10. Mechanisms of Contaminant Transport in Aquifers

This chapter focuses on the mechanisms that govern the propagation of contaminants in aquifers. A qualitative and analytical description of the main hydrological, physico-chemical and biological process is provided. The hydrological mechanisms responsible for the transport and spreading of contaminants derive from the presence and movement of groundwater. The first of such processes is advection, according to which a compound is transported along the main direction of flow at seepage velocity. Molecular diffusion, instead, is responsible for the migration of the contaminant from high to low concentration areas, as a result of thermal agitation of water molecules. The last hydrological process is mechanical dispersion, which is a consequence of microscale heterogeneities present in the porous medium and results in a non-uniform velocity distribution relative to seepage velocity and the emergence of a transverse velocity component. During their migration within an aquifer, chemical compounds can also undergo chemical reactions that can lead to their transformation or degradation. The main reaction models and the most common types of reactions that are likely to occur in groundwater (i.e., acid-base reactions, complexation, hydrolysis, dissolution and precipitation, radioactive decay) are illustrated. Contaminant transformation and degradation can also be biologically mediated, primarily through microbial activity; such reactions are often described through first-order reaction kinetic models or Monod’s model. Finally, contaminant concentration in groundwater is also affected by sorption, a process by which compounds are removed from solution and transferred to the solid phase through a partitioning process typically characterized through isotherms. All these processes are described individually in this chapter, although in reality they occur simultaneously. The chapters that follow describe contaminant transport accounting for the concomitance of these processes.