Field demonstration of in situ treatment of buried low-level radioactive solid waste with caustic soda and soda ash to immobilize /sup 90/Sr

Abstract

A low-level radioactive solid waste disposal trench was injected on four occasions with solutions of caustic soda, soda ash, caustic soda, and lime/soda ash, respectively. Because investigations had indicated that /sup 90/Sr could be coprecipitated with soil calcium carbonate by treatment with soda ash, this demonstration was undertaken as a test of its technical feasibility. After concentrations of /sup 90/Sr and water hardness decreased within the intratrench monitoring wells; one well at the foot of the trench decreased from over 100 to a persistent level of less than 10 kBq of /sup 90/Sr per liter. Recharge of /sup 90/Sr from the trench to a sump immediately below was reduced by about 90%. Water hardness and /sup 90/Sr concentrations were strongly correlated through time within each monitoring well, indicating that /sup 90/Sr behaved as a tracer for soil calcium and magnesium. The disappearance of /sup 90/Sr from the trench water, therefore, was an in situ water softening. Soil samples retrieved from the trench indicated that as much as 98% of the total /sup 90/Sr was present as a coprecipitate with calcium carbonate. The hydrologic characterization of this trench indicated an average void space of 41% and an average trench-wall hydraulic conductivity of 3.4 x 10/sup -7/ m/s. Sampling of the trench's discharge contamination plume indicated that it had resulted from a combination of subsurface seepage and bathtub overflow during infrequent periods of intense precipitation. A generic assessment of soda ash treatment indicated that treatment would be most effective for soils of high cation exchange capacity with either low (< 20%) or high (> 80%) basic cation saturation of that cation exchange capacity.