A Large Undisturbed Column Method to Study Flow and Transport in Macropores and Fractured Media

Abstract

Intact soil columns can bridge the gap between field studies and idealized laboratory investigations of flow and transport in macropores and fractured media. However, the value of intact column studies is often hampered by shortcomings such as lack of column intactness, small column size, and column rim flow, which can cause serious artifacts and hamper system understanding. The flexible‐wall pressurized large undisturbed column (LUC) method overcomes these limitations and is a valuable approach to analyze fluid flow and solute transport in macroporous and fractured geological formations. The method investigates subsurface processes in complex media, mimicking in situ conditions and facilitating the control of system boundary conditions including effective stress. In recent years, considerable experience has been gained through different applications of the LUC approach. Modeling tools have also been developed for a detailed interpretation of flow and transport processes in LUC systems. This paper describes the steps of the LUC method from column excavation in the field to experimental setup in the laboratory. The description encompasses the key features of the sampling of LUCs in field excavations, the laboratory setup, the procedure for hydraulic and transport experiments, as well as practical challenges and potential issues during operation of an LUC system. Application examples with a fully three‐dimensional numerical model of LUC tracer experiments are also presented to illustrate the quantitative interpretation of transport processes in macroporous clayey tills.

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

“…A detailed and comprehensive description of the LUC approach is provided by Jørgensen et al. (2019).…”

“…The two LUCs from clayey tills were excavated and installed in the laboratory for further testing following the procedure described by Jørgensen et al. (2019). The experiments conducted with the two LUCs included the following steps: Hydraulic tests at fixed flow rates to determine the bulk hydraulic conductivity based on the observed hydraulic gradient (piezometers at inlet and outlet of the LUC) Long‐term flow‐through experiments with pulse injections of solutions containing a conservative ionic tracer (bromide) and a sorbing pesticide (tebuconazole), followed by a flushing period with tap water (similar chemical composition as the groundwater at the site) Injection of a dye tracer solution until visible breakthrough in the effluent water to determine active flow paths.…”

“…The LUC method was developed and applied in several studies (Jørgensen, McKay, et al, 1998;Jørgensen, Urup, et al, 2004;O'Hara et al, 2000). A detailed and comprehensive description of the LUC approach is provided by Jørgensen et al (2019).…”

Estimation of the representative elementary volume of a fractured till: a field and groundwater modeling approach

“…Hydraulic properties of till (and glaciolacustrine clay) have been estimated at the large-core scale (Grisak et al 1980;Jørgensen et al 1998Jørgensen et al , 2002Helmke et al 2005a,b;Jørgensen et al 2019) and at the field scale (Grisak and Cherry 1975;Keller et al 1988;McKay et al 1993;McKay and Fredericia 1995). Large-core studies demonstrate that preferential flow of fluid and solutes occurs in till fractures, but whether data from large cores (~ 0.5 m diameter, and ~ 0.5 m long) can be used to scale these phenomena to large-scale groundwater flow systems is unknown.…”

Are Visible Fractures Accurate Predictors of Flow and Mass Transport in Fractured Till?

“…Thus, determining the extent of fracture contribution to the groundwater flow system should be a routine component of site characterization in till-dominated regions. However, such determinations are rarely made, because measuring the hydraulic properties of fractured till is exceptionally difficult and presents a number of challenges at both the field and laboratory scale (McKay et al 1993;Jørgensen et al 1998Jørgensen et al , 2019Helmke et al 2005a).…”