Analytical procedures for measuring the amount and distribution of radioactive constituents in soils

Abstract

For a research project on the evaluation and distribution of radioactive pollutants in Biscay's plants and soils, it was necessary to establish an analytical procedure to detect and measure extremely low amounts of radioactive elements. For the project 15 cm cores of fourteen soils were taken and divided into three depth levels. Some of these soil samples (<2 mm) were i) air‐dried before radioactive measurements. Additionally, a few samples were measured again after being ii) selectively extracted with ammonium acetate at pH 7 (exchangeable), acetic acid (carbonate), hydroxylamine (associated to Fe‐Mn oxides) and hydrogen peroxide (related to sulphide and organic matter fraction) to estimate the distribution of constituents according to mineral soil species or iii) classified as clay+silt, fine and coarse sand, to study size partition. Radioactive measurements were made with a low‐level gamma spectrometer (Ge‐HP detector + analyser with 8000 channels) whose resolution was 1.64 keV with a relative efficiency of 24.8 per cent for the 1330 keV Co‐60 line. Calibration was made against a standard solution containing nine radio‐nuclides with gamma‐ray energies in the range from 88.03 to 1836 keV. The shielding was 15 cm thick of pre‐war iron. Original or derived soil samples were put into 0.25 liter Marinelli flasks and measured during 24 hours periods. Double distilled water background was discounted. Net peak areas were then analysed at 95% confidence by the Mariscotti method. K‐40 and several radionuclides from U‐Th series were identified as presumably natural constituents, but significant amounts of Cs‐137, a fission product, were also found in most of depth levels of the studied soils. The distribution of these radioactive constituents in some treated samples, according to soil species or fraction size is shown.