Press Release

Table of Contents

Acknowledgements

Summary and Recommendations

1. Introduction

2. The concept of the critical group and the maximally exposed individual

3. Description of the subsistence farmer scenario

4. International use of the subsistence farmer approach

5. Reasonableness of the subsistence farmer scenario on occupational grounds

6. Relation of the subsistence farmer scenario to Radionuclide Soil Action Levels (RSALs) at Rocky Flats

7. Erosion of the subsistence farmer scenario

8. The Radioactive Wildlife Refuge

9. Enforcement for the eons

10. Conclusions and Recommendations

11. References

Health risks to people living near a site that has been decommissioned may arise from a number of different sources, such as:

• Direct gamma radiation from residual radionuclides, and in some cases also neutron and beta radiation
• drinking contaminated water
• eating food grown using contaminated water for irrigation
• eating contaminated soil or ingesting it during periods when the air is dusty or via food
• breathing air containing contaminated soil that has re-suspended due to high winds
• breathing contaminants entering the air during fires
• exposure in utero via the mother's diet

These sources of risk are not static or independent. One of the most important sources of the evaluation of total risk and the distribution of doses via specific pathways is the residual contamination in the soil. For instance, the contamination in the soil acts as a reservoir for potential contamination of water that would be used for drinking or irrigation. As another example, the amount of radioactivity that is present in the air during periods of heavy wind, such as those that occur commonly at Rocky Flats, depends directly on the residual soil contamination, as does uptake of radioactivity by plants. Both of course, depend on other factors as well.

These points were illustrated by the Risk Assessment Corporation (RAC) in their analysis of RSALs at Rocky Flats. Their conclusions were that the most important exposure pathway at Rocky Flats was the inhalation of contaminated soil that had been resuspended by gusts of wind.⁵⁰ In addition, their recommended RSAL of 35 pCi/g does not assume a 100% probability of a large grass fire that would enhance the resuspension of contaminated soil. If this were the case, the RSAL would be even lower than 35 pCi/g.⁵¹ This analysis also admits shortcomings in its investigation into the groundwater exposure pathway.⁵²

Because of the crucial connection of residual soil contamination to a number of dose pathways, the residual concentration of long-lived radionuclides in the soil is a parameter of central importance in assessing the efficacy of clean up in protecting future populations. A number of radionuclides, such as tritium and strontium-90 are known to migrate rapidly through the soil. It had been the conviction of the DOE and its contractors for several decades that plutonium would not migrate rapidly through the soil. However, evidence has been accumulating for over two decades that, under a variety of conditions, the ion-exchange property of the soil that would bind plutonium and greatly retard its migration is overwhelmed by countervailing phenomena: migration of plutonium in colloidal form, the mobilization of plutonium by natural organic materials in the soil and spilled or dumped solvents, and complexing of plutonium with compounds present in the soil.⁵³

For instance, experience at Oak Ridge has shown that organic materials in the soil can mobilize plutonium by forming complexes with it causing rapid movement through the soil and into groundwater. The rate of plutonium migration under such conditions was estimated in an Oak Ridge National Laboratory report to be 100 to 1,000 times faster "than predicted from batch adsorption studies in the literature."⁵⁴

Assumptions in the early years that insoluble forms of plutonium would remain that way in the environment for long periods of time or remain bound by ion exchange in the soil for hundreds of thousands of years are being shown to be contrary to actual experience under a variety of circumstances. One fundamental reason is that the chemistry of plutonium is extremely complex. According to a Los Alamos scientific evaluation of the properties of plutonium, "[n]o other element displays such a complex chemistry."⁵⁵

Specifically, the Los Alamos paper describes, among other things, the behavior of plutonium in oxidation state IV, which is the oxidation state of plutonium dioxide. This is the most insoluble form of plutonium and it is also the form that has been found at Rocky Flats Pad 903.⁵⁶ But insolubility does not guarantee that plutonium will remain relatively immobile, an assumption that has been made in evaluations of Rocky Flats.

Insoluble plutonium can be mobilized and can move rapidly through the vadose zone into groundwater in colloidal form. This has been found not only at the Nevada Test Site as noted above, but has been noted to be a specific property of plutonium in the IV valence state found at Rocky Flats. According to the Los Alamos study:

A growing body of careful research shows that the migration of plutonium in the environment is dependent not only on the oxidation state of plutonium but on the environmental conditions in which that oxidation state is present. A changing environment will change the potential for plutonium mobility. .

Even if almost all the plutonium were to be in this insoluble form today, there is no guarantee that it will remain so in the future. Complexing with carbonate ions, for instance, can mobilize plutonium. Use of Rocky Flats as a wildlife preserve may considerably increase the amount of vegetable, animal, and related organic matter over the decades at Rocky Flats, creating new and unforeseen mechanisms for complexing and mobilization of plutonium. Natural organic matter has been known to mobilize plutonium at least one DOE site (Oak Ridge).⁵⁸ Hence if the site is first used as a wildlife refuge and then as a residential site, a ranch or a farm, the potential for harm may actually increase in comparison to a cleanup of the soil to a level corresponding to a subsistence rancher or farmer scenario.

Further evidence explaining the rapid migration of plutonium in groundwater is illustrated by the work of Haschke, Allen, and Morales.⁵⁹ Their experiments have shown that the water-catalyzed oxidation of plutonium dioxide (PuO₂) in air yields PuO_2+x in which plutonium is in its Pu(VI) valence state and therefore in a soluble form. The increase in solubility would increase mobility in groundwater. This might further explain the rapid migration of plutonium (1.3 km in 30 years) described by Kersting, et al.

The current contamination of groundwater at Rocky Flats with americium-241 and plutonium-239/240 is generally regarded as minimal. For instance, the reported maximum contamination levels in the fall of 2000 were 0.0354 and .0193 picocuries per liter respectively.⁶⁰ On an annual basis, these concentrations would result in doses of 1.7 and 0.9 millirem per year from drinking water alone, using EPA Federal Guidance Report 11 dose conversion factors.⁶¹ These add up to 2.6 mrem per year, or more than half of the drinking water limit of 4 mrem per year set for beta emitters.⁶² A two-fold increase would result in the drinking water dose exceeding 4 mrem per year. A six-fold increase in transuranic contamination would result in a drinking water dose exceeding the 15 mrem per year limit used by RAC for its calculations.

For a 500 pCi per gram of soil residual plutonium level, plus the associated americium-241 of about 55 pCi per gram of soil, RAC analysis estimated a water pathway dose of 88 mrem/year, mainly from drinking water.⁶³ For the 35 pCi/gram suggested as the plutonium RSAL by RAC, the dose would be about 6 mrem/year, which is in considerably excess of the safe drinking water limit for most beta emitters. (See footnote .) . The RSAL based on a 4 mrem per year dose limit to the bone surface corresponding to this calculation would be about 1.2 pCi/gram, or about 30 times lower than that recommended by the RAC team.. While this is not the current way that safe drinking water limits are defined, it is a reasonable to assume that limits for alpha emitters, which are today set according to dose estimation procedures that are 40 years old, will, in the future, be brought into line with the methods now used in all other regulations, or even more current methods.⁶⁴

The RAC analysis used a low solubility assumption for plutonium and did not account for colloidal transport, which is the subject of ongoing investigations, which it cited. (Most of the RAC water dose is from the residual americium-241.) These calculations assume low plutonium mobility. RAC did recognize that plutonium may become more mobile than it assumed, but the complexity of the problem, the ongoing nature of the debate on plutonium migration, and the limited scope of the project that RAC undertook meant that a more sophisticated groundwater calculation was not done.⁶⁵ The RAC assumption about plutonium mobility was based on analyses of the present chemical form of plutonium in the 903-pad soil at Rocky Flats.⁶⁶ Corresponding to these assumptions, RAC concluded that plutonium would probably not reach groundwater within the calculation period of 1000 years and, hence, that plutonium would not be likely to contribute to the peak dose via the groundwater pathway. Only americium-241 would contribute to the groundwater dose.⁶⁷

The assumption of low plutonium mobility cannot be supported for the long-term in the absence of a more detailed environmental analysis, as the RAC team recognized. The analysis above regarding the complexity of plutonium migration under real-world conditions in the natural environment indicates that the possibility that water pathway doses could be an order of magnitude or more greater in the long-term than estimated by RAC cannot be and should not be ruled out. Indeed, that possibility could be enhanced by the designation of Rocky Flats as a wildlife refuge. Yet no study to date has addressed the potential synergism between such a designation and the long-term water pathway dose.

This analysis of the water pathway dose indicates the crucial importance of using the subsistence farmer scenario as the basis for protection of future populations. It is unrealistic to assume that site control and specific current site uses will endure for long periods of time. The evolution of the contamination over time could result in far greater threats to future populations than if a thorough cleanup were carried out in the first place corresponding to a subsistence farmer scenario.

Next: 7. Erosion of the subsistence farmer scenario

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