Release characteristics of Sr and Cs from Hanford Sediments contaminated under caustic conditions. Inadvertent leaks of caustic high level reactive waste from underground storage tanks at the Hanford Site (WA, USA) released radioactive ¹³⁷Cs and ⁹⁰Sr to the surrounding sediments. The caustic nature of this waste (high pH: 8 to 14, high ionic strength: 2 to 16 M NaNO₃) has accelerated weathering of the sediments leading to the precipitation of various feldspathoid secondary phases, which have significantly retarded the movement of Cs and Sr contamination. Plans to remove the caustic waste source will eventually expose the zone of contamination to neutral and low ionic strength porewater derived from rainfall, possibly triggering release of the sorbed Cs and Sr. To investigate this scenario, we reacted pristine Hanford sediments with Cs and Sr under caustic conditions (pH >13, 2M NaNO₃) for 6 months and 1 yr to created a “contaminated” source. We have then examined the release characteristics of Cs and Sr when exposed (1) continuous flow and (2) wet-dry oscillations of a neutral, low ionic strength porewater solution (BPW) in 2-ml mini columns. Characteristics of the release curves from the continuous flow experiments will allow us to predict the principal mechanisms of the Cs and Sr in the contaminated minerals. Characteristics of Cs and Sr release from the wet-dry oscillations will be discussed relative to oscillation frequency and duration. These experimental results will be compared the model simulations of desorption using the reactive transport model Crunch.