The origin of I in soil and the 129I problem.

Abstract

It is widely believed that the I in soil is derived from the I in oceans by evaporating-atmospheric transport-washout by rain over land, rather than from the I in rock when the latter is weathered into soil. Since human I burdens are derived largely from soil by plant root uptake into food, this implies that the specific activity of 129I in the I of human thyroids will be the same as that in the oceans, whereas it would be similar to that in rock (much lower) if the I in soil were derived from weathering of rock as is the case for other heavy trace elements. The bases for the oceanic origin theory are analysed and found not to be in accord with modern data. Recent data shows no correlation between concentration of I in soil and proximity to oceans, which argues strongly against the oceanic origin theory. Some problems with long-distance atmospheric transport are pointed out. It is shown that the balance between I input to soil during soil formation by rock weathering and output from soil by erosion leaves little room for contributions from oceanic sources. It is concluded that only a small fraction of the I in soil is of oceanic origin. Most estimates of very long-term effects of 129I on human health are therefore several times too high.