Transport of Tracers and Pesticides Through Fractured Clayey Till: Large Undisturbed Column Experiments and Model‐Based Interpretation

Leaching of contaminants through fractured aquitards such as clayey tills may occur due to typically slow matrix advection and diffusion, or it can be dramatically enhanced in the presence of preferential flow through fractures and macropores. Sorption also plays a crucial role since it can significantly impact contaminant leaching and residence times. These different mass‐transfer mechanisms imply a distinct transport behavior and very different time scales for contaminant leaching toward underlying aquifer systems. However, the prevalent controls on contaminant transport and their effects are difficult to assess. This paper shows a detailed characterization of flow and transport processes under water‐saturated conditions in two large undisturbed columns (LUCs) collected from two visually similar fractured/macroporous clayey tills typical for the Northern hemisphere. Flow‐through tracer and pesticide experiments revealed a contrasting transport behavior. In one column, transport through fractures/macropores was dominant, whereas matrix advection and diffusion had a distinct influence on solute transport in the other column. Detailed 3D discrete‐fracture‐matrix models were developed to illuminate prevalent controls on contaminant transport and to quantitatively interpret the flow‐through experiments with a minimal number of fitting parameters. Nonequilibrium sorption kinetics were included to reproduce the transport behavior of the considered pesticide. The parameters determined from the two LUC experiments were integrated in a vertical cross‐section model to investigate the influence of varying fracture properties on vertical solute transport through surficial clayey till aquitards. The analysis showed that small fracture apertures in deeper parts of the aquitard could substantially prolong solute migration times and control solute fluxes.