The geochemistry of caesium in the secondary environment around the St Austell granite

Interest in the geochemistry of caesium (Cs) in the secondary environment stems from the need to monitor the behaviour of ¹³⁷Cs resulting from nuclear accidents. Caesium abundance is generally low in crustal rocks (2.6 ppm) but is known to be strongly enriched in the south-west England granites. Granites from the St Austell pluton can contain in excess of 50 ppm Cs, while the Trelavour Down pegmatite contains 132 ppm. In soils over the St Austell granite, Cs levels generally range from 50 to 100 ppm, with the highest concentrations frequently being found in the deepest soil horizons where Cs contents often exceed those found in the parent material. As Cs is concentrated in the finest soil fractions it is probable that as the element is released during weathering so it is preferentially adsorbed by clays. Vegetation samples from the St Austell area show species variation for Cs uptake but the average content of the plants analysed is 2.3 ppm. While the Rb/Cs ratio for the granite is 27, similar to that found for literature data, the mean ratio for topsoils is 7.2 indicating a strong relative enrichment of Cs. The mean Rb/Cs ratio of plants (47) points to a preferential uptake of Rb.