Aquifer Vulnerability to Pesticide Migration Through Till Aquitards

Simulating seasonal variations of tile drainage discharge in an agricultural catchment

“…In the models presented here, only matrix flow was simulated in the porous media toward the drains, although field evidence shows the presence of macropores and fractures in the topsoil and clayey matrix. Sand lenses, macropores, and fractures have been shown to be major preferential water flow pathways for near surface deposits in Danish soils [Jørgensen et al, 2004;Kessler et al, 2012]. A more detailed representation of these heterogeneities in the model could have [2015].…”

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

“…Flow and transport simulations were run on 2D vertical cross sections with four equally spaced fractures on a domain width of 2 m, hence, a spacing of 0.5 m according to the size of an LUC and as a spacing common in clayey tills (Jørgensen, McKay, et al., 2004). A periodic system was considered and the symmetry was exploited by simulating one 0.5 m fracture‐matrix interval and mirroring it horizontally for further evaluations of flow and transport in the considered domain.…”

“…Hence, advective transport in open macropores and major fractures happens relatively fast compared to transport in the clayey till matrix where diffusion is the dominant transport process (Jin et al., 2014; Koestel & Larsbo, 2014; Muniruzzaman & Rolle, 2019; Parker et al., 2008; Rolle et al., 2013) However, the appearance of large‐aperture fractures (few tens to a few hundred micrometers) and macropores (e.g., earthworm burrows, root channels) is often limited to the few uppermost meters of the soils. The fracture density often decreases below that (Harrar et al., 2007; Jørgensen, McKay, et al., 2004; Mckay & Fredericia, 1995) and smaller apertures can be observed (Helmke et al., 2005a; Young et al., 2019), with few to no macropores. For long residence times, matrix diffusion can considerably slow down contaminant transport through fractures due to a continuous transfer of the solute from the fractures to the matrix (e.g., Carrera et al., 1998; Grisak & Pickens, 1980; Lipson et al., 2005; Mosthaf et al., 2018), with the solute flux into the matrix that may be enhanced by sorption to solid phase organic carbon (e.g., Parker et al., 1994).…”

“…The LUC method aims at a minimal disturbance of natural fractures and at mimicking field conditions (confining stress and temperature) in the laboratory. The method allows the determination of parameters required for assessing the risk for leaching of contaminants, such as pesticides, chlorinated organic compounds, or nitrate (Jørgensen, Broholm, et al., 1998; Jørgensen, McKay, et al., 2004; Jørgensen, Urup, et al., 2004; O’Hara et al., 2000). Typical features such as macropores, fractures, and clayey‐till matrix properties and their influence on solute transport can be investigated, since such features are well represented and well defined in the large column samples (diameter of 0.5 m and height of 0.5 m).…”

Seasonal variations in groundwater upwelling zones in a Danish lowland stream analyzed using Distributed Temperature Sensing (DTS)

“…The clayey tills, with low matrix hydraulic conductivity, form an aquitard in the area. However, studies have shown that the upper 5-10 m are fractured, with relatively high hydraulic conductivity, thus forming a shallow groundwater aquifer in the top layer of the till (Jørgensen et al, 2004;Rosenbom et al, 2009). This heterogeneity played a crucial role in shaping the remaining local scale interactions represented by the highly dynamic stream temperature observations and influenced by the hydrogeological setting as well as the permeability and width of the riparian area aquifer determining the flow path distribution.…”