Within the past decade it has become widely recognized that contaminants stored in lower permeability zones can sustain groundwater contaminant plumes long after primary sources have been depleted by natural processes or remedial efforts or isolated via physical or hydraulic means. This can be of particular consequence for chlorinated solvents, where regulatory limits are generally several orders of magnitude lower than historical source concentrations (e.g. aqueous solubility for contaminants released as DNAPLs). Recognition of this problem has lead to efforts to improve site characterization methods to collect appropriate data, and then to apply numerical models allowing better prediction of effects of contaminant storage and release from lower permeability zones and improved decision making. A primary challenge is that diffusion processes are controlled by small-scale concentration gradients and the distribution and heterogeneity of lower permeability zones where diffusion processes dominate require incorporating much higher resolution, both for site characterization and also in model applications, than what is commonly practiced. This presentation will explore validity of using high resolution numerical modeling to capture dissolved contaminant mass transfer to and from low permeability zones within unconsolidated porous media aquifers. The necessary condition for matching the lab experimental data and observed field conditions to the numerical model simulations was to use fine scale spatial and temporal discretization to resolve flow fields and accurately represent both advective transport in the sands and diffusive interactions with the lower permeability zones and minimize numerical dispersion. This suggests numerical models can be used to address field-scale scenarios given sufficient characterization of the flow domain and lower permeability zone geometry, hydrogeologic properties, and with reasonable estimates of prior source history for model parameterization and boundary conditions. An overview of selected field case study results will be presented, where high resolution characterization of contaminant distributions in high and lower permeability zones were conducted at sites where the source zones have been isolated or depleted and numerical models were applied to provide insights on longer-term effects of contaminant release from lower permeability strata.