Distribution and retention of 137Cs in sediments at the Hanford Site, Washington

(137)Cesium and other contaminants have leaked from single-shell storage tanks (SSTs) into coarse-textured, relatively unweathered unconsolidated sediments. Contaminated sediments were retrieved from beneath a leaky SST to investigate the distribution of adsorbed Cs-137(+) across different sediment size fractions, All fractions contained mica (biotite, muscovite, vermiculatized biotite), quartz, and plagioclase along with smectite and kaolinite in the clay-size fraction. A phosphor-plate autoradiograph method was used to identify particular sediment particles responsible for retaining Cs-137(+). The Cs-bearing particles were found to be individual mica flakes or agglomerated, smectite, mica, quartz, and plagioclase. Of these, only the micaceous component was capable of sorbing Cs+ strongly. Sorbed Cs-137(+) could not be significantly removed from sediments by leaching with dithionite citrate buffer or KOH, but a fraction of the sorbed Cs-137(+) (5-22%) was desorbable with solutions containing an excess of Rb+. The small amount of Cs-137(+) that might be mobilized by migrating fluids in the future would likely sorb to nearby micaceous clasts in downgradient sediments.