Can chlorofluorocarbon sorption to black carbon (Char) affect groundwater age determinations?

Although adsorption is not generally considered important in low f _oc (fraction organic carbon) aquifers, we show that chlorofluorocarbon (CFC) adsorption to black carbon (BC) is sufficiently large to retard transport and affect groundwater ages obtained with CFCs. Sorption isotherms of CFC-11, -12, and -113 to synthetic wood char were nonlinear (Freundlich n = 0.71-0.94) while humic acid isotherms were linear. Moreover, sorption to char was 10-1000 times greater than to humic acid for all three CFCs at the lowest observed concentrations, C_w/S ∼ 10 ^-8-10^-7. We used the observed isotherms for char and humic acid to represent sorption to BC and amorphous organic matter, respectively, in a dual mode model to estimate retardation factors for a low f_oc aquifer (= 0.06% gC g^-1). The estimated retardation factors for the char-containing aquifer (presumed BC fraction = 9% of f_oc) were ∼6.8-10.6 at C_w/S = 10^-8 and >5 times those estimated assuming amorphous organic matter partitioning only. The results indicate that unless CFC adsorption to BC is evaluated in transport, the groundwater age determined may be biased toward older than true ages. The CFC data archived in BC-containing aquifers may contain information about its adsorbent properties that could be useful to predict retardation of other chlorinated organic contaminants.