Using Gamma-Ray Signatures to Identify Horizon Boundaries and Soil Properties within Soil Profiles.

Gamma radiometric data consist of the passive gamma radiation that is naturally emitted from rocks and the soil. The sensor, with either a sodium iodide or bismuth germinate oxide 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 rays 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 will investigate the capabilities of gamma spectrometers to assess variation in soil properties with depth from soils of two different geologies. Gamma surveys were conducted in grid of 5 columns wide by 20 rows deep where the columns are 25 cm wide and 10 cm deep the total area surveyed was 125 cm by 200 cm. The two soils that we examined are 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. The samples were analyzed for soil texture, pH, total potassium and organic carbon content. These data will be used to assess whether gamma radiometric data can be used to (i) identify horizon boundaries or (ii) characterize selected chemical properties within a soil profile.