“…For instance, first-order degradation reactions of a single species can be included into PTMs by assigning to every particle a variable mass, which develops in time according to first-order kinetics [Kinzelbach, 1987;Wen and Gomez-Hernandez, 1996]. When all species share the same transport operator, certain reactions in chemical equilibrium can be easily simulated with particle tracking by using conservative components [Molins et al, 2004;Kr€ autle and Knabner, 2005;De Simoni et al, 2005;FernandezGarcia et al, 2008;Fernandez-Garcia and Sanchez-Vila, 2011], i.e., a linear combination of the species concentrations that can be used to decouple the system of equations into simpler problems. Fast kinetic reactions have been properly simulated by applying simple proximity relationships between nearby particles [Edery et al, 2009[Edery et al, , 2010.…”