Retardation of dissolved radiocarbon through a carbonated matrix

Spiked solutions (14C and 3H) were passed through two columns, one filled with silica and one with a mixture of silica and carbonate sand. Significant delay of 14C vs. 3H appeared in the second material and leads to a reduction of the relative flow rate which reaches 40%. Three types of effects causing modifications of the 14C transfer are analysed: (i) flow rate, (ii) sediment aging, (iii) solution chemical composition and load. The residence time distributions show that the retardation of carbon with respect to water is governed by a linear process (assuming a constant number of reacting sites on the matrix) and kinetic process (where mass transfer effects are significant). It is also shown that much less than one mono-molecular layer of solid carbonate is involved and a distribution coefficient was estimated for these dynamic runs. Under field conditions, the interaction between 14C species (in the mobile and in the stationary phases) make ages obtained by 14C overestimated. (author)