Urban context remediation: a targeted and sustainable hydrogeochemical technique to face chlorinated solvent plumes

In urban contexts where contamination persists in areas with limited accessibility, the use of sustainable and non-invasive remediation technologies becomes crucial to effectively address and mitigate environmental risks. The presence of chlorinated solvents in the environment raises significant concerns due to their persistent nature and potential health risks. These solvents, characterized by high density and limited solubility, are classified as DNAPLs (dense non-aqueous phase liquids). During their downward migration, they tend to become trapped in the microporosities of saturated and unsaturated zones, persisting in an adsorbed form. This phenomenon results in a gradual and slow-release secondary source, which contributes to the formation of long lasting contamination plumes. This study focuses on a heavily anthropized area affected by chlorinated solvents. In particular, this paper outlines a meticulous approach to remediating a tetra-chloroethylene (PCE) plume within an urban district characterized by a complex hydrological context and limited accessibility. The process included fundamental steps. Initially, an integrated geodatabase was reconstructed, combining all hydrogeochemical characterization data such as geological borehole, membrane interface probe (MIP) investigations, and chemical analyses on water samples. This facilitated detailed geomodelling, merging geological and hydrochemical information to reveal the hydrogeological structure of the subsurface, providing valuable information on groundwater quality and the evolution of the contamination plume. In addition, the incorporation of high-resolution site characterization data obtained with the MIP technique improved and parameterized the multi-source model. The fusion of the hydrogeological and physico-chemical data culminated in the development of a comprehensive conceptual site model (CSM). The CSM functions as a robust, data-driven decision support system that enables the design and customisation of two innovative and non-invasive remediation technologies. These technologies include coaxial groundwater circulation (CGC) wells with air sparging (AS) for the removal of chlorinated solvents from environmental matrices into a gaseous stream that is treated at the surface, and the injection of micrometric zero-valent iron (S-MicroZVI®) and colloidal activated carbon (PlumeStop®) to enhance chemical reduction and adsorption in situ. Hydrochemical monitoring serves as a valuable tool to unveil the intricate dynamics involved in decontamination processes. As a result, the physical approach reveals the efficacy of contamination containment, while the chemical-biological approach demonstrates the potential to reduce contaminant concentrations in urban groundwater. These results underline the importance of remediation geology via a coupled hydrogeochemical methodology to address complex contamination scenarios. This approach is key to shaping an efficient remediation strategy and promoting innovative, adaptable, sustainable, and effective solutions specifically designed for remediation actions in urban industrialised areas.