Rate-limited mass transfer or macrodispersion: Which dominates plume evolution at the Macrodispersion Experiment (MADE) site?

We present a model of solute transport that explains the large-scale behavi or of the solute tracer-test plumes at the Macrodispersion Experiment (MADE ) site as the result of advection and rate-limited mass transfer between mo bile and small-scale immobile domains. This model does not consider the pro cess of dispersion and yet provides an alternative explanation of the evolu tion of the observed concentration profiles. Compared to the macrodispersio n model, the mass transfer model better represents the change in mobile dis solved mass with time, the peak of the concentration profile, and the profi le asymmetry. Specifically, unlike the macrodispersion model, the mass tran sfer model explains the facts that the observed mass of the plume was great er than the injected mass in early snap shots of the plume and less than th e injected mass at late times. We suggest that the injected mass advects th rough the mobile domain and diffuses into and out of the immobile domain. T he immobile domain consists of a combination of low-permeability zones on t he scale of centimeters to decimeters (the Darcy-scale immobile domain), an d intragranular porosity, dead-end pores, and surface sorption (the pore-sc ale immobile domain). We suggest that the mobile domain was sampled prefere ntially when water was extracted. Therefore, at early times, relatively cle an water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations. Similarly, the mass at late times was u nderestimated because solute trapped in the pore-scale immobile domain was not extracted during sampling and therefore ignored. The combination of adv ection and slow mass transfer is consistent with the fact that the peak of the plume migrated only similar to 5 m by the termination of the experiment , as well as the different behavior of bromide and tritium tracers.