Modelling groundwater fractal flow with fractional differentiation via Mittag-Leffler law D. P. Ahokposi, Abdon Atangana and D. P. Vermeulen Eur. Phys. J. Plus, 132 4 (2017) 165 Published online: 13 April 2017 DOI: 10.1140/epjp/i2017-11434-8