Back-Diffusion phenomena from low permeability layers of aquifers: laboratory investigations and numerical model

The aim of this thesis is to study the redistribution process of contaminants from low to high-permeability zones of aquifers in order to improve groundwater remediation. When the plume emitted by the primary source of contamination encounters low permeability layers of aquifer generates a concentration gradient between the high and low aquifer permeability zones. A molecular diffusive flux hence is induced causing pollutant accumulation in the low permeable layers (Forward-Diffusion). After the primary source of contamination is reduced, the pollutant concentration in the transmissive zone decreases and the concentration gradient reverses. Previously stored contaminant is released by the so-called “Back-Diffusion” process, which causes a long plume tail. Thus, contaminated low permeability zones of aquifer represent long-term sources of contamination, today considered as one of the main obstacles to effective groundwater remediation. The Back-Diffusion phenomenon was reproduced at laboratory scale reconstructing an aquifer characterized by a high-permeability layer and three different low-permeability zones. The measurement of contaminant fluxes released from the low permeability lenses by image analysis technique permitted to evaluate the influence of the lenses granulometry on the pollutant redistribution process. The results show that the lower the average grain dimension of the formation is, the higher the flux is and the longer the time for which the values of the released concentration can constitute a source of contamination. It is known the large use of pumping technologies to restore groundwater, so the efficiency of the traditional pumping technology to remediate contaminated low permeability zones of aquifer was investigated by laboratories tests. The remediation time of the three lenses were calculated under different groundwater velocities using the diffusive flux values obtained by image analysis. In this way, a relation between “groundwater velocity” and “remediation time” was determined for each lens. This relation shows the low efficiency of the traditional pumping system to restore contaminated low permeability zones of aquifers, demonstrating the need to use alternative technologies. The Groundwater Circulation Well (GCW) system was investigated as an alternative remediation technology. The GCW system creates vertical groundwater circulation cells by drawing groundwater through a one screened section of a multiscreen well and discharging it through another screened section. The suitability of this technology to restore contaminated low permeability layers was investigated by laboratory test. The collected data were used to calibrate a model created to simulate the Back-Diffusion process and to evaluate the effect of pumping technologies on the depletion time of that process. Results show that GCW develops a hydraulic gradient inside the aquifer, which supports the diffusive flow into the lenses and increases the concentration gradient at the interface between low and high permeability zones of aquifers. The efficiency of the GCW system appears dependent on the position of the low permeability zones, but it seems always higher than traditional pumping technology one.