  We present a hybrid approach to groundwater transport modeling,
"CTRW-on-a-streamline", that allows continuous-time random walk (CTRW) particle
tracking on large-scale, explicitly-delineated heterogeneous groundwater
velocity fields. The combination of a non-Fickian transport model (in this
case, the CTRW) with general heterogeneous velocity fields represents an
advance of the current state of the art, in which non-Fickian transport models
or heterogeneous velocity fields are employed, but generally not both. We
present a general method for doing this particle tracking that fully separates
the model parameters characterizing macroscopic flow, subscale advective
heterogeneity, and mobile-immobile mass transfer, such that each can be
directly specified a priori from available data. The method is formalized and
connections to classic CTRW and subordination approaches are made. Numerical
corroboration is presented.
