Uncertainty assessment with multi-conceptual models and sedimentology in a key role – The MADE case 

Reliable modelling of contaminant spreading remains a formidable challenge in hydrogeology. The MADE (macro-dispersion) natural-gradient tracer field experiments and related data provide an excellent opportunity to test methods for solute transport and related uncertainty.Most published MADE- models have been based on a single conceptualisation and on spatial hydraulic conductivity structures derived from large volumes of published data. At best, this approach allows parametric uncertainty to be determined, but it provides no assessment of the impact of conceptual uncertainty.Herweijer et al. (2026) conducted a ‘back to basics’ review of the original MADE reports and concluded that there are significant unexplored conceptual issues that influenced the migration of the tracer plume and or biased observations. These issues include geology (sedimentological heterogeneity at three scale levels), unreliable measurement of hydraulic conductivity, biased tracer concentrations, and a non-stationary flow field. As a result, we developed a framework of knowns and unknowns, with the latter category being very important for further uncertainty analysis.We demonstrate that, with limited drilling and hydraulic conductivity data, but using sedimentological inferences, a 3D spatial hydrogeological architecture can be established for MADE. Using this architecture and lithological information a heterogenous hydraulic conductivity field can be established. The latter involves some alternate conceptual models reflecting sedimentological uncertainty, which can be further constrained using affordable and reliable piezometric data. Additional conceptual models are proposed to test uncertainty in boundary conditions and data validity.The known-and-unknown framework yields an ensemble of numerical and analytical models that can be built to address the underdetermined nature of modelling results arising from multiple concepts and imperfect data. It is concluded that the ensemble result would provide a more holistic assessment of transport uncertainty Herweijer J.C., S. C Young, P. Hayes, and O. Batelaan, 2026, A multi-conceptual model approach to untangling the MADE experiment, Accepted for Publication in Groundwater.