Controls on the persistence of aqueous-phase groundwater contaminants in the presence of reactive back-diffusion

The persistence of groundwater contaminants is influenced by several interacting processes. Physical, physico-chemical, and (bio-)chemical processes all influence the transport of contaminants in the subsurface. However,for a given hydrogeologicalsystem, itisgenerally unclear to whichdegree each of these phenomena acts as a con-trol on plume behaviour. Here, we present a comprehensive investigation of these processes and their influenceson plume behaviour and persistence in layered sedimentary systems. We investigate different scenarios that rep-resentfundamental configurationsof common contaminantsituations. A confined aquifer over- and underlain byaquitard layers is investigated in a source-removal scenario and a constant-source equilibrium scenario. Addi-tionally, an aquitard overlain and underlain by high permeability units is investigated in a source-removal sce-nario. In these investigations, we vary layer thickness, as well as parameters governing advection, (back-)diffusion, sorption, and degradation. Extensive analysis of these results enables quantification of the influenceof these parameters on maximum down-gradient concentration, plume persistence duration, and plume spatialextent. Finally, parameterspace dimensionality reductionisused to establishtrends and regimes inwhichcertainprocesses dominate as controls. A lower limit to plume extent as a function of a novel constructed parameter isalso determined. These results provide valuable quantitative information for the assessment of the fate ofgroundwater contaminants and are applicable to a wide range of aqueous-phase solutes.