  Model applicability to core-scale solute transport is evaluated using
breakthrough data from column experiments conducted with conservative tracers
tritium (H-3) and sodium-22, and the retarding solute uranium-232. The three
models considered are single-porosity, double-porosity with single-rate
mobile-immobile mass-exchange, and the multirate model, which is a
deterministic model that admits the statistics of a random mobile-immobile
mass-exchange rate coefficient. The experiments were conducted on intact
Culebra Dolomite core samples. Previously, data were analyzed using single- and
double-porosity models although the Culebra Dolomite is known to possess
multiple types and scales of porosity, and to exhibit multirate
mobile-immobile-domain mass transfer characteristics at field scale. The data
are reanalyzed here and null-space Monte Carlo analysis is used to facilitate
objective model selection. Prediction (or residual) bias is adopted as a
measure of the model structural error. The analysis clearly shows single- and
double-porosity models are structurally deficient, yielding late-time residual
bias that grows with time. On the other hand, the multirate model yields
unbiased predictions consistent with the late-time -5/2 slope diagnostic of
multirate mass transfer. The analysis indicates the multirate model is better
suited to describing core-scale solute breakthrough in the Culebra Dolomite
than the other two models.
