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Posted: 07/06/2010

Topic:
Modelling exposure to Acacia trees (EMRAS II Little Forest Burial Ground Scenario)

From:
Marko Černe
Jožef Stefan Institute
Slovenia

Question:
Acacia has it's roots 0-2m deep in the waste (part in Zone 1 and part in Zone 2) and has also aboveground part. How to use occupancy factors in this case? The major part of the tree is above the soils, so in that case we will use on soil, but the dose from ionising radiation here is a consequence of underground direct contact and aboveground part, but according to permitted occupancies for tree reference organisms (ERICA help) we could use occupancy factor just on soil? Does this approach include also the roots? How to deal with such cases?

Answer:
This question is in regard to one of the IAEA EMRAS II programme Biota Modelling Group (WG4) scenarios - Little Forest Burial Ground, Australia.

The Zone 1 referred to in the question is contamination at >1 m depth into which it is assumed the Acacia roots. An interesting question as you are correct in that the ERICA Tool considers plants to be on the soil surface and in the case of trees the assumed geometry is the trunk. Therefore, the default tree reference organism within the ERICA Tool will not model this scenario very well. It would be a fair criticism of the currently available models to say that perhaps the dosimetry for plants requires some further thought. However, scenarios such as this are proposed to make you think and identify problems and solutions. It's not the purpose of the exercise but there are ways you could try to get around the limitations of the ERICA Tool (could you create an organism that is representative?) although you would then need to consider how to interpret the results.

Answered by Nick Beresford 08/06/2010