Radiometric Analysis of Soil Profiles to Assess Horizonation, Particle Size Distribution, and Elemental Composition.

Gamma radiometric data consist of the passive gamma radiation that is naturally emitted from rocks and soil. The sensor, with either a sodium iodide, bismuth germinate oxide, or cesium iodide crystal inside, is used to detect concentrations of four elements (five isotopes): ¹³⁷Cs, ²³²Th, ⁴⁰K, ²³⁸U, and ²³⁵U. These elements are relatively abundant in the environment, have long half-lives, and produce gamma radiation of measurable concentration at variable but characteristic energy levels. Gamma radiometric data are known to be related to mineralogy and geochemistry (e.g., soil parent material, weathering and pedogenesis, geomorphic activity, erosion and deposition). Previous work with gamma radiometric data has demonstrated that mappable soil properties include parent materials, soil texture, nutrients and other elements, soil organic matter, pH, bulk density, water retention, and saturated hydraulic conductivity. We investigated the capabilities of a gamma spectrometer to assess variation in soil properties with depth in vertical profiles from soils of two different geologies. Gamma surveys were conducted using a rectangular grid (5x20), where the total area surveyed was 125 cm by 200 cm. The two soils that we examined were classified as Typic Paleudalfs (residuum weathered from limestone interbedded with shale) and Aeric Endoaqualfs (residuum weathered from black shale). After the surveys, soil samples were collected from each measurement point, then dried and ground to 2 mm. Particle size analysis was used to determine sand, silt, and clay content, and a portable X-ray fluorescence (XRF) analyzer was used to determine elemental (Fe, Mg, Al, Ca, P, S, Ca, K, Si, Mn) composition. These data were used to assess whether gamma radiometric data can be used to (i) identify horizon boundaries and (ii) characterize selected physical and chemical properties within a soil profile.