Nonideal transport of reactive solutes in heterogeneous porous media 6. Microscopic and macroscopic approaches for incorporating heterogeneous rate-limited mass transfer
Z. Li et Ml. Brusseau, Nonideal transport of reactive solutes in heterogeneous porous media 6. Microscopic and macroscopic approaches for incorporating heterogeneous rate-limited mass transfer, WATER RES R, 36(10), 2000, pp. 2853-2867
Two major approaches have been used to incorporate heterogeneous rate-limit
ed mass transfer into mathematical models for solute transport. One focuses
on processes operative at the microscopic scale and associated grain-scale
heterogeneity, while the other stresses the macroscopic variability of the
medium and the field-scale behavior of solute transport. In this paper, we
examine the conceptual framework and model formulation of these two approa
ches in an attempt to evaluate potential commonality. Numerical solvers are
developed for both sets of governing equations, and the performance of the
se two models is tested for two systems, each incorporating one of two type
s of mass transfer mechanisms. The results show that despite differences in
conceptualization and formulation, the models produce comparable behavior
for smaller-scale systems. However, greater deviations are observed at larg
er scales. This suggests that caution should be exercised when using mathem
atical modeling for elucidating the specific processes that may be influenc
ing reactive-solute transport for a given system. We also evaluate the impa
ct of microscopic-scale mass transfer heterogeneity on field-scale transpor
t in systems for which hydraulic conductivity is spatially variable. The re
sults show that inclusion of locally heterogeneous mass transfer does not a
ppear to significantly influence the mean transport behavior for systems wi
th field-scale heterogeneity. However it does appear to influence low-conce
ntration tailing. For simulations of reactive transport over extended dista
nces, models with locally heterogeneous mass transfer may "preserve" the no
nequilibrium effects associated with rate-limited mass transfer better than
models incorporating locally uniform mass transfer when both pore-scale an
d field-scale heterogeneity coexist.