Migration of dissolved heavy metal compounds and PCP in the presence of colloids through a heterogeneous calcareous gravel and a homogeneous quartz sand - Pilot scale experiments

In preparation to field experiments with in situ mobilized colloids, a set of pilot scale aquifer tank experiments was performed to gain an insight into the mass transfer of several heavy metal ions and pentachlorophenol (PCP) in a heterogeneous gravel aquifer and a homogeneous sandy aquifer. The experiments simulate a spill of dissolved contaminants and their subsequent transport with in situ mobilized colloids that are not yet in equilibrium with the contaminant. Contaminants with a weak tendency to adsorb to the stationary sediment matrix, e.g., PCP, establish a dynamic equilibrium with the mobile colloidal phase. The fraction of contaminants sorbed to colloids remains constant along the observed flow path. In contrast, for contaminants that strongly sorb to the sediment matrix, the fraction associated with colloids increases with increasing flow distance. In either case, the filtration of colloids is the limiting factor for the colloidal mass transfer. It is also shown that the recovery rates decrease rapidly within both sediments, and that the transport parameters (flow velocity, dispersivity) are only representative for the mobile fraction of the contaminant. On the given experimental scale, there was no evidence of a preferred colloidal transport, or even a transport of colloids ahead of the conservative tracer.