Geochemical and mineralogical properties of Copper Ridge and Chepultepec Regolith at The Oak Ridge National Laboratory Reservation-West Chestnut Ridge Site

The nature and properties of regolith overlying the Copper Ridge and Chepultepec formations of the Knox Group at the Oak Ridge National Laboratory Reservation-West Chestnut Ridge Site (WCRS) were investigated to determine (a) their natural ability to adsorb dissolved waste, (b) the weathering products of siliceous carbonates, and (c) processes leading to pedogenic horizonation.

With the exception of ion exchange capacity, standard soil methods were used to determine particle size distribution; pH and salt pH; bulk chemical composition (<2mm) using XRF; mineralogical composition using XRD of <2mm size-fraction, <2μm size-fraction, and magnetic nodules; and extractable amorphous oxyhydroxides of Fe, Mn, Si, and A1 analyzed by AA and colorimetric techniques.

The results of this study include identification of a maximum clay concentration at approximately 3 meters, a relatively high concentration of sand and silt near the surface, and deeper, variable concentrations of sand, silt, and gravel layers inherited from the parent rock. The pH values ranged from 4.5 to 5.7. Bulk mineralogy (<2mm) was predominately quartz with minor amounts of kaolinite, illite, and aluminum hydroxy-interlayered vermiculite (HIV). Clay fraction mineralogy, in order of abundance, was kaolinite, quartz, illite, and HIV. The magnetic sand-sized nodules were most concentrated in the upper soil horizons and were composed of maghemite, hematite, and quartz. Bulk chemical composition revealed relatively high concentrations of the soluble cations Mg⁺⁺, K⁺, Ca⁺⁺, and Rb⁺⁺⁺ at the surface; concentrations decreased with depth until below 2 meters where they became highly variable. Iron trends were similar to Ti trends; A1 trends were inversely related to Si trends; and A1 closely correlated to clay content. Cation exchange capacities ranged from 30 to 20C mmol (½ Ba⁺⁺)•kg^-1 or 3 to 20 meq/100g; anion exchange capacities were very low, below the detection sensitivity of the XRF method. Amorphous oxyhydroxides were most concentrated near the surface with highly variable concentrations below 2 meters.

The adsorption capacity of the regolith, dependent upon amorphous oxyhydroxides and clay content, is likely to change once it has been subjected to chemical waste due to the chemical vulnerability of oxyhydroxides of Fe, Mn, Si, and A1. Anionic species are not significantly adsorbed by the regolith at WCRS. Detrital illite is converted into HIV by weathering which results in the stripping-out of the K⁺ ion, exemplified by high concentrations of HIV in the upper soil horizons which decrease with depth while correspondingly low concentrations of illite in the upper soil horizons increase with depth. Soluble cations of Mg⁺⁺, K⁺, Ca⁺⁺, and Rb⁺⁺⁺ are concentrated near the surface due to biocycling by plants and recent agricultural practices.