We consider the application of methods based on deriving the probability density function (PDF) of the space-time evolution of the concentration of a solute undergoing linear and nonlinear transport processes in a geo-chemically heterogeneous porous medium. We start by formulating the governing equations for a class of reactive transport problems in porous media, and then derive the corresponding PDF equations. The latter step requires a closure approximation. We show that traditional perturbation expansions fail to provide robust closures, and that the accuracy of such closures deteriorates with time. An alternative closure that we pursue here is based on a Large Eddy Diffusivity (LED) approximation. The PDF equations are analysed in detail for the case of advective transport in a uniform velocity field, with a solute undergoing linear heterogeneous reactions. We present analytical solutions for the first and second ensemble moments of the solute resident concentration as well as closed-form analytical expressions for the time-dependent macro-dispersion coefficient and the effective reaction rate.