“…These include, for example, Monte Carlo methods (e.g., [28,29,20,15]) and perturbation techniques, such as the moment equation methods (e.g., [5,38,33,17,34,37]) and the so called first-order second-moment methods based on Taylor's expansions (e.g., [30,35,2]). All of these methods, however, are computationally demanding when applied to flow and transport problems of realistic size, mostly because they all require solving large numbers of partial differential equations on very fine spatial discretizations in order to predict the impact of the small-scale heterogeneous dynamics (e.g., [5,4,15,11,12]). …”