Applications of Caesium-137 in Soil Erosion and Sedimentation Studies (an introduction) at Geography Department University of Exeter

o What is caesium-137?o Why uses caesium-137?
o Basic of caesium-137 for soil erosion study o Caesium-137 on the web
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oWhat is caesium-137?

As the expression indicated, caesium-137, cesium-137, or Cs-137 is one of radioisotopes of caesium with a nominal atomic mass of 137. It is an artificial radionuclide with a half-life of 30 years or so, which was released into the stratosphere by the testing of above ground thermonuclear weapons in the late 1950s and early 1960s and deposited as fallout.

oWhy uses caesium-137?

Its high affinity to fine soil particles, relatively long half-life, and world-wide distribution have made it almost a universal environmental tracer for studying upslope soil erosion and downstream sedimentation.

Globally speaking, the temporal patterns of caesium-137 input are characterised by: 1) detectable caesium-137 began in 1954; 2) the first peak appeared in 1958/1959; 3) the second peak occurred at 1962-1964; 4) and the termination of caesium-137 input around mid-1980s (
diagram). Some areas had additional input in 1986 after the Chernobyl incident. This annual variation has been successfully used to determine sediment accumulation rates in a wide variety of depositional environments including reservoirs, lakes, wetlands, coast areas, and flood plains.

In the study of rates and patterns of erosion and deposition on agricultural land, the following conceptual benefits of the caesium-137 technique were identified:

1). Permits retrospective assessment of medium-term erosion rates
2). Both rates and patterns of soil redistribution may be quantitatively assessed
3). The rates and patterns estimated represented the sum of all erosive processes
4). Soil redistribution rates estimated are less influenced by extreme events

With a single field visit, caesium-137 technique can provide spatially distributed data for the parameterisation and validation of physically-based erosion and sediment yield models. Meanwhile, existing measurement techniques including experimental plot / trap and field survey posses important limitations in terms of both temporal and spatial sampling constraints and the nature of the data provided


oBasic of caesium-137 technique

Basis of the caesium-137 technique

The basis of the caesium-137 technique can be summarised as follow:

1) Caesium-137 was deposited as fallout primarily during the late 1950s and the 1960s and in most environments was rapidly and strongly absorbed by soil particles at the ground surfaces.
2). Subsequent redistribution of the radiocaesium reflects the movement of soil particles since the caesium-137 remains absorbed and moves in associated with the soil particles.
3). If it is assumed that the initial distribution of the caesium-137 fallout input was uniform, the deviations in the measured distribution of caesium-137 from the local fallout inventory represent the net impact of soil redistribution during the period since caesium-137 deposition. Higher inventory indicates deposition and lower inventory indicates erosion. Redistribution of caesium-137 in agricultural environments was illustrated by a
schematic representation of the agricultural landscape and examples of typical measured caesium-137 profiles associated with cultivated soils .

4) If a relationship between caesium-137 loss and gain and soil loss and gain can be established, it will be possible to estimate rates of soil erosion and aggradation from caesium-137 measurement

Units used for measuring radioactivity

Becquerel, symbol 'Bq', is the SI unit of radioactivity. One becquerel is the activity of a quantity of radioactive material in which one nucleus decays per second. It replaces the curie (Ci). One curie is equal to 3.7 x 10^10 becquerels. Other common units include:

mBq = 0.001 Bq

Pci = 10 ^ (-12) Ci

GBq = 10 ^ 9 Bq

TBq = 10 ^ 12 Bq

Some formula used in the caesium-137 technique

1). Calculation of detector's efficiency (DE in percent)

where 'A' is the counts, 'T' is the counting time in seconds, 'Sc' is the source concentration in 'Pci' when it was prepared, and 'TL' is the time that has gone by in year

2). Calculation of caesium-137 activity per unit mass (Is in mBq g^(-1))

where 'Wd' is the mass of the sample under detection in grams, DE is the detector efficiency

3). Calculation of caesium-137 inventory (I in mBq cm^(-2))

where 'Wt' is total sample mass in gram (usually < 2 mm), 'Is' is the calculated caesium-137 activity per unit mass, and 'At' is sampling area area in cm*cm.

If you have known the detect efficiency and are using a Javascript-abled browser, this small calculator can help you to estimate caesium-137 activity per unit mass or caesium-137 inventory

Typical procedure for using caesium-137 technique on a field

Following stages are involved in the application of the caesium-137 technique for erosion and deposition studies on field scale

1). Collection of soil core samples from the study field and from undisturbed 'reference' sites in the surrounding area. Cored samples can be sliced to show the depth distribution of caesium-137 activity. A motorised percussion corer may be used if large amount of samples are to be collected.
2). Samples are air- or oven-dried and then disaggregated by hand or machine and sieve through a mesh (usually 2 mm). After this, subsamples are taken and packed into Marinelli beakers or small plastic pots
3). Measurement of total remaining caesium-137 fallout to the location by analysis of soil samples from the 'reference' sites. Measurement of the caesium-137 inventory (activity per unit area) for each location sampled in the study field. A gamma spectrometry is indispensable.
4). Identification of caesium-137 loss and gain for each sampled location in the study field by comparison of the measurement from the reference sites with those from the study field.
5). Development of site-specific 'calibration' relationships between caesium-137 loss and gain, and soil erosion and deposition rates. There are some conversion models available for thiss purpose. Choice of suitable models and specification of the relevant parameters are the key.
6). Use of the calibration relationships from stage 5 with the caesium-137 loss and gain data from stage 4 to estimate the rate of soil erosion or deposition rate for each location in the study field.
7). Integration of the point erosion and deposition data to obtain field-based estimates of soil erosion and deposition.


oCaesium-137 on the WWW

A literature list which includes the application of caesium-137 technique
Spatial Analysis of Cesium in Sediments of Watts Bar Reservoir
Radioisotopes in Industry
Jerry Ritchie's homepage where a well-maintained publication list related to the application of cesium in erosion and sedimentation can be found
Sediment and Cesium-137 Transport Modeling

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