As a result of highly publicized incidences of groundwater contamination from industrial wastes, hydrocarbons, and agricultural chemicals, during the last four decades or so, significant attention was given to develop tools for risk assessment and cleanup of affected sites. Due to a large investment in research, the field has advanced to a stage where sites can be remediated to partially or fully meet cleanup and regulatory goals. However, challenges still remain at some sites to meet closure standards due to multiplicity of factors that include geologic heterogeneity, unknown sources, and knowledge gaps in the understanding of complex reactive processes, specifically with chemical mixtures. Add to this, during the last several years, a suite of new contaminants that include recalcitrant chemicals, emerging contaminants such as PFAS and pharmaceuticals, nano- and microparticle contaminants, microorganisms, etc. are receiving attention. These contaminants have produced many additional challenges that need to be overcome to attain the level of success that has been possible with traditional contaminants. Questions related to the mobility and persistence of these chemicals in both the unsaturated and unsaturated zones have to be addressed. As the regulatory standards for some of these chemicals are much more stringent (concentrations in the order of parts per trillion), the ability to whether existing modeling tools developed for traditional contaminants can be used or adapted to make accurate predictions of ultra-low concentrations have to be questioned. The questions of how best to physically and chemically characterize these contaminated sites have to be visited. This session seeks papers on both basic research on process understanding through laboratory and field research, modeling, and site characterization to address challenges associated with groundwater contamination from these new chemicals.