Contribution of gamma ground spectrometry to the textural characterization and mapping of floodplain sediments

Abstract

The possible contribution of ground gamma-ray spectrometry (GRS) to the study of topsoils was tested in the Rieti floodplain (Central Italy), an area characterized by a complex sedimentary evolution and a long lasting interference between human activities and hydrological setting.A significant inverse correlation between total γ count and sand content (R=−0.901; p<0.01), with the latter acting as a diluent for the radioisotopes concentration in the topsoils, suggested to integrate the textural data of 289 topsoil samples by using a ground gamma ray survey. An extensive survey (1906 total γ emission measurements at a density of 21.1measurements/km2) was carried out with a portable NaI(Tl) scintillometer by combining hand-held and car-borne techniques.The total γ emission compared to the spatial distribution of measured soil textures and to the relatively small altimetrical variations of the topographic surface along selected transects, allowed to have a better insight of the Holocene sedimentary evolution.The total γ counts were also used in conjunction with a series of geographic variables uniquely linked to the geographic and physiographic setting of the floodplain, such as the distance from rivers and floodplain edges, the topographic curvature and the geographic position of sampled points (X, Y, Z), to carry out a discriminant analysis that allowed to correctly assign soils to their USDA texture groups in 59.2% of the cases. The same analysis carried out on a subgroup of 101 soil samples, where some common pedological parameters were also included (pH, total organic carbon, total carbonates and field capacity), allowed to increase the discriminant score to 76.2% showing how the statistical contribution of total γ counts remains always relevant independently from the suite of variables that is chosen. The gamma survey allowed to improve the understanding of the spatial distribution of the topsoil textures which is also the basis for a more efficient management of agricultural land in the study area. Total γ emission recorded near the edge of the floodplain, in the distal sectors of some alluvial fans, resulted significantly affected by the presence of calcareous rock fragments in the topsoil and was therefore reliably used to measure their abundance.The results of this study encourage the extensive use of ground GRS in the field of applied sedimentology and topsoil mapping of floodplains as a fast and low cost technique capable to usefully integrate the traditional surveys based on the execution of laboratory analyses. In fact this survey technique can be potentially addressed to different surface sedimentary environments by using expert knowledge to interpret the recorded doses in the light of the local lithological setting.