Comparison of Batch and Column Methods for Determining Strontium Distribution Coefficients for Unsaturated Transport in Basalt

Contaminant distribution coefficients determined under saturated conditions are often used to model transport under unsaturated conditions. Although the distribution coefficients are assumed to be consistent under different moisture conditions, this is rarely tested. Column and batch adsorption tests were used to determine strontium distribution coefficients in crushed basalt. Column tests were conducted at saturated and unsaturated moisture contents. Batch tests were conducted at several solid/liquid ratios. Preliminary column tests using bromide as a conservative tracer indicated the presence of immobile water in the column and pointed toward the use of a two-region model to determine the Sr distribution coefficients. Use of a single-region model (no immobile water), however, resulted in an average value (3.09 mL/g) not significantly different than the average value determined using the two-region model (3.41 mL/g). Moisture content had no significant effect on Sr distribution coefficients determined by applying either model to the column test data. The batch test distribution coefficient determined at the recommended standard solid/liquid ratio (0.25 g/mL) was less than the column test values and decreased significantly with increasing solid/liquid ratio. The results indicate that K ds determined with this method will not accurately reflect Sr transport in unsaturated or saturated basalt.