“…In recent two decades, some researches indicated that the classical model was inadequate to simulate many real situations, where a particle plume spreads slower than predicted by the integer-order diffusion equation. See, e.g., Adams and Gelhar (1992), Berkowitz, Scher, and Silliman (2000), Giona, Gerbelli, and Roman (1992), Hatano, Y. and Hatano, N. (1998), Nigmatullin (1986), Xiong, Huang, and Huang (2006), Zhou and Selim (2003), and the references therein. Such slow diffusion is called anomalous subdiffusion, and one model equation is the time fractional diffusion equation given as…”