Soil radioecological study in the Lomonosov diamond mining area (Arkhangelsk region, Russia)

Abstract

The aim of the study was to obtain new data and on their basis to analyze soil radioactivity in the Lomonosov diamond mining area (Arkhangelsk region, Russia).
 Location and time of the study: Field studies were carried out in the summer of 2018 in the Lomonosov diamond mining area (Arkhangelsk region, Russia).
 Methodology: To accomplish the task, 10 soil profile pits were cut, and 69 soil samples were taken from them. In the field, a description of soil morphology was carried out. Under laboratory conditions, the samples were dried at 105° C to constant mass. After weighing, the soil samples were placed in a Marinelli vessel to measure the specific activity of isotopes (cesium (137Cs), potassium (40К), radium (226Ra) and thorium (232Th)) by gamma spectrometry. Registration of gamma radiation from a counting soil sample, as well as processing of spectra was carried out using the Progress-gamma software and hardware complex FVKM.412131.002-03. The energy calibration of the gamma spectrometer to control the safety of the setup parameters was carried out after each measurement using a combined control source OISN-137-1 in a Marinelli vessel. The minimum exposure time of the counting sample was 3600 seconds. However, depending on the activity of the sample, the exposure time in individual samples with weak activity increased.
 Main results: As a result of field studies, it was found that Podzols, characteristic of the northern taiga, prevail in this area. The maximum activity of 137Cs was concentrated in the litter-peat (11.4–71.8 Bq/kg) and the humus horizon (26.7–105 Bq/kg). In the podzolic and Al-Fe-humus horizons there was a sharp decrease in the activity, reaching the lowest measured values in the parent rock (2–3 Bq/kg). The general tendency for the accumulation of natural isotopes 40K, 226Ra, and 232Th in the soil horizons was found to be weakly related to soil type or subtype. In some soil pits an increase in the natural radionuclides with depth was observed, being mostly the characteristic of 40K as its activity changed downwards from 82.6 to 652 Bq/kg. The 226Ra and 232Th activities in the soil profiles varied from 2.4 to 47.2 Bq/kg and from 2.9 to 40.6 Bq/kg, respectively. This is due to the fact that thorium is strongly sorbed by soil; therefore, its migration along the soil profile is poorly expressed, and the element is deposited in soils in the form of hydroxides. Loamy and clayey soils, were shown to be more enriched in radium, whereas sandy soils were poor in this element. The amount of radioactive elements, contained in a soil, is largely determined by the concentration of their isotopes in the underlying rocks. 
 Conclusion: The peaks of increasing concentration in the Al-Fe-humus horizons (BF, BFg) were associated with an increase in the content of the clay fraction, Fe, Al hydroxides, which increase the sorption capacity of soils. The litter-peat and the humus horizon act as a biogeochemical barrier that retains radioactive elements in the soil for a long time. Due to the high filtration capacity of podzols, the underlying horizons are not reliable barriers. 137Cs and other radioisotopes can bind with humic substances, oxides and hydroxides of iron and aluminum, form humus-ferruginous complexes, precipitate as new soil formations, e.g. become part of the mineral composition of hard pans. The distribution of natural radioisotopes 40K, 226Ra and 232Th was found to be more chaotic and more related to the composition of the constituent rocks. The mobility and accumulation of radionuclides along soil profiles depends on the particle size distribution, the content of organic matter, sesquioxides, mineralogical composition, as well as on the temperature and water regime.