Technetium Accumulation within Reduced Sediment Horizons.

On the Department of Energy’s Savannah River Site (SRS), located near Aiken, SC, acidic wastes containing metals and radioactive materials were discharged into a series of unlined seepage basins from 1955 to 1988. The acidic plume eventually reached the water table aquifer and migrated to a seepline adjacent to the Fourmile Branch (FMB) stream, a tributary to the Savannah River. Local regions of tree mortality were observed along the FMB seepline during the 1980s and 1990s. Recently a set of soil cores were collected from the seepline along FMB in an area with a documented history of low-level ⁹⁹Tc contamination.  Additional soil cores were collected from a non-contaminated area along FMB for comparison.  The soil cores were immediately transferred to an anoxic chamber for storage prior to extraction. Samples representing distinct layers were subjected to an operational extraction procedure in an effort to discriminate between soluble/exchangeable ⁹⁹Tc (i.e., 0.01 M CaCl₃) and less soluble forms that have precipitated due to chemical reduction (i.e., 2 M HNO₃ combined with 10% H₂O₂). Following extraction, ⁹⁹Tc was determined by both liquid scintillation counting (LSC) and inductively coupled plasma – mass spectrometry (ICP-MS).  Although the ⁹⁹Tc levels were quite low, even for soil cores from the impacted seepline, there was evidence for limited ⁹⁹Tc accumulation within organic-rich, reduced sediment layers at the seepline surface.