“…Two alternative differential equations have been developed [see, e.g., Gardiner , 1983]: the forward Fokker‐Planck equation (FPE) and the backward‐in‐time Kolmogorov equation (BKE) [ Kolmogorov , 1931]. The use of forward and backward models for location and travel time probability has become a classical mathematical approach for contaminant transport characterization and prediction [ Uffink , 1989; LaBolle et al , 1998; Varni and Carrera , 1998; Neupauer and Wilson , 1999; Spivakovskaya et al , 2005]. The forward version of the Fokker‐Planck equations points to the future, and the transformation from probabilities and particle densities to a physical concept of a solute concentration is straightforward.…”