Biorisanamento in situ di sorgenti storiche da DNAPL. Pozzi a ricircolazione per la ottimale distribuzione di donatori di elettroni e mobilizzazione di contaminanti da zone a bassa permeabilità

In this paper, an example, on a full scale and on the Italian territory, of the design and management of an in situ bioremediation intervention and the mobilization of chlorinated solvents, in a historically contaminated industrial site characterized by a complex hydrogeological conformation is reported. In particular, the presence of historical accumulations of chlorinated substances in layers with very limited permeability, with consequent slow back diffusion phenomena, has been addressed with the use of groundwater circulation wells, IEG-GCW®, which allow the mobilization and external treatment of contaminants retained in portions of the aquifer that cannot be attacked with traditional pumping systems. Furthermore, the occurrence of reductive dechlorination phenomena limited by the electron donor deficiency was addressed by using recirculation as a system for their continuous distribution, generated by the fermentation of a biodegradable polymer (PHA). The aim of the work is also to underline the relevance of a 3D hydrogeochemical model for the representation and understanding of the contamination dynamics and decontamination mechanisms in a highly contaminated industrial site. A multi-phase approach was followed allowing the management and release of data during the various phases of remediation, from the characterization of the site, through the conduct of a pilot test, up to full-scale remediation, thus allowing to monitor, analyze and manipulate the information in 4D space-time. Multi-source and multi-temporal scenarios reveal the impact of current hydraulic dynamics and describe the decontamination mechanisms in relation to the interventions implemented over time, quantifying the overall performance of the strategies adopted in terms of reducing the concentrations of contaminants present in groundwater. The results illustrate the effectiveness of the recirculation system in the progressive reduction of the mass of contaminants in the secondary sources identified, both by mobilization of contaminants and by enhancement of the in situ biological reductive dechlorination processes. It also confirms the usefulness in the management of interventions and in the representation of the results of the integrated georeferenced model

Gestione sostenibile di un plume di contaminazione da solventi clorurati combinando interventi in situ fisici, chimici e biologici

The presence of chlorinated solvents in groundwater in industrialized and urbanized areas represents an environmental problem. An industrial plant in a densely populated urban neighborhood of a large Italian city has historically generated a widespread plume of tetrachlorethylene (PCE) in the downgradient aquifer. The management of the plume in the external areas required a thoughtful approach due to the complex hydrological context and limited accessibility. Through a multiscale and coupled hydrogeochemical approach, a remediation strategy was developed consisting in the application of different technologies that intercept the plume in consideration of the level of concentration and of the specific logistic and hydrogeological characteristics. On the one hand, coaxial groundwater circulation wells (CGC) coupled with air sparging (AS) have been used, in the areas with the highest concentration, to promote the in situ transfer of PCE from the contaminated matrices into a gaseous flow which is then treated on site. On the other hand, the injection of reagents into the groundwater has the aim of stimulating chemical reduction combined with rapid adsorption in situ, creating adsorbing areas of contaminants and stimulating the dechlorinating activity, in areas of intermediate concentration. The monitored natural attenuation was instead used in areas with a lower concentration. The development of an integrated conceptual model of the site (MCS) captures the specific geological and hydrochemical peculiarities of the site and supports the decision-making process. The multi-source model guides the configuration and localization of an advanced remediation strategy that is adapted to particular physicochemical conditions. The encouraging results obtained in the first phase of the activity show how, on the basis of a sufficiently robust conceptual model of the contaminated site, it was possible to identify the most suitable intervention technologies in a residential urban area with logistical constraints and particularly challenging potential secondary impacts