Chlorinated solvent plume remediation in urban context: a targeted hydrogeochemical approach

The presence of chlorinated solvents in the environment is a major concern due to their persistent nature and potential health risks. These solvents, characterised by high density and low solubility, belong to the class of so-called DNAPL (dense non-aqueous phase liquids). They can remain trapped in the microporosities of the unsaturated and saturated zones as they migrate downwards, persisting in an adsorbed form. This phenomenon leads to the creation of a slow-release secondary source generating persistent contamination plumes. In urban water systems, the presence of chlorinated solvents presents a challenge for non-invasive remediation technology, therefore it is important to identify interventions that combine a strategic methodological approach that is compatible with logistical difficulties also in terms of accessibility. This study addresses the remediation of a chlorinated solvent contamination plume in an urban area by adopting a multi-phase approach for the gradual refinement of a conceptual model, the deployment of a remediation strategy, and the monitoring of remedy performance. The first stage involved the reconstruction of an integrated geodatabase that joined all the hydrogeochemical 1 characterisation data. The second phase focused on geomodelling, in which geological and hydrochemical data were interpolated to provide information on the hydrogeological structure of the subsurface, giving insights into groundwater quality and the evolution of the contamination plume. Data from the Membrane Interface Probe (MIP) investigations were also integrated into the model, providing high-resolution site characterisation. The integration of the hydrogeological and physicochemical data led to the development of a comprehensive conceptual site model (CSM) that serves as a data-driven decision support system. This multi-source model has guided the implementation of targeted remediation solutions. These technologies include coaxial groundwater circulation (CGC) wells with air sparging (AS), and the injection of micrometric zero-valent iron (S-MicroZVI®) and colloidal activated carbon (PlumeStop®) to enhance chemical reduction and adsorption in situ. The 3D Conceptual Site Model (CSM), outlines the site-specific geological framework, groundwater circulation pattern, source of secondary contamination, and plume geometry. The CSM was fundamental to understanding the spatio-temporal patterns of contaminants dynamics, and in combination with groundwater monitoring, monitored the effectiveness of interventions in containing contamination and reducing concentrations through a combination of chemical, physical and biological processes. The success of data-driven interventions is widely demonstrated by this dual achievement. Thus, by combining a comprehensive conceptual model, continuous groundwater monitoring and a series of effective intervention strategies, it is possible not only to understand and observe the behaviour of contamination, but also to actively manage to reduce its impact.