Reactive Sandpacks for the Removal of Strontium-90 in Groundwater

Abstract

Reactive sandpacks have been proposed as an alternative treatment method for Sr-90 in groundwater. In concept, reactive sandpacks are installed around the screens of dewatering wells, replacing the non-reactive conventional sand pack, such that the contaminant is altered or sorbed in the ground during the dewatering process. While the concept appears to be useful, it has not been known whether reaction kinetics are fast enough in the fast moving water near the well-screen for satisfactory retention of contamination in a dewatering application. This study tested the concept of reactive sandpacks under realistic pumping conditions, by conducting in situ column experiments. Two sets of column experiments were conducted for the period of 49-55 days, with each set composed of two 10 cm columns placed in a well at the Chalk River site. The well was screened in an actual Sr-90 groundwater plume. Columns were filled with clinoptilolite (i.e., a natural zeolite), and groundwater was pumped through the columns at the velocities expected near the pumping well (33-200 m/day). Measurements of gross beta for the effluent water showed that the columns with higher flow velocities had effluents with greater radioactivities than the columns with lower velocities. The starting time and extent of the breakthrough of Sr-90 was proportional to the flow velocity. Distribution coefficients, determined by fitting the normalized aqueous gross beta data to the 1-D advection-dispersion equation, varied inversely with flow velocity, suggesting that the sorption of Sr-90 onto clinoptilolite is kinetically-controlled in this high velocity range. Radioactivities for the solid samples, retrieved from the columns after the operation, showed that sorption fronts were advancing in proportion to the flow velocity, consistent with the aqueous data. The results showed that the concept can be applied to dewatering projects if the sandpack is properly designed.