Feature

Radioactive Soil Studies

Trace radioactivity from atmospheric testing has its uses as an indicator of soil movement and erosion.

By Dr Murray Matthews

The fission product caesium-137 is now being widely used as a tracer of soil movement. Even in New Zealand, interest in this rather unlikely "benefit" of nuclear weapons testing has been growing over recent years.

DSIR Land Resources is now actively investigating the technique as part of sustainable land use research, with the National Radiation Laboratory making a major contribution through provision of historical fallout data and low-level radioactivity measurements.

Caesium-137, along with all the other products of nuclear fission, was injected into the atmosphere during atmospheric weapons tests conducted mainly in the late 1950s and early 1960s.

Much of the debris entered the stratosphere, which acted as a reservoir from which very small amounts of the longer-lived members are still being deposited back on the Earth's surface, at a steadily decreasing rate from the peak deposition year of 1964.

Fixed In Soils

The fate of fallout radionuclides once deposited depends primarily on the degree to which they are "fixed" by soil minerals. If not firmly bound by ion exchange processes or surface absorption, they may be washed out of the soil or taken up by plants.

Caesium-137 is one which is usually firmly fixed by clay minerals, and measurements of the ¹³⁷Cs profile in different soils have shown that it is quantitatively retained in the top 20-40 centimetres of soil. This firm fixing results in relatively little being taken up by grass and entering dairy foodchains.

The volcanic soils of Taranaki and, to a lesser extent, the Waikato, are deficient in caesium-fixing minerals, particularly illite, and so result in higher levels of ¹³⁷Cs in milk than in other regions of New Zealand.

Soil Movement Indicator

Assuming, then, that all ¹³⁷Cs deposited on the soil is retained there, the amount present in the soil today should represent the total deposition since the early 1950s, when artificial radioactive fallout first appeared in New Zealand, correcting for radioactive decay. It follows that if the total amount deposited is known from fallout monitoring records, then any deficiency or excess in soil ¹³⁷Cs content can be equated with topsoil loss or gain respectively.

The technique involves determining the amount of ¹³⁷Cs in the soil and comparing this value with known deposition records, or with levels in nearby undisturbed sites. It can be used in relative terms, comparing caesium levels in a transect across a slope or valley system. In a transect from the top, shoulder, slope and bottom of a depression, caesium levels may follow the trend 60, 20, 45 and 80 millibecquerels per square centimetre.

This indicates that the greatest soil loss is on the shoulder, with some loss on the slope, and an accumulation of soil on the bottom of the depression. The applications in the assessment of the effects of agricultural practices are obvious.

Work in New Zealand has so far been restricted to studies of ¹³⁷Cs, but overseas other fallout radionuclides are used as well. Naturally occurring beryllium-7 and lead-210, in particular, are under study in Australia.

Beryllium-7 is produced in the upper atmosphere by cosmic ray bombardment. It has a 53-day half-life, and is used as an indicator of transport in the top millimetre of soil, such in areas of strong winds. Lead-210, with a 22-year half-life, is used in similar applications to 137Cs.

This description of the use of radioactive tracing in assessing soil erosion is rather simplistic, and it would be wrong to assume there are no difficulties or discrepancies in practice.

However, the use of fallout radionuclides in soil transport studies is showing great promise and should prove to be an important tool in the development of sustainable agricultural practices in New Zealand, a country with significant erosion control problems.

Dr Murray Matthews is a scientist at the National Radiation Laboratory.