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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
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3. Description of the subsistence farmer scenario
How should the critical group be defined? What are the criteria that must be used? Here also the basic thrust of historical practice has been to take a conservative, but reasonable approach that corresponds to the idea of the maximally exposed individual. We seek to define such a group at a time when we cannot know whether there may be radiation doses from other sources. Lack of knowledge in this regard has always meant that the maximum dose limit be kept well below the allowable exposure from man-made sources.
When the main route of exposure over long time periods is uncertain, it is the general practice to use the subsistence farmer scenario for calculating risk, or the level of permissible exposure. This approach assumes that a person would unknowingly use contaminated water for drinking and farming and would grow all their own food. Further, it assumes that such exposure would last a lifetime, and not just a few years. The people in the critical group spend most of their time on the contaminated site. In addition, it assumes that the diets as well as food and water intake of future populations will be similar to those of today. People are considered protected if their lifetime exposure is less than an assigned limit. The reasoning is that in such a case all other people would be protected since their doses would be lower than that of the hypothetical subsistence farmer.
The assumption that the risk of illness to all individuals within a population will be below that of the hypothetical subsistence farmer is not a prediction, of course. It is an estimate that, with some unknown, but small likelihood, may turn out to be wrong. The subsistence farmer scenario is a conservative, stringent, and practically bounding approach to calculating future regulatory dose limits. However, it should be recognized that, in general, it excludes the most extreme doses that it is possible to calculate. For instance, it is common to exclude extreme diets consisting only of the most contaminated foods. While such diets cannot be ruled out, they may reasonably be considered as improbable, unless there is some evidence to the contrary. The subsistence farmer scenario is akin to and based on the maximally exposed individual concept that we have discussed above, but for the purpose of long-term calculations.
One concept within the subsistence farmer scenario is the notion that radionuclides, once in the environment, can move up the food chain. This food chain concept was incorporated into regulations in Table 2 of 10 CFR 20, a regulation of the Atomic Energy Commission. This table, which still exists today in updated form, deals with the possible exposure of people who may live near a licensed nuclear facility and was initially a regulation of the Atomic Energy Commission, the predecessor to the Department of Energy, and still exists today (in updated form). It codifies the permissible concentration of various radionuclides in air and water based upon the allowable quantity of each radionuclide in the body. In preparing this table the AEC assumed that the individual continuously breathes contaminated air and only drinks contaminated water. The subsistence farmer scenario is one step beyond this one in that food is grown using the contaminated water. The one exception in 10 CFR 20 is the maximum permissible concentration for iodine-131. This regulation takes into account airborne radioiodine being deposited onto the ground and taken up by grass which cows then eat. The iodine is then concentrated in cow's milk and consumed by an infant.20
Much of the development of the subsistence farmer scenario was done by Bruce Napier and William Kennedy at the Hanford Pacific Northwest Laboratory in the 1980s. The early version of this model was known as the "backyard farmer" scenario.21 In their analysis of allowable residual contamination levels (ARCL) at Hanford, they assumed that restricted use of the site for 100 years, controlled use for another 300, and unrestricted use of the facilities afterward. All of their assumptions are based on the ingestion characteristics of the " standard Hanford maximally-exposed individual," a construct that fits the description of the average adult male.22 For unrestricted use, they assumed that an individual would have "free access to any remaining facilities or radionuclides on the site."23
"...for the far-term scenario, it may be assumed that people will eventually move onto the waste site. This is not intended to imply that future populations are unintelligent or technologically inferior, but only that records of the waste sites are forgotten or ignored."24
This individual is "assumed to raise a large fraction of his own fruits and vegetables for personal consumption."25 Calculations were carried out to determine doses at ten kilometers from the site, one kilometer from the site, and on-site. It was assumed that the individual would live downwind and downstream from the site. Because doses were found to be much smaller offsite than on, the onsite exposure scenarios were deemed the most critical.26
By the late 1980s, this model had been refined even further into three different scenarios.27 These are the resource-recycle scenario, the residential/home-garden scenario, and the agriculture scenario. The resource-recycle scenario bases its assumptions on an individual who recycles materials that were salvaged at a destroyed facility after institutional controls are lost.
The home-garden scenario is based on an individual who resides on-site and operates a home garden for 50 years. This person constructs a basement where the greatest contamination associated with a facility would occur. It is assumed that this person spends twelve hours per day outside where s/he is exposed to radiation from the soil and can inhale resuspended contamination from the soil surface. Also, twenty-five percent of the individual's fruit and vegetable intake is assumed to come from a backyard garden that is located on contaminated soil.
The agricultural scenario, a slight variation of the home-garden scenario, was designed to assess exposure resulting from eating agricultural products whose roots come into contact with buried radioactive materials.28 In common with the home-garden scenario, Napier, et al. assume that only twenty-five percent of the diet would be from food grown on-site. While the home-garden model is only designed for one person, the agriculture system assumes that a family of four would get twenty-five percent of its total fruit and vegetable supply from the land. As a result, it is assumed that the land would be 0.1 to 1 hectare in size. It is assumed that 50 square meters would be used for above ground vegetables, 200 square meters would be used for root vegetables and grains, and slightly more than 200 square meters would be used for fruit trees. Homegrown animal products are not included in this scenario because it is assumed that one hectare of land would not be enough to grow animals as well as crops.
These scenarios were eventually adopted as official protocol for the Hanford site.29 This is apparent in DOE's 1987 Final Environmental Impact Statement: Disposal of Hanford Defense High-Level, Transuranic and Tank Wastes. In appendix R, a description is given of a "full garden scenario" that is very similar to the agricultural scenario described above, the only difference being that it assumes a small two hectare farm instead of the smaller ones described above.30 While none of the scenarios described here is exactly like the subsistence farmer model, the DOE's official analysis has been along the lines of a subsistence-farmer-like model for quite sometime.
As another example, the Yucca Mountain Project has, in the past, based estimated future doses on subsistence farmers using computer modeling in the biosphere scenario.31 The U.S. Nuclear Regulatory Commission (NRC) has also performed calculations to assess the risk of exposure to future populations due to geologic disposal at Yucca Mountain. In its calculations, the NRC has used a hypothetical self-sufficient farm family of three who obtain all of their water from a contaminated well. This same water is used to grow the family's crops, and their meat and milk is obtained from farm animals raised on vegetation that is irrigated by it.32 The NRC also did not restrict the location of the critical group to currently populated areas, but it is assumed to live at the boundary of the controlled area.33 This is consistent with ICRP 43 recommendations for calculating doses from major sources because the recommendations do not specify occupancy parameters. They only state that the chosen parameters be "appropriate."34
Other projects that have used the subsistence farmer scenario or variants thereof include the Waste Isolation Pilot Plant (WIPP) and Sandia National Laboratories.35 The WIPP project was formed to dispose of transuranic waste in bedded salt, while Sandia ran an evaluation of spent fuel in a tuff repository. The Hanford repository program uses the subsistence farmer approach to calculate exposure when the locations and other traits of exposed individuals are unknown. So, a strong precedent has been set for the use of the subsistence farmer scenario when the location and lifestyles of the exposed population are unknown. Finally, in regulatory terms, the EPA in establishing Superfund regulations has used the subsistence farmer scenario.36
Next: 4. International use of the subsistence farmer approach 
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Institute for Energy and Environmental Research