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The fallout map is not a mushroom map

7 minJames

Revised 11 augusti 2026

At five Gävle sampling sites in 2020, the median caesium-137 activity in monitored trattkantarell ran from 116 Bq/kg fresh weight at Eskön to 1,100 at Hemlingby. The same municipal series put Viksjölandet at 180 and Oslättfors at 643. Those are not competing maps of the same fact. They are five small, repeated observations inside a part of Sweden where the 1986 fallout was unusually high, and they show why a broad fallout map cannot be read as the value of a fruiting body in the hand (Gävle kommun, 2020).

The band was drawn by rain

The phrase Gävle fallout belt makes it sound as if there were a neat stripe, laid down with a ruler. There was not. In late April 1986, material from the Tjernobyl accident reached Sweden in moving air masses. Where those masses met rain, radionuclides were washed out efficiently. The pattern followed weather at the time, not county borders, forest type or the places people happened to know well.

The national picture was assembled quickly. SGU flew radiometric surveys through the spring and autumn of 1986; the formal map of caesium-137 on the ground was produced that autumn. Its purpose was to describe deposition: how much was laid onto land, expressed per unit area. It was never a map of individual organisms. The highest deposition was concentrated in broad parts of southern and middle Norrland, from northern Uppland and Västmanland through to Västerbotten, with prominent high areas around Gävle and the Sundsvall–Härnösand coast (SGU; Livsmedelsverket).

That first distinction does most of the work. A deposition map gives the starting material in a landscape. A fungal measurement gives activity in a particular fruiting body, after years of movement through litter, mineral soil, roots, mycelium, weather and the biology of one taxon. Both are measurements. They do not have the same unit, scale or question.

Strålsäkerhetsmyndigheten's current map preserves this distinction. The ground-deposition layer is based on airborne measurements and shows the distribution in 1986; its comparison layer estimates the amount remaining after physical radioactive decay. The agency says the geographic pattern is still broadly the same, while the amount has more than halved. That is useful historical geography. It is not a reading of the ground beneath one moss patch, and it is not a reading of a particular svamp (Strålsäkerhetsmyndigheten, 2024a; 2024b).

Caesium enters a living nutrient system

Caesium-137 is a radionuclide, but it is also caesium: a positively charged ion with chemical behaviour sufficiently close to potassium that living uptake systems can move it. Potassium is not an incidental comparison. Fungi and plants require it, and their transport systems are built to acquire and redistribute ions in soil water. Caesium can enter that traffic without being a nutrient the organism needs.

That resemblance is an explanation, not a complete mechanism. A controlled study of ectomycorrhizal association in Japanese pine seedlings found that colonisation by one fungal partner increased caesium in shoots, and also increased potassium uptake. The researchers were careful: soil nutrient conditions, tree species and fungal species all mattered. It is a study of seedlings in Japan, not of Swedish fruiting bodies in barrskog, so it cannot supply a transfer number for Gävleborg. It does establish the biological reason that potassium and fungal partnerships belong in the discussion (Ogo et al., 2018).

The forest makes the route longer. Part of the deposited caesium becomes fixed to mineral particles and can become much less available for biological uptake. Strålsäkerhetsmyndigheten attributes much of the sharp fall in surface radiation and in many environmental concentrations to downward movement in soil and to this binding. Other caesium remains in the organic, biologically active upper layers. Mycelium runs through precisely those layers, gathering water and ions over an area much larger than a fruiting body.

Swedish work led by Mykhaylo Vinichuk has examined caesium alongside potassium and rubidium in bulk soil, the soil-root interface, mycelium and sporocarps in a Swedish forest. The choice of compartments matters. It rejects the simple picture of a fruiting body as a passive cup filled by whatever lies directly below it. Caesium is retained, released and moved through an ecological system. The IAEA's forest synthesis reaches the same broad conclusion from several affected regions: soil properties and fungal species are central to transfer into mushrooms (Vinichuk, 2010; Belli et al., 1996).

Accumulation is a pattern, not a field character

Some fungal taxa repeatedly show higher caesium-137 concentrations than others under comparable broad conditions. Livsmedelsverket specifically identifies sandsopp and rimskivling as taxa that take up more caesium than many other fungi. That is a monitoring observation worth keeping separate from the old habit of treating all svamp as one environmental compartment (Livsmedelsverket, 2026).

But “accumulator” is not a stable label that turns a name into a number. A taxon's physiology, its mycorrhizal or saprotrophic way of living, the depth and chemistry of its mycelium, and the available rather than total caesium in soil all intervene. The literature has good evidence that taxa differ. It has a much weaker basis for saying precisely which mechanism dominates in each Swedish forest and in each year.

The variation is not only between taxa. In its 2024 collection project, Strålsäkerhetsmyndigheten stated plainly that concentrations in fruiting bodies can vary substantially between years, between species, between individuals of the same species and even between fruiting bodies of the same individual. Its public map reports Bq/kg fresh weight. That last phrase matters scientifically: water content contributes to variation between fresh specimens, while dried material is converted to an estimated fresh-weight value for display (Strålsäkerhetsmyndigheten, 2024c; 2025).

This is where the Gävle figures become more instructive than a national average. The municipal series shows a declining long-term median in its monitored material, but also a wide spread among five sites in one kommun in one year. A county label cannot remove that spread. Nor can an apparently precise species name, particularly when the observation is not tied to a verified specimen, a sampling protocol and an analysis.

This is not an identification question. A name in a report is a label attached to a monitored sample, not a means of determining a specimen in the field. Questions of identification belong with a svampkonsulent, not with a fallout map.

Four decades changed the amount more than the outline

The physical half-life of caesium-137 is about 30 years. In 2024, Strålsäkerhetsmyndigheten estimated that about 42 per cent of the caesium-137 deposited in 1986 remained. The arithmetic is exact once the starting amount is set. Ecology is not. Surface radiation has fallen more than physical decay alone would suggest because the caesium has moved down and is shielded by overlying soil; biological availability has its own course in each soil (Strålsäkerhetsmyndigheten, 2024b).

The older Swedish record explains why this distinction survived the first decade after the accident. Johanson, von Bothmer, Bergström and Kardell reported mean activity in mushroom samples far above the plant values they measured in a Heby area with known deposition. They also found no significant fall in moose concentrations from 1986 to 1990. That was an early warning against assuming that a short calendar interval would erase forest transfer pathways (Johanson et al., 1991).

The later record does show substantial decline. Gävle kommun measured blåbär, lingon, gul kantarell and trattkantarell at recurring sites for years after the accident and reported lower medians by 2020. Livsmedelsverket's 2023 collection, published in 2025, likewise found generally low concentrations in its limited national material, although it emphasised large variation between samples. These are useful observations of change. Neither turns physical half-life into a universal ecological half-life (Gävle kommun, 2020; Livsmedelsverket, 2025).

The map therefore still matters most where the original contrast is large. Gävleborg, Västernorrland, Jämtland, Västerbotten, Uppsala, Västmanland and Södermanland were the län chosen for Strålsäkerhetsmyndigheten's 2024 fungal collection because they were affected by the fallout. In län well outside the high-deposition pattern, the historic layer supplies less discrimination between broad areas. In the affected belt it remains an important prior condition for monitoring and for interpreting a measured result.

A map can set the question but cannot settle the sample

There is a temptation, forty years later, to call the old map obsolete because many measured concentrations have declined. The opposite error is to treat a red patch on it as a present-tense verdict. Both mistakes confuse deposit with uptake.

The useful reading is narrower. A deposition map says that two areas began with different burdens after Tjernobyl, and that this broad contrast has not been spatially rearranged. It can explain why Gävle, Heby and the Norrland coast remain relevant places in monitoring. It can guide the design of a sampling programme: compare the same taxon, season, preparation and landscape across a gradient in historical deposition. It cannot infer the activity of an unsampled fruiting body.

The same restraint applies to larger databases. Strålsäkerhetsmyndigheten's cesiumdatabas contains about 100,000 individual measurements, chiefly from the hardest-hit län and mainly from 1986 to 2002. The agency warns that variation can be large even within a small area, normally reports means only with at least five samples, and notes that sampling was not always random. As time passed, low-deposition places may have been underrepresented. Those are not defects to be hidden; they are the conditions under which the record can be read (Strålsäkerhetsmyndigheten, 2024d).

Settled: the 1986 weather left an uneven, still legible national deposition pattern; caesium-137 remains in forest systems; fungal taxa and individual samples differ greatly; and decay has reduced the total without making every ecological pathway decline at the same rate.

Not settled: the best species-specific mechanism for a particular Swedish ståndort, the transferable ecological half-life for a given patch, and the activity of any unmeasured specimen. A map can retain the memory of the fallout. It cannot replace a measurement.

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