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Radiological Screening Values for Effects on Aquatic Biota at the Oak Ridge Reservation

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.

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Radiological Screening Values for Effects on Aquatic Biota at the Oak Ridge Reservation

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  1. Radiological Screening Values for Effects on Aquatic Biotaat 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

  2. 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.

  3. 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.

  4. 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

  5. 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 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).

  6. 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).

  7. 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.

  8. 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.

  9. 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).

  10. 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

  11. 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

  12. 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)

  13. 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

  14. 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.

  15. 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

  16. CW (Measured) External Radiation from Water Uptake (CW x BCF) Internal Radiation Adsorption (CW x K d ) CO (Estimated) 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.

  17. CW (Estimated) External Radiation from Water Uptake (CW x BCF) Internal Radiation Desorption (CS / K d ) CO (Estimated) 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.

  18. 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)

  19. 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.

  20. 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

  21. 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)

  22. 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

  23. Copies of these and other ORNL Benchmarks are available on the internet at http://www.hsrd.ornl.gov/ecorisk/ecorisk

  24. References Barnthouse, L. W. 1995. Effects of Ionizing Radiation on Terrestrial Plants and Animals: A Workshop Report, ORNL/TM-13141, Oak Ridge Natl. Lab., Oak Ridge, Tenn. Blaylock, B. G., M. L. Frank, and B. R. O’Neal. 1993. Methodology for Estimating Radiation Dose Rates to Freshwater Biota Exposed to Radionuclides in the Environment, ES/ER/TM-78, Oak Ridge Natl. Lab., Oak Ridge, Tenn. DOE (U.S. Department of Energy). 1997. Report on the Remedial Investigation of Bear Creek Valley at the Oak Ridge Y-12 Plant, Oak Ridge, Tennessee, DOE/OR/01-1455/V3&D2, Lockheed Martin Energy Systems, Inc., Oak Ridge, Tenn. Garten, C. T., Jr. 1981. Comparative uptake of actinides by plants and rats from the shoreline of a radioactive pond, J. Environ. Qual., 10:487–91. Garten, C. T., Jr., E. A. Bondietti, J. R. Trabalka, R. L. Walker, and T. G. Scott. 1987. Field studies on the terrestrial behavior of actinide elements in East Tennessee, Pages 109–19 in: Environmental Research on Actinide Elements(eds. J. E. Pinder III, J. J. Alberts, K. W. McLeod, and R. G. Schreckhise), CONF-841142, U.S. Department of Energy, Washington, D.C. IAEA (International Atomic Energy Agency). 1976. Effects of Ionizing Radiation on Aquatic Organisms and Ecosystems, IAEA Technical Report Series 172, Vienna, Austria. IAEA (International Atomic Energy Agency). 1979. Methodology for Assessing Impacts of Radioactivity on Aquatic Ecosystems, IAEA Technical Report Series 190, Vienna, Austria. ICRP (International Commission on Radiological Protection). 1983. Radionuclide Transformations: Energy and Intensity of Emissions, ICRP Publication No. 38, Vienna, Austria. Kaye, S. V., and P. B. Dunaway. 1962. Bioaccumulation of radioactive isotopes by herbivorous small mammals, Health Phys., 7:205–17. NCRP (National Council on Radiation Protection and Measurements). 1991. Effects of Ionizing Radiation on Aquatic Organisms, NCRP Report No. 109, National Council on Radiation Protection and Measurements, Bethesda, Md. Suter, G. W., II. 1995. Guide for Performing Screening Ecological Risk Assessments at DOE Facilities, ES/ER/TM-153, Oak Ridge Natl. Lab., Oak Ridge, Tenn. Yu, C., C. Loureiro, J. J. Cheng, L. G. Jones, Y. Y. Wang, Y. P. Chia, and E. Faillace. 1993. Data Collection Handbook to Support Modeling Impacts of Radioactive Material in Soil, Argonne National Laboratory, Argonne, Ill.

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