RD&D-Programme 2004 - SKB

the evolution of wetlands. It is above all forests in depressions, for example swamp forests, that
will be of interest with respect to the effects of a deep repository, and not highland forests such
as flat-rock pine forests.
Mires and wetlands are important recipients for the hypothetical sitings. In SR 97, mires were
identified as the typical ecosystem that gives the potentially highest dose to man, higher for
many radionuclides than the doses that can be obtained from wells. At the same time, the
assumptions that were made in SR 97 were simplified and probably overestimates. Furthermore,
several steps of changes of mires are required to result in an exposure of humans, for example
drainage ditching, cultivation or burning of peat. It was further noted in SR 97 and Safe that
mires are common in many areas. They are a probable discharge point from the geosphere and
a probable result of the natural future evolution of the biosphere after postglacial land uplift
in a coastal area. Furthermore it has been common to drain wetlands by ditching in order to
obtain agricultural land in parts of northern Uppland /20-50/ and in the Simpevarp area. It is
therefore important to obtain a more profound understanding of mires and wetlands and to
study processes that can affect radionuclide transport and potential exposure pathways.
Conclusions in RD&D 2001 and its review
See section 20.2.
Newfound knowledge since RD&D 2001
The primary production is the accumulation of organic matter due to photosynthesis. In order
to grow, plants must be able to obtain nutrients from the transpiration stream or actively, for
example by secreting complexing agents etc to dissolve nutrients from mineral particles, clay
particles, etc. Radionuclides, which have properties similar to those of nutrients, will then be
taken up by the plant. The rate of nutrient uptake is proportional to the growth rate, which
means that radionuclide uptake, as an analogue of nutrient uptake, must also be proportional to
the growth rate. The mass flow of radionuclides in plants can thus be modelled with the aid of
two variables: growth rate and transpiration. These two variables are scalable to climate change
and vegetation type and are an alternative approach to the root uptake factors that are used in
radioecology. Similar ideas have recently been applied to e.g. radium and radon uptake in plants
within other disciplines /20-51/.
A development of models is also necessary in order to be able to apply this alternative approach.
A powerful model that can handle transpiration, growth and nutrient uptake for vegetation
has been developed by SLU and KTH (the so-called CoupModel, /20-52/). The CoupModel,
or its predecessor Soil, has been used successfully by several research teams to study various
phenomena within forestry, agriculture, hydrology and climate, see bibliography in /20-52/.
The advantage of the CoupModel is that it is largely based on processes – such as heat balance,
mass flow, transpiration, primary production – and that it has a large database for parameters
collected by SLU over a long period of time from various parts of Sweden. The model has been
modified at SKB’s request to include transport of other substances, such as radionuclides. The
CoupModel is currently undergoing testing. If the results are good, the model will be used for
all types of land, i.e. forest, agricultural land and wetland.
A compilation is currently being made at the Department of Botany at Stockholm University
of how different substances get into plants via root transport /20-124/. The intention is to
classify substances into those that can be expected to passively accompany the water flows,
those that are actively prevented from entering the plants, and those that are actively taken up
and accumulated. Experimental work has also been done in greenhouses with sallow shoots at
the Department of Botany. Uptake of a dozen stable analogue elements has been correlated to
the willow’s transpiration and primary production. Transpiration is the water flow that passes
through the plant; substances can passively accompany this stream of water into the plant, but
due to evaporation the substances accumulate in the plant. The plant is able to discriminate
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