The size of aquatic humic substances (HS) influences many aspects of their physicochemical behavior, including proton and metal binding, organic pollutant partitioning, and adsorption onto mineral surfaces. However, in addition to differences in size, HS display a wide range of chemical structures across their molecular weight (MW) distribution. Approaches are needed to cut through this complexity to develop reliable conceptual and quantitative models of HS behavior. This dissertation deals specifically with HS transport in sandy porous media, and asks whether MW could be used as a key parameter to help predict HS mobility.  To answer this question, we conducted a series of column experiments using aquatic HS, polystyrene sulfonate (PSS) MW standards of various sizes, and natural aquifer sand. Column influent and effluent were analyzed to track changes in the HS MW distribution over time. Differences in retardation of various size fractions of HS and PSS suggested that MW could indeed be considered a key parameter, although questions remained as to whether different MW fractions interact with one another, leading to behavior that differs from the simple sum of the component parts. Additional experiments using mixtures of PSS standards revealed that models must take interactions between different components into consideration.  Finally, transport of HS was modeled using a continuous time random walk (CTRW) approach. By explicitly considering differences in the transport rates of different HS fractions, the CTRW model captured HS transport behavior more accurately than the conventional advection-disperision equation (ADE) under all conditions examined. This suggests that the multi-component nature of HS is critical to understanding their mobility in the environment, although adsorption kinetics and interactions among HS components are also likely to play a role.  In summary, MW is one of several parameters that control HS adsorption and transport. While a strong correlation between MW and HS mobility exists, knowledge of MW alone is not sufficient to predict HS transport. However, a basic understanding of variability in transport behavior across the MW distribution can provide insight into how different fractions of HS - and substances associated with those fractions - travel through the subsurface.