Experiment at laboratory scale to study the Back Diffusion phenomena from low permeability layers

Today the presence of contaminants in low permeability zones can represent a real limitation for a complete groundwater restoration. Low permeability layers store contaminants by molecular diffusion when plume of contaminant gets in touch with them, creating a concentration gradient between low and high permeability zones. After removal of the primary source of contamination, concentration gradient became inverse bringing a slow re-distribution of contaminant in groundwater (Back Diffusion). Therefore low permeability zones became a secondary source of contamination. This process is the primary cause of long-term plume tailing. Due to the enormous difficulties to carry out field tests, the experiments at laboratory scale assume an important role in the definition of the suitable mathematical relation to implement a correct treatment approach of these contamination scenarios. A Representation of the process was carried out using a Plexiglas tank and Sodium Fluorescein as fluorescent tracer. In the tank an aquifer represented by means of a high permeability layer with low permeability lenses was reproduced. The lenses were saturated with a solution containing dissolved Fluorescein and inside the aquifer was flushed clean water at different velocities to investigate the effects of pumping water on the low permeability lenses rate of release. An Image Analysis procedure based on the intensity of emitted light by the tracer is used to evaluate the concentration and the mass of Fluorescein released from low permeability zones. By this procedure different relations to estimate the duration of the phenomenon and the amount of mass release from the lenses are obtained. The results show a not-significant decrease of the time affected by Back Diffusion process and a slight increase of contaminants removal from low permeability lenses. The collected data demonstrate the need to manage the Back Diffusion phenomenon using alternative remediation technologies, such as groundwater circulation wells or thermal treatments.