Subsurface resources, such as aquifers and the vadose zone, are subject to multiple sources of contamination that may pose risk to human health and the functioning of ecosystems. Sources of pollution include waste disposal facilities, accidental spills of toxic substances, landfill leachate, agricultural activities, industrial water discharge, deep nuclear waste repositories, mining waste management (e.g. acid mine drainage) and seawater intrusion. Improved understanding of the mechanisms controlling plume dispersion and dilution is critical to better manage the subsurface environment in an effective manner. However, predicting the fate and transport of these substances in the subsurface environment and estimating the associated risks are challenging tasks given the presence of hydrogeological heterogeneity at a broad range of scales and multiple sources of uncertainty stemming from the incomplete characterisation of the subsurface. This session aims to attract contributions that focus on developing tools that 1) address fundamental problems in contaminant transport in the subsurface environment at different scales, 2) quantify uncertainty in model predictions, 3) identification of pollution sources using tracer techniques, and 4) provide practical solutions to site management. The topics covered in this session are well aligned with the "23 Unsolved Problems in Hydrology" presented in Blöschl et al. (2019). This session will bring together experts from hydrogeology, uncertainty quantification and risk analysis and invites contributions ranging from analytical and numerical modelling of contaminant transport in the subsurface environment and data analysis of contaminant sites.