Sediment characteristics of the 2800 meter Atlantic nuclear waste disposal site: Radionuclide retention potential

The sediments of the abandoned 2800 meter Atlantic nuclear waste dumpsite have been analyzed for texture mineral composition, physical properties, cation exchange capacity and factors effecting sediment deposition, as part of an extensive program by the Environmental Protection Agency to evaluate ocean disposal as an alternative nuclear waste disposal method. The sediments physical and chemical properties are evaluated in the light of the geologic setting for their potential role in retarding radionuclide leachate migration from the waste drums to the water column. The sediments are relatively uniform silty clays and clayey silts comprised of approximately one-third biogenous carbonate materials, one-third terrigenous materials and one-third clay minerals. The biogenous materials in the sand and upper silt-size fraction are predominantly foraminifera with minor amounts of diatoms while coccoliths dominate the finer silt and clay size fractions. The terrigenous materials in the course sediment fractions are predominantly quartz and feldspar with minor amounts of mica, glauconite, and heavy minerals. Clay minerals, of the clay-size fraction, in order of abundance, include illite, kaolinite, chlorite and montmorillonite. Relatively high cation exchange capacity in the sediment (15.2-25.4 meq/100g) is attributed to the clay minerals comprising approximately one-third of the sediment. Correspondingly high Kd values might also be expected as a result of sorption of radionuclides onto clay minerals with most favorable conditions related to pH, Eh, and other environmental factors. The biogenous fraction might also be expected to retain some strontium-90 by isomorphous substitution of this radionuclide for calcium. Diagnostic heavy minerals in the sand-size fraction reflect the source areas as predominantly the adjacent continental shelf, and provide important clues concerning the mechanisms effecting transport and deposition of the sediment. Longshore currents along the coast funnel sediment into the Hudson Canyon and turbidity currents transport sediment down the submarine canyon; some of this sediment is advected in a southwesterly direction from the submarine canyon by contour currents for deposition along the continental rise. The net deposition at the waste site thus consists of the 'rain' of biogenous microorganisms, the transport of sediment from the coastal and continental shelf area by turbidity currents via submarine canyons, and transport of sediment along the continental rise by prevailing contour currents. The effectiveness of the sediment barrier relates to timely burial of the waste drum prior to leachate release from ruptured or corroded drums as well as freedom from 'short circuiting' effects such as bioturbation or other mechanisms capable of providing migration pathways for the radionuclides. (author)