blaylock et al. (1993). Indented radionuclides are the
daughter products of the preceding long-lived radionuclide, as presented in Blaylock et al. (1993). Yields, halflives, 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 Kd
Cs = 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 reevaluating 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
CW
(Measured)
External Radiation
from Water
Uptake
(CW x BCF)
Internal
Radiation
CO
(Estimated)
External Radiation
from Sediment
CS
(Measured)
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
CW
(Measured)
External Radiation
from Water
Uptake
(CW x BCF)
Internal
Radiation
CO
Adsorption
(Estimated)
(CW x K d )
External Radiation
from Sediment
CS
(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.
CW
(Estimated)
External Radiation
from Water
Uptake
(CW x BCF)
Internal
Radiation
CO
Desorption
(Estimated)
(CS / K d )
External Radiation
from Sediment
CS
(Measured)
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 1
Single-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 the
calculation of hazard quotients (HQs) and hazard indices (HIs)
Radionuclide
Strontium-90
Technetium-99
Thorium-228
Thorium-230
Thorium-232
Uranium-233/234
Uranium-235
Uranium-238
Americium-241
Cesium-137
Technetium-99d
Thorium-228
Thorium-230
Thorium-232
Uranium-234
Uranium-235
Uranium-238
Concentrationa Benchmarkb
Water (pCi L-1)
1.33
6.29e+04
327
1.94e+06
0.144
6.01e+01
0.117
4.13e+02
0.081
4.78e+02
37.9
4.00e+03
2.33
4.37e+03
83.1
4.55e+03
HI - Water
-1
Sediment (pCi g )
0.06
1.67e+06
0.18
9.32e+04
8.74
N/A
1.45
3.31e+04
1.03
1.12e+08
0.99
5.47e+04
16.77
1.00e+08
0.73
2.96e+05
27.38
1.75e+06
HI - Sediment
HI - Total
a
Water and sediment concentrations are from DOE (1997).
b
Benchmarks are the Water(W) and Sediment(S) values for small fish.
c
HQc
2.11e-05
1.69e-04
2.40e-03
2.83e-04
1.69e-04
9.48e-03
5.33e-04
1.83e-02
3.13e-02
3.59e-08
1.93e-06
N/A
4.38e-05
9.20e-09
1.81e-05
1.68e-07
2.47e-06
1.56e-05
8.22e-05
3.14e-02
The hazard quotient is the media concentration divided by the benchmark value. The hazard index is the sum of the hazard quotients.
d
Technetium-99 does not have a Sediment(S) benchmark because external exposure is not a significant pathway.
Example 2
Multi-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 the
calculation of hazard quotients (HQs) and hazard indices (HIs)
Radionuclide
Strontium-90
Technetium-99
Thorium-228
Thorium-230
Thorium-232
Uranium-233/234
Uranium-235
Uranium-238
Americium-241
Cesium-137
Technetium-99
Thorium-228
Thorium-230
Thorium-232
Uranium-234
Uranium-235
Uranium-238
Concentrationa Benchmarka
Water (pCi L-1)
1.33
5.80e+04
327
1.94e+06
0.144
5.93e+01
0.117
4.13e+02
0.081
4.49e+02
37.9
4.00e+03
2.33
4.36e+03
83.1
4.55e+03
HI - Total
Sediment (pCi g-1)
0.06
5.83e+03
0.18
7.13e+03
8.74
9.69e+03
1.45
5.90e+02
1.03
4.13e+03
0.99
4.40e+03
16.77
2.02e+02
0.73
2.18e+02
27.38
2.27e+02
HI - Total
a
Water and sediment concentrations are from DOE (1997).
b
Benchmarks are the Water(W+S) and Sediment(S+W) values for small fish.
c
HQc
2.29e-05
1.69e-04
2.43e-03
2.83e-04
1.80e-04
9.48e-03
5.34e-04
1.83e-02
3.14e-02
1.03e-05
2.52e-05
9.02e-04
2.46e-03
2.49e-04
2.25e-04
8.30e-02
3.35e-03
1.21e-01
2.11e-01
The 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